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seancostello · 2nd July 2009

The Black Hole reverb points out one of the fundamental issues with allpass delays in series, or for that matter any sort of filtering operation in series: As the order of the series increases, the impulse response of the output begins to resemble a Gaussian bell curve more and more. This causes a very distinctive "fade in" sound that can be used to great effect if intended, as in the Black Hole reverb, but will cause problems if not intended.

I have heard simpler digital reverbs, such as the SilverVerb in Logic, that are clearly based around comb filters with a lot of series allpasses embedded in the delay loop, and a single output tap at the end. The output of such reverbs can be very "open," but will also demonstrate that fading in quality, as opposed to an impulsive attack followed by an exponential decay.

The solution is to tap in between allpass stages, as shown in the Gardner and Dattorro papers, and later on in the Dahl/Jot papers with the absorbant allpasses.* This allows you to grab the signal before it becomes overly "Gaussian." Of course, this also creates the parallel allpass artifacts which Casey refers to above. So there is no neat and tidy solution, just a bunch of compromises and heuristic tricks.

Or, you can avoid using allpasses altogether. Lots of reverbs only use short allpasses outside of the feedback loop (if at all), and rely on a combination of parallel combs, feedback delay networks, and/or cleverly arranged output taps to get the required echo density. Having said that, so many of the classic digital reverbs seemed to use the allpasses embedded inside of recursive loops, that the artifacts are probably considered desirable for many listeners out there.

I was starting to write about this stuff on my blog (The Halls of Valhalla), but it seems like there are <10 people out there that are really actively pursuing this stuff right now, and most of them are posting to this thread.

Sean

* "Absorbant allpasses" are subject to a patent by Creative Labs. However, the use of them embedded within feedback delay network structures (which can include a single comb) was first mentioned in Jot's 1992 PhD thesis, so the validity of the claims concerning those allpasses is questionable. Plus, Gerzon showed frequency dependent allpasses in his 1972 and 1976 papers, and Date/Tozuka showed analog circuits for implementing these as early as 1966.

2nd July 2009	#177
seancostello ValhallaDSP Joined: Feb 2009 Location: Pacific NW Posts: 1,713	The Black Hole reverb points out one of the fundamental issues with allpass delays in series, or for that matter any sort of filtering operation in series: As the order of the series increases, the impulse response of the output begins to resemble a Gaussian bell curve more and more. This causes a very distinctive "fade in" sound that can be used to great effect if intended, as in the Black Hole reverb, but will cause problems if not intended. I have heard simpler digital reverbs, such as the SilverVerb in Logic, that are clearly based around comb filters with a lot of series allpasses embedded in the delay loop, and a single output tap at the end. The output of such reverbs can be very "open," but will also demonstrate that fading in quality, as opposed to an impulsive attack followed by an exponential decay. The solution is to tap in between allpass stages, as shown in the Gardner and Dattorro papers, and later on in the Dahl/Jot papers with the absorbant allpasses.* This allows you to grab the signal before it becomes overly "Gaussian." Of course, this also creates the parallel allpass artifacts which Casey refers to above. So there is no neat and tidy solution, just a bunch of compromises and heuristic tricks. Or, you can avoid using allpasses altogether. Lots of reverbs only use short allpasses outside of the feedback loop (if at all), and rely on a combination of parallel combs, feedback delay networks, and/or cleverly arranged output taps to get the required echo density. Having said that, so many of the classic digital reverbs seemed to use the allpasses embedded inside of recursive loops, that the artifacts are probably considered desirable for many listeners out there. I was starting to write about this stuff on my blog (The Halls of Valhalla), but it seems like there are <10 people out there that are really actively pursuing this stuff right now, and most of them are posting to this thread. Sean * "Absorbant allpasses" are subject to a patent by Creative Labs. However, the use of them embedded within feedback delay network structures (which can include a single comb) was first mentioned in Jot's 1992 PhD thesis, so the validity of the claims concerning those allpasses is questionable. Plus, Gerzon showed frequency dependent allpasses in his 1972 and 1976 papers, and Date/Tozuka showed analog circuits for implementing these as early as 1966.