Hi, > > These patches add support for libsoxr resampler library to PulseAudio. The > resampler is exposed in four selectable quality presets: lq, mq, hq and vhq > (in the order of increasing quality). The patches update the code, build > system and docs. > > > > libsoxr[1] is a resampler library extracted from the SoX utility[2]. It > offers very good output quality and is very fast. Quality comparisons are > available here: > > > > - http://src.infinitewave.ca/ - "SoX 14.4 High Quality" corresponds to hq > quality. "SoX 14.4 VHQ Linear Phase" is close to vhq, the passband for vhq is > slightly lower. > > - http://sox.sourceforge.net/SoX/Resampling - there are graphs for impulse > response comparison with libsamplerate available at the bottom of the page. > > - http://lastique.github.io/src_test/ - I ran some more tests to compare > with Speex resampler at different levels. Some performance measurments are > also present at the end. should have checked your evaluation before, impressive amount of work! so the claim is soxr looks good for these "odd" rate conversions: 44.1kHz -> 8000*N (for useful values of N) > > In short, libsoxr is almost always faster than speex, and introduces much > less distortions. Its passband frequency is slightly lower than speex though, > and it can add a delay up to 20 ms in some cases. > > 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. > > 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. > > Statement about speed is based on a direct CPU usage measurement by "top > -d 10" on this CPU (forced to run at 933 MHz): > > Intel(R) Core(TM) i3 CPU M 370 @ 2.40GHz > > Baseline (48 kHz playback without resampling): 0.7% (0.1% on null sink) > Speex-float-5 (44100 -> 48000 Hz): 5.6% (4.5% on null sink) > Speex-float-3 (44100 -> 48000 Hz): 4.3% (3.1% on null sink) > Speex-float-1 (44100 -> 48000 Hz): 2.5% (1.9% on null sink) > Soxr-hq (44100 -> 48000 Hz): 2.4-3.3% (1.6% on null sink) > Soxr-mq (44100 -> 48000 Hz): 2.5-3.1% (1.5% on null sink) > Soxr-lq (44100 -> 48000 Hz): 2.1-3.4% (1.7% on null sink) > > One obvious thing here is that, while Speex-based resamplers yield > stable performance on a real hardware sound card, Soxr CPU usage > oscillates rather significantly. This effect does not exist on a null > sink. Also, higher quality does not mean higher CPU usage, which is strange. > > 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. > > -- Peter Meerwald +43-664-2444418 (mobile)
