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.
Read more about this topic: Capture Effect
Famous quotes containing the words amplitude, modulation, immunity, capture and/or effect:
“Imagination, which in truth
Is but another name for absolute power
And clearest insight, amplitude of mind,
And reason, in her most exalted mood.”
—William Wordsworth (1770–1850)
“Every accent, every emphasis, every modulation of voice, was so perfectly well turned and well placed, that, without being interested in the subject, one could not help being pleased with the discourse; a pleasure of much the same kind with that received from an excellent piece of music. This is an advantage itinerant preachers have over those who are stationary, as the latter can not well improve their delivery of a sermon by so many rehearsals.”
—Benjamin Franklin (1706–1790)
“When the peace is a trade route, figures
For the budget, reduction of population,
Life grown sullen and immense
Lusts after immunity to pain.”
—Allen Tate (1899–1979)
“Because the young child feels with such intensity, he experiences sorrows that seem inconsolable and losses that feel unbearable. A precious toy gets broken or a good-bye cannot be endured. When this happens, words like “sad” or “disappointed” seem a travesty because they cannot possibly capture the enormity of the child’s loss. He needs a loving adult presence to support him in his pain but he does not want to be talked out of it.”
—Alicia F. Lieberman (20th century)
“Power is poison. Its effect on Presidents had always been tragic.”
—Henry Brooks Adams (1838–1918)