[sdiy] Article "Analyzing the Moog Filter"

Donald Tillman don at till.com
Fri Aug 23 19:20:04 CEST 2019

On Aug 22, 2019, at 6:07 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
> [...] And the State Variable makes a reasonable appearance by virtue of its versatility, but is more common in modular synths than in any classic keyboard.As we know, Oberheim was the only manufacturer who pushed 2-pole filters, and even then the later instruments offered that as an option, not the only choice.

The State Variable Filter was introduced by the brilliant Dennis Colin in the ARP 2500 VCF.  And his article is one of the classics:

    Dennis Colin
    The Electrical Design and Musical Applications of an Unconditionally Stable Combination Voltage Controlled Highpass, Bandpass, Lowpass, Band Reject Filter/Resonator
    Journal of the Audio Engineering Society, Dec 1971

He didn't invent the SVF, but he was the first to voltage control it and apply it to music.

The State Variable Filter has a long history, back to analog computers.  It's also the same mechanism as the simple harmonic motion of a mass, spring, and friction.  So it has a wonderful "universal" quality.

The Oberheim filter, the Rhodes Chroma filter, and others, are mostly variations on this.

  -- Don
Donald Tillman, Palo Alto, California

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