Bit Rate Comparison - DTT v DSat
 

Hi

Apologies if this question has been asked before...

Obviously, satellite has plenty of bandwidth whereas each DTT multiplex
is quite limited and there are few, if any, spare frequencies for
additional multiplexes. Are individual channels transmitted at a lower
bit rate on DTT (and so are more compressed) than on DSat or is there a
standard level of compression for both? Presumably the charge BSkyB
make for broadcasting a channel is data rate dependent.

Also, do the signals leave the broadcaster as analogue (RGB or CVBS) and
get converted to MPEG for digital or as MPEG for conversion to CVBS for
analogue transmitters?

Many thanks,

Nick

Nick wrote:
Hi

Apologies if this question has been asked before...

Obviously, satellite has plenty of bandwidth whereas each DTT multiplex
is quite limited and there are few, if any, spare frequencies for
additional multiplexes. Are individual channels transmitted at a lower
bit rate on DTT (and so are more compressed) than on DSat or is there a
standard level of compression for both? Presumably the charge BSkyB
make for broadcasting a channel is data rate dependent.

Bit rates for DTT and D-Sat are in the same ball park for BBC
transmissions, ITV and C4 seem to use lower bit rates on D-Sat than DTT,
but also their D-Sat pictures are at only at a horizontal resolution of
540, compared with 720/704 for the other major channels. C5 D-Sat is far
better than DTT.

BBC,ITV,and C4 make their own uplinking arrangements. Sky have nothing
to do with these, and they certainly do not (and can not) dictate bit
rates. Sky's involvement only goes as far as EPG data handling and in
ITV/4/5's case encryption services. Only C5 use Sky's uplink facilities.

Also, do the signals leave the broadcaster as analogue (RGB or CVBS) and
get converted to MPEG for digital or as MPEG for conversion to CVBS for
analogue transmitters?

The output of all major TV stations is high bit rate, low or zero
compression digital component video. Usually SDI 270Mb/s (no
compression). This signal is coded directly to emission rate MPEG for
digital platforms, and into PAL for analogue transmission. Most
distribution of signals between studio centres, and to the analogue
transmitters is now digital.

--
Mark
Please replace invalid and invalid with gmx and net to reply

Mark Carver wrote:
Nick wrote:

Hi

Apologies if this question has been asked before...

Obviously, satellite has plenty of bandwidth whereas each DTT
multiplex is quite limited and there are few, if any, spare
frequencies for additional multiplexes. Are individual channels
transmitted at a lower bit rate on DTT (and so are more compressed)
than on DSat or is there a standard level of compression for both?
Presumably the charge BSkyB make for broadcasting a channel is data
rate dependent.


Bit rates for DTT and D-Sat are in the same ball park for BBC
transmissions, ITV and C4 seem to use lower bit rates on D-Sat than DTT,
but also their D-Sat pictures are at only at a horizontal resolution of
540, compared with 720/704 for the other major channels. C5 D-Sat is far
better than DTT.

BBC,ITV,and C4 make their own uplinking arrangements. Sky have nothing
to do with these, and they certainly do not (and can not) dictate bit
rates. Sky's involvement only goes as far as EPG data handling and in
ITV/4/5's case encryption services. Only C5 use Sky's uplink facilities.

Thanks for this. A couple more questions:

- You say that the main broadcasters make their own uplink arrangements
- does this mean that they separately contract with transponder owners
on Astra to rebroadcast their signal and just get Sky in effect to tell
their boxes where to find the channels? I had assumed that BSkyB owned
Astra - is that not so?

- Presumably ITV/C4/5 do their own encryption if the signal does not go
through Sky - the Sky EPG just passes on the decryption keys.

Regards,

Nick

Nick wrote:

BBC,ITV,and C4 make their own uplinking arrangements. Sky have nothing
to do with these, and they certainly do not (and can not) dictate bit
rates. Sky's involvement only goes as far as EPG data handling and in
ITV/4/5's case encryption services. Only C5 use Sky's uplink facilities.

