[sdiy] Resonator type filters

[Richie Burnett]

The Q factor of each filter is obviously higher for something like a 30 band 
graphic EQ where the bands are 1/3rd octave spaced, than for something like 
a 10 band EQ with 1 oct wide bands.  As you've said the Q factors aren't 
particularly high though in a typical graphic EQ.  In fact I don't think 
they are even fixed in low-cost GEQs, because i've seen much more expensive 
GEQs that are specifically marketed as "constant Q" and are supposedly 
superior.

If you want to simulate body resonances of musical instruments the graphic 
EQ probably isn't the most precise tool to use, and may even fall well short 
in terms of the required Q factor.  I've never tried, but it's quite a 
coarse tool.  I have tried using graphic EQs to correct for nasty room modes 
over the years, and it doesn't work very well.  In both cases I suspect that 
a decent multi-band parametric EQ would be much more precise and better 
suited to the job.  Room modes at bass frequencies tend to have quite high 
Q-factors and are very narrowband, so they demand very narrow notches from 
an equaliser, if room equalisation is your thing.  A graphic EQ is nowhere 
near precise enough in my experience, at least for small home studios.

In the graphic equaliser schematics that I have seen, I don't think you can 
"tap into" the band filters in the way that you mean Mike.  They are usually 
wired such that they appear in the feed-forward / feed-back path of an 
op-amp, so with the sliders set mid-way you get neither gain boost nor gain 
cut.  I don't know off the top of my head how you would make a voltage 
controlled analogue graphic equaliser with VCAs.

In the digital world it's done differently, and everything is just 
coefficients in a big calculation, so you can automate centre frequencies, Q 
factors, boost/cut amounts etc, for each band if you wanted to.

-Richie,


-----Original Message----- 
From: Ian Fritz
Sent: Thursday, June 29, 2017 7:11 PM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] Resonator type filters

Yes, but I have never never seen one with the high resonance and the
large number of stages needed for a string filter -- do you have some
specific examples?  I would love to have a VST string filter but cannot
find any.

Ian


On 6/29/2017 9:46 AM, Richie Burnett wrote:
> A graphic equaliser (analogue or digital) is really just a bank of 30 or 
> so resonant filters.
>
> -Richie,
>
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