ematically represented as O'(t)= Bx(t) (2.8) where is a constant, known as the frequency modulation index. The instantaneous frequency of the carrier wave is hence changed according to the amplitude of the information signal. FM is more resistant to noise because random interfer- ence is more likely to affect amplitude than frequency of the signal. In phase modulation (PM), the phase of the modulated signal, O(t), is directly propor- tional to the information signal, x(t). Mathematically, the relationship can be represented O(t) )=Bpx(t) (2.9) where Bp is a constant, known as the phase modulation index. In angle modulation, the information is conveyed by the instantaneous phase of the signal. Consequently, phase distortion in the transmission path causes attenuation distortion of the received signal. The differential delay of the transmission path must therefore be closely controlled over the bandwidth required to transmit the signal. However, because the information is not conveyed by the amplitude of the signal, the receiver can contain a limiter to maintain constant signal amplitude. Consequently, nonlinear distortion in the transmission path does not cause distortion of the demodulated output signal; nor does attenuation/frequency distortion. In PM, the frequency deviation is proportional to the frequency of the modulating signal as well as to its amplitude. Consequently, for signals such as speech that have the major proportion of their energy at the lower end of the baseband, PM makes inefficient Data Transmission Basics use of the transmission path bandwidth compared with FM. Moreover, to demodulate a PM signal, the receiver must compare the phase of the incoming carrier with that of a locally generated carrier, which must be very stable. FM is therefore preferred to PM for the transmission of analogue signals. It is used whenever sufficient bandwidth can be pro- vided. However, PM is widely used for the transmission of digital signals, as described in the next section. Digital Modulation A digital signal 