This is an encapsulation spec for the Opus codec in [Matroska]. There are a number of outstanding functional issues with muxing Opus in Matroska, and until those are resolved, use of this spec is NOT RECOMMENDED.
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? The SimpleBlock timestamp is signed 16 bits, so the format can signal about half of the pre-skip if playback timestamps are to start at zero.
One could set an incorrect timestamp on the skipped blocks, and rely on the decoder to drop them based on the OpusHead preskip value. As long as the initial blocks are timestamped <= start of output this shouldn't affect seeking.
How important is it that timestamps start at zero in a Matroska file?
The SimpleBlock structure also has an 'invisible' bit, which tell the player to decode, but not display, the contained frames. This lets the muxer signal the pre-skip semantics with frame accuracy, but not sample accuracy. If players implement this it will at least help with sync. Libav does not appear to support the invisible bit.
Seeking in Opus files requires a pre-roll (recommended to be at least 80 ms). However, currently Matroska requires its index entries to point directly to the data whose timestamp matches the corresponding seek point, not some place arbitrarily before that timestamp. These two requirements are incompatible, and mean that seeking in Opus will be broken in all existing Matroska software. In particularly unlucky cases (e.g., around a transient), playing back audio decoded without any pre-roll can produce extremely loud (possibly equipment-damaging) results.
How can sample-accurate end-time trimming work in Matroska? Currently all software encapsulating Vorbis in Matroska is broken in this regard, and muxing a Vorbis file in Matroska causes it to get longer (i.e., produce more audio output than the original Ogg file). It would be unfortunate to repeat this disaster for Opus.