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How to add two add two correlated signals, but NOT have comb filtering...

Started by sparafucile17 August 20, 2008
Jerry Avins wrote:
..
> That is why I suggested using two snare samples instead of repeating the > one. All non-coherent sources do, though, is make the cancellation > notches move around. I suspect that's good. Aren't comb filters used to > create "chorus"? >
Chorus uses one or more variable combs; usually it is also used to spread a mono signal (coming out of a synth, say) into stereo, so there will be at least two, with the variable delays modulated with a phase offset to make the sound swim between left and right in a, um, phasey sort of way. The modulation rate is typically very low ( a few Hz) and shallow so pitch warping is very slight, so the movement is easily audible (especially when using a mere sine as the lfo waveform); but one can of course easily push it into more experimental realms. A 10msec delay is however more in the boundary between the phaser and flanger effects; it is ~conceivable~ one could diminish the obvious fixed comb effect by some more-or-less rapid phaser-style processing, but all this means is that one is replacing one colouring effect with another. Richard Dobson
> Jerry
On Thu, 21 Aug 2008 08:37:48 -0700 (PDT), Greg Berchin
<gberchin@sentientscience.com> wrote:

>sparafucile17 wrote: > >> Any ideas on how to sum two delay signals, preserve the delay, but NOT >> have comb filtering? Any idea or suggestion is welcome. > >It is not clear exactly what you are trying to achieve. The comb >filtering is a direct result of the delay. Remove the delay, and the >comb filtering goes away. Preserve the delay and the comb filtering >stays. You can't keep one and eliminate the other, except within some >limited frequency band(s) by manipulating the phase, which you have >apparently already tried and rejected. > >Greg
To the OP: Allpass/phase-shift filters have been used in rack-based chorus effects to avoid complete cancellation of all harmonically-related signals (otherwise they could practically null out a certain musical pitch). I'd expect this to have a different affect on drums (less 'harmonic' correlation of component frequencies), but allpass is the obvious thing to try. Why didn't it work?
Rob wrote:
> On Thu, 21 Aug 2008 08:37:48 -0700 (PDT), Greg Berchin > <gberchin@sentientscience.com> wrote: > >> sparafucile17 wrote: >> >>> Any ideas on how to sum two delay signals, preserve the delay, but NOT >>> have comb filtering? Any idea or suggestion is welcome. >> It is not clear exactly what you are trying to achieve. The comb >> filtering is a direct result of the delay. Remove the delay, and the >> comb filtering goes away. Preserve the delay and the comb filtering >> stays. You can't keep one and eliminate the other, except within some >> limited frequency band(s) by manipulating the phase, which you have >> apparently already tried and rejected. >> >> Greg > > To the OP: Allpass/phase-shift filters have been used in rack-based > chorus effects to avoid complete cancellation of all > harmonically-related signals (otherwise they could practically null > out a certain musical pitch). I'd expect this to have a different > affect on drums (less 'harmonic' correlation of component > frequencies), but allpass is the obvious thing to try. > > Why didn't it work?
I think it didn't work because he didn't equalize the delay. Instead, he used the allpass to approximately *remove* the delay. A second drum sample could be made from the first by scrambling the phase with a few allpass filters with transitions at different frequencies, but the delay they introduce has to be accounted for. While our ears aren't very sensitive to phase for continuous sound, messing with the phase of percussive sounds might not make for happiness. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;