[sdiy] HPFs and ADSRs

[Aaron Lanterman]

On Jun 6, 2010, at 4:52 AM, Antti Huovilainen wrote:

>> That second version can be understood that way - it gives insight into how it works - but it does look like a "Bach" version of a Sallen-Key (where there are two buffers instead of one) to me. At least that's what I've been calling it in lecture, and giving homework assignments on... :)
> 
> Yes, it is the same as the "Bach" filter (for one particular resonance setting). However, nowhere on that page does it say anything about the "Bach" being a version of Sallen-Key. I don't consider it one either as the design equations are different, and the filter even works differently (as Sallen-Key is not two identical RC sections in series).

Fair enough. 

I wondered if the Korg folks knew about this "Bach" filter... or if they looked at a lowpass Sallen-Key, wondered what happeded if you replaced the resistors with OTAs, and then worked out the frequency response. I've only seen this "Bach" thing a few papers and I think one textbook. 

I've also seen it called something entirely different - named after a person - but I can't remember where I saw that or what the name was.

- Aaron