[sdiy] A question about phasers/notch filters

[Richie Burnett]

Hi Tom,

The allpass filter stages in the phaser need to go in series.  Each stage passes
all frequencies with equal amplitude but imparts a phase shift curve to the
signal as you go through it's "cutoff frequency".  As you said each time you
pass the signal through another allpass stage it imparts more phase shift.  All
of the allpass stages are typically of the same design so the phase shifts
contributed by each stage all add up.

The aim of the game with a phase if you like is to bugger about with the phase
of the incoming signal as much as you can by passing it through an allpass
filter many times.  Then mix it with the original signal again so that you get
lots of constructive and destructive interference.  You get notches when the
phase of the "buggered about signal" is 180 degrees relative to the original
signal because they cancel out when added.  However if you can shift the phase
of the signal by as much as 540 degrees or even 900 degrees you will get
destructive interference again and another produce another notch!

The more allpass stages you put in series the greater the total final phase
shift and the more chance that you will have several notches occuring somewhere
within the audio band.  Think of how a chorus or flanger works:  Here you delay
one version of the signal by lots of milliseconds before combining it with the
original and it introduces hundreds of peaks and notches!

If you did put lots of analogue allpass stages in PARALLEL and combined their
outputs, each stage would contribute at most 180 degrees of phase shift, so you
could only have a single notch at best.  Much better to cascade them to impart
the maximum phase shift to one signal, and leave the original signal un-shifted
before mixing the two together.

-Richie,