Frequency-shift keying

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An example of binary FSK

Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The most common form of frequency shift keying is 2-FSK. As suggested by the name, 2-FSK uses two discrete frequencies to transmit binary (0's and 1's) information. With this scheme, the "1" is called the mark frequency and the "0" is called the space frequency. The time domain of an FSK modulated carrier is illustrated in the figures at right. One interesting facet of the 2-FSK (binary FSK) modulation scheme is that when it is generated using a quadrature modulator (see Quadrature phase), the baseband waveform consists of two sinusoids which use phase shifting to move from one symbol to the next. In this scenario, baseband I and Q signals adjust from 90 degrees to 270 degrees out of phase to mark each symbol of the FSK baseband waveform. For that reason, the binary FSK modulation scheme is sometimes referred to as "phase reversal" as well. Minimum frequency-shift keying or minimum-shift keying (MSK) is a particularly spectrally efficient form of coherent frequency-shift keying. In MSK the difference between the higher and lower frequency is identical to half the bit rate. As a result, the waveforms used to represent a 0 and a 1 bit differ by exactly half a carrier period. This is the smallest FSK modulation index that can be chosen such that the waveforms for 0 and 1 are orthogonal. A variant of MSK called GMSK is used in the GSM mobile phone standard. FSK is commonly used in Caller ID and remote metering applications: see FSK standards for use in Caller ID and remote metering for more details. Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented as changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone. Normally, the transmitted audio alternates between two tones: one, the "mark", represents a binary one; the other, the "space", represents a binary zero. AFSK differs from regular frequency-shift keying in that the modulation is performed at baseband frequencies. In radio applications, the AFSK-modulated signal is normally used to modulate an RF carrier (using a conventional technique, such as AM FM or ACSSB(R)(LM Mode(R)) for transmission. AFSK is not generally used for high-speed data communications, as it is much less efficient in both power and bandwidth than most other modulation modes. In addition to its simplicity, however, AFSK has the advantage that encoded signals will pass through AC-coupled links, including most equipment originally designed to carry music or speech.

Applications

Most early telephone-line modems used audio frequency-shift keying to send and receive data, up to rates of about 300 bits per second. The common Bell 103 modem used this technique, for example. Some early microcomputers used a specific form of AFSK modulation, the Kansas City standard, to store data on audio cassettes. AFSK is still widely used in amateur radio, as it allows data transmission through unmodified voiceband equipment. Radio control gear uses FSK, but calls it FM and PPM instead. AFSK is also used in the United States' Emergency Alert System to transmit warning information. It is used at higher bitrates for Weathercopy used on Weatheradio by NOAA in the U.S., and more extensively by Environment Canada. The CHU shortwave radio station in Ottawa, Canada broadcasts an exclusive digital time signal encoded using AFSK modulation.

See also

• Amplitude-shift keying
• Multiple frequency-shift keying (MFSK)
• Orthogonal frequency division multiplexing
• Phase-shift keying
• Dual-tone multi-frequency (DTMF) is another encoding technique which represents data using pairs of audio frequencies.

References

• Federal Standard 1037C
• MIL-STD-188