The ratio of the frequency deviation to the highest modulating audio frequency iscalled the modulation index. For good fidelity, it should be at least 1: 1. But it shouldnot be much more; that would waste spectrum space.Pulse modulationStill another method of modulation works by varying some aspect of a constant streamof signal pulses. Several types of pulse modulation (PM) are briefly described below.They are diagrammed in Fig. 26-12 as amplitude-versus-time graphs. The modulatingwaveforms are shown as curvy lines, and the pulses as vertical lines.Pulse modulation48526-12Pulse modulation. At A, pulse amplitude modulation; at B, pulse widthmodulation; at C, pulse interval modulation; and at D, pulse codemodulation.Pulse amplitude modulationIn pulse amplitude modulation (PAM), the strength of each individual pulse variesaccording to the modulating waveform. In this respect, PAM is very much like ordinaryamplitude modulation. An amplitude-versus-time graph of a hypothetical PAM signal isshown in Fig. 26-12A.