"Jim Logajan" wrote in message
.. .
"Vaughn Simon" wrote:
But changing to FM would require a new radio to be
simultaneously installed in every cockpit in the world. The only way
to accomplish that would be for every plane with a new radio to
transmit in "parallel" (as someone already suggested) for a period of
years on both the new mode and the old mode.

That need not be the case, as evidenced by dual-mode cell phones that allow
access to analog and digital cell sites,

The problem I was thinking of that is solved by parallel operation is where
you have two planes in the same pattern who can't hear each other because their
radios are not compatible. The only way I know to solve that is dual (parallel)
operation.

A good example of that concept is what they are doing with TV today. Many TV
stations are transmitting in both analog and digital (HD) so that we are covered
no matter what type of receiver we happen to own.

Vaughn