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Frequency-Shift Keying (FSK)

Like FM, frequency-shift keying (FSK) shifts the carrier frequency of the transmitter. Unlike FM, however, FSK shifts the frequency between just two separate, fixed points. The higher frequency is called the mark frequency while the lower of the two frequencies is called the space frequency. (By contrast, an FM signal can swing to any frequency within its deviation range.)

FSK was originally developed to send text via radioteleprinter devices, like those used by the TeleType Corporation. The shifting of the carrier between the mark and space was used to generate characters in the Baudot code, which can be thought of as a more elaborate version of the Morse code. At the receiver, the Baudot signals were used to produce printed text on printers and, later, video screens.

As technology improved, FSK was used to transmit messages in the ASCII code used by computers; this permitted the use of upper and lower case letters and special symbols. The introduction of microprocessors made it possible to use FSK to send messages with automatic error detection and correction capabilities. This is done by including error checking codes into messages and allowing the receiving station to request a retransmission of a message if the message and its error checking code are in conflict (or if the code is not received.) Among the most common such FSK modes are amateur teleprinting over radio (AMTOR) and forward error correction (FEC).

FSK is the fastest way to send text by radio, and the error-correcting modes offer high accuracy and reliability. The frequency space occupied depends on the amount of shifting, but typical FSK signals occupy less than 1.5 kHz of space. The big disadvantage of FSK is the more elaborate receiving gear required.

Special receiving terminals and adapters are available to let you "see" FSK modes. Many of these work in conjunction with personal computers.