Radio broadcasting

Radio broadcasting is the transmission of audio (sound), often in conjunction with relevant information, through radio waves with the goal of reaching a large number of people. Terrestrial radio broadcasting involves the transmission of radio waves by a land-based radio station, while satellite radio broadcasting involves the transmission of radio waves by a satellite in Earth orbit. The listener must be equipped with a broadcast radio receiver in order to hear the material (radio). In many cases, broadcasters are connected with a radio channel which provides material in a similar radio format, either through broadcast syndication, simulcasting, or both modes. AM radio stations transmit in AM (amplitude modulation), FM radio stations transmit in FM (frequency modulation), which are both older analogue audio standards, whereas newer digital radio stations transmit in several digital audio standards, including DAB (digital audio broadcasting), HD radio, and DRM (digital rights management) (Digital Radio Mondiale). Television broadcasting is a distinct service that, like radio broadcasting, utilises radio frequencies to transmit television signals (video).

In the beginning, radio stations were only radiotelegraphy systems, and they did not transmit sounds. It was necessary to add electrical detection and amplification equipment in order for audio broadcasts to be conceivable.

The thermionic valve (a kind of vacuum tube) was designed by the English scientist John Ambrose Fleming in 1904, and is still in use today. "Oscillation valve" is the name he gave to a gadget he invented (because it passes current in only one direction). When the filament, or cathode, was heated to a high temperature, it was capable of thermionic emission of electrons, which would flow to the plate, or anode, when the voltage was increased. In contrast, electrons were unable to move through the plate in the other direction because it was not heated, and hence was not capable of thermionic emission of electrons. It was later referred to as the Fleming valve, and it was capable of serving as a rectifier of alternating current as well as a radio wave detector. This significantly enhanced the crystal set, which rectified the radio signal using an early solid-state diode based on a crystal and a so-called cat's whisker to achieve higher frequencies. But an amplifier was still necessary in order to complete the setup.