Modulation

Origin

• 1. the action of treating, regulating, or varying something so as to achieve due measure and proportion; variation of light, line, form, etc., with regard to artistic effect; an act of softening, tempering, or toning down.
• 2. Musical inflection of the voice; variation of the quality of one's voice, or of the sound of a musical instrument, with respect to tone, pitch, and intensity. Also: a particular inflection or intonation.
• 3. The action or process of passing from one key to another in the course of a piece of music; the result of this, as an element in the harmony of a piece; a change of key.
• 4. Physics: The process of modulating an electromagnetic wave or other oscillating signal, or a beam, esp. in order to impress a signal on it; the extent to which a modulated carrier wave is varied; the modulated waveform or signal itself.

Description

In electronics, modulation is the process of varying one or more properties of a high frequency periodic waveform, called the carrier signal, with respect to a modulating signal. This is done in a similar fashion as a musician may modulate a tone (a periodic [waveform) from a musical instrument by varying its volume, timing and pitch. The three key parameters of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also occur.

In telecommunications, modulation is the process of conveying a 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 is used to transform a baseband message signal to a passband signal, for example a radio-frequency signal (RF signal). In radio communications, cable TV systems or the public switched telephone network for instance, electrical signals can only be transferred over a limited passband frequency spectrum, with specific (non-zero) lower and upper cutoff frequencies. Modulating a sine wave carrier makes it possible to keep the frequency content of the transferred signal as close as possible to the centre frequency (typically the carrier frequency) of the passband. When coupled with demodulation, this technique can be used to, among other things, transmit a signal through a channel which may be opaque to the baseband frequency range (for instance, when sending a telephone signal through a fiber-optic strand).

In music synthesizers, modulation may be used to synthesise waveforms with a desired overtone spectrum. In this case the carrier frequency is typically in the same order or much lower than the modulating waveform. See for example frequency modulation synthesis or ring modulation.

A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (short for "Modulator-Demodulator").[1]