arduino-audio-tools: Mixing Effects with the Input Signal

Yesterday, a user was proposing an Arduino Sketch for a Guitar Effects Pedal which is mixing the original input with an effect. Setting up a simple effect is easy, but I have never thought about how to mix the effect with the original input. He was proposing the following process chain (which I think is just ingenious):

                               |-> AudioEffectStream --| 
I2SStream -copy-> MultiOutput -|                       |-> OutputMixer -> I2SStream
                               |-----------------------|

It took me quite some time to figure out why this is not working: The OutputMixer requires consistent write sizes of all it’s input components and the AudioEffectStream seemed to be writing individual samples. Fortunately this can be easily fixed by adding a buffer into the chain:

                 |-> AudioEffectStream -> BufferedStream --| 
... MultiOutput -|                                         |-> OutputMixer ...
                 |-> BufferedStream -----------------------|

The BufferedStream makes sure that the OutputMixer gets fed with a consistent number of samples.

The corresponding sketch for a LyraT board look as follows:

#include "AudioTools.h"
#include "AudioTools/AudioLibs/AudioSTK.h"
#include "AudioTools/AudioLibs/AudioBoardStream.h"

AudioInfo info(44100, 2, 16);
AudioBoardStream i2s(LyratV43);  // or use I2SStream
MultiOutput split;
OutputMixer<int16_t> mixer(i2s, 2);  // output mixer with 2 outputs
BufferedStream effects_buffer(1024, mixer);
BufferedStream direct_buffer(1024, mixer);
AudioEffectStream effects(effects_buffer);
StreamCopy copier(split, i2s, 1024); // copy i2sStream 
Fuzz fuzz(6.5, 500);
STKChorus chorus(6000);
PitchShift pitch(0.5);
Boost boost(1.0);

// Arduino Setup
void setup(void) {
  Serial.begin(115200);
  AudioToolsLogger.begin(Serial, AudioToolsLogLevel::Warning);

  // setup multioutput
  split.add(effects);
  split.add(direct_buffer);

  // setup effects
  //effects.addEffect(fuzz);
  //effects.addEffect(chorus);
  //effects.addEffect(boost);
  //effects.addEffect(pitch);
  effects.begin(info);

  // setup i2s
  auto cfg = i2s.defaultConfig(RXTX_MODE);
  cfg.copyFrom(info);
  cfg.sd_active = false;
  cfg.input_device = ADC_INPUT_LINE2;
  cfg.buffer_size = 256;
  cfg.buffer_count = 3;
  i2s.begin(cfg);
  i2s.setVolume(1.0);

  // setup mixer
  //mixer.setWeight(0, 1.0); // dynamically adjust weights
  //mixer.setWeight(1, 1.0);
  mixer.begin(1024);
}

// Arduino loop - copy data
void loop() {
  copier.copy();
}

I was selecting the LyraT because there it is possible to define the aux input source properly.

It might be a good idea to replace the 1024 values that are scattered across the Sketch with one constant that is defined in the beginning. I have the gut feeling that it’s possible to decrease this value to optimize the RAM use.

I am still impressed that it is possible to write such a complex logic with only 50 lines!