"One makes a filter resonate by providing the right kind of feedback. Amplify the output of the VCF and feed it back to the input. If that doesn't work, do it again but invert the signal. Note that only filters with even numbers of poles oscillate [see the comments in this post]. The classic Moog filter is a 4 pole.
David Hillel Wilson
Curator,
New England Synthesizer Museum
www.synthmuseum.com/nesm"
Friday, December 14, 2007
5 comments:
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MATRIXSYNTH - EVERYTHING SYNTH
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MATRIXSYNTH - EVERYTHING SYNTH
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ummm...
ReplyDeleteThe TB 303 is a 3 pole filter and it self oscillates. The 777 has a 7 pole filter and I am pretty sure it self oscillates too.
WRONG! That "even numbers of poles" comment is completely, completely wrong. If you amplify the output of a VCF and feed it back to the input, then ANY filter with 3 or more poles will oscillate, MOST two-pole filters will oscillate, and even SOME one-pole filters will.
ReplyDeleteControl theory says for any system with three or more poles, the root locus eventually leads (at least) two poles into the right-half plane. Bam! Oscillation. There's no even/odd bullshit involved.
Isn't the 303 filter a little weirder than just being 3-pole? I heard something somewhere about it being a 4-pole topology with the first pole tuned up an octave, which gave an 18dB/oct slope overall... was that incorrect?
ReplyDeleteAlso, many references I've read state that the 303 filter doesn't technically self-oscillate, going off into distortion instead.
(On the other hand, logically speaking, if you have a 2-pole filter that is in self-oscillation and you add an identically controlled 1-pole filter to it, you have a 3-pole topology that will self-oscillate.)
EMS Synthi / VCS3 also has a 3-pole filter, oscillates beautifully.
ReplyDeleteGwenhwyfaer,
ReplyDeleteThe 303 filter is 3 pole. It is a ladder like the moog filter, but the stages are not well isolated. This makes one of the poles close to zero. So you have to "tune" the filter stages to keep the poles more aligned. This is all real bad engineering from a mathematical perspective, but it is a musical happy accident.
The filter is odd in so many ways. It is too bad there are not more unusual filter designs in production. The Wasp is the only other unorthodox filter design I can think of that was mass produced.