FAQ: The basic basics

2old4this

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#1
Here's my FAQ/glossary as originally posted on this board, here:
http://www.satellites.co.uk/php-bin/forum/showthread.php?s=&threadid=2740

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FAQ: MPEG-DVB-tech sat info
2old4this
v2
16-May-2001
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PIDs
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PIDs = Packet IDentifiers. They are parameters which identify particular packets (188-byte chunks) of the digital MPEG-2 data-stream... They are 4-digit integers represented in 13 bits. So max 8191 values. 0000 is reserved to identify the Program Association Table (PAT). 8191 is reserved to identify "null packets" used as fillers to keep the transport bit stream constant (I assume these are not used in the case of the less comon variable bit-rate streams). So the actual real values in use for audio, video, etc. are 0001-8190.

APID = Audio PID (a tag on the packet which says "this packet encodes some audio data")

VPID = Video PID (likewise for the video info)

PCRPID = Program Clock Reference PID (used to synchronise video & audio. Not commonly used).

TEXTID = the "PID" of any teletext info

SID = the "PID" of the SI (DVB service Information) packets. Such packets are used for various purposes like program info. Actually, SI specifies all the data required by the receiver to demultiplex and decode the programs and services in the
transport stream. Values x'0000' through x'0014'. It is metadata (data about data).

NID = the "PID" of the NIT (Network Information Table). The NIT typically contains pointers to other related transmitted data for the same network/provider.

There are a few common conventions with respect to PIDs but don't be misled. For example - the PCRPID is rarely used and defaults to the value of the VPID. The Audio PID is usually 1 higher than the VPID. Etc. But this is just convention. In theory they could all have arbitary values.
Often a provider retains the same frequency, symbol-rate, polarisation and FEC, but for reasons known only to themselves changes one or more PIDs. This will result in the receiver losing sound, picture - whatever - for the changed channel. The solution is to rescan the transponder, since the scan funciton usually pulls in the PIDs from the data stream. Some providers do not transmit the PIDs in the data stream. What I mean is; the PIDs are of course used (each packet has its appropriate identifier) but that the actual PID values are not provided in the data stream. So the scan functions will not be able to determine the PIDs and will not "find" the channel. An example of such a channel was the now defunct Irish "Tara" on Intel601. For those, one must manually create the channel in the reciever's settings - i.e. the receiver must have a "manual PID entry" function. Manaul PID entry can also be used to correct channels whose PIDs had changed thus avoiding a total re-scan of the transponder.


FEC
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FEC = Forward Error Correction. It is the proportion (fraction) of data which is used for the actual data. The rest is redundant, used only for correcting any transmission errors. The lowest value is 1/2 which means that fully half of the data is there for error-correction purposes only. The highest value is 7/8 which measn that seven-eighths (nearly 84%) of the data is "real". IE only 16% is for error correction...
Possible values are 1/2, 2/3, 3/4, 4/5, 6/7. 7/8. If you see an "auto" function for this setting it means that your receiver is capable of automatically detecting the FEC in use.


SR/SYMBOL RATE
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Symbol-rate is a measure of the amount of data bits sent per unit time. The unit is Megasymbols per second... NOT megabits, but megasymbols. Actually, a "symbol" is equivalent to 2 bits. So for example, the commonly used symbolrate of "27.500" is actually 55 Mb/s. The lowest symbol rates used by satellite tv transmissions are around 1.000, below the threshold of most consumer receivers. The highest values are around 35.000. The Echostar receivers can process signals with symbol rates ranging from about 1.500 to 40.000 (formally, the lower limit is 2.000 but in practise they go lower).


POLARITY
--------
This is a feature of the microwave wave-form.
Polarity can be linear or circular. Linear polarity is "Horizontal" or "Vertical". Circular is "Left" or "Right".
In fact, satellite receivers only ever work in horizontal or vertical settings. When you tune to a crircularly polarised signal, you must therefore choose either H or V. No receivers have L or R settings. But it works becuase the circularly polarized signals have both V and H components (ever studied vectors?). So tuning to "H" pulls out the H vector/component of the L or R signal, although with a 3db loss.
You can get dialectric plates for your LNB which "convert" the circularly polarised signals to linear signals, but they then cause 3db loss on the linear signals. It's one or the other... (see the separate FAQ I wrote).


