[sdiy] Formant filters, yay!

Elain Klopke functionofform at gmail.com
Thu Mar 9 18:15:23 CET 2017


Ah, thank you! The equation helps a lot. I think I chose 100k as an example
because that was the last thing I remember seeing in an input mixing stage
somewhere.

On Thu, Mar 9, 2017 at 10:48 AM, <mskala at ansuz.sooke.bc.ca> wrote:

> On Thu, 9 Mar 2017, Elain Klopke wrote:
> > For example,
> > Tenor 'o'        f1   f2   f3   f4   f5
> > frequency (Hz)   400  800 2600 2800 3000
> > Amp (dB)         0   -10  -12  -12  -26
> > bw (Hz)          40   80   100  120  120
> >
> > I am, of course, assuming that the signal goes to the five filters in
> > parallel and then the outputs get mixed back together into, say, an op
> amp.
> > f1 would be going through a 100k resistor and then the rest would be
> > progressively higher values of resistors. Yes?
>
> That's one way to do it.  You want the voltage gains to be the square
> roots of the power gains shown, and voltage gains are in turn inverse to
> the resistor values.  So if you're using 100kOhm for amplitude 0dB, then
> for amplitude -10dB you can calculate antilog(-(-10)/20) = 3.162,
> resistance = 316.2kOhm.  Substitute other decibel numbers for the -10 in
> that equation as appropriate.  The overall minus sign accounts for the
> inverse relationship between resistance and gain, and the 20 accounts for
> the square root operation and the conversion from decibels to "bels"
> (powers of ten).
>
> However, using such large resistances is going to introduce some thermal
> noise.  It would be preferable, if you aren't constrained by needing very
> high input impedance for interfacing or something, to start with some
> smaller resistance, like 10kOhm or even less, for 0dB and scale everything
> (including the feedback resistor) accordingly.
>
> --
> Matthew Skala
> mskala at ansuz.sooke.bc.ca                 People before principles.
> http://ansuz.sooke.bc.ca/
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