Amplitude modulation and its improvement

The panel of any modern radio has an AM-FM switch. As a rule, the average consumer does not think about what these letters mean, he just has to remember that his favorite VHF radio broadcasting a signal in stereo sound and with excellent quality is on FM, and you can catch "Mayak" on AM. If you look at the technical details at least at the level of user instructions, it turns out that AM is amplitude modulation, and FM is frequency. What are they different?

In order for music to sound from the loudspeaker of the radio, 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 communication engineers learned to transmit voice and music programs on air. The American Fessenden in 1906, with the help of a mechanical generator, received oscillations of 50 kilohertz, which became the first carrier frequency in history. Next, he solved the technical problem in the simplest way by installing a microphone at the output of the winding. When sound waves were applied to the carbon 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. Thus, amplitude modulation was invented, i.e., a change in the span of the carrier signal such that the shape of the envelope line corresponded to the form of the transmitted signal. In the twenties, mechanical generators were replaced by electronic tube lamps. This greatly reduced the dimensions and weight of the transmitters.

Frequency modulation differs from amplitude modulation in that the sweep of the carrier wave remains unchanged, its frequency varies. As the electronic base and circuit technology developed, other methods appeared, with the help of which the information signal "sat down" on the frequency of the radio range. The change in phase and pulse width gave the name phase and pulse width modulations. It seemed that the amplitude modulation as a method of radio transmission was obsolete. But it turned out otherwise, she retained her position, albeit in a slightly modified form.

The growing demands on information frequency saturation prompted engineers to look for ways to increase the number of channels transmitted on the same wavelength. The possibilities of multichannel translation are determined by the Kotel'nikov theorem and the Nyquist barrier, however, in addition to quantizing the signal, 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 on one frequency, shifted in phase relative to each other by 90 degrees. Four-phase forms a quadrature or a combination of two components described by trigonometric functions sin and cos, hence the name.

Quadrature amplitude modulation has become widespread in digital communications. In its essence, it is a combination of phase and amplitude modulation.