FREQUENCY BANDS
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The frequency range of signals is split arbitrarily into various bands. The C-band is the lower end (3.7 to 4.1 GHz) with the Ku-band higher (10.7 to 18 GHz, although only used to 12.75 in practise). Still higher is the Ka-band (18.3 to 1.2 GHz) used for data transmissions, not for TV signals.
Higher frequencies can be more tightly focussed than lower, so that the signal at a given location in its footprint is stronger and the dish required is smaller. So Ku-band dishes tend to be smaller than C-band dishes. The huge dishes one sees in America or Asia are usually that big because they have to be - those guys use a lot of C-band.


Hope this helps
See the following sites for more info:

http://www.coolstf.com/mpeg/index.html
http://www.dvb.org


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2old4this

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#2
My FAQ as originally posted at this board, here: http://www.satellites.co.uk/php-bin/forum/showthread.php?s=&threadid=1794

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FAQ: The basic basics
2old4this
v1
2-Oct-2001
--------------------------------------------

Satellite channels are carried on microwave signals beamed down from a string of geostationary satellites orbiting above the equator. That orbit is called the Clarke Belt. From the earth's surface they appear to be aligned along an arc which is the bit of the Clarke belt visible above the horizon from wherever you happen to be. If you were at the equator, this arc would stretch from horizon to horizon and pass directly overhead. At N.European latitudes, it is "lower" in the sky (and a bit smaller).

There are many satellites positioned along the arc. They each appear staionary. For many markets, only one satelllite (or cluster of co-located satellites) is typically in use. For example, the Sky-digital market in UK use the cluster of "Astra2" satellites positioned at 28.2 degrees east. Most continental European markets use the Astra1 cluster at 19.2east, or the Hotbird cluster at 13east.

In contrast to a typical terrestrial tv arial, a satellite dish is highly directional. And perhaps paradoxically, the bigger the dish the more precisely it needs to be aimed (analogous to a higher powered telescope having a smaller field of view). If a dish is just a few centimeters too far right or left or up or down of the target position then you will miss that satellite altogether.

For the typical average viewer of satellite TV, a single dish aimed permanently at one of the mentioned positions will suffice.
That is referred to as a "fixed dish" arrangement. That dish collects the incoming signals and reflects them back to its focal point, where a clever device called an LNB (Low Noise Block down-converter) is situated. That device converts the frequency of the signal to something that can efficiently be propogated along high-grade co-axial cable to the satellite tuner/receiver.


Multi-satellite reception
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Then there are a number of ways of being able to receive from more than just one position.

The most obvious is to have more than one fixed dish, each pointing in a different direction.

The most popular (avoiding the "one dish only" planning restrictions many regions have in force) is to use a single dish but to augment it with additional LNBs, each angled slightly differently so they pick up signals which are arriving at the dish from the different satellites and being bounced left or right of the dish centre. A switching device controls which of those multiple LNBs is in use at any given moment. The satellite receiver sends control signals to the switch as you zap between channels on the various satellites (those control signals most commmonly follow a protocol called DiSEqC)

The most flexible but also the most complicated and often most expensive solution is to use a "motorised" ("steerable") system. Basically this is just a dish mounted on a motor that spins left or right by a given amount according to instructions it receives from a "positioner". The positioner can be a stand-alone box, or integrated into the receiver.
A motorised dish is mounted at such an angle that when it is moved left or right by the motor, it actually traces out the arc along which the satellites appear to lie. Same principle as an equatorial telescope mount if you've ever been into amateur astronomy.

In fact, there are two main types of mototised system. The "traditional" type is a setup whereby a positioner powers a motor directly and sends (and counts) pulses to move it left or right. Several cables are involved, stretching from the positioner (or integrated positioner/receiver) to the motor.
Becoming more popular these days is the DiSEqC motor. This type of motor is operated entirely on power and control pulses fed up the same single coaxial cable that is already running between receiver and LNB. It is popular with manufacturers because their receivers no longer need to include a specific positioner - they just need software which generates the right control signals. The protocol of control signals used is called DiSEqC v1.2
It is popular with consumers since the cost is lower and no extra cables are required - although a specific "DiSEqC" motor is needed, or else a conversion box for operating with a traditional motor.

Signal types and encryption
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A basic receiver without bells and whistles will typically be able to receive either "analogue" or "digital" signals. Analogue is being phased out in Europe since many more channels can be squeezed into the available bandwidth using digital techniques. Analogue systems are typically PAL or SECAM. Digital transmissions use MPEG compression, and the satellite standard is called DVB.

