mirror of
https://github.com/mollyim/webrtc.git
synced 2025-05-17 07:37:51 +01:00
b0ed12099f
As part of go/coil update code search links to not point to the "master" branch. Bug: chromium:1226942 Change-Id: I0ae9e84ecc660f789a69fe0b226f93bbc39a8a66 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/226081 Commit-Queue: Tony Herre <toprice@chromium.org> Reviewed-by: Harald Alvestrand <hta@webrtc.org> Cr-Commit-Position: refs/heads/master@{#34531}
54 lines
3.4 KiB
Markdown
54 lines
3.4 KiB
Markdown
<?% config.freshness.owner = 'alessiob' %?>
|
|
<?% config.freshness.reviewed = '2021-04-21' %?>
|
|
|
|
# The WebRTC Audio Mixer Module
|
|
|
|
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`](https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/audio/audio_mixer.h)
|
|
and it includes the definition of
|
|
[`AudioMixer::Source`](https://source.chromium.org/search?q=symbol:AudioMixer::Source%20file:third_party%2Fwebrtc%2Fapi%2Faudio%2Faudio_mixer.h),
|
|
which describes an incoming audio stream, and the definition of
|
|
[`AudioMixer`](https://source.chromium.org/search?q=symbol:AudioMixer%20file:third_party%2Fwebrtc%2Fapi%2Faudio%2Faudio_mixer.h),
|
|
which operates on a collection of
|
|
[`AudioMixer::Source`](https://source.chromium.org/search?q=symbol:AudioMixer::Source%20file:third_party%2Fwebrtc%2Fapi%2Faudio%2Faudio_mixer.h)
|
|
objects to produce a mix.
|
|
|
|
## AudioMixer::Source
|
|
|
|
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()`](https://source.chromium.org/search?q=symbol:AudioMixer::Source::GetAudioFrameWithInfo%20file:third_party%2Fwebrtc%2Fapi%2Faudio%2Faudio_mixer.h)
|
|
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](https://tools.ietf.org/html/rfc3550#section-3)).
|
|
|
|
## AudioMixer
|
|
|
|
The interface allows to add and remove sources and the
|
|
[`AudioMixer::Mix()`](https://source.chromium.org/search?q=symbol:AudioMixer::Mix%20file:third_party%2Fwebrtc%2Fapi%2Faudio%2Faudio_mixer.h)
|
|
method allows to generates a mix with the desired number of channels.
|
|
|
|
## WebRTC implementation
|
|
|
|
The interface is implemented in different parts of WebRTC:
|
|
|
|
* [`AudioMixer::Source`](https://source.chromium.org/search?q=symbol:AudioMixer::Source%20file:third_party%2Fwebrtc%2Fapi%2Faudio%2Faudio_mixer.h):
|
|
[`audio/audio_receive_stream.h`](https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/audio/audio_receive_stream.h)
|
|
* [`AudioMixer`](https://source.chromium.org/search?q=symbol:AudioMixer%20file:third_party%2Fwebrtc%2Fapi%2Faudio%2Faudio_mixer.h):
|
|
[`modules/audio_mixer/audio_mixer_impl.h`](https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/modules/audio_mixer/audio_mixer_impl.h)
|
|
|
|
[`AudioMixer`](https://source.chromium.org/search?q=symbol:AudioMixer%20file:third_party%2Fwebrtc%2Fapi%2Faudio%2Faudio_mixer.h)
|
|
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.
|
|
|
|
[^2]: [`audio/utility/channel_mixer.h`](https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/audio/utility/channel_mixer.h)
|
|
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.
|