Modulation is the process of incorporating information inside a periodic waveform by varying a certain characteristic or property of the periodic waveform. The periodic waveform is called the 'carrier signal', since it carries the information, while the information comes from another waveform of lower frequency known as the 'information signal'.  The purpose of modulation is to be able to use the carrier signal in the efficient transmission of the information. 

The carrier signal used for transmissions is usually a high-frequency sinusoidal waveform.  A sine wave is often defined in terms of three major parameters: its amplitude, its frequency, and its phase. Any of these three parameters may be varied or modulated to carry information.

Using a waveform of high frequency to carry the signal offers the following advantages: 1) lower loss and dispersion during propagation; 2) minimal interference from other signals; 3) smaller antenna requirements; 4) possibility of multiplexing for simultaneous transmission of multiple signals.  The typical carrier frequency ranges used by various systems today are as follows: 550-1600 KHz for AM Radio, 88-108 MHz for FM Radio, 52-216 MHz for VHF TV, and 470-900 MHz for UHF TV.  

During modulation of a given parameter of a carrier signal, the parameter is made to vary in accordance with how the information signal varies.  For example, in amplitude modulation (AM), the amplitude of the carrier signal is made to change so that it follows changes in the shape of the information signal.  In frequency modulation (FM), it is the frequency of the carrier signal that is continuously varied according to the information waveform. In phase modulation (PM), the phase of the carrier waveform is shifted to match variations of the information signal in time.  A device that modulates a carrier frequency is known as a modulator, while a demodulator is a device that retrieves the information from a modulated carrier signal.

In the above examples of modulation, the information comes from an analog signal, i.e., a signal that is continuous in time.  When a carrier signal is modulated continuously in time by an analog signal, it is referred to as 'analog modulation'.  There is another type of modulation, wherein the carrier signal is modulated by a stream of digital bits. This modulation, known as digital modulation, is used in the transmission of digital data over an analog channel.  Thus, digital modulation may also be considered as a form of digital-to-analog-conversion, while the recovery of the digital bits by demodulation after transmission over the analog channel may be considered as analog-to digital conversion.  Sending digital data over a telephone line via a modem is an example of this.

Many techniques exist for both analog and digital modulation. In fact, amplitude modulation alone can be implemented in many ways including: Double-Sideband Modulation (DSB), Single-Sideband Modulation(SSB), Vestigial Sideband Modulation (VSB), etc. Each of these AM techniques has many different kinds too. The analog modulation techniques of FM and PM (which were defined earlier in this article), are kinds of angle modulation.  

Techniques used in digital modulation include:

1) Phase-Shift Keying (PSK), wherein data are represented by modulating the phase of the carrier signal, i.e., a finite number of phases are used, each one corresponding to a unique pattern of binary bits;

2) Frequency-Shift Keying (FSK), wherein data are represented by discrete changes in the frequency of the carrier signal, e.g., a certain frequency is used to represent '1' while another is used for '0'; and

3) Amplitude-Shift Keying (ASK), wherein data are represented by discrete levels of the amplitude of the carrier signal; and

Another modulation technique, known as 'Quadrature Amplitude Modulation'  or QAM, modulates the amplitudes of two carrier waves that are out of phase with each other by 90 degrees (hence the term 'quadrature'). QAM can be employed both in digital and analog modulation by ASK or AM, respectively.

Note that each of these major analog and digital techniques also comes in many different and special forms, each providing a solution to the vast requirements of the enormous telecommunications industry.

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