 modulator. So, in 4-ASK, the output of the modulator is two bit durations long. You'll want to notice that each waveform created in a 4-ASK modulator lasts twice as long as a waveform created in a B-ASK modulator. That's because each waveform in 4-ASK is representing two bits (each of duration T), and SO will last a total time of T = 2T;; meanwhile, each waveform in B-ASK is repre- senting only one bit (of duration T), and SO it will last for a time T = . Output waveform Input bits Output waveform (shorthand form) so(t) = -3A cosw t iT<t<(i+1)T s((t) = -A COS Wct, iT<t<<i+1)T S(t)=Acoswt, iTSt<(i+1)T = 53(t) = Table 5.2 4-ASK Getting It from Here to There: Modulators and Demodulators Some say "size matters" or "bigger is better." To those who do, we offer you 8- ASK. A simple extension of the ideas of B-ASK and 4-ASK, in 8-ASK three bits are input to the modulator at the same time, and the modulator outputs one of eight possible waveforms. Table 5.3 summarizes what's going on. As you can see here, when bits 000 enter the modulator, it outputs -7Acos(wit) ; if the next 3 bits entering the modulator are 111, then it outputs +7Acos(w)t. And SO on. Output waveform Input bits Output waveform (shorthand form) s(t)==7Acos oft,ITS = s,(t) = -5A Wt, S2(t)=-3Acos Wt,iTSt<(i+1)7 = S3(t) = s(t)=3AcosWt,iTSt<(i+1)T = Table 5.3 8-ASK Of course, things don't stop with 8-ASK. You could make a 16-ASK modulator, a 32-ASK modulator, or even a 5092-ASK modulator. The tables just get bigger, but the idea stays the same. The most popular of the bandpass modulators are the PSK modulators, short for phase shift-keying modulators. With these, input bits are mapped into output waveforms of the form s(t) = Acos(at+0), , and the information bits are stuffed in the phase O. We'll start with the simplest case first, BPSK (binary PSK). In BPSK, when bit 0 goes into the modulator, the modulator spits out the wave- form Acos(w,++0°). If bit 1 struts into the modulator, it pops out Acos(@,++180°). Figure 5.16 shows you what happens when bits 010