> 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.
�����������������������������������������������������������������������
Reply by Rob●August 22, 20082008-08-22
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?
Reply by Richard Dobson●August 22, 20082008-08-22
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
Reply by Ico●August 22, 20082008-08-22
sparafucile17 <sparafucile17@hotmail.com> wrote:
>>
>><grouch mode>
>>
>>This is an amusing dilemma. If you mean what you say, then you don't
>>know what you're talking about. :-) To quote you again:
>>o Let Signal A be a periodic snare hit at 100ms intervals
>>o Let Signal B be a periodic snare hit at 110ms intervals
>>o Both signals start at the same time: T=0
>>
>>Signal A occurs at t = 0, 100, 200, ... 100*n, ...
>>Signal B occurs at t = 0, 110, 220, ... 110*n, ...
>>
>>If you don't mean than, then you don't mean what you say.
>>
>>A simple delay of n seconds is a shift register or circular buffer that
>
> Again what I'm trying to avoid is that when both signals are present and
> being summed, a comb effect occurs effectively REDUCING the amplitude.
The comb effect is inherent to what you're trying to do. If you want to
avoid this, use two non-correlated samples instead of one.
You might have a chance by distorting the phase of one of the snares
with some kind of all-pass filter; this reduces correlation between the
two samples, but will not be very obvious to the human ear.
Ico
--
:wq
^X^Cy^K^X^C^C^C^C
Reply by Jerry Avins●August 22, 20082008-08-22
glen herrmannsfeldt wrote:
> Jerry Avins wrote:
> (snip)
>
>> Have you listened to the two snare hits summed? There's a comb-filter
>> effect when a drummer plays that way. The difference is that the
>> sounds are produced by a physical mechanism and are therefore not
>> absolutely identical. The comb-filter effect is part od the difference
>> in timbre between a single violin and a whole violin section. It's
>> part of the way we hear music, so simple summation might sound
>> natural. If it doesn't, the best cure is a secand snare sample.
>
> Back to the coherence question again.
>
> Two violins won't be tuned exactly the same on the time
> scale required for a concert. If you electronically
> generate violin sounds and combine them, they might be
> coherent. It would seem that this problem would come
> up more often in electronically generated music.
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"?
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Mark●August 21, 20082008-08-21
well I think the comb filtering and time delay are orthogonal views of
the same thing related by the FT and therefore you can't have one
without the other...
it is also interesting to consider therefore that complicated
reverberation in a room MUST also imply a very complicated and messy
frequency response..
and again you can't have one without the other..
Mark
Reply by glen herrmannsfeldt●August 21, 20082008-08-21
Jerry Avins wrote:
(snip)
> Have you listened to the two snare hits summed? There's a comb-filter
> effect when a drummer plays that way. The difference is that the sounds
> are produced by a physical mechanism and are therefore not absolutely
> identical. The comb-filter effect is part od the difference in timbre
> between a single violin and a whole violin section. It's part of the way
> we hear music, so simple summation might sound natural. If it doesn't,
> the best cure is a secand snare sample.
Back to the coherence question again.
Two violins won't be tuned exactly the same on the time
scale required for a concert. If you electronically
generate violin sounds and combine them, they might be
coherent. It would seem that this problem would come
up more often in electronically generated music.
-- glen
Reply by sparafucile17●August 21, 20082008-08-21
>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
>
Greg,
Yes I guess I am trying to defy physics once again... I just thought
there might be an approach to summing delayed correlated signals, maintain
the delay, and somehow UNDO the combing amplitude effects. I've just been
tinkering around with adding an IIR comb filter to sorta "fill-in" the
holes produced by the standard FIR comb (original prob), with little
success.
Yes I did reject the limited frequency band approach because, as I said,
it removed the time delay that I want to preserve. After realizing this,
it seems like it's not worth allpasses and I might as well time delay
signal B to remove the comb.
In the end I may be stuck here and will have to tradeoff delay for good
magnitude response as you indicated. But I figure I'd float this out there
for all the group, in case there is a innovative approach to the problem.
Thanks,
Jeff
Reply by Jerry Avins●August 21, 20082008-08-21
sparafucile17 wrote:
>> <grouch mode>
>>
>> This is an amusing dilemma. If you mean what you say, then you don't
>> know what you're talking about. :-) To quote you again:
>> o Let Signal A be a periodic snare hit at 100ms intervals
>> o Let Signal B be a periodic snare hit at 110ms intervals
>> o Both signals start at the same time: T=0
>>
>> Signal A occurs at t = 0, 100, 200, ... 100*n, ...
>> Signal B occurs at t = 0, 110, 220, ... 110*n, ...
>>
>> If you don't mean than, then you don't mean what you say.
>>
>> A simple delay of n seconds is a shift register or circular buffer that
>> is n*(sample rate) stages long. No approximations.
>>
>> Jerry
>> --
>> Engineering is the art of making what you want from things you can get.>>
>
> Hmm... yes, I may have used a bad example. What I meant to say that
> Signal B always is 10ms delay from Signal A. So, using you're timing
> nomenclature:
>
> Signal A occurs at t = 0, 100, 200, ... 100*n
> Signal B occurs at t = 0, 110, 210, ... (100*n) + 10
>
> And for arguments sake, lets assume that the duration of the snare hit is
> 30ms in length. That means there will be some overlap when both signals
> are summed. Here is what I would want to see:
>
> After sum:
> @ T=100:110, my orginal Signal A snare hit
> @ T=110:130, a snare hit that has *MORE* amplitude than original
> @ T=130:140, my orginal Signal B snare hit
>
> Again what I'm trying to avoid is that when both signals are present and
> being summed, a comb effect occurs effectively REDUCING the amplitude.
>
> Sorry, for my inconsistent post. It was after a long day of hair-pulling
> on this subject....
No problem. I just didn't want to address the wrong issue. I still don't
see what you want the delay to do. (I don't think you mean correlated in
the technical sense, but I may be wrong.)
Have you listened to the two snare hits summed? There's a comb-filter
effect when a drummer plays that way. The difference is that the sounds
are produced by a physical mechanism and are therefore not absolutely
identical. The comb-filter effect is part od the difference in timbre
between a single violin and a whole violin section. It's part of the way
we hear music, so simple summation might sound natural. If it doesn't,
the best cure is a secand snare sample.
Jerry
--
Engineering is the art of making what you want from things you can get.
Reply by Greg Berchin●August 21, 20082008-08-21
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