modulation
modulation 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 (see multiplexing).
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 communication except in some modems as quadrative amplitude modulation (QAM).(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. See also phase shift keying.(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.The term shift keying, as in frequency shift keying, denotes specialized modulation techniques in which the modulating signal is digital rather than analog.
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 communication except in some modems as quadrative amplitude modulation (QAM).(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. See also phase shift keying.(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.The term shift keying, as in frequency shift keying, denotes specialized modulation techniques in which the modulating signal is digital rather than analog.
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