Thursday, July 13, 2006

AM versus FM Radio

It occured to me while making a long trip up to the Canadian border last weekend and listening to a lot of radio, that the difference between AM radio and FM radio must have something to do with the stations I was receiving on my car's radio. I remembered something from a teacher in highschool about those fundamental differences, but my memory of the matter was like swiss cheese.

In simplest terms AM radio is radio which uses amplitude modulation to disseminate information. FM radio uses frequency modulation. This means that both methods use radio waves, comparatively long, slow wavelengths in the electromagnetic spectrum. These are at the opposite end from gamma and x-rays, if you consider that wavelengths in the visual spectrum fall somewhere in the middle.

Amplitude modulation refers to the height of the wave. If you were to draw a wave from an AM tranmission, it would look like the cross-section of an ocean wave, with peaks and valleys, except that some of the peaks would be higher than others. The change in amplitude, peak and valley height and depth, is what translates, with an AM receiver, into a radio broadcast. Its the equivalent of the bumps on a record. Tiny bumps in the vinyl grooves of a record are interpreted by the needle and arm on your record player to produce different pitches. An infinite combination of pitches is possible, and so long as we can hear them, any pitch in the audible range can be heard. Voila, a radio broadcast.

Frequency modulation alters the radio wave in the opposite way to produce a radio broadcast. Frequency refers to the number of peaks and valleys in the wave occur within a given period of time. FM radio uses a carrier frequency, which is actually a range of frequencies, and this range and the allocation of frequencies to different stations varries by country. The carrier frequency is overlayed with signal, i.e. what you want to hear, and the signal changes the frequency of the carrier to make larger or smaller gaps between peaks and valleys in the wave. These gaps correspond to the bumps in the grove of the record, same idea as AM radio, just the opposite implementation.

Interestingly, AM radio is generally of lower sound quality, as smaller changes in the amplitude tend to be lost as noise, and therefore AM radio is usually only used for talk radio broadcasts. The range of AM radio is different during the day and the night, and this is where it gets really interesting. during the day, AM radio signals reverberate across the surface of the Earth, and follow its contours, this is why AM radio reception may be so poor during the day if you are a long way from the source. Distortions in the Earth's surface, i.e. a mountain range, will impede the progress of the radio signal to your device. At night however, changes in the atmostphere which occur when the sun's rays are absent, allow AM radio to travel higher in the air, effectively carrying the AM signal farther from its source, over obstructions. This phenomenon applies to the more commonly used medium and short wave AM signals.

FM radio is clearer over longer distances from its source because its signal is transmitted using only changes in frequency. What this means is that the height of the FM waves is always the same, and only their spacing changes. If you get an FM signal during the day, it will be the same signal you get at night.

The energy of radio signals is conserved along the lines of the law of conservation of energy though, and no radio signal is infinite, as the power it is given at its source will dissipate the farther you are from that source. Increasing the power of a signal though is simply a matter of turning up the juice and pumping more electricity in. This is why stations with a larger budget have better reception, they can afford a higher energy bill.

Radio may also be transmitted in stereo, that means that two different parts of the broadcast may come out of two different speakers, adding a spatial demention to the sound you hear. This is an even more complicated topic however, and one who's explanation belongs to another time.


At 7:03 AM, May 25, 2007, Anonymous Anonymous said...

awesome post!

it's surprising, even as a curious scientist, how much i continuously take for granted. radio is definitely a big one that's remained overlooked.

thx for the background :)


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