What actually affects digital picture quality?

[leecovuk]

Hello,

If you know and confidently understand how digital broadcast signals work, maybe you can clarify this for me;

Assuming I/we are getting a signal that is strong enough to lock without obvious picture or sound breakup, I understand that the broadcast bit rate and compression used can cause 'blocking' on the resulting picture, often varying at different moments due to the content of other neighbouring channels.

My first question is:-
Does the available broadcast bandwidth tie in directly with this?
For example, do UK Freeview channels (terrestrial digital) always have less bandwidth to start with than their satellite versions, meaning that the 'best' bit rate will always be less than on satellite?

My second question is; again assuming a consistently reasonable received signal strength:-
Is any picture quality gained if I/we adjust anything at our end to get a slightly 'stronger' signal? Or, are digital signals either 'there' or 'not at all' ?

Thanks very much,
Lee

 

[HB13DISH]

Hello,

If you know and confidently understand how digital broadcast signals work, maybe you can clarify this for me;

Assuming I/we are getting a signal that is strong enough to lock without obvious picture or sound breakup, I understand that the broadcast bit rate and compression used can cause 'blocking' on the resulting picture, often varying at different moments due to the content of other neighbouring channels.

My first question is:-
Does the available broadcast bandwidth tie in directly with this?
For example, do UK Freeview channels (terrestrial digital) always have less bandwidth to start with than their satellite versions, meaning that the 'best' bit rate will always be less than on satellite?

My second question is; again assuming a consistently reasonable received signal strength:-
Is any picture quality gained if I/we adjust anything at our end to get a slightly 'stronger' signal? Or, are digital signals either 'there' or 'not at all' ?

Thanks very much,
Lee

I will not attempt to answer your questions because for one thing, I can't receive the Freeview channels, but perhaps this might help.

Based only on satellite experience, I found a correlation between the video and audio bitrate that a channel transmits and how pleasing/rich/alive the picture looks and sounds.
If you can measure the bitrate of the channels that you are interested in (with a Dreambox or perhaps a DVB-S PC card), you might find the answer to your questions.
Alternatevely, post the channel names and which satellite and someone might help you.

 

[leecovuk]

Hello HB13DISH,

Thanks, however I'm looking to grasp the actual technicalities I asked about, rather than focusing on specific channels.
Certainly, I agree that low bit rate channels are generally quite poor to watch.

Lee

 

[Llew]

Hello,

If you know and confidently understand how digital broadcast signals work, maybe you can clarify this for me;

Assuming I/we are getting a signal that is strong enough to lock without obvious picture or sound breakup, I understand that the broadcast bit rate and compression used can cause 'blocking' on the resulting picture, often varying at different moments due to the content of other neighbouring channels.

My first question is:-
Does the available broadcast bandwidth tie in directly with this?
For example, do UK Freeview channels (terrestrial digital) always have less bandwidth to start with than their satellite versions, meaning that the 'best' bit rate will always be less than on satellite?

My second question is; again assuming a consistently reasonable received signal strength:-
Is any picture quality gained if I/we adjust anything at our end to get a slightly 'stronger' signal? Or, are digital signals either 'there' or 'not at all' ?

Thanks very much,
Lee

In answer to the second question, yes, the DVB-T Mux bandwidths are normally 8MHz compared to satellite (~30MHz).

Example bitrates are as shown in the pics. The first pic shows my graph of bitrates received on mux1 (Channel 42) from Sandy Heath terrestial transmitter an hour or so ago.. BBC1 is a consistent 4.6Mb/s. others much lower. Adding each together gives ~15Mb/s. Adding channel CBBC and BBC Red Button (not shown) gives a total bitrate of the mux of around 18Mb/s.

The other pic gives the bitrates when scanning 10.773H (BBC channels) on Astra 28E. I'll leave you to add the total, but will be about 33Mb/s (The total bitrate of that TP).

As for picture quality comparison, that of course will depend on transmitted channel resolution, receiver picture artefacts etc.

Llew

 

[leecovuk]

Hello Lew and thanks for that;
I have a fairly technical mind but I am consistently left at a loss when I try to read up on these things. I suspect my brain has become very hazy through under-use of late.

...
the DVB-T Mux bandwidths are normally 8MHz compared to satellite (~30MHz).

Assuming a channel is prepared to pay for the highest possible bandwidth, making sure to make and route programmes to output matching the available broadcast bandwidth, how much of each 8 or 30Mhz is required by that one TV channel?

Does the 'bit rate' determine how much 'bandwidth' is used by one channel?

As for picture quality comparison, that of course will depend on transmitted channel resolution, receiver picture artefacts etc.

ok sure;
What I am getting at though is;
Given the same channel and bandwidth/bit rate, is there any received picture quality improvement if we adjust the aerial or LNB (for example) and bring in 90% signal strength/quality over a previous 60%?
I stress, I am not meaning that the picture was previously breaking up or freezing. Nor am I talking about any limitations of the TV in terms of the resolution it is capable of. Let's assume we have a set which transparently displays whatever it is fed, and is capable of all broadcast SD or HD resolutions.

I hope you get what I mean? I always seem to have trouble getting an answer to this wherever I post! That may of course be due to how I try to describe it, although I struggle to do it any clearer.
In short, when does improving received digital signal strength and quality have _no further impact_ on what the viewer sees on the screen, in terms of crispness, blockiness or sharpness?

