AM/FM
If you look at the tuning controls of a shortwave radio or in the broadcast station listings of a newspaper, you'll see the notations AM and FM. These mean amplitude modulation and frequency modulation, respectively. By itself, a radio signal doesn't carry much information. The information has to be added somehow. Modulation is the process of adding that information and demodulation is the process by which the information is recovered. The oldest and simplest way of adding information to a radio wave is to make the wave stronger or weaker in a way that represents the information you're trying to communicate. The strength of a radio wave is called its amplitude so adding information to the wave by varying its strength is called amplitude modulation or AM for short.
Say the word hullabaloo out loud and note the pattern of loud and soft sounds, as well as the different frequencies in your voice. If you were speaking into a microphone for broadcast to an AM radio, by modulating the radio wave's amplitude with these various frequencies and the loud and soft patterns of your voice, the information you're conveying is added to the radio wave and transmitted far and wide. If you watch a meter that measures transmitter power while you speak into the microphone of an AM transmitter, you'll see the meter's needle jump up and down as your voice gets louder and softer, just like on a tape recorder. Because broadcast stations using AM are grouped together between 550 and 1700 kHz this is referred to as the AM band. You may notice when you listen to AM radio that there is a bit of interference as well as static crashes. This is because the various noises from natural resources such as lightning and from manmade sources such as your cars spark plugs are amplitude variations, just like signals. Your radio can't tell the difference between the two. Frequency modulation, or FM, adds desired information to a signal by varying the signal's frequency. In fact, the amplitude of an FM signal doesn't change at all, not matter how loud or soft the modulating information may be. The only thing that changes is the frequency of the signal. An FM receiver can just ignore AM signals, because it only cares about variations in frequency. This makes the recovered signals almost noise free. This is one of the advantages of FM and a reason for its popularity. FM is the most widely used modulation of all, despite complexity of FM radio equipment. Like AM broadcast stations, FM stations are grouped together in their own band 88 to 108 MHz just between TV channel 6 and the aircraft band. This range of frequencies is called the FM band. An AM/FM radio receives AM in the lower range of frequencies and FM in the higher range. |
