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  Modulation, applied to communication systems in electrical engineering, is the transfer of information in a low frequency signal, for example speech, to a higher frequency carrier signal for transmission. The carrier signal (so-called because it ‘carries’ the information) requires demodulation at the receiver to recover the information.

Amplitude modulation (AM) was in use on the telephone networks by the 1920s, and was part of the multiplexed carrier systems which permitted multiple separate communication channels to be used all at once (see multiplexing). This enabled the telephone companies to connect more callers to the network at one time and thereby increase their revenue and quality of service.

AM radio broadcasting had also been pioneered by 1915, although problems of unwanted noise affected the transmission. With the invention of frequency modulation (FM) by E.H. Armstrong in 1933, the noise problems were significantly reduced, and at the present time many radio stations have changed from AM to FM broadcasting.

The implementation of modulation techniques required prior advances in the theory of transmission systems, for example J.R. Carson\'s invention of single sideband transmission. This method of transmitting part of a modulated signal saved electrical power compared to other forms of modulated transmission. Of more importance was the invention of the electrical filter by G.A. Campbell in 1909, which allows selected transmission or blocking of certain signal frequencies, without which modulation schemes could not be practically realized. AC

See also transmission line theory.



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