Filter Cutoff Modulation

There are multiple sources of cutoff modulation built into the SynthLab example synths and detailed in the synth book. Here is the update() function that implements this modulation; no other code needs to be modified. Note the use of key track, EG and bipolar modulation along with the necessary bounding function to make sure the cutoff frequency is within range. Notice how the modulation input array values are accessed for the bpFmodSemitones and egFmodSemitones variables.

bool FilterModule::update()
{
     // --- to be modulated
     double filterFc = parameters->fc;

     // --- bipolar freqmod (0.5 is to split the total range)
     double bpFmodSemitones = 0.5*freqModSemitoneRange * getModulationInput()->getModValue(kBipolarMod);

     // --- EG input here
     double egFmodSemitones = freqModSemitoneRange *  getModulationInput()->getModValue(kEGMod);

     // --- setup keytrack fc mod
     double ktFmodSemotones = 0.0;

     // --- key tracking
     if (parameters->enableKeyTrack)
     {
          // --- key track amount
          ktFmodSemotones = getModKnobValueLinear(parameters->modKnobValue[MOD_KNOB_A], -48.0, +48.0);

          // --- overwrite fc
          filterFc = midiPitch;
     }

     // --- sum modulations
     double fcModSSemis = bpFmodSemitones + egFmodSemitones + ktFmodSemotones;

     // --- multiply by pitch shift factor
     filterFc *= pow(2.0, fcModSSemis / 12.0);
     boundValue(filterFc, freqModLow, freqModHigh);

     // --- setup biquad structures and load with coefficients depending on filter type
     if (parameters->filterIndex == vicLPF2)
     {
          -- Rest of function is identical from this point onward...
//

Testing the Modulation
Notice how the EG and bipolar modulation values are parsed and used in the update equation (the theory is in the synth book). For testing, we can either manipulate those values directly or connect the filter to the LFO the way you did in the MinSynth voice object. We will manipulate them directly here. An easy place to do this is in the code for updating the filter. Start with the modulators at 0.0 each before trying different values. The modulation code does not include the intensity control so they are technically running at 100% (1.0). When you adjust either of the modulation values a bit, there will be a large swing in the new cutoff frequency. Set a breakpoint in the calculation to watch this happen. Note that the filter's fc can go right up to the Nyquist frequency so if using the bandpass filter, you may hear nothing if the center frequency is outside the bounds of the oscillator's spectrum.

// --- get parameters
std::shared_ptr<SynthLab::FilterParameters> filterModuleParameters = filterModule->getParameters();
if (filterModuleParameters) // should never fail
{
     // --- set the variable
     filterModuleParameters->filterIndex = 1; // LPF2
     filterModuleParameters->fc = 500.0;
     filterModuleParameters->Q = 20.0;

     // --- set modulation values
     filterModule->getModulationInput()->setModValue(SynthLab::kEGMod, 0.0);
     filterModule->getModulationInput()->setModValue(SynthLab::kBipolarMod, 0.0);
     
     // --- update phase
     filterModule->update();
}
//