Advantages of FM over AM for Sound Signal Transmission

Frequency modulation has the following advantages over amplitude modulation- Noise Reduction, Transmitter Efficiency, Less adjacent Channel and co-channel Interference etc.

The greatest advantage of FM is its ability to eliminate noise interference and thus increase the signal to noise ratio. In FM, amplitude variations of the modulating (message) signal cause frequency deviations and not a change in the amplitude of the carrier. Noise interference results in amplitude variations of the carrier and thus it can be easily removed by the use of amplitude limiters. The amplitude limiter in the FM section of the receiver works on the principle of passing the stronger signal and eliminating the weaker. In this manner, a relatively weak interfering signal (as sound signals  generated at nature has low frequency) or any pick-up from a co-channel station (a station operating at the same carrier frequency) gets eliminated in a FM system.

In an FM receiver, if two signals are being received simultaneously, the weaker signal will be eliminated almost entirely if it possesses less than half the amplitude of the other stronger signal. However, in AM the interfering signal or station can be heard or received even when a 100 : 1 relationship exists between their amplitudes.

The amplitude of the FM wave is independent of the depth of modulation, whereas in AM it is dependent on this parameter. This means that low level modulation can be used in FM and all succeeding amplifiers can be class ‘C’ which are more efficient. Thus, unlike AM, all amplifiers handle constant power and this results in more economical FM transmitters.