TRANSMISSION SYSTEM FOR TELETYPEWRITER 531 



The first requisite is accomplished b>' mainlaiiiing a high degree of 

 synchronism between the sending and receiving devices during the 

 transmission of each character. The second is accompHshed by 

 providing satisfactory transmission facilities so that the mid-portion 

 of each received signal element is the same as the corresponding signal 

 element at the sending end. 



Timing Arrangements 



The sending and receiving devices are driven by motors which run 

 at approximately the same speed. The receiving device is driven 

 through a friction clutch so that it normally may be idle even though 

 its motor is running. When a signal is received the receiving selector 

 is released, makes one complete revolution, and again comes to rest. 

 With this arrangement it is necessary to maintain synchronism only 

 while one character is being transmitted, because a fresh start is made 

 for each character, and the time intervals for the selecting impulses 

 are measured from this starting point. Cumulative lack of syn- 

 chronism, therefore, over long periods of time does not affect the 

 accuracy of transmission. This is called the start-stop system. 



The advantages of this arrangement are as follows : 



(a) No elaborate means of synchronizing are required. 



Ih) The lag in the line is automatically taken care of because the receiving machme 



does not start until the first signal of a code combination is received, 

 (c) Multisection circuits and conference connections can be set up without any 



special line-up. 

 {d) Machines can be started and shut down without any special adjustment, 

 (e) Local power sources can be used for driving the subscriber's machine. 



In actual practice speed is maintained within ± 0.75 per cent in either 

 of two ways: 



1. Where regulated frequency a-c. power is available, synchronous motors 

 ordinarily will maintain the speed within ±0.17 per cent, which is well within the 

 limit necessary for satisfactory transmission. 



2. Where regulated frequency a-c. power is not available, governed motors are 

 used for either alternating or direct current. These governors are designed so as to 

 maintain the speed within ±0.75 per cent without attention over long periods of time. 

 If the speed of the sending machine is out in one direction and that of the receiving 

 machine in the opposite, the maximum difference may be 1.5 per cent. 



Assuming no deformation of the wave shape between the sender 

 and the receiver, the start-stop teletypewriter operating at 60 words 

 per minute will stand about 7 per cent speed discrepancy before errors 

 occur. In practice, however, there is deformation and therefore the 

 speed discrepancy must be kept as low as practicable. 



Sending and Receiving Arrangements 

 The sending arrangement in a teletypewriter is required to do 

 three things: 



