It really depends on how good your old analogue NTSC was. For a
noiseless picture, you would need around 43dB CNR, but pictures were
still more-than-watch-able at 25dB, and the picture was often still
lockable at ridiculously low CNRs (when you certainly wouldn't bother
watching it). Digital signals can work at SNRs down to around 15dB for
64QAM and 20dB for 256QAM (although if it's a little below this, and you
will suddenly get nothing).


That has not been our experience. We had a number of customers here in
the DC area who had great pictures on NTSC sets, but got either heavy
pixilation or no picture at all when the switchover occurred. We sent
them to a company which does tv antenna installations (we do a lot of
low voltage, including tv - but not antennas). In every case,
installing a better outdoor antenna solved the problem.

No one said the NTSC had to be noiseless. But the 43dB is a bit high,
even for older sets. Input from the cable tv company to our equipment
was 10-20dB; we tried to push 10dB to all of the outputs but never had a
problem even down to 7dB (the lowest we would let it drop to).

That makes no sense; a 7dB CNR would be pretty much unwatchable on
analogue, it would be a very very noisy picture, if it even locked at all!

Jeff