Re: decodeAudioBuffer, XHR and MIME headers

On 24 Jul 2013, at 23:26, "K. Gadd" <kg@luminance.org> wrote:

> I cannot comment on precisely why the mime-type is important; I've never actually written an MP3 decoder. But I know that the lack of mime type information has at least caused problems with Web Audio in Firefox thus far.
> 
> https://bugzilla.mozilla.org/show_bug.cgi?id=865553
> 
> From what I understand, the problem is that they must automatically figure out what type an audio file is, and this is very difficult without having the assistance of mime-type. This certainly meshes with my understanding of things, given that canPlayType and <audio>/<video> sources all use mime types as part of their infrastructure for deciding what to play and how to play it. Given the amount of engineering effort that seems to be going on related to that bug, I assumed there was some sort of challenge involved here. Maybe it's actually a super easy problem to solve and I'm just missing something.

I'm not an expert either, but I have at least implemented an MP3 decoder (and a few other common formats) and have a bit of insight into the problem.

After reading the bug, it seems like they know what the issues are, and how to solve them, just that there is a bit of effort involved that could (not counting licensing reasons) be solved with copying the code from for example ffmpeg.

a) One problem is that they are currently sniffing for an ID3 tag, not an MPEG-1 elementary audio stream. You unfortunately cannot expect an MPEG-1 stream to contain an ID3 tag (many do not), you should sniff for audio frames.

b) They also have not implemented MPEG-2(.5) support for MP3 yet, which is another stream format for MP3 (and layer 1/2), with the same MIME-type (audio/mpeg). You need to separate these two via sniffing, they have a version field in the headers for each frame.

I also think that ADTS (AAC audio) may have the same MIME-type (audio/mpeg) which means that it is even more diluted.

FLAC/Vorbis in an Ogg container would also share MIME-type (audio/ogg).

Hugs,
Jens Nockert