What is frequency modulation

At first, it may seem that the term "frequency modulation" is used only by those people whose work is related to the repair or maintenance of electronic radio transmitting equipment. In fact, this is a common misconception. Frequency modulation is familiar to most, simply because of the use of the English-language name, many do not even suspect that in everyday life it is often mentioned. Now the names of radio stations are on everyone's lips. This is a different "radio FM". Have you ever wondered what the "FM" prefix means? This is the frequency modulation, only in the abbreviated abbreviation - Frequency Modulation, that is FM.

To modulate any analog signal carrying useful information, several types of its formation are used. The most widespread types of modulation: frequency and amplitude. Let's consider each of them in more detail.

The term "analogue" means continuous in time, unlike a discrete digital, transmitted by alternating pulses. Let us recall the fundamentals of physics and the sinusoidal alternating current. Cartesian coordinates in the plane: horizontal and vertical lines. Time is plotted horizontally, and the vertical displays the intensity (current value). Sinusoid are continuous waves, as if strung on the axis of time. In order to transmit information using a sinewave, it is necessary to perform a certain coding, that is, to convert the wave in a conventional way.

Imagine that we can take a sinusoid in two sides (two arbitrary points at a distance from each other) and compress it like an accordion. As a result, the waves will become sharper, the area of each of them will decrease, but their number, passing per unit time, will become larger. In fact, the sine wave will be condensed. That is, the frequency was changed. However, such a sinusoid is not a useful signal: if you take it to a radio receiver, you can hear only a uniform background (squeak, hum).

For this carrier frequency to contain a useful component, you need to perform encoding or modulation, which is the same. Imagine now that we in one place squeezed the waves more strongly than in the other, and somewhere in general they stretched the sinusoid, that is, they formed a certain order. As a result, the background became modulated, changing the key (when played back by a speaker). If you change your tone often, you can send useful information (sound). This is the frequency modulation. Of course, no one stretches the hands of a sinusoid - this is the task of special converters.

A similar principle uses amplitude modulation of the signal (AM, Amplitude Modulation), with the only difference being that the change is not in the frequency of the waves, but in their height. Mentally, imagine a sinusoid: grab hold of several waves, located above and below the horizontal axis of time. Now we stretch or compress them about the axis. As a result, the height changes. The next group of waves is also changed in a similar way, but with a smaller (or greater) intensity. If you send such a "crumpled" sinusoid to the speaker, you can hear a change in tone: where the waves are higher, there is more signal intensity, and where lower - there is less. Further everything is simple: we compress (stretch) the wave in the required way, thereby encoding the information. As in the case of frequency modulation, here, too, nothing is stretched by hand - this is the work of the corresponding instruments.

The method using amplitude modulation of the signal is strongly limited by the width of the spectrum, therefore, stereo transmission is of little use. True, because of different wavelengths and radiated power in FM and AM, the first requires frequent placement of repeaters. In turn, for AM, the radio is characterized by a large covered distance and a clearly distinguishable useful component, even in the presence of noise.