I' ve got this technical question:
why a multimode filter can't be 24 dB?
the more technical the answer the better please! (as much as this forum allows)
Thanks in advance
Nikos
why a multimode filter can't be 24 dB?
- space_nerd
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The amount of electronic parts = money.
keep on turning these Moog knobs
Sequence:
Prodigy * minimoog '79 * Voyager * MF102 * MF103 * MF104z * MP201 * Taurus 3 * Minitaur * Sub Phatty * MF105 * Minimoog 2017+ MUSE * One 16
Sequence:
Prodigy * minimoog '79 * Voyager * MF102 * MF103 * MF104z * MP201 * Taurus 3 * Minitaur * Sub Phatty * MF105 * Minimoog 2017+ MUSE * One 16
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- space_nerd
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Thank you both for the answer,
although I was thinking that the answer was more technical related.
I remember a synth, was called "the aviator"
http://machines.hyperreal.org/incoming/ ... tor.review
that had two filters one multimode 12dB and one lowpass (Moog-clone I think) 24dB
although I was thinking that the answer was more technical related.
I remember a synth, was called "the aviator"
http://machines.hyperreal.org/incoming/ ... tor.review
that had two filters one multimode 12dB and one lowpass (Moog-clone I think) 24dB
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- Kevin Lightner
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I think we're talking basic structure.
How one simultaneously changes freq and/or resonance is up to the implementation.
One pot could feed both sections the same control voltage and one pot could control two gain stages (via OTA, vactrol ,etc) for resonance or just provide an inverted feedback to the original input.
How one simultaneously changes freq and/or resonance is up to the implementation.
One pot could feed both sections the same control voltage and one pot could control two gain stages (via OTA, vactrol ,etc) for resonance or just provide an inverted feedback to the original input.
The technical answer is that nobody has yet to find a successful working design for a 24dB multimode VCF.
The standard 12dB multimode filter is a product of very clever math manipulation. It works by rearranging the transfer function of a high pass filter, which is the ratio of two quadratic functions. It just so happens that the rearrangement of the terms can permit simultaneous lowpass mode and consequently bandpass, notch, etc with the addition of extra opamps. Unfortunately the transfer functions for 12dB and 24dB filters are radically different enough that the same alchemy isn't possible.
On part count, 12dB multimode filters aren't very efficient compared to other filter topologies. The 24dB equivalent - two cascaded multimodes - would be worse part-count wise, and would require exotic matched OTA circuits for voltage control of frequency and resonance of each half. It just isn't cost-effective.
The standard 12dB multimode filter is a product of very clever math manipulation. It works by rearranging the transfer function of a high pass filter, which is the ratio of two quadratic functions. It just so happens that the rearrangement of the terms can permit simultaneous lowpass mode and consequently bandpass, notch, etc with the addition of extra opamps. Unfortunately the transfer functions for 12dB and 24dB filters are radically different enough that the same alchemy isn't possible.
On part count, 12dB multimode filters aren't very efficient compared to other filter topologies. The 24dB equivalent - two cascaded multimodes - would be worse part-count wise, and would require exotic matched OTA circuits for voltage control of frequency and resonance of each half. It just isn't cost-effective.
MC wrote:The standard 12dB multimode filter is a product of very clever math manipulation. It works by rearranging the transfer function of a high pass filter, which is the ratio of two quadratic functions.
A nice geeky EE explanation!
I'd like to see this on paper. I can understand the math and I'm curious as to how this works out.MC wrote:It just so happens that the rearrangement of the terms can permit simultaneous lowpass mode and consequently bandpass, notch, etc with the addition of extra opamps. Unfortunately the transfer functions for 12dB and 24dB filters are radically different enough that the same alchemy isn't possible.
cheers,
theglyph
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