Oversampling – DSP and Plugin Development Forum

I am currently developing a saturation plugin and thought, it would be a good idea to use oversampling, so I am using the HIIR library with 4x oversampling. But when I create a spectrum plot with Audacity, I see no difference between the 1x and the 4x output.

1x (DAW at 44,1 KHz)

hsat 1x.jpg

4x (DAW at 44,1 KHz)

hsat 4x.jpg

Interesting enough, I thought the peak around 20 KHz is aliasing, but when I turn of Dithering in audio export of Bitwig, the peak is gone for 1x as well as 4x.

I then tried my 1x plugin, setting the DAW to 96 KHz:

hsat 1x 96KHz.jpg

This gives a much smoother curve. I would have expected the same curve for 4x oversampling. Am I wrong, or is there a problem with the HIIR library?