fm

Also known as frequency-shift keying (FSK). A signal modulation technique in which the frequency of the analog carrier sine wave is varied to distinguish between a 1 bit and a 0 bit. For example, FM is the sole technique used in low-speed, Hayes-compatible modems. When no bits are transmitted, the carrier is at a reference frequency of 1700 Hz. A unibit FM technique impresses one bit on each baud by shifting the carrier to 2200 Hz when transmitting a 1 bit, and to 1200 Hz when transmitting a 0 bit. At 2400 baud, therefore, the transmission rate is 2400 bps.The benefits of dibit transmission can be realized by defining four frequencies, with each sine wave or set of sine waves representing a 2-bit pattern (00, 01, 10, and 11).Thereby, at 2400 baud, the transmission rate is 4800 bps. Figure F-3 illustrates unibit frequency modulation, with the carrier waveform modulated to a relatively high frequency to represent a 1 bit and to a relatively low frequency to represent a 0 bit. FM often is used in conjunction with amplitude modulation (AM) and phase-shift keying (PSK). A variation on the FSK theme is Gaussian frequency-shift keying (GFSK), which is used in DECT and Bluetooth wireless networks. See also AM, analog, Bluetooth, carrier, DECT, GFSK, modulation, PSK, sine wave, and transmission rate.