Issue with mixed generated noise output after mixed mp3 stops playing

[level0dev]

As a prefix, I'm new to this library, ESP32 in general and even coding with C, so please excuse my inexperience. This might be an expected behavior, but I want to check before I try to solve the issue on my own.

I'm working on a project where I would add "radio noise" to mp3 files stored on SD card, my project is not too dissimilar from @alastaira's London Calling Prop. The project uses the A1S audiokit board.

My current approach is to generate static noise and mix it with an mp3 file stored on SD card. I achieved this with the below code based on this discussion.

The code mixes noise with the mp3 file as expected, but when the mp3 file has finished playing the noise turns into a clicking sound. I would expect the noise to keep playing as normal after the file has finished.

Is this clicking sound/change to the noise output an expected behavior or a bug?

• if a bug I'm happy to open an issue.
• If expected, how can I keep the generated noise playing normally after the mp3 stops playing?

#include <SPI.h>
#include <SD.h>
#include "AudioTools.h"
#include "AudioTools/AudioLibs/AudioBoardStream.h"
#include "AudioTools/AudioCodecs/CodecMP3Helix.h"


const int chipSelect=PIN_AUDIO_KIT_SD_CARD_CS;
File audioFile;
AudioBoardStream i2s(AudioKitEs8388V1); // final output of decoded stream
InputMixer<int16_t> mixer;

EncodedAudioStream decoder(&i2s, new MP3DecoderHelix());

PinkNoiseGenerator<int16_t> Noise;
GeneratedSoundStream<int16_t> NoiseIn(Noise);

VolumeStream volumeSong(decoder);
VolumeStream volumeNoise(NoiseIn);

StreamCopy copier(i2s, mixer);

// Arduino Setup
void setup(void) {
  Serial.begin(115200);
  while (!Serial)
    ;
  
  auto config = i2s.defaultConfig(TX_MODE);
  config.sd_active = true;
  i2s.begin(config);

  SD.begin(chipSelect);
  audioFile = SD.open("/test2.mp3");
  
  if (!audioFile) {
    Serial.println("Failed to open MP3 file!");
    stop();
    return;
  } else {
    Serial.println("Opened the MP3 file");
  }
  Serial.println("Card Mount succeeded");

  NoiseIn.begin(config);
  Noise.begin();

  volumeSong.setVolume(0.8);
  volumeSong.begin();
  volumeNoise.setVolume(0.3);
  volumeNoise.begin();
  decoder.setStream(audioFile);
  decoder.transformationReader().setResultQueueFactor(14);
  decoder.begin();

  mixer.add(volumeSong);
  mixer.add(volumeNoise);
  mixer.begin(config);
  copier.begin();
}

void loop() {
  copier.copy();
}

[pschatzmann]

Please follow the advice given here

[level0dev]

Thanks - so it's an expected behavior due to the mixer expecting equal amount of data from its sources.

What is the proper way to dynamically change the amount of sources a mixer is expecting?

I would like to change what the mixer is expecting each time a file finishes, so the mixer plays the remaining source(s) correctly. Then, when a new mp3 starts playing, I would change it back to expect the extra source.

Is simply calling mixer.remove() and mixer.add() when the mp3 stops/starts the answer?