Opus has few signaling requirements, so a simple mapping:
- CodecID is A_OPUS - SampleFrequency is always 48000 - Channels is 1 or 2 based on what the muxer knows about the input - CodecPrivate is void.
However, this doesn't work for multistream. Supporting multistream requires signalling the number of Opus streams packed in each frame and the mapping from those to output channels through the container.
To support multistream, we place the complete 'OpusHead' header packet from OggOpus, as defined there, in the CodecPrivate element. This provides the number of streams and the channel mapping table, as well as related features like pre-skip and gain which improve the chances of lossless remuxing between the two encapsulations.
- CodecID is A_OPUS - SampleFrequecy is 48000 - Channels is number of output PCM channels - CodecPrivate is the 'OpusHead' packet, identical to the Ogg mapping
The second 'OpusTags' header packet from OggOpus is not used in the Matroska encapsulation. Matroska has its own system for tag metadata, and this avoids duplicating it and the need for sub-framing to index multiple packets within the CodecPrivate element.
If the CodecPrivate is empty, players should treat it as the simpler mapping, I guess.
Should we say muxers MAY or SHOULD NOT produce simple streams without filling in CodecPrivate?
How does the OpusHead pre-skip field interact with the timestamps. Blocks which are skipped aren't part of playback, but they must go in the file with presentation timestamps anyway, and timestamps are unsigned, so we can't use negative values. Can we leave off the timestamp elements? set them all to zero? I fear many players will get this wrong.