Editing Repeater (section)

=== Telephone repeater ===
This is used to increase the range of telephone signals in a telephone line.

:* '''Land line repeater'''
They are most frequently used in [[Telecommunications link|trunkline]]s that carry [[Long-distance calling|long distance]] calls. In an [[Analog signal|analog]] telephone line consisting of a pair of wires, it consists of an amplifier circuit made of [[transistor]]s which use power from a DC current source to increase the power of the [[alternating current]] audio signal on the line. Since the telephone is a [[duplex (telecommunications)|duplex]] (bidirectional) communication system, the wire pair carries two [[audio signal]]s, one going in each direction.  So telephone repeaters have to be bilateral, amplifying the signal in both directions without causing feedback, which complicates their design considerably.  Telephone repeaters were the first type of repeater and were some of the first applications of amplification. The development of telephone repeaters between 1900 and 1915 made long-distance phone service possible. Now, most telecommunications cables are [[fiber-optic cable]]s which use optical repeaters (below).

Before the invention of electronic amplifiers, mechanically coupled [[Microphone#Carbon microphone|carbon microphones]] were used as amplifiers in telephone repeaters. After the turn of the 20th century it was found that negative resistance mercury lamps could amplify, and they were used.<ref name="Sungook">{{cite book
  | last = Sungook
  | first =  Hong
  | title = Wireless: From Marconi's Black-Box to the Audion
  | publisher = MIT Press
  | year = 2001
  | pages = 165
  | url = https://books.google.com/books?id=UjXGQSPXvIcC&pg=PA165
  | isbn = 0262082985}}</ref>  The invention of [[audion tube]] repeaters around 1916 made transcontinental telephony practical.  In the 1930s [[vacuum tube]] repeaters using [[hybrid coil]]s became commonplace, allowing the use of thinner wires.  In the 1950s [[negative resistance|negative impedance]] gain devices were more popular, and a [[transistor]]ized version called the E6 repeater was the final major type used in the [[Bell System]] before the low cost of digital transmission made all [[voiceband]] repeaters obsolete. [[Frequency frogging]] repeaters were commonplace in frequency-division multiplexing systems from the middle to late 20th century.

:* '''Submarine cable repeater'''
This is a type of telephone repeater used in underwater [[submarine telecommunications cable]]s.