[FFmpeg-devel] mpegts_write_header(), the buffering and thus reordering of audio samples.

Michael Niedermayer michaelni at gmx.at
Tue Nov 1 03:41:23 CET 2011

On Mon, Oct 31, 2011 at 06:24:07PM -0700, Will Korbe wrote:
> Hello,
> I hopefully have a minor question about the
> libavformat/mpegtssec.c:mpegts_write_header() implementation. My hours
> of online searching and specification reading for the answer has been
> unsuccessful.
> I see there is an efficiency built in to bundle multiple audio samples
> into one PES packet and was wondering if someone could point out a
> paragraph from the MPEG2 specification which indicates the reordering
> of TS Packets this causes is okay? I have a video with many 41 byte
> audio samples at the start, and this seems to push the first audio
> samples 5 seconds into the output. At this point, the PTS and DTS for
> the audio samples are beyond the last PCR value by 5 seconds. The
> video plays fine in QuickTime, the audio and video are precisely in
> sync, so I assume QuickTime has logic to buffer the video frames until
> seeming the first audio sample to account for this, but I wonder if
> most MPEG2TS players handle this the same way. If this is spelled out
> in the specification, I would sure feel more comfortable with it.
> Any information would be useful.

I dont remember a tight limit on the PCR-DTS distance in the spec but
then i dont remember the spec very well ...
but having this large is bad because this means a long startup delay.
So 5 seconds is IMHO not appropriate by default.
Of course that delay would only affect actual broadcast and not playing
from a local file with a modern player.
the user parameter max_delay should be able to limit this, if not
a bugreport & patch are welcome

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Asymptotically faster algorithms should always be preferred if you have
asymptotical amounts of data
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