Add support for description audio tracks in HLS

[OwenEdwards]

Description

An audio track can be included in an HLS mux which contains description for blind or low-vision users. This track is indicated by including a CHARACTERISTICS key, with value public.accessibility.describes-video.

This PR, and m3u8-parser PR 14, mark the kind of the audio track to indicate that it is main-desc (main and description), so that the player can specifically handle this track in a different way.

Please describe the change as necessary.

Use CHARACTERISTICS of an audio track when determining its kind.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[OwenEdwards]

If/when PR 14 gets pulled into m3u8-parser and released, I'll update the package.json file to reflect the version of that package that is needed to support this.

[gesinger]

Thanks for the PR! Might be good to have a couple tests though (probably easiest to have audioTrackKind_ as a function outside of MPC's class and can export it to make testing it in isolation easier).

[gesinger]

Minor, but can just have one conditional:

if (properties.characteristics &&
    properties.characteristics.indexOf('public.accessibility.describes-video') >= 0)

[OwenEdwards]

Sure - I wasn't sure how to deal with the line getting too long! I'll change that.

[gesinger]

Is it possible for there to be two tracks with 'public.accessibility.describes-video' (something like an alt-desc)?

[OwenEdwards]

In theory that's possible; I'd assume it would need to follow the same rules as, say, a CAPTIONS or SUBTITLES tracks:

https://tools.ietf.org/html/draft-pantos-http-live-streaming-20#section-4.3.4.1.1:

All EXT-X-MEDIA tags in a Playlist MUST meet the following
constraints:

o All EXT-X-MEDIA tags in the same Group MUST have different NAME
attributes.

o A Group MUST NOT have more than one member with a DEFAULT
attribute of YES.

o All members of a Group whose AUTOSELECT attribute has a value of
YES MUST have LANGUAGE [RFC5646] attributes with unique values.

So you could tell them apart by name (label) and language. But it wouldn't change the audio track's kind. Is that what you were wondering about?

[OwenEdwards]

I'll take a look at how to set up some tests.

[mjneil]

Thanks for the contribution @OwenEdwards