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Date:      Sun, 21 Dec 1997 22:59:47 -0500
From:      Randall Hopper <rhh@ct.picker.com>
To:        Amancio Hasty <hasty@rah.star-gate.com>
Cc:        multimedia@freebsd.org
Subject:   MPEG audio/video sync & Re: These mtv video pauses are murder
Message-ID:  <19971221225947.08124@ct.picker.com>
In-Reply-To: <199712220329.TAA00402@rah.star-gate.com>; from Amancio Hasty on Sun, Dec 21, 1997 at 07:29:13PM -0800
References:  <19971221222047.08726@ct.picker.com> <199712220329.TAA00402@rah.star-gate.com>

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Amancio Hasty:
 |Yes, I know about the mtv plus the sound driver issue . Will try again
 |this week to sort it out with Tristan. We left the issue that Tristan
 |was supposed to use a new ioctl provided by Hannu once that gets
 |iron out and it works on linux, I will be happy to implement the
 |new sound driver ioctl for freebsd.

Ok, sounds like plan.

 |Got to admit that once fxtv supports YUV it is going to be totally
 |awesome -- we should try to load up mpeg movies --- for instance
 |I think that we need a FreeBSD mpeg theme --- Perhaps a Daemon watching
 |Fxtv 8)

There's an idea!  It's got some PR possibilities too.  An eye catcher for
sure, and would promote FreeBSD in the process.  (Now we just got to find us
someone with good cartooning skills :-)


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.

What I'm a little concerned about is that for longer recordings, without
these attachments, the strea audio will likely slip out of sync with the
video during playback (due to a dropped frame or two per second [or more]
in the original recording).  Fxtv does try to buffer the video stream so
that it's 30fps, regardless of how many frames it had to skip during
capture, but if this process is off by, for example let's say, 1 frame per
second, after 30 seconds the audio is going to be out of sync with the
video by 1 second.  It's accuracy is better than that, but I know it's not
perfect.

If this is possible to address, I'd like to look at it for the next batch
of mods.

Really, its kind of a nutty process that goes on now because of the tools
that are available.  First fxtv writes a RAW capture file that has the
audio/video interleaved (so we know the audio/video relationship).  Then we
separate those out into separate audio and video streams (video padded up
to 30fps) so mpeg_encode can be used to build an MPEG video stream, and
mpeg_audio can be used to encode an MPEG audio stream.  Finally those
streams are merged together with mplex into a MPEG system stream, where we
hope the video still "lines-up" with the audio.

Randall



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