[sdiy] (OT?) FEEDBACK as a topic in electronics, music, science, etc?? BOOKS?

[Richard Gorbutt]

According to my (limited) understanding and research into video feedback and video synthesis, the actual feedback loop operates at lightspeed, and it is the limited rate of change and persistence-of-glowiness of the phosphor coating on the inside of the CRT that slows the evolving patterns down to a visually perceptible speed. Lcd/tft monitors operate in a different manner, and give different effects. Tube based cameras (Ie ones that use vidicons, trinicons etc) provide different results to ccd sensors.
Ive been building a large analogue video modular for the last couple of years (time flies when youre having fun!) and the different types of feedback texture are fascinating to watch and play with. Direct feedback paths (I e out of the output mixer and patched back in to a spare input) give completely different effects from feedback patches involving cameras and monitors. Combining the two approaches is even more fun. Im currently working on a way of micing up the picture tube to capture the buzzing tones from the LOPT transformer on the top of the picture tube (which audibly changes, depending on the picture being displayed) and feed them back into the system. It's a lot of fun. Further research directions will be involving video delay networks and filters patched into the feedback paths. Im guessing that even milli or micro second delays will be enough to produce useable/interesting results due to the frequencies involved.
Cheers
rich

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Scott Nordlund
Sent: Thursday, September 08, 2011 5:58 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] (OT?) FEEDBACK as a topic in electronics, music, science, etc?? BOOKS?



> Great thread and since I've been meaning to ask about this as well - I
> thought this question might be relevant. Hope you don't mind Dan.
>
> Can anyone comment on the main qualities affecting 'musical' (or, I
> guess, in the case of video - 'artistic') feedback?
>
> Is it...
>
> a) The time delay - e.g. the time it takes a signal to get from the
> speakers of my amp and back into my guitar pickups
> b) The change in the shape of the waveform e.g. small inaccuracies that start to stack up infinitely
> c) Other...

I think what generally makes it interesting is chaotic behavior. Rather than a stable oscillation (limit cycle), it can, for instance, transition between a number of different states at unpredictable intervals. But this is really general. It can happen on an audio timescale (producing a noisy waveform) or over minutes. And this isn't necessarily musical or non-musical by itself, it's far more dependent on how you're using it (the way it's mapped to audible parameters). To listen directly to the output may sound awful, but you can use it to modulate pitch or something else. The interesting part is that the sound essentially determines its own behavior and produces complex output without direct user intervention. The "composition" is the algorithm; the specifics take care of themselves. You can do this with complex synth patching, mixer feedback, neural networks, feedback through effect processors, self-observing and adaptive systems, etc.
Video feedback can produce more immediately interesting results. The mapping of the sensor's pixels to the monitor's pixels, the nonlinear control of brightness via AGC and the time delay from frame to frame can easily produce fractal and cellular automata type things.
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