See Also: Pulse Amplitude Modulation
In Communications, process in which some characteristic of a Wave (the carrier wave) is made to vary in accordance with an information-bearing
signal wave (the modulating wave); demodulation is the process by which the
original signal is recovered from the wave produced by modulation. In modulation
the carrier wave is generated or processed so that its amplitude, Frequency, or some other property varies. Amplitude modulation (AM), widely used in Radio, is constant in frequency and varies the intensity, or amplitude, of the
carrier wave in accordance with the modulating signal. In other words, the
information-bearing signal looks like the outline of the modulated signal. Frequency
modulation (FM) is constant in amplitude and varies the frequency of the carrier
wave in such a way that the change in frequency at any instant is proportional
to another Time-varying signal. The principal application of FM is also in radio, where it
offers increased Noise (see Interference) immunity and greater sound fidelity at the expense of greatly increased
bandwidth. In pulse modulation the carrier wave is a series of pulses that are all
of the same amplitude and width and are all equally spaced. By controlling one
of these three variables, a modulating wave may impress its information on the
pulses. In pulse code modulation (PCM) it is the presence or absence of
particular pulses in the carrier stream that constitutes the modulation.