Noise Shaping

[phoboslab]

I've added some very simple noise shaping to the encoder (to the noise_shaping branch). This does not change the decoder or the data format. The noise shaping should help to move quantization noise into the higher, less audible frequencies.

Here's a comparison page with all samples with and without noise shaping: https://phoboslab.org/files/qoa-samples/noiseshaping.html

The difference for some sample is night & day. Listen to 32_triangles-triangle_roll_stereo at 00:43 or 35_glockenspiel_arpegio_melodious_phrase_stereo at 00:39.

However, this noise shaping has an adverse effect for some other samples. I tried to contain it by only applying most of the shaping when our prediction is "bad" anyway. But still, I feel that some samples sound more "crunchy" now. Listen to 21_trumpet_arpegio_melodious_phrase_stereo right at the beginning for instance. Vocals in julien_baker_sprained_ankle and others also seem to have lost a bit of "smoothness".

Maybe someone with better ears (and/or equipment :D) can take a listen? What's the usual strategy here, to adaptively correct for quantization noise?

[mattdesl]

Maybe something that could be optional or configurable? Some of the audio files I’m thinking of using QOA for have a lot of noise to begin with that I would like to keep more or less intact, almost treating it like data rather than audible signal. But perhaps I misunderstand what this shaping does.

I can’t hear the difference in your example page with my cheapo headphones but in Audacity comparing files with Invert it becomes more noticeable. Will need to do some more tests.

[phoboslab]

This should only affect the quantization noise that is added by the encoder; it won't remove any noise that is present in the source. But yes, making it optional is certainly the right idea!

[p0nce]

audio-formats has TDPF (courtesy of MIT-licensed Airwindows, tuned and modified by me to fit WAV) dithering in its QOA encoder now: https://github.com/AuburnSounds/audio-formats/blob/master/source/audioformats/qoa.d#L724
It was tuned for WAV. imo It's more important to get dithering levels right rather than get the best dithering. I will finetune the dither level for QOA encoding. (EDIT: errr, disabled for now, it sounds worse than without dithering)