[Speex-dev] Increasing the speed of speex playback

Steve Underwood steveu at coppice.org
Tue Oct 19 18:37:06 PDT 2010

  Hi Bill,

Any attempt to alter speed by simple insert or dropping produces poor 
results. Even if you can get it to sound smooth, the resulting pitch 
shift is horrible. You really need to use a transform that alters speed 
smoothly, while maintaining the original pitch of the voice. If you look 
in my spandsp library you will find a module which does exactly this, 
using an algorithm called PICOLA. You can speed up or slow down a voice 
in fine speed steps using this module, and the resulting voice is almost 
the same quality as the original. There is a test program for it, which 
should function as an example of how you need to call the library to 
initialise and use it.


On 10/20/2010 05:21 AM, Bill Cox wrote:
> Here's one clue about whatever is causing the low quality speech.
> Speech sounds terrible at 1.01X faster, and it sounds excellent at
> normal speed (1.0X).  So, the main problem is something that breaks
> with any change in frame size in the decoder.  Any idea what that
> might be?
> Thanks,
> Bill
> On Tue, Oct 19, 2010 at 5:14 PM, Bill  Cox<waywardgeek at gmail.com>  wrote:
>> 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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