Audio Streaming – The Opus Codec

Codecs play an important role in audio streaming, as we have seen in one of my last blogs.

I have spent some time to make the Opus Codec available as Arduino Library and I am providing some simple integration from my Arduino Audio Tools library.

Opus is a codec for interactive speech and audio transmission over the Internet.

Opus can handle a wide range of interactive audio applications, including Voice over IP, videoconferencing, in-game chat, and even remote live music performances. It can scale from low bit-rate narrowband speech to very high quality stereo music.

Opus packets are not self-delimiting, but are designed to be used inside a container of some sort which supplies the decoder with each packet’s length. Opus was originally specified for encapsulation in Ogg containers.

Nevertheless we can test the codec w/o container.

Arduino Sketch

We can test the codec e.g. with the following Sketch which makes sure that the encoded packets are subsequently decoded again:

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

int sample_rate = 24000;
int channels = 2;  // The stream will have 2 channels

SineWaveGenerator<int16_t> sineWave( 32000);  // SoundGenerator max amplitude of 32000
GeneratedSoundStream<int16_t> sound( sineWave); // Stream generated from sine wave
MeasuringStream out; 
OpusAudioEncoder enc;
EncodedAudioStream decoder(&out, new OpusAudioDecoder()); // encode and write 
EncodedAudioStream encoder(&decoder, &enc); // encode and write 
StreamCopy copier(encoder, sound);     

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

  // Setup sine wave
  auto cfgs = sineWave.defaultConfig();
  cfgs.sample_rate = sample_rate;
  cfgs.channels = channels;
  cfgs.bits_per_sample = 16;
  sineWave.begin(cfgs, N_B4);

  // Opus decoder needs to know the audio info
  decoder.begin(cfgs);

  // configure and start encoder
  enc.config().application = OPUS_APPLICATION_VOIP;
  enc.config().complexity = 5;
  encoder.begin(cfgs);

  Serial.println("Test started...");
}

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

Performance Overview

Please note that the values of the valid supported sample rates is limited to 8000,12000,16000 ,24000,48000 and there are quite a few encoder settings that we can play with, but the most important ones are:

• application
• complexity
• data type (defined in opus_config.h)

Conclusion

By adjusting the complexity parameter we can improve the performance to achieve the rate that is needed by real time processing.
Using the fixed point is faster, so I used this as default setting in the library.

Next Steps

As the next step I need to support of Ogg containers.