Add support to ARIB subtitles

[saumarh]

While CEA-608/708 closed caption standards are used by the ATSC system in North America, DVB Subtitles / DVB Teletext defined in DVB standard are used in Europe and many parts of the world, Japan established its own TV broadcasting standard ISDB that includes a kind of caption service defined in ARIB STD-B24 by the Association of Radio Industries and Businesses (ARIB).

Brazil also adopted ISDB-T International for their broadcasting by establishing Brazilian version SBTVD / ISDB-Tb based on the Japanese standard, which has been widely used in South America countries and around the world. Brazilian version also includes a caption service for Latin languages defined in ABNT NBR 15606-1 which is modified from ARIB STD-B24 specification. Philippines also adopted ISDB-T International based on the Brazilian standards, but uses UTF-8 for caption encoding based on the Japansese specification ARIB STD-B24.

Though ISDB-based TV broadcasting has been operating for about 20 years, ARIB based caption is still lacking support in general players.

(above text copied from https://github.com/xqq/libaribcaption - this library is in use in ffmpeg and vlc projects)

Proposed solution

Create a subtitle decoder for ARIB subtitles and specify it when needed: DefaultExtractorsFactory extractorsFactory = new DefaultExtractorsFactory(); extractorsFactory.setTsSubtitleFormats(ImmutableList.of( // new Format.Builder().setSampleMimeType(MimeTypes.APPLICATION_ARIB).build(), new Format.Builder().setSampleMimeType(MimeTypes.APPLICATION_CEA608).build()));

Alternatives considered

Right now our player supports 2 implementations: libVLC and ExoPlayer. We're using VLC for multicast streams coming from a tuner-to-IP gateway. This gateway is just a bypass of tuner input. Resulting streams are untouched. VLC is able to decode ARIB subtitles.

ExoPlayer is in use for DRM protected content. We'd like to keep ExoPlayer as our only player implementation - reducing code maintenance and app size.

[icbaker]

Thanks for the request! Unfortunately we are unlikely to invest in adding support for this format because it's not really used in the digital & streaming formats that are the main focus of this library.

We would accept high quality PRs for this, but with some important caveats:

• The work is probably large enough that we should have some design discussion first. If you'd like to pursue this, it's probably worth starting by sharing a doc for comments either publicly or with android-media-github@google.com (please let us know on this issue if you've done this, otherwise we won't see it).
  • This should help to quickly answer some big questions before investing in significant coding.
• It should either be implemented in Java with no additional external dependencies, or it must be in an extension (i.e. not added next to where the existing SubtitleParser implementations live).
  • If you're thinking of building on top of an external dependency, please check with us first - it's likely that we will not be able to accept the dependency, even from an extension, so by checking first you can save wasted work.
• The PR must come with a suite of suitable test assets (probably very short MPEG-TS streams with ARIB subtitles muxed in) that can be used with our extractor and playback tests. You must be able to include these assets in the CLA you sign as part of the PR process.