Amplitude-modulation radio

(communications) Also known as AM radio. The system of radio communication employing amplitude modulation of a radio-frequency carrier to convey the intelligence. A receiver used in such a system.

Radio communication employing amplitude modulation of a radio-frequency carrier wave as the means of conveying the desired intelligence. In amplitude modulation the amplitude of the carrier wave is made to vary corresponding to the fluctuations of a sound wave, television image, or other information to be conveyed. See also Amplitude modulation; Radio.

Amplitude modulation (AM), the oldest and simplest form of modulation, is widely used for radio services. The most familiar of these is broadcasting; others include radiotelephony and radiotelegraphy, television picture transmission, and navigational aids.

European and Asian countries use low frequencies in the range 150–255 kilohertz (kHz) for some broadcast services. An advantage of these frequencies is stable and relatively low-attenuation wave propagation. When not limited by atmospheric noise, large areas may be served by one station. In the United States these frequencies are reserved for navigational systems and so are not available for broadcasting.

The frequencies in the range from 535 to 1605 kHz are reserved all over the world for AM (standard) broadcasting. In the Western Hemisphere this band is divided into channels at 10-kHz intervals, certain channels being designated as clear, regional, and local, according to the licensed coverage and class of service. European medium-frequency (mf) broadcasting channels are assigned at 9-kHz intervals.

Small bands of high frequencies between 3000 and 5000 kHz are used in tropical areas of high atmospheric noise for regional broadcasting. This takes advantage of the lower atmospheric noise at these frequencies and permits service under conditions where medium frequencies have only severely limited coverage.

The first radiotelephony was by means of amplitude modulation, and its use has continued with increasing importance. Radiotelephony refers to two-way voice communication. Amplitude modulation and a modified form called single-sideband are used almost exclusively for radiotelephony on frequencies below 30 megahertz. Above 30 MHz, frequency or phase modulation is used almost exclusively, a notable exception being 118–132 MHz, where amplitude modulation is used for all two-way vhf radiotelephony in aviation operations.

The least expensive method known for communicating by telephony over distances longer than a few tens of miles is by using the high frequencies of 3–30 MHz. Furthermore, since radio is the only way to communicate with ships and aircraft, hf AM radiotelephony has remained essential to these operations, except for short distances that can be covered from land stations using the very high frequencies.

Single-sideband (SSB) hf telephony is a modified form of amplitude modulation in which only one of the modulation sidebands is transmitted. In some systems the carrier is transmitted at a low level to act as a pilot frequency for the regeneration of a replacement carrier at the receiver. Since 1933 most transoceanic and intercontinental telephony has been by single-sideband reduced-carrier radio transmission on frequencies between 4000 and 27,000 kHz. In time, SSB will gradually displace AM radiotelephony to reduce serious interference due to overcrowding of the radio spectrum.

Amplitude-modulated radio has a dominant role in guidance and position location, especially in aviation, which is almost wholly under radio guidance.

Amplitude modulation is used everywhere for the broadcasting of the picture (video) portion of television. In England, France, and a few other places amplitude modulation is also used for the sound channel associated with the television picture, but frequency modulation is more commonly used for sound.

Countries of the Western Hemisphere, Japan, Philippines, Thailand, and Iran broadcast television video in an emission band of 4.25 MHz; the English video bandwidth is 3 MHz; the French system, 10 MHz. The rest of continental Europe (except the Soviet Union) use a bandwidth of 5.25 MHz. The carrier frequencies employed are between 40 and 216 MHz, and 470 to 890 MHz. A channel allocation includes the spectrum needed for both sound and picture. Japan and Australia also use 88–108 MHz for television broadcasting.

A number of systems for stereophonic AM radio broadcasting exist that allow stations to transmit two channels of information in the same spectrum space where only one could exist previously. The result is similar to that of stereophonic FM, audio cassette, and other binaural entertainment media. To transmit in AM stereo, a broadcast station employs a device known as an exciter to adapt its existing transmitter. The exciter has left- and right-channel audio inputs as well as summed audio- and radio-frequency outputs. Stereophonic audio program material is connected to the audio inputs, while the outputs attach to the AM transmitter. Stereophonic AM broadcasting methods all have their roots in a system known as quadrature multiplexing. This system allows the transmission of two channels of information on a single carrier frequency, but true quadrature transmissions are inherently incompatible with the majority of radios available to consumers. The differences between various systems employed to transmit stereophonic AM broadcasts all relate to the methods used to overcome this incompatibility. See also Amplitude modulation; Radio; Stereophonic radio transmission.