[vlc-devel] [PATCH] mpeg4 packetizer

[Andreas Junghans]

Hi list,

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: 
"http://www.lucid-cake.net/cytv/mpeg4sample_orig.ts".

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: 
"http://www.lucid-cake.net/cytv/mpeg4sample.ts".

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.

Regards,

   Andreas Junghans

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