spiney
Guest
- Joined
- Apr 29, 2005
- Messages
- 1,514
- Reaction score
- 1
- Points
- 0
- My Satellite Setup
- Pace 2200 Sky digibox with ftv card, Comag SL65 FTA sat receiver, 40cm Sky minidish, Setpal terrestrial receiver (for free uk tv only!).
- My Location
- Midlands
Er, this "guide" is overdue for tidying up! Please be patient .....
Contents:
1) Mini guide, quickie HDTV "what's what", quick links for FAQs and current info.
2) Main guide, complete HDTV technology starting from first principles, links to various technical information.
(will update guides as/when I've time, use the links just below for latest news!).
(Below, there's lots of weblinks! If any website owner objects to a link, please say so, and I'll remove it).
MINI GUIDE: INTRODUCTION, BEGINNERS' INFO, QUICK LINKS:
(quick links also further below, in "main guide"):
(Intro).
1. HD receivers.
2. HD screens.
3. Quick links.
4. UK info.
HDTV pictures are "far more detailed" than older format analogue and digital tv. Very roughly, there's up to twice as much "resolution" - both horizontally and vertically - or, put slightly differently - up to four times as much "total picture information".
(((Hence, delivering such pictures requires up to 4 times the previous "bandwidth", see further below. Mostly, this is "got round" by using advanced data compression techniques, but a significantly bigger capacity transmission system is also needed!))).
Receiving/Viewing HDTV requires:
1) Receivers.
An HDTV receiver (capable of decoding the newer format HD signals). Can be a satellite or cable type of "box".
(HDTV on UK digital terrestrial is technically achievable, but unlikely in the immediate future, due to current system capacity constraints).
Cable HDTV is "deliverable" over existing cable systems, both older coaxial and newer ADSL enabled telephone lines, but - obviously - requires a newer type cable tv receiver.
(Whether cable HD is available in your area depends on how quickly the local company's distribution equipment gets "upgraded").
Sky HD can re-use existing Sky dishes, but also requires a newer type receiver. Note that this is a twin sat antenna input "PVR" device - same as Sky+ - so requires a twin (or quad) output lnb, and 2 cables from dish (presumably, Sky will upgrade existing dishes where necessary).
(Without the 2nd antenna input, a Sky HD receiver will still work, from an existing dish, but can't then record 2 channels at once!).
Some free-to-air satellite HD is also due to start - eg BBC sports transmissions, etc - and such can be received on suitable non-Sky DVB-S2/MEG4 satellite receivers (not requiring 2 antenna feeds!). However, only Sky proprietary receivers can get the encrypted Sky subscription channels (whether standard or high definition)!
"Real time" HDTV is also theoretically possible over the Internet, but there are bandwidth capacity problems, although just transferring HD MPEG4 content data files - slower than "real time" - should work.
2) Screens
An "HD ready" newer type tv screen. Most HD receivers will also output to older type tv sets, but there's not much point in buying expensive new HD equipment, then doing only that!
(Note, "HD ready" means a tv screen will accept and display the newer format signals. It does NOT necessarily mean the picture will be displayed in the maximium possible resolution, since this depends on how many "dots" (pixels) a particular screen has, better is usually more expensive! There's also issues - with various different screen types - of picture quality, lifetime, reliability, etc).
Most HD ready tv "screens" are either LCD or Plasma flat ("hang-on-wall") ones, but projection types are also avaliable.
The 2 main HD broadcast picture "formats" are: 1280x720, and 1920x1080. These refer to (horizontal)x(vertical) "number of dots" (or pixels), and are ususally abbreviated as 720 or 1080 (the corresponding horizontal figure being implied). A "very good" screen will be able to display the full higher resolution mode, lower resolution ones won't show as much detail!
(There's also several possible picture rates, and i (interlaced) and p (progressive) scan modes, for explanations see further below).
(Most HD capable tv screens can accept a wider variety of input modes, eg various computer screen formats, etc).
The HD video signal is sent between HD receiver and HD screen in digital form, via a special new type of connecting cable, HDMI/DVI, confusingly there's 2 different plug types, but all "HD ready" receivers and screens using this connection system should be compatible.
(Older HD ready tv screens use the "obsolete" DVI plug, which doesn't carry sound, so then an additional audio cable is needed, but you'd probably do that anyway, since the Dolby 5.1 surround sound is best with a separate "home theatre" audio system!).
There's an issue with the HDCP copy protection system used on HD broadcasts. Any receivers/screens labelled "HD ready" should be fine, but some older receivers and screens - including computer equipment - able to accept and display the different HD video formats and show the necessary picture resolution, but not having HDCP capability, may not be able to show most HD material (see below, copy protection).
(((Much current Internet HDTV info refers to the USA, where hd IS broadcast terrestrially, but using the ATSC system (all European digital terrestrial tv uses the entirely different COFDM system), so be aware of the difference, ATSC equipment is useless in Europe!))).
3) Links.
UK HDTV quick guide (faqs), up-to-date: High Definition Television (HDTV) FAQ for the UK (Sky HD, Telewest, BBC World Cup ) .
Latest UK HDTV news (services, products, etc); scroll a little down the page for "categories", different subjects and archives: HDTV UK .
TV screen types compared:
CNET's quick guide to TV types - CNET reviews .
Connecting cables: Welcome to DVIHDMICables.com - The DVI HDMI SVGA Information Source .
4 ) CURRENT UK SITUATION (updated 14/07/2006).
SKY. About 15 HD channels currently available. Sky HD home installations have now been going for a few months.
UK Sky HD info: Sky HD - Wikipedia, the free encyclopedia .
(official website): Sky HD - High Definition TV (HDTV) in the UK from Sky .
Unfortunately, there are various reports of a high problem rate with Sky HD receivers! For more details, check the Sky HD sections/threads in our forums, and/or search for further info using Google, etc.
CABLE. The merged NTL/Telewest "tv drive" - with HD capability - has been available for some months, with HD channels and some “playback on demand”. Use of the HD facility requires an extra “premium” payment (as you’d expect!).
Telewest - TV Drive .
(There’s now also various ASDL services down your phone landline – including TV – for details of HDTV currently offered, consult the various companies’ websites).
BBC HD is now established, with a good range of actual HD material (some broadcasters are just “scaling up”, standard material, which admittedly gives slightly better viewing results, but isn’t really HDTV!). This is basically free-to-air (some World Cup was encrypted, for copyright reasons). However, it is being described as a “trial”, with no guaranteed continuation beyond the I year trial period.
No firm news yet on ITV ch4/5, and other UK “maybe” free-to-air HD broadcasts.
EUROPEAN (non UK) SATELLITE HD. A number of channels are broadcasting an HD service, in addition to the older SD one (eg, Sat1 HD). These free-to–air HD channels show many American/British films, but with a dubbed soundtrack! (An exception is Arte, where the French version often – not always! - shows “classic” USA/Brit films with original soundtrack, but the in-video French subtitles will look rather distracting on the HD version!).
ENCRYPTED Sky HD channels can only be received on Sky HD boxes (of course!). Otherwise, presumably, non-Sky encrypted European channels can be received on HD capable sat receivers, if you “know how” (nudge, wink).
HD CAPABLE SAT RECEIVERS are now becoming available and better known, with a slowly increasing range and choice, eg the popular Humax and Pace receivers, for which see relevant threads on these forums.
* * ** * * * * * * * ** * * * * * * * ** * * * * * * * ** * * * * * * * *
MAIN GUIDE.
(intro)
1) Preconditions for any tv picture.
2) Standard definition television (SDTV).
3) Digitising SDTV (bandwidth implications).
4) Compression for digitised SDTV (MPEG).
5) HDTV delivery/receivers .
6) The HDTV receiver/screen link.
7) HDTV display devices.
8) HDTV storage/recording.
9) HDTV copy protection.
(here's an brief "World HDTV standards overview": High-definition television - Wikipedia, the free encyclopedia ).
PURPOSE OF THIS GUIDE:
Meant to be a "very roughly what's what" info source, for people "unfamiliar with the territory". It's a "whistlestop tour", going - perhaps rather slowly! - through basic principles, to actual implementations. So, just look for the particular things you want, more practical stuff is further down.
(The basic technical principles apply to all HDTV, which is a "worldwide standard", but details of particular broadcasters and/or delivery systems are UK specific, since this is a UK site!).
(((As in my other guides, I'm trying to "pitch" at about halfway between 1 page "consumer faqs", and technical papers; there's lots of each type on the Internet, but not that much in between!
It's a Spiney guide, so extremely "wordy" and OTT, I'd rather annoy people that way round than by being too cryptic! Brevity may be a virtue, but ambiguity isn't!))).
Warning/disclaimer, I'll try to be accurate, but no guarantees (obviously), so carefully check specific info (especially before any financial commitiments!).
(Hopefully, other people will soon be adding more guides, general and specific. I'm really "Mr Theory", and anyway - obviously - can't go into exact details on lots of particular products. Where weblinks mention specific products, these are just particular examples I found, and not any sort of endorsement ...... ).
Please forgive obvious "howlers", and "deliberate mistakes" (!).
(((the added weblinks - for more info - are hopefully a) fairly clear;
reasonably fast; c) have helpful pictures. But, I can't spend forever "web-trawling" for the "very best" ones, and of course webpages can change or vanish ..... where links do vanish, then wikipedia often - but not always - has a good entry, it's fairly up to date on tv technology. see:
Categoryigital television - Wikipedia, the free encyclopedia .
Category:High-definition television - Wikipedia, the free encyclopedia .
Material on webpages may be copyright, but there's a "right to view". However, if any website owner objects to a link on this guide, please say so and I'll remove it))).
GENERAL INFO:
(HDTV is already going in USA, eg see :: HDTV Galaxy :: HDTV Schedule ).
(to see HDTV user problems in North America, there's www hidefforum com . I've not made that a direct weblink, in case Rolf objects (but hey, aren't they using the same website software?). Note, "Direc" and "Dish" sat tv systems are roughly equivalent to British Sky. Also - importantly - in North America HDTV is currently being broadcast terrestrially, and you can get standalone and PC plugin card receivers which get both standard def and hi def broadcasts, however these all decode only ATSC modulation, and would be useless in any EU countries, as in Europe we use COFDM modulation instead for terrestrial digital. Also, HD on UK Freeview/ Topup is very unlikely before analogue switchoff frees up some extra bandwidth!).
There's this UK site: HDTV UK (useful for latest news; note my specific info on the rapidly changing UK situation - below - probably isn't "bang up to date"!)
(Some useful UK FAQs at: High Definition Television (HDTV) FAQ for the UK (Sky HD, Telewest, BBC World Cup ) )
European satellite HDTV already exists (EURO1080:::::European High Definition Media Group) but older type (DVB-S1/MPEG2) receivers will be incompatible with any upcoming UK "commercial" services using DVB-S2/MPEG4.
Some DVB-S2 MPEG4 satellite receivers are becoming available, but unlikely to receive much (if any) Sky HD, due to Sky's proprietry encryption! A PC plugin card has also just become available, but of course nearly all current sat receiver PC cards don't have a CI interface for conditional access.
(How much free-to-air - or at least non-Sky - satellite HD will be available in the UK remains to be seen!).
"HD READY". This logo, on "screens", means they can accept a TMDS input (see below), and therefore have either a DVI socket, or HDMI sockets (preferably HDMI, being the newer version, though "converter plugs" are reasonably cheap). But, beware! There are rumours of some shops putting "hd ready" labels on stuff that isn't, if unsure then check that the correct type sockets are on that particlar item. Be especially careful, becase now HD is coming, some older "not-HD-ready" flat screens are being "sold off" at reduced prices!
(Note, HD Ready does NOT necessarily mean a screen can show the HD content at full resolution, but only that it can accept the signals; what a particular screen can show depends on its own maximum resolution!).
HDCP (high density content protection) - insisted on by hi def tv programme broadcasters - is being universally implemented on all HDTV reception equipment (any "hd ready", ie, having TMDS inputs/outputs), and might make copying and storing HD stuff difficult in future.
KEY HDTV CONCEPTS.
The two main ones are "resolution" (amount of detail in picture) and "bandwidth" (ability of system to handle wide frequency range, especially high frequencies!). These are related, in that higher resolution requires greater bandwidth. I'll try to show how, starting with SDTV .....
(Note, some bandwidths are suggested below, but merely as illustrations and only approximately, actual "real" TV systems will have various bandwidths specific to them ..... ).
PRECONDITIONS FOR A TV PICTURE.
Call separate pictures "frames".
For illusion of continuous motion, at least 12 frames/sec are required (eg, Zoetrope, RANDOM MOTION: Zoetrope Animation by Ruth Hayes ). But faster is better. Early cinema used 16 frames/sec, and later 24/sec (current standard).
However, the eye still sees flickering! So, cinema projectors show each frame twice (ie, 48 pictures/sec; some 70mm projectors showed each frame 3 times, though this film standard is now obsolete!).
48 frames/sec - or faster - gives "tolerable" flicker, so this was used for analogue tv systems. However, the rate had to be sychronised with local mains power frequency (otherswise, you got horizontal "bars" slowly going up/down screen, from early technology limitations). So, frame rate was actually 50/sec Europe, and 60/sec USA.
Picture transmission. Sound - audio - is a single quantity varying with time, hence easily transmissible (down a "communications channel"). But a picture's an area, which you can't "send all together at once", so it must be broken up into tiny dots, and these are then sent sequentially
TV picture resolution. This can be considered as a "rectangular dot pattern", with format: (number1 Horizontal) x (number2 Vertical). Obviously, for a fixed screen size, the more pixels (dots!) there are, the more detailed the picture. However, as resolution increases (ie dots get smaller), you're multiplying 2 numbers together, so total number of dots required increases very rapidly, according to square of the screen size, hugely increasing the required bandwidth (one of HDTV's "little problems" - er- in fact the main one!).
