[sdiy] Pole-mixing, Highpass filters, and switching the first stage off

[Richie Burnett]

I think you can do that Tom.  Switching out the first capacitor (really this 
just reduces the capacitance here and moves the first pole way up to a very 
high frequency,)  is really just another way of getting access to the raw 
input waveform whilst using only a 4 input mixer.  This is instead of having 
to have a mixer with five weighted inputs instead of four.  The price you 
pay is that you can't achieve a 4-pole HPF response though, unless you mix 
the input with a weighted mix of the 4 separate pole outputs.

The "pole-mixing" is a cheap way to get multi-mode responses but the 
high-pass and band-pass responses are a little funky looking when resonance 
is introduced, because of the way the poles split as feedback is applied. 
Not really what you would expect them to look like on paper, but good enough 
for making music ;-)

Also you can get strange responses like a "1st order 6dB/oct" filter with 
adjustable resonance.  Of course a 1st order filter technically cannot be 
resonant at all because that requires pairs of poles, and the response is 
really 4th order as soon as any feedback is applied around the loop. 
Pspice, Multisim or MATLAB are great for graphing all of these different 
responses!

-Richie,



-----Original Message----- 
From: Tom Wiltshire
Sent: Friday, September 05, 2014 11:19 PM
To: synthdiy diy
Subject: [sdiy] Pole-mixing, Highpass filters,and switching the first stage 
off

Hi All,

I've got a question about pole-mixing filters of the Oberheim Xpander/Matrix 
type. Olivier Gillet gives a nice explanation of the concept:

http://mutable-instruments.net/static/documentation/pole_mixing.pdf

and the schematic..

http://mutable-instruments.net/static/schematics/Shruthi-Analog-PoleMixing-v02.pdf

Olivier's filter does the same as the original Xpander filter and switches 
out the first stage for some of the responses.

The point of this seems to be to allow access to the input signal directly, 
since now the output of the first stage is just the input (or the input 
inverted, anyway) which is required for the high pass filters.

The question is this - given that we're just adding poles into a mixer, why 
not just add a resistor directly from the input, instead of going to all 
this trouble of switching the first stage out?
What am I missing? Is my assumption about the purpose of the switch wrong?

Thanks,
Tom


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