panel of any modern radio is a switch AM-FM.As a rule, the ordinary consumer does not think about the meaning of those letters, it is enough to remember that it has an FM favorite FM radio stations broadcasting a signal in stereo and with great quality, and AM you can catch the "Lighthouse".If we look into the technical details at least at the level of the user manual, you will find that the AM - amplitude modulation is, as FM - frequency.How do they differ?
To the speaker of the radio sounded the music, the sound must undergo certain changes.Primarily it should be made suitable for a radio broadcast.Amplitude Modulation is the first way that engineers learned signalmen transmit voice and music programs on the air.American Fessenden in 1906 by mechanical vibrations in the generator was 50 kilohertz, which became the first in the history of the carrier frequency.Then he decided to technical problem in the simplest way, by setting the microphone output winding.Under the influence of sound waves on the carbon powder inside the membrane of the box changes its resistance, and the magnitude of the signal from the generator to the transmitting antenna, reduce or increase depending on them.Thus was invented amplitude modulation, i.e. the carrier signal amplitude variation in such a way that the envelope shape consistent with the shape of the line transmission signal.In the twenties, mechanical generators were driven electron-lamp.This greatly reduced the size and weight transmitters.
frequency modulation amplitude is different from the fact that the scope of the carrier wave remains constant, changing its frequency.With the development of electronic circuit technology base and there were other ways in which the information signal is "sit" on the frequency of the radio spectrum.Changing the phase and the pulse width was given the name of the phase and pulse width modulation.It seemed that the amplitude modulation radio broadcasting as a way out of date.But things turned out differently, it has retained its position, albeit in a somewhat modified form.
increasing demands for information saturation frequency encourage engineers to look for ways to increase the number of channels transmitted on the same wavelength.Features a multi-channel broadcasting are determined Nyquist Theorem and Nyquist barrier, however, in addition to the quantization of the signal, it is possible to increase the data load on the communication channel by changing the phase.Quadrature amplitude modulation - is a transmission method in which a single frequency transmitted different signals, phase shifted relative to each other by 90 degrees.Four-phase quadrature forms or a combination of two components, described by trigonometric functions sin and cos, hence the name.
quadrature amplitude modulation is widely used in digital communication.At its core, it is a combination of phase and amplitude modulation.
