Another method of modulation works by varying some aspect of a constant stream of signal pulses. Several types of pulse modulation (PM) are briefly described in the following sections. They are diagrammed in below figure as amplitude-versus-time graphs. The modulating waveform in each case is shown as a dashed curve, and the pulses are shown as vertical gray bars.
Time-domain graphs of various modes of pulse modulation. At A, pulse amplitude modulation; at B, pulse width modulation; at C, pulse interval modulation; at D, pulse code modulation.
Pulse Amplitude Modulation
In pulse amplitude modulation (PAM), the strength of each individual pulse varies according to the modulating waveform. In this respect, PAM resembles AM. An amplitude-versus-time graph of a hypothetical PAM signal is shown in Fig. 25-8A. Normally, the pulse amplitude increases as the instantaneous modulating signal level increases (positive PAM). But this can be reversed, so higher audio levels cause the pulse amplitude to go down (negative PAM). Then the signal pulses are at their strongest when there is no modulation. The transmitter works a little harder if negative PAM is used.
Pulse Width Modulation
Another way to change the transmitter output is to vary the width (duration) of the pulses. This is called pulse width modulation (PWM) or pulse duration modulation (PDM), and is shown in above figure B. Normally, the pulse width increases as the instantaneous modulating-signal level increases (positive PWM). But this can be reversed (negative PWM). The transmitter must work harder to accomplish negative PWM. Either way, the peak pulse amplitude remains constant.
Pulse Interval Modulation
Even if all the pulses have the same amplitude and the same duration, modulation can still be accomplished by varying how often they occur. In PAM and PWM, the pulses are always sent at the same time interval, known as the sampling interval. But in pulse interval modulation (PIM), also called pulse frequency modulation (PFM), pulses can occur more or less frequently than they do when there is no modulation. A hypothetical PIM signal is shown in Fig. 25-8C. Every pulse has the same amplitude and the same duration, but the time interval between them changes. When there is no modulation, the pulses are evenly spaced with respect to time. An increase in the instantaneous data amplitude might cause pulses to be sent more often, as is the case in above figure C ( positive PIM). Or, an increase in instantaneous data level might slow down the rate at which the pulses are sent (negative PIM).
Pulse Code Modulation
In recent years, the transmission of data has been done more and more by digital means. In digital communications, the modulating data attains only certain defined states, rather than continuously varying. Digital transmission offers better efficiency than analog transmission. With digital modes, the signal-to-noise (S/N) ratio is better, the bandwidth is narrower, and there are fewer errors. In pulse-code modulation (PCM), any of the above aspects—amplitude, duration, or interval—of a pulse sequence (or pulse train) can be varied. But rather than having infinitely many possible states, there are finitely many. The number of states is a power of 2, such as 4, 8, or 16. The greater the number of states, the better the fidelity. An example of 8-level PCM is shown in above figure D