Thanks again,
Lee

 

[HB13DISH]

A satellite transponder can transmit TV channels, radio channels and data.
The owner of the leased transponder can do whatever he wants with it in terms of number of channels, picture resolution and bitrate, audio quality (AC3), etc. and I am sure that they can determine the bitrate of each channel individually so that the sports channel gets the highest bitrate, while his FTA promo channel can have a lot less bitrate and resolution.
As a rule of thumb, a transponder can have in general ten TV channels and several radio channels.
If they go beyond this number, they are bound to decrease the quality of the picture to a point that the subscribers can start complaing.
The best picture quality on Hot Bird 13E is coming from the SRG (Swiss)
provider on TP #17 F=11,526 H
_http://www.lyngsat.com/hotbird.html
They have a stunnig picture quality with measured bitrate of about 6500 kb/s as they have allowed ONLY four TV channels and just eight radio channels.
On the other hand, TP #155 F=11,604 H has an enormous number of TV channels ~29 and I am sure that some of the channels are barely worth watching as they have both low resolution and bitrate.
Hope that I haven't added to the confusion.
Thanks Llew for the great graphs and excellent post. :-worship

 

[Llew]

Yes, that's a good example for comparison HB. A provider often uses statistical multiplexing to maintain the quality of a chosen channel where many channels are crammed into the multiplex.

Llew

 

[leecovuk]

Hello and thanks again.
So, for the sake of absolute clarity and to bring this section to a conclusion, can I please force and check the following statements ...

Does the 'bit rate' determine how much 'bandwidth' is used by one channel?

Answer = Yes.

Given the same channel and bandwidth/bit rate, is there any received picture quality improvement if we adjust the aerial or LNB (for example) and bring in 90% signal strength/quality over a previous 60%?
I stress, I am not meaning that the picture was previously breaking up or freezing.

Answer = No. No further improvement can be gained by doing that.

Thanks,
Lee

 

[compufunk]

Answer = Yes.

Correct, A higher bitrate will use a higher proportion of the available bandwidth.

Answer = No. No further improvement can be gained by doing that.

Correct, From my understanding of things. Your either receiving the signal with little or no errors, or your not.

That extra %20 in signal quality will help if there is any interference in the signal from something like heavy rain. It wont improve the quality of the picture.

You seem to be confusing two separate things here, Signal quality has no bearing on the quality a picture is broadcast/received at.

I'm trying to think of a good analogy, but I cant at the minute (Monday blues)

The signal is either strong enough to establish a one way link or its not. This has no bearing on the amount of bits that are pumped down that link for a given channel.

 

[leecovuk]

thanks, compufunk.

Yes, I loosely understand the principles but I was looking for that clarification which you gave. I have become confused in recent times, due to some things people on various forums have told me, although given with good intent.

Lee

 

[HB13DISH]

There is one more thing that I can think of which can affect the picture quality, which is the equipment that the provider is using in order to transmit the broadcasting.
For example, looking at Amos 3 4W:
_http://www.lyngsat.com/amos3.html
TP #6 seems to have several sections and each section has a frequency allocation
TP #6 F=11,647 V
There are only TWO TV channels in the whole section
IBA Channel 1
IBA Channel 33
Take my word for it, but the picture quality is terrible and I can only think that the reason being that our national broadcasting has outdated equipments, in spite of all the money that the population is forced to pay every year for the TV license.
Or they are not utilizing the whole bandwidth for some odd reason.
(maybe they asked specifically for a narrower bandwidth to reduce the cost???)
On the other hand, another section of TP #6 with the frequency 11,658 V which also has only two channels
Channel 2
Channel 10
has a much better picture than the IBA channels shown above, which I can only relate to the fact that these two channels have better equipments.

 

[ralphmagno]

well there is a lot involved.
on the satellite one transponder can send out 10 to 14 tv channels
using MPEG-2 standard deff.
using the mpeg-4 about 20 to 26 channels depending on bit rate.
news can run lower rate and sports need a higher rate.
so viewing sports and they are kicking a ball it will not go to little squares
(macro blocking).
signal strength will not make the picture better on a digital signal it will make the picture and sound run on or if the level is too low the picture will lock up,macro block and sound break up.
freeze framing will happen as well.
some places transfer the digital packet from the satellite and convert the
transponder fron a QPSK (FM ) to a QAM(AMdigital )packet.
a 36 mhz wide FM signal gets transcoded to a 8 mhz wide digital packet.
the same information is available in the transcoded packet and is sent over underground cable service.
we are still talking channels in the clear.
so its the bit rate of each channel that gives a cleaner picture.
MPEG 2 the standard.mpeg 4 part 2 is better that mpeg 2 and mpeg 4-part 10 and H264 is the best and gives the better picture and a fixed rate.
the sling box running at mpeg4-2 at 1.5 megebits/sec is ok
but the digizon box at h264 at1.5 megabits/sec has better quality.
there is a picture compairson on the web that shows the difference.
hpoe that was not to confusing.
ralph

 

[Channel Hopper]

Apologies, a bit late for this one

The quality of digital data reception is dependent on the manufaturing specifications of the receive equipment. Regardless of the advertising blurb, many receivers designed to 'work' between 1000 and 45000 symbols will not be able to, owing to failures of the individual tuner filtering and demod chip design. This is especially important with HD transmissions where there is little margin for error when the transponder is at full capacity.

Most broadcasters would rather you believe it has nothing to do with them if your subscriber kit fails to provide a decent (as in error free, not crap/handbagging) 90 minutes of football though.

And don't forget, with any system, too much signal can cause degradation of the quality of the digital stream, as the detectors within the tuner can also 'fall over' and fail to pick up the key components of the final video/audio/data.