(system bandwidth; the more detailed the picture - the more "dots" there are - then the faster brightness must be able to change, since more rapid changes create higher signal frequencies (try to "picture" sine waves; the steeper the slope, the higher the frequency!). If higher frequencies are missing, then the "dots" can't change so quickly, and the picture gets progressively more blurred from increasing lack of detail. So, the "transmission system" must be able to convey the very highest frequencies needed!).
(bandwidth, see: Bandwidth - Wikipedia, the free encyclopedia , Television Bandwidth ).
For analogue tv, horizontal rows of dots instead become "scan lines", and horizontal resolution gets fixed by the particular system bandwidth (ie, how fast scanning spot brightness can actually change!), vertical resolution being fixed by number of lines.
For analogue, screen size 21 inches diagonal was considered "ok", then the scan lines had to "just disappear" at the "normal viewing distance". Using amplitude modulation (am) - then the only "realistic" broadcasting method available- this determined the transmission bandwidth needed (which was still large (!), so vestigal sideband (vsb) transmission was universally used, giving some - but acceptable - picture distortion).
(See: www.scte.org.uk/photo/technical/channel.jpg ). This shows an analogue tv channel, although on a non-UK VHF frequency!).
(For "baseband" video - just sending the brightness along a cable - SDTV requires roughly 6Mhz bandwidth. Using vsb am transmission - for analogue broadcasting - only 8 MHz is required, just 2Mhz more, a good result for "older technology"!).
Interlace. At this resolution, using vestigal sideband, the required bandwidth for analogue tv was still too high at 16 Mhz (making tv receivers difficult to design and expensive, and limiting the number of broadcast tv channels). So, interlace was universally used (reducing bandwidth to 8 Mhz). On the 50 pics/sec or more (to reduce flicker), each was sent with only half the number of total lines, with the lines on even/odd pictures in slightly different positions (but there's still 25 or more frames/sec, to give adequate "continuous movement" illusion! See: http://www.sparkysworld.co.uk/images/interlace.gif ).
(Interlace is now obsolete, by preference not used in digital SD or HDTV systems, and this leaves behind some "legacy" conversion/ compatibility problems).
Colour. Best explained in "joke format"(!) Good news, due human eye physiology any colour can be created by mixing 3 primary colours, red/green/blue. Bad news, that still requires 3 separate tv pictures (needs 3x previous bandwidth). Good news, eye percieves colours in much less detail, so not that much extra bandwidth needed. Bad news, even so, the extra bandwidth still isn't there. Good news, you can "invisibly include" the colour info inside existing analogue monochrome tv channels (also ensuring backwards compatibility!), then no more bandwidth needed.
This is how: A colour camera/video source actually does output 3 separate "primary colour" pictures, red, green, blue. These are multiplexed (added/subtracted/filtered), giving Y (monochrome/fine detail), and R-Y and B-Y (two "colour difference" signals). The monochrome (Y) signal is then transmitted exactly as before, but R-Y and B-Y are first bandwidth limited - to form U and V signals - these are then encoded onto a supressed carrier am signal, at around freq 4MHz ("inside" the 8 MHz tv channel). Thus, this "subsidiary colour signal" (subcarrier) occupies "the same signal space" as the still-the-same monochrome signal, but at this high frequency only "appears" on screen as a very fine - almost invisible - "dot pattern", on any type of analogue tv set.
So, now a monocrome tv still works as before, but a colour receiver then also decodes the subcarrier, extracts the U and V signals, and de-multiplexes these from Y to re-form red, green, blue, pictures (terminology; monochrome Y signal is "luminance", combined U/V colour subcarrier is "chrominance", both these combined - with synchronising pulses then added - is "composite video").
(primary colours see: Television Production: Principles of Color ).
(subcarrier - PAL version - see Software-based PAL colour decoding ).
(Digital tv systems don't have a "subcarrier", that's an analogue only technique! However, they all - including HDTV - retain the Y,U,V system, this still being this most economical way to encode/send colour information. Since monochrome Y needs more detail resolution than colour U, V, more Y "samples" are always sent, the particular Y.U.V ratio depending on chosen digital format, eg: 4.2.0, 4.2.2, etc.....
(see: Chroma subsampling - Wikipedia, the free encyclopedia ).
STANDARD DEFINITION (SD) TV.
Using above techniques, the world ended up with 2 analogue SDTV formats, 525lines;30pics/sec (for 60 Hz mains power supplies) and 625;25 (for 50 Hz). Both with 6Mhz video bandwidth, nicely fitting into a 8mhz vsb-am "tv channel", the channels being on VHF and/or UHF frequency bands (depending on country).
Colour was then "slotted in" later, slightly different systems but variants of the above (colour tv folklore: NTSC = "never the same colour", SECAM = "system contrary to American method", PAL = "peace at last").
(Basically, NTSC is colour tv as above, PAL is a later variant of it, SECAM sends U and V sequentially instead of together).
(Here's a countries list:
http://avconvert.com/video/world_tel...standards.html ).
(Here's the 3 systems compared: www.scatmag.com/technical/worldtv.pdf ).
DIGITISING SDTV.
The next step - as a precursor to HDTV - was (and is!) digitising SDTV. There's many reasons to want to. Digital systems are stable, and you can repeatedly "regenerate" the signal, allowing infinite copying, transmit over huge distances without deterioration, etc. (But, analogue systems need frequent adjustments and calibration, recording and re-copying slowly degrades the original, and quality degrades with transmission distance).
However, a price is paid for these "digital advantages", in the form of hugely increased bandwidth requirements.
In sampling a 6 Mhz video signal, first there's Nyquist's critereon, sampling must be at least 12 Mhz, preferably significantly higher, otherwise we get alaising (unwanted "visual artifacts"").
(see: http://redwood.berkeley.edu/bruno/npb261/aliasing.pdf .
Also: Nyquist...mpling theorem - Wikipedia, the free encyclopedia ).
Let's say we sample at 12 MHz (still slightly too low!). Then, the video signal level may be sampled in 256 "steps", so in binary each sample can be represented as a single "byte", ie 8 bits. However, transmitting the bits directly, 8 requires multiplying the sampling frequency by 4 (bandwidth is half bitrate!), giving a whopping 48 Mhz!
But hang on, we haven't finished, that's just the Y signal (monocrome only picture)! There's also U and V, which we need to re-derive the red/green/blue pictures. Luckily (!) we said that U and V are bandwidth limited, so - doing the above calculation again - the digitised versions of them will only require 24 MHz each. Finally, add this all together, and the digitised SDTV colour picture requires - at least! - 96 Mhz of bandwidth, just to transmit the baseband ("raw"!) video (for example, down a coaxial cable). Never mind finding some way to broadcast it to tv receivers (100 MHz is approx a quarter of the entire UK tv UHF broadcast band!).
So, "straighforward" digital SDTV is "unbroadcastable"! But, 100MHz will travel down coax cables (and can be infinitely regenerated in "repeaters"), and by now most broadcast studio cameras and video recorders are in fact digital (with a fair proportion bit of them also being digital HD, which when fed down coax cables requires a "mind-blowing" 800 Mhz bandwidth!)
(see: CCIR 601 - Wikipedia, the free encyclopedia ).
(also:Digital VCR Formats Compared: Tabular Specifications ).
(also:Tabletop Productions - HD Video Formats ).
(also: Serial Digital Interface - Wikipedia, the free encyclopedia ).
(Now, even "digital videotape" is becomming obsolete, with video material likely to be stored on computer "servers" ......).
(But hang on again, in analogue tv systems, isn't the colour subcarrier already "contained inside" the 8Mhz signal, so couldn't you just digitise the whole thing at once, instead of doing Y, U, V separately, saving lots of bandwidth? Well ....er....yes! In fact, this is actually done, eg in D3 video recorders and on some SDI links. But, you still end up with 50Mhz bandwidth, and this "digitised composite analogue" has some problems, so keeping Y,U,V completely separate is better, and essential for compression .....)
VIDEO COMPRESSION (MPEG).
So, we can make and record digital sdtv pictures, but not transmit them! However ....
Redundancy. TV pictures contain a huge amount of "spacial redundancy" (large areas of the picture being exactly the same), and "temporal redundancy" (large parts of picure not moving over time). Once the picture has been digitised, then it's possible to use computing techniques to identify these parts, and work out numbers for the "brightness/colour values" and "fill-in area size".
Amazingly, using such methods (and others), it's possible to decrease the SDTV bitrate from 200 MB/s (100MHZ) down to 2 MB/s (1MHz), a "decrease factor" of 100!
This is the basis of MPEG2, as currently used for digital (SD!) television broadcasting (whether satellite, terrestrial, cable, or "other"). At 1 Mhz per tv channel, you can get 4 - or even 6 - standard definition channels into the 8 MHz bandwidth previously occupied by only 1 analogue "channel", and that's what's been done. Each "set of channels" is called a "multiplex" (or "mux"). Single muxes are then delivered digitally over the same previously exisiting analogue channels. An amazing achievement!
(Er, 100:1 compression ratio is "extreme", and would only work on "favourable" material, more realistic is maybe 40:1. Unfortunately, commercial pressure often forces broadcasters to "squeeze in" more than is wise, with consequent "artifacts" - eg, jerkiness, reduced resolution - on digital tv brooadcasts).
(see: http://www.fh-friedberg.de/fachberei...g/beginnzi.htm.
Also: MPEG-2 - Wikipedia, the free encyclopedia ).
(Much of the research into DCT compression for bit rate reduction was done at MIT/ Bell labs. This then allowed the formulation of the JPEG and MPEG picture compression standards. Meanwhile - around 1985-1993 - all sorts of "weird and wonderful" schemes were being developed for sending HDTV via existing systems, which very suddenly became obsolete once MPEG2 appeared!).
(see: HDTV By HDTV News Online ; written from a "USA viewpoint", but conveys the worldwide confusion there's been in formulating HDTV standards!).
(In Europe, the British IBA's MAC became the EU "official" method for satellite tv broadcasting, but it got marginalised by Sky starting up in UK using PAL. Nevertheless, as part of "Eureka", the EU still pressed ahead with upgraded HD-MAC for possible HDTV, with widespread equipment demos, and broadcasts of the 1992 Olympic Games, before finally abandoning it when MPEG2 appeared.)
For SDTV MPEG2 muxes, current delivery systems are:
terrestrial (DTT, or ATSC), 1 mux per 8MHz UHF channel.
satellite (DVB-s) 1 mux per 27Mhz sat channel.
cable 1 mux per (8 Mhz) cable channel.
(Also, recently, 1 MPEG2 channel down an adsl line, channel switching being done at landline phone exchange by service providor).
TOWARDS HDTV (yes, I'm getting there, really ...!).
For an analogue HD picture, increase resolution x2 vert and hori, so that's x4 no. of pixels (as was explained), giving a "baseband" bandwidth of 6x4 = 24 Mhz. Transmitted using vsb-am, that would require 30MHz, or same as 4 standard tv channels, which is why it was never done (also, the receiver design would be difficult!)
(Now of historical interest only, the world's first HDTV system was NHK's Hi Vision (analogue HDTV, 40MHz bandwidth), the broadcastable verison being MUSE (8 Mhz bandwidth, via satellite), achieved using analogue video compression. A fine achievement, but incompatible with later digital HDTV, hence a dead-end, and is being phased out).
(see: Analog high-definition television system - Wikipedia, the free encyclopedia ).
((( Hmmm .... all remaining Internet stuff on Muse is gobbledygook, so here's how it worked. NHK developed Hi Vision, a "straightforward" analogue HDTV system with working cameras, video recorders, and display monitors. But, as said, 24Mhz video bandwidth is "unbroadcastable", so MUSE was developed. The HD video was sampled at a quarter the "minimum" Nyquist frequncy, at slighty different positions on each picture, and then each 4 picture sequence made a "complete" single HD picture, providing nothing moved! Where there was movement, careful choice of the sampling frequency ensured the aliasing patterns were "least visible". Motion was also blurred - at quarter resolution - but movement looks blurred anyway! But to help, additional digital "motion vector" info was sent, further improving movement.
Colour used YUV - as usual - but these were transmitted sequentially per line - with U and V also time compressed - so there were no cross chrominance/luminance problems.
Then, the video samples were kept as PAM (pulse amplitude modulation) - not digitised! - and these analogue pulses modulated an fm carrier. Digital encoded sound was also added, using "NICAM like" compression.
It worked quite well, and all fitted into a single 8MHz broadcast tv channel, which compares very well with the - maybe - 2 MPEG4 video bitstreams we now "squeeze into" the same bandwidth, despite using much more powerful technology!)))
HDTV is (approx) double the horizontal and vertical resolution of SDTV, that's 2x2 = 4 times as many pixels (as said above, pixels number increases as screen area squared!). However, we've already got 4 SDTV channels in the MPEG2 "mux", so it looks as if we might get 1 HDTV channel into the same "space", and indeed we can! MPEG2 inclues a HDTV picture format "profile" (see last-but-one above link), and the first experimental digital HDTV broadcasts have been via satellite this way.
However - in the meantime - improved MPEG4 ("video objects", and H264 coding) much denser compression was developed, allowing more HDTV channels-per-bandwidth to be sent than via MPEG2, and "commerical" already exisiting and just starting services will be using this newer method, almost exclusively.
Despite denser MPEG4 compression ratios, nevertheless HDTV still has a higher data rate. Very roughly, you might get 1 or 2 HDTV channels into the same bandwidth that previously carried 4 or 6 SDTV ones. Note, MPEG4 doesn't have "muxes", as such - being a far more general standard - intead for HDTV it's configured with a (number of) H264 video stream(s).
