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Pulse-amplitude modulation (PAM), is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. It is an analog pulse modulation scheme in which the amplitudes of a train of carrier pulses are varied according to the sample value of the message signal. Demodulation is performed by detecting the amplitude level of the carrier at every symbol period. ==Types== There are two types of pulse amplitude modulation: # Single polarity PAM: In this a suitable fixed DC bias is added to the signal to ensure that all the pulses are positive. # Double polarity PAM: In this the pulses are both positive and negative. Pulse-amplitude modulation is widely used in modulating signal transmission of digital data, with non-baseband applications having been largely replaced by pulse-code modulation, and, more recently, by pulse-position modulation. In particular, all telephone modems faster than 300 bit/s use quadrature amplitude modulation (QAM). (QAM uses a two-dimensional constellation). The number of possible pulse amplitudes in analog PAM is theoretically infinite. Digital PAM reduces the number of pulse amplitudes to some power of two. For example, in 4-level PAM there are possible discrete pulse amplitudes; in 8-level PAM there are possible discrete pulse amplitudes; and in 16-level PAM there are possible discrete pulse amplitudes. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pulse-amplitude modulation」の詳細全文を読む スポンサード リンク
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