[Speex-dev] Increasing the speed of speex playback

[Jean-Marc Valin]

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
>