(note, mpeg4 - specfically H264 coding - isn't just about HDTV, but more generally about reducing all video bitrates (and therefore bandwidths, and also storage requirements). So, for example, we're now seeing things like tiny "pocket" MPEG4 tv players which can hold several hours of SD video on a USB "memory stick", etc, unthinkable just a few years ago .....).
(see: MPEG-4 description
also: H.264/MPEG-4 AVC - Wikipedia, the free encyclopedia ).
THE NEW DIGITAL HDTV VIDEO FORMATS.
There's a number of constraints here:
1) Must be "transmissible" via actual broadcasting systems (ie, already exisiting cable and satellite and terrestrial systems, since we don't want to "rip everything out" and completely re-equip!).
2) Must be "backwards compatible" with SDTV, ie, easily back - convertible "in real time", since although the "video source" is now HD, much broadcasting will remain in - and many people still watch in - the lower resolution SD version.
3) Easy real-time conversion between the different HD and SD production and transmission formats.
This has resulted in (horizontal x vertical):
720x480i (NTSC), 720x576i (PAL) - to show existing SDTV digital formats (from terrestrial, cable, satellite, etc);
ED ("enhanced definition") 720x480p (NTSC progressive), 720x576p (PAL progressive) - see below;
HDTV: 1280x720p, 1920x1080i (1920x1080p).
Notes:
i = interlace (as above), p = progressive scan (each separate picture straight down from top to bottom, no interlace). Progressive looks better on rapid motion, especially sports, but much material is still originated in interlace (for easier backwards compatibility). It's better to show things in "original format", rather than digitally converting between them, as the result can be "juddery".
(progressive scan also uses 30 pictures/sec, rather than 25, which further helps with fast movement. However - using same bandwidth - this means the resolution must be reduced, hence 720 instead of 1080 (v)).
(But, here's a funny thing! Although scanning progressively - instead of interlaced - makes no actual difference to the bandwidth, as well as motion being better rendered, for some rerason the eye also "sees" double the picture resolution, although obviously it's not really there (at least, not when the picture's completely static). It's just one of those peculiar "psycho-physical quirks", very useful when you're trying to reduce the required bit rate for compression!).
(So, if progressive scan always looks better, why not always use it? Alas, real-time backwards-conversion to SD is much better from an interlaced source, as was said!).
(note that the picture rate now depends on what's wanted, the previous connection with local power systems has - at last! - been broken).
((( hmmm ... it seems that Europe is/will be broadcasting 720p/25 mode, instead of 720p/30. Exactly why I'm not sure, since dropping from 1080 to 750 resolution is precisely in order to increase the picture rate within the same transmission bandwidth! However, there may be distribution equipment reasons. Unfortunately, this means any USA derived 720/30 material will first need very careful "digital domain" standards conversion, before being re-broadcast as 720/25, otherwise it will look rather juddery ....... ))).
(Standards Conversion: In analogue systems, different line standards may be inter-converted by "line interpolation". The different picture rates are more complicated, demanding some real-time computer power! Or, you can just drop or repeat every 4th or 5th picture - depending which way you're converting - but the result looks "juddery"!
In digital (SD or HD), the easiest thing to do is real-time convert the "raw" format video picture, lines by interpolation, pictures by the 5/4 USA/Europe different rates ratio. Paraodxically, the compressed video formats are potentially easier, because you can interpolate the "motion vectors" at 5/4 ratio. But, whichever method is used, very careful design is needed to avoid "visual artifacts", especially on HD resolutions).
(see: www.snellwilcox.com/community/knowledge_center/engineering_guides/estandard.pdf ; warning: 350K pdf file, fast Internet connection required!).
(But, the 24/25 USA/Euro picture rates are "almost" the same, and might be "converted" just by replaying at a slightly different speed, which would avoid standards conversion problems, although sound pitch changes slightly!)
The highest possible resolution - 1920x1080p (30pics/s) - requires extra bandwidth (of course!), and seems unused at present (in USA).
In practice, the main formats used would be SDTV (which one depends on country), and the HDTV formats. EDTV is unlkely to be used.
Note, these are just "video format standards"! What's actually seen on screen depends - obviously - on the "physical resolution" (total number of dots!) on the particular screen you're watching; better is more expensive! Inside the screen, electronics "re-map" the video mode being used onto the actual screen pixels.
See: High-definition television - Wikipedia, the free encyclopedia (note, that's the same link as at top of this page!).
Also: www.ebu.ch/en/technical/trev/trev_299-ive.pdf .
Also: EBU Technical Review .
RECEIVER/SCREEN SPLIT.
Analogue tv was (mostly, in UK anyway) off-air, and it made sense to put the tuner/demodulator and display all in one box ( a "tv set"!).
With digital, several different delivery systems became available (terr, sat, cable, adsl), and the previous "tv set" now often functions as just a "screen" (internal analogue tuner unused!), connected to the actual receiver via aerial socket or SCART cable.
With HDTV, it no longer makes much sense to have a "tv set", so receivers and screens have now become separate devices.
RECEIVER/SCREEN CONNECTION.
It's certainly possible to have a sort of "super SCART" - feeding the 3 colour picture analogue waveforms separately - but messy, as there's now several screen formats! Also, the 30 MHz analogue bandwidth per picture (either Y/U/V, or "full bandwidth" R/G/ won't "travel so well" down connecting cables (high freq attenuation, fine detail blurring). And converting from digital to analogue - then back again - just for the receiver/screen link risks creating additional "visual artifacts". So, a digital link format was decided on.
((Note, many HD screens still have analogue component video inputs, either 3 line Y/U/V with composite sync pulses, or sometimes R/G/B//H/V full bandwidth primary colour pictures with separate horiz/vert syncs (most "screens" with such inputs should be able to auto-detect the HD video format being used from the sync pulse timings). Using such analogue connections should be considered a "last resort", due to the above mentioned problems, although the actual end result may be fine!))
The digital system chosen is TMDS (transition minimised differential signalling). Transition minimising means there's a minimum number of actual voltage transitions, lowering the required bitrate - hence bandwidth - making the digital link work physically better (the signals will travel further, down cheaper cables!).
(Transition minimising - to reduce bitrate and bandwidth - is fairly commonly used, eg, the EFM re-coding used in the original CD format. Although - to be strictly accurate - EFM is a "run length limited code").
(TMDS is also "synchoronous", ie, the data transmission rate is determined by a "reference clock", so a separate clock signal is sent - as well as the data - which was previously unusual in consumer equipment, but is quite common on various "professional" broadcast link formats).
(The "differential" bit refers to 2 wires used for each signal - instead of one wire and "common earth" - which allows high common mode (outside electrical interference) rejection, again increasing the usable cable length. This is often called "circuit balancing").
DVI (digital visual interface). This was originally for a single PC base to PC monitor connection, so carries just a single TDMS HDTV video format channel.
Per pixel, the 3 colour pictures are sent as 3 signals (on 3 sets of 2 "differential" wires), at 8 bit resolution. The master clock signal (on 4th set of 2 wires) may be up to 170MHz, which allows up to a 1920x1080i (and p?) format picture to be sent.
(Note, DVI was originally a PC monitor standard, so doesn't convey sound, just video. You have to separately connect the HD receiver audio output(s) to the screen and/or surround sound system).
HDMI. (high density multimedia interface). This is a superset of (ie, "with knobs on") DVI, hence backwards compatible ("converter adaptors" - or alternatively leads with different type plugs on each end - are reasonably cheap!).
As well as the above DVI type link, this also has several digital sound channels (eg, for Dolby 5.1, etc), forwards data signalling, and a reverse direction data signalling channel (possibly for remote controls).
((( hmmm ... this system of digitally transmitting the 3 colour pictures in parallel "harks back to" the earlier format broadcasting links, later abandoned in favour of HD-SDI (see above). However, this way a lower bitrate is required for each picture, which allows simpler unequalised cables to be used, in a less complicated "consumer format"))).
(Basically, DVI is a single-device one-way receiver to screen link. HDMI is a more a "bus system", allowing different simultaneous digital video/audio/data streams - in both directions - through several "daisy chained" devices. HDMI is better - since more recent - hence preferred).
(Also, the "nominal" maximum working distance is 5m for DVI and 15m for HDMI. Why so different I don't know, but possibly because better "driver transistor" configurations have been used in the chipsets, since the distance between an HD receiver and screen would typically be greater than between a PC base and monitor. Note that, "nominal distance" means under "worst case" conditions, normally you'd expect a greater maximum working length before "digital fallover" occurs).
(Since many DVI "hd ready" tv screens have been sold, a fairly common "household situation" will be connecting a hi def receiver HDMI ouptut to a DVI input screen. In which case, even if the different plugs were completely pin compatible (they're not!), the DVI input still won't have the correct chip sets to handle the additional audio/ data channels!).
HDMI is "tristate", ie, you should be able to "loopthrough" devices that are completely unpowered and disconnected from the mains supply. However, having a "signal regenerator" inside a device on "standby" would increase the working distance. At these high frequncies, I'm not sure what max cable lengths would be for "household distribution", but "signal regenerators" (digital versions of booster amps) ought to be fairly cheap.
(In theory, a "crude but usable" HDMI cable regenerator/ range extender would require just a phase lock loop and noise gate, maybe 3 or 4 ICs, and should be about the size/cost of current sat lnb line amps, But, the cheapest I've seen - there's much more expensive ones - is HDMI extender HDMI repeater. Extend HDMI DVI connection to 150ft) , still at rather a high price! Well, OK, I suppose most people aren't going to have several HDTV screens "around the house", and if they do then the extra $100 is certainly "affordable"!).
(note, HDMI cables are fairly expensive, as at the required frequencies they're "not just a bit of wire", but a carefully designed transmission line. However, any that work should be fine (!) - either the digital pulses travel far enough, or they don't - and stuff like "gold plating" etc seems a bit silly .....
However, over short distances only, you could "get away with" poorly designed cables, so maybe we'll see short-distance usable (8 foot!) ones appearing in "pound shops", OK "for most purposes"!
Basically, it's difficult to distribute the digital HDTV video formats any distance "on copper", due to the extremely high frequencies involved. So, sending HD "around the house" is going to be a problem! Either you use horribly expensive HDMI, or send as much lower bitrate MPEG4 then decode that at the receiving point.
The "logical answer" to distributing these high bitrates is optical fibre, but that's expensive, and there's practical problems for domestic systems ..... ).
(see High-Definition Multimedia Interface - Wikipedia, the free encyclopedia, Digital Visual Interface - Wikipedia, the free encyclopedia ,
Welcome to DVIHDMICables.com - The DVI HDMI SVGA Information Source ).
(also Balanced line - Wikipedia, the free encyclopedia , www.highfrequencyelectronics.com/Archives/Sep04/HFE0904_Tutorial.pdf ) . This is explaining balanced baseband 2-wire analogue audio links, but the principles of (electrically) balanced transmission apply equally to high speed digital links!).
(((Hmmm .... with HDMI cable lengths, you might think "what's the problem"?, since 300MHz is at bottom end of the UHF tv band, and we commonly distribute such frequencies fairly large distances over coax, with no problem! However, a single tv channel only occupies 8 MHz bandwidth, whereas the TMDS signal for only a single HD video channel occupies (up to) 3x150 MHz, very different! With such a wide bandwidth, dispersion becomes significant, and pulses lose their "sharp edges" over relatively short distances, causing intersymbol interference.
See: http://dar.ju.edu.jo/mansour/723/Lec...s/image002.jpg .
Note, dispersion is an entirely different thing from signal loss on sat cable at higher sat i.f. frequencies of 1-2 GHz, this being due to dielectric absorbtion!))).
DISPLAY SCREEN DEVICES (TECHNOLOGIES).
SDTV was worked out to make the pixels "just disappear" at "normal viewing distance", on an approx 21 inch diagonal screen. So, to see the higher HD resolution, really you need a larger screen (that's not mentioning the huge difficulty of actually making any screens at all with "smaller dots"!).
The original tv display device was - of course - the cathode ray tube (CRT), and colour variant the shadowmask tube (and later equivalents). Despite the higher line frequency (with larger "flyback" voltages), it's certainly possible to have HDTV CRT displays (production sets were built for MUSE and later EUREKA 1250). However, for various reasons - again including multiple standards - manufacturers have decided not to go down this route.
(Interestingly, although the 600 line analogue PAL picture is very good, most people have never seen it! Most "consumer product" (built to a price) CRT colour tv sets have tended to have low resolution; largish screen pixels, and Y cutoff above 3.5 MHz. But now, in shops, the (digital) SD picture is being displayed on "HD ready" sets, and it looks quite good!).
A CRT "scans" the screen area with a "light point", hence is subject to flicker (although high persistance phosphors and 100 HZ scanning help a lot!). With newer pixel technologies, all the pixels are "working" all the time, and just get "updated" by the successive video stream pictures. Also, pixels are digital - basically "on" or "off" - so instead of a CRT varying beam current, brightness is adjusted by changing the mark/space ratio of a high frequency switching waveform, and there should be no noticable "flickering" at the freqencies used.
LCD. Liquid crystal display (tv screens) are similar - in principle - to the older technology used in digital wristwatches and instrument displays. The crystals are normally transparent - light goes straight through them - but become opaque when a dc voltage is applied. So, the tv version has a "backlighter" lightsource panel (probably electro-illuminsecent type), covered with narrow coloured stripes (red, green, blue). Then, the lcd flat panels contain electrode grids, forming "crosspoints" where the separate pixels are.
