TRANSMISSION SYSTEM FOR TELKTYPEWRITKR 535 



second. In the illustration in Fig. 2, this frecjucncy would fall between 

 that of curves d and c in the left-hand column and the character 

 theoretically could be interpreted correctly with the transmission 

 in correct phase relation of only the components up to and including; 

 the fourth harmonic (curve o in the right-hand column). As pre- 

 viously stated, however, while transmission of such a limited fre- 

 quency range could be interpreted without error by an ideal receiving 

 device, practical considerations of over-all economy make it desirable 

 to transmit the wider frequency range mentioned. 



Types of Distortion 



In order to design a satisfactory teletypewriter transmission system 

 it is desirable to understand the effects of various types of distortion 

 and mechanical variations in the sending and receiving mechanisms. 

 Figure 3 shows schematically that part of the receiving mechanism 

 which is of interest in explaining the effect of signal distortion on 

 correct interpretation of the message. This includes a receiving 

 selector magnet with its associated armature and armature extension, 

 a locking lever, a stop latch, and a selector cam driven by a friction 

 clutch. In the idle condition the selector magnet is energized and the 

 magnet armature and armature extension are in the position shown, 

 the selector cam being held from rotating by the stop latch. When a 

 train of impulses representing a character is received the start pulse 

 (spacing) allows the armature and armature extension to move to a 

 position shown by the dotted lines and at the same time releases the 

 stop latch. This latter operation permits the selector cam to start 

 rotating. The speed of rotation and the starting position of the 

 selector cam are normally so adjusted that the first depression (shown 

 by A) will arrive at the locking lever at the time the middle of the first 

 selecting impulse is being received. The locking lever will then fall 

 into this depression and the locking wedge B will move toward the 

 armature extension and lock it in the position it occupies at this 

 instant. This determines which of the 2 line conditions will be 

 recorded for this signal element. Immediately thereafter mechanical 

 arrangements (not shown) will operate to transfer this information 

 to the selection storing mechanism. This process will then be re- 

 peated for each of the other 4 selecting impulses. 



After the 5 selecting impulses have been received the slightly longer 

 stop impulse is received. During the latter part of this impulse an 

 arm C on the receiving selector cam will strike the stop latch and the 

 cam will be held until the reception of the start impulse for the next 

 character. 