Thanks for this. A couple more questions:

- You say that the main broadcasters make their own uplink
arrangements - does this mean that they separately contract with
transponder owners on Astra to rebroadcast their signal and just get Sky
in effect to tell their boxes where to find the channels?

Correct. Astra (aka SES) owns the satellites and transponders. The BBC,
ITV, and C4 all directly rent their transponders from SES. (As do
Viacom, aka MTV etc)

just get Sky
in effect to tell their boxes where to find the channels?

Yep, spot on !

I had assumed
that BSkyB owned Astra - is that not so?

Certainly not !

Presumably ITV/C4/5 do their own encryption if the signal does not
go through Sky - the Sky EPG just passes on the decryption keys.

Yes, there is a data link between the broadcasters and Sky for the EPG,
I assume the keys are probably also present on this.

--
Mark
Please replace invalid and invalid with gmx and net to reply

"Mark Carver" wrote in message
...
Nick wrote:
Hi

Apologies if this question has been asked before...


Bit rates for DTT and D-Sat are in the same ball park for BBC
transmissions, ITV and C4 seem to use lower bit rates on D-Sat than DTT,
but also their D-Sat pictures are at only at a horizontal resolution of
540, compared with 720/704 for the other major channels. C5 D-Sat is far
better than DTT.

There are times when the compression/multiplexing can't keep up with the input
signals. i.e. the trasmitted digital pictures can not be perfectly resolved.

Logically one might think that the encoding equipment ought to know when this is
happening.

Are any statistics kept? In the BBC's case there would also be some
justification for publication.


--

Michael Chare

Michael Chare wrote:
"Mark Carver" wrote in message
...
Nick wrote:
Hi

Apologies if this question has been asked before...


Bit rates for DTT and D-Sat are in the same ball park for BBC
transmissions, ITV and C4 seem to use lower bit rates on D-Sat than
DTT, but also their D-Sat pictures are at only at a horizontal
resolution of 540, compared with 720/704 for the other major
channels. C5 D-Sat is far better than DTT.

There are times when the compression/multiplexing can't keep up with
the input signals. i.e. the trasmitted digital pictures can not be
perfectly resolved.

Logically one might think that the encoding equipment ought to know
when this is happening.

Are any statistics kept? In the BBC's case there would also be some
justification for publication.


If any statistics were kept the BBC wouldn't let you see them. Why?
Because the BBC is always right when it comes to technological
decisions, and will not subject itself to questioning on such issues and
will ignore criticism.

If you actually look at the BBC's bit rate allocations then there's
masses of (self-serving) examples of hypocrisy. It actually beggars
belief why other broadcasters look to the BBC as being a good
broadcaster in terms of technical issues, because they're borderline
incompetent.

An example is the 40 Mbps of capacity on DSat for BBCi and just 2 Mbps
for all their national radio stations on DSat. If they wanted to they
could provide their national radio stations on DSat at very high audio
quality if they just re-allocated 0.5 Mbps of the BBCi capacity. But
they choose to provide average audio quality on most of their national
stations and low audio quality on the rest, almost certainly to limit
the difference in audio quality between DAB and DSat. Can't have that,
oh no.

Another issue with the BBC spending our dosh is that they spend a hell
of a lot of money providing internet audio and video streams for the
rest of the world's consumption, which has a knock-on effect on the bit
rates we can receive the internet audio and video streams at, because
bandwidth costs money.


--
Steve - www.digitalradiotech.co.uk - Digital Radio News & Info

Find the cheapest Freeview, DAB & MP3 Player Prices:
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The output of all major TV stations is high bit rate, low or zero
compression digital component video. Usually SDI 270Mb/s (no
compression).

So we can see how much info is thrown away in the MPEG process!...

This signal is coded directly to emission rate MPEG for
digital platforms, and into PAL for analogue transmission. Most
distribution of signals between studio centres, and to the analogue
transmitters is now digital.


Yes at that rate coded to PAL no wonder PAL looks better:))

--
Tony Sayer