[FFmpeg-devel] soundtouch filter?

Pavel Koshevoy pkoshevoy at gmail.com
Sun Apr 29 22:56:56 CEST 2012

On 04/29/2012 09:50 AM, Reimar Döffinger wrote:
> Hello Pavel,
> it seems I can't send anything to the list at the moment, so direct
> answer instead...
> On Sun, Apr 29, 2012 at 08:36:02AM -0600, Pavel Koshevoy wrote:


>> SoundTouch is LGPL.  However, I am fine with GPL.  What existing
>> avfilter code is a good reference for me to use for scaletempo port?
> I don't really know, but af_aresample does the kind of pts fiddling you
> asked about and is not that large since most of the "heavy lifting" code is in
> libswresample.
> So that might be a good starting point.

OK, I had a quick look at scaletempo, it implements WSOLA.  I had a look at 
SoundTouch, it appears to implement SOLA.  I looked at WSOLA paper, it's not 
terribly enlightening.  This article  
http://www.surina.net/article/time-and-pitch-scaling.html by SoundTouch author 
is much easier to understand.  I am thinking I may not need to port this filter 
from mplayer, I'll try to implement it from scratch first, maybe use ffmpeg rdft 
functions for cross correlation calculation.

It also occurs to me that this filter needs to modify timestamps not only for 
audio but also for video and subtitles, otherwise they'll go out of sync.  Is 
this going to be a problem?

Thank you,

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