Proposal to include channelCount constraint/capability

Hello Media Capture,

I'm writing to you with the first of several proposals from the Audio WG
that relate to our earlier Last Call comments. We've worked with the Media
Capture co-chairs to revise our earlier ideas and make sure they fit in
cleanly with the spec as it stands today, and with the history of previous
decisions by the task force.

Summary:

The proposed change will associate a channel count with an audio
MediaTrack, allowing both device discovery (via constraints) and device
description (via capabilities and settings) to account for the number of
audio channels in a track.

Rationale:

Channel counts are fundamental descriptors of audio tracks, as width or
height are fundamental to video tracks (for input, at any rate). Knowing
the potential and real channel counts of device streams are essential for
audio-based apps to present or make meaningful choices between devices, and
to adjust an application's behavior to best fit a given device.

The Audio WG did consider using an enumerated type for channel layouts
(e.g. "mono", "5.1", etc.) but has settled on channel counts because they
are always knowable and discoverable in the UA's native environment, while
knowledge of actual channel layouts are neither universally available nor
standardized. Channel layout constraints could be added in the future if
needed.

Definitions:

The channel count of an audio track is the number of independent channels
of sound that it contains, i.e. the number of audio samples per sample
frame.

Proposed changes:

- Add a *boolean channelCount* attribute to MediaTrackSupportedConstraints.
- Add a *long channelCount* attribute to MediaTrackSettings
- Add a *long/LongRange channelCount* attribute to MediaTrackCapabilities
- Add a *ConstraintLong channelCount* attribute to MediaTrackConstraintSet

Thank you very much for your consideration.

Best,
.            .       .    .  . ...Joe

*Joe Berkovitz*
W3C Audio WG Co-chair

President, *Noteflight LLC*
49R Day Street / Somerville, MA 02144 / USA
phone: +1 978 314 6271
www.noteflight.com
"Your music, everywhere"