[sdiy] Article "Analyzing the Moog Filter"
don at till.com
Fri Aug 23 08:05:06 CEST 2019
On Aug 22, 2019, at 6:07 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
> I don’t like to disagree with you, Don, but I’m not sure what you’re thinking. Moog or SVF are definitely *not* the only two filter options.
> OTA+cap-to-ground+buffer? VCA+Integrators?
Consider a description of a filter as a sort of "taxonomy" with three layers:
Top Layer: the filter spec, number of poles, response
Second Layer: the topology that implements that filter function
Bottom Layer: implementation details, including the control element
So a Moog Ladder would be:
Top Layer: 4 pole, low-pass, with resonance
Second Layer: 4 single-pole low-pass sections in series, with feedback
Bottom Layer: the ladder circuit
And a State Variable filter would be:
Top Layer: 2 pole, multi-mode
Second Layer: 2 integrators and an inverter, in a loop
Bottom Layer: the circuit, perhaps OTAs
And so forth.
This analysis also works really well with oscillators and other functions.
Here's a Moog style VCO:
Top Layer: VCO with sine, square, triangle, sawtooth waves
Middle Layer: block diagram with exponential current source, sawtooth core, waveshapers
Bottom Layer: the circuit details
So if I dismiss the implementation details, as defined this way, it limits the number of unique filter designs.
You know I'm a big fan of implementation details. And you'd want to make sure that the implementation details didn't have a significant functional effect as you draw these lines. That's part of the craft.
But if I'm characterizing filter types, I think it's reasonable to pay attention to the implementation topology and ignore the implementation details.
Donald Tillman, Palo Alto, California
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