I found a free beta VST plugin called FeedBug which creates feedback.
It’s OK but it’s not extreme enough to create the sustained squeals I’m looking for.
Can anyone suggest some Nyquist code for deliberate feedback ?
[ it’s going to have to have parameters for delay and the amount of feedback,
a limiter on the output would be a nice touch to stop the output going off the chart ].
The Feedback-delay function comes to mind.
Unfortunately that does not allow you to insert other processing within the feedback loop - to do that I think you would need to create your own feedback function, which you would probably need to implement as a DSP class (see http://www.cs.cmu.edu/~rbd/doc/nyquist/part19.html#index1328), which is quite complicated.
Doh ! 
But like you say there is no temporal variability possible with the command “Feedback-delay” : the effect is constant like a comb. I tried combining a few [below] thinking they may interact to change the effect with time , but no joy 
(sim s
(feedback-delay s 0.001 0.01)
(feedback-delay s 0.003 0.03)
(feedback-delay s 0.005 0.05)
(feedback-delay s 0.010 0.01))
Still usable though.
What about varying the ratio that each feedback-delay contributes to the final result according to (3?) LFOs, each oscillating at a prime-number frequency so there is no repetition.
So the ratio of the notches is always changing , (not quite the same as a flanger), e.g. …
It’s an approximation of the guitarist moving about on stage getting feedback from different monitor-speakers, each with their own constant resonant-frequency.
The second argument of feedback-delay can be a sound too:
(feedback-delay s 0.001 (pwl 0.5 1.0003 1 0.0 1))
This gives a sound like:
For a “true” feedback loop, the decay sound must temporarily go over 1.0.
You can of course pre- and post-process the sound to get e.g. changing frequencies.
Thanks Robert ,
is it possible to have a Low Frequency Oscillator as the second argument of “feedback-delay” to provide Amplitude Modulation of the combed content ?
I had to use this arrangement to amplitude-modulate each of the "feedback-delay"s by a different LFO …
(setq A1 (hzosc 0.5))
(setq A2 (hzosc 0.333))
(setq A3 (hzosc 0.1234))
(sim
(mult A1 -0.5) (mult A2 -0.5) (mult A3 -0.5)
(mult (sum 0.5 (feedback-delay s 0.01 0.015)) A1)
(mult (sum 0.5 (feedback-delay s 0.03 0.05)) A2)
(mult (sum 0.5 (feedback-delay s 0.05 0.07)) A3))
