[vlc-devel] Re: vlc skipping frames - looks very vlc related

[Gildas Bazin]

On Friday 26 November 2004 15:00, Marius Kjeldahl wrote:
> I'm having trouble with vlc skipping frames for certain streams. This is 
> probably related to either the amount of data in the stream and/or 
> format of the stream. For reference, I've captured a stream that 
> demonstrate the problem and tested it on at least one other player.
> 
> When I run the captured stream in vlc, there are noticeable jumps which 
> is probably the result of vlc skipping frames (the message window 
> messages about frames skipped seems to coincide nicely with the jumps in 
> the video).
> 
> When I run the same stream on the same hardware using mplayer, I get no 
> messages about skipped frames and the visual defects do not seem to 
> appear either.
> 
> The stream in question seems to be variable rate with a very high peak 
> bit rate, but the problem seem to appear on both the very different 
> machines I've tried it on (Windows, one AMD64 3200+ and one Mobile 
> Pentium 4 1.8MHz). On the same hardware, mplayer has no problems. I also 
> ran the same stream under vlc on linux (version 0.8), with no visible 
> improvements (still skipping, although in different places).
> 
> AFAIK, mplayer uses libmpeg2 (that's what is says when playing the 
> stream), and I've tried forcing vlc to use it also, but I can see no 
> improvement in the vlc output, so I'm guessing the mpeg decoding is not 
> to blame (ignoring the issue of possible different versions of libmpeg2 
> for now).
> 
> I've put up a copy of the stream on:
> 
> http://kjeldahl.net/vlcstuff/
> 
> if anybody would like to take a look at it. Please let me know if you 
> figure out anything interesting.
> 

I had a look at the stream and the issues get solved by increasing the 
caching in VLC (--file-caching 600). This usually indicated a problem with 
the PCR timestamps in the stream. Mplayer doesn't use PCRs so won't be 
affected but the demuxing layer of VLC uses them to know when to send the 
raw data to the codecs so it relies on them being valid (this is for 
instance the information that allows VLC to use a caching option in 
miliseconds. Most players only allow to specify a caching size in bytes but 
thanks to the information carried with the PCRs, VLC can be cleverer than 
this).

--
Gildas

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