New resampler: aliasing when downsampling

[SDLBugzilla]

This bug report was migrated from our old Bugzilla tracker.

These attachments are available in the static archive:

• sine sweep test case (sweep96k.wav, audio/x-wav, 2017-11-11 18:44:49 +0000, 192044 bytes)

Reported in version: 2.0.7
Reported for operating system, platform: Other, x86

Comments on the original bug report:

On 2017-11-11 18:44:49 +0000, Eric Wasylishen wrote:

Created attachment 3084
sine sweep test case

The issue is SDL_ResampleAudio currently lowpass filters at 0.5*inrate, even in the case when outrate < inrate. In other words, the part of the "band-limited resampling" paper that says - "if rho < 1... the step-size through the filter table is reduced.." is not implemented.

Here is a test case with a 0-48kHz sine sweep sampled at 96kHz.

testresample sweep96k.wav sweep44100.wav 44100 1
sox sweep44100.wav -n spectrogram -o sweep44100.png
open sweep44100.png

(careful with the volume if you play these!)

It should be resampled as a single upward sweep that stops at the 22kHz nyquist limit with silence after that, but currently you get a siren effect due to the aliasing.
I have a patch in the works for this.

[0x1F9F1]

I could have a go at implementing this if it would be useful, though I'm not sure any improvements would outweigh the increased complexity.
It only applies to downsampling (dst_rate < src_rate), which (ignoring pitch shifting) I don't think SDL does much beyond something like 48000 -> 44100 (where I imagine the difference would be minimal).

[icculus]

Using the test case in the original bug report...

SDL2's resampler:

SDL3's resampler:

I think I'm okay with this outcome...?