[sdiy] Formant filters, yay!

Richie Burnett rburnett at richieburnett.co.uk
Thu Mar 9 19:12:04 CET 2017

I'm a bit suspicious of those tabulated formant values...  The f3, f4 and f5 
formants are all very close together, and those combinations of centre 
frequency vs bandwidth result in some pretty high Q values!  Maybe they are 
right but something seems a bit fishy about them to me.

Also, if those "Amp (dB)" values are the amplitudes of the formant peaks in 
the frequency spectrum, then they may or may not take into account what the 
excitation waveform was.  If you go about making a filter bank and drive it 
from a sawtooth oscillator for example, the sawtooth waveform will already 
have a -6dB/oct rolloff to it's spectrum before you start boosting and 
cutting areas with the formant filters.  Something to bear in mind.


-----Original Message----- 
From: Elain Klopke
Sent: Thursday, March 09, 2017 5:15 PM
To: mskala at ansuz.sooke.bc.ca
Subject: Re: [sdiy] Formant filters, yay!

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 

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.

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