Telegraphy

As its name indicates, it is a telecommunication system based on the transmission of telegraphic messages. It was noted earlier that transduction consists in matching a graphic symbol from a given set with a special graphic symbol, an electrical signal: the Morse language or telegraphic alphabet. In the case of telegraphy, transduction may therefore consist in assigning electrical representations to the symbols of an alphabet.

Manual and Automatic Transcription

This transliteration may be carried out manually or automatically through a transmitting device. The manual key consists of a simple lever; by pressing it, the operator produces combinations of short electrical impulses (dots) and long impulses (dashes), which form the Morse alphabet, named after the inventor of the telegraph. Automatic manipulators may adopt various forms; the more modern versions include a keyboard similar to that of a typewriter, where each key corresponds to two possible transliterations. These two options are selected through a special key that functions like the shift key on a typewriter. The machine includes a total of 31 keys, enabling the transmission of 62 distinct characters. It may employ the Morse telegraphic alphabet or the so-called five-element code. In this latter system, the time interval assigned to one symbol (100 milliseconds) is divided into five elements, each of which may be transmitted with the circuit either open or closed. This yields 32 possible combinations, although one of them (open circuit throughout the five elements) is not used for transmission.

Emission and Intermediate Recording

Transmission can be performed directly through the manual or automatic key, through a transmitter coupled to it, or through an independent transmitter. In this last case, there is an intermediate recording stage between manipulation and emission in which the result of the manipulation is registered on a paper tape by perforation (magnetic tape may be used for this purpose in the future) or by suitable mechanical organs. The transmitter then includes a reading device sensitive to this record and controls the circuit contacts accordingly. Introducing this intermediate recording stage generally provides a significant advantage, since the time required for manipulation is always greater than that needed for transmission, allowing better use of the communication line. In the Baudot system, for instance, a single transmission line serves four manipulators, enabling four messages to be sent in the time ordinarily required for one. The communication line may be wired or radio-based. In older telegraphic systems, the wired line consisted of a single conductor with ground return, a mono-wire configuration that is now disappearing.

Receiving Mechanisms

The simplest telegraphic receiver includes a basic electromagnet with a movable armature, which is attracted when current flows through the line and released when it does not. The receiving mechanism thus mirrors the positions of the transmitting key. In this arrangement there is no actual written record; the operator interprets the signals by ear and reconstructs the message manually. In other systems, the receiving electromagnet drives a pen that traces a sinuous line on a paper tape according to the movement of the armature. This device, known as an undulator, records the dots and dashes of the telegraphic alphabet, and translation is performed by the operator.

Teleprinters and Synchronous Operation

The translating mechanism is sensitive to the positions of the five elements and controls the hammers of a typewriter that prints the transmitted text directly on a tape (tape printers) or a sheet (page printers). The typewriting machines used in automatic telegraphic systems are known as teleprinters or teletypes. They may operate synchronously or asynchronously. In synchronous systems, the transmitting and receiving machines function in unison, so the operator (or automatic transmitter) must emit each character at a uniform rate for the receiving machine to interpret it correctly.

Asynchronous Systems: Start-Stop Operation

In asynchronous machines, no such synchronization is required. Here, the transmitting mechanism (or intermediate recorder) remains at rest until a key is pressed. The emission of the five code elements is preceded by a special signal (start signal) that activates the receiving machine, which remains stopped until this signal is received. Once the character has been transmitted, the transmitter stops again, but not before sending another special signal (stop signal) that halts the receiving machine. In this way, transmission can occur without maintaining a fixed cadence. This operating mode explains the designation “start-stop” applied to asynchronous systems.