| 单词 | AM (Amplitude Modulation) |
| 释义 | AM (Amplitude Modulation) Synonymous with ASK (Amplitude Shift Keying).A signal modulation technique in which the amplitude of the analog carrier sine wave is modulated to represent one or more 1 bits or 0 bits. The transmitting computer outputs a baseband electrical signal, defining 1 bits and 0 bits as discrete voltage levels. Using a unibit (single-bit) AM technique as illustrated in Figure A-4, each 1 bit entering the transmitting modem is expressed as one or more relatively high-amplitude sine waves, and each 0 bit is expressed as one or more low-amplitude sine waves. The high and low levels are defined in terms of a reference level or by the relative difference between the levels. At 2400 baud, this unibit technique yields a transmission rate of 2400 bps, with one bit transmitted per baud, that is to say that the bit rate equals the baud rate. In a dibit coding scheme, it is possible to express 2 bits with a single baud by defining 4 levels of amplitude, so the bit rate is double the baud rate. A tribit coding scheme expresses 3 bits per baud by defining 8 levels of amplitude, and a quadrabit coding scheme expresses 4 bits by defining 16 levels of amplitude. Amplitude modulation often is used in conjunction with frequency modulation (FM) and phase-shift keying (PSK) in electrically based networks. Digital fiber optics transmission systems (FOTS) use amplitude modulation. See also AM, amplitude, analog, baseband, baud, baud rate, bit, bit rate, carrier, digital, FOTS, modem, PSK, and sine wave. 74570c01.qxd 9/12/07 12:34 AM Page 30 |
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