Modulation

As noted in the previous section, an ordinary audio signal has a low frequency (below about 20 kHz), and low-frequency signals of this kind simply cannot be transmitted efficiently over large distances -- partly because of the impractically long antenna they would require, and partly because, if many different transmitters tried to send their raw low-frequency audio signals directly, those signals would inevitably mix together and become impossible to separate at the receiver. The solution is to use a high-frequency wave, called a carrier wave, and to change one of its characteristics -- its amplitude, its frequency, or its phase -- in accordance with the amplitude of the (low-frequency) signal to be sent. This process of impressing a low-frequency signal onto a high-frequency carrier is called modulation. Because different transmitters can be assigned different carrier-wave frequencies, modulation also solves the mixing problem: without modulated carriers of distinct frequencies, audio signals broadcast directly by different transmitters would indeed overlap and become jumbled together.

Modulation can be carried out in three distinct ways, depending on which property of the carrier is varied while the other two are held fixed, in proportion to the amplitude (or intensity) of the signal wave: (i) amplitude modulation (AM), in which the carrier's amplitude is varied; (ii) frequency modulation (FM), in which the carrier's frequency is varied; and (iii) phase modulation (PM), in which the carrier's phase is varied. Fig. 13.9 shows a representative carrier wave (a), a representative signal wave (b), and the resulting AM, FM and PM waveforms (c, d and e respectively) that would carry that signal. …