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Delay effects theory

5/26/1998 7:35 PM
David B.
Delay effects theory
I'm interested in the theory behind delay based effects especially phasers and flangers.  
ANYTHING on the subject is GREATLY appreciated.  
THanx, David
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5/27/1998 6:04 AM
Both phasers and flangers operated by doing time delays that are comparable to the time of one or a few cycles of sound in the band of interest. This delay means that there are always certain frequencies delayed by integer multiples of 360 degrees or odd multiples of 180 degrees. Much longer delays do the same, but you also hear the delayed transients like pick attack as discrete echoes. Phasers and Flangers are too short a delay to easily hear echoes of the transients.  
When mixed back with the "dry" signal, the 360 degree delays reinforce at those frequencies, and make a 3db hump, not all that noticeable, but the 180 degree shifts make 20 or more db notches, VERY audible. Varying the amount of delay makes the notches even more aubible. The EH Electric Mistress also had a setting that killed the variation of delay and made for a "filter matrix" sound.  
If you add feedback around the delay line, the 3db humps get reinforced, and you can get  
peaks on those frequencies, to make a sharper, more resonant sound.  
Phasers get their delay from the group delay of RC filters, which is itself a frequency variable effect, and flangers get their delay from pure time delay. Phasers get one notch/hump from each pair of RC filter sections, flangers have a whole "comb" of notches/humps spaced out depending on the amount of delay, so flangers sound more intense in almost all cases, as the number of notches/humps is larger, and the spacing depends on the delay.  
The Journal of the Audio Engineering Society published some articles on phasing/flanging/ chorus, etc back in the late 70's and early 80's if you have a technical library you can pore through.

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