Editing Continuous wave (section)

===Persistence of radio telegraphy===
[[File:Bencher paddle.jpg|thumb|250px|A commercially manufactured paddle for use with electronic keyer to generate Morse code]]
Early radio transmitters could not be [[Modulation|modulated]] to transmit speech, and so CW radio telegraphy was the only form of communication available. CW still remains a viable form of radio communication many years after voice transmission was perfected, because simple, robust transmitters can be used, and because its signals are the simplest of the forms of [[modulation]] able to penetrate interference. The low bandwidth of the code signal, due in part to low information transmission rate, allows very selective filters to be used in the receiver, which block out much of the radio noise that would otherwise reduce the intelligibility of the signal.

Continuous-wave radio was called [[radiotelegraphy]] because like the [[telegraph]], it worked by means of a simple switch to transmit [[Morse code]]. However, instead of controlling the electricity in a cross-country wire, the switch controlled the power sent to a radio [[transmitter]]. This mode is still in common use by [[amateur radio]] operators due to its narrow bandwidth and high [[signal-to-noise ratio]] compared to other modes of communication.

In military communications and [[amateur radio]] the terms "CW" and "Morse code" are often used interchangeably, despite the distinctions between the two. Aside from radio signals, Morse code may be sent using [[direct current]] in wires, sound, or light, for example. For radio signals, a carrier wave is keyed on and off to represent the dots and dashes of the code elements. The carrier's amplitude and frequency remain [[constant envelope|constant]] during each code element. At the receiver, the received signal is mixed with a [[heterodyne]] signal from a BFO ([[beat frequency oscillator]]) to change the radio frequency impulses to sound. Almost all commercial traffic has now ceased operation using Morse, but it is still used by amateur radio operators. [[Non-directional beacon|Non-directional beacons (NDB)]] and [[VHF omnidirectional range|VHF omnidirectional radio range (VOR)]] used in air navigation use Morse to transmit their identifier.