Machine Telegraphy
Machine telegraphy started with the 5-unit code system and worked very well. However, the matching of the sending and receiving mechanisms was very critical. To overcome this difficulty the start-stop principle was introduced in machine telegraphy.
In the start-stop principle, for each character, the transmitter is automatically. Prefixes the group of code elements with a start signal and terminates with a stop signal. Before any character is sent, a start (space) signal of 20 ms duration is transmitted which shows the start of a character. Then the five code signals, each of 20 ms duration (100 ms) are transmitted which is followed by a-slop (mark) signal of 30 ms duration, showing the end of a character.
The synchronization of the transmitter and receiver needs only to be for the duration of one character. The length of the stop signal is 1.5 times (20 ms x 1.5 30 ms) that of the other units, to allow the receiving mechanism to identify it in the case of continuous transmission. With this system, the minimum character duration is 150 ms. most machines work at 50 baud, although newer machines can work at 75 baud, and electronic machines can work at 100 baud. The start-stop formal for the five-unit code is shown in the figure.
In the early days of computers, input and output were handled through electromechanical teleprinters rather than the screens and keyboards we associate with modern PCs. For example:
- Teletype Model 33 - A popular teleprinter terminal used with early minicomputers and mainframes in the 1960s/70s. It printed text on paper and allowed typing input. Data speeds reached 10 characters per second.
- Computer Networks - Teleprinters were used to connect terminals to primitive computer networks prior to wide-area networking. For example, CompuServe began in the 1960s using teletypewriter equipment.
- Timeshare Systems - Early timeshare computers used banks of teleprinters to allow multiple users to access the mainframe. Users typed in commands and code which printed responses.
- Telex Channels - Some computer systems could utilize telex teleprinter networks internationally to exchange data between remote terminals.
- TTY Interface - Early Unix operating systems had a TTY (teletypewriter) interface to handle text input/output to terminals. Many commands referenced this legacy.
So essentially, teleprinters were an early solution to providing text input and printed output for computers. Much slower than modern networks and terminals, but revolutionary for early experimentation in connecting computers over distance and between multiple users. The capabilities of teleprinter equipment paved the way for more advanced computer interfaces and networking.
Characteristics of Machine Telegraphy
- Transmits text messages by modulating audio frequency tones rather than Morse code
- Uses electromechanical teleprinters to convert typed text into tone codes
- Tone frequencies typically in the range of 600-3000 Hz
- Faster transmission than manual Morse code
- More reliable printing of text directly on paper tape
Applications of Machine Telegraphy
- News wire services for transmitting news articles and reports
- Stock market tickers to send financial data
- Business communications for orders, transactions, memos
- Early computer networking for sending data between terminals
- Telex network for international text-based messages
Merits And Demerits Of Machine Telegraphy
Merits:
- The machine telegraph uses the equal length code.
- The synchronization of the sending and receiving mechanism is made with the help of start-stop signals.
- Machine telegraphy is speedier and more efficient.
- In machine telegraphy, a local record of the message can be Recorded on paper as well as on perforated tape in the coded form.
- With the start-stop mechanism, the machine can go out of synchronization for one character because each character is independent of the previous one.
- Automatic encoding and decoding of typed text
- Printed output eliminates errors in transcribing Morse code
- Can interface with teletypewriters, keyboards, and computers
- International compatibility using standard 5-bit Baudot code
Demerits:
- In machine telegraphy, any disturbance on the line upsets Synchronization.
- The speeds of the transmitter and receiver are be kept with Close limits.
- The speed of signal transmission through manual means is limited by the Speed of keyboard separation by the operator.
- The transmission media also introduces some amount of distortions.
- Electromechanical systems prone to reliability issues
- Limited to text-only communication
- Restricted to point-to-point usage
- Inferior to modern digital communication protocols
- Lacks encryption resulting in minimal security
Examples of Machine Telegraphy
Teleprinter
Popular teleprinters like the Teletype Model 15 were used extensively for news wires and business communication. The typed text was converted to a 5-bit Baudot code that modulated audio tones.
Hellschreiber
Used a simple FSK system that was robust to noise. Primarily used by the military for Morse code transmission. Printed on a paper strip.
Telex Network
An international subscriber network using teleprinters. Allowed businesses and news agencies to exchange text-based messages worldwide.
Stock Ticker
Printed stock prices, trades, and financial news on a paper ticker tape. Transmitted from a central exchange to brokerage houses.
Computer Terminals -
Early computers used teletypewriter machines for input/output. Linked computers could exchange data over telegraph wires.
Railroad Communications -
Trains were dispatched via teleprinters. Messages were typed between stations and locomotive crews.
News Wire Services
Press associations like AP and Reuters relied on teleprinters to distribute news stories to publications worldwide.
In summary, machine telegraphy enabled faster and more reliable text transmission over long distances using tone codes. While revolutionary in its time, it was an intermediate analog technology before the advent of modern digital communications.