First line should be blanked for NTSC with closed captions

[atsampson]

NTSC closed caption data overlaps the first line of the active region, so it's visible (and distracting) in the output:

If valid CC is detected in the video, ld-chroma-decoder should blank the first line, and ld-dropout-correct should ignore the first line, both for fixing dropouts and as a source of replacements.

I think the easiest way to do this would be to adjust the first line of the active region, checking that this doesn't change the output padding for ld-chroma-decoder. Currently the active range is hardcoded in all the tools that need to know it...

[simoninns]

Technically speaking; it's a deliberate feature of the closed caption system; it's placed in a visible line since it means any system/processing in between the viewer's closed caption decoder and the source should never remove it. I imagine a number of PC based players can generate subtitles based on it, although I've not checked. I guess the active region could be provided by ld-decode into the metadata; and the tools changed to use it instead of the hard-coded values.

[Gamnn]

My first thought is you can simply crop it out with ffmpeg or other tools if needed, but it's a fair point that it should be excluded from DOC stuff.

I'm not sure if any PC-based players can actually decode it at present. I think VLC includes libzvbi for decoding, but it only works when viewing from a capture card as it expects fields rather than frames, and only looks on line 21 of each field. Other players I've found that support CC require it attached in pre-decoded text form as metadata.

[atsampson]

The other reason for excluding it early is that you don't want ld-chroma-decoder's 2D/3D filters seeing it (since it would affect the decoded colour on the surrounding lines).