Pulse amplitude modulation (PAM) is the transmission of data by varying the amplitude s ( voltage or power levels) of the individual pulses in a regularly timed sequence of electrical or electromagnetic pulses. The number of possible pulse amplitudes can be infinite (in the case of analog PAM), but it is usually some power of two so that the resulting output signal can be digital . For example, in 4-level PAM there are 2^2 possible discrete pulse amplitudes; in 8-level PAM there are 2^3 possible discrete pulse amplitudes; and in 16-level PAM there are 2^4 possible discrete pulse amplitudes.
In some PAM systems, the amplitude of each pulse is directly proportional to the instantaneous modulating-signal amplitude at the time the pulse occurs. In other PAM systems, the amplitude of each pulse is inversely proportional to the instantaneous modulating-signal amplitude at the time the pulse occurs. In still other systems, the intensity of each pulse depends on some characteristic of the modulating signal other than its strength, such as its instantaneous frequency or phase .
PAM is only one of several forms of pulse modulation. Other methods include varying the durations (or widths), the frequencies, the positions, or the intervals of the individual pulses in a sequence.