Editing Demodulation (section)

== History ==
Demodulation was first used in [[radio receivers]].  In the [[wireless telegraphy]] radio systems used during the first 3 decades of radio (1884–1914) the transmitter did not communicate audio (sound) but transmitted information in the form of pulses of radio waves that represented text messages in [[Morse code]].{{Citation needed|date=March 2023}} Therefore, the receiver merely had to detect the presence or absence of the radio signal, and produce a click sound.  The device that did this was called a [[detector (radio)|detector]].  The first detectors were [[coherer]]s, simple devices that acted as a switch. The term ''detector'' stuck, was used for other types of demodulators and continues to be used to the present day for a demodulator in a radio receiver.

The first type of [[modulation]] used to transmit sound over radio waves was [[amplitude modulation]] (AM), invented by [[Reginald Fessenden]] around 1900.  An AM radio signal can be demodulated by [[rectifying]] it to remove one side of the carrier, and then filtering to remove the radio-frequency component, leaving only the modulating audio component. This is equivalent to peak detection with a suitably long time constant. The [[amplitude]] of the recovered audio frequency varies with the modulating audio signal, so it can drive an earphone or an audio amplifier.  Fessendon invented the first AM demodulator in 1904 called the [[electrolytic detector]], consisting of a short needle dipping into a cup of dilute acid.  The same year [[John Ambrose Fleming]] invented the [[Fleming valve]] or [[thermionic diode]] which could also rectify an AM signal.