[linux-dvb] Re: Pcr question

[Holger Waechtler <holger@convergence.de>]

Paolo Rossi wrote:
> Hi,
>
>
> I want know if PCR PID is used by the receiver card to decode the mpeg2 
> stream. I have read the standard, i have dump the SI table in 
> transponder 13 east,and i have see that the programs define a PCR pid 
> not equal 8191 ( the pid for null packets).
> Can someone help me?
The Program Clock Reference transmitted on the PCR PID is used to 
synchronize the receivers MPEG clock with the clock on the transmitter 
side in the playout center.

If you would not do so the decoder buffers would overflow or underflow 
because the sender is transmitting the stream faster or slower than the 
receiver decodes them.

Usually the PCR pid is pointing to the video PID, sometimes to the audio 
PID and in some cases also to a seperate PID which transmits the 
reference clock for multiple channels. The PCR is encoded in the MPEG TS 
header before the payload starts.

For playback from disk the PCR is not required, you can fetch the data 
as fast as you want and decode with a constant clock.

Holger



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