Use a subcarrier mixed additively to the video and send manchester data.
The receiver will be able to recover it. Choice of carrier is yours. I'd
suggest color subcarrier if you send B/W. You can send 115kBps on the
color subcarrier w/o problems.

There is way to put data on unused lines in the video, but the speed
required puts it beyond PIC range for most people. Look into VITC and
SMTP/ESMTP (nothing top do with email this time ?) encoding (used for time
code and close captioning and other things in professional video).

Peter

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