Editing Superheterodyne receiver (section)

=== Early radio ===

Early [[Morse code]] radio broadcasts were produced using an [[alternator]] connected to a [[spark gap transmitter|spark gap]]. The output signal was at a [[carrier frequency]] defined by the physical construction of the gap, modulated by the [[alternating current]] signal from the alternator. Since the output frequency of the alternator was generally in the audible range, this produces an audible [[amplitude modulated]] (AM) signal. Simple [[Detector (radio)|radio detectors]] filtered out the high-frequency carrier, leaving the modulation, which was passed on to the user's [[headphone]]s as an audible signal of dots and dashes.

In 1904, [[Ernst Alexanderson]] introduced the [[Alexanderson alternator]], a device that directly produced radio frequency output with higher power and much higher efficiency than the older spark gap systems. In contrast to the spark gap, however, the output from the alternator was a pure carrier wave at a selected frequency. When detected on existing receivers, the dots and dashes would normally be inaudible, or "supersonic". Due to the filtering effects of the receiver, these signals generally produced a click or thump, which were audible but made determining dots from dashes difficult.

In 1905, Canadian inventor [[Reginald Fessenden]] came up with the idea of using two Alexanderson alternators operating at closely spaced frequencies to broadcast two signals, instead of one. The receiver would then receive both signals, and as part of the detection process, only the [[Beat (acoustics)#Mathematics and physics of beat tones|beat frequency]] would exit the receiver. By selecting two carriers close enough that the beat frequency was audible, the resulting Morse code could once again be easily heard even in simple receivers. For instance, if the two alternators operated at frequencies 3&nbsp;kHz apart, the output in the headphones would be dots or dashes of 3&nbsp;kHz tone, making them easily audible.

Fessenden coined the term "[[heterodyne]]", meaning "generated by a difference" (in frequency), to describe this system. The word is derived from the Greek roots ''hetero-'' "different", and ''-dyne'' "power".