The WebRTC audio mixer module is responsible for mixing multiple incoming audio streams (sources) into a single audio stream (mix). It works with 10 ms frames, it supports sample rates up to 48 kHz and up to 8 audio channels. The API is defined in api/audio/audio_mixer.h and it includes the definition of AudioMixer::Source, which describes an incoming audio stream, and the definition of AudioMixer, which operates on a collection of AudioMixer::Source objects to produce a mix.
A source has different characteristic (e.g., sample rate, number of channels, muted state) and it is identified by an SSRC[^1]. AudioMixer::Source::GetAudioFrameWithInfo() is used to retrieve the next 10 ms chunk of audio to be mixed.
[^1]: A synchronization source (SSRC) is the source of a stream of RTP packets, identified by a 32-bit numeric SSRC identifier carried in the RTP header so as not to be dependent upon the network address (see RFC 3550).
The interface allows to add and remove sources and the AudioMixer::Mix() method allows to generates a mix with the desired number of channels.
The interface is implemented in different parts of WebRTC:
AudioMixer is thread-safe. The output sample rate of the generated mix is automatically assigned depending on the sample rate of the sources; whereas the number of output channels is defined by the caller^2. Samples from the non-muted sources are summed up and then a limiter is used to apply soft-clipping when needed.
is used to mix channels in the non-trivial cases - i.e., if the number of channels for a source or the mix is greater than 3.