[sdiy] what factors can be changed to give a LP VCF variable Q

[Aaron Lanterman]

> Help i am doing a project on analogue synthesizers I understand what Q is and 
> how to derive it from the cut of frequency but do not understand how variable 
> Q filters are designed. Any direction to books, web pages or a full answer 
> would be ACE.

Hi Colin - the cutoff frequency and Q are really separate things. There's 
a specific definitio for two-pole filters, but for other poles, the 
definition of Q varies.

See my lectures at users.ece.gatech.edu/~lanterma/ece4803 - look at the 
VCF lectures on Sallen-Key and State variable.

Basically, both kinds of filters have feedback that you need to make 
voltage controllable. With a SK, Q increases with feedback; with a state 
variable, Q decreases with feedback.

- Aaron

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Dr. Aaron Lanterman, Asst. Prof.
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