Also known as: Morse Code, CW
Morse code is a method of encoding text characters as sequences of short and long signalling events — dots (dits) and dashes (dahs) — where the timing of on and off periods carries the meaning. On radio it is almost always sent as continuous wave (CW): an unmodulated carrier switched on and off with a key, the simplest form of on-off keying.1 A single tone, turned on and off in a disciplined rhythm, is enough to pass a full text message across a noisy channel that would defeat wider modes.
How it works
Morse is built entirely on a single time unit. A dot is one unit of carrier-on; a dash is three. Within a character, elements are separated by a one-unit off period; letters are separated by three units of silence, and words by seven. Each character maps to a distinct dot/dash pattern — E is a single dot, T a single dash, and common letters get the shortest codes so ordinary text sends efficiently. Speed is quoted in words per minute (WPM), benchmarked against the word “PARIS.” Because the receiver only has to detect presence or absence of a tone, Morse holds up in weak-signal and interference conditions where schemes that must recover phase or many amplitude levels would fail; a trained operator’s ear (or a narrow filter and simple detector) does the decoding.
International Morse standardises the letter, digit, and punctuation patterns, and adds prosigns — special element groups like AR (end of message) or SK (end of contact) — for procedural signalling. There is no forward error correction: robustness comes from the extreme narrowness of the signal and the redundancy of natural language.
Relevance to SDR
Morse/CW is still heavily used in amateur radio and lingers in navigation-beacon and station identifiers, so it is a common sight on any HF or VHF waterfall — a dashed line blinking in a few-hertz-wide slot. In a software-defined receiver it is trivial to demodulate: mix the CW carrier down to an audio beat note (BFO), filter narrowly, and detect the on/off envelope; a continuous-wave tone under on-off keying is about the simplest signal a decoder can face. Many SDR programs (Fldigi, CW Skimmer, and the like) automate the timing-to-text step.
GopherTrunk is focused on digital trunked land-mobile systems and does not include a Morse decoder; CW is out of its scope. It is worth understanding here because CW is the historical root of the on/off keyed digital modes GopherTrunk does touch, and it frequently shares the bands a wideband SDR sweeps.
Sources
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Morse code — Wikipedia, for the dot/dash timing rules, WPM/PARIS convention, prosigns, and CW keying on radio. ↩