Morse code
Morse code is a telecommunications method which encodes text characters as standardized sequences of two different signal durations, called dots and dashes, or dits and dahs. It is named after Samuel Morse, one of several developers of the system. Morse's preliminary proposal for a telegraph code was replaced by an alphabet-based code developed by Alfred Vail, the engineer working with Morse. Vail's version was used for commercial telegraphy in North America. Friedrich Gerke simplified Vail's code to produce the code adopted in Europe, and most of the alphabetic part of the "Morse" is copied from Gerke's revision.
The ITU International Morse code encodes the 26 basic Latin letters to, one accented Latin letter, the Indo-Arabic numerals to, and some punctuation and messaging procedural signals. There is no distinction between upper and lower case letters. Each code symbol is formed by a sequence of dits and dahs. The dit duration can vary for signal clarity and operator skill, but for any one message, once the rhythm is established, a half-beat is the basic unit of time measurement. The duration of a dah is three times the duration of a dit. Each dit or dah within an encoded character is followed by a period of signal absence, called a space, equal to the dit duration. The letters of a word are separated by a space of duration equal to three dits, and words are separated by a space equal to seven dits.
Morse code can be memorized and sent in a form perceptible to the human senses, e.g. via sound waves or visible light, such that it can be directly interpreted by persons trained in it. Morse code is usually transmitted by on-off keying of an information-carrying medium such as electric current, radio waves, visible light, or sound waves. The current or wave is present during the time period of the dit or dah and absent during the time between dits and dahs. Since many natural languages use more than the 26 letters of the Latin alphabet, Morse alphabets have been developed for those, largely by transliteration of existing codes.
To increase transmission efficiency, Vail designed the original alphabetic code so the duration of each symbol was approximately inverse to the frequency that the character it represents occurs in English text. After revisions that led to ITU Morse code, the assignment of codes to characters in a few cases became non-optimal, although many encodings are. For instance, the most common letter in English,, has the shortest code a single dit. Because code elements are specified relatively, by proportion, rather than fixed duration, the code is usually sent at the highest rate the receiver is capable of decoding.
Development and history
Pre-Morse telegraphs and codes
Early in the nineteenth century, European experimenters made progress with electrical signaling systems, using a variety of techniques including static electricity and electricity from Voltaic piles producing electrochemical and electromagnetic changes. These experimental designs were precursors to practical telegraphic applications.File:Telegraph key and sounder, L.C.T. and Co., 8 Dey Street, NY - Bennington Museum - Bennington, VT - DSC08636.JPG|thumb|250px|right|Telegraph key and sounder; the signal is "on" when the knob is pressed, and "off" when it is released, length and timing of the dits and dahs are entirely controlled by the telegraphist
Following the discovery of electromagnetism by Hans Christian Ørsted in 1820 and the invention of the electromagnet by William Sturgeon in 1824, there were developments in electromagnetic telegraphy in Europe and America. Pulses of electric current were sent along wires to control an electromagnet in the receiving instrument. Many of the earliest telegraph systems used a single-needle system which gave a very simple and robust instrument. However, it was slow, as the receiving operator had to alternate between looking at the needle and writing down the message. In Morse code, a deflection of the needle to the left corresponded to a dit and a deflection to the right to a dah. The needle clicked each time it moved to the right or left. By making the two clicks sound different, transmissions on the single needle device became audible as well as visible, which led in turn to the Double Plate Sounder System.
William Cooke and Charles Wheatstone in Britain developed an electrical telegraph that used electromagnets in its receivers. They obtained an English patent in June 1837 and demonstrated it on the London and Birmingham Railway, making it the first commercial telegraph. Carl Friedrich Gauss and Wilhelm Eduard Weber as well as Carl August von Steinheil used codes with varying word lengths for their telegraph systems. In 1841, Cooke and Wheatstone built a telegraph that printed the letters from a wheel of typefaces struck by a hammer.
Samuel Morse and Alfred Vail
The American artist Samuel Morse, the American physicist Joseph Henry, and mechanical engineer Alfred Vail developed an electrical telegraph system. The simple "on or off" nature of its signals made it desirable to find a method of transmitting natural language using only electrical pulses and the silence between them. Around 1837, Morse therefore developed such a method, an early forerunner to the modern International Morse code.The Morse system for telegraphy was designed to make indentations on a paper tape when electric currents were received. Morse's original telegraph receiver used a mechanical clockwork to move a paper tape. When an electrical current was received, an electromagnet engaged an armature that pushed a stylus onto the moving paper tape, making an indentation on the tape. When the current was interrupted, a spring retracted the stylus and that portion of the moving tape remained unmarked.
