Amplitude Modulation Immunity To Capture Effect
In FM demodulation the receiver tracks the modulated frequency shift of the desired carrier while discriminating against any other signal since it can only follow the deviation of one signal at a time. In AM modulation the receiver tracks the signal strength of the AM signal as the basis for demodulation. This allows any other signal to be tracked as just another change in amplitude. So it is possible for an AM receiver to demodulate several carriers at the same time, resulting in an audio mix.
If the signals are close but not exactly on the same frequency the mix will not only include the audio from both carriers but depending on the carrier separation a whistle might be heard as well representing the difference in the carrier frequencies. This mix can also occur when an AM carrier is received on a channel that is adjacent to the desired channel. The resulting overlap forms the high pitched whistle (about 10 Kilohertz) that can often be heard behind an AM station at night when other carriers from adjacent channels are traveling long distances due to atmospheric bounce.
Since AM assumes short term changes in the amplitude to be information, any electrical impulse will be picked up and demodulated along with the desired carrier. Hence lightning causes crashing noises when picked up by a AM radio near a storm. FM radios suppress short term changes in amplitude and are therefore much less prone to noise during storms and during reception of electrical noise impulses.
For digital modulation schemes it has been shown that for properly implemented on-off keying/amplitude-shift keying systems, co-channel rejection can be better than for frequency-shift keying systems.
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