At first it may seem that the term "frequency modulation" is used only by people whose work involves the repair or maintenance of electronic radio transmitting equipment.In fact it is a common misconception.Frequency modulation is familiar to most, simply because of the use of English-language titles, many do not realize that in everyday life it is often mentioned.Now the station names on everyone's lips.This is different, "FM radio".But have you ever thought what is the prefix "FM"?This is the frequency modulation only Russified abbreviation - Frequency Modulation, ie FM.
to modulate any analog signal carrying useful information, use several varieties of its formation.The most widely used the following types of modulation: frequency and amplitude.Let us consider each of these in more detail.
term "analog" refers to a continuous in time, in contrast to the discrete digital transmissible alternating pulses.Let us remember the basics of physics and sine wave AC.Cartesian coordinates on the plane: horizontal and vertical lines.Horizontal delayed time and the vertical shows the intensity (current value).Sine - a continuous wave, as if strung on a time axis.In order to start using a sine wave to convey information, it is necessary to perform a certain way encoding, that is able to convert the conditional wave.
imagine that we can take a sine wave of the two sides (two points at a distance from each other) and squeeze it like an accordion.As a result, the wave will be more acute, the area of each reduced, but the number passing per unit time becomes larger.In fact, sine compacted.That is, the frequency has been changed.However, such a sine wave is not useful signal: if its take on the radio, you can hear only a uniform background (squeak, hum).
to the carrier frequency contain useful component, perform coding and modulation, which is the same.Imagine now that we are in one place squeezed wave is stronger than the other, and somewhere even stretched sine wave that is shaped a certain order.In the background was the result of a modulated, change the tone (when playing a speaker).If we now frequently change the tone, you can send useful information (sound).This is the frequency modulation.Of course, no one sinusoid arms stretched - this is a task of special converters.
similar principle using amplitude modulation signal (AM, Amplitude Modulation), with the only difference that undergoes no change in the frequency of the waves and their height.Let us imagine a sine wave: hands grab several waves above and below the horizontal time axis.Now, with respect to the axis of stretch or shrink.As a result, the height varies.The next group of waves also change in a similar way, but with smaller (or larger) intensity.If you send a "battered" sine wave to the speaker, you can hear the change in tone: where the waves are higher, there is the signal intensity is greater and where less - there is less.Then everything is simple: squeeze as required (stretch) of the wave, thereby encoding information.As in the case of frequency modulation, there is also no one hand does not stretch - the operation of the instruments.
method using amplitude modulation signal is very limited width of the spectrum, so little use to transmit stereo.However, due to different wavelengths, and the radiated power in the FM and AM, the first requires frequent placement of transponders.In turn, for AM radio is characterized by a large and covers a distance clearly distinguish between useful component, even in the presence of noise.
