Implement WaveShaper node

[jdm]

Spec: https://webaudio.github.io/web-audio-api/#waveshapernode

Reference processing code: https://searchfox.org/mozilla-central/source/dom/media/webaudio/WaveShaperNode.cpp

[jdm]

For ease of implementation, don't both supporting oversampling at first.

[khodzha]

I'd like to take a shot at oversampling, but how should it be implemented? Is adapting speex resampler code ok? Or maybe its better to use some existing lib?

[jdm]

Adapting the speex code seems like a reasonable choice.

[khodzha]

Ok, thanks, I'm not sure in my ability but I'll try.

[jdm]

Yeah, https://github.com/xiph/speexdsp/blob/master/libspeexdsp/resample.c looks pretty complicated :( It might be worth trying out https://github.com/suhr/resample/blob/master/src/lib.rs?

[khodzha]

@jdm i arrived at some 2x upsampling implementation but im not sure how to structure Block with 2x buffer len.

Right now block's buffer is hardcoded to store channels*FRAMES_PER_BLOCK_USIZE (so channels*128) elements.
I suppose I could store Tick(256) inside an upsampled block; but since this comment says Tick is time taken for a single frame using Tick(256) looks wrong to me, since this is just 2x more samples per same Tick(128).

So idk how to proceed here, should I store something like frequency multiplier in a block?

[Manishearth]

@khodzha don't, use 2x or 4x the blocks before wave shaping. Firefox does something similar. WaveShaper is supposed to downsample after upsampling so it's fine if a block is temporarily representing only half or quarter of a frame.

[khodzha]

To be honest I'm kind of stuck wondering if my oversampling is useful at all, probably I'm missing something? I dont have any experience with digital signal processing

Right now my 2x oversampling implementation upsamples like in https://github.com/suhr/resample/blob/master/src/lib.rs using method described here: https://ccrma.stanford.edu/~jos/resample/
(basically every even output sample is a copy of corresponding input sample; every odd sample is linear combination of input samples weighted with kaiser-windowed sincs)

So I upsample with this process, apply waveshaping curve and then downsample by simply throwing out every odd sample.

The problem is since both upsampling and downsampling leave every even sample as it is this whole process outputs same numbers whether there is oversampling or not.

[Manishearth]

Huh. This is true, the spec slightly mentions that filters can be applied but it's frustratingly vague.

Perhaps file a spec bug? And/or look at the Firefox code.

[khodzha]

I want to try out oversampling with libspeex-dsp first

[khodzha]

Using https://github.com/rust-av/speexdsp-rs I get noise with 2x oversampling (and even more with 4x) both with quality=0 (used in firefox) and quality=5 (and right-padding a block with 0f32 since quality=5 requires more than 128 insamples to produce 256 outsamples) 😞

[khodzha]

@Manishearth should we close this?