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.
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