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== Draft revision ==
Move this into the article once it's tidy enough.
An audio filter is a frequency dependent amplifier circuit, working in the audio frequency range, 0 Hz to beyond 20 kHz. Audio filters can amplify (boost), pass or attenuate (cut) some frequency ranges.
== Donald Tillman's filter taxonomy ==
== Filter architectures (sorry this is crudely laid out, 1st step was capture info, 2nd pass will organize) == ▼Common filter types used in audio synthesis include:
* Low-pass (high-cut) filter: removes higher frequencies
* High-pass (low-cut) filter: removes lower frequencies
* Band-pass filter: removes frequencies outside a given band
* Notch filter: removes frequencies within a given band
* Shelf filters: raise or lower frequencies above or below a cut off point.
* Peak filters: Raise frequencies within a given band
* Formant filters: raise multiple peaks, often in such a way that they mimic the human voice.
There are many ways to implement most filters, and each has its own specific audio characteristics or flavour. Some common features across most filter types include:
* A cut-off frequency - the frequency at which the filter begins to remove frequencies. This is often [[voltage control|voltage-controllable]], and can changes over time (e.g. controlled by an [[Envelope generator|envelope]], or an [[Low frequency oscillator|LFO]]).
* resonance - how much the filter boosts the frequency at the cut-off point. This may also be voltage-controllable.
* frequencies beyond the cut-off often have their [[phase]] affected.
<br />
▲== Filter architectures (sorry this is crudely laid out, 1st step was capture info, 2nd pass will organize) ==
Consider a description of a filter as a sort of "taxonomy" with three layers:
* '''Top Layer''': the filter spec, number of poles, response
* '''Top Layer''': 2 pole, multi-mode
* '''Middle Layer''': 2 integrators and an inverter, in a loop
* '''Bottom Layer''': the circuit, perhaps OTAs<ref name="aamf">[https://synth-diy.org/pipermail/synth-diy/2019-August/171529171536.html Article "Analyzing the Moog Filter"] reply by Donald Tillman, Synth-diy mailing list, 23:05, 22 August 2019</ref>
=== Moog ladder ===
The [[Moog ladder filter]] is like that Zen Koan that all students of the
synthesizer electronics temple meditate upon.
"Oren Leavit"<ref name="aamf">[https://synth-diy.org/pipermail/synth-diy/2019-August/171529.html Article "Analyzing the Moog Filter"], Synth-diy mailing list, August 2019</ref>
"Oren Leavit"<ref name="aamf"/>
<nowiki>https://www.allaboutcircuits.com/technical-articles/analyzing-the-moog-filter/</nowiki>
Saitama, JAPAN.
== References ==
{{reflist}}
== External links ==
* [https://sound-au.com/articles/active-filters.htm Active Filters - Characteristics, Topologies and Examples] by Rod Elliott (ESP)
[[Category:Synthesizer components]]
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