Synchronous Reception is a term that refers to circuitry in a shortwave radio that attempts to eliminate distortion in an AM signal primarily caused by selective fading. Rather than using a diode to convert the signal to audio (envelope detection) the signal is run through a mixer that converts it down to audio frequencies. This type of detector is more complicated than an envelope detector because the oscillator that is fed to the mixer must be locked to the frequency of the original carrier (which may have faded to nothing).
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