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MatroskaOpus

3,801 bytes added, 13:32, 12 April 2013
Handling Pre-skip data
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 (TODO) Define layout of 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.
== Muxing Recommendations ==
== Open Questions ==
<ul> <li>Should we say muxers MAY or SHOULD NOT produce simple streams without filling in CodecPrivate?</li>
How does <ul> <li>If the OpusHead pre-skip field interact with the timestamps? The SimpleBlock timestamp CodecPrivate is empty or not present and Channels is signed 16 bits1 or 2, so the format can signal about half players MAY treat it as a sane set of the defaults, I guess. e.g. channel mapping family 0, no pre-skip if playback timestamps are to start at zeroor gain. Moritz suggests this won't work because the resolution of For Channels > 2 the timestamps is controlled by the muxertrack MUST be rejected, so since there's no way to map the SimpleBlock timestamp offset isn't sample accurate anywayencoded substreams to channels.[[http: </li> </lists.matroska.org/pipermail/matroska-devel/2012-September/004254.html ref]]ul>
One could set an incorrect <li>How does the OpusHead pre-skip field interact with the timestamps?</li> <ul> <li>The SimpleBlock timestamp on is signed 16 bits, so the skipped blocks, and rely on format can signal about half of the decoder pre-skip if playback timestamps are to drop them based on the OpusHead preskip valuestart at zero. As long as Moritz suggests this won't work because the initial blocks are timestamped <= start resolution of output this shouldnthe timestamps is controlled by the muxer, so the SimpleBlock timestamp offset isn't affect seekingsample accurate anyway.[[http://lists.matroska.org/pipermail/matroska-devel/2012-September/004254.html ref]] </li>
How important is it that timestamps <li>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 at zero in a Matroska file?of output this shouldn't affect seeking. </li>
<li>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. </li> </ul>
<li>How can sample-accurate end-time trimming work in Matroska? Currently all software encapsulating Vorbis in Matroska important is broken it that timestamps start at zero 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.?</li>
<li>How can sample-accurate end-time trimming work in Matroska?</li>  <ul> <li>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. </li> </ul>  <li> 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? </li> <ul> <li>SeekPreRoll should work for rolling intra video.</li> </ul> </ul> == Handling Pre-skip data == <ul> <li>Use Cases:</li> <ul> <li>UC1: Playback starts from the beginning of the stream. Source stream time starts at 0.</li> <li>UC2: Playback starts from the beginning of the stream. Pre-skip data ends in middle of compressed packet.</li> <li>UC3: Playback starts from the middle of the stream > SeekPreRoll time.</li> <li>UC4: Playback starts from the middle of the stream < SeekPreRoll time.</li> </ul></ul>  <ul>  <li>one: Timeshift the timestamps by pre-skip data <ul> <li> The Opus audio stream pre-skip data starts from time 0 and adds the pre-skip time to the normal audio time, like how Opus files are muxed into ogg files. We would add a new element to the TrackEntry element, PreSkip, and the player would adjust the timestamps of the decoded samples by subtracting PreSkip. All use cases should be covered. </li>  <li>Cons:</li> <ul> <li>The timestamp of the Block does not match the timestamp of the playback position.</li> <li>Does not generalize known "decode, but not render" data.</li> <li>Forces the player to handle the pre-skip samples. I.e. not the decoder.</li> </ul>  </ul>  <li>two: Add pre-skip data to CodecPrivate.</li> <ul> <li> On every discontinuity the decoder would need to decode and throw away the pre-skip data. </li>  <li>Cons:</li> <ul> <li>UC2 will throw away valid data and the AV sync will be off.</li> <li>UC3 will redundantly decode the pre-skip data.</li> </ul>  </ul>  <li>three: Add TimeToDiscard to Block.</li> <ul> <li> Add an element to the Block element, TimeToDiscard in nanoseconds. A value of -1 would not render the whole Block, which would have the same effect as setting the invisible bit. How would this affect the Block timestamp? Maybe the new element should be SamplesToDiscard or DataToDiscard? </li>  <li>Cons:</li>  </ul>  <li>four: Blocks that contain pre-skip data will set invisible flag.</li> <ul> <li> Blocks that contain pre-skip data have timestamps from the beginning of the stream. Blocks that only contain normal data have timestamps from the playback position. </li>  <li>Cons:</li> <ul> <li>Forces the player to handle the pre-skip samples. I.e. not the decoder.</li> <li>UC2 will throw away valid data and the AV sync will be off. Other use cases should be fine.</li> </ul>  </ul>  <li>five: Force pre-skip packets to be prepended to the first normal packet in the first Block.</li> <ul> <li> The first Block's timestmap will be set to the start time of the source playback position. We would add a new element to the TrackEntry element, PreSkip. All use cases should be covered. </li>  <li>Cons:</li> <ul> <li>Does not generalize known "decode, but not render" data.</li> <li>Forces the player to handle the pre-skip samples. I.e. not the decoder.</li> </ul>  </ul>  <li>six: Create a new codec, OPUS_MKV.</li> <ul> <li> Basically the codec will wrap Opus packets with data telling the decoder what type of Opus packet it contains. Essentially we would be creating a new codec to handle pre-skip data within the decoder. </li>  <li>Cons:</li> <ul> <li>There will be two types of Opus data streams!</li> <li>Does not generalize known "decode, but not render" data.</li> </ul>  </ul>  </ul>
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