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Date: Mon, 22 Dec 1997 12:09:02 -0500
From: Randall Hopper <rhh@ct.picker.com>
To: Petri Helenius <pete@sms.fi>
Cc: Amancio Hasty <hasty@rah.star-gate.com>, multimedia@FreeBSD.ORG
Subject: Re: MPEG audio/video sync & Re: These mtv video pauses are murder
References: <19971221222047.08726@ct.picker.com> <199712220329.TAA00402@rah.star-gate.com> <19971221225947.08124@ct.picker.com> <199712220916.LAA22145@silver.sms.fi>
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Petri Helenius:
 |Randall Hopper writes:
 | > 
 | > Say, something I've been meaning to ask you and the group.  Is there
 | > something in the MPEG system stream format that you know of which allows
 | > you to "attach" (or suggest a relation) between a particular point in the
 | > video and the audio streams.  Or is that implicit in the interleaving.
 |
 |MPEG system stream contains "PTS"s which stands for "Presentation Time
 |Stamp". I cannot recall from top of my head at which point the data
 |should be going at the same time when the PTS hits. It's either the
 |last or the first byte of the pack and it was supposed to either play
 |back or enter the decoder at the time of PTS. I've to check the spec.

Interesting.  Thanks for the info.  I'll look this up and see if I can drop
these breadcrumbs in mpeg_encode's direction and get it to carry them
through for each video frame.

 |It's possible to address, however doing PTS's is not a perfect way
 |since some decoders drop or fill in frames to keep the sync. However,
 |it's much better than no sync at all.

Right.  That's really all I want.  I know mtv or whatever decoder is going
to have to skip frames, particularly for slower machines.  I just want
to be able to give it hints as to where to skip "to".  Even if if no frames
need to be skipped during decoding, a frame or two now and then might need
to be tossed to try and stay in sync with the audio stream.  Sounds like
PTS is what I was looking for.

 |I think carrying the timestamps trough the elementary stream phase
 |might be difficult.

Ok.  I'll take a look.  Thanks.

Randall