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Encoding and Decoding of Audio

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Phil Schatzmann edited this page May 19, 2022 · 174 revisions

Compressed Audio

Unfortunately the available memory on Microcontrollers is quite restricted and we do not get very far by storing a (uncompressed) WAV file e.g. in program/flesh memory, so I started to look into compressed audio formats: The compression and decompression can be done with the help of Codecs. Codecs are also important if you need to transmit audio data at a high sampling rate over a line with limited capacity.

On the desktop we can use the FFmpeg project which comes with a rich set of functionality. Unfortunately the situation is much more fragmented for Microcontrollers.

I started to collect the relevant libraries and in order to make things simple to use I also added a simple C++ API on top of the available libraries:

Supported Codecs

Project Class Include Function Format
- Decoder8Bit AudioCodecs/Codec8Bit.h Decoding PCM
- Encoder8Bit AudioCodecs/Codec8Bit.h Encoding PCM
- WAVDecoder AudioCodecs/CodecWAV.h Decoding WAV
- WAVEncoder AudioCodecs/CodecWAV.h Encoding WAV
libhelix MP3DecoderHelix AudioCodecs/CodecMP3Helix.h Decoding MP3
libhelix AACDecoderHelix AudioCodecs/CodecAACHelix.h Decoding AAC
libvorbis-idec VorbisDecoder AudioCodecs/CodecVorbis.h Decoding OGG Vorbis
fdk-aac AACEncoderFDK AudioCodecs/CodecAACFDK.h Encoding AAC
libmad MP3DecoderMAD AudioCodecs/CodecMP3MAD.h Decoding MP3
liblame MP3EncoderLAME AudioCodecs/CodecMP3LAME.h Encoding MP3
libflac FLACDecoder AudioCodecs/CodecFLAC.h Decoding FLAC
libflac FLACEncoder AudioCodecs/CodecFLAC.h Encoding FLAC
minimp3 MP3DecoderMini AudioCodecs/CodecMP3Mini.h Decoding MP3
libsbc SBCDecoder AudioCodecs/CodecSBC.h Decoding SBC
libsbc SBCEncoder AudioCodecs/CodecSBC.h Encoding SBC
liblc3 LC3Decoder AudioCodecs/CodecLC3.h Decoding LC3
liblc3 LC3Encoder AudioCodecs/CodecLC3.h Encoding LC3
libopenaptx APTXDecoder AudioCodecs/CodecAPTX.h Decoding APTX
libopenaptx APTXEncoder AudioCodecs/CodecAPTX.h Encoding APTX
libopus OpusDecoder AudioCodecs/CodecOpus.h Decoding Opus
libopus OpusEncoder AudioCodecs/CodecOpus.h Encoding Opus
libgsm GSMDecoder AudioCodecs/CodecGSM.h Decoding GSM
libgsm GSMEncoder AudioCodecs/CodecGSM.h Encoding GSM
libg722 G722Decoder AudioCodecs/CodecG722.h Decoding G.722
libg722 G722Encoder AudioCodecs/CodecG722.h Encoding G.722
codec2 Codec2Decoder AudioCodecs/CodecCodec2.h Decoding Codec2
codec2 Codec2Encoder AudioCodecs/CodecCodec2.h Encoding Codec2

Decoding

I am also providing an integration into my Arduino Audio Tools where you can use these libraries with the EncodedAudioStream class:

#include "Arduino.h"
#include "AudioTools.h"
#include "AudioCodecs/CodecMP3Helix.h"
#include "BabyElephantWalk60_mp3.h"

MemoryStream mp3(BabyElephantWalk60_mp3, BabyElephantWalk60_mp3_len); // MP3 data source
I2SStream i2s; // final output of decoded stream
EncodedAudioStream dec(&i2s, new MP3DecoderHelix()); // Decoding stream
StreamCopy copier(dec, mp3); // copy in to out

void setup(){
  Serial.begin(115200);

  i2s.begin();
  dec.begin();
}

void loop(){
  if (mp3) {
    copier.copy();
  } 
}

The above stream is implementing the following flow: mp3 MemoryStream -copy-> EncodedAudioStream -> I2SStream This method is used by most codecs.

Streaming Decoding

Ogg and Vorbis are using an alternative methods which is pulling the data directly from an input stream:

#include "AudioTools.h"
#include "AudioCodecs/CodecVorbis.h"

const char* ssid = "ssid";
const char* pwd = "password";
URLStream url(ssid, pwd);
VorbisDecoder dec;
I2SStream i2s;

void setup() {
  Serial.begin(115200);
  AudioLogger::instance().begin(Serial, AudioLogger::Info);  

  i2s.begin(i2s.defaultConfig(TX_MODE));

  url.begin("http://marmalade.scenesat.com:8086/bitjam.ogg","application/ogg");
  dec.setInputStream(url);
  dec.setOutputStream(i2s);
  dec.begin();
}

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

Encoding

The encoding of audio data to a different format is also done with the help of the EncodedAudioStream class. The only difference (to the decoding examples) is that we pass an Encoder as argument.

Here is the related Arduino sketch:

#include "AudioTools.h"
#include "SdFat.h"

uint16_t sample_rate = 44100;
uint8_t channels = 2;                                             // The stream will have 2 channels 
NoiseGenerator<int16_t> noise(32000);                             // subclass of SoundGenerator with max amplitude of 32000
GeneratedSoundStream<int16_t> in_stream(noise);                   // Stream generated from sine wave
SdFat SD;
File audioFile;                                                   // final output stream
EncodedAudioStream out_stream(&audioFile, new WAVEncoder());             // encode as wav file
StreamCopy copier(out_stream, in_stream);                                // copies sound to out

void setup(){
  Serial.begin(115200);
  AudioLogger::instance().begin(Serial, AudioLogger::Info);  

  auto cfg = noise.defaultConfig();
  cfg.sample_rate = sample_rate;
  cfg.channels = channels;
  cfg.bits_per_sample = 16;
  noise.begin(cfg);
  in_stream.begin();

  // we need to provide the audio information to the encoder
  out_stream.begin(cfg);
  // open the output file
  SD.begin(SdSpiConfig(PIN_CS, DEDICATED_SPI, SD_SCK_MHZ(2)));
  audioFile = SD.open("/test/002.wav", O_WRITE | O_CREAT);
}

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

We create an input stream which is based on some sound generator. In the out_stream we indicate the final output stream (which is a file in the example) and the encoder that is used when the data is written: GeneratedSoundStream -copy-> EncodedAudioStream -> File

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