Modulating A Sound Wave
The simple transmission scheme outlined above cannot be used for commercial broadcasting. If a dozen stations all transmitted sounds by the mechanism described above, a receiving station would pick up a garbled combination of all transmissions. To prevent interference from a number of transmitting stations, all broadcast radio waves are first modulated.
Modulation is the process by which a sound wave is added to a basic radio wave known as the carrier wave. For example, an audio signal can be electronically added to a carrier signal to produce a new signal that has undergone amplitude modulation (AM). Amplitude modulation means that the amplitude (or size) of the wave of the original sound wave has been changed by adding it to the carrier wave.
Sound waves can also be modulated in such a way that their frequency is altered. For example, a sound wave can be added to a carrier signal to produce a signal with the same amplitude, but a different frequency. The sound wave has, in this case, undergone frequency modulation (FM).
Both AM and FM signals must be decoded at the receiving station. In either case, the carrier wave is electronically subtracted from the radio wave that is picked up by the receiving antenna. What remains after this process is the original sound wave, encoded, of course, as an electrical signal.
All broadcasting stations are assigned characteristic carrier frequencies by the Federal Communications Commission. This system allows a number of stations to operate in the same area without overlapping. Thus, two stations a few kilometers apart could both be sending out exactly the same program, but they would sound different (and have different electric signals) because each had been overlaid on a different carrier signal.
Receiving stations can detect the difference between these two transmissions because they can tune their equipment to pick up only one or the other carrier frequency. When you turn the tuning knob on your own radio, for example, you are adjusting the receiver to pick up carrier waves from station A, station B, or some other station. Your radio then decodes the signal it has received by subtracting the carrier wave and converting the remaining electric signal to a sound wave.
The identifying characteristics by which you recognize a radio station reflect its two important transmitting features. The frequency, such as 101.5 megahertz (or simply "101.5 on your dial") identifies the carrier wave frequency, as described above. The power rating ("operating with 50,000 watts of power") describes the power available to transmit its signal. The higher the power of the station, the greater the distance at which its signal can be picked up.
See also Wave motion.
Bloomfield, Louis A. How Things Work: The Physics of Everyday Life. 2nd ed. New York: John Wiley & Sons, 2000.
Davidovits, Peter. Communication. New York: Holt, Rinehart and Winston, Inc., 1972.
David E. Newton