[sdiy] Multi-output comb filter?

cheater00 cheater00 cheater00 at gmail.com
Wed May 23 06:34:22 CEST 2018


Ah yes, of course, interesting insight about the sieve algorithm.

I like to faff around, Neil. Don't spoil my fun!
Besides, unlike FFT this uses no multiplication or exponentiation, just
integer addition, and each thing being added is just added 0, 1, or -1
times. I'd say that's a pretty big win, wouldn't you?

I don't think any important discovery was made without faffing :-)

On Tue, 22 May 2018 14:07 Tom Wiltshire, <tom at electricdruid.net> wrote:

> You’re basically running the sieve algorithm for finding prime numbers
> there, so if you find a pattern or a better method, you’re in line for the
> Fields Medal.
>
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
>
> > On 22 May 2018, at 11:00, cheater00 cheater00 <cheater00 at gmail.com>
> wrote:
> >
> >   So what I thought might be feasible would be to eg get a single peak at
> >   1 Hz, create a 1 Hz comb filter, then subtract a 2 Hz comb from it so
> >   you're left with peaks at 1, 3, 5, 7, etc Hz, then subtract a 3 Hz
> >   comb, so you're left with 1, 5, -6, 7, 9, 11, -12, etc Hz, then
> >   subtract a 5 Hz comb etc. If you look at the first 249 such filters
> >   (that's what I could quickly calculate), you will get these
> >   coefficients:
> >
> >   +--0-+-00+-0-++0-0-0++
> >   -00+00---0+++0-++0---0
> >   0+-000+0-0+0++-0-+00+-
> >   -0+--0-+00+--00+-0+++0
> >   -0+0+++0-000---0-+-0--
> >   +0--+00++00++000-0+--0
> >   ++00---0+++0++00-0-00-
> >   +0-++0+0-0-+-00-00--00
> >   ++-0--+0+-+00--0-+-0-0
> >   -0+++0++00++-0+++0+++0
> >   +--00+-0---0-0+0+--0-0
> >   000-+0
> >
> >   Meaning: 1 * comb at 1 Hz, -1 * comb at 2 Hz, -1 * comb at 3 Hz, 0 *
> >   comb at 4 Hz, -1 * comb at 5 Hz, 1 * comb at 6 Hz, etc.
> >
> >   I assume this can be found for each nth peak, and a lot of the data
> >   will be shared. Note you still need (nyquist/f) delays with 1 bucket, 2
> >   buckets, ..., nyquist buckets. That's a good 468 megabytes at a 44.1
> >   kHz sampling frequency. I wonder if this too can be shared somehow.
> >
> >   I don't really see a pattern to the coefficients - i used a program to
> >   find them - does anyone else see any rule?
> >
> >   The program can be found here:
> >   [1]https://lpaste.net/6557071818560110592
> >
> >   You have to use the deepseq package.
> >   Change endHz to 250 and run firstPeakOnly from the repl. I'm not sure
> >   if the last coeff will be the same as with 22050, as I haven't
> >   calculated it up to that number, and I don't really have time to
> >   optimize this any more.
> >
> >   On Tue, 22 May 2018 07:37 , <[2]rsdio at audiobanshee.com> wrote:
> >
> >     On May 21, 2018, at 10:04 PM, Corey K <[3]coreyker at gmail.com> wrote:
> >> �  � Option 2: write out the Z-transform for your comb filter.
> >     There will be
> >> �  � one pole per peak, and these will be spaced at equal angles
> >     around a
> >> �  � circle in the complex place. Add a zero to perfectly cancel
> >     the pole
> >> �  � corresponding to the peak you want removed. This will in turn
> >> �  � correspond to an FIR filter that you can run before or after
> >     your comb
> >> �  � filter.
> >     Here�s an exercise that might prove interesting:
> >     Write out the Z-transform for an ideal comb filter. Then convert
> >     this to an FIR filter without any modification.
> >     My question is this: Is the math perfect enough that you�d end up
> >     with a single delay tap? I mean, that�s what a comb filter is, so I
> >     wonder whether the Z-transform to FIR would actually guide you to
> >     the simplest implementation. Granted, the FIR would probably be
> >     equivalent to a string of individual Z-1 delays, but they�re
> >     equivalent to a single, long delay so long as nothing taps the
> >     intermediate samples.
> >     I have a suspicion that things wouldn�t quite work out so perfectly,
> >     and there�s be lots of taps, each with their own weight, and it
> >     wouldn�t come out looking like the simple single-delay-plus-mixer
> >     circuit.
> >     Has anyone gone through the exercise to see how it turns out?
> >     Brian
> >     _______________________________________________
> >     Synth-diy mailing list
> >     [4]Synth-diy at synth-diy.org
> >     [5]http://synth-diy.org/mailman/listinfo/synth-diy
> >
> > References
> >
> >   1. https://lpaste.net/6557071818560110592
> >   2. mailto:rsdio at audiobanshee.com
> >   3. mailto:coreyker at gmail.com
> >   4. mailto:Synth-diy at synth-diy.org
> >   5. http://synth-diy.org/mailman/listinfo/synth-diy
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at synth-diy.org
> > http://synth-diy.org/mailman/listinfo/synth-diy
>
>
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