On the panel of any modern radio there is an AM-FM switch. As a rule, an ordinary consumer does not think about what these letters mean, it is enough for him to remember that there is his favorite VHF radio station on FM, broadcasting a signal in stereo sound and with excellent quality, and on AM you can catch Mayak. If you delve into the technical details at least at the level of the user manual, it turns out that AM is amplitude modulation, and FM is frequency modulation. How are they different?
In order for music to sound from the radio speaker, the sound signal must undergo certain changes. First of all, it should be made suitable for radio broadcasting. Amplitude modulation was the first way that communications engineers learned to transmit speech and music programs over the air. The American Fessenden in 1906, using a mechanical generator, received oscillations of 50 kilohertz, which became the first carrier frequency in history. He further solved the technical problem in the simplest way possible by installing a microphone at the output of the winding. When sound waves acted on coal powder inside the membrane box, its resistance changed, and the magnitude of the signal,coming from the generator to the transmitting antenna, decreased or increased depending on them. This is how amplitude modulation was invented, that is, changing the amplitude of the carrier signal so that the shape of the envelope line matches the shape of the transmitted signal. In the 1920s, mechanical generators were replaced by vacuum tube ones. This greatly reduced the size and weight of the transmitters.
Frequency modulation differs from amplitude modulation in that the amplitude of the carrier wave remains unchanged, its frequency changes. As the electronic base and circuitry developed, other methods appeared by which the information signal "sat down" on the frequency of the radio range. The change in the phase and width of the pulse gave the name to phase and pulse-width modulation. It seemed that amplitude modulation as a way of broadcasting was outdated. But it turned out differently, she retained her positions, although in a slightly modified form.
The growing demands for information saturation of frequencies prompted engineers to look for ways to increase the number of channels transmitted on one wave. The possibilities of multichannel translation are determined by the Kotelnikov theorem and the Nyquist barrier, however, in addition to signal quantization, it became possible to increase the information load on the communication channel by changing the phase. Quadrature amplitude modulation is a transmission method in which different signals are transmitted at the same frequency, shifted in phase relative to each other.friend 90 degrees. Four-phase forms a quadrature or a combination of two components described by the trigonometric functions sin and cos, hence the name.
Quadrature amplitude modulation is widely used in digital communications. At its core, it is a combination of phase and amplitude modulation.
