D. Peter Maus wrote:

Yeah, actually, it does. The digital system is virtually a separate
system, requiring separate transmitters and towers.

No, actually it doesn't. Or at least it usually doesn't require a new
transmitter.

As long as the existing transmitter has an extra 10% of power headroom
to overcome combiner losses, you can do high-level combining and you do
not need a new transmitter (or tower).

If you have to buy a new transmitter then of course the cost goes way up
but you still don't need a new tower.

They are not separate systems, either virtually or in reality. No
stations at all would be broadcasting HD if it required separate
transmitters and towers.

John can answer the question as to how many stations have transmitters
with that 10% of headroom, but apparently many do.