[Speex-dev] Increasing the speed of speex playback

[Jean-Marc Valin]

What you are trying to achieve is just *not* applicable to Speex or any
codec I know that has a bit-rate above 4 kb/s. That's just not how it
works. No amount of changes to Speex is going to help. You need to look
elsewhere.

	Jean-Marc

On 10-10-19 11:11 PM, Bill Cox wrote:
> Hi, Jean-Marc, and thanks for the quick reply.  Let me just say I'm a
> huge fan of speex, and the work you've done.  I actually barely
> understand what I'm reading so far in the source code and
> documentation, just enough to understand just how cool the algorithms
> are.
> 
> LPC10 and MELP allow me to speed up speech with a simple hack on the
> decoder frame size.  Playing fewer samples per frame speeds up the
> speech, without effecting the excitation.  It works well, but not as
> well as I would like.  I've attached a sample of a female voice sped
> up with MELPe.  I fully understand basic LPC10.  Simply reducing the
> frame size in the decoder is exactly the right way to speed up LPC10
> speech without changing the pitch. I would like to figure out how to
> apply some of the innovations in CELP to sped up speech.  Frankly,
> this is the limit of my current knowledge, and I am clueless as to how
> to apply CELP concepts to high speed playback.
> 
> Bill
> 
> 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
>>>
>>
>>
>>
>>