I have an old digital synthesizer that has a Butterworth-like 2nd order filter with resonance. However, with high frequency tones it appears there is no steep cutoff near Nyquist (I am sure it is the Nyquist as the tones fold beyond that point), as is usual with the bilinear-transformed Butterworth filter. This poses few questions.
- Is the attenuation caused by aliasing or something different, and what is it called?
- Are there digital alternatives to the Butterworth or bilinear transform that do not suffer from the attenuation near Nyquist, or is this a usual feature?
- What filter could an old resource-strained synthesizer have used to achieve a response without the attenuation?
Clarification: By Butterworth-like response it is meant that the response appears similar at lower frequencies, I have not confirmed that there is no ripple or anything like that. Nonetheless, a 2nd order filter of some sort.