oherent transmission. Consequently, the method is used whenever its com- demodulator, consisting of a balanced modulator supplied with a plexity is justified by the saving in bandwidth (Pappenfus, 1964). locally generated carrier as shown in Figure 18.6(b) instead of the An important example is the use of SSBSC for multichannel carrier 18/6 Analogue modulation telephone systems (Kingdom, 1991). An error in the frequency of does on each component of the signal. Consequently, the signal to the local carrier of the demodulator results in a corresponding shift noise ratio at the output of the demodulator is the same as the input in the frequencies of the components in the baseband output signal. signal to noise ratio. For speech transmission, frequency shifts of the order of 10Hz are For a DSBSC signal, the output of the demodulator produced by not noticeable, but the errors that can be tolerated for telegraph and a signal component at f + is in phase with that produced by the data transmission are less. The CCITT specifies that the frequency component at fc-fm.Thus," the output signal voltage is twice the shift should be less than +2Hz. input signal voltage (c.f. Equation 18.15 with 0 = 0) and the output A SSBSC signal can be generated by using a balanced modulator signal power is four times that for SSBSC. For noise, the outputs and a bandpass filter, as for DSBSC. However, the filter in Figure produced by components at and f have a random phase 18.6(a) is designed to pass only one of the sidebands, instead of difference. Thus, the output noise power is twice the input noise both. power. Consequently, the output signal to noise ratio is twice the An alternative method of generating a SSBSC signal is the quad- input signal to noise ratio, an apparent improvement of 3dB over rature method shown in Figure 18.7. This uses two product modu- SSBSC. However, the bandwidth required for DSBSC transmission lators. The baseband signal, and the carrier, y c' are applied to is twice that for SSBSC. This doubles the received noi