Block timestamps will match how all other Codecs are handled. I.e. The Block timestamp is the starting time of the first PCM sample position in nanoseconds.
If the CodecPrivate is empty or not present and Channels is 1 or 2, players MAY treat it as a sane set of defaults, I guess. e. g. channel mapping family 0, no pre - skip or gain. For Channels > 2 the track MUST be rejected, since there's no way to map the encoded substreams to channels.
== Open Questions ==
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. We need a new element to signal this , e. g . Track : : TrackEntry : : PreRoll.
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. Moritz suggests this won't work because the resolution of the timestamps is controlled by the muxer, so the SimpleBlock timestamp offset isn't sample accurate anyway.[[http:/ /lists.matroska.org/pipermail/matroska-devel/2012-September/004254.html ref]]
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 tells 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.
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. This needs a new element specifying the number of samples to trim, perhaps a new BlockGroup child.
If new elements are required, can they be defined so as to enable correct seeking in rolling intra (a.k.a intra refresh) video as well?