[PD] allpass~ and phase shifting

[Sean Costello]

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 at yahoo.co.uk>
To: <pd-list at 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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