Modulation is the Method the transmitter uses to send the data containing
the position of the sticks to the receiver.
Although there are several types of transmission system The signal transmitted to the servos has not changed since the early days of R/C
Each servo channel consists of a pulse which is varied by the stick associated
with that channel. This pulse varies from 1 millisecond to 2 millisecond (1.5 being centre position)
This is repeated for each servo channel every 20 ms (50 times a second)
AM: Stands for Amplitude Modulation which transmits by a variation in the
amplitude of signals. In R/C it basically switches the transmitted signal
off and on. It is subject to interference more than FM. Mostly found
on 27 MHz sets.
AM transmitter output
FM: Stands for Frequency Modulation which transmits signals by varying
the frequency of the transmitted signal ,this reduces the risk
of "glitches" due to signal interference making it preferable to AM.
The 2GHz transmitters use a different Transmission system (see 2GHz system) although the output to the servos is the same as the previous systems