by Harry Dalek » Sat Jun 16, 2018 4:43 pm
Its been done but still not much in use if ever these days that i know ,the ham radio operators tend to like colour digital SSTV these days .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.