Clarification needed for handling raw AAC bitstreams without description

[ChunMinChang]

According to the AAC registration in WebCodecs, when the description is not present in AudioDecoderConfig, the bitstream is assumed to be in ADTS format. Otherwise, it's assumed to be in AAC. However, there's a case (https://webvideo-js.pages.dev/) where raw AAC data is provided to the audio decoder without supplying description data in configure beforehand (#826 seems a similar issue for opus).

Currently, Chrome can decode this raw AAC data, and Firefox has patches to support this behavior as well (by generating a AudioSpecificConfig).

This issue can be easily reproduced by adding a test case in audioDecoder-codec-specific.https.any.js WPT that replicates the properties of MP4_AAC_DATA but omits the description. I would like to ask if this behavior should be officially mentioned or detailed in the WebCodecs spec. If this use case is something that should be supported, it might be worthwhile to include a corresponding WPT for this scenario.

[dalecurtis]

Hmm, Chromium doesn't intentionally generate an audio specific config AFAIK. FFmpeg may be doing something under the hood though -- I doubt this works if we end up having to use a platform decoder. I don't know enough about AAC to say if there are forms where AAC can be decoded w/o the ASC.

[padenot]

I don't know enough about AAC to say if there are forms where AAC can be decoded w/o the ASC.

Very often you can make a good guess if you have the channel count / sample rate, and get something to decode fine by generating extradata. For example, here's some code that does it: https://searchfox.org/mozilla-central/source/dom/media/platforms/agnostic/bytestreams/Adts.cpp#273-287.

Related: #826 (comment)