Forums > MaxMSP

Feedback Problem in Bandpass-Setting

February 19, 2006 | 8:58 pm

Hi list,
i have quite a problem:
Im working on a piece, where i use a bank of bandpass filters (four
parallel cascades of two biquads at gain 1 and Q bout 15 on every one
of the four input channels) to loosely imitate the resonances of an
instrument. The output of the filter should then be reverbed a lot to
generate a cloud of airy sound. Here seems to be the problem: Reverbing
the filteroutput more or less amplifies it, as it is present over
longer a time and gets accumulated. Thus the whole thing is extremely
prone to feeding back into the filter…
I’ve tried building feedbackdestroyers with fft, but when those shut
enough to kill the feedback, i cant get anything else past them into
the filter. i’ve tried working on the reverb settings (used newverb) as
well, but when i got the feedback under control, the reverb level was
almost impercrptible over the direct sound from the instruments –
timpani i forgot to mention…
Could it just be a problem of the very unfavorable accoustics of the
room in which i’ m working, – its small and the instruments are in a
corner, so i get a lot of reflection from the walls and the ceiling,
and the huge membranes reflect everythig into the mics. Its not just a
problem of the newverb, if tried other devices as well…
Any ideas how to get a sort of tuneable resonator/filter bank without
the self exitation problem?
i’ll try to get an isolated patch without any externals missing for
posting…

Titus Bellwald


February 19, 2006 | 9:59 pm

Something you might try is doubling the number of filters, and reducing
the Q. This will make them be less prone to ringing. Also, since
you’re using the reverb to amplify the filters, you might also try
reducing the gain.

i.e.

input
|
reson~ 1. 400. 7.5
|
reson~ 1. 400. 7.5
|
output

Also, have you considering routing the audio in the opposite manner
(reverb, then EQ)? Depends on how you’re using it, so this might not
work stylistically…

Additionally, I’d consider a compressor/limiter on the input and output
stages to manage volume levels.

Peter McCulloch


February 20, 2006 | 7:12 am

I’m not fully sure what you are doing, but I get the impression that
you are micing the timpani, processing and then play back using
loudspeakers. If so, the first thing I would consider is the distance
and position of mic and loudspeakers, and also what kind of mics you
are using. Sorry if these suggestions are to basic.

Another thing to try is to let the center frequency of the filters
drift a little bit. This might prevent them from being triggered at
the exact same frequency.

One more thing you could try is introducing freqshift~ to shift
frequency downward by a few Hz. This is a common way of avoiding
feedback, as any frequency that starts building up through feedback
will be pushed downward frequency-wise and hence gradually pushed
away from of the resonant frequency. It might introduce unwanted
detuning and artifacts though.

It’s well worth checking out if you might benefit from limiters or
compressors as well.

Best,
Trond


February 20, 2006 | 9:12 pm

Titus Bellwald wrote:
> Hi list,
> i have quite a problem:
> Im working on a piece, where i use a bank of bandpass filters (four
> parallel cascades of two biquads at gain 1 and Q bout 15 on every one of
> the four input channels) to loosely imitate the resonances of an
> instrument. The output of the filter should then be reverbed a lot to
> generate a cloud of airy sound.

You might consider to combine the two things. As the resonance of an
instrument is in general a simplified reverb, you could build the reverb
from scratch and ignore the main problem of reverbs, the avoidance of
resonances. Thus its easier to build (less delay lines) and you get your
resonances as well. It requires some math to find the reflection length
aka delay times to get your resonances. You could still put some filters
into the feedback, but these could be simple onepole~s.

Just a suggestion…

Stefan

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Stefan Tiedje
Klanggestalter
Electronic Composition
&
Improvisation

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