[Stk] Wah-wah filter? UPDATE for STK using BiQuad

[Perry Cook]

Howdy!!  My recent hacking of the Raspberry Pi have renewed my
enthusiasm for, and appreciation of, STK.  

Today's Wah Wah update for the STK people.  I dug into ChucK's ResonZ 
filter source code, and it turns out that it (of course) inherits from BiQuad.

I did a test comparing ResonZ and BiQuad (in ChucK), and they seem
to generate basically the same thing as long as I set the coefficients correctly.  
Duh, huh?

I hacked a new version of the STK effects project, adding WahWah as
a radio button and condition.  Fairly minimal changes to effects.cpp, 
and adding BiQuad.o to the Makefile.  That new project all lives at:

http://www.cs.princeton.edu/~prc/STKHacks/effectsWithWah.zip

Punchline:

You can make a BiQuad (or subclass it) , and set equal gain zeroes,
and set the frequency and radius thusly:

// given a float value named "temp" ranging from 0.0 to 1.0
// and an STK BiQuad filter named wahwah,
// in the main controller service functions, add this:

case 22: // effect parameter change 1
.
.  OTHER STUFF for Other EFX
.

{  // new block for wahWah servicing
          float freqs[3] = {460.0,840,2250}; // JOS data
          float Qs[3] = {9.4,4.0,2.0};      // from his pedal
          float myFreq, myQ;
          if (temp < 0.5) {
             myFreq = freqs[0] +
                ((freqs[1]-freqs[0])*2.0*temp);
             myQ = Qs[0] +
                ((Qs[1]-Qs[0])*2.0*temp);
          }
          else  {
             myFreq = freqs[1] +
                ((freqs[2]-freqs[1])*2.0*(temp-0.5));
             myQ = Qs[1] +
                ((Qs[2]-Qs[1])*2.0*(temp-0.5));
          }
          data->wahwah.setEqualGainZeroes();
          float pFreq = myFreq * TWO_PI / SRATE;
          float bandwidth = pFreq / myQ;
          float radius = 1.0 - (bandwidth * 0.5);
          data->wahwah.setResonance(myFreq,radius,1);
      }

********************************************************
// For the bilingual, here's my ChucK test program to
// compare the ResonZ to BiQuad:

Noise nz => ResonZ wah => WvOut left => dac;
nz =>       BiQuad wah2 => WvOut right => dac;
"left.wav" => left.wavFilename;
"right.wav" => right.wavFilename;
    
0.3 => nz.gain;

    [460.0,840,2250] @=> float freqs[]; // JOS data
    [9.4,4.0,2.0] @=> float Qs[];      // from his pedal
    
1 => wah2.eqzs;

for (0 => int i; i < 3; i++)  {
    (freqs[i],Qs[i]) => wah.set;
    freqs[i]*6.28/44100.0 => float pFreq;
    pFreq / Qs[i] => float bandwidth;
    1.0 - (bandwidth * 0.5) => float radius;
    freqs[i] => wah2.pfreq;
    radius => wah2.prad;
    (1.0 - radius*radius) * 0.5 => wah2.gain;
    2.0 :: second => now;
}

left.closeFile();
right.closeFile();

/// END