532 BELL SYSTEM TECHNICAL JOURNAL 



1. It must transmit a signal which will start the selecting cycle of the distant 

 machine. 



2. It must apply the proper current condition to the line for each of the 5 accu- 

 rately spaced selecting time intervals. 



3. It must send a signal which will return the line to the normal idle condition. 



The teletypewriter operates in a local circuit in which current is 

 flowing during the normal idle condition. The transmitting is done by 

 opening and closing this circuit, causing zero current or normal 

 current in it, the two conditions being referred to as "open" and 

 "closed." The selecting cycle of the distant machine is initiated by 

 opening the circuit at the sending teletypewriter. This is called the 

 "start" signal. The five selecting signals follow and the line current 

 during each of these time intervals depends upon the character which 

 is being transmitted. Since the normal idle condition of the line is 

 closed, the "stop" signal which is sent last in the train of signals is a 

 "closed" signal. 



The selecting arrangement in a receiving teletypewriter is also 

 required to do three things: 



1. It must start timing the signals when the start signal is received. 



2. It must determine the line condition at the midpoint of each selecting interval. 



3. It must come to rest during the stop interval following the 5 selecting signals. 



A single electromagnet in the receiving machine converts the elec- 

 trical pulses into mechanical operations of the selecting mechanism. 

 This magnet controls an armature which is energized for the closed 

 line condition and de-energized during the open line condition. By 

 this means the 2 line conditions are converted into 2 positions of the 

 magnet armature. 



Theory of Teletypewriter Signal Transmission 



In teletypewriter signal transmission at 60 words per minute 

 (hereafter called 60-speed) the start pulse and each of the 5 selecting 

 signal elements are normally of 0.022 second duration. The minimum 

 length of the stop pulse is 0.031 second. In keyboard sending the 

 maximum length of stop pulse depends upon the time the operator 

 hesitates between the striking of the individual keys of the teletype- 

 writer. Any lengthening or shortening of the signal elements in 

 transmission is referred to as distortion and is expressed as a per- 

 centage of the normal length of a signal element. The fundamentals 

 of signal transmission have been discussed thoroughly by various 

 writers.^' ^- •* A few of these principles are enumerated here without 

 any attempt to discuss them thoroughly. 



1. With the transmitting arrangements usually employed the complete change in 

 line condition at the sender is practically instantaneous. 



