[sdiy] A question about phasers/notch filters

[Mattias Rickardsson]

It's easy to believe that notch filters can be connected in parallel
because we tend to think that a notch has a particular "sound". In
fact it has not, and that's the whole point here. (-:

/mr

On 7 October 2011 09:14, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> 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,
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