Questions about VCF's in general

[Tommy Killander]

Hi there!

First of all; thanks for all mail regarding your own experiences with
TB-303 replicas, much appreciated I ensure you...


I have a couple of questions on how to implement a low-pass filter,
which is usable in a traditional analog synth.

1/   It seems like the most common ways of implementing a LP VCF is
to use either "custom" IC's like the Curtis chips, SSM chips from
Analog Devices or a discrete Filter Ladder (like the TB-303, Moog etc).

Are there any other ways of implementing a filter (using only op-amps or
switched capacitor filters ? I think that controllable parameters could
be restricted to only Cutoff Frequency and Resonance.

2/   What is Filter Resonance in technical terms ? It seems like a
oscillation at the filters cutoff frequency, but is it the same ripple
as found in a Chebychev filter near the cutoff frequencey, or is it
something else (like a induced oscillation caused by 180 degr. phase
shifted feedback) ?

3/   Does anybode have any experience in using Analog FPGAs ? They
are quite expensive and require a CPU for reprogramming, but they
could provide a generic filter building block, without the need to 
use a A/D -> DSP -> D/A signal chain.

Check out http://www.impweb.com/ for more info...

Kindest Regards
/Tommy K 
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