Modulation is the process of varying a carrier signal in order to use that signal to convey information. The three key parameters of a sinusoid are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal. A device that performs modulation is known as a modulator and a device that performs the inverse operation of demodulation is known as a demodulator. A device that can do both operations is a modem (a contraction of the two terms).

In digital modulation, the changes in the signal are chosen from a fixed list (the modulation alphabet) each entry of which conveys a different possible piece of information (a symbol). The alphabet is often conveniently represented on a constellation diagram.

In analog modulation, the change is applied continuously in response to the data signal. The modulation may be applied to various aspects of the signal as the lists below indicate.

Analog modulation techniques

Angular modulation

· Phase modulation (PM)

· Frequency modulation (FM)

Amplitude modulation (AM)

· Single-sideband modulation (SSB, or SSB-AM), very similar to single-sideband suppressed carrier modulation (SSB-SC)

· Vestigial-sideband modulation (VSB, or VSB-AM)

· Sigma-delta modulation (∑Δ)

Digital modulation techniques

Any form of digital modulation necessarily uses a finite number of distinct signals to represent digital data.

· In the case of PSK, a finite number of phases are used.

· In the case of FSK, a finite number of frequencies are used.

· In the case of ASK, a finite number of amplitudes are used. This is very similar to pulse code modulation

Pulse modulation

These are hybrid digital and analogue techniques.

· Pulse-code modulation (PCM)

· Pulse-width modulation (PWM)

· Pulse-amplitude modulation (PAM)

· Pulse-position modulation (PPM)

· Pulse-density modulation (PDM)

Miscellaneous techniques

· The use of on-off keying to transmit Morse code at radio frequencies is known as continuous wave (CW) operation.

· Adaptive modulation

· Wavelet modulation

· Trellis coded modulation (TCM) also known as trellis modulation