mxf2raw: extend `--rdd6` option description

[philipnbbc]

Fixes https://github.com/bbc/bmx/issues/44

[thomasheritage]

Thanks, it's very useful to have this documented more thoroughly.

A few thoughts:

• There's no indication given if the frames value was insufficient to allow complete program descriptions to be extracted for all programs
• If there were multiple programs in the bitstream can they in practice be included in the XML? It looks like the XML only supports a single program description.
  • I don't think we have a use case for multiple programs -- if mxf2raw can't fully support this then this could be noted in the help text
• Presumably mxf2raw assumes that the RDD 6 metadata is static except for both frame_count and description_text i.e. there are two non-static fields
  • Presumably mxf2raw only reads RDD 6 metadata (except for those two fields) from the first frame indicated in the frames commandline parameter? i.e. won't notice if other values actually vary
• Perhaps remove the "Set the ..." paragraph from https://github.com/bbc/bmx/tree/main/meta/rdd6_xml_creator to avoid conflicting text / having to maintain the text in two places

[philipnbbc]

I've updated the PR to

• improve the option description following your comments,
• log a warning if the description text is not complete for all programs,
• remove the redundant text in the rdd6_xml_creator app

The RDD 6 XML schema defines the desc_text element to have at most 8 programs. See table 6.2 of the RDD 6 spec.. E.g. if there is 5.1 and stereo audio then that is represented as 2 RDD 6 programs.

[thomasheritage]

That all sounds very good, thanks for all the improvements.

Just one outstanding item:

presumably mxf2raw only reads RDD 6 metadata (except for those two fields) from the first frame indicated in the frames commandline parameter? i.e. won't notice if other values actually vary

On this topic, the help text says:

The output XML will contain the static RDD 6 metadata

Perhaps you can just add something to convey whether this RDD 6 metadata being static is just an assumption, or whether mxf2raw actually checks that it's static (for the frames that it reads)?

[philipnbbc]

Just one outstanding item:

presumably mxf2raw only reads RDD 6 metadata (except for those two fields) from the first frame indicated in the frames commandline parameter? i.e. won't notice if other values actually vary

Yes, it assumes it is static and doesn't check if it isn't.

On this topic, the help text says:

The output XML will contain the static RDD 6 metadata

Perhaps you can just add something to convey whether this RDD 6 metadata being static is just an assumption, or whether mxf2raw actually checks that it's static (for the frames that it reads)?

I've changed it to say it assumes it is static and that it uses the first and subsequent frames to accumulate the description text.

[thomasheritage]

LGTM