Printing Telegraphs
Printing Telegraphs
MORSE'S telegraph opened up the frontiers of electronic communications, but it had many shortcomings. First and foremost, the original Morse design allowed for only one conversation on the line at one time. Wire was handmade then, brittle, and very expensive. Installing the wire along the railroad tracks was time-consuming and often dangerous work. Several inventors, including Thomas Edison, put themselves to the task of inventing a multiplex telegraph—one that would allow several telegraph operators to use the same line at the same time. (Remember, Alexander Bell and Elisha Gray were both attempting to invent the harmonic telegraph—a form of multiplex telegraph—when they turned their attention to the telephone instead.)
Multiplexing made telegraph service more efficient and cost-effective, but a larger obstacle still remained: Morse's code itself. Sending messages via Morse code required a trained operator at each end of the wire. Western Union and its competitors were keen to develop a system that did not require constant human intervention.
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As early as 1846 (only two years after Morse's first successful telegraph demonstration), a man with the unlikely name of Royal House invented a printing telegraph. Unfortunately, House's machine had its own set of problems. Although House claimed his machine was "twice as fast as Morse," it required two operators at each end of the line.
Several other inventors worked on printing telegraph machines, but French inventor Emile Baudot made many of the breakthroughs. Baudot's printing telegraph was the first to use a typewriterlike keyboard, and it allowed eight machines to share a single wire. More importantly, Baudot's machines did not use Morse code. Baudot's five-level code sent five pulses down the wire for each character transmitted. The machines themselves did the encoding and decoding, eliminating the need for operators to become proficient at Morse code. For the first time, electronic messages could be sent by nearly anyone.
English inventor Donald Murray expanded and improved on Baudot's work, and Murray sold the American rights to his inventions to Western Union and Western Electric. The Murray patents became the basis for the teletypewriter, also known by AT&T's brand name Teletype and by its generic nickname, TTY.
Western Union applied the new technology on its own network. Over time, the teletypewriter replaced the Morse key and sounder in most of Western Union's offices. Western Union also used the teletypewriter technology to provide a service called telex. Telex service allows subscribers to exchange typed messages with one another. Until the advent of the fax machine in the 1980s, telex service was widely used in international business.
AT&T operated a similar service called the Teletypewriter Exchange (TWX). Like telex, TWX service consisted of a teletypewriter connected to a dedicated phone line. TWX had the advantage of access to AT&T's wide-reaching telephone network. Like telex, TWX usage peaked in the 1960s and 1970s. In 1972, AT&T sold the TWX service to its old nemesis, Western Union.
In the 1930s and 1940s, several schemes were developed to allow the transmission of Teletype signals via shortwave radio. Radio Teletype, or RTTY, uses a technique called frequency shift keying (FSK) to simulate the on and off voltage used by conventional teletypes. In FSK, a signal on one frequency indicates ON, and a signal on the other indicates OFF. Because radio signals can be keyed on and off very quickly, RTTY signals run at speeds similar to land-line teletypewriters.
RTTY signals broadcast via shortwave radio allow many stations to receive the same signal. RTTY was widely used by United Press International (UPI) and the Associated Press (AP) wire services before cheaper, more reliable satellite links became available in the 1980s. RTTY in various forms is still used today for ship-to-shore telex service and for marine and aeronautical weather information.
The Teletypewriter
For 50 years after its invention at the turn of the century, the teletypewriter was the mainstay of nonvoice electronic communications. Teletypewriters were frequently connected in a round-robin circuit. In this configuration, the original signal is sent from one point on the circuit and received by all the other machines on the circuit. This type of circuit was widely used by news wire services such as the Associated Press and United Press International.
Unlike the Baudot code, Morse code uses characters of unequal length and size. For example, the letter E is expressed as one dot, but the number 0 is expressed as five dashes. This inequality of size makes Morse easy to detect by ear but very difficult to decode mechanically.
The Baudot code uses five equal-length elements (Morse would have called them "dots") to define each character of the alphabet. Five elements can define only 25, or 32, different combinations—not enough to print the entire alphabet plus numerals and punctuation marks. To overcome this problem, two special nonprinting characters, called FIGS and LTRS, shift the printing mechanism between letters (A–Z) mode and figures (numbers and punctuation marks) mode. The two modes allow the code to represent a total of 62 characters.