Re: [Cellar] matroska and side data vs timecode

[Derek Buitenhuis <[e-mail]>]

Comments inlined below.

On 29/10/2019 16:04, Dave Rice wrote:
> _Implementation Considerations:_
>
> Thinking ahead of this work, I think it would be worthwhile to add a dedicated section to the Matroska specification on timecode and describe two methods for adding a reference timecode to a Matroska Segment.
>
> 1. Using tags. Make an official tag “TIMECODE” to store a string of the timecode expression that correlates to the Matroska timestamp of 0. This approach is limited to storing timecode that is incremental and continuous. Defaults for the incrementation and the behavior of the timecode could be described as part of the tag definitions and feasibly another tag or tags could be reserved for timecode flags when the timecode behavior is not a default. Defining a use of a “TIMECODE” tag would provide the benefit of standardizing an existing practice in FFmpeg’s Matroska muxer, where rewrapping from a timecode-supportive format to Matroska carries the timecode value over as a tag.

To me, requiring timecodes to be incremental and continuous pretty much defeats a good chunk of the point of timecodes, no?

> 2. Using BlockAdditions as described above. This adds more overhead as each timecode value would be written, but would support non-continuous timecode. In FFmpeg, timecode is sometimes carried as side-data such as from the decklink device via https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/0946c0ec177dc48ef0677f890aa42d95e667c417. I’d be interested if the Block Addition Mapping described above could be used to carry the timecode side data from the decklink device (or other incoming stream with timecode side data) and store the result within the written Matroska BlockGroups.

As long as it's an optional element, I don't see a downside to the small overhead - especially considering the large size of many/most mezzanines that carry timecodes.

Unrelated to the above two comments: How do you intent to rectify the historical (and confusing) use of 'timecode' in Matroska to be timestamp with the newly introduced timecode functionality?

Cheers,
- Derek