1321246 - Opus in MP4, not taking into account Discard padding

Status: NEW

(Core :: Audio/Video: Playback, defect, P3)

Description

[Jean-Yves Avenard [:jya]]

As seen in bug 1319992.

The opus decoder requires specific fields to be set to handle pre-skip and discard padding to be set.

The MP4 demuxer doesn't appear to set anything...

It's also the same for other audio codec in MP4, but I'll open another bug for that.

Comment 1

[Ralph Giles (:rillian)]

The mp4 demuxer provides pre-skip and codec delay values in the codec-specific just like the webm demuxer does. Is this code not working?

https://dxr.mozilla.org/mozilla-central/source/media/libstagefright/binding/MP4Metadata.cpp#805
https://dxr.mozilla.org/mozilla-central/source/media/libstagefright/binding/mp4parse_capi/src/lib.rs#482

Comment 2

[Jean-Yves Avenard [:jya]]

ok. So it's good for preskip. 

However, there's definitely no allowance for the number of frames to trim at the end of the media. That is an information that should be set on the last demuxed frame only.

Webm has a specific entry on the block you need to apply it to (https://www.matroska.org/technical/specs/index.html DiscardPadding) and for ogg it's always the last block

Comment 3

[Ralph Giles (:rillian)]

I think one is supposed to use the EDIT list for end trimming in mp4. https://opus-codec.org/docs/opus_in_isobmff.html#4.4

Right now the rust parser just copies stagefright's duration calculation, so it's possible they're both wrong.

Comment 4

[Jean-Yves Avenard [:jya]]

The opus decoder makes no consideration of the packet duration. Which would be difficult anyway as the two containers where opus is most used have no duration concept.

We need an explicit way in the demuxer to flag the specific sample where the trimming is to occur. 
Long term we want to remove the concept of block duration too.

With fragmented MP4, it gets further complicated in that the last sample of a block may not need to be trimmed. The specific handling of webm is better here. There's an explicit trimming value set.