But a basic receiver (analogue OR digital) will only typically be able to cope with unencrypted, or Free-To-Air (FTA) signals. Most newer stuff is actually encrypted these days. So a FTA receiver will not, for example, be able to receive most of the stuff in the Sky Digital package, or Holland's "Canal Digitaal"/Canal+ package. Encryption is used to limit the reception to a paying subscriber base. A subscriber typically receives a smartcard which is activated with some "keys" to open up the channels paid for. The smartcard is inserted into a decoder (decryption) module of some description, and it is the combination of decoder module and smartcard which allows the channel(s) to be viewed.

Some decoders are external boxes that plug into the receiver. Example: D2MAC decoders for certain anlaogue channels.
Some are "embedded" in the receiver.
Example: the mebedded Irdeto of a Humax5400 digital receiver.
Some are modular, and removable.
Example: the irdeto CA modules of early Nokia boxes.

The most modern type are modular, but also receiver-independent - ie interchangeable with any "Common Interface" enabled receiver. These are called CI modules. Example: the Astoncrypt CAMs (for emulated decryption of SECA's "mediaguard" system).

All modular decoding devices are generically referred to as CAMs (Conditional Access Modules)

CAMs are like mini-computers. They internally consist of a bunch of chips including a chip at the heart which contains the 'firmware'. In some CAMs, that firmware can be replaced. Usually that process is referred to as "flashing". Most CAMs can only be flashed by the manufacturer or satellite service provider (for example, to enable slight improvements in security of the system from time to time). But a few CAMs can be flashed by the hobbyist, to enable other interesting features. One example of a flashable (reprogrammable) CAM is the original series of Irdeto CI CAMs. Some clever person once created some new firmware for that CAM that is called "FreeCAM". A CAM with FreeCAM firmware loaded can decrypt Irdeto transmissions with or without a card. Some other clever person later even tweaked the FreeCAM firmware to allow command signals from not just Irdeto but also Viaccess ad Mediaguard systems to be passed to any inserted card. That tweak was originally FreeCAM2 v0.15, and later v0.16 and most recently 0.17. So your "FreeCAM2 0.17" is an original series Irdeto CAM reflashed with firmware that - used in conjunction with a suitably programmed pirate card - can give access to channels encrypted under Irdeto, Mediaguard ("seca") and Viaccess as well as being able still to decrypt the Irdeto ones without any card at all.

The "ADD PLAINKEY" etc that you mentioned are functions not of the receiver but of the FreeCAM module. A plainkey is a particular kind of Irdeto decryption key. The FreeCAM firmware contains decryption algorithms and a set of known plainkeys for certain channels. But those keys can change, and the "enter plainkey" function allows you to update them when hey do (although you need much more knowledge on what to enter and why, before you start experimenting with that...)


Channel settings in the receiver:
---------------------------------
Most receivers allow the customer to "scan" (sometimes called "searching") for new channels or changes in the channel line up. The method of doing this varies from receiver to receiver, and most receivers allow different degrees of scanning (eg fuill scan, or scanning just on one particular frequency).
It may come as a surprise to learn that there are such changes happening daily up there. Often a satellite dealer selling a receiver will kick things off by loading a then up-to-date list of channels into the receiver. But this is just a snapshot in time. Within a few weeks, there will typically have been several changes - even on a single satellite. It's also possible that the dealer gets it wrong and introduces channels with the "wrong" parameters, so that when you actually try to tune to that channel you won't see anything. So there are all kinds of reasons that you may be "missing" channels.

Having said that, you will never of course be able to receive channels that are beyind the reach of your dish/LNB or outside the technical/decryption capabilities of your equipment. If you have a fixed dish and Fox Sports is not on the satellite you have fixed it towards, then you will never receive it without repositioning the dish. If it is transmitted on "your" satellite but under an encryption system you do not have a module and card for then you will "see" the signal but get no picture/sound.

There are many parameters that define the channels. For digital transmissions, the most essential are the frequency, the polarisation (eg, horizontal or vertical), and the symbol-rate (SR). Others include Forward-Error-Correction (FEC) but a receiver can usually determine that automatically if you command it to scan a particular freq/pol/SR. There is also a bunch of "packet identifiers" (PIDs) which the receiver uses to separate out from the huge amount of incoming digital info those bits which belong together for a particular channel. But all receivers will find PIDs automatically during scanning.
Often people refer to the combination of frequency and polarisation (and sometimes SR) as "transponder". So a receiver might have a "transponder scan" in its menu, for example. A transponder is actually an electronic device that transmits the signal, but that's another story...
 
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