wn as amplitude shift keying (ASK) or on off keying (OOK). phone circuits with 8-level ASK using VSB transmission. The method is widely used in wireless telegraphy. Another import- ant example is its use in optical communication systems. At the 19.3.2 Frequency shift keying sending end, a binary electrical signal switches on and off a laser or light emitting diode. At the receiving end, a photodiode is switched If a digital signal is used to modulate a carrier in frequency, this is on and off to reproduce the binary electrical signal. known as frequency shift keying (FSK). It is used in voice fre- In ASK, the spectrum of the transmitted signal consists of a quency telegraph systems (Chittleburgh et al., 1957) and for wire- component at the carrier frequency, together with an upper sideband less telegraphy in the h.f. band (Hills and Evans, 1973). It is also which is a translated version of the baseband spectrum and a lower widely used for data transmission over telephone connections in sideband which is an inverted version. It was shown earlier that the public switched telecommunication networks (Bennett and Davey, minimum bandwidth required to transmit the baseband signal is W 1965; Lucky et al., 1968). A typical data modem for use on tele- = 1/2B (where B is the digit rate in bauds). It therefore follows that phone circuits uses frequencies of 1.3kHz and 1.7kHz to transmit at the minimum bandwidth required to transmit the ASK signal is 2W 600 bauds or 1.3kHz and 2.1kHz to transmit at 1200 bauds (CCITT = B. If the baseband signal has a raised cosine spectrum with 100% Recommendation V.23). roll off, the bandwidth of the ASK signal is increased to 2B. As shown in Figure 19.5, binary FSK is equivalent to applying If the carrier amplitude is the power when it is switched on ASK to two carriers of frequencies f, and f one being switched on corresponds to 1/2V 2. If the on and off states are equiprobable, the when the baseband signal is '0' and the other when it is '1'. In fact, mean power is equal to half this value