The loudness of a given audio stream on Fuchsia is a product of five factors:
- Usage of the stream
- System-global volume setting for that Usage
- System-global gain adjustment for that Usage
- Stream-local gain adjustment for the stream
- Mute state
Usages are a hint to the system about
the meaning of the audio to a user. Two supported Usages are
MEDIA, for media content such as
music and videos, and
INTERRUPTION for things like alarms that interrupt the user's task. Each
Usage has a separate volume control.
Audio streams such as
AudioCapturers must be tagged with a Usage.
Volume is a floating point value between 0 and 1, where 0 is muted and 1 is the maximum volume for the stream.
MEDIAis set to 0.5 volume and
INTERRUPTIONto 1.0, a stream tagged as
INTERRUPTIONwould play back at half its loudness if played back instead as a
INTERRUPTIONis set to 0 volume, all streams tagged as
INTERRUPTIONare inaudible to the user.
Usage Gain Adjustment
To realize the stream, the Fuchsia audio subsystem must translate volume settings to gain in dbfs for each output device. Since devices have different ranges of gain and different mappings from volume to gain, this translation may result in a different value for each output device.
After this translation, the usage's gain adjustment is applied. The gain adjustment is a persistent setting in units of gain dbfs.
This is useful to enforce deltas between two usages when they are at the same volume.
For example, if
INTERRUPTION are both set to 0.7 volume, but
MEDIA has a gain
adjustment of -10db, a stream tagged as
INTERRUPTION would not be as loud as if it is played back
Stream Gain Adjustment
Another gain adjustment can be applied, directly to the stream. This gain adjustment value is local.
For example, if two
AudioRenderers exist on the system and one has a gain adjustment of -5db, the
other, if unmodified, still has a no-op gain adjustment of 0db.
A stream, or usage, may be muted. When a stream is muted, it is not output to the user. When a usage is muted, no stream tagged with that usage is output to the user.
During mute, other settings such as volume and gain are retained; muted is not the same as 0 volume because the volume may be changed while muted but the stream remains inaudible.
When unmuted, streams will resume output at their previous loudness settings if those were not modified during mute.
Gain Adjustment Considerations
Volume is not a precisely linear mapping to loudness but it is relatively close for the human ear.
Gain is logarithmic, so a gain adjustment has a significantly different effect on loudness depending on the current loudness of the stream to which it is applied.
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