The process of varying one signal, called the carrier, according to
the pattern provided by another signal. The carrier is usually an analog signal
selected to match the characteristics of a particular transmission system.
Modulation signals and techniques may be combined to produce composite signals
carrying many independent channels of information
The primary types of modulation are as follows:
(a) Amplitude modulation (AM) -- the strength or amplitude of the carrier signal
is varied. This form of modulation is not often directly used in computer
(b) Frequency modulation (FM) - the frequency of the carrier is varied. This
technique is often used by modems.
See also frequency shift keying.
(c) Phase modulation (PM) - the phase of the carrier wave is varied. This
technique is often used together with amplitude modulation in high-speed modems.
(d) Pulse code modulation (PCM) - an analog signal
is encoded as a series of pulses in a digital data stream. This
technique is used by codecs.
(e) Spread-spectrum modulation (SSM) -the carrier wave is frequency modulated by
the analog or digital data signal in conjunction with a third or code signal.
This technique is used by packet radio networks for military applications.
The term shift keying, as in frequency shift keying, denotes specialized
modulation techniques in which the modulating signal is digital rather than