Home > Quadrature amplitude modulation

Alternately, this can be regarded (using complex number notation) as simple amplitude modulation of a complex-valued carrier wave by a single complex-valued signal.

What this actually means is that the amplitude and the phase of the carrier wave are simultaneously changed according to the information you want to transmit.

QAM is used in NTSC and PAL television systems, where the in-phase and 90° components carry the components of chroma (colour) information. "Compatible QAM" or C-QUAM is used in AM stereo radio to carry the stereo difference information.

It is also used extensively in modems, and other forms of digital communication over analogue channels. In digital applications, the modulating signal is generally quantised in both its in-phase and 90° components. The set of possible combinations of amplitudes, as shown on an x-y plot, is a pattern of dots known as a QAM constellation.

This constellation, and therefore the number of bits which can be transmitted at once, can be increased for higher bitrates and faster throughputThroughput in theory of constraints is the rate at which a system produces money, in contrast to output which may be sold or stored in a warehouse. The signal provided by throughput is received (or not) at the point of sale exactly the right time. Output, or decreased for more reliable transmissionIn communications, transmission is the act of transmitting electrical messages (and the associated phenonomena of radiant energy that pass through media). In telecommunication, the term transmission has the following meanings: 1. The dispatching, for rece with fewer bit errorBit error indicates the number of bits of a data stream over a communication channel that have been altered by noise. The bit error is often written as the ratio of the number of failed bits to the total number of bits sent over the channel.s. The number of "dots" in the constellation is given as a number before the QAM, and is always an integerThe integers consist of the positive natural numbers (1, 2, 3, …) the negative natural numbers (−1, −2, −3,. and the number zero. The set of all integers is usually denoted in mathematics by Z (or Z in blackboard bold, ), which st power of two -- from 2¹ (2QAM) to 2¹² (4096QAM). 256QAM is often used in digital cableDigital cable is a term for a type of cable digital television that delivers more channels than possible with analog cable by using digital video compression. Digital cable also enables two-way communication, enabling services such as an on-screen program television and cable modemA cable modem is a special type of modem that is designed to modulate a data signal over cable television infrastructure. The term cable internet (or simply cable refers to the delivery of internet service over this infrastucture. Cable modems should not applications.

See also:

• modulationFor the musical use of "modulation", see modulation (music . Modulation describes a range of techniques for encoding information on a carrier signal typically a sine-wave signal. A device that performs modulation is known as a modulator . Modulation techn for other examples of modulation techniques