[linux-dvb] Re: Audio sync

[Felix Domke <tmbinc@elitedvb.net>]

Régis Bossut wrote:

> PCR means Program Clock Reference. Packets are sent at regular
> intervals. Between packets, the clock is deduced from these timesyncs
> packets from an internal PLL synchronized on them. This is one
> mechanism.
ok

> An other one is the DTS (decoding timestamp) which is carried

> on audio and video packets in the MPEG headers. These DTS are generally
> sent in the TS header of the packet starting the beginning of a frame 
PES-header, not TS-header.

> (this is the case for DVD MPEG streams, which do not contain a PCR). 
PCR is for TS what SCR (system clock reference) is for PS. As DVD 
contain PS, they don't contain PCRs. But they contain SCRs.
(Esentially they are the same, but one transport stream often has 
multiple programs, and thus, there are multiple clocks, so they aren't 
called *System* Clock Reference)

BUT:
Syncing your local "system" or "program" clock to the one in the stream 
is only neccessary when having a "live" input, where the incoming 
bitrate is fixed. Because normally, your local clock will NEVER be 
EXACTLY the same as the one of the broadcaster, you use that PLL to 
ensure they are at the same frequency, so that frames are consumed 
exactly as fast as they are produced.

> The question:
> Which clock does the ARM use for audio/video synchronization ? I guess
> it's DTS if present, PCR if not. But I'm not a guru...
Well, DTS are plain "decoding time stamps". You can, however, take the 
first DTS as your clock reference, and let your local clock just "run", 
for example using a normal timer, or even udelay() (bad idea :) or 
whatever. In that case, your "sync master" is the video stream. (You can 
do the same for audio) But this only works as long as your buffer fills 
in the same rate as your system decodes and displays them. If you can 
load the stream in the speed you want (DVD for example, or a recording) 
this is no problem, since you only keep the buffers "full", thus loading 
the stream in the rate you display the frames.
But if you get a "live" stream (for example, real DVB), you have to 
display the frames in the right speed. that's only possible with the 
correct PCR and the PLL.

So in a "broadcasting environment", PCR evaluation is essential to avoid 
buffer over- or underruns.

(A real pain is to sync to PCRs when your delivery network has variable 
latencies, for example the internet, for example in a multicast-environment)

Please correct my if i'm wrong.

And after all, i never used a AV7110-based card, so i can't say anything 
to that matter.

felix




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