[vlc-devel] Re: VLC audio output FIFO questions ...

John Michael Zorko j.zorko at att.net
Fri May 16 03:24:44 CEST 2003


> This is a known issue
> http://bugzilla.videolan.org/cgi-bin/bugzilla/show_bug.cgi?id=115

I think this is a completely different problem -- the issue i'm seeing 
is not specific to Mac OSX -- it happens on VLC 0.5.3 on RedHat Linux 9 
on Intel, and VLC 0.5.4 (built from CVS as of this past Monday on Mac 
OSX) also exhibits the problem.  Basically, a lot of seeking around 
using the rc interface (or using my ff / rewind commands, which 
basically seek like the rc seek command does i.e. call input_Seek()) 
causes the audio to play back faster or slower for several seconds (the 
pitch is altered), then return to normal, i.e. the audio 'wobbles.'  
Continued seeking makes the problem happen again (you have to seek a 
lot, and at least once every second or two -- which is exactly what my 
ff / rewind commands do as long as they're active).

I was looking at the audio output code (for OSS / ALSA / etc.) and I 
don't think it's in there, as it does the same thing on very different 
sound architectures (OSS and ALSA on RH9, CoreAudio on Mac OSX).  I 
don't think it's a resampling issue because I changed the max 
resampling percentage in vlc_config.h from 10 to 2, and the problem did 
not go away (besides, the wobble seems much greater than +/- 10%).  I 
can make the wobble far less pronounced by changing how often I seek 
when doing ff / rewind (i.e. calling input_Seek() less often), which 
leads me to believe it has something to do with a FIFO on the decoder / 
playback end that is time-based, much like the decoder discussion in 
the VLC API documentation describes.

So i've been looking for the piece of code that might be responsible 
for altering the pitch of the audio being played back, under whatever 
circumstances it does it in, and i've not found it yet.  Can someone 
illuminate me as to where it lies?



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