AM broadcasting
|
|
AM radio is radio broadcasting using amplitude modulation. It was the dominant method of broadcasting during the first two thirds of the 20th century and remains widely used into the 21st.
Because of its susceptibility to atmospheric interference and generally lower fidelity sound, AM broadcasting is better suited to talk radio and news radio, while music radio and public radio have shifted in recent decades to FM broadcasting.
AM radio technology is simpler than either FM radio or DAB. An AM receiver detects amplitude variations in the radio wave. It then amplifies changes in the signal voltage to drive a loudspeaker or earphones. The earliest crystal radio receivers used a crystal diode detector with no amplification.
AM radio was used for small-scale voice and music broadcasts before World War I. The great increase in the use of AM radio came the following decade. The first commercial radio services began on AM in the 1920s (the first American radio station was started by Frank Conrad: KDKA in Pittsburgh, Pennsylvania). Radio programming boomed during the "Golden Age of Radio" (1920s–1950s). Dramas, comedy and all other forms of entertainment were produced, as well as broadcasts of news and music.
AM radio is broadcast in on several frequency bands:
- Long wave is 153–279 kHz
- Medium wave is 530–1,710 kHz.
- Short wave is 2,300–26,100 kHz, divided into 15 broadcast bands.
The allocation of these bands is governed by the ITU's Radio Regulations and, on the local level, by each country's national telecommunications administration – for instance, in the US, the FCC.
- Long wave is used for commercial radio broadcasting in Europe, Africa, Asia, and Australasia (ITU regions 1 and 3). In the Americas this band is reserved for aeronautical navigation. Due to the propagation characteristics of long wave signals, the frequencies are used most effectively in latitudes north of 50°.
- Medium wave is by far the most heavily used band for commercial broadcasting. This is the "AM radio" that most people are familiar with.
- Short wave is used by radio services intended to be heard at great distances from the transmitting station. The long range of short wave broadcasts comes at the expense of lower audio fidelity. The mode of propagation for short wave is different (see High frequency). AM is used mostly by broadcast services – other shortwave users may use a modified version of AM such as SSB or an AM-compatible version of SSB such as SSB with carrier reinserted. In many parts of the world short wave radio also carries audiable, encoded messages of unknown purpose from numbers stations.
Frequencies between the broadcast bands are used for other forms of radio communication, such as baby monitors, walkie talkies, cordless telephones, radio control, "ham" radio, etc.
Medium wave and short wave radio signals act differently during daytime and nighttime. During the day, AM signals travel by groundwave, refracting around the curve of the earth over a distance up to a few hundred kilometres (or miles) from the signal transmitter. However, after sunset, changes in the ionosphere cause AM signals to travel by skywave, enabling AM radio stations to be heard much farther from their point of origin than is normal during the day. This phenomenon can be easily observed by scanning an AM radio dial at night. As a result, many broadcast stations are required as a condition of license to reduce their broadcasting power significantly after sunset, or even to suspend broadcasting entirely during nighttime hours. The hobby of listening to long distance signals is known as DX or DX'ing, from an old telegraph abbreviation for "distant". Several non-profit hobbyist clubs are devoted exclusively to DXing the AM broadcast band, including the National Radio Club and International Radio Club of America. Similarly, people listening to short wave transmissions are SWLing.
AM radio signals can be disrupted in large urban centres by skyscrapers and other sources of radio frequency interference (RFI). FM signals, however, are not affected as much by these types of interference. As a result, AM radio has lost its dominance as a music broadcasting service, and in many cities is now relegated to news, sports and talk radio stations.
Stereo transmissions are possible (see AM stereo), and there is work underway to add digital radio services to currently-existing AM transmissions. In the United States, the iBiquity company is developing a proprietary standard for medium wave transmissions, while Digital Radio Mondiale is a more open effort, and can be used alongside many AM broadcasts.
While FM radio can also be received by cable, AM radio cannot be, although an AM station can be converted into an FM cable signal. In Canada, cable operators that offer FM cable services are required by the CRTC to distribute all locally-available AM stations in this manner.
See also
External link
- Table of Voltage, Frequency, TV Broadcasting system, Radio Broadcasting, by Country (http://www.salestores1.com/woreltab.html).
- Listen to live AM radio transmissions (http://www.dxtuners.com).