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seancostello · 16th April 2009

Regarding the sound at small sizes, one important thing is what your total modal density is at that size setting. The length of the longest delay is used in some reverbs to determine size, but I would think that it would be more important to try and match the sum of the delay lengths to the room size. Of course, a large hall has a few orders of magnitude more resonances than what a digital reverb usually has, but perhaps there is some metric that would be useful.

One possibility is to have the Size control match up with the predicted Schroeder frequency of a space. In other words, your average density of resonances per Hz for your artificial reverberator (which, for a conventional delay based single band reverberator, is constant across all frequencies) matches the modal density at the Schroeder frequency of an "ideal" rectangular space with the longest dimension corresponding to size. Someone who has had more sleep than me can probably explain this better, or better explain why my idea is totally wrong.

EDIT: OK, my idea is probably wrong, in that I think that the Schroeder frequency is DEFINED by having X amount of resonances per Hz. So, maybe a better idea is to match the resonance density of the reverb (totalDelay/samplingRate) to the resonance density of the modeled space at some particular frequency, like 1000 Hz or so.

Of course, the structure that Casey was discussing was loosely based on the EMT-250, which didn't have enough modal density to emulate anything but the tiniest of spaces. The modulation tricks Casey is discussing are used to turn the discrete resonances into statistical peaks. The goal of the modulation is that the peaks become wide enough to "fill in the gaps" between each other, thus emulating the effect of having far more peaks per Hz. Again, others can explain this better than I can, with Barry Blesser's work being a good start. I always view the resonances as a picket fence, with the modulation moving the fence back and forth such that it appears to be a solid surface with some degree of transparency.

Another idea is to not worry about emulating a physical space with this algorithm, and make it more of a "plate." Some recent papers I have read suggest that plates have a fairly constant modal density across the audio range - around 1.27 resonances per Hz on average. So, give yourself 60K of total delay time for a 48K sampling rate, and then put in taps and allpasses and scattering and stuff until it sounds like a plate. Just that simple.

I love this thread, BTW. Love love love it. I have been following along with my VST code, and my algorithms are improving as a result. My EMT-250 type algorithm sounds OK, although I am having some technical issues with the modulation on my end, but working on this resulted in some breakthroughs on modulation in general that I can apply to my existing code.

16th April 2009	#68
seancostello ValhallaDSP Join Date: Feb 2009 Location: Pacific NW Posts: 1,536	Regarding the sound at small sizes, one important thing is what your total modal density is at that size setting. The length of the longest delay is used in some reverbs to determine size, but I would think that it would be more important to try and match the sum of the delay lengths to the room size. Of course, a large hall has a few orders of magnitude more resonances than what a digital reverb usually has, but perhaps there is some metric that would be useful. One possibility is to have the Size control match up with the predicted Schroeder frequency of a space. In other words, your average density of resonances per Hz for your artificial reverberator (which, for a conventional delay based single band reverberator, is constant across all frequencies) matches the modal density at the Schroeder frequency of an "ideal" rectangular space with the longest dimension corresponding to size. Someone who has had more sleep than me can probably explain this better, or better explain why my idea is totally wrong. EDIT: OK, my idea is probably wrong, in that I think that the Schroeder frequency is DEFINED by having X amount of resonances per Hz. So, maybe a better idea is to match the resonance density of the reverb (totalDelay/samplingRate) to the resonance density of the modeled space at some particular frequency, like 1000 Hz or so. Of course, the structure that Casey was discussing was loosely based on the EMT-250, which didn't have enough modal density to emulate anything but the tiniest of spaces. The modulation tricks Casey is discussing are used to turn the discrete resonances into statistical peaks. The goal of the modulation is that the peaks become wide enough to "fill in the gaps" between each other, thus emulating the effect of having far more peaks per Hz. Again, others can explain this better than I can, with Barry Blesser's work being a good start. I always view the resonances as a picket fence, with the modulation moving the fence back and forth such that it appears to be a solid surface with some degree of transparency. Another idea is to not worry about emulating a physical space with this algorithm, and make it more of a "plate." Some recent papers I have read suggest that plates have a fairly constant modal density across the audio range - around 1.27 resonances per Hz on average. So, give yourself 60K of total delay time for a 48K sampling rate, and then put in taps and allpasses and scattering and stuff until it sounds like a plate. Just that simple. I love this thread, BTW. Love love love it. I have been following along with my VST code, and my algorithms are improving as a result. My EMT-250 type algorithm sounds OK, although I am having some technical issues with the modulation on my end, but working on this resulted in some breakthroughs on modulation in general that I can apply to my existing code. Last edited by seancostello; 16th April 2009 at 06:47 PM.. Reason: I was tired and had the tired stupids going on.