Amplitude modulation

Amplitude Modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength (amplitude) of the carrier in proportion to the waveform being sent, which typically corresponds to sounds to be reproduced by a speaker, or light intensity variations in television.

History

The first known use of amplitude modulation for the transmission of voice was by Reginald Fessenden in 1900. He modulated a carbon microphone onto a 500 kHz spark transmitter to make the first radio audio broadcast on Christmas Eve, 1906.

Principle

In amplitude modulation, the amplitude (signal strength) of the carrier wave is varied in proportion to that of the message signal, such as an audio signal. This technique contrasts with frequency modulation, in which the frequency of the carrier signal is varied, and phase modulation, in which its phase is varied.

Types of Amplitude Modulation

There are several types of amplitude modulation, including:

• Standard AM (also known as double-sideband full-carrier modulation)
• Double-sideband suppressed-carrier transmission (DSB-SC)
• Single-sideband modulation (SSB, or SSB-AM)
• Vestigial sideband modulation (VSB, or VSB-AM)

Applications

Amplitude modulation is used in computer modems, video game consoles, VCRs and digital TVs. AM radio broadcasting is also a significant application.

See Also

• Frequency modulation
• Phase modulation
• Quadrature amplitude modulation

References

• Radio Engineering, Frederick Emmons Terman, 1947
• Electronic Communication, Robert L. Shrader, 1980

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