[sdiy] filter resonance

[Magnus Danielson]

From: <jays at aracnet.com>
Subject: Re: [sdiy] filter resonance
Date: Thu, 11 Nov 2004 08:59:56 PST
Message-ID: <200411111659.iABGxudD003788 at jade.spiritone.com>

> John,
> 
> Yes and no.
> 
> It really depends on the type of filter. Some filters decrease res with more feedback and some increase the res with more. But in general yes putting a VCA in the feedback loop can control the res. Another issue is what type of scale do you want log or linear? Some filters give better control with log scaling on the res some linear.

To be clear. Traditional 4-pole filters (Moog-ladder etc) have increased Q with
feedback of the *correct* sign. That is, negative feedback is not what you
want but positive feedback is what you are after, unless there is another
sign-change in there lurking. You *quickly* find out. Increasing feedback gives
you increased Q. Linear control gives you pretty much what you want. Antilog is
perferred.

For 2-pole state-variables it works quite the opposite. The feedback goes from
a middle tap and the feedback dampens the Q. The amount is working in
reciprocal as I recall it (too many years ago I did the math). Check the ASM-1
VCF and naturally the Oberheim SEM as you see fit.

> Has anyone put together anything saying what type of feedback and scaling works best with different filters?

Probably.

Cheers,
Magnus