Richard Fry wrote:
"But--isn`t program recovered in FM systems a question of carrier
deviation only?"

Certainly so when the signal is ample. That`s about the biggest
advantages of FM and essential to the success of repeaters.

I`ve found and aligned countless FM microwave paths. I always started by
transmitting signal from an audio generator directly on the baseband at
one end of the path. On the receive end of the path the tone comes out
of the order-wire loudspeaker when the dishes are aligned well enough to
find some. It does not go from none to some instataneoisly as if keyed
on. It fades in. Initially, the signal is too weak to register any
increase in limiter current. The tone gets louder when the dishes come
on path, though no one has changed the deviation. You just have more
signal and it behaves much as it would were it an AM signal while it is
weak. When you can hear the tone well you begin to have limiter current
increase so I usually attached an external limiter meter and either sent
it or carried it up the tower to refine the dish position for maximum
limiter current.

Driving across the country listening to FM on the fringes one may notice
background noise, scratchy audio, sudden changes to the station being
received, and a gradual change in the audio level from the loudspeaker.
It is not nearly the change one notices in audio versus distance in AM
reception, but I notice it and adjust my volume control accordingly.
More things happen at the FM improvement threshold than appearance of
noise sputter.

Best regards, Richard Harrison, KB5WZI