Amplitude Modulation (AM) is a mode for transmitting voice over the air. It consists of a HF carrier whose amplitude is varied (i.e. modulated) in proportion to the transmitted audio signal (i.e. the baseband signal). Spectrally, an AM signal contains this carrier and two sidebands above and below the carrier; the upper sideband is a frequency-shifted copy of the baseband signal, while the lower sideband is a frequency-shifted and frequency-inverted copy of the baseband signal. While it was the first audio mode ever developed, and was the first audio mode used on amateur radio, AM is not very popular in the present day because FM is more resilient against interference on VHF/UHF and SSB is more spectrally efficient, saving space on the narrow HF bands on which it is used.
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