[Speex-dev] FFT Resampler

[Alexander Chemeris]

On 5/29/08, Thorvald Natvig <thorvald at natvig.com> wrote:
> Alexander Chemeris wrote:
> > On 5/29/08, Thorvald Natvig <thorvald at natvig.com> wrote:
> > >  I've done listening tests when converting wb_male.wav to 44.1, 48 and 8khz,
> > > and there aren't any obvious artifacts. I also did a 16=>16 test, and the
> > > results are delayed by 10ms and within +/- 1 (basically, rounding errors
> > > from the FFT).
> > >
> > >
> >
> > Do you plan to use it in VoIP environment? If so, 10ms of additional
> latency
> > is a big cost for resampling. Do you think there are any ways to reduce
> it?
> >
> >
>  Yes, I plan to use it in a VoIP environment if I can get latency reduced to
> an acceptable level :)
>  The latency depends directly on the overlap parameter, which also controls
> the quality. Higher quality => higher latency. You could set the overlap to
> 0, but that would give you some nasty artifacts.
>  You can also resample with smaller block sizes. In the example I used 20ms
> blocks and 50% overlap. If you use 10ms blocks and 50% overlap, latency
> sinks to 5ms.

How quality and CPU usage depends from block size?
Looking at edge case - 1 sample blocks - I should have <1 sample latency,
but something should be  wrong here, e.g. required CPU power may be
infinite, no? :)

>  It could be 50% overlap is complete overkill. It could be it's not enough.
> It could be how much overlap you need depends on the block size. I need to
> do some quality testing before I can say for sure :)

*nod*
Also comparison with standard resampler would be very useful. Probably
the best way would be to take standard resampler as a reference
and see how much latency/CPU is needed to reach each of its complexity
levels. This would give clear idea of what's going, even for people
who unaware of implementation details.

> > >  +/** Get the latency in input samples introduced by the resampler.
> > >  + * @param st Resampler state
> > >  + * @return Latency in input samples.
> > >  + */
> > >  +int
> > >
> speex_block_resampler_get_input_latency(SpeexBlockResamplerState
> > > *st);
> > >  +
> > >  +/** Get the latency in output samples introduced by the resampler.
> > >  + * @param st Resampler state
> > >  + * @return Latency in output samples.
> > >  + */
> > >  +int
> > >
> speex_block_resampler_get_output_latency(SpeexBlockResamplerState
> > > *st);
> > >
> > >
> >
> > What is input and output latency? As a user, I think there is only one
> latency,
> > latency between data I passed to resampler and data I've got from it.
> > I suppose there may be some internal idea behind this division of latency,
> > but is end user interested in it?
> >
> >
>  It's copied directly from the speex_resampler.h; it's the same latency, but
> measured in input and output samples. So if I resample from a blocksize of
> 320 to 960 with 50% overlap, the input latency is 160 samples and the output
> latency is 480 samples.

Aha, got it. Documentation is very unclear at this point. =\ It should be
something like what you've written, e.g. "Latency, measured in samples
at input (output) sample rate".

-- 
Regards,
Alexander Chemeris.

SIPez LLC.
SIP VoIP, IM and Presence Consulting
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