The First Printing Telegraphs |
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| As we previously discussed, the first electric telegraph machines were invented in 1837 by Sir Charles Wheatstone in England and Samuel Finley Breese Morse in America. Morse's machine was eventually adopted as the standard, because it was simpler, easier to construct, and more reliable. Morse's original machines kept a record of incoming messages using an electromechanically controlled pencil that made marks on a moving strip of paper. The paper was driven by clockwork, while the lengths of the marks corresponded to the dots and dashes used in Morse Code. However, operators quickly realized that they could recognize the message by sound alone, so Morse's recording devices returned to the nether regions from whence they came. |
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| Throughout the rest of the 1800s there continued to be a strong interest in the idea of a printing telegraph. Much of the work toward realizing this dream was based on the concept of a wheel with characters embossed around the periphery. The idea was to use the incoming telegraph signals to spin the wheel by fixed steps until the correct character faced the paper, and to then propel that character onto an inked tape located in front of the paper. | There were a variety of techniques for controlling the wheel, such as a single pulse for 'A', two pulses for 'B', three for 'C', and so on, with the wheel returning to a home position after each character, but this technique was very slow in terms of words-per-minute. Later techniques used a five-bit code created by the French inventor Jean Maurice Emile Baudot in 1880, which soon became known as the Baudot Code. The two-channel paper tape technique pioneered by Sir Charles Wheatstone was subsequently extended to handle the Baudot Code.
Paper tape with 5-bit Baudot Code |
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| Using a five-bit code, it is possible to obtain 32 different combinations of holes and blanks (no holes). In the case of the Baudot Code, twenty-six of these combinations were used for letters of the alphabet, leaving eight spare combinations for an idle code, a space code, a letter-shift code, and so on. The problem was that there weren't enough spare combinations left over to represent the numbers '0' through '9' or any punctuation characters. |
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| To solve this dilemma, the letter-shift code was used to emulate the shift key on a typewriter by instructing the receiver that any subsequent codes were to be treated as uppercase characters (in this context, uppercase was used to refer to numbers, punctuation, and special symbols). A second letter-shift code could subsequently be used to return the receiver to the alphabetical character set. The five holes and blanks for each character were transmitted as a sequence of pulses and gaps, and decoded and printed at the receiving end by a variety of different techniques. Note the special characters such as BELL, which actually rang a bell on the receiver to alert the operator. | The early systems required the operator to use a keypad with five separate keys, and to simultaneously push whichever keys were required to form a character. Later systems were based on a typewriter-style keyboard, whereby each typewriter key activated the five transmitting keys (or a paper tape punch) to establish the correct pattern. Unfortunately, none of these systems were tremendously robust or reliable, and they all suffered from major problems in synchronizing the transmitter and the receiver such that both knew who was doing what and when they were doing it. | |
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| The
original Baudot code became known as
the International Telegraph Code No. 1. Sometime around 1900, another 5-bit code
called the Murray Code was invented. The Murray Code eventually
displaced the Baudot Code and became known as the International Telegraph Code No. 2.
Unfortunately, everyone was hopelessly confused by this time -- to the extent that
Murray's name sank into obscurity, while Baudot's name became associated with almost every
5-bit code on the face of the planet, including the International Telegraph Code No. 2. See also: The first commercial typewriter and The first teleprinters |
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| These notes are abstracted from the book Bebop BYTES Back (An Unconventional Guide to Computers) Copyright Information |
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