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DRAFT encapsulation spec for the Opus codec in [Matroska].

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.

The Channel Matroska element's value should match that of the corresponding field of the OpusHead data; the Matroska SampleFrequency remains fixed at 48000.

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.