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acreil · 22nd May 2010

I'm really happy to see an open discussion of this. I've read through the thread (and related threads) several times and carefully taken notes, and I'm trying to digest all the papers and patents I can find on the subject. I was going to just work on it by myself, but it seems more in the spirit of things to share my observations and questions.

I'm building reverberators and other effects in Pure Data. After a few initial experiments, a couple "cloned" algorithms, and some increasingly stupid ideas, I think I can safely clear all effect processors, hardware and software, from my wanted list. That probably sounds excessive, but my tastes tend toward the exotic, so I don't mind making things that turn single piano notes into underwater reverse bowed Balinese gong tone clusters. Making something that resembles an actual acoustic space seems quite difficult, but "outlandish" can be a lot more forgiving. I've only been messing with these for a few weeks but I'm really surprised at how quickly I'm able to make and refine them. As I become accustomed to the freedom I'm allowed here, I suspect even the most powerful Eventide/TC boxes would feel limiting.

Some ideas and things that I've tried:

1. Frequency shifters in feedback loops. I suspect Isao Tomita may have done this with a Bode frequency shifter and AKG BX20, results are interesting but it's perhaps too tempting to take it to total excess.

2. Dispersion filters- I saw that Sean participated in a paper modeling spring reverb (I don't have the full paper), and I saw a patent (4955057) suggesting 20-ish allpass analog phase shifter stages in a reverb feedback loop, which I tried. It's interesting in comparison to normal APFs, since an impulse is spread into a chirp, but it's sort of computationally intensive. Gary Kendall wrote a paper (The Decorrelation of Audio Signals and Its Impact on Spacial Imagery, 1995)
suggesting time variant decorrelating filters made from long chains of allpass biquads- maybe this is another option? I imagine sticking these inside normal APFs could be useful, computing power permitting. Anyone tried it?

3. That random delay Yamaha patent that Sean mentioned (553150)- I didn't expect a lot but I tried it with 16 delays and got interesting results. A single "comb" made this way can't handle much feedback due to the amplitude modulation of the crossfade, but modulated APFs and additional feedback paths (sort of a naive FDN) can actually make things quite smooth. Without feedback it can be used as a weirdly modulating "early reflection" FIR. It's really interesting and I'll have to play with it a lot more.

4. Long APF chain with random taps: I chained 16 APFs per channel and randomly crossfade between taps at each stage for output, feedback, etc. This can be used in a relatively sane way, or to make reverbs with a sort of dynamic "envelope", building slowly or quickly or panning from right to left or whatever. It's pretty cool so far, but I need to extend it.

Some questions and things:

1. I've had trouble finding some papers. I have a healthy pile of user manuals, service manuals, patents, papers that the authors have made available on their websites, DAFX ones, etc. but I'm missing some Computer Music Journal/JAES/etc. ones that are probably fairly important- in particular Schroeder, Blesser, Jot, Gerzon, and the Agnello one describing the Eventide SP2016. Anyone got these handy?

2. I've gotten the impression that it's best to modulate "in the loop" APF delay times (one paper suggested modulating feedback coefficients, though Sean described poor results), but are there limits to this? Is it a logical conclusion, given adequate processing power, to just "modulate everything"? Does it make more sense to use one n-phase sinusoidal or triangular modulator for everything, or to break it down into separate 2/3/4 phase modulators running at different rates? Or independent randomized (piecewise linear or whatever) modulators? Is it preferred to apply complementary modulation to consecutive APFs? There are a huge number of options, and I'm not sure I'm perceptive enough to find the "best".

3. I'm not a huge fan of FIR "early reflections", but they do come in handy sometimes. I've found they're quite hard to make from scratch and generating them randomly just results in 10000 flavors of crap. I've gotten a few from papers and such, but is there any sort of repository or some that people wouldn't mind sharing?

3.14159... Speaking of which, Moorer notes that an APF with a non-allpass filter in the loop can be made allpass if preceded by its time reverse. It seems like a multitap FIR type thing used this way could take care of early reflections and diffusion all at once- anyone tried it?

4. Regarding multi-band reverbs, is there much advantage here besides independent control over reverb times? I know Yamaha did this a lot, but did anyone else? Do different delay times for different bands result in a smoother sound?

Christ, I've nearly written a doctoral thesis myself...

