Frequency modulation( FM) is a modulation method in which a high- frequency carrier oscillation is changed in its frequency in rhythm with the low-frequency signal frequency. The frequency-modulated carrier signal theoretically consists of an infinite number of spectral lines, unlike amplitude modulation, where it is a matter of sidebands modulated through. The individual spectral lines can be calculated by the Bessel functions.
In frequency modulation, the maximum deviation of the carrier frequency by the modulation frequency is proportional to the amplitude of the modulation signal - not the modulation frequency - and is called the frequency deviation. Thus, for a frequency modulated audio signal, the volume determines the frequency deviation, not the pitch. The frequency of the sound signal, i.e. the pitch, is reflected in the frequency deviation. When frequency modulated signals are demodulated, they are first converted to amplitude modulated signals and then rectified.
High-quality transmissions use a large frequency deviation. This FM technique, which is used in FM broadcasting, is called Wide Frequency Modulation (WFM). This contrasts with Narrow Frequency Modulation( NFM), which operates with frequency deviations below 50 kHz. NFM technology is used in wireless systems, radio microphones, radiotelephony and amateurradio.
Frequency modulation has the advantage over amplitude modulation that it is less susceptible to interference. Interference spikes that build up on the amplitudes of the modulated carrier frequency on the radio path can be removed by a limiter that cuts off the amplitudes of all carrier oscillations at a certain level threshold.