Tecsun PL-330 User Manual page 27

These forms of amplitude modulation, which cannot be demodulated by the
envelope detector, arise from fairly common situations. Although the
broadcast signal is transmitted as a conventional double-sideband AM wave,
the detuning and skywave reflection of the radio will change this AM signal
into one or a combination of the forms shown in the chart.
When there is adjacent channel interference, TV horizontal scanning
harmonics, interference of the carrier frequency and the like, then using the
upper sideband or lower sideband to receive the unaffected sideband can
significantly reduce the interference. This is because these types of
interference usually only affect one sideband of the AM signal, and the
content of AM radio broadcasts exists in two sidebands, in which each
sideband contains the same content.
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