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* 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
* '''Middle Layer''': the topology that implements that filter function
* '''Bottom Layer''': implementation details, including the control element
So for example a Moog Ladder would be:
* '''Top Layer''': 4 pole, low-pass, with resonance
* '''Middle Layer''': 4 single-pole low-pass sections in series, with feedback
* '''Bottom Layer''': transistor cascade ladder
And a state variable filter would be:
* '''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/171529.html Article "Analyzing the Moog Filter"], Synth-diy mailing list, 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"/>
<nowiki>https://www.allaboutcircuits.com/technical-articles/analyzing-the-moog-filter/</nowiki>
<nowiki>https://www.allaboutcircuits.com/technical-articles/small-signal-open-loop-transfer-function-moog-filter/</nowiki>
Tim Stinchcombe's paper, "Analysis of the Moog Transistor Ladder and Derivative Filters," where he compares transistor and diode ladder filters.
<nowiki>http://www.timstinchcombe.co.uk/synth/Moog_ladder_tf.pdf</nowiki>
ladder filter variations: http://www.till.com/blog/archives/2005/03/ladder_filter_v.html
=== Roland MS20 ===
=== Ian Fritz threeler ===
=== Mutant vactrol ===
3-pole, 4-pole, etc - without cascading 2-pole SVFs.
Leapfrog topology as implemented by Matthew Skala. The original ARP2600 filter is a clone of the Moog Ladder.
The later ARP2600 filter has the same filter topology, but implemented inelegantly to get around the patent.
<nowiki>https://files.northcoastsynthesis.com/msk-007.pdf</nowiki> (page 69 of the PDF)
One of the important properties of this topology is that it in some sense
minimizes component dependence - which is important for keeping the shape
of the curve reasonably consistent when tuning it, given that I'm trying
to keep five OTAs tracking each other. If you like implementation
details, you might also like the way I'm using different linearizing-diode
currents to set the fixed proportion between the different integrator time
constants.<ref name="aamf"/>
=== Steiner Parker ===
The Steiner Parker is a rare exception. It's a classic Sallen-Key filter hacked up with biased diodes as controlled resistors.
=== Wasp ===
The Wasp filter is a State Variable with 4069 inverters replacing the inverting opamp in the integrators. Certainly the overdrive characteristics of the 4069 inverter are different than an opamp, but it's in a local feedback loop, and in a global feedback loop, and I think the OTA overdrive will predominate anyway.
=== EMS diode ladder ===
The [[EMS diode ladder filter]] is the same as the Roland diode ladder, and they're both Moog Ladder knockoffs, knocked off sufficiently to get around the patent.
=== Arp 2600 ===
The original [[ARP 2600 filter]] is a clone of the Moog Ladder. The later ARP2600 filter has the same filter topology, but implemented inelegantly to get around the patent.
programmable op amp filters based on the lm4250?
https://modularsynthesis.com/kuzmin/polivoks/polivoks_vcf.htm Okay, that's a State Variable Filter with programmable op amps for the integrators.
And a programmable op amp is basically an OTA, with an integration cap, and an output stage.
SVF
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
<nowiki>http://www.guitarfool.com/ARP2500/DennisCollinPaper.pdf</nowiki>
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.
The Colin/ARP SVF puts the integrating cap in the integrator following the OTA.
The Rossum/Oberheim SVF puts the integrating cap at the output of the OTA,
shunted to ground.
Then there's the ARP 4075 VCF based around the LM3900 Norton amplifier.
The first low distortion low noise high fidelity VCF. I just read the
patent (US 4,011,466) on the 4075 and it is a fascinating read. That's a
unique topology.
One thing the article fails to touch on is the feedback architecture.
Feedback has a big impact on the sound, and the feedback design changed
between Moog synth models. That is why players back then complained that
the new models didn't sound like the Minimoog. My post in SDIY way back in
1997 here:
<nowiki>http://search.retrosynth.com/synth-diy/search/lookit.cgi?-v9710.272</nowiki>
Alisa 1377 and 1387 have Sallen-Key ladder filters.
<nowiki>http://www.ruskeys.net/eng/base/alisa1377.php</nowiki>
<nowiki>http://www.ruskeys.net/eng/base/alisa1387.php</nowiki>
I wrote blog articles on them. Although they are in Japanese, they may help you.
<nowiki>https://houshu.at.webry.info/200704/article_1.html</nowiki>
<nowiki>https://houshu.at.webry.info/201101/article_3.html</nowiki>
Osamu HOSHUYAMA
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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