Define how audio elements are "matched up" when multiple SampleGroups are used about iamf

[shawnisingh]

Hello Thomas, can you please remind us what was the example situation that raised this issue? Thank you in advance!

[tdaede]

The streams in a file are continuous - other than at the beginning and end of a file, when there is preskip and end padding, the decoders are not re-initialized. If we have 3 substreams, 0, 1, and 2, and then remove (1) partway through the file (i.e. via IA_STATIC_META), we want the remaining two streams to continue to be fed to the (0) and (2) decoder.

This is a potential case where having substream IDs would make this easy. It's not the only way to solve this issue, though. We could also just have a fixed set of rules to how streams are mapped (e.g. you're only allowed to add or remove from the end). We could also require complete decoder re-initialization every time IA_STATIC_META changes, though this has implications for being gapless through the transition.

[sunghee-hwang]

Simply, we may need to restrict that every Substreams have the same duration.

[tdaede]

This will be solved with the Sync OBU changes when those land, hopefully.