Colum Mylod wrote in
:

I was surprised that the "digital cliff" is err "wider" than expected:
it's not so much a now you see it now you don't but a varying
disruption which shows up as mass blockiness. I had expected a more
binary works/fails, the current atmospherics show up as unreliability
that comes and goes (prob. due to fading in and out).

This is because digital works a bit differently.

I'm no expert (Pye VT14 circa 1960 was my forte), but now they use a sort
of differential signal to save bandwidth by not transmitting picture areas
that don't change (much)from frame to frame.

Total signal loss would give a perfect still frame.

I don't know the details, but it's the basic reason

mike