The process of manipulating the frequency or amplitude of a carrier in relation to an incoming video, voice or data signal. Modulator is a device which modulates a carrier. Modulators are found as components in broadcasting transmitters and in satellite transponders. Modulators are also used by CATV companies to place a baseband video television signal onto a desired VHF or UHF channel. Home video tape recorders also have built-in modulators which enable the recorded video information to be played back using a television receiver tuned to VHF channel 3 or 4.

No matter which technique but in any modulation scheme, the data and the carrier frequency (the main frequency on which a voice, data, or video signal is sent) must be available and known. It is often important that the receiving device also knows about the carrier frequency, depending on the method of demodulation.

There are different schemes of modulation for digital and analogue purposes), each having it‘s own specifications, positive and negative points. Here we list some of them:

Amplitude Modulation (AM): The baseband (data) signal is caused to vary the amplitude or height of the carrier wave to create the desired information content.

Frequency Modulation (FM): A modulation method whereby the baseband signal varies the frequency of the carrier wave.

Phase Modulation (PM): A modulation method, somehow similar to FM from technical point of view, but the variations made by the baseband signal is applied to the phase information of the carrier wave.

Pulse Amplitude Modulation (PAM): Consists of sending discrete amplitude levels (symmetric about 0 volts) at a regular rate.

Pulse Width Modulation (PWM): Sends pulses with equal height and equal rating position (respect to time). However the amplitude of the sampled (analogue or digital) signal determines the width of each pulse. That is the bigger the value of the sampled signal, the wider that output pulse results and the smaller the amplitude results narrower output pulse.

Pulse Code Modulation (PCM): This method possesses similar procedure specified as sampling, quantizing and binary coding. Therefore the input can be analogue or digital and the output is a train of digital bytes in a specific rate. PCM is greatly used in phone network to reduce the data rate required for voice grade audio to less than 64Kbps.

Amplitude Shift Keying (ASK): Where the zero and one effect of the digital data is applied to the carrier so that, when the data is in the zero state, there is no output and in one state, the carrier itself (we will not cover digital schemes in this section).

Frequency Shift Keying (FSK): Where two different frequencies are used to indicate zero and one.

Phase Shift Keying (PSK): Where a single frequency in two different phase forms are used to indicate zero and one.

Quadrature Amplitude Modulation (QAM): In this method, 16 wave conditions can be included in one carrier cycle. These conditions are produced by 4 different amplitude levels and 4 different phase information. Therefore 4 bits can be transmitted by each cycle.

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