ACTION-290: Describe the history for how we got to the signalling needed for mpeg that came from dece for issue-305.

[nigel]: Nigel to add any requirements from other places.

The ISO-layer namespace and schema_location fields were defined by DECE as a way to provide additional ISO-layer signaling about the content of an XML document track - that is what the instance document(s) conformed to. Arguably it could also be what decoder requirements were, but that was not explicitly considered then. FYI, whether the track is a CFF text profile or a CFF image profile was signaled in the metadata. But the document could also contain SMPTE closed caption information and other extension namespaces not easily enumerated in advance without a registry.

XML signals instance document conformance through the use of namespaces and schemas. A list of namespaces in use in the document is useful and was added as a signal field. This would be able to signal that a track document had both. e.g. tt and smpte namespaces.

But it was felt that more information about any profiling of the namespaces would be (optionally) better.

The TTML "profile" was attractive except that: 1) it is currently specific to TTML and not XML in general, and 2) see: https://www.w3.org/AudioVideo/TT/tracker/issues/263

The schema_location field could uniquely identify a profile of the namespace that the document conformed to. The schema_location values are not a managed value space, but a fully qualified URI would accomplish that if there was concern over collisions.

When later defining the ISO sample entry for XML-based timed text in ISO 14496-12, AMD2 & 14496-30 it was deemed desirable to remain compatible with the DECE syntax in use.

There is currently no ISO sample entry specific to TTML. Tracks with TTML documents are minimally signaled with namespace="http://www.w3.org/ns/ttml".