[sdiy] Filter slopes

[David G Dixon]

Hi Elain,
 
You should check out the frequency responses for the Intellijel Polaris
filter (which I designed) here:
 
https://intellijel.com/wp-content/uploads/2015/12/Polaris-Frequency-Response
s.pdf
 
You will notice that three of the bandpass modes (B, C, and E) are
asymmetrical, with different slopes on either side of the corner frequency.
The bandpass modes can put different numbers of poles on either side.
Hence, for a four-pole filter, the possibilities are: 1/1, 1/2, 1/3, 2/1,
2/2, and 3/1.  The five bandpass modes available on the Polaris are as
follows:
 
BP A = 1/1 (symmetric)
BP B = 2/1 (asymmetric)
BP C = 3/1 (asymmetric)
BP D = 2/2 (symmetric)
BP E = 1/3 (asymmetric)
 
The 1/2 bandpass mode is not available on the Polaris, because the
powers-that-be at Intellijel didn't want to include it for whatever reason
(it was probably too similar-sounding to one of the others).  These modes
are obtained by summing the four individual stage outputs of a
cascaded-stage (Roland-type) filter in precise gain ratios.
 
Also, there are some interesting notch and double-notch modes available on
the Polaris which will give something like asymmetric bandpass filtering
when resonance is employed.  Some of these modes may get you close to the
instrumental timbres you are looking for.
 
Cheers,
Dave Dixon



  _____  

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Elain
Klopke
Sent: Sunday, June 10, 2018 10:43 PM
To: *SYNTH DIY
Subject: [sdiy] Filter slopes


Hey all,

I was reading an article about the spectral content of various instruments
(woodwinds and strings) and while they didn't have any circuits, there were
some tables showing cutoff frequencies and high and low slopes. Several of
the pictures looked like bandpass filter responses with different slopes on
each side. How would I go about doing that? Is the slope determined by the
gain of the op amp in an active filter? If it's that easy, would it be a
highpass filter followed by a lowpass filter each with their own gain
settings? 

-Ian

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