[whatwg] Background audio channels

[Wesley Johnston]

In most situations, when the user puts a webpage in the background, any media being played by the page should be paused. Any attempts to play audio by a background page should also be prevented. However, for some sites (music or radio apps) the user would like to continue to hear the app while they do something else. These pages should be able to designate their audio as a type that should keep playing while in the background. The useragent should also attempt to avoid having the stream killed by the operating system if possible. This is especially true on mobile devices, but the problem is also already prevalent on desktop.

I think semantically we need a way to describe to the useragent how to play a particular track. I'd suggest we add an optional attribute to media elements, "audiochannel", designating the output and priority of this audio. The channel attribute can potentially take on three different values. "normal", "background", and "telephony". 

"normal" channels are the default for all media elements. Using them doesn't require any special permissions. Audio playing with these channels is paused when the web page moves into the background. In addition, calling play on an media element with this channel while in the background will put the element into the paused for user interaction state (i.e. playback won't start until the webapp is brought to the foreground)?

"background" channels will continue to play when the page is put into the background. Trying to play a background channel while in the background should also work. The ability to play audio on this channel may require requesting permission from the UA first (i.e. possibly a prompt when the audio is first played or when moving to the background). If the user doesn't grant permission, these should throw a MediaError (MEDIA_ERR_CHANNEL_PERMISSION_NOT_GRANTED?) so that the page can know what has happened and do something appropriate.

"telephony" channels are similar to "background" channels and can play even if the page is in the background. Playing audio on a telephony channel may cause any audio playing on "normal" or "background" channels to be paused or have their volume severely decreased. They also, on devices where its supported, will likely play over handset speakers rather than normal speakers. Similar to "background", these may require permission from the UA.

Note: This is all based rather loosely on the AudioChannels implementation written for B2G recently [1]. It includes a few other use-cases on its wiki page, along with definitions of additional channels to accomadate them. I've been trying to simplify it down to handle the most common use cases. Finding the correct terminology here is difficult though. For instance, it seems likely that games will see the background channel and think its an appropriate place to play game background music, the exact type of audio you'd like to have paused when you leave the game. Ideas for better ways to describe it are welcome.

[1] https://wiki.mozilla.org/WebAPI/AudioChannels