Radio broadcasting
The thermionic valve, a kind of vacuum tube, was invented in 1904 by the English physicist John Ambrose Fleming.
The heated filament, or cathode, was capable of thermionic emission of electrons that would flow to the plate (or anode) when it was at a higher voltage.
The triode (mercury-vapor filled with a control grid) was created on March 4, 1906, by the Austrian Robert von Lieben;[3][4][5] independently, on October 25, 1906,[6][7] Lee De Forest patented his three-element Audion.
[9][10] However, an early audio transmission that could be termed a broadcast may have occurred on Christmas Eve in 1906 by Reginald Fessenden, although this is disputed.
[11] While many early experimenters attempted to create systems similar to radiotelephone devices by which only two parties were meant to communicate, there were others who intended to transmit to larger audiences.
In 1920, wireless broadcasts for entertainment began in the UK from the Marconi Research Centre 2MT at Writtle near Chelmsford, England.
Radio Argentina began regularly scheduled transmissions from the Teatro Coliseo in Buenos Aires on August 27, 1920, making its own priority claim.
This station continued regular broadcasting of entertainment, and cultural fare for several decades.
[17] In line to ITU Radio Regulations (article1.61) each broadcasting station shall be classified by the service in which it operates permanently or temporarily.
Many stations broadcast on shortwave bands using AM technology that can be received over thousands of miles (especially at night).
For example, the BBC, VOA, VOR, and Deutsche Welle have transmitted via shortwave to Africa and Asia.
If a signal is strong enough, not even a power source is needed; building an unpowered crystal radio receiver was a common childhood project in the early decades of AM broadcasting.
During the night, absorption largely disappears and permits signals to travel to much more distant locations via ionospheric reflections.
At the time that AM broadcasting began in the 1920s, this provided adequate fidelity for existing microphones, 78 rpm recordings, and loudspeakers.
[20] Bob Carver created an AM stereo tuner employing notch filtering that demonstrated that an AM broadcast can meet or exceed the 15 kHz baseband bandwidth allotted to FM stations without objectionable interference.
The ground wave propagation at these frequencies is little affected by daily changes in the ionosphere, so broadcasters need not reduce power at night to avoid interference with other transmitters.
The improved fidelity made available was far in advance of the audio equipment of the 1940s, but wide interchannel spacing was chosen to take advantage of the noise-suppressing feature of wideband FM.
Additional unused capacity is used by some broadcasters to transmit utility functions such as background music for public areas, GPS auxiliary signals, or financial market data.
At the time FM was set up, the available frequencies were far higher in the spectrum than those used for AM radio - by a factor of approximately 100.
Using these frequencies meant that even at far higher power, the range of a given FM signal was much shorter; thus its market was more local than for AM radio.
The change to the current frequencies, 88 to 108 MHz, began after the end of World War II and was to some extent imposed by AM broadcasters as an attempt to cripple what was by now realized to be a potentially serious threat.
Its history can be traced back to the unlicensed nature of the transmission, but historically there has been occasional use of sea vessels—fitting the most common perception of a pirate—as broadcasting bases.
Digital radio broadcasting has emerged, first in Europe (the UK in 1995 and Germany in 1999), and later in the United States, France, the Netherlands, South Africa, and many other countries worldwide.
Commercial broadcasters may simply see a business opportunity to sell advertising or subscriptions to a broader audience.
A second reason is to advance a nation's foreign policy interests and agenda by disseminating its views on international affairs or on the events in particular parts of the world.
Another station, such as HCJB or Trans World Radio will carry brokered programming from evangelists.
In the case of the Broadcasting Services of the Kingdom of Saudi Arabia, both governmental and religious programming is provided.
For instance, the U.S. Federal Communications Commission designates the 88–92 megahertz band in the U.S. for non-profit or educational programming, with advertising prohibited.
As technology for sound recording improved, an increasing proportion of broadcast programming used pre-recorded material.
Some stations now operate without direct human intervention by using entirely pre-recorded material sequenced by computer control.
