dulation It is possible to combine ASK and PSK to obtain hybrid modula- tion, as shown in Figure 19.12. The 8-level scheme shown in Figure 19.12(b) has a similar error threshold voltage to the PSK scheme of Figure 19.11. However, it has a lower mean transmitted power, because four of the eight states have a smaller carrier amplitude than those in Figure 19.11. Signal to noise ratio (dB) These hybrid modulation schemes can be implemented using quadrature amplitude modulation (QAM), i.e. modulation of two carriers in quadrature as shown in Figure 19.13. If a 2-level signal Figure 19.9 Bit error probability for various digital sytems is applied to each modulator, the output has a constant amplitude, as shown in Figure 19.12(a). It therefore produces quadrature phase shift keying (QPSK). 19.3.5 Quadrature amplitude modulation If a 3-level signal (s, = -1, = =0,s3=+1) is applied to each of the two modulators, the sum of their outputs has three values of its If a carrier is modulated by a digital signal, its amplitude and phase in phase component and three values of its quadrature component. can only have a finite number of values. These can be represented This permits duobinary partial response coding and the system is in a signal space diagram, as shown in Figures 19.10 and 19.11. known as a quadrature partial response (QPRS) system (Taub and These show an 8-level ASK signal and an 8-level PSK signal. The Schilling, 1986). circles show the maximum permissible perturbation from the ideal If a 4-level signal is applied to each of the two modulators, the sum signal before an error may occur in the transmission. It is obvious of their outputs has four values of its in phase component and four that, in this respect, PSK is superior to ASK. values of its quadrature component. This produces the sixteen state signal constellation shown in Figure 19.12(c). The demodulators reproduce the two 4-level signals at their outputs. Four state QAM is used to produce QPSK to transmit data at 2400bit/s over switched telephone circuits w