 network which has a rising gain-frequency by frequency modulation (B = 5.0) characteristic across the baseband. The receiver contains a de-em- phasis network with the inverse gain-frequency characteristic, to 18.4.2 obtain a channel having a flat overall gain-frequency characteristic Pulse amplitude modulation and a uniform noise spectrum. For single channel telephony or broadcasting this provides a better output signal to noise ratio. A basic PAM system is shown in Figure 18.16. If a train of pulses When FM is used to transmit a wideband signal consisting of a as shown in Figure 18.17(b) is amplitude modulated by the base- block of telephone channels assembled by frequency division multi- band signal shown in Figure 18.17(a) the resulting PAM signal is plexing, the use of pre-emphasis is essential. Otherwise, there shown in Figure 18.17(c). The baseband signal is represented by a would be a large difference between the signal to noise ratios of sequence of samples of it, SO the process is also called sampling. channels at the top and the bottom of the band. A train of pulses having a pulse repetition frequency (PRF) f may be represented by a Fourier series, as in Equation 18.54. It has been assumed above that amplitude variations due to noise have no effect on the output signal of a FM receiver. For this assumption to be valid, the input signal to noise ratio must exceed a threshold of approximately 12dB (Brown and Glazier, 1974). When Sampling Baseband Baseband Transmission output the input signal to noise ratio decreases below this, peaks of noise input begin to obliterate the carrier and the output signal to noise ratio deteriorates rapidly, as shown in Figure 18.15. Radio links used for commercial telecommunications must therefore have signal to noise ratios well above the threshold. Figure 18.16 Basic pulse amplitude modulation system Pulse modulation with one direction of transmission only 18.4.1 General (18.54) In the preceding sections, it has been assumed that the carrier wave cos n is sinusoidal. Howe