(see What is TFT LCD TV and LCD Monitor Panel? ).
Plasma. Basically, this is an updated version of the old flourescent "gas discharge" lamp, as until recently used in kitchens (and currently "energy saver" bulbs), etc. The flat glass panel contains a low pressure gas, with electrode grids once again forming a crosspoint matrix. 400 v dc (approx) between 2 electrodes causes local gas ionisation (a "plasma"), the gas then emits UV radiation which causes visible-light phosphors coated onto the front panel to glow.
Plasma TV Science.org: Inner working of Plasma Displays .
Projection. The 3 primary coloured pictures are produced separately, and only "mixed" on the projection screen. Older devices tended to use 3 CRTs though coloured filters, newer (including HDTV) ones use coloured lamps shining through 3 led pixel arrays (there are other technologies, but normally used in cinemas etc, not domestic stuff!).
(see Howstuffworks "How Projection Television Works" ).
Screen lifetime/reliability. LCD technology - including waveform modulated brightness - has been around for about 30 years, and lcd computer and mobile phone screens for a while, tv versions following later due to solving "slow response time" and contrast problems. So, these should be reliable. The screen part most likely to fail is the backlighter, which (at least on some models) is replaceable. Plasma - like its flourescent lamp ancestors - slowly wears out with use, the panel "half-life" being time for emissivity to fall by factor 2 (although this would be - maybe - 15,000 hours). Plasma is newer (reliability unknown!) technology, and seems more inherently prone to fail due to 400v dc on the panels, but can currently give a brighter and higher image contrast than LCD.
(On all HD screen types - as on all consumer electronics - the one part by far most likely to fail is the switch mode power supply, which is usually fairly easily replacable/repairable).
"Missing" Pixels. A CRT colour tube is made "photographically" - a bit like using a darkroom photographic enlarger (remember those?) - and the phosphor pattern should be "perfect". Producing flat screens is more like making ics - thin film deposition, monolithic fabrication, etc - and on such a large area some faults can occur. The few missing pixels - per screen - should be black, hence unnoticable (where lcd pixels fail by losing all power, they go permanently transparent to the backlighter, but this should be very obvious!).
For more screen type details, see: HDTV Display Technology Shoot-Out .
For more on the various different flat screen technologies, see:
Telenet - de gevraagde pagina kan niet gevonden worden
(Also note, UK magazine Television has run a series on different Plasma and LCD screen technologies, starting - I think - March 2005).
For some flat screen "pros and cons", see:
CNET's quick guide to TV types - CNET reviews .
For particular product comparisons, try:
Compare Plasma, LCD and Projector HDTVs available in the UK (1280x720, 1920x1080, 720p, 1080i, 1080p) .
HDTV BROADCAST DELIVERY SYSTEMS.
The general principle: existing broadcast distribution systems carry "digital bandwidth", so can usually be re-used for the new HD services, but new receivers will be required to decode MPEG4 instead of MPEG2 (and sometimes a newer modulation system, since - despite denser MPEG4 compression - HDTV still requires a higher bitrate per channel!).
(Note, as with all digital systems, HDTV delivery is subject to the "cliff" or "sudden fallover" effect. With a decreasing signal level, error correction still gives you a "perfect" picture, until a particular point where you suddenly get nothing! Cable operators will send at an "adequate" signal level, otherwise - as for SDTV - it's a question of big enough dishes, aerials, etc ....).
Satellite. Existing dishes/lnbs, cables, line amplifiers, multiswitches, etc, should still be fine, but new receivers will be needed to decode MPEG4 and DVB-S2. MPEG2 and MPEG4 tv channels can co-exist on the same satellite (different transponders), and in some DVB-S2 formats it's possible to "piggy back" an MPEG4 channel onto an existing MPEG2 mux (but I don't know if this is going to be done!).
(see DVB-S2 - Wikipedia, the free encyclopedia , www.ebu.ch/en/technical/trev/trev_300-morello.pdf ).
((satellite has a huge bandwidth - 4 Ghz per oribital position - so is the "best" - and will likely be the most popular - way of delivering HDTV)).
(Note, re-use of existing dishes - eg Sky minidishes - for newer format signals require they have the same "link budget", restricting the broadcaster to only some DVB-S2 "modes").
Cable (coax). For digital tv, exisiting 8Mhz fdm cable channels were updated from vestigal am (1 analogue channel) to QAM (1 MPEG2 mux). There's no difficulty about delivering MPEG4 over the same systems (if necessary, slightly upping the QAM signalling rate). Of course, new receivers will be required for any new HDTV, meanwhile SDTV will continue (analogue/ MPEG2/ MPEG4 can all co-exist on a single coax cable system, using different channels, though analogue is now being progressively withdrawn for "bandwidth economy" reasons!).
(These days, the "main trunk" part of a coax cable system will almost certainly be fibre opic, allowing much greater bandwidths - to include broadband Internet etc - but "endpoint" connections into homes still use a coax cable with frequency division multiplexing).
(see www.scatmag.com/technical/techarticle-june05.pdf
(Adsl (phone landline). At present, this delivers a single MPEG2 channel, switching being done in phone exchange. There should be no difficulty in delivering a MPEG4 channel instead, possibly using updated adsl2 modulation (but a new receiver is needed anyway, for the MPEG4!).
(since adsl tv channel switching is done "at source" - then unlike sat/terr/coax cable - the receiver doesn't contain a tuner, just a demodulator and picture decoder).
(adsl is very attractive to cable providors. Most houses have a telephone landline, and any complex switching/delivery equipment only needs installing at the phone exchange, unlike for coax you don't need lots of street distribution cabinets (which can fail!). However, phone lines were not meant to carry high frequencies, with the disadvantage that adsl only works up to maybe 3 miles from exchange, whereas coax covers much greater distances).
(see: Asymmetric Digital Subscriber Line - Wikipedia, the free encyclopedia ).
(UK) Terrestrial (DTT-T, COFDM delivery). Unfortunately, Freeview/ Topup bandwidth is currently "stretched", and any MPEG4/ HDTV services at all (apart from experimental) seem unlikely before analogue is switched off! (I'm not sure what's happening in other EU countries!). Obviously, new MPEG4 receivers would be required, but COFDM modulation will continue being used, so the reception area would be roughly the same as before (where there's existing DTT!).
Internet. Possible in principle, but unlikely in practice, since - even with "broadband" home connections - the data rate still isn't sufficient for "real-time" MPEG2, never mind MPEG4! However - of course - there's no problem with transferring files, which may have either MPEG2 or MPEG4 content (for later replay).
(The various Internet protocols "use up" lots of bandwidth, that's why this doesn't work over the same ADSL line which can deliver MPEG2/4, since ADSL itself sends almost "pure data" with fewer overheads).
(However, it remains possible to deliver MPEG2/4 bitstreams directly over ATM and frame router systems, in real time, where dedicated links exist!).
CURRENT UK SITUATION (updated 14/07/2006).
SKY. About 17 HD channels currently available. Sky HD home installations have now been going for a few months.
UK Sky HD info: Sky HD - Wikipedia, the free encyclopedia .
(official website): Sky HD - High Definition TV (HDTV) in the UK from Sky .
Unfortunately, there are various reports of a high problem rate with Sky HD receivers! For more details, check the Sky HD sections/threads in our forums, and/or search for further info using Google, etc.
CABLE. The merged NTL/Telewest "tv drive" - with HD capability - has been available for some months, with HD channels and some “playback on demand”. Use of the HD facility requires an extra “premium” payment (as you’d expect!).
Telewest - TV Drive .
(There’s now also various ASDL services down your phone landline – including TV – for details of HDTV currently offered, consult the various companies’ websites).
BBC HD is now established, with a good range of actual HD material (some broadcasters are just “scaling up”, standard material, which admittedly gives slightly better viewing results, but isn’t really HDTV!). This is basically free-to-air (some World Cup was encrypted, for copyright reasons). However, it is being described as a “trial”, with no guaranteed continuation beyond the I year trial period.
No firm news yet on ITV ch4/5, and other UK “maybe” free-to-air HD broadcasts.
EUROPEAN (non UK) SATELLITE HD. A number of channels are broadcasting an HD service, in addition to the older SD one (eg, Sat1 HD). These free-to–air HD channels show many American/British films, but with a dubbed soundtrack! (An exception is Arte, where the French version often – not always! - shows “classic” USA/Brit films with original soundtrack, but the in-video French subtitles will look rather distracting on the HD version!).
ENCRYPTED Sky HD channels can only be received on Sky HD boxes (of course!). Otherwise, presumably, non-Sky encrypted European channels can be received on HD capable sat receivers, if you “know how” (nudge, wink).
HD CAPABLE SAT RECEIVERS are now becoming available and better known, with a slowly increasing range and choice, eg the popular Humax and Starsat (Linux) receivers, for which see relevant threads on these forums.
HDTV RECORD/REPLAY.
Videotape? As said above, video recording has been digital for a while, in "professional" formats. However, it's certainly possible to digitally record a single H264 prog stream on "consumer format" tapes. So, will there be any? Probably not, since optical discs have many advantages, amd there's been no announcements ......
Optical disc has many advantages (mechanically robust, quick random access, films mass-distribution by "stamping", etc), so is attractive to retailers and consumers. However, current DVD cannot achieve sufficient storage density, which requires a smaller wavelength laser and larger lens (fourier optics). So, the newer HD versions are:
Blu-ray (currently "up and going").
(see: Blu-ray Disc - Wikipedia, the free encyclopedia ).
DVD-HD (expected soon).
(see: HD DVD - Wikipedia, the free encyclopedia ).
(There should also be single (R) and multiuse (W/R) writable versions of both, using the same laser burn and dye techniques).
Magnetic recording. On "winchester" sealed computer drives, these days there's no problem with either storage capacity or data transfer rates! Currently, a single SDTV MPEG2 prog stream is "encapsulated" within the native file system of whatever device you're using (computer or PVR), and there should be no difficulty with the slightly higher data rate/storage capacity (about 70%) required for H264 streaming instead, which is still "well inside" the hardware capabilities.
PC software is currently available for MPEG2 SDTV recording/ storage/ replay, and should also become available for (real time) MPEG4 streaming, so in principle you just add the appropriate PC cards. However, not many people would want to view HD on the smallish computer monitor screen, so for viewing presumably you'll need a card with HDMI output sockets - for transferring the HD video format directly to a viewing screen - and there's also the copy proetction issue, see below.
(Note, special on-card hardware would usually be required to turn MPEG4/H262 into a "real time" HDTV video, as using the computer's main processor would slow things down a lot, even top range Pentiums might not manage it too well!).
Also, of course there's PVRs, dedicated tv tuners with magnetic disc storage (Sky's first HD receiver includes one!).
(UK HD PC/PVR situation at time of writing, date: 27/01/06. Right now - as an Internet search will show - there's a fair number of HDTV PVRs and PC cards/ recording software available in USA/Canada. Unfortunately, these nearly all involve terrestrial HDTV using the ATSC system. Europe - including UK - uses COFDM instead, so all such devices would be useless in the UK (and anyway there's no plans for UK terrestrial HDTV before analogue switchoff). So, we'll just have to wait and see what devices become available here. Meanwhile, one DVB-S2/MPEG4 sat receiver plug-in PC card has been announced, but it's unlikely to be any use for Sky's upcoming HD services, which almost certainly will be entirely subscription only via proprietry encryption).
(Obviously, PCs are a single worldwide standard, and - of course -the same software will function anyplace, but that's not much use if there's no local HD broadcasts and/or no tuner cards!).
(However - that warning given - then strictly for information only, there's various public domain and commercial software for PC based HDTV recording in the USA. For instance, see:
EFF: Cooking with EFF: KnoppMyth r5a10 and pcHDTV for DTV Liberation
SnapStream Beyond TV: PC PVR software
www.zenith.com/sub_prod/downloads_pdf/HDR230.pdf .
COPY PROTECTION.
Although it doesn't have to be (!), much MPEG4 HDTV will be subscription, so sent encrypted, and received as usual via conditional accesss (cams and cards, etc).
CD and DVD have a "copy protect bit" in the data structure - which supposedly stops direct digital copying, but it's not always implemented, and somewhat irrelevant anyway as copying is usually done in "analogue domain" (take CD/DVD analogue output to 2nd device, then record that by re-digitising).
However, for HDTV there's an additional system called HDCP (high density content protection). This is a bit dire, as it allows you to record on a PVR type HD receiver, and watch as many times as you like, but not make permanent copies via HDMI or component video outputs (the hd picture shows on your hd screen, but an external recording device won't record it, either not at all, or only in lower def mode!).
(HDCP re-encrypts the HD format video output, before sending it by TMDS down the HDMI cable. The system used is public/private key encryption, with public keys sent openly, but "device embedded" private keys kept hidden. Any device without HDCP fitted can't - (obviously!) - decode the signal, even if given the public key. And all "HDCP compliant" recording devices will refuse to record in HD format (or at all!), if "instructed" to behave this way by the programme's original broadcaster).
(public/private key cryptography is already used for encryption, with CAMS etc, the difference here is that the HD CAM decrypted MPEG4 broadcast is then re-encrypted locally in HD video format - using HDCP instead - and the higher resolution stays "hidden", unless the recieving device is "authorised").
http://www.hometheaterhifi.com/volum...s-11-2004.html .
High-Bandwidth Digital Content Protection - Wikipedia, the free encyclopedia .
Exact use of this is currently the subject of legal battles in USA, and meanwhile it looks as if the Sky HD receiver will be using it ....
Well, making good tv programmes costs money, and we should expect to pay for them. But - I think - preventing customers making their own permanent copies is going a bit too far!
