The loudness of a given audio stream on Fuchsia is a product of five factors:
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 AudioRenderers and 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.
Some examples:
MEDIA is set to 0.5 volume and INTERRUPTION to 1.0, a stream tagged as INTERRUPTION would play back at half its loudness if played back instead as a MEDIA stream.INTERRUPTION is set to 0 volume, all streams tagged as INTERRUPTION are inaudible to the user.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 MEDIA and 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 tagged as MEDIA.
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.
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.