[vlc-devel] [PATCH] mpeg4 packetizer
aj at lucid-cake.net
Wed Mar 9 23:54:59 CET 2005
I'm the developer of CyTV
("http://www.lucid-cake.net/cytv/index_en.html"), a streaming solution
for certain TV tuners on OS X. It's based on VLC for playback, and I'm
very greatful for this terrific player!
During the last few weeks, I wrestled with MPEG-4 streams produced by
Plextor's ConvertX unit in combination with elgato's EyeTV software for
OS X. These streams consist of MPEG-4 video embedded in an MPEG-2
transport stream. A sample is available here:
The first problem was that there are no PCRs in the stream. I've
already encountered this in the past, and I've solved it by generating
PCRs based on the streams' audio timestamps on the fly to make them
playable. The sample stream mentioned above, but with these artificial
PCRs inserted, is available here:
It would be nice if VLC wasn't so dependent on PCRs. Are clock
differences between systems really so bad? I would assume (perhaps
naively) that playback would work just fine with an internal clock that
just keeps running (without synchronization with the stream's PCRs).
Even if there are differences in the millisecond range, they shouldn't
be noticeable (especially since it's only a question of playback speed,
not audio/video sync). I'd appreciate some comments about this.
After I generated PCRs artificially, the stream sort of worked. Audio
was fine, but video was stuttering badly (looking "scratched" with some
sequences shown too early and some too late). However, transcoding the
stream with ffmpeg worked fine, so it seemed like the stream was OK. A
few long debug sessions later, it turned out that there are problems in
modules/packetizer/mpeg4video.c - the timestamps are simply not handled
correctly (even in the latest svn version).
Attached is a patch (against the current trunk, freshly checked out)
that fixes playback of the ConvertX streams for me (at least after PCRs
have been inserted). It's basically a port of ffmpeg's mpeg4 timestamp
handling to VLC's mpeg4 packetizer. However, it's not quite ready since
I've completely ignored DTS for now. I'd appreciate any insight into
how the decoding timestamps are used by VLC. Simply setting them to the
same value as PTS seems to work just fine, so that's what I'm currently
doing. It would be great if someone could explain to me how these are
actually used, so I could come up with a proper patch.
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