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'On-off keying' ('OOK') is a type of modulation that represents digital data as the presence or absence of a carrier wave. In its simplest form, the presence of a carrier for a specific duration represents a binary one, while its absence for the same duration represents a binary zero. Some more sophisticated schemes vary these durations to convey additional information.
On-off keying is most commonly used to transmit Morse code over radio frequencies (referred to as continuous wave operation), although in principle any digital encoding scheme may be used. OOK has been used in the ISM bands to transfer data between computers, for example.
OOK is not very spectrally efficient due to the abrupt changes in amplitude of the carrier wave. At low to medium signalling speeds, this can be mitigated by adjusting the rise and fall rates of the carrier's amplitude. At high speeds, more efficient modulation modes (such as frequency-shift keying) are normally used instead.
In addition to RF carrier waves, OOK is also used in optical communication systems (e.g. IrDA).