e bit is a 1, the signal level changes (e.g., from +V to -V). Figure 5.4 shows you what I mean. mapped to mapped to Figure 5.3 NRZ-L Chapter Five Send either change in signal level mapped to no change in signal level mapped to Figure 5.4 NRZ-M described RZ Modulators Leaving the NRZ family, we come to its well-known rivals, the RZs. As you'll likely guess, RZ is short for return-to-zero. These modulators make sure that, for at least some of the time, the transmitted signal sits at 0. First is unipolar RZ. Figure 5.5 shows you what happens to a 1 and what happens to 0 after it passes through this type of RZ modulator. Next, there's bipolar RZ, another straightforward modulator whose output waveform is shown in Figure 5.6. And finally, we've got RZ-AMI (alternative mark inversion), in which a 1 corresponds to an alternating symbol, and a 0 is sent as 0 (Figure 5.7). Getting It from Here to There: Modulators and Demodulators Tb/2 Tb/2 Figure 5.5 Unipolar RZ Tb/2 Tb/2 Tb/2 Tb/2 Figure 5.6 Bipolar RZ Chapter Five (alternates) Tb/2 Tb/2 Tb/2 Tb/2 Figure 5.7 RZ-AMI Phase-encoded Modulators Finally, we come to the modulator family known as the phase-encoded group. Among the most popular is the Manchester Coding modulator, in which bit 1 is mapped to a waveform that starts at +V and ends up at 0, and where bit 0 becomes the waveform starting out at 0 and ending at +V (Figure 5.8). Another popular member of the phase- encoded group is the Miller Coding modulator. This one believes in choice, SO it lets bit 0 become one of two waveforms and also lets bit 1 be one of two possible wave- forms (Figure 5.9). The decision as to which of the two possible waveforms to output is based on this simple rule: always make sure that there's a transition (e.g., from +V to - -V) between bits (Figure 5.9). Getting It from Here to There: Modulators and Demodulators Figure 5.8 Manchester Coding Tb/2 Tb/2 Tb/2 Tb/2 Figure 5.9 Miller Coding Tb/2 Tb/2 Tb/2 Tb/2 Always a +V to -V or -V to +V transition at each new bit Chapter Five Which Modulator