Nice PDF Describing Digital Implementation of Phase Shift/Delay Effects

[DiyFreaque]

I picked this link up from the Synth DIY list - Antti Huovilainen has authored a very nice paper on the digital phase shifting and chorus/flanging entitled "Enhanced Digital Models for Analog Modulation Effects".

http://dafx05.ssr.upm.es/Proc_DAFx05/P_155.pdf

Even though the thrust is towards digital implementation, there is a lot of good info on what flanging, chorus, phasing and companding are all about. 

Back when I was working on the Dim C, Antti sent me samples of a digital implementation of the DC-2 he'd worked up (he was wondering how close it came) - it sounded incredible - very close, if not spot on.  Made me wish I had the chops to do that kind of stuff.

RG Keen is mentioned in the References section.

Cheers,
Scott

[puretube]

 

[bioroids]

Thanks for sharing! Very interesting

Miguel

[bassmeister]

Lots of PDFs on similar subjects can be found at www.dafx.de. The site is in english, in spite of the top domain name. Perhaps not aimed at DIY analog stombox makers, but it might be worth checking out.

[puretube]

missing: Mike Beigel & Elliott Randall in the props...

[edit] and, err, Sean Costello mighta coulda been in there, too 

[SeanCostello]

[edit] and, err, Sean Costello mighta coulda been in there, too 

Nah, I don't need to be in there. Mike Beigel definitely should have been referenced, though. The Julius Smith paper on phasing uses 2nd order sections, which is nifty, but totally different than your standard analog phase shifter. The Beigel paper from 1979(?) refers to phase shifting using 1st order allpasses, bilinearly transformed from analog to digital. Dennis Cronin published a nice paper on the subject, but the Beigel one is the earliest paper on a direct digital equivalent of a phase shifter that I have seen. I would have quoted William Hartmann as well, but that is just me. So, Antti shouldn't have proped me, but he should have proped some of my props. 

I'm surprised that Antti just glossed over the issues with the placement of the resonant peaks with feedback in discrete time phase shifters. He seems to think it isn't a big deal, but I know that at least one high-end plugin company (Universal Audio) felt that the behavior deviated enough from continuous time that they ended up deriving their own filter equations to synthesize the output of the phase shifter directly, instead of via the standard allpass cascade mixed with direct signal. Personally, I don't think that the precise placements of the peaks is all that important either, but this tends to be the sort of issue that is important in academia. Of course, the vast majority of DSP engineers in academia don't know about the audio uses of time-varying allpass filters, let alone the difference between the Tom Oberheim Maestro phaser and the Bob Moog "silver wedge" Maestro phaser.

I think that the simulation of nonlinear analog phase shifters is very interesting, but the simulation of BBD-based effects I find pretty boring. In digital, you can program chorus effects that are far more interesting than any BBD-based effect, as well as simulating classic string ensemble choruses, tape flanging, and good stuff like that. No matter how accurate a simulation of that Dimension-D one has, I would still take the 6-voice chorus from the Lexicon 224XL any day of the week (and that is a digital effects unit from 1979!).

Sean Costello

[A.S.P.]

information lost...