Fundamentals of Teletypewriters Used in the Bell System

7 I T H the growth of Teletypewriter Exchange Service and the * * general increase in the use of teletypewriters in private line services of various types, questions frequently asked are: How do teletypewriters operate? W h a t is the " s t a r t - s t o p " system? Why is it used? W h a t is a regenerative repeater? This article will attempt to answer some of these questions and explain also the fundamental principles and features of teletypewriters and their auxiliary arrangements as now employed in the Bell System. These have been developed to meet the needs of customers for a typed or similar record form of communication and at the same time be suitable for operation in connection with the Bell System plant. CODE For economical transmission over long distances it is fundamental that only a single wire or transmission channel be required to carry the signals. Furthermore, long experience with manual telegraphy on land lines has proved t h a t reliable and efficient operation is secured by using not more than two conditions on the line, such as current and no current or positive impulses and negative impulses, as contrasted with the use of three or more conditions, or current values. The entire telegraph plant of the Bell System as well as practically all other land line telegraph systems have been built on this two-condition basis. The familiar Morse code uses sequences of dots and dashes to represent the different characters of the alphabet and meet the above conditions. This code is not well adapted for teletypewriter control, however, since the signals for different characters vary widely in the time they require, from a single dot for the letter E to a combination of 620 FUNDAMENTALS OF TELETYPEWRITERS 621 several dots and dashes for some of the less frequently used letters or numerals.