[Speex-dev] Increasing the speed of speex playback

Stuart O Anderson stuart at whispersys.com
Tue Oct 19 14:54:39 PDT 2010

Along those lines - what you want it to find a high quality time-scale shift
algorithm.  If you're batch processing samples offlines, I believe Audacity
can do this base on a STFT that includes phase information.  For fast online
shifting, PICOLA, as provided by the SpanDSP library, might work well for
you, although I haven't tried it at an 8x speedup.


2010/10/19 Jean-Marc Valin <Jean-Marc.Valin at usherbrooke.ca>

> You're asking the wrong question. The question is not "why does it
> would bad with Speex?", but "why does it sound good with LPC10 and
> MELP?". And the answer is that both are vocoders. Try dropping
> frames/subframes with anything else (Vorbis, MP3, G.729, u-law, ...)
> and it'll sound terrible as well. The only reason it sounds good with
> vocoders is because the codec parameters are in fact synthesizer
> parameters that don't have a direct connection with the signal.
>    Jean-Marc
> Bill Cox <waywardgeek at gmail.com> a écrit :
> > I was able to easily hack in an option to play back at different
> > speeds.  For example, using "speexdec --speed 2.0 file.enc file.wav"
> > plays back encoded file.enc at 2X speed.  What I did was divide
> > st->frameSize and st->subFrameSize by the speedup, and added a
> > SPEEX_SET_SPEED decoder control for the nb_celp decoder.  This
> > produced speech that was 2X faster than the original.
> >
> > However, the quality is very poor.  This is where it gets harder for
> > me, as the quality is impacted by so many parts of the code.  Can
> > anyone guess which part of the decoder is leading to such poor quality
> > when I cut the frame size in half?  This hack works very well in
> > LPC10, and fairly well in MELPe.
> >
> > I've attached two outputs from speex: the decoded playback at normal
> > speed, and the 2X speed version.
> >
> > Thanks,
> > Bill
> >
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