Description
For MLB Advanced Media, timed metadata is an important part of what we do (and even more importantly, it is part of how we get paid).
So, it is mission critical to our streams. For the most part, this metadata is sparse - appearing at most 1 or 2 times a segment, and is a minimal amount of data (on the order of 10's of bytes). At most, we have a 24Kbyte image for our radio only broadcasts.
We feel that CMAF has not sufficiently dealt with this subject, the only mention of it is in section 7.3.3, in the section on CMAF headers.
We feel that the solution of timed metadata as a separate media stream is not optimal; it requires yet another stream that has to be download (increasing player complexity, increased traffic over the CDN, additional complexity in the manifest, etc).
We feel that there are 2 different solutions that would work (both in the audio stream): either an additional trak, or emsg.
While we understand the desire to separate out streams between audio/video and subtitles, we feel that the sparse nature of most timed metadata will not hinder this effort. We would find restrictions such as a separate MDAT for metadata be more than sufficient to allow for existing restrictions.
Even if that was to be taken, timed metadata must be more formally introduced other than a throw away line. Either way, it should be given a trak, sample entry description, and be referenced in at least Annex G.
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We're also concerned what to do about sparse tracks that have no data (either subtitle or timed metadata, if the separate track approach is taken). We feel that CMAF should use our suggestion of an "empty" sample with duration for this.