The relative levels of the sidebands is determined by the modulation index, how much the modulated frequencyf the carrier varies around its unmodulated level. The spectrum of an FMed carrier consists of the carrier plus an infinite number of sidebands spreading out on either side of the carrier at integral multiples of the modulating frequency. The broadcaster determines the bandwidth of the signal as transmitted by choosing the high-frequency cutoff of the modulating audio. The transmitted signal is therefore distributed in frequency over a bandwidth that is equal to twice the highest frequency in the signal. When an audio signal is superimposed on an RF carrier, sidebands at the sum and difference of the carrier frequency are produced. The carrier is said to have an envelope, which can be displayed on the screen of an oscilloscope. The carrier’s frequency is unchanged, but its amplitude oscillates, at any instant relating to the amplitude of the audio signal, which itself varies in amplitude and frequency. The true situation is slightly more complex because, in amplitude modulation, a much lower audio frequency modulates the carrier. When speaking of a broadcast station’s frequency, it is generally the carrier’s frequency that is referenced. A simple and fundamental example is amplitude modulation and its close relatives, frequency and phase modulation. Often, but not always, the term is applicable to a situation where one frequency is superimposed on, mixed with, or modulates a different frequency. Bandwidth has an array of meanings, depending on the context.
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