FREQUENCY MODULATION (FM)

Frequency modulation (FM) is the scheme in which the frequency of the carrier signal is varied in proportion to the instantaneous amplitude of the baseband signal, while the carrier's own amplitude and phase are both held fixed. A louder or larger-amplitude moment in the baseband signal compresses the carrier's cycles closer together (raising its instantaneous frequency), while a weaker or negative-going moment stretches the cycles further apart (lowering it) -- producing alternating compressions and rarefactions in the modulated wave's frequency spectrum that exactly mirror the shape of the original baseband signal. When the baseband signal's amplitude is momentarily zero (no input), the carrier sits at its own normal, unshifted frequency, called the centre frequency or resting frequency -- practically, this is simply the frequency the FM transmitter is officially allotted to broadcast on. FM offers clear advantages over AM: because the message is coded in frequency rather than amplitude, most amplitude-disturbing noise picked up in transit has little effect on the recovered signal, giving FM a large decrease in noise and a correspondingly better signal-to-noise ratio; its usable operating range is comparatively large; its transmission efficiency is very high, since essentially all the transmitted power is put to useful work; and because commercial FM bandwidth is wide enough to cover the entire range of frequencies the human ear can hear, FM radio delivers noticeably better audio quality than AM. These benefits com …