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Bug 3955 - New resampler: aliasing when downsampling
Summary: New resampler: aliasing when downsampling
Status: NEW
Alias: None
Product: SDL
Classification: Unclassified
Component: audio (show other bugs)
Version: 2.0.7
Hardware: x86 Other
: P2 normal
Assignee: Ryan C. Gordon
QA Contact: Sam Lantinga
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2017-11-11 18:44 UTC by Eric Wasylishen
Modified: 2017-11-11 18:44 UTC (History)
0 users

See Also:

Attachments
sine sweep test case (187.54 KB, audio/x-wav)
2017-11-11 18:44 UTC, Eric Wasylishen 		Details
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Description Eric Wasylishen 2017-11-11 18:44:49 UTC 
Created attachment 3084 [details]
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.