[Hls-interest] Use of characteristic tags for Subtitles in HLS streaming

[James Hart <[e-mail]>]

Hello,

I am trying to determine the correct use of the characteristics tags for
subtitles in HTTP LIve Streaming and i am a little confused by the various
sources of information and hoping some clarifications can be provided on
what devices require:

>From RFC-8216-09
<https://datatracker.ietf.org/doc/html/draft-pantos-hls-rfc8216bis-09#section-4.4.6.1>
i
see:

A SUBTITLES Rendition MAY include the following characteristics:
> "public.accessibility.transcribes-spoken-dialog",
> public.accessibility.describes-music-and-sound", and
> "public.easy-to-read" (which indicates that the subtitles have been edited
> for ease of reading).


>From the HLS Authoring Specification
<https://developer.apple.com/documentation/http_live_streaming/hls_authoring_specification_for_apple_devices>
for Apple Devices i see only one characteristic defined:


4.5. If a subtitles track is intended to provide accessibility for people
> who are deaf or hard of hearing, it MUST be marked with the attribute
> CHARACTERISTICS="public.accessibility.describes-music-and-sound".
> (Subtitles with this attribute value are treated the same as closed
> captions.)


Then if i compare this with the sample streams on the Developer Site
<https://developer.apple.com/streaming/examples/> i can see one such
example contains subtitles
<https://developer.apple.com/streaming/examples/basic-stream-osx-ios5.html>with
the characteristics "public.accessibility.transcribes-spoken-dialog,
public.accessibility.describes-music-and-sound" within the top level m3u8,
which does not appear aligned fully with the HLS authoring specification.
For reference this was the example i used:

$ curl '
> https://devstreaming-cdn.apple.com/videos/streaming/examples/bipbop_16x9/bipbop_16x9_variant.m3u8
> '
> #EXTM3U
>
> #EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="bipbop_audio",LANGUAGE="eng",NAME="BipBop
> Audio 1",AUTOSELECT=YES,DEFAULT=YES
> #EXT-X-MEDIA:TYPE=AUDIO,GROUP-ID="bipbop_audio",LANGUAGE="eng",NAME="BipBop
> Audio 2",AUTOSELECT=NO,DEFAULT=NO,URI="alternate_audio_aac/prog_index.m3u8"
>
>
>
> #EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID="subs",NAME="English",DEFAULT=YES,AUTOSELECT=YES,FORCED=NO,LANGUAGE="en",CHARACTERISTICS="public.accessibility.transcribes-spoken-dialog,
> public.accessibility.describes-music-and-sound"
> ,URI="subtitles/eng/prog_index.m3u8"
> < - - snip - - >



Appreciate some pointers on the correct signaling here.

Many thanks,

James Hart