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

[Eryk Vershen <[e-mail]>]

Thanks for your inquiry.

I can see how the Authoring Specification rule 4.5 might cause confusion. The intent of the rule is that "public.accessibility.describes-music-and-sound" should be used in addition to "public.accessibility.transcribes-spoken-dialog". I'll make sure the rule is revised to be clearer.

The three characteristics relevant to subtitle renditions are (as draft-pantos-hls-rfc8216bis-09 says):
	"public.accessibility.transcribes-spoken-dialog"
	"public.accessibility.describes-music-and-sound"
	"public.easy-to-read"

Characteristics are also known as AVMediaCharacteristics (see https://developer.apple.com/documentation/avfoundation/avmediacharacteristic <https://developer.apple.com/documentation/avfoundation/avmediacharacteristic>).

A subtitle rendition may have none of these. For example, if only portions of the dialog are subtitled.
If the rendition is subtitled throughout, it should be marked "public.accessibility.transcribes-spoken-dialog".
If, in addition, the rendition has sound effects & music described, then it should also be marked "public.accessibility.describes-music-and-sound"

The last characteristic ("public.easy-to-read") is defined in the AVFoundation documentation above (AVMediaCharacteristicEasyToRead) as "'easy reader' captions (per the CEA-608 specification)".

regards,
-eryk

> On Jul 22, 2021, at 8:44 AM, James Hart <[e-mail]> wrote:
> 
> 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 <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 
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