in stationary. The maximum of the demodulator, as shown in Figure 19.25(a). However, the DM quantising error is thus + A whereas in PCM it is I 1/2 . Conse- demodulator consists of an integrator, SO differentiation of the quently, the quantisation noise power is four times that given by output can be obtained by simply omitting it. The demodulator then Equation 19.48 i.e. as in Equation 19.68. consists only of a low pass filter, as shown in Figure 19.25(b). Slope overload of the delta modulator within the delta sigma (19.68) modulator occurs when dv./dt is too large. However V is given by E2-342 Equation 19.72, resulting in Equation 19.73, and slope overload of this delta modulator corresponds to amplitude limiting of the input signal to the delta sigma modulator. Consequently, the signal ampli- However, the quantisation noise power spreads over a wider tude that can be handled and the SQNR are both independent of frequency band than for PCM, because of the higher sampling frequency (Wallace et al., 1991). If it is assumed that the noise frequency. power is uniformly distributed up to frequency f , then the fraction (19.72) of the noise power reaching the output is F m/fs, where F is the cut off frequency of the baseband output channel, resulting in Equation 19.69. dvi=kum (19.73) (19.69) For a sinusoidal baseband signal of maximum amplitude, it can be shown (Johnson, 1968) that, SQNR is given by Equation 19.74. For a sinusoidal signal which just avoids slope overload, Vm is given by Equation 19.67 and the output signal power is given by (19.74) Equation 19.70 and the SQNR by Equation 19.71. (19.70) Thus, as with simple DM, the quantisation noise power decreases with the cube of the sampling frequency. (19.71) 19.6.4 Conversion filters The various digital modulation schemes can be represented by a general theoretical model (Flood and Hawksford, 1971), which For example, applies time and amplitude quantising to an analogue modulated SQNR is 36.6dB. This is similar to that obtained with 64kbit/s PCM, signal. For PC