Morse code: the original variable-length encoding

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Morse codeCommunicationEncoding

Morse code (1830s) is one of the earliest electrical communication codes. Its short/long pulse encoding is a classic example of variable-length coding and a precursor to modern entropy coding.

The basics

Each character is a sequence of dots (·) and dashes (–):

A: · –
B: – · · ·
C: – · – ·
SOS: · · ·   – – –   · · ·
  • Dot: 1 time unit.
  • Dash: 3 time units.
  • Inter-symbol gap inside a letter: 1 unit silence.
  • Inter-letter gap: 3 units silence.
  • Inter-word gap: 7 units silence.

Frequency-driven length: variable-length coding’s origin

Samuel Morse assigned shorter codes to more frequent letters:

LetterCodeFrequency in English
E·12.7% (highest)
T–9.1%
A· –8.2%
O– – –7.5%
I· ·7.0%

The shortest codes (1 unit) go to E and T; rare letters like Q and Z get five-unit codes.

This is the prototype of entropy coding — Huffman coding, arithmetic coding, and modern compression algorithms inherit the idea of “shorter codes for common symbols”.

International Morse vs American Morse

Two historical variants:

  • American Morse (1838) — early code with some idiosyncratic characters.
  • International Morse (1865) — standardized at an international telegraph conference; the version used today.

When people say “Morse code” now, they mean the international version.

SOS isn’t an acronym

The international distress signal · · · – – – · · · (adopted 1906) was chosen because:

  • It’s nine continuous dots and dashes, easy to remember.
  • It has a distinctive rhythm even through noise.
  • It’s transmitted as a single signal, not as the letters S-O-S.

“Save Our Souls” / “Save Our Ship” are folk etymologies after the fact.

Numbers and punctuation

0: – – – – –
1: · – – – –
2: · · – – –
3: · · · – –
4: · · · · –
5: · · · · ·
6: – · · · ·
7: – – · · ·
8: – – – · ·
9: – – – – ·

Numbers are uniform 5-unit codes.

International Morse also defines punctuation:

.   : · – · – · –
,   : – – · · – –
?   : · · – – · ·
@   : · – – · – ·   (added in 2004)

Where Morse code still shows up

Telegraphy is long gone, but Morse hasn’t fully disappeared:

1. Amateur radio

Hams use CW (continuous wave) mode. It’s narrow-bandwidth and goes long distances on minimal power.

2. Aviation VOR identification

VOR navigation aids broadcast a three-letter station identifier in Morse code.

3. Military / survival

Backup communication when voice is unavailable or jammed.

4. Accessibility

Some assistive input devices let users type with Morse code.

5. Visual / audio signalling

Flashlight blinks or mirror flashes for SOS in emergencies — still taught as a survival skill.

Memorizing tips

Anchors:

  • E = · (shortest)
  • T = –
  • I = · ·
  • M = – –
  • S = · · ·
  • O = – – –
  • H = · · · ·

“Common letters are short” gets you most of the way.

Encoding/decoding

A simple lookup-based implementation:

const morseTable = {
	A: '.-',
	B: '-...',
	C: '-.-.'
	// ... all letters
};

function toMorse(text) {
	return text
		.toUpperCase()
		.split('')
		.map((c) => morseTable[c] ?? '')
		.join(' ');
}

function fromMorse(morse) {
	const reverse = Object.fromEntries(Object.entries(morseTable).map(([k, v]) => [v, k]));
	return morse
		.split(' ')
		.map((m) => reverse[m] ?? '')
		.join('');
}

For timing-accurate output, treat each unit as a time duration and feed it to Web Audio.

Summary

  • 1830s code, still alive in narrow contexts.
  • Variable-length encoding driven by letter frequency — the conceptual ancestor of Huffman.
  • International Morse is the modern standard.
  • SOS was chosen as an easy-to-recognize pattern, not as an acronym.

To translate between text and Morse, the Morse code tool on this site converts both ways.