Radios transmit sine waves (smooth and repetitive oscillations) into space, but sine waves can't carry information on their own. They need to be modulated in some way, so that information -- say, the voices on your favorite radio station -- can be encoded on the waves. That's where the "M" comes from in AM and FM radio: modulation. There are three types of modulation:
- Pulse modulation (PM) simply turns sine waves on and off to transmit information. A common use for PM is maintaining the correct time on radio controlled clocks.
- Amplitude modulation (AM) changes the amplitude (the peak-to-peak voltage) of a signal to produce sound. AM radio stations and television pictures use AM to encode signals.
- Frequency modulation (FM) relies on changing the sine wave frequency (the number of times it oscillates up and down per second) to transmit, based upon the information signal. FM signals tend to be clear and static-free.
The technical designations for transmission of AM and FM aren't arbitrary. In the radio spectrum, frequencies are assigned for specific purposes. The Federal Communications Commission (FCC) issues licenses for radio broadcasting stations, police, air traffic controllers and other groups to transmit only within unique frequency ranges. Frequency modulated (FM) radio stations occupy a range of frequencies between 88 megahertz and 108 megahertz. Amplitude modulated (AM) radio stations can transmit over frequencies in the range of only 535 kilohertz to 1,700 kilohertz. This assignment has its basis in history. FM radio became popular only in the 1960s, when the lower frequencies had already been allocated for other purposes. The first AM radio broadcasts occurred in the 1920s, when electronic technology was relatively nascent, and lower frequencies were available. This is why our FM and AM radio station numbers top out where they do on the dial.
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