Voltage controlled filter: Difference between revisions
Jump to navigation
Jump to search
Content added Content deleted
(revert to 29 June 2019 version, moved the mess to ''Draft revision'' on talk page) |
|||
Line 16: | Line 16: | ||
* resonance - how much the filter boosts the frequency at the cut-off point. This may also be voltage-controllable. |
* 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. |
* 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]] |
[[Category:Synthesizer components]] |