# [sdiy] Multimode pole-mixing - building notch responses

Andrew Simper andy at cytomic.com
Sun Sep 16 14:47:43 CEST 2018

```Hi Tom,

I think if you want a visual understanding you need to get to grips with
Laplace domain pole zero plots and what amplitude and phase plots mean in
terms of them. Check out this document for starters:
http://www.ee.ic.ac.uk/pcheung/teaching/ee2_signals/Lecture%209%20-%20Poles%20Zeros%20&%20Filters.pdf

In the end, for notches, you end up having zeros at +-jw and also need some
poles to balance them out, but not be quite as strong (further away from
the j axis), and their position determines how things are shaped away from
the notch.

Cheers,

Andy

On Sun, 16 Sep 2018 at 19:35, Tom Wiltshire <tom at electricdruid.net> wrote:

> Hi Richie,
>
> Thanks for this.
>
> Yes, I’ve got some calculations you sent me and an excel sheet David D
> did, so I’m good for doing the math. What I felt I was lacking was any real
> visual understanding of what’s going on. The equations alone aren’t enough
> for my brain to get it - not a maths brain in that sense. I need to be able
> to see it in my mind’s eye, and for that reason your rubber sheet helps a
> lot!
>
> Tom
>
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
>
> > On 16 Sep 2018, at 00:08, Richie Burnett <rburnett at richieburnett.co.uk>
> wrote:
> >
> > Hi Tom,
> >
> > Notch filters have pairs of complex zeros (roots of the transfer
> function's numerator) that pull the frequency response down towards zero.
> Visualise a rubber sheet suspended above the ground with a pair of tent
> pegs pinning it to the ground at particular points.  (When you walk along
> the jw axis in s-domain the closer you go to a "zero" the deeper the notch
> is.)
> >
> > Conversely, resonant filters have pairs of complex poles (roots of the
> transfer function's denominator) that push the magnitude response up.
> Visualise a rubber sheet with tent "poles" pushing it up to great height at
> particular points.  (When you walk along the jw asix in the s-domain the
> closer you get to a pole the more the gain peaks at that frequency due to
> "resonance.")
> >
> > Combinations of poles and zeros allow you to get particular frequency
> responses, and allow you to control things like notch width and depth.
> >
> > The equations Don posted are s-domain prototype filter equations.  The
> cutoff frequency is 1 rad/s.  In practice the s terms are scaled to move
> the cutoff frequency to an arbitrary number of Hz, rad/s or whatever.
> Often scaled exponentially with voltage ;-)
> >
> > The best way to calculate all the transfer functions from that
> pole-mixing arrangement is to first work out what the s-domain transfer
> function (TF) is for each of the taps between the cascaded poles without
> any global feedback:
> >
> > A = 1
> > B = 1 / (s+1)
> > C = 1 / (s+1)^2
> > D = 1 / (s+1)^3
> > E = 1 / (s+1)^4
> >
> > Then rewrite all of these TFs with a common denominator of (s+1)^4 and
> expand them...
> >
> > A = (s^4 + 4*s^3 + 6*s^2 + 4*s + 1) / (s^4 + 4*s^3 + 6*s^2 + 4*s + 1)
> > B = (s^3 + 3*s^2 +3*s + 1) / (s^4 + 4*s^3 + 6*s^2 + 4*s + 1)
> > C = (s^2 + 2*s + 1) / (s^4 + 4*s^3 + 6*s^2 + 4*s + 1)
> > D = (s + 1) / (s^4 + 4*s^3 + 6*s^2 + 4*s + 1))
> > E = 1 / (s^4 + 4*s^3 + 6*s^2 + 4*s + 1)
> >
> > You can then work out what weightings you need to apply to each of the
> taps when mixing to get whatever TF you want.  I think that's how I did it.
> >
> > I thought I sent you an Excel sheet with my calculations for this a
> couple of years ago, Tom?
> >
> > -Richie,
> >
> >
> >
> > -----Original Message----- From: Tom Wiltshire
> > Sent: Saturday, September 15, 2018 7:44 PM
> > To: Donald Tillman
> > Cc: SDIY List
> > Subject: Re: [sdiy] Multimode pole-mixing - building notch responses
> >
> > Hi Don,
> >
> > Sorry, I didn’t explain myself well enough. I’m talking about a
> pole-mixing filter like the Xpander (multimode that way) not a
> state-variable filter (multimode a different way).
> >
> > State variable I get well enough, but I hadn’t really considered the
> notch response there either, so thanks for the explanation. The S domain
> maths is the same for the pole-mixing case, so I’m looking for  (s^2 + 1) /
> (s^2 + s + 1) for a 2-pole notch, right?
> >
> > What does the equation for a 4-pole notch look like? Can you do a 3-pole
> notch? What does that look like?
> >
> > Thanks,
> > Tom
> >
> > ==================
> >      Electric Druid
> > Synth & Stompbox DIY
> > ==================
> >
> >> On 15 Sep 2018, at 19:24, Donald Tillman <don at till.com> wrote:
> >>
> >>
> >>> On Sep 15, 2018, at 10:46 AM, Tom Wiltshire <tom at electricdruid.net>
> wrote:
> >>>
> >>> Can anyone explain pole-mixing notch responses to me in a way that I
> can “get”?
> >>>
> >>> Most of the responses from a multimode pole-mixing filter I
> understand, but I’m struggling with notches.
> >>>
> >>> The bandpass responses, for example, are the same as the highpass
> responses, but “shifted right” to effectively make them into a highpass and
> a lowpass in series - a bandpass. That’s easy to understand. Intuitive,
> even.
> >>
> >> Hey Druid,
> >>
> >> That's not really accurate.  Assuming a 2-pole multimode filter, the
> lowpass will have a 12dB/oct slope down.  The highpass will have a 12dB/oct
> slope up.  But the bandpass will have a 6dB/oct slope on each side.
> >>
> >> So it's not like a highpass and lowpass in series.  It's much more like
> a lowpass tilted counterclockwise with a 6dB/oct boost throughout.
> >>
> >>> I guess the trouble is I don’t really know what a notch filter looks
> like in terms of the s-domain equations, so it’s hard to see how the
> pole-mixing leads to that result. Even the Allpass responses are easier,
> but in that case, I *do* know what the s domain equation is supposed to
> look like (s-1/s+1 for the one-pole, s^2-s+1 / s^2+s+1 for the two-pole,
> right?).
> >>
> >> The notch function simply adds the lowpass and highpass signals
> together. The secret is that, at resonance, the lowpass response shifts the
> phase 90 degrees behind, the highpass response shifts the phase 90 degrees
> forward, leaving a 180 degree difference between them, and adding them
> together cancels completely
> >>
> >> For the equation, we add the highpass "s^2" term and the lowpass "1"
> term, and the "s" term isn't used:
> >>
> >> (s^2 + 1) / (s^2 + s + 1)
> >>
> >>
> >> Since "s" is complex frequency, s^2 has a value of -1 at resonance, and
> the -1 and +1 cancel for the notch.
> >>
> >> -- Don
> >> --
> >> Donald Tillman, Palo Alto, California
> >> http://www.till.com
> >>
> >
> >
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