Broadcast Signals

Radio communication is typically in the form of AM radio or FM Radio transmissions. The broadcast of a single signal, such as a monophonic audio signal, can be done by straightforward amplitude modulation or frequency modulation. More complex transmissions utilize sidebands arising from the sum and difference frequencies which are produced by superposition of some signal upon the carrier wave. For example, in FM stereo transmission, the sum of left and right channels (L+R) is used to frequency modulate the carrier and a separate subcarrier at 38 kHz is also superimposed on the carrier. That subcarrier is then modulated with a (L-R) or difference signal so that the transmitted signal can be separated into left and right channels for stereo playback. In television transmission, three signals must be sent on the carrier: the audio, picture intensity, and picture chrominance. This process makes use of two subcarriers. Other transmissions such as satellite TV and long distance telephone transmission make use of multiple subcarriers for the broadcast of multiple signals simultaneously.

AM Radio

AM radio uses the electrical image of a sound source to modulate the amplitude of a carrier wave. At the receiver end in the detection process, that image is stripped back off the carrier and turned back into sound by a loudspeaker.

When information is broadcast from an AM radio station, the electrical image of the sound (taken from a microphone or other program source) is used to modulate the amplitude of the carrier wave transmitted from the broadcast antenna of the radio station. This is in contrast to FM radio where the signal is used to modulate the frequency of the carrier.

The AM band of the Electromagnetic spectrum is between 535 KHz and 1605 kHz and the carrier waves are separated by 10 kHz.

A radio receiver can be tuned to receive any one of a number of radio carrier frequencies in the area of the receiver. This is made practical by transferring the signal from the carrier onto an intermediate frequency in the radio by a process called heterodyning. In a heterodyne receiver, most of the electronics is kept tuned to the intermediate frequency so that only a small portion of the receiver circuit must be retuned when changing stations.

FM Radio

FM radio uses the electrical image of a sound source to modulate the frequency of a carrier wave. At the receiver end in the detection process, that image is stripped back off the carrier and turned back into sound by a loudspeaker.

When information is broadcast from an FM radio station, the electrical image of the sound (taken from a microphone or other program source) is used to modulate the frequency of the carrier wave transmitted from the broadcast antenna of the radio station. This is in contrast to AM radio where the signal is used to modulate the amplitude of the carrier.

The FM band of the electromagnetic spectrum is between 88 MHz and 108 MHz and the carrier waves for individual stations are separated by 200 kHz for a maximum of 100 stations. These FM stations have a 75 kHz maximum deviation from the center frequency, which leaves 25 kHz upper and lower "gaurd bands" to minimize interaction with the adjacent frequency band. This separation of the stations is much wider than that for AM stations, allowing the broadcast of a wider frequency band for higher fidelity music broadcast. It also permits the use of sub-carriers which make possible the broadcast of FM Stereo signals.

Frequency Modulation

The change in frequency, which is greatly exaggerated here, is proportional to the amplitude of the signal. An FM radio carrier around 100 MHz is limited to modulation of +/- 0.1 MHz. Normal FM stereo broadcast is within +/- .053 MHz.