Modulation, Frequency modulation, Signal transmission, Signal modulation

Modulation is the addition of information (or the signal) to an electronic or optical signal carrier. Modulation can be applied to direct current (mainly by turning it on and off), to alternating current, and to optical signals. One can think of blanket waving as a form of modulation used in smoke signal transmission (the carrier being a steady stream of smoke).

A computer with an online or Internet connection that connects over a regular analog phone line includes a modem. This term is derived by combining beginning letters from the words modulator and demodulator. In a modem, the modulation process involves the conversion of the digital computer signals (high and low, or logic 1 and 0 states) to analog audio-frequency (AF) tones. Digital highs are converted to a tone having a certain constant pitch; digital lows are converted to a tone having a different constant pitch. These states alternate so rapidly that ,if you listen to the output of a computer modem, it sounds like a hiss or roar. The demodulation process converts the audio tones back into digital signals that a computer can understand. directly.