Voltage controlled filter: Difference between revisions
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{{for|electronic filter circuits rather than synthesizer filters|Filter}} |
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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. |
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The '''voltage-controlled filter''' '''(VCF)''' is an [[filter|electronic filter]] whose operating characteristic can be accurately [[CV/gate|voltage controlled]] over a wide audio range.<ref>''[https://books.google.co.uk/books?id=3hjvWzkMK-sC&pg=PA27#v=onepage&q=filter&f=false Analog Days: The Invention and Impact of the Moog Synthesizer]'' by Trevor Pinch and Frank Trocco, Harvard University Press, 2004, {{ISBN|0674016173}}</ref><ref>[https://web.archive.org/web/20191104124000/https://synth-diy.org/pipermail/synth-diy/2019-August/171532.html Article "Analyzing the Moog Filter"] reply by Donald Tillman, Synth-diy mailing list, 23 August 2019</ref> |
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Common filter types used in audio synthesis include: |
Common filter types used in audio synthesis include: |
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* Low-pass (high-cut) filter - removes higher frequencies |
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* |
* High-pass (low-cut) filter - removes lower frequencies |
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* |
* Band-pass filter - removes frequencies outside a given band |
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* |
* Notch filter - removes frequencies within a given band |
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* |
* Shelf filters - raise or lower frequencies above or below a cut off point. |
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* |
* Peak filters - raise frequencies within a given band |
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* Formant filters - raise multiple peaks, often in such a way that they mimic the human voice. |
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* Peak filters: Raise frequencies within a given band |
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* State variable filters - e.g. include simultaneous low-pass, high-pass and band pass outputs as well as voltage controllable cut off frequency and resonance (Q) |
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* Formant filters: raise multiple peaks, often in such a way that they mimic the human voice. |
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* Comb filter - adding a delayed version of a signal to itself, causing constructive and destructive interference. |
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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: |
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: |
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* A cut-off frequency - the frequency at which the filter begins to remove frequencies. This is often voltage-controllable and can changes over time (e.g. controlled by an [[Envelope generator|envelope]], or an [[Low frequency oscillator|LFO]]). |
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* Resonance - how much the filter boosts the frequency at the cut-off point. This may also be voltage-controllable. |
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* Phase shift - frequencies beyond the cut-off often have their [[phase]] affected. |
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== References == |
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* 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]]). |
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{{reflist}} |
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* resonance - how much the filter boosts the frequency at the cut-off point. This may also be voltage-controllable. |
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* frequencies beyond the cut-off often have their [[phase]] affected. |
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<br /> |
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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]] |
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Latest revision as of 15:32, 2 July 2023
The voltage-controlled filter (VCF) is an electronic filter whose operating characteristic can be accurately voltage controlled over a wide audio range.[1][2]
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.
- State variable filters - e.g. include simultaneous low-pass, high-pass and band pass outputs as well as voltage controllable cut off frequency and resonance (Q)
- Comb filter - adding a delayed version of a signal to itself, causing constructive and destructive interference.
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-controllable and can changes over time (e.g. controlled by an envelope, or an LFO).
- Resonance - how much the filter boosts the frequency at the cut-off point. This may also be voltage-controllable.
- Phase shift - frequencies beyond the cut-off often have their phase affected.
References
- ^ Analog Days: The Invention and Impact of the Moog Synthesizer by Trevor Pinch and Frank Trocco, Harvard University Press, 2004, ISBN 0674016173
- ^ Article "Analyzing the Moog Filter" reply by Donald Tillman, Synth-diy mailing list, 23 August 2019