[pulseaudio-discuss] [PATCH 0/4] Add support for libsoxr resampler

[Andrey Semashev]

On Wed, Nov 12, 2014 at 12:26 PM, Alexander E. Patrakov
<patrakov at gmail.com> wrote:
>
> We generally don't need a zoo of resamplers. But you have definitely changed
> something important from an earlier submission by Peter Meerwald so that the
> CPU figure became much better. I guess, that's SOXR_LINEAR_PHASE - that's
> the only obvious change.

SOXR_LINEAR_PHASE is 0, I added it mostly for documentation purpose,
in case if someone wants to add more modes with different phase
response presets. There is also another difference in that I
explicitly specify interleaved sample formats, although it is
numerically equivalent to those without the _I suffix.

I think, the essential difference is quality. Peter's patches use
SOXR_QQ, while mine use other presets. I didn't add QQ because I don't
see much point in this mode.

I also noticed unusual relation between quality and CPU consumption.
You can see that in my results LQ actually appears the most consuming
in some cases. I don't have a good explanation for this, my guess is
that this could be related to different math precision and
optimization. I think the library uses different precision for math on
different quality levels (i.e. double in VHQ, float in HQ, etc.), and
it does have some optimizations for SSE. It's possible that LQ path is
not as optimized. One would probably have to inspect libsoxr code to
tell for sure.

> Here are the facts, applicable to 44100 -> 48000 Hz resampling, as required
> by some sound cards when playing back CD rips:
>
>  * Speex-float-5 never introduces audible distortions, even on
> specifically-crafted testcases at insane volume in an otherwise absolutely
> quiet room. So "even better quality" never makes sense - unless we are
> talking about non-human listeners.
>  * Speex-float-1 (the current default) does not introduce audible
> distortions on music that you can buy in shops (or download), but does
> create audible artifacts on specially prepared test cases.
>  * SOXR, even at its LQ setting (which, according to prior tests, is good
> enough) and with THIS set of patches, is indeed slightly faster than
> speex-float-1.
>
> Statements about distortion audibility are based on the model described in
> this scientific paper:
>
> http://www.mp3-tech.org/programmer/docs/6_Heusdens.pdf
>
> Distortions caused by removal of high frequencies, as well as any other
> distortions, would have been counted as audible if a person could detect
> that in an ABX test. The nuance here (with "auditory masking" as its
> scientific name) is that the ear becomes less sensitive to very high
> frequencies when there is something else in the air - and in typical music,
> this "something else" definitely exists in a sufficient quantity.

Thanks for the reference and all the information.

I didn't do a blind test to detect the audible difference, all I can
say is my own impressions. I have a locally patched PulseAudio 4.0 and
I'm using soxr-vhq preset resampling everything to 96kHz (I have
records in 96kHz that I don't want to resample). The difference with
speex-float-5, which I used previously, is indeed rather subtle. I'd
say, the sound is a bit more clear, instrument separation is better,
especially in bass. But that's all subjective, of course, so it should
be taken with a big grain of salt. In any case, soxr-vhq is clearly
faster than speex-float-5, so at the very least it's a performance
improvement for me.

> I will recheck the quality separately later today, in order to verify that
> it is still as good as in the previous tests. Please don't merge the patches
> until this is done.

Thanks, I'd be interested in the results.