Second Order All Pass Filter

Adds a new filter class called Soap. Soap is a second order all pass filter that is configured to be either a bandpass or a band reject with independent center frequency and q input parameters. A detailed writeup by Tom Erbe that discusses the theory of the filter can be found here:

http://synthnotes.ucsd.edu/wp4/index.php/2019/11/09/second-order-allpass-filter/

http://synthnotes.ucsd.edu/wp4/index.php/2019/11/10/bandpass-and-bandreject-derived-from-allpass-filter/

Example Output

An electric guitar playing an E major chord is the input to a daisy pod. The q, which is adjustable on Pot 2, is fixed at 50%. The center frequency, which is adjustable on Pot 1, is manually and continuously swept during the video to show the bandpass filter effect.

https://user-images.githubusercontent.com/120678973/230170626-837b8f2a-159d-43d7-beac-8c1e3230b663.mp4

Similarly, an electric guitar playing an E major chord is input into a daisy pod. This time the q on Pot 2 is fixed at 15%, causing the bandpass's bandwidth to be more narrow than the previous example. The center frequency on Pot 1 is manually and continuously swept during the video to show the bandpass filter effect.

https://user-images.githubusercontent.com/120678973/230170715-6f01dcbb-91a1-4150-bf4a-e6b39fb5fce6.mp4

Example Code

This is the source used to generate the videos above. Note that a Daisy Pod was used for its rotary encoder and potentiometers. Turn the rotary encoder until both LED's on the Daisy Pod are BLUE to access the Soap filter. This code is based off of the MultiEffect Daisy Pod example written by Ben Sergentanis.

// Project: SoapFilter Example
//
// Written by: Brian Tice, 5/2/2023

#include "daisysp.h"            // Open Source DSP Library
#include "daisy_pod.h"          // libDaisy, hardware abstraction library for
                                // the daisy platform

#define PI 3.141592653589793

#define TRM 2   // Tremolo Test State. This is just a tremolo with depth and rate
                // LED's are pink in this state. POT1 is rate, POT2 is depth.

#define BNP 1   // Band Pass SVF filter Test State. LED's are GREEN in this state.
                // A test of the built in SVF.
                // POT1 is center frequency, POT2 is q.

#define SAB 0   // Second Order All Pass Bandpass Filter 
                // LED's are BLUE in this state.
                // POT1 is center frequency, POT2 is q. 

using namespace daisysp;
using namespace daisy;

static DaisyPod pod;

static Tremolo                                   trem;
static Svf                                       filt;
static Tone                                      tone;
static Soap                                      soap;
int                                              mode = SAB;

//Helper functions
void Controls();

void GetTremoloSample(float &outl, float &outr, float inl, float inr);

void GetBandPassSample(float &outl, float &outr, float inl, float inr);

void GetSoapSample(float &outl, float &outr, float inl, float inr);

void AudioCallback(AudioHandle::InterleavingInputBuffer  in,
                   AudioHandle::InterleavingOutputBuffer out,
                   size_t                                size)
{
    float outl, outr, inl, inr;

    Controls();

    //audio
    for(size_t i = 0; i < size; i += 2)
    {
        inl = in[i];
        inr = in[i + 1];

        switch(mode)
        {

            case TRM: GetTremoloSample(outl, outr, inl, inr); break;
            case BNP: GetBandPassSample(outl, outr, inl, inr); break;
            case SAB: GetSoapSample(outl, outr, inl, inr); break;

            default: outl = outr = 0;
        }

        // left out
        out[i] = outl;

        // right out
        out[i + 1] = outr;
    }
}

int main(void)
{
    // initialize pod hardware and oscillator daisysp module
    float sample_rate;

    // Inits and sample rate
    pod.Init();
    pod.SetAudioBlockSize(4);
    sample_rate = pod.AudioSampleRate();

    trem.Init(sample_rate);
    filt.Init(sample_rate);
    tone.Init(sample_rate);
    soap.Init(sample_rate); 

    // tremolo parameters
    trem.SetFreq(2);
    trem.SetDepth(0.75);

    // SVF bandpass filter settings
    filt.SetFreq(300.0);
    filt.SetRes(0.85);
    filt.SetDrive(0.8);

    // Soap filter settings
    soap.SetCenterFreq(400.0);
    soap.SetFilterBandwidth(50.0);

    // start callback
    pod.StartAdc();
    pod.StartAudio(AudioCallback);

    while(1) {}
}

void UpdateKnobs(float &k1, float &k2)
{
    k1 = pod.knob1.Process();
    k2 = pod.knob2.Process();

    switch(mode)
    {

        case TRM:
            trem.SetFreq(k1*3);
            trem.SetDepth(k2);
        case BNP:
            filt.SetFreq(k1*3000);
        case SAB:         
            soap.SetCenterFreq(k1*1000.0);           
            soap.SetFilterBandwidth(k2*100.0);
    }
}

void UpdateEncoder()
{
    mode = mode + pod.encoder.Increment();
    mode = (mode % 3 + 3) % 3;
}

void UpdateLeds(float k1, float k2)
{
    pod.led1.Set(
        k1 * (mode == 2), k1 * (mode == 1), k1 * (mode == 0 || mode == 2));
    pod.led2.Set(
        k2 * (mode == 2), k2 * (mode == 1), k2 * (mode == 0 || mode == 2));

    pod.UpdateLeds();
}

void Controls()
{
    float k1, k2;

    pod.ProcessAnalogControls();
    pod.ProcessDigitalControls();

    UpdateKnobs(k1, k2);
    UpdateEncoder();
    UpdateLeds(k1, k2);
}

void GetTremoloSample(float &outl, float &outr, float inl, float inr)
{
    outl = trem.Process(inl);
    outr = outl;
}

void GetBandPassSample(float &outl, float &outr, float inl, float inr)
{

    filt.Process(inl);
    outl = filt.Band();
    outr = outl;

}

void GetSoapSample(float &outl, float &outr, float inl, float inr) 
{   

    soap.Process(inl);
    outl = soap.Bandpass() * 3;
    outr = outl;
}

Future Work

Remove trig calculations and make a lookup table for the coefficients for c and d. this should speed up the algorithm

electro-smith / DaisySP

Add a Second Order All Pass Filter called Soap #182

Second Order All Pass Filter

Example Output

Example Code

Future Work