[sdiy] Article "Analyzing the Moog Filter"

Donald Tillman 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.

  -- Don
Donald Tillman, Palo Alto, California

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