(hmmm ........ well, HD pictures will have to be free of those annoying on-screen logos ("dogs"), so just one HD pirate copy could then be "infinitely regenerated" with no trouble (!), so copyright owners are being very careful, but I still think a better system could have been devised!).
--------------------------------------------------------------------------------
Contents:
1) Mini guide, quickie HDTV "what's what", quick links for FAQs and current info.
2) Main guide, complete HDTV technology starting from first principles, links to various technical information.
(will update guides as/when I've time, use the links just below for latest news!).
(Below, there's lots of weblinks! If any website owner objects to a link, please say so, and I'll remove it).
MINI GUIDE: INTRODUCTION, BEGINNERS' INFO, QUICK LINKS:
(quick links also further below, in "main guide"):
(Intro).
1. HD receivers.
2. HD screens.
3. Quick links.
4. UK info.
HDTV pictures are "far more detailed" than older format analogue and digital tv. Very roughly, there's up to twice as much "resolution" - both horizontally and vertically - or, put slightly differently - up to four times as much "total picture information".
(((Hence, delivering such pictures requires up to 4 times the previous "bandwidth", see further below. Mostly, this is "got round" by using advanced data compression techniques, but a significantly bigger capacity transmission system is also needed!))).
Receiving/Viewing HDTV requires:
1) Receivers.
An HDTV receiver (capable of decoding the newer format HD signals). Can be a satellite or cable type of "box".
(HDTV on UK digital terrestrial is technically achievable, but unlikely in the immediate future, due to current system capacity constraints).
Cable HDTV is "deliverable" over existing cable systems, both older coaxial and newer ADSL enabled telephone lines, but - obviously - requires a newer type cable tv receiver.
(Whether cable HD is available in your area depends on how quickly the local company's distribution equipment gets "upgraded").
Sky HD can re-use existing Sky dishes, but also requires a newer type receiver. Note that this is a twin sat antenna input "PVR" device - same as Sky+ - so requires a twin (or quad) output lnb, and 2 cables from dish (presumably, Sky will upgrade existing dishes where necessary).
(Without the 2nd antenna input, a Sky HD receiver will still work, from an existing dish, but can't then record 2 channels at once!).
Some free-to-air satellite HD is also due to start - eg BBC sports transmissions, etc - and such can be received on suitable non-Sky DVB-S2/MEG4 satellite receivers (not requiring 2 antenna feeds!). However, only Sky proprietary receivers can get the encrypted Sky subscription channels (whether standard or high definition)!
"Real time" HDTV is also theoretically possible over the Internet, but there are bandwidth capacity problems, although just transferring HD MPEG4 content data files - slower than "real time" - should work.
2) Screens
An "HD ready" newer type tv screen. Most HD receivers will also output to older type tv sets, but there's not much point in buying expensive new HD equipment, then doing only that!
(Note, "HD ready" means a tv screen will accept and display the newer format signals. It does NOT necessarily mean the picture will be displayed in the maximium possible resolution, since this depends on how many "dots" (pixels) a particular screen has, better is usually more expensive! There's also issues - with various different screen types - of picture quality, lifetime, reliability, etc).
Most HD ready tv "screens" are either LCD or Plasma flat ("hang-on-wall") ones, but projection types are also avaliable.
The 2 main HD broadcast picture "formats" are: 1280x720, and 1920x1080. These refer to (horizontal)x(vertical) "number of dots" (or pixels), and are ususally abbreviated as 720 or 1080 (the corresponding horizontal figure being implied). A "very good" screen will be able to display the full higher resolution mode, lower resolution ones won't show as much detail!
(There's also several possible picture rates, and i (interlaced) and p (progressive) scan modes, for explanations see further below).
(Most HD capable tv screens can accept a wider variety of input modes, eg various computer screen formats, etc).
The HD video signal is sent between HD receiver and HD screen in digital form, via a special new type of connecting cable, HDMI/DVI, confusingly there's 2 different plug types, but all "HD ready" receivers and screens using this connection system should be compatible.
(Older HD ready tv screens use the "obsolete" DVI plug, which doesn't carry sound, so then an additional audio cable is needed, but you'd probably do that anyway, since the Dolby 5.1 surround sound is best with a separate "home theatre" audio system!).
There's an issue with the HDCP copy protection system used on HD broadcasts. Any receivers/screens labelled "HD ready" should be fine, but some older receivers and screens - including computer equipment - able to accept and display the different HD video formats and show the necessary picture resolution, but not having HDCP capability, may not be able to show most HD material (see below, copy protection).
(((Much current Internet HDTV info refers to the USA, where hd IS broadcast terrestrially, but using the ATSC system (all European digital terrestrial tv uses the entirely different COFDM system), so be aware of the difference, ATSC equipment is useless in Europe!))).
3) Links.
UK HDTV quick guide (faqs), up-to-date: High Definition Television (HDTV) FAQ for the UK (Sky HD, Telewest, BBC World Cup ) .
Latest UK HDTV news (services, products, etc); scroll a little down the page for "categories", different subjects and archives: HDTV UK .
TV screen types compared:
CNET's quick guide to TV types - CNET reviews .
Connecting cables: Welcome to DVIHDMICables.com - The DVI HDMI SVGA Information Source .
4 ) CURRENT UK SITUATION (updated 14/07/2006).
SKY. About 15 HD channels currently available. Sky HD home installations have now been going for a few months.
UK Sky HD info: Sky HD - Wikipedia, the free encyclopedia .
(official website): Sky HD - High Definition TV (HDTV) in the UK from Sky .
Unfortunately, there are various reports of a high problem rate with Sky HD receivers! For more details, check the Sky HD sections/threads in our forums, and/or search for further info using Google, etc.
CABLE. The merged NTL/Telewest "tv drive" - with HD capability - has been available for some months, with HD channels and some “playback on demand”. Use of the HD facility requires an extra “premium” payment (as you’d expect!).
Telewest - TV Drive .
(There’s now also various ASDL services down your phone landline – including TV – for details of HDTV currently offered, consult the various companies’ websites).
BBC HD is now established, with a good range of actual HD material (some broadcasters are just “scaling up”, standard material, which admittedly gives slightly better viewing results, but isn’t really HDTV!). This is basically free-to-air (some World Cup was encrypted, for copyright reasons). However, it is being described as a “trial”, with no guaranteed continuation beyond the I year trial period.
No firm news yet on ITV ch4/5, and other UK “maybe” free-to-air HD broadcasts.
EUROPEAN (non UK) SATELLITE HD. A number of channels are broadcasting an HD service, in addition to the older SD one (eg, Sat1 HD). These free-to–air HD channels show many American/British films, but with a dubbed soundtrack! (An exception is Arte, where the French version often – not always! - shows “classic” USA/Brit films with original soundtrack, but the in-video French subtitles will look rather distracting on the HD version!).
ENCRYPTED Sky HD channels can only be received on Sky HD boxes (of course!). Otherwise, presumably, non-Sky encrypted European channels can be received on HD capable sat receivers, if you “know how” (nudge, wink).
HD CAPABLE SAT RECEIVERS are now becoming available and better known, with a slowly increasing range and choice, eg the popular Humax and Pace receivers, for which see relevant threads on these forums.
* * ** * * * * * * * ** * * * * * * * ** * * * * * * * ** * * * * * * * *
MAIN GUIDE.
(intro)
1) Preconditions for any tv picture.
2) Standard definition television (SDTV).
3) Digitising SDTV (bandwidth implications).
4) Compression for digitised SDTV (MPEG).
5) HDTV delivery/receivers .
6) The HDTV receiver/screen link.
7) HDTV display devices.
8) HDTV storage/recording.
9) HDTV copy protection.
(here's an brief "World HDTV standards overview": High-definition television - Wikipedia, the free encyclopedia ).
PURPOSE OF THIS GUIDE:
Meant to be a "very roughly what's what" info source, for people "unfamiliar with the territory". It's a "whistlestop tour", going - perhaps rather slowly! - through basic principles, to actual implementations. So, just look for the particular things you want, more practical stuff is further down.
(The basic technical principles apply to all HDTV, which is a "worldwide standard", but details of particular broadcasters and/or delivery systems are UK specific, since this is a UK site!).
(((As in my other guides, I'm trying to "pitch" at about halfway between 1 page "consumer faqs", and technical papers; there's lots of each type on the Internet, but not that much in between!
It's a Spiney guide, so extremely "wordy" and OTT, I'd rather annoy people that way round than by being too cryptic! Brevity may be a virtue, but ambiguity isn't!))).
Warning/disclaimer, I'll try to be accurate, but no guarantees (obviously), so carefully check specific info (especially before any financial commitiments!).
(Hopefully, other people will soon be adding more guides, general and specific. I'm really "Mr Theory", and anyway - obviously - can't go into exact details on lots of particular products. Where weblinks mention specific products, these are just particular examples I found, and not any sort of endorsement ...... ).
Please forgive obvious "howlers", and "deliberate mistakes" (!).
(((the added weblinks - for more info - are hopefully a) fairly clear;
reasonably fast; c) have helpful pictures. But, I can't spend forever "web-trawling" for the "very best" ones, and of course webpages can change or vanish ..... where links do vanish, then wikipedia often - but not always - has a good entry, it's fairly up to date on tv technology. see:Category
igital television - Wikipedia, the free encyclopedia .Category:High-definition television - Wikipedia, the free encyclopedia .
Material on webpages may be copyright, but there's a "right to view". However, if any website owner objects to a link on this guide, please say so and I'll remove it))).
GENERAL INFO:
(HDTV is already going in USA, eg see :: HDTV Galaxy :: HDTV Schedule ).
(to see HDTV user problems in North America, there's www hidefforum com . I've not made that a direct weblink, in case Rolf objects (but hey, aren't they using the same website software?). Note, "Direc" and "Dish" sat tv systems are roughly equivalent to British Sky. Also - importantly - in North America HDTV is currently being broadcast terrestrially, and you can get standalone and PC plugin card receivers which get both standard def and hi def broadcasts, however these all decode only ATSC modulation, and would be useless in any EU countries, as in Europe we use COFDM modulation instead for terrestrial digital. Also, HD on UK Freeview/ Topup is very unlikely before analogue switchoff frees up some extra bandwidth!).
There's this UK site: HDTV UK (useful for latest news; note my specific info on the rapidly changing UK situation - below - probably isn't "bang up to date"!)
(Some useful UK FAQs at: High Definition Television (HDTV) FAQ for the UK (Sky HD, Telewest, BBC World Cup ) )
European satellite HDTV already exists (EURO1080:::::European High Definition Media Group) but older type (DVB-S1/MPEG2) receivers will be incompatible with any upcoming UK "commercial" services using DVB-S2/MPEG4.
Some DVB-S2 MPEG4 satellite receivers are becoming available, but unlikely to receive much (if any) Sky HD, due to Sky's proprietry encryption! A PC plugin card has also just become available, but of course nearly all current sat receiver PC cards don't have a CI interface for conditional access.
(How much free-to-air - or at least non-Sky - satellite HD will be available in the UK remains to be seen!).
"HD READY". This logo, on "screens", means they can accept a TMDS input (see below), and therefore have either a DVI socket, or HDMI sockets (preferably HDMI, being the newer version, though "converter plugs" are reasonably cheap). But, beware! There are rumours of some shops putting "hd ready" labels on stuff that isn't, if unsure then check that the correct type sockets are on that particlar item. Be especially careful, becase now HD is coming, some older "not-HD-ready" flat screens are being "sold off" at reduced prices!
(Note, HD Ready does NOT necessarily mean a screen can show the HD content at full resolution, but only that it can accept the signals; what a particular screen can show depends on its own maximum resolution!).
HDCP (high density content protection) - insisted on by hi def tv programme broadcasters - is being universally implemented on all HDTV reception equipment (any "hd ready", ie, having TMDS inputs/outputs), and might make copying and storing HD stuff difficult in future.
KEY HDTV CONCEPTS.
The two main ones are "resolution" (amount of detail in picture) and "bandwidth" (ability of system to handle wide frequency range, especially high frequencies!). These are related, in that higher resolution requires greater bandwidth. I'll try to show how, starting with SDTV .....
(Note, some bandwidths are suggested below, but merely as illustrations and only approximately, actual "real" TV systems will have various bandwidths specific to them ..... ).
PRECONDITIONS FOR A TV PICTURE.
Call separate pictures "frames".
For illusion of continuous motion, at least 12 frames/sec are required (eg, Zoetrope, RANDOM MOTION: Zoetrope Animation by Ruth Hayes ). But faster is better. Early cinema used 16 frames/sec, and later 24/sec (current standard).
However, the eye still sees flickering! So, cinema projectors show each frame twice (ie, 48 pictures/sec; some 70mm projectors showed each frame 3 times, though this film standard is now obsolete!).
48 frames/sec - or faster - gives "tolerable" flicker, so this was used for analogue tv systems. However, the rate had to be sychronised with local mains power frequency (otherswise, you got horizontal "bars" slowly going up/down screen, from early technology limitations). So, frame rate was actually 50/sec Europe, and 60/sec USA.
Picture transmission. Sound - audio - is a single quantity varying with time, hence easily transmissible (down a "communications channel"). But a picture's an area, which you can't "send all together at once", so it must be broken up into tiny dots, and these are then sent sequentially
TV picture resolution. This can be considered as a "rectangular dot pattern", with format: (number1 Horizontal) x (number2 Vertical). Obviously, for a fixed screen size, the more pixels (dots!) there are, the more detailed the picture. However, as resolution increases (ie dots get smaller), you're multiplying 2 numbers together, so total number of dots required increases very rapidly, according to square of the screen size, hugely increasing the required bandwidth (one of HDTV's "little problems" - er- in fact the main one!).
