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Sound Definitions

Sound effects : modulation

Modulation is the process of varying a carrier signal, typically a sinusoidal signal, in order to use that signal to convey information. One of the three key characteristics of a signal are usually modulated: its phase, frequency or amplitude. A device that performs modulation is known as a modulator and a device that performs 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 analogue 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.

Modulation is generally performed to overcome signal transmission issues such as to allow

Easy (low loss, low dispersion) propagation as electromagnetic waves
Multiplexing — the transmission of multiple data signals in one frequency band, on different carrier frequencies.
Smaller, more directional antennas

Carrier signals are usually high frequency electromagnetic waves.

Analogue modulation techniques

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

Phase-shift keying (PSK)
Frequency-shift keying (FSK) and audio frequency-shift keying (AFSK)
- Minimum-shift keying (MSK)
- Gaussian minimum-shift keying (GMSK)
- Very minimum-shift keying (VMSK)
Amplitude-shift keying (ASK) and its most common form, on-off keying (OOK)
Quadrature amplitude modulation (QAM), a combination of PSK and ASK.
Trellis coded modulation (TCM) also known as trellis modulation

MSK and GMSK are particular cases of continuous phase modulation (CPM). Indeed, MSK is a particular case of the sub-family of CPM known as continuous phase-frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one symbol-time duration (total response signaling).

Often incorrectly referred to as a modulation scheme, orthogonal frequency division multiplexing (OFDM) usually takes advantage of one of the digital techniques. It is also known as discrete multi tone (DMT). When OFDM is used in conjunction with channel coding techniques, it is described as Coded orthogonal frequency division multiplexing (COFDM). OFDM is strictly a channel access method and not a modulation scheme.