Lowpass filter.
Can be used before lossy encoding, if your encoder of choice doesn't have lowpass filter with arbitrary frequency.
[attachment=5844:foo_dsp_...ss_0.0.2.7z]
Sources (LGPL):
[attachment=5845:foo_dsp_....0.2_src.7z]
[attachment=6366:foo_dsp_...ss_0.0.4.zip]
[attachment=6367:foo_dsp_....0.4_src.7z]
Discussion topic (https://hydrogenaud.io/index.php/topic,125596.0.html)
Attached is a version with fixes to Default button and channel mask retaining. And added features are dark mode and 64-bit foobar2000 support.