(system bandwidth; the more detailed the picture - the more "dots" there are - then the faster brightness must be able to change, since more rapid changes create higher signal frequencies (try to "picture" sine waves; the steeper the slope, the higher the frequency!). If higher frequencies are missing, then the "dots" can't change so quickly, and the picture gets progressively more blurred from increasing lack of detail. So, the "transmission system" must be able to convey the very highest frequencies needed!).
(bandwidth, see: Bandwidth - Wikipedia, the free encyclopedia , Television Bandwidth ).
For analogue tv, horizontal rows of dots instead become "scan lines", and horizontal resolution gets fixed by the particular system bandwidth (ie, how fast scanning spot brightness can actually change!), vertical resolution being fixed by number of lines.
For analogue, screen size 21 inches diagonal was considered "ok", then the scan lines had to "just disappear" at the "normal viewing distance". Using amplitude modulation (am) - then the only "realistic" broadcasting method available- this determined the transmission bandwidth needed (which was still large (!), so vestigal sideband (vsb) transmission was universally used, giving some - but acceptable - picture distortion).
(See: www.scte.org.uk/photo/technical/channel.jpg ). This shows an analogue tv channel, although on a non-UK VHF frequency!).
(For "baseband" video - just sending the brightness along a cable - SDTV requires roughly 6Mhz bandwidth. Using vsb am transmission - for analogue broadcasting - only 8 MHz is required, just 2Mhz more, a good result for "older technology"!).
Interlace. At this resolution, using vestigal sideband, the required bandwidth for analogue tv was still too high at 16 Mhz (making tv receivers difficult to design and expensive, and limiting the number of broadcast tv channels). So, interlace was universally used (reducing bandwidth to 8 Mhz). On the 50 pics/sec or more (to reduce flicker), each was sent with only half the number of total lines, with the lines on even/odd pictures in slightly different positions (but there's still 25 or more frames/sec, to give adequate "continuous movement" illusion! See: http://www.sparkysworld.co.uk/images/interlace.gif ).
(Interlace is now obsolete, by preference not used in digital SD or HDTV systems, and this leaves behind some "legacy" conversion/ compatibility problems).
Colour. Best explained in "joke format"(!) Good news, due human eye physiology any colour can be created by mixing 3 primary colours, red/green/blue. Bad news, that still requires 3 separate tv pictures (needs 3x previous bandwidth). Good news, eye percieves colours in much less detail, so not that much extra bandwidth needed. Bad news, even so, the extra bandwidth still isn't there. Good news, you can "invisibly include" the colour info inside existing analogue monochrome tv channels (also ensuring backwards compatibility!), then no more bandwidth needed.
This is how: A colour camera/video source actually does output 3 separate "primary colour" pictures, red, green, blue. These are multiplexed (added/subtracted/filtered), giving Y (monochrome/fine detail), and R-Y and B-Y (two "colour difference" signals). The monochrome (Y) signal is then transmitted exactly as before, but R-Y and B-Y are first bandwidth limited - to form U and V signals - these are then encoded onto a supressed carrier am signal, at around freq 4MHz ("inside" the 8 MHz tv channel). Thus, this "subsidiary colour signal" (subcarrier) occupies "the same signal space" as the still-the-same monochrome signal, but at this high frequency only "appears" on screen as a very fine - almost invisible - "dot pattern", on any type of analogue tv set.
So, now a monocrome tv still works as before, but a colour receiver then also decodes the subcarrier, extracts the U and V signals, and de-multiplexes these from Y to re-form red, green, blue, pictures (terminology; monochrome Y signal is "luminance", combined U/V colour subcarrier is "chrominance", both these combined - with synchronising pulses then added - is "composite video").
(primary colours see: Television Production: Principles of Color ).
(subcarrier - PAL version - see Software-based PAL colour decoding ).
(Digital tv systems don't have a "subcarrier", that's an analogue only technique! However, they all - including HDTV - retain the Y,U,V system, this still being this most economical way to encode/send colour information. Since monochrome Y needs more detail resolution than colour U, V, more Y "samples" are always sent, the particular Y.U.V ratio depending on chosen digital format, eg: 4.2.0, 4.2.2, etc.....
(see: Chroma subsampling - Wikipedia, the free encyclopedia ).
STANDARD DEFINITION (SD) TV.
Using above techniques, the world ended up with 2 analogue SDTV formats, 525lines;30pics/sec (for 60 Hz mains power supplies) and 625;25 (for 50 Hz). Both with 6Mhz video bandwidth, nicely fitting into a 8mhz vsb-am "tv channel", the channels being on VHF and/or UHF frequency bands (depending on country).
Colour was then "slotted in" later, slightly different systems but variants of the above (colour tv folklore: NTSC = "never the same colour", SECAM = "system contrary to American method", PAL = "peace at last").
(Basically, NTSC is colour tv as above, PAL is a later variant of it, SECAM sends U and V sequentially instead of together).
(Here's a countries list:
http://avconvert.com/video/world_tel...standards.html ).
(Here's the 3 systems compared: www.scatmag.com/technical/worldtv.pdf ).
DIGITISING SDTV.
The next step - as a precursor to HDTV - was (and is!) digitising SDTV. There's many reasons to want to. Digital systems are stable, and you can repeatedly "regenerate" the signal, allowing infinite copying, transmit over huge distances without deterioration, etc. (But, analogue systems need frequent adjustments and calibration, recording and re-copying slowly degrades the original, and quality degrades with transmission distance).
However, a price is paid for these "digital advantages", in the form of hugely increased bandwidth requirements.
In sampling a 6 Mhz video signal, first there's Nyquist's critereon, sampling must be at least 12 Mhz, preferably significantly higher, otherwise we get alaising (unwanted "visual artifacts"").
(see: http://redwood.berkeley.edu/bruno/npb261/aliasing.pdf .
Also: Nyquist...mpling theorem - Wikipedia, the free encyclopedia ).
Let's say we sample at 12 MHz (still slightly too low!). Then, the video signal level may be sampled in 256 "steps", so in binary each sample can be represented as a single "byte", ie 8 bits. However, transmitting the bits directly, 8 requires multiplying the sampling frequency by 4 (bandwidth is half bitrate!), giving a whopping 48 Mhz!
But hang on, we haven't finished, that's just the Y signal (monocrome only picture)! There's also U and V, which we need to re-derive the red/green/blue pictures. Luckily (!) we said that U and V are bandwidth limited, so - doing the above calculation again - the digitised versions of them will only require 24 MHz each. Finally, add this all together, and the digitised SDTV colour picture requires - at least! - 96 Mhz of bandwidth, just to transmit the baseband ("raw"!) video (for example, down a coaxial cable). Never mind finding some way to broadcast it to tv receivers (100 MHz is approx a quarter of the entire UK tv UHF broadcast band!).
So, "straighforward" digital SDTV is "unbroadcastable"! But, 100MHz will travel down coax cables (and can be infinitely regenerated in "repeaters"), and by now most broadcast studio cameras and video recorders are in fact digital (with a fair proportion bit of them also being digital HD, which when fed down coax cables requires a "mind-blowing" 800 Mhz bandwidth!)
(see: CCIR 601 - Wikipedia, the free encyclopedia ).
(also:Digital VCR Formats Compared: Tabular Specifications ).
(also:Tabletop Productions - HD Video Formats ).
(also: Serial Digital Interface - Wikipedia, the free encyclopedia ).
(Now, even "digital videotape" is becomming obsolete, with video material likely to be stored on computer "servers" ......).
(But hang on again, in analogue tv systems, isn't the colour subcarrier already "contained inside" the 8Mhz signal, so couldn't you just digitise the whole thing at once, instead of doing Y, U, V separately, saving lots of bandwidth? Well ....er....yes! In fact, this is actually done, eg in D3 video recorders and on some SDI links. But, you still end up with 50Mhz bandwidth, and this "digitised composite analogue" has some problems, so keeping Y,U,V completely separate is better, and essential for compression .....)
VIDEO COMPRESSION (MPEG).
So, we can make and record digital sdtv pictures, but not transmit them! However ....
Redundancy. TV pictures contain a huge amount of "spacial redundancy" (large areas of the picture being exactly the same), and "temporal redundancy" (large parts of picure not moving over time). Once the picture has been digitised, then it's possible to use computing techniques to identify these parts, and work out numbers for the "brightness/colour values" and "fill-in area size".
Amazingly, using such methods (and others), it's possible to decrease the SDTV bitrate from 200 MB/s (100MHZ) down to 2 MB/s (1MHz), a "decrease factor" of 100!
This is the basis of MPEG2, as currently used for digital (SD!) television broadcasting (whether satellite, terrestrial, cable, or "other"). At 1 Mhz per tv channel, you can get 4 - or even 6 - standard definition channels into the 8 MHz bandwidth previously occupied by only 1 analogue "channel", and that's what's been done. Each "set of channels" is called a "multiplex" (or "mux"). Single muxes are then delivered digitally over the same previously exisiting analogue channels. An amazing achievement!
(Er, 100:1 compression ratio is "extreme", and would only work on "favourable" material, more realistic is maybe 40:1. Unfortunately, commercial pressure often forces broadcasters to "squeeze in" more than is wise, with consequent "artifacts" - eg, jerkiness, reduced resolution - on digital tv brooadcasts).
(see: http://www.fh-friedberg.de/fachberei...g/beginnzi.htm.
Also: MPEG-2 - Wikipedia, the free encyclopedia ).
(Much of the research into DCT compression for bit rate reduction was done at MIT/ Bell labs. This then allowed the formulation of the JPEG and MPEG picture compression standards. Meanwhile - around 1985-1993 - all sorts of "weird and wonderful" schemes were being developed for sending HDTV via existing systems, which very suddenly became obsolete once MPEG2 appeared!).
(see: HDTV By HDTV News Online ; written from a "USA viewpoint", but conveys the worldwide confusion there's been in formulating HDTV standards!).
(In Europe, the British IBA's MAC became the EU "official" method for satellite tv broadcasting, but it got marginalised by Sky starting up in UK using PAL. Nevertheless, as part of "Eureka", the EU still pressed ahead with upgraded HD-MAC for possible HDTV, with widespread equipment demos, and broadcasts of the 1992 Olympic Games, before finally abandoning it when MPEG2 appeared.)
For SDTV MPEG2 muxes, current delivery systems are:
terrestrial (DTT, or ATSC), 1 mux per 8MHz UHF channel.
satellite (DVB-s) 1 mux per 27Mhz sat channel.
cable 1 mux per (8 Mhz) cable channel.
(Also, recently, 1 MPEG2 channel down an adsl line, channel switching being done at landline phone exchange by service providor).
TOWARDS HDTV (yes, I'm getting there, really ...!).
For an analogue HD picture, increase resolution x2 vert and hori, so that's x4 no. of pixels (as was explained), giving a "baseband" bandwidth of 6x4 = 24 Mhz. Transmitted using vsb-am, that would require 30MHz, or same as 4 standard tv channels, which is why it was never done (also, the receiver design would be difficult!)
(Now of historical interest only, the world's first HDTV system was NHK's Hi Vision (analogue HDTV, 40MHz bandwidth), the broadcastable verison being MUSE (8 Mhz bandwidth, via satellite), achieved using analogue video compression. A fine achievement, but incompatible with later digital HDTV, hence a dead-end, and is being phased out).
(see: Analog high-definition television system - Wikipedia, the free encyclopedia ).
((( Hmmm .... all remaining Internet stuff on Muse is gobbledygook, so here's how it worked. NHK developed Hi Vision, a "straightforward" analogue HDTV system with working cameras, video recorders, and display monitors. But, as said, 24Mhz video bandwidth is "unbroadcastable", so MUSE was developed. The HD video was sampled at a quarter the "minimum" Nyquist frequncy, at slighty different positions on each picture, and then each 4 picture sequence made a "complete" single HD picture, providing nothing moved! Where there was movement, careful choice of the sampling frequency ensured the aliasing patterns were "least visible". Motion was also blurred - at quarter resolution - but movement looks blurred anyway! But to help, additional digital "motion vector" info was sent, further improving movement.
Colour used YUV - as usual - but these were transmitted sequentially per line - with U and V also time compressed - so there were no cross chrominance/luminance problems.
Then, the video samples were kept as PAM (pulse amplitude modulation) - not digitised! - and these analogue pulses modulated an fm carrier. Digital encoded sound was also added, using "NICAM like" compression.
It worked quite well, and all fitted into a single 8MHz broadcast tv channel, which compares very well with the - maybe - 2 MPEG4 video bitstreams we now "squeeze into" the same bandwidth, despite using much more powerful technology!)))
HDTV is (approx) double the horizontal and vertical resolution of SDTV, that's 2x2 = 4 times as many pixels (as said above, pixels number increases as screen area squared!). However, we've already got 4 SDTV channels in the MPEG2 "mux", so it looks as if we might get 1 HDTV channel into the same "space", and indeed we can! MPEG2 inclues a HDTV picture format "profile" (see last-but-one above link), and the first experimental digital HDTV broadcasts have been via satellite this way.
However - in the meantime - improved MPEG4 ("video objects", and H264 coding) much denser compression was developed, allowing more HDTV channels-per-bandwidth to be sent than via MPEG2, and "commerical" already exisiting and just starting services will be using this newer method, almost exclusively.
Despite denser MPEG4 compression ratios, nevertheless HDTV still has a higher data rate. Very roughly, you might get 1 or 2 HDTV channels into the same bandwidth that previously carried 4 or 6 SDTV ones. Note, MPEG4 doesn't have "muxes", as such - being a far more general standard - intead for HDTV it's configured with a (number of) H264 video stream(s).
