Voltage controlled filter: Difference between revisions

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== Filter architectures (sorry this is crudely laid out, 1st step was capture info, 2nd pass will organize) ==
== Filter architectures (sorry this is crudely laid out, 1st step was capture info, 2nd pass will organize) ==
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Consider a description of a filter as a sort of "taxonomy" with three layers:
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This section began from an excellent thread on <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>
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* '''Top Layer''': the filter spec, number of poles, response
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* '''Middle Layer''': the topology that implements that filter function
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* '''Bottom Layer''': implementation details, including the control element
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So for example a Moog Ladder would be:
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* '''Top Layer''': 4 pole, low-pass, with resonance
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* '''Middle Layer''': 4 single-pole low-pass sections in series, with feedback
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* '''Bottom Layer''': the ladder circuit
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And a state variable filter would be:
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* '''Top Layer''': 2 pole, multi-mode
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* '''Middle Layer''': 2 integrators and an inverter, in a loop
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* '''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 ===

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The [[Moog ladder filter]] is like that Zen Koan that all students of the
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Tillman: 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
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And a State Variable filter would be:
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Top Layer: 2 pole, multi-mode
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Second Layer: 2 integrators and an inverter, in a loop
Bottom Layer: the circuit, perhaps OTAs<ref name="aamf"/>



Moog Ladder

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The Moog Ladder filter is like that Zen Koan that all students of the
synthesizer electronics temple meditate upon.
synthesizer electronics temple meditate upon.
"Oren Leavit"
"Oren Leavit"<ref name="aamf"/>


<nowiki>https://www.allaboutcircuits.com/technical-articles/analyzing-the-moog-filter/</nowiki>
<nowiki>https://www.allaboutcircuits.com/technical-articles/analyzing-the-moog-filter/</nowiki>
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<nowiki>http://www.timstinchcombe.co.uk/synth/Moog_ladder_tf.pdf</nowiki>
<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
ladder filter variations: http://www.till.com/blog/archives/2005/03/ladder_filter_v.html


=== Roland MS20 ===


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=== Ian Fritz threeler ===
MS20,

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Ian Fritz Threeler,

Mutant Vactrol Filter ...



=== Mutant vactrol ===


3-pole, 4-pole, etc - without cascading 2-pole SVFs.
3-pole, 4-pole, etc - without cascading 2-pole SVFs.
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details, you might also like the way I'm using different linearizing-diode
details, you might also like the way I'm using different linearizing-diode
currents to set the fixed proportion between the different integrator time
currents to set the fixed proportion between the different integrator time
constants.
constants.<ref name="aamf"/>


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Steiner Parker,


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=== Steiner Parker ===
The Steiner Parker is a rare exception. It's a classic Sallen-Key filter hacked up with biased diodes as controlled resistors.
The Steiner Parker is a rare exception. It's a classic Sallen-Key filter hacked up with biased diodes as controlled resistors.


=== Wasp ===

Wasp filter.

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.
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 ===
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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 ===
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EMS diode ladder The EMS diode ladder is the same as the Roland diode ladder, and they're both Moog Ladder knockoffs, knocked off sufficiently to get around the patent.
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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.

Arp 2600 filters The original ARP2600 filter is a clone of the Moog Ladder.

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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?
programmable op amp filters based on the lm4250?
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https://modularsynthesis.com/kuzmin/polivoks/polivoks_vcf.htm Okay, that's a State Variable Filter with programmable op amps for the integrators.
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.
And a programmable op amp is basically an OTA, with an integration cap, and an output stage.



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


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The Colin/ARP SVF puts the integrating cap in the integrator following the OTA.

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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,
The Rossum/Oberheim SVF puts the integrating cap at the output of the OTA,
shunted to ground.
shunted to ground.



Then there's the ARP 4075 VCF based around the LM3900 Norton amplifier.
Then there's the ARP 4075 VCF based around the LM3900 Norton amplifier.
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patent (US 4,011,466) on the 4075 and it is a fascinating read. That's a
patent (US 4,011,466) on the 4075 and it is a fascinating read. That's a
unique topology.
unique topology.



One thing the article fails to touch on is the feedback architecture.
One thing the article fails to touch on is the feedback architecture.
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