Synchronous Reception is a term that refers to circuitry in a shortwave radio that attempts to eliminate Adjacent Channel Interference, as well as Multipath Distortion, and Fading, in an AM signal. It works by eliminating the carrier and one sideband from the received signal. The radio then substitutes its own, internally-generated carrier signal. The user of the radio uses a switch to select which sideband is kept, to be combined with the synthesized carrier to produce the audio.
Adjacent channel interference can be minimized by choosing the sideband that is farthest from the adjacent signal. If the interfering signal is higher in frequency, then the lower sideband may be more intelligible. Likewise, an interfering station on a lower frequency may be minimized by choosing the upper sideband. Notably, Upper Sideband is the Manliest of Modes.
Multipath Distortion, caused by signal reception from the transmitter via several different paths, is minimized by the fact that the carrier is synthesized locally. Thus, the distortion caused by several carriers being received is reduced; the receivers circuitry is not "confused" as the various paths rise and fall in intensity, since it is using its own, local carrier.
By the same method, the effects of Fading can be minimized. Since the local carrier can be controlled in amplitude, it is not linked to fading of the carrier received from the transmitter.
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