[PD] feedback stability..?

Derek Holzer derek at umatic.nl
Mon Sep 15 22:34:56 CEST 2008

Yeah, this has been covered here before. Look up "DSP loops" in the 
archives. In short, you can't ask a subpatch/abstraction/whatever to 
compute output based on it's input simultaneously. Unlike analog 
electronics, where electrons move almost simultaneously, in DSP it's 
logically impossible. So your feedback loop must contain a one block 
delay. [send] and [receive] do that.

However, if your problems are with something like a delay line or filter 
inside [blackbox~] saturating and "blowing up", then you might look at 
some kind of limiter, waveshaper or clipper to keep the incoming audio 
within certain limits.


Damian Stewart wrote:
> hey pd,
> i'm trying to do feed forward in pd. i think i'm tired and not thinking 
> this through...
> so, i have a [blackbox~] that does stuff to the audio. it uses feedback 
> internally and is currently a bit unstable. can i make it more stable by 
> going like this:
> [...]   [r~ fb]
> |       |
> |       [*~ -0.1]
> | ______/
> |/
> [blackbox~]
> |\______
> |       \
> |       [s~ fb]
> |
> [...]
> is there some funky combination of delays i can achieve this? i get the 
> feeling this only works in analog electronics because you can say that on 
> some level everything happens at exactly the same time - the delays 
> inherent in digital systems mean that this doesn't work...
> what alternative DSP techniques are there for damping unstable feedback loops?
> thanks
> d

derek holzer ::: http://www.umatic.nl ::: http://blog.myspace.com/macumbista
---Oblique Strategy # 110:
"Lowest common denominator"

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