(note, mpeg4 - specfically H264 coding - isn't just about HDTV, but more generally about reducing all video bitrates (and therefore bandwidths, and also storage requirements). So, for example, we're now seeing things like tiny "pocket" MPEG4 tv players which can hold several hours of SD video on a USB "memory stick", etc, unthinkable just a few years ago .....).
(see: MPEG-4 description
also: H.264/MPEG-4 AVC - Wikipedia, the free encyclopedia ).
THE NEW DIGITAL HDTV VIDEO FORMATS.
There's a number of constraints here:
1) Must be "transmissible" via actual broadcasting systems (ie, already exisiting cable and satellite and terrestrial systems, since we don't want to "rip everything out" and completely re-equip!).
2) Must be "backwards compatible" with SDTV, ie, easily back - convertible "in real time", since although the "video source" is now HD, much broadcasting will remain in - and many people still watch in - the lower resolution SD version.
3) Easy real-time conversion between the different HD and SD production and transmission formats.
This has resulted in (horizontal x vertical):
720x480i (NTSC), 720x576i (PAL) - to show existing SDTV digital formats (from terrestrial, cable, satellite, etc);
ED ("enhanced definition") 720x480p (NTSC progressive), 720x576p (PAL progressive) - see below;
HDTV: 1280x720p, 1920x1080i (1920x1080p).
Notes:
i = interlace (as above), p = progressive scan (each separate picture straight down from top to bottom, no interlace). Progressive looks better on rapid motion, especially sports, but much material is still originated in interlace (for easier backwards compatibility). It's better to show things in "original format", rather than digitally converting between them, as the result can be "juddery".
(progressive scan also uses 30 pictures/sec, rather than 25, which further helps with fast movement. However - using same bandwidth - this means the resolution must be reduced, hence 720 instead of 1080 (v)).
(But, here's a funny thing! Although scanning progressively - instead of interlaced - makes no actual difference to the bandwidth, as well as motion being better rendered, for some rerason the eye also "sees" double the picture resolution, although obviously it's not really there (at least, not when the picture's completely static). It's just one of those peculiar "psycho-physical quirks", very useful when you're trying to reduce the required bit rate for compression!).
(So, if progressive scan always looks better, why not always use it? Alas, real-time backwards-conversion to SD is much better from an interlaced source, as was said!).
(note that the picture rate now depends on what's wanted, the previous connection with local power systems has - at last! - been broken).
((( hmmm ... it seems that Europe is/will be broadcasting 720p/25 mode, instead of 720p/30. Exactly why I'm not sure, since dropping from 1080 to 750 resolution is precisely in order to increase the picture rate within the same transmission bandwidth! However, there may be distribution equipment reasons. Unfortunately, this means any USA derived 720/30 material will first need very careful "digital domain" standards conversion, before being re-broadcast as 720/25, otherwise it will look rather juddery ....... ))).
(Standards Conversion: In analogue systems, different line standards may be inter-converted by "line interpolation". The different picture rates are more complicated, demanding some real-time computer power! Or, you can just drop or repeat every 4th or 5th picture - depending which way you're converting - but the result looks "juddery"!
In digital (SD or HD), the easiest thing to do is real-time convert the "raw" format video picture, lines by interpolation, pictures by the 5/4 USA/Europe different rates ratio. Paraodxically, the compressed video formats are potentially easier, because you can interpolate the "motion vectors" at 5/4 ratio. But, whichever method is used, very careful design is needed to avoid "visual artifacts", especially on HD resolutions).
(see: www.snellwilcox.com/community/knowledge_center/engineering_guides/estandard.pdf ; warning: 350K pdf file, fast Internet connection required!).
(But, the 24/25 USA/Euro picture rates are "almost" the same, and might be "converted" just by replaying at a slightly different speed, which would avoid standards conversion problems, although sound pitch changes slightly!)
The highest possible resolution - 1920x1080p (30pics/s) - requires extra bandwidth (of course!), and seems unused at present (in USA).
In practice, the main formats used would be SDTV (which one depends on country), and the HDTV formats. EDTV is unlkely to be used.
Note, these are just "video format standards"! What's actually seen on screen depends - obviously - on the "physical resolution" (total number of dots!) on the particular screen you're watching; better is more expensive! Inside the screen, electronics "re-map" the video mode being used onto the actual screen pixels.
See: High-definition television - Wikipedia, the free encyclopedia (note, that's the same link as at top of this page!).
Also: www.ebu.ch/en/technical/trev/trev_299-ive.pdf .
Also: EBU Technical Review .
RECEIVER/SCREEN SPLIT.
Analogue tv was (mostly, in UK anyway) off-air, and it made sense to put the tuner/demodulator and display all in one box ( a "tv set"!).
With digital, several different delivery systems became available (terr, sat, cable, adsl), and the previous "tv set" now often functions as just a "screen" (internal analogue tuner unused!), connected to the actual receiver via aerial socket or SCART cable.
With HDTV, it no longer makes much sense to have a "tv set", so receivers and screens have now become separate devices.
RECEIVER/SCREEN CONNECTION.
It's certainly possible to have a sort of "super SCART" - feeding the 3 colour picture analogue waveforms separately - but messy, as there's now several screen formats! Also, the 30 MHz analogue bandwidth per picture (either Y/U/V, or "full bandwidth" R/G/
won't "travel so well" down connecting cables (high freq attenuation, fine detail blurring). And converting from digital to analogue - then back again - just for the receiver/screen link risks creating additional "visual artifacts". So, a digital link format was decided on.((Note, many HD screens still have analogue component video inputs, either 3 line Y/U/V with composite sync pulses, or sometimes R/G/B//H/V full bandwidth primary colour pictures with separate horiz/vert syncs (most "screens" with such inputs should be able to auto-detect the HD video format being used from the sync pulse timings). Using such analogue connections should be considered a "last resort", due to the above mentioned problems, although the actual end result may be fine!))
The digital system chosen is TMDS (transition minimised differential signalling). Transition minimising means there's a minimum number of actual voltage transitions, lowering the required bitrate - hence bandwidth - making the digital link work physically better (the signals will travel further, down cheaper cables!).
(Transition minimising - to reduce bitrate and bandwidth - is fairly commonly used, eg, the EFM re-coding used in the original CD format. Although - to be strictly accurate - EFM is a "run length limited code").
(TMDS is also "synchoronous", ie, the data transmission rate is determined by a "reference clock", so a separate clock signal is sent - as well as the data - which was previously unusual in consumer equipment, but is quite common on various "professional" broadcast link formats).
(The "differential" bit refers to 2 wires used for each signal - instead of one wire and "common earth" - which allows high common mode (outside electrical interference) rejection, again increasing the usable cable length. This is often called "circuit balancing").
DVI (digital visual interface). This was originally for a single PC base to PC monitor connection, so carries just a single TDMS HDTV video format channel.
Per pixel, the 3 colour pictures are sent as 3 signals (on 3 sets of 2 "differential" wires), at 8 bit resolution. The master clock signal (on 4th set of 2 wires) may be up to 170MHz, which allows up to a 1920x1080i (and p?) format picture to be sent.
(Note, DVI was originally a PC monitor standard, so doesn't convey sound, just video. You have to separately connect the HD receiver audio output(s) to the screen and/or surround sound system).
HDMI. (high density multimedia interface). This is a superset of (ie, "with knobs on") DVI, hence backwards compatible ("converter adaptors" - or alternatively leads with different type plugs on each end - are reasonably cheap!).
As well as the above DVI type link, this also has several digital sound channels (eg, for Dolby 5.1, etc), forwards data signalling, and a reverse direction data signalling channel (possibly for remote controls).
((( hmmm ... this system of digitally transmitting the 3 colour pictures in parallel "harks back to" the earlier format broadcasting links, later abandoned in favour of HD-SDI (see above). However, this way a lower bitrate is required for each picture, which allows simpler unequalised cables to be used, in a less complicated "consumer format"))).
(Basically, DVI is a single-device one-way receiver to screen link. HDMI is a more a "bus system", allowing different simultaneous digital video/audio/data streams - in both directions - through several "daisy chained" devices. HDMI is better - since more recent - hence preferred).
(Also, the "nominal" maximum working distance is 5m for DVI and 15m for HDMI. Why so different I don't know, but possibly because better "driver transistor" configurations have been used in the chipsets, since the distance between an HD receiver and screen would typically be greater than between a PC base and monitor. Note that, "nominal distance" means under "worst case" conditions, normally you'd expect a greater maximum working length before "digital fallover" occurs).
(Since many DVI "hd ready" tv screens have been sold, a fairly common "household situation" will be connecting a hi def receiver HDMI ouptut to a DVI input screen. In which case, even if the different plugs were completely pin compatible (they're not!), the DVI input still won't have the correct chip sets to handle the additional audio/ data channels!).
HDMI is "tristate", ie, you should be able to "loopthrough" devices that are completely unpowered and disconnected from the mains supply. However, having a "signal regenerator" inside a device on "standby" would increase the working distance. At these high frequncies, I'm not sure what max cable lengths would be for "household distribution", but "signal regenerators" (digital versions of booster amps) ought to be fairly cheap.
(In theory, a "crude but usable" HDMI cable regenerator/ range extender would require just a phase lock loop and noise gate, maybe 3 or 4 ICs, and should be about the size/cost of current sat lnb line amps, But, the cheapest I've seen - there's much more expensive ones - is HDMI extender HDMI repeater. Extend HDMI DVI connection to 150ft) , still at rather a high price! Well, OK, I suppose most people aren't going to have several HDTV screens "around the house", and if they do then the extra $100 is certainly "affordable"!).
(note, HDMI cables are fairly expensive, as at the required frequencies they're "not just a bit of wire", but a carefully designed transmission line. However, any that work should be fine (!) - either the digital pulses travel far enough, or they don't - and stuff like "gold plating" etc seems a bit silly .....
However, over short distances only, you could "get away with" poorly designed cables, so maybe we'll see short-distance usable (8 foot!) ones appearing in "pound shops", OK "for most purposes"!
Basically, it's difficult to distribute the digital HDTV video formats any distance "on copper", due to the extremely high frequencies involved. So, sending HD "around the house" is going to be a problem! Either you use horribly expensive HDMI, or send as much lower bitrate MPEG4 then decode that at the receiving point.
The "logical answer" to distributing these high bitrates is optical fibre, but that's expensive, and there's practical problems for domestic systems ..... ).
(see High-Definition Multimedia Interface - Wikipedia, the free encyclopedia, Digital Visual Interface - Wikipedia, the free encyclopedia ,
Welcome to DVIHDMICables.com - The DVI HDMI SVGA Information Source ).
(also Balanced line - Wikipedia, the free encyclopedia , www.highfrequencyelectronics.com/Archives/Sep04/HFE0904_Tutorial.pdf ) . This is explaining balanced baseband 2-wire analogue audio links, but the principles of (electrically) balanced transmission apply equally to high speed digital links!).
(((Hmmm .... with HDMI cable lengths, you might think "what's the problem"?, since 300MHz is at bottom end of the UHF tv band, and we commonly distribute such frequencies fairly large distances over coax, with no problem! However, a single tv channel only occupies 8 MHz bandwidth, whereas the TMDS signal for only a single HD video channel occupies (up to) 3x150 MHz, very different! With such a wide bandwidth, dispersion becomes significant, and pulses lose their "sharp edges" over relatively short distances, causing intersymbol interference.
See: http://dar.ju.edu.jo/mansour/723/Lec...s/image002.jpg .
Note, dispersion is an entirely different thing from signal loss on sat cable at higher sat i.f. frequencies of 1-2 GHz, this being due to dielectric absorbtion!))).
DISPLAY SCREEN DEVICES (TECHNOLOGIES).
SDTV was worked out to make the pixels "just disappear" at "normal viewing distance", on an approx 21 inch diagonal screen. So, to see the higher HD resolution, really you need a larger screen (that's not mentioning the huge difficulty of actually making any screens at all with "smaller dots"!).
The original tv display device was - of course - the cathode ray tube (CRT), and colour variant the shadowmask tube (and later equivalents). Despite the higher line frequency (with larger "flyback" voltages), it's certainly possible to have HDTV CRT displays (production sets were built for MUSE and later EUREKA 1250). However, for various reasons - again including multiple standards - manufacturers have decided not to go down this route.
(Interestingly, although the 600 line analogue PAL picture is very good, most people have never seen it! Most "consumer product" (built to a price) CRT colour tv sets have tended to have low resolution; largish screen pixels, and Y cutoff above 3.5 MHz. But now, in shops, the (digital) SD picture is being displayed on "HD ready" sets, and it looks quite good!).
A CRT "scans" the screen area with a "light point", hence is subject to flicker (although high persistance phosphors and 100 HZ scanning help a lot!). With newer pixel technologies, all the pixels are "working" all the time, and just get "updated" by the successive video stream pictures. Also, pixels are digital - basically "on" or "off" - so instead of a CRT varying beam current, brightness is adjusted by changing the mark/space ratio of a high frequency switching waveform, and there should be no noticable "flickering" at the freqencies used.
LCD. Liquid crystal display (tv screens) are similar - in principle - to the older technology used in digital wristwatches and instrument displays. The crystals are normally transparent - light goes straight through them - but become opaque when a dc voltage is applied. So, the tv version has a "backlighter" lightsource panel (probably electro-illuminsecent type), covered with narrow coloured stripes (red, green, blue). Then, the lcd flat panels contain electrode grids, forming "crosspoints" where the separate pixels are.
(see What is TFT LCD TV and LCD Monitor Panel? ).
