Hi Stefan:
The term "allpass" refers to a filter that has a gain of 1 for all frequencies. There are lots of topologies (first order, second order, lattice), but the main types you will encounter in computer music are as follows:
- Delay-based allpasses, where the allpass filter contains a delay line with
a large number of samples. These are usually used in reverb construction.
- Lower-order filter allpasses, such as first-order and second-order
allpasses. These are less common in computer music languages. Csound has them (phaser1 and phaser2 - I programmed them a few years back), and MAX/MSP has a 2nd order allpass (called phaseshift~). Usually, several of these are arranged in series, and the output of the series is mixed with the input signal to produce the classic phase shifting effect.
I am not sure if anyone has programmed first or second order allpasses for PD. You can definitely use biquad~ to obtain a 2nd-order allpass response. For that matter, you could get 2 first order allpasses out of biquad~ as well - you need to multiply together the transfer functions to get the proper coefficients.
The one issue with using biquad~ (or phaseshift~, for that matter), is that the classic analog phase shifter uses a bunch of first order sections in series, with feedback around the entire network. The feedback needs to have a vector size of 1 to sound correct, while the filter coefficients do not necessarily have to be computed at the sampler (they can be computed at a slower rate).
Sean Costello
----- Original Message ----- From: "Stefan Turner" stefan_turner@yahoo.co.uk To: pd-list@iem.at Sent: Friday, April 09, 2004 3:01 AM Subject: [PD] allpass~ and phase shifting
Hi list,
I am trying to figure out how to write an average guitar-pedal-style (to start off with) phase shifter using allpass~, but from what I have found about digital phase shifters, you need to use allpass filters where the delay of each input sine component is dependant upon the frequency. As far as I can tell, allpass~ only allows a constant delay. So while you can do a comb filter using it, I don't see how phase-shifting is possible. Any help would be appreciated!
Cheers Stefan
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