In his earliest design for a code, Morse had planned to transmit only numerals, and to use a codebook to look up each word according to the number which had been sent. However, the code was soon expanded by Alfred Vail in 1840 to include letters and special characters, so it could be used more generally. Vail estimated the English language letter frequency by counting the movable type he found in the type cases of a local newspaper in Morristown, New Jersey. The shorter marks were called "dots" and the longer ones "dashes", and the letters most commonly used were assigned the shortest sequences of dots and dashes. This code, first used in 1844, was what later became known as Morse landline code, American Morse code, or Railroad Morse, until the end of railroad telegraphy in the U.S. in the 1970s.
Operator-led change from graphical to audible code
In the original Morse telegraph system, the receiver's armature made a clicking noise as it moved in and out of position to mark the paper tape. Early telegraph operators soon learned that they could translate the clicks directly into dots and dashes, and write these down by hand, thus making the paper tape unnecessary. When Morse code was adapted to radio communication, the dots and dashes were sent as short and long tone pulses.Later telegraphy training found that people become more proficient at receiving Morse code when it is taught "like a language", with each code perceived as a whole "word" instead of a sequence of separate dots and dashes, such as might be shown on a page.
With the advent of tones produced by radiotelegraph receivers, the operators began to vocalize a dot as dit, and a dash as dah, to reflect the sounds of Morse code they heard. To conform to normal sending speed, dits which are not the last element of a code became voiced as di. For example, the,, is voiced as. Morse code was sometimes facetiously known as "iddy-umpty", a dit lampooned as "iddy" and a dah as "umpty", leading to the word "umpteen".
Gerke's refinement of Morse's code
The Morse code, as specified in the current international standard, International Morse Code Recommendation, ITU-R M.1677-1, was derived from a much-improved proposal by Friedrich Gerke in 1848 that became known as the "Hamburg alphabet", its only real defect being the use of an excessively long code for the frequently used vowel.Gerke changed many of the codepoints, in the process doing away with the different length dashes and different inter-element spaces of American Morse, leaving only two coding elements, the dot and the dash. Codes for German umlauted vowels and were introduced. Gerke's code was adopted in Germany and Austria in 1851.
This finally led to the International Morse code in 1865. The International Morse code adopted most of Gerke's codepoints. The codes for and were taken from a code system developed by Steinheil. A new codepoint was added for since Gerke did not distinguish between and. Changes were also made to,, and. The codes for the digits in International Morse were completely revised from both Vail's original and Gerke's revised encodings. This left only four codes identical to the original Morse code, namely,,, and ; and and had their original short dahs extended to full length.
Radiotelegraphy and aviation
In the 1890s, Morse code began to be used extensively for early radio communication before it was possible to transmit voice. In the late 19th and early 20th centuries, most high-speed international communication used Morse code on telegraph lines, undersea cables, and radio circuits.Although previous transmitters were bulky and the spark gap system of transmission was dangerous and difficult to use, there had been some early attempts: In 1910, the U.S. Navy experimented with sending Morse from an airplane. However the first regular aviation radiotelegraphy was on airships, which had space to accommodate the large, heavy radio equipment then in use. The same year, 1910, a radio on the airship America was instrumental in coordinating the rescue of its crew.
During World War I, Zeppelin airships equipped with radio were used for bombing and naval scouting, and ground-based radio direction finders were used for airship navigation. Allied airships and military aircraft also made some use of radiotelegraphy.
However, there was little aeronautical radio in general use during World War I, and in the 1920s, there was no radio system used by such important flights as that of Charles Lindbergh from New York to Paris in 1927. Once he and the Spirit of St. Louis were off the ground, Lindbergh was truly incommunicado and alone. Morse code in aviation began regular use in the mid-1920s. By 1928, when the first airplane flight was made by the Southern Cross from California to Australia, one of its four crewmen was a radio operator who communicated with ground stations via radio telegraph.
Beginning in the 1930s, both civilian and military pilots were required to be able to use Morse code, both for use with early communications systems and for identification of navigational beacons that transmitted continuous two- or three-letter identifiers in Morse code. Aeronautical charts show the identifier of each navigational aid next to its location on the map.
In addition, rapidly moving field armies could not have fought effectively without radiotelegraphy; they moved more quickly than their communications services could put up new telegraph and telephone lines. This was seen especially in the blitzkrieg offensives of the Nazi German Wehrmacht in Poland, Belgium, France, the Soviet Union, and in North Africa; by the British Army in North Africa, Italy, and the Netherlands; and by the U.S. Army in France and Belgium, and in southern Germany in 1945.