telecommunications, modulation is the process of varying one or more properties of a periodic
waveform, called the
carrier signal, with a modulating signal that typically contains information to be transmitted. Most radio systems in the 20th century used
frequency modulation (FM) or
amplitude modulation (AM) to make the carrier carry the radio broadcast.
In general telecommunications, modulation is a process of conveying message signal, for example, a digital bit stream or an
analog audio signal, inside another signal that can be physically transmitted. Modulation of a sine waveform transforms a narrow frequency range
baseband message signal into a moderate to high frequency range
passband signal, one that can pass through a filter.
A modulator is a device that performs modulation. A demodulator (sometimes detector or demod) is a device that performs
demodulation, the inverse of modulation. A
modem (from modulator–demodulator) can perform both operations.
The aim of analog modulation is to transfer an
analog baseband (or
lowpass) signal, for example an audio signal or TV signal, over an analog bandpass channel at a different frequency, for example over a limited radio frequency band or a cable TV network channel.
The aim of digital modulation is to transfer a
digital bit stream over an analog
channel, for example over the
public switched telephone network (where a
bandpass filter limits the frequency range to 300–3400 Hz) or over a limited radio frequency band.
Analog and digital modulation facilitate
frequency division multiplexing (FDM), where several low pass information signals are transferred simultaneously over the same shared physical medium, using separate passband channels (several different carrier frequencies).
The aim of digital baseband modulation methods, also known as
line coding, is to transfer a digital bit stream over a baseband channel, typically a non-filtered copper wire such as a
serial bus or a wired
local area network.
The aim of pulse modulation methods is to transfer a
narrowband analog signal, for example, a phone call over a
wideband baseband channel or, in some of the schemes, as a bit stream over another
digital transmission system.
In music synthesizers, modulation may be used to synthesize waveforms with an extensive overtone spectrum using a small number of oscillators. In this case, the
carrier frequency is typically in the same order or much lower than the modulating waveform (see
frequency modulation synthesis or
ring modulation synthesis).