titus bellwald

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 

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 - 

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 

i'll try to get an isolated patch without any externals missing for 

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...

feedback-problem-in-bandpass-setting

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 

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 

Additionally, I'd consider a compressor/limiter on the input and output 

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.

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.

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 

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 

It's well worth checking out if you might benefit from limiters or 

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.

> 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 

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.

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.

  [][]  [][][]  [][]  [][][]
[][][][][][][][][][][][][][][]

         Stefan Tiedje
         Klanggestalter
     Electronic Composition
               &
         Improvisation

            /~~~~~
     \   /|() ()|
     ))))) )|  |  |( \
     ///     _/)/ )))))
             ___/   ///

-------------------------x----
--_____-----------|-----------
--(_|_ ----|-----|-----()----
-- _|_)----|-----()-----------
----------()------------x-----

14, Av. Pr. Franklin Roosevelt,
94320 Thiais, France
Phone at CCMIX +33-1-49 77 51 72


Feedback Problem in Bandpass-Setting