22nd May 2010	#525
acreil Lives for gear Joined: May 2010 Posts: 722	hello I'm really happy to see an open discussion of this. I've read through the thread (and related threads) several times and carefully taken notes, and I'm trying to digest all the papers and patents I can find on the subject. I was going to just work on it by myself, but it seems more in the spirit of things to share my observations and questions. I'm building reverberators and other effects in Pure Data. After a few initial experiments, a couple "cloned" algorithms, and some increasingly stupid ideas, I think I can safely clear all effect processors, hardware and software, from my wanted list. That probably sounds excessive, but my tastes tend toward the exotic, so I don't mind making things that turn single piano notes into underwater reverse bowed Balinese gong tone clusters. Making something that resembles an actual acoustic space seems quite difficult, but "outlandish" can be a lot more forgiving. I've only been messing with these for a few weeks but I'm really surprised at how quickly I'm able to make and refine them. As I become accustomed to the freedom I'm allowed here, I suspect even the most powerful Eventide/TC boxes would feel limiting. Some ideas and things that I've tried: 1. Frequency shifters in feedback loops. I suspect Isao Tomita may have done this with a Bode frequency shifter and AKG BX20, results are interesting but it's perhaps too tempting to take it to total excess. 2. Dispersion filters- I saw that Sean participated in a paper modeling spring reverb (I don't have the full paper), and I saw a patent (4955057) suggesting 20-ish allpass analog phase shifter stages in a reverb feedback loop, which I tried. It's interesting in comparison to normal APFs, since an impulse is spread into a chirp, but it's sort of computationally intensive. Gary Kendall wrote a paper (The Decorrelation of Audio Signals and Its Impact on Spacial Imagery, 1995) suggesting time variant decorrelating filters made from long chains of allpass biquads- maybe this is another option? I imagine sticking these inside normal APFs could be useful, computing power permitting. Anyone tried it? 3. That random delay Yamaha patent that Sean mentioned (553150)- I didn't expect a lot but I tried it with 16 delays and got interesting results. A single "comb" made this way can't handle much feedback due to the amplitude modulation of the crossfade, but modulated APFs and additional feedback paths (sort of a naive FDN) can actually make things quite smooth. Without feedback it can be used as a weirdly modulating "early reflection" FIR. It's really interesting and I'll have to play with it a lot more. 4. Long APF chain with random taps: I chained 16 APFs per channel and randomly crossfade between taps at each stage for output, feedback, etc. This can be used in a relatively sane way, or to make reverbs with a sort of dynamic "envelope", building slowly or quickly or panning from right to left or whatever. It's pretty cool so far, but I need to extend it. Some questions and things: 1. I've had trouble finding some papers. I have a healthy pile of user manuals, service manuals, patents, papers that the authors have made available on their websites, DAFX ones, etc. but I'm missing some Computer Music Journal/JAES/etc. ones that are probably fairly important- in particular Schroeder, Blesser, Jot, Gerzon, and the Agnello one describing the Eventide SP2016. Anyone got these handy? 2. I've gotten the impression that it's best to modulate "in the loop" APF delay times (one paper suggested modulating feedback coefficients, though Sean described poor results), but are there limits to this? Is it a logical conclusion, given adequate processing power, to just "modulate everything"? Does it make more sense to use one n-phase sinusoidal or triangular modulator for everything, or to break it down into separate 2/3/4 phase modulators running at different rates? Or independent randomized (piecewise linear or whatever) modulators? Is it preferred to apply complementary modulation to consecutive APFs? There are a huge number of options, and I'm not sure I'm perceptive enough to find the "best". 3. I'm not a huge fan of FIR "early reflections", but they do come in handy sometimes. I've found they're quite hard to make from scratch and generating them randomly just results in 10000 flavors of crap. I've gotten a few from papers and such, but is there any sort of repository or some that people wouldn't mind sharing? 3.14159... Speaking of which, Moorer notes that an APF with a non-allpass filter in the loop can be made allpass if preceded by its time reverse. It seems like a multitap FIR type thing used this way could take care of early reflections and diffusion all at once- anyone tried it? 4. Regarding multi-band reverbs, is there much advantage here besides independent control over reverb times? I know Yamaha did this a lot, but did anyone else? Do different delay times for different bands result in a smoother sound? Christ, I've nearly written a doctoral thesis myself...