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Date: 1991 

Could you explain modulation as it relates to radio waves and what is a radio carrier signal? This questions from a 6th grade sc.

Hold up a flashlight, turn it on, and point it at the blackboard. The light that you see is a "carrier signal", "carrier beam", or whatever. Now turn the light on and off a few times. You are "modulating" the carrier beam when you do this. Navy ships at sea "talk" to each other with light signals using the method I described. Radio waves are "kinds" of light beams.

Jack L. Uretsky

Hi Adam-- Here is another analogy. Suppose someone is singing a single note for a length of time, but sometimes sings it softer and sometimes louder -- that is, the pitch (frequency) does not change over time, but the volume (amplitude) does. The single note corresponds to the carrier wave in AM (amplitude modulation), and its loudness vs. time corresponds to the signal conveyed by the carrier wave. If you plotted a graph of the wave, the signal would form a sort of "envelope" for the high-frequency carrier. In this way a complex signal can be carried by a simple wave. The same analogy can be used for FM (frequency modulation). Now imagine that the singer sings a note (say, with constant loudness, though this is not important) which does not quite remain at a single pitch; it varies some, but not too much, from a "central" pitch. The central pitch corres- ponds to the carrier, and the manner in which the wave deviates from the central pitch conveys the signal. If you plotted a graph of the wave, it would look much like a regular sine wave, except that the frequency would not be constant: in some places the wave would look slightly "scrunched" and in other places slightly "stretched". In both cases, you tune your radio to the carrier frequency.


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