Plasma. Basically, this is an updated version of the old flourescent "gas discharge" lamp, as until recently used in kitchens (and currently "energy saver" bulbs), etc. The flat glass panel contains a low pressure gas, with electrode grids once again forming a crosspoint matrix. 400 v dc (approx) between 2 electrodes causes local gas ionisation (a "plasma"), the gas then emits UV radiation which causes visible-light phosphors coated onto the front panel to glow.
Plasma TV Science.org: Inner working of Plasma Displays .
Projection. The 3 primary coloured pictures are produced separately, and only "mixed" on the projection screen. Older devices tended to use 3 CRTs though coloured filters, newer (including HDTV) ones use coloured lamps shining through 3 led pixel arrays (there are other technologies, but normally used in cinemas etc, not domestic stuff!).
(see Howstuffworks "How Projection Television Works" ).
Screen lifetime/reliability. LCD technology - including waveform modulated brightness - has been around for about 30 years, and lcd computer and mobile phone screens for a while, tv versions following later due to solving "slow response time" and contrast problems. So, these should be reliable. The screen part most likely to fail is the backlighter, which (at least on some models) is replaceable. Plasma - like its flourescent lamp ancestors - slowly wears out with use, the panel "half-life" being time for emissivity to fall by factor 2 (although this would be - maybe - 15,000 hours). Plasma is newer (reliability unknown!) technology, and seems more inherently prone to fail due to 400v dc on the panels, but can currently give a brighter and higher image contrast than LCD.
(On all HD screen types - as on all consumer electronics - the one part by far most likely to fail is the switch mode power supply, which is usually fairly easily replacable/repairable).
"Missing" Pixels. A CRT colour tube is made "photographically" - a bit like using a darkroom photographic enlarger (remember those?) - and the phosphor pattern should be "perfect". Producing flat screens is more like making ics - thin film deposition, monolithic fabrication, etc - and on such a large area some faults can occur. The few missing pixels - per screen - should be black, hence unnoticable (where lcd pixels fail by losing all power, they go permanently transparent to the backlighter, but this should be very obvious!).
For more screen type details, see: HDTV Display Technology Shoot-Out .
For more on the various different flat screen technologies, see:
Telenet - de gevraagde pagina kan niet gevonden worden
(Also note, UK magazine Television has run a series on different Plasma and LCD screen technologies, starting - I think - March 2005).
For some flat screen "pros and cons", see:
CNET's quick guide to TV types - CNET reviews .
For particular product comparisons, try:
Compare Plasma, LCD and Projector HDTVs available in the UK (1280x720, 1920x1080, 720p, 1080i, 1080p) .
HDTV BROADCAST DELIVERY SYSTEMS.
The general principle: existing broadcast distribution systems carry "digital bandwidth", so can usually be re-used for the new HD services, but new receivers will be required to decode MPEG4 instead of MPEG2 (and sometimes a newer modulation system, since - despite denser MPEG4 compression - HDTV still requires a higher bitrate per channel!).
(Note, as with all digital systems, HDTV delivery is subject to the "cliff" or "sudden fallover" effect. With a decreasing signal level, error correction still gives you a "perfect" picture, until a particular point where you suddenly get nothing! Cable operators will send at an "adequate" signal level, otherwise - as for SDTV - it's a question of big enough dishes, aerials, etc ....).
Satellite. Existing dishes/lnbs, cables, line amplifiers, multiswitches, etc, should still be fine, but new receivers will be needed to decode MPEG4 and DVB-S2. MPEG2 and MPEG4 tv channels can co-exist on the same satellite (different transponders), and in some DVB-S2 formats it's possible to "piggy back" an MPEG4 channel onto an existing MPEG2 mux (but I don't know if this is going to be done!).
(see DVB-S2 - Wikipedia, the free encyclopedia , www.ebu.ch/en/technical/trev/trev_300-morello.pdf ).
((satellite has a huge bandwidth - 4 Ghz per oribital position - so is the "best" - and will likely be the most popular - way of delivering HDTV)).
(Note, re-use of existing dishes - eg Sky minidishes - for newer format signals require they have the same "link budget", restricting the broadcaster to only some DVB-S2 "modes").
Cable (coax). For digital tv, exisiting 8Mhz fdm cable channels were updated from vestigal am (1 analogue channel) to QAM (1 MPEG2 mux). There's no difficulty about delivering MPEG4 over the same systems (if necessary, slightly upping the QAM signalling rate). Of course, new receivers will be required for any new HDTV, meanwhile SDTV will continue (analogue/ MPEG2/ MPEG4 can all co-exist on a single coax cable system, using different channels, though analogue is now being progressively withdrawn for "bandwidth economy" reasons!).
(These days, the "main trunk" part of a coax cable system will almost certainly be fibre opic, allowing much greater bandwidths - to include broadband Internet etc - but "endpoint" connections into homes still use a coax cable with frequency division multiplexing).
(see www.scatmag.com/technical/techarticle-june05.pdf
(Adsl (phone landline). At present, this delivers a single MPEG2 channel, switching being done in phone exchange. There should be no difficulty in delivering a MPEG4 channel instead, possibly using updated adsl2 modulation (but a new receiver is needed anyway, for the MPEG4!).
(since adsl tv channel switching is done "at source" - then unlike sat/terr/coax cable - the receiver doesn't contain a tuner, just a demodulator and picture decoder).
(adsl is very attractive to cable providors. Most houses have a telephone landline, and any complex switching/delivery equipment only needs installing at the phone exchange, unlike for coax you don't need lots of street distribution cabinets (which can fail!). However, phone lines were not meant to carry high frequencies, with the disadvantage that adsl only works up to maybe 3 miles from exchange, whereas coax covers much greater distances).
(see: Asymmetric Digital Subscriber Line - Wikipedia, the free encyclopedia ).
(UK) Terrestrial (DTT-T, COFDM delivery). Unfortunately, Freeview/ Topup bandwidth is currently "stretched", and any MPEG4/ HDTV services at all (apart from experimental) seem unlikely before analogue is switched off! (I'm not sure what's happening in other EU countries!). Obviously, new MPEG4 receivers would be required, but COFDM modulation will continue being used, so the reception area would be roughly the same as before (where there's existing DTT!).
Internet. Possible in principle, but unlikely in practice, since - even with "broadband" home connections - the data rate still isn't sufficient for "real-time" MPEG2, never mind MPEG4! However - of course - there's no problem with transferring files, which may have either MPEG2 or MPEG4 content (for later replay).
(The various Internet protocols "use up" lots of bandwidth, that's why this doesn't work over the same ADSL line which can deliver MPEG2/4, since ADSL itself sends almost "pure data" with fewer overheads).
(However, it remains possible to deliver MPEG2/4 bitstreams directly over ATM and frame router systems, in real time, where dedicated links exist!).
CURRENT UK SITUATION (updated 14/07/2006).
SKY. About 17 HD channels currently available. Sky HD home installations have now been going for a few months.
UK Sky HD info: Sky HD - Wikipedia, the free encyclopedia .
(official website): Sky HD - High Definition TV (HDTV) in the UK from Sky .
Unfortunately, there are various reports of a high problem rate with Sky HD receivers! For more details, check the Sky HD sections/threads in our forums, and/or search for further info using Google, etc.
CABLE. The merged NTL/Telewest "tv drive" - with HD capability - has been available for some months, with HD channels and some “playback on demand”. Use of the HD facility requires an extra “premium” payment (as you’d expect!).
Telewest - TV Drive .
(There’s now also various ASDL services down your phone landline – including TV – for details of HDTV currently offered, consult the various companies’ websites).
BBC HD is now established, with a good range of actual HD material (some broadcasters are just “scaling up”, standard material, which admittedly gives slightly better viewing results, but isn’t really HDTV!). This is basically free-to-air (some World Cup was encrypted, for copyright reasons). However, it is being described as a “trial”, with no guaranteed continuation beyond the I year trial period.
No firm news yet on ITV ch4/5, and other UK “maybe” free-to-air HD broadcasts.
EUROPEAN (non UK) SATELLITE HD. A number of channels are broadcasting an HD service, in addition to the older SD one (eg, Sat1 HD). These free-to–air HD channels show many American/British films, but with a dubbed soundtrack! (An exception is Arte, where the French version often – not always! - shows “classic” USA/Brit films with original soundtrack, but the in-video French subtitles will look rather distracting on the HD version!).
ENCRYPTED Sky HD channels can only be received on Sky HD boxes (of course!). Otherwise, presumably, non-Sky encrypted European channels can be received on HD capable sat receivers, if you “know how” (nudge, wink).
HD CAPABLE SAT RECEIVERS are now becoming available and better known, with a slowly increasing range and choice, eg the popular Humax and Starsat (Linux) receivers, for which see relevant threads on these forums.
HDTV RECORD/REPLAY.
Videotape? As said above, video recording has been digital for a while, in "professional" formats. However, it's certainly possible to digitally record a single H264 prog stream on "consumer format" tapes. So, will there be any? Probably not, since optical discs have many advantages, amd there's been no announcements ......
Optical disc has many advantages (mechanically robust, quick random access, films mass-distribution by "stamping", etc), so is attractive to retailers and consumers. However, current DVD cannot achieve sufficient storage density, which requires a smaller wavelength laser and larger lens (fourier optics). So, the newer HD versions are:
Blu-ray (currently "up and going").
(see: Blu-ray Disc - Wikipedia, the free encyclopedia ).
DVD-HD (expected soon).
(see: HD DVD - Wikipedia, the free encyclopedia ).
(There should also be single (R) and multiuse (W/R) writable versions of both, using the same laser burn and dye techniques).
Magnetic recording. On "winchester" sealed computer drives, these days there's no problem with either storage capacity or data transfer rates! Currently, a single SDTV MPEG2 prog stream is "encapsulated" within the native file system of whatever device you're using (computer or PVR), and there should be no difficulty with the slightly higher data rate/storage capacity (about 70%) required for H264 streaming instead, which is still "well inside" the hardware capabilities.
PC software is currently available for MPEG2 SDTV recording/ storage/ replay, and should also become available for (real time) MPEG4 streaming, so in principle you just add the appropriate PC cards. However, not many people would want to view HD on the smallish computer monitor screen, so for viewing presumably you'll need a card with HDMI output sockets - for transferring the HD video format directly to a viewing screen - and there's also the copy proetction issue, see below.
(Note, special on-card hardware would usually be required to turn MPEG4/H262 into a "real time" HDTV video, as using the computer's main processor would slow things down a lot, even top range Pentiums might not manage it too well!).
Also, of course there's PVRs, dedicated tv tuners with magnetic disc storage (Sky's first HD receiver includes one!).
(UK HD PC/PVR situation at time of writing, date: 27/01/06. Right now - as an Internet search will show - there's a fair number of HDTV PVRs and PC cards/ recording software available in USA/Canada. Unfortunately, these nearly all involve terrestrial HDTV using the ATSC system. Europe - including UK - uses COFDM instead, so all such devices would be useless in the UK (and anyway there's no plans for UK terrestrial HDTV before analogue switchoff). So, we'll just have to wait and see what devices become available here. Meanwhile, one DVB-S2/MPEG4 sat receiver plug-in PC card has been announced, but it's unlikely to be any use for Sky's upcoming HD services, which almost certainly will be entirely subscription only via proprietry encryption).
(Obviously, PCs are a single worldwide standard, and - of course -the same software will function anyplace, but that's not much use if there's no local HD broadcasts and/or no tuner cards!).
(However - that warning given - then strictly for information only, there's various public domain and commercial software for PC based HDTV recording in the USA. For instance, see:
EFF: Cooking with EFF: KnoppMyth r5a10 and pcHDTV for DTV Liberation
SnapStream Beyond TV: PC PVR software
www.zenith.com/sub_prod/downloads_pdf/HDR230.pdf .
COPY PROTECTION.
Although it doesn't have to be (!), much MPEG4 HDTV will be subscription, so sent encrypted, and received as usual via conditional accesss (cams and cards, etc).
CD and DVD have a "copy protect bit" in the data structure - which supposedly stops direct digital copying, but it's not always implemented, and somewhat irrelevant anyway as copying is usually done in "analogue domain" (take CD/DVD analogue output to 2nd device, then record that by re-digitising).
However, for HDTV there's an additional system called HDCP (high density content protection). This is a bit dire, as it allows you to record on a PVR type HD receiver, and watch as many times as you like, but not make permanent copies via HDMI or component video outputs (the hd picture shows on your hd screen, but an external recording device won't record it, either not at all, or only in lower def mode!).
(HDCP re-encrypts the HD format video output, before sending it by TMDS down the HDMI cable. The system used is public/private key encryption, with public keys sent openly, but "device embedded" private keys kept hidden. Any device without HDCP fitted can't - (obviously!) - decode the signal, even if given the public key. And all "HDCP compliant" recording devices will refuse to record in HD format (or at all!), if "instructed" to behave this way by the programme's original broadcaster).
(public/private key cryptography is already used for encryption, with CAMS etc, the difference here is that the HD CAM decrypted MPEG4 broadcast is then re-encrypted locally in HD video format - using HDCP instead - and the higher resolution stays "hidden", unless the recieving device is "authorised").
http://www.hometheaterhifi.com/volum...s-11-2004.html .
High-Bandwidth Digital Content Protection - Wikipedia, the free encyclopedia .
Exact use of this is currently the subject of legal battles in USA, and meanwhile it looks as if the Sky HD receiver will be using it ....
Well, making good tv programmes costs money, and we should expect to pay for them. But - I think - preventing customers making their own permanent copies is going a bit too far!
(hmmm ........ well, HD pictures will have to be free of those annoying on-screen logos ("dogs"), so just one HD pirate copy could then be "infinitely regenerated" with no trouble (!), so copyright owners are being very careful, but I still think a better system could have been devised!).
--------------------------------------------------------------------------------