Parametric EQ Schem

[gstopp at fibermux.com]

>
>Have someone noted that I kind of enjoy fiddling about with filters? :)
>

Oh yes we notice, happy to hear your stories! Certainly a worthy field of 
study... many interesting places to go on the S-plane. Yet another example of 
imaginary numbers giving real results.

Although it is interesting to speculate on the utilization of state-variable 
filters as parametric EQ elements, there comes a point where we must stop and 
ask ourselves exactly what kind of box we are trying to design. In other words, 
what are the user specifications? I just realized that I don't even have a 
paremetric EQ anywhere in the depths of my equipment pile, except maybe for an 
SPX-90 patch or a D-50 effect. All the EQ's on my Mackies are fixed.

What are the controls and capabilities of a standard parametric? Of a cheap one?
Of a high-end one? Anybody got schematics for the Moog unit?

- Gene
gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: Re: Re[2]: Parametric EQ Schem
Author:  Magnus Danielson <magda at it.kth.se> at ccrelayout
Date:    3/7/97 6:18 AM

     <much filter math snipped>
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As for Gene's whish to have a variable bandwidth once the Q value is 
low (sigma is way up there, the poles have little influence) you could 
start playing with mixing in some LP or HP, but this is *very* 
uncommon... also, recall that the bandwidth is a function of where the 
-3db points happends show up. Are you sure you are looking for 
diffrent bandwidth? I could go on a deeper discussion (beware!) on 
this, but I want to be sure that we talk about the same things. 
Besides, moving the zeros from the endpoints will just sharpen the 
bandwidth, so I don't see how that would help out. An sideeffect of 
moving the zeros is that you will no longer have a ture bandpass 
filter, so the response migth increase at other places (base and top) 
than intended to modify. The only thing that would seriously act as 
Gene think is a 4 pole EQ, but I am not sure that this is what he 
really means or that an 4 pole EQ would be musically usefull except 
for making nasty things if sweepable...
     
The lowpass/highpass solution that Gene where talking about is in a 
way equalent to just fuddeling about with the output summings of LP, 
BP, HP and input signal that I described, but I think that the output 
summing case is much easier to get into shape with combind with normal 
Q values.
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Have someone noted that I kind of enjoy fiddling about with filters? :)