Voltage controlled filter: Difference between revisions
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(Captured many links from a synth-diy mailing list thread about VCF architectures) |
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* frequencies beyond the cut-off often have their [[phase]] affected. |
* frequencies beyond the cut-off often have their [[phase]] affected. |
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== Filter architectures (sorry this is crudely laid out, 1st step was capture info, 2nd pass will organize) == |
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This section began from an excellent thread on the synth-diy mailing list in August 2019: https://synth-diy.org/pipermail/synth-diy/2019-August/171529.html |
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Tillman: Consider a description of a filter as a sort of "taxonomy" with three layers: |
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Top Layer: the filter spec, number of poles, response |
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Second 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 a Moog Ladder would be: |
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Top Layer: 4 pole, low-pass, with resonance |
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Second 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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Second Layer: 2 integrators and an inverter, in a loop |
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Bottom Layer: the circuit, perhaps OTAs |
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Moog Ladder |
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The Moog Ladder filter is like that Zen Koan that all students of the |
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synthesizer electronics temple meditate upon. |
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"Oren Leavit" |
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<nowiki>https://www.allaboutcircuits.com/technical-articles/analyzing-the-moog-filter/</nowiki> |
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<nowiki>https://www.allaboutcircuits.com/technical-articles/small-signal-open-loop-transfer-function-moog-filter/</nowiki> |
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ladder filter variations: http://www.till.com/blog/archives/2005/03/ladder_filter_v.html |
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MS20, |
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Ian Fritz Threeler, |
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Mutant Vactrol Filter ... |
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3-pole, 4-pole, etc - without cascading 2-pole SVFs. |
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Leapfrog topology as implemented by Matthew Skala. 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. |
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<nowiki>https://files.northcoastsynthesis.com/msk-007.pdf</nowiki> (page 69 of the PDF) |
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One of the important properties of this topology is that it in some sense |
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minimizes component dependence - which is important for keeping the shape |
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of the curve reasonably consistent when tuning it, given that I'm trying |
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to keep five OTAs tracking each other. If you like implementation |
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details, you might also like the way I'm using different linearizing-diode |
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currents to set the fixed proportion between the different integrator time |
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constants. |
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Steiner Parker, |
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The Steiner Parker is a rare exception. It's a classic Sallen-Key filter hacked up with biased diodes as controlled resistors. |
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Wasp filter. |
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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. |
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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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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. |
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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. |
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And a programmable op amp is basically an OTA, with an integration cap, and an output stage. |
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SVF |
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The State Variable Filter was introduced by the brilliant Dennis Colin in the ARP 2500 VCF. And his article is one of the classics: |
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Dennis Colin |
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The Electrical Design and Musical Applications of an Unconditionally Stable Combination Voltage Controlled Highpass, Bandpass, Lowpass, Band Reject Filter/Resonator |
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Journal of the Audio Engineering Society, Dec 1971 |
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<nowiki>http://www.guitarfool.com/ARP2500/DennisCollinPaper.pdf</nowiki> |
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He didn't invent the SVF, but he was the first to voltage control it and apply it to music. |
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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. |
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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 |
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OTA. |
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The Rossum/Oberheim SVF puts the integrating cap at the output of the OTA, |
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shunted to ground. |
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Then there's the ARP 4075 VCF based around the LM3900 Norton amplifier. |
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The first low distortion low noise high fidelity VCF. I just read the |
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patent (US 4,011,466) on the 4075 and it is a fascinating read. That's a |
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unique topology. |
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One thing the article fails to touch on is the feedback architecture. |
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Feedback has a big impact on the sound, and the feedback design changed |
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between Moog synth models. That is why players back then complained that |
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the new models didn't sound like the Minimoog. My post in SDIY way back in |
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1997 here: |
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<nowiki>http://search.retrosynth.com/synth-diy/search/lookit.cgi?-v9710.272</nowiki> |
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<br /> |
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[[Category:Synthesizer components]] |
[[Category:Synthesizer components]] |