You are certainly talking about DAM...
Dynamic Amplitude Modulation....
a process that is used with very high power
transmitters to save energy.....
When there is no modulation, the carrier
level is moved UP, in order to keep the
signal to noise ratio on the channel at
a better level for the listener ( i.e. no background
noise or co-channel stations... you require
26 dB signal to noise ratio on an AM system
that has between 4 and 5 kiloHertz bandwidth
in order to provide full quieting ...)
AND YES.... there are many high power
AM transmitters nowadays that are facing
very poor operation !!!
73 and DX
Arnie Coro
CO2KK