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This paragraph is somehow misleading.
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. However, during times of lower than usual speeds their needs to be an automation that allows for the mitigations to occur [1]
The abrupt changes in amplitude of the carreir wave can be viewed as a square wave modulation which leads to frequency expansion. However, this is inherited in most of the digital modulation scheme such as FSK, ASK and PSK. Therfore, going for FSK does not solve the problem. Besides, Frequency-Shift Keying is not intended for a specturally efficient modulation.
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The article states that OOK is used to transmit Morse code. Morse code utilizes pulse-width modulation. From the persepctive of detection theory, OOK and PWM are different and have different error characteristics. There ares some radar transponders which transmit short data messages in OOK (see ADS-B, for example). — Preceding unsigned comment added by 130.76.64.115 (talk) 01:56, 26 January 2012 (UTC)