Amplitude modulation

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In electronics and radio engineering, amplitude modulation (AM) is a method of injecting information onto an oscillating analog signal, such as a radio wave, by changing the height (amplitude) of the peaks of the periodic waveform.

The process of mixing the information signal and the carrier wave produces sidebands at the sum and difference of the carrier signal. Hence, a modulated signal with no additional processing contains at least four frequencies, with different power levels:

  1. Carrier frequency
  2. Information frequency
  3. Upper sideband of the sum of the carrier and information frequencies
  4. Lower sideband of the difference of the carrier and information frequencies

The modulation process, or the oscillator that generates the carrier wave, may produce integer multiples, called harmonics, of any or all of these signals.

Conventional AM radio broadcasting sends all four signals. Two-way communications systems are usually more limited in transmitted power, so they usually process the signal for more efficient use of the available signal.

Since a substantial part of the energy of the combined signals is in the unchanging carrier frequency, there are various techniques of suppressing the carrier before the modulated signal is amplified for transmission. The most basic is double sideband with suppressed carrier, which still sends duplicate information in the two sidebands. Single sideband, where the carrier and one of the sidebands is suppressed, is more common.

All of these methods for suppressing signal components require more complex and expensive transmitters and receivers, as opposed to a basic AM system. Since there is price sensitivity in broadcast receivers, simplicity and cost of the receiver is traded off against efficient power use.