Support for various audio formats

[HadrienGardeur]

In order to maximize compatibility, we probably want to restrict the audio formats that can be used in the audiobook manifest.

Given the large number of formats available, this can be a tricky choice (the situation in browsers regarding video file formats is pretty much a nightmare).

[HadrienGardeur]

Any thoughts? cc @daryanypl @pettarin

[pettarin]

Methodologically, first I would establish the use cases this thing should cover. Assuming the Web platform and browser-based apps are the primary target, I would start by examining the "Browser compatibility" table in https://developer.mozilla.org/en-US/docs/Web/HTML/Supported_media_formats Commercially, I have mostly seen MP3 (and ZIP[MP3]), because basically anything can read an MP3 file. At least in the EU, patents on MP3 encoders/decoders have expired, so MP3 might be considered effectively a patent-free format in the EU. Wikipedia says patents still hold in the USA until the end of 2017 --- I am not familiar with the details outside the EU. (See: Licensing, ownership and legislation in https://en.wikipedia.org/wiki/MP3 ) AAC in MPEG4 is covered by patents, so I think it should not be a primary choice. This cascades to Apple's M4B, which is basically the counterpart of ZIP[MP3]. PCM WAVE is rarely used due to its uncompressed nature, but the format is so simple and supported by everyone that it is safe including it. FLAC is unfortunately poorly supported in browsers, but libflac works on any OS and there are several applications to read/write FLAC files, since it is BSD-licensed. Speaking of more "exotic" codecs, Vorbis and Opus are patent-free and widely supported, so I would include them as well. In particular Opus, which is a general-purpose codec that superseded older speech-specific codecs (like Speex). Some public libraries have collections encoded with even more speech-optimized codecs like AMR-WB+ (see this discussion: w3c/epub-specs#645 ) however it is unlikely that e.g. browsers will ever support them natively, as the delta with Opus is small (in terms of quality/bits) and their adoption is negligible (at Web scale). HTH, AP

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On 04/01/2017 07:30 PM, Hadrien Gardeur wrote: Any thoughts? cc @daryanypl <https://github.com/daryanypl> @pettarin <https://github.com/pettarin> — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#2 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AD5Zkz389FWM3vccWuHDGZ3ODN4VECf9ks5rrom6gaJpZM4Ji-zF>.

[HadrienGardeur]

Thanks @pettarin that's very useful.

I would indeed consider the Web to be a primary target, along with major mobile (Android, iOS) and desktop (Windows, macOS, Linux) platforms.

[daryanypl]

DRM-encrypted AAC is a common format for audiobooks. I'd like to include it as a possible media type. Even if we cannot handle it right now, we'll want to be able to identify it.