Montage Acoustics: Medium-wave frequency
Medium-wave (medium frequency, MF) and short-wave (high frequency, HF) radio signals act differently during daytime and nighttime. During the day, MF signals travel by groundwave, diffracting around the curve of the earth over a distance up to a few hundred kilometers from the signal transmitter. However, after sunset, changes in the ionosphere cause MF signals to travel by skywave, enabling 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 the medium wave radio dial at night. As a result, many broadcast stations are required as a condition of license to reduce their broadcasting power significantly (or use directional antennas) after sunset, or even to suspend broadcasting entirely during nighttime hours. Such stations are commonly referred to as daytimers. In Australia medium wave stations are not required to reduce their power at night and consequently stations such as the 50,000-watt 3LO can be heard in some parts of New Zealand at night.
Modulation
Frequency modulation is a form of modulation which conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant). In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. This form of modulation is commonly used in the FM broadcast band.Montage Acoustics HD9001
Montage Acoustics BT4480: Pre-emphasis and de-emphasis
Random noise has a triangular spectral distribution in an FM system, with the effect that noise occurs predominantly at the highest frequencies within the baseband. This can be offset, to a limited extent, by boosting the high frequencies before transmission and reducing them by a corresponding amount in the receiver. Reducing the high frequencies in the receiver also reduces the high-frequency noise. These processes of boosting and then reducing certain frequencies are known as pre-emphasis and de-emphasis, respectively.
Limitations
Because of its relatively low audio quality due to audio bandwidth limitations, and its susceptibility to atmospheric and electrical interference, AM broadcasting now attracts mainly talk radio and news programming, while music radio and public radio mostly shifted to FM broadcasting in the late 1970s in the developed countries. However, in the late 1960s and 1970s, top 40 rock and roll stations in the U.S. and Canada such as WABC and CHUM transmitted highly processed and extended audio to 11 kHz, successfully attracting huge audiences. In the UK during the 1980s, the national speech station, BBC Radio 4, had an FM location, whereas BBC Radio 1, a music station, was confined to AM broadcasts. Frequency response is typically 40 Hz–5 kHz with a 50 dB Signal to noise(S/N) ratio. Montage Acoustics
Montage Acoustics BT4480:AM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation (AM). AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is authorized in the medium wave band worldwide, and also in parts of the long wave and short wave bands. Radio broadcasting was made possible by the invention of the amplifying vacuum tube, the Audion (triode), by Lee de Forest in 1906, which led to the development of inexpensive vacuum tube AM radio receivers and transmitters during World War I. Commercial AM broadcasting developed from amateur broadcasts around 1920, and was the only commercially important form of radio broadcasting until FM broadcasting began after World War II. This period is known as the "Golden Age of Radio". Today, AM competes with FM, as well as with various digital radio broadcasting services distributed from terrestrial and satellite transmitters. In many countries the higher levels of interference experienced with AM transmission have caused AM broadcasters to specialize in news, sports and talk radio, leaving transmission of music mainly to FM and digital broadcasters.
Medium-wave (medium frequency, MF) and short-wave (high frequency, HF) radio signals act differently during daytime and nighttime. During the day, MF signals travel by groundwave, diffracting around the curve of the earth over a distance up to a few hundred kilometers from the signal transmitter. However, after sunset, changes in the ionosphere cause MF signals to travel by skywave, enabling 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 the medium wave radio dial at night. As a result, many broadcast stations are required as a condition of license to reduce their broadcasting power significantly (or use directional antennas) after sunset, or even to suspend broadcasting entirely during nighttime hours. Such stations are commonly referred to as daytimers. In Australia medium wave stations are not required to reduce their power at night and consequently stations such as the 50,000-watt 3LO can be heard in some parts of New Zealand at night.
Modulation
Frequency modulation is a form of modulation which conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant). In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. This form of modulation is commonly used in the FM broadcast band.Montage Acoustics HD9001
Montage Acoustics BT4480: Pre-emphasis and de-emphasis
Random noise has a triangular spectral distribution in an FM system, with the effect that noise occurs predominantly at the highest frequencies within the baseband. This can be offset, to a limited extent, by boosting the high frequencies before transmission and reducing them by a corresponding amount in the receiver. Reducing the high frequencies in the receiver also reduces the high-frequency noise. These processes of boosting and then reducing certain frequencies are known as pre-emphasis and de-emphasis, respectively.
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Limitations
Because of its relatively low audio quality due to audio bandwidth limitations, and its susceptibility to atmospheric and electrical interference, AM broadcasting now attracts mainly talk radio and news programming, while music radio and public radio mostly shifted to FM broadcasting in the late 1970s in the developed countries. However, in the late 1960s and 1970s, top 40 rock and roll stations in the U.S. and Canada such as WABC and CHUM transmitted highly processed and extended audio to 11 kHz, successfully attracting huge audiences. In the UK during the 1980s, the national speech station, BBC Radio 4, had an FM location, whereas BBC Radio 1, a music station, was confined to AM broadcasts. Frequency response is typically 40 Hz–5 kHz with a 50 dB Signal to noise(S/N) ratio. Montage Acoustics
Montage Acoustics BT4480:AM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation (AM). AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is authorized in the medium wave band worldwide, and also in parts of the long wave and short wave bands. Radio broadcasting was made possible by the invention of the amplifying vacuum tube, the Audion (triode), by Lee de Forest in 1906, which led to the development of inexpensive vacuum tube AM radio receivers and transmitters during World War I. Commercial AM broadcasting developed from amateur broadcasts around 1920, and was the only commercially important form of radio broadcasting until FM broadcasting began after World War II. This period is known as the "Golden Age of Radio". Today, AM competes with FM, as well as with various digital radio broadcasting services distributed from terrestrial and satellite transmitters. In many countries the higher levels of interference experienced with AM transmission have caused AM broadcasters to specialize in news, sports and talk radio, leaving transmission of music mainly to FM and digital broadcasters.