Which modulation type is characterized by varying the amplitude of the carrier wave?

Amplitude Modulation (AM) is characterized by varying the amplitude of the carrier wave while keeping the frequency constant. In AM, the information or audio signal changes the height of the carrier signal's waveform, which allows the variations in amplitude to convey the information being transmitted. This modulation type is widely used in AM radio broadcasting, where the audio signal from a microphone is used to modulate the carrier wave's amplitude for transmission to a radio receiver.

In contrast, Frequency Modulation (FM) varies the frequency of the carrier wave in accordance with the information signal, while Phase Modulation (PM) alters the phase of the carrier signal. Pulse Code Modulation (PCM) is a digital method that encodes audio signals into a series of pulses, which does not involve continuous variation of amplitude, frequency, or phase like the other methods. The emphasis in AM on altering amplitude directly relates to its definition and application, distinguishing it clearly from these other modulation techniques.