Pulse amplitude modulated

Pulse-amplitude modulation (PAM), is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulse. It is an analog pulse modulation scheme in which the amplitudes of a train of carrier pulses are varied according to the sample value of the message signal. Demodulation is performed by detecting the amplitude level of the carrier at every single period. PAM is a technique that is also used in PCM.

There are two types of pulse amplitude modulation:

Pulse-amplitude modulation is widely used in modulating signal transmission of digital data, with non-baseband applications having been largely replaced by pulse-code modulation, and, more recently, by pulse-position modulation.

In particular, all telephone modems faster than 300 bit/s use quadrature amplitude modulation (QAM). (QAM uses a two-dimensional constellation).

The number of possible pulse amplitudes in analog PAM is theoretically infinite. Digital PAM reduces the number of pulse amplitudes to some power of two. For example, in 4-level PAM there are ${\displaystyle 2^{2}}$ possible discrete pulse amplitudes; in 8-level PAM there are ${\displaystyle 2^{3}}$ possible discrete pulse amplitudes; and in 16-level PAM there are ${\displaystyle 2^{4}}$ possible discrete pulse amplitudes.

...
Wikipedia