e amplitude modulation (QAM). In practice, the Alternatively, subtracting from V2 gives Equation 18.19. method is not used for analogue baseband signals since small errors in the phases of the local carriers cause a fraction of the signal of (18.19) each channel to appear as crosstalk in the output from the other. However, the method is used for transmitting digital signals. It is straightforward to provide a 90° phase shift at the single carrier frequency, but not over the band of frequencies of a modu- 18.2.3 Vestigial sideband modulation lating signal. Instead, the baseband signal is applied to the modula- tors through a pair of networks whose phase shifts differ by If the sideband signal extends down to very low frequencies, as in approximately 90° over the required band (Coates, 1975). The television, it is almost impossible to suppress the whole of the quadrature method is not widely used, since small phase errors unwanted sideband without affecting low frequency components in result in unwanted frequency components in the output signal. the wanted sideband. Use is then made of vestigial sideband (VSB) A coherent demodulator is required for demodulating a SSBSC transmission instead of SSBSC. A conventional AM signal (as signal. Since only one sideband is present, the demodulated output shown in Figure 18.1(b)) is first generated and this is then applied signal, from Equation 18.15, is as in Equation 18.20. to a filter having a transition between its pass and stop band that is skew symmetric about the carrier frequency. This results in an (18.20) output signal having the spectrum shown in Figure 18.1(e). If a coherent demodulator is used, the original baseband signal can be recovered without distortion. It is also possible to use a simple If a SSBSC signal is corrupted by noise, the coherent demodulator envelope demodulator for VSB, but some non linear distortion then acts on each component of the noise spectrum in the same way as it results (Black, 1953). VSB transmission does, of course, require a Frequenc