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Multiple Identity Filter

Courtesy/Thanks to: Craig Anderton

Description: HERE IS A FILTER STRUCTURE that uses only three ICs, has no commercially available equivalent, is only slightly more costly than standard filter modules that do a lot less, and is amazingly versatile. How versatile? Here are some of the responses obtainable with this module:
Lowpass: Choice of 24 18 12 or 6dB/octave slope; includes voltage-controlled (VC) resonance option in 24dB/octave mode.
Highpass: Choice of 24, 18, 12, or 6dB/octave slope; includes VC resonance option in 24dB/octave mode.
Phase shifter (allpass): Single or dual notch response. VC resonance option in dual notch mode.
Maximum rolloff lowpass: Similar to 24dB/octave lowpass, but with faster rolloff. A typical application would be getting rid of clock feedthrough from analog delay line outputs.
Notch filter: Choice of broad or sharp notch response.
Bandpass: Choice of sharp or broad bandpass response.
Lowpass plus notch: Fig. 1 shows the response curve for this type of filter.
Highpass plus notch: Another new type of response. See Fig. 2.

Since this module can assume the identity of many other filter structures, the term “Multiple Identity Filter” (MIF for short) seems very appropriate. But there's still more; it can act as a stand-alone signal phaser, and can put different filter blocks in parallel for highly unusual responses (for example, you could have a I2dB/octave lowpass filter in parallel with a single notch phase shifter, or you could parallel four phase shift sections). There's even a straight channel option with switchable polarity (more on this later). And it's also a sine wave oscillator.... Sound incredible? Well, I still haven't discovered all the possible sounds obtainable from this module, so I don't think you'll get tired of it too easily. You do pay a price for this versatility, the time required to learn how to use the device to best advantage. However, you can always treat the thing like a standard lowpass filter, and learn about the many extras later on at your own pace.


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   Article from "CONTEMPORARY KEYBOARD/NOVEMBER 1979" explaining the MIF and how to build it.
Parts used
   Voltage Controlled Filter
   Operational Amplifier
   Operational Amplifier
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