At first, it may seem that the term “frequency modulation” is used only by people whose work is associated with the repair or maintenance of electronic radio transmitting equipment. This is actually a common misconception. Frequency modulation is familiar to most, simply because of the use of the English name, many do not even suspect that it is often mentioned in everyday life. Now the names of the radio stations are on everyone’s lips. These are various "FM radio". Have you ever wondered what the prefix "FM" means? This is frequency modulation, only in the Russified acronym - Frequency Modulation, that is, FM.
To modulate any analog signal that carries useful information, several varieties of its formation are used. The most widely used are the following types of modulation: frequency and amplitude. Let's consider each of them in more detail.
The term "analog" means continuous in time, in contrast to a discrete digital, transmitted by the alternation of pulses. Recall the basics of physics and the sine wave of alternating current. Cartesian coordinates on the plane: horizontal and vertical lines. Horizontal time is plotted, and the vertical displays the intensity (current value). A sine wave is a continuous wave, as if strung on the time axis. In order to transmit information using a sine wave, it is necessary to perform encoding in a certain way, that is, to conditionally convert the wave.
Imagine that we can take a sinusoid by 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 of time will become larger. In fact, the sine wave is compacted. That is, the frequency has been changed. However, such a sinusoid is not a useful signal: if it is received on a radio receiver, then you can hear only a uniform background (squeak, hum).
In order for this carrier frequency to contain a useful component, it is necessary to perform encoding or modulation, which is one and the same. Imagine now that in one place we squeezed the waves more strongly than in another, but somewhere in general we stretched a sinusoid, that is, we formed a certain order. As a result, the background became modulated, changing the key (when playing a speaker). If now you often change the tone, then you can convey useful information (sound). This is frequency modulation. Of course, nobody stretches the sinusoid with their hands - this is the task of special converters.
A similar principle is used by amplitude modulation of the signal (AM, Amplitude Modulation), with the only difference being that it is not the frequency of the waves that undergoes the change, but their height. Imagine a sine wave: grab with your hands several waves located above and below the horizontal axis of time. Now we stretch or compress them relative to the axis. As a result, the height changes. The next group of waves is also changed in a similar way, but with less (or more) 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 waves in the required manner, thereby encoding the information. As in the case of frequency modulation, here, too, no one is stretching anything with his hands - this is the work of the corresponding devices.
A method using amplitude modulation of the signal is very limited by the width of the spectrum, therefore, it is of little use for stereo transmission. However, due to the different wavelength and radiated power in the FM and AM, the first requires frequent placement of repeaters. In turn, AM in radio is characterized by a large covered distance and a clearly distinguishable useful component, even in the presence of noise.