166 BELL SYSTEM TECHNICAL JOURNAL 



E. Sources of Test Signals 

 Bell System telegraph circuits are tested at present with both 

 substantially perfect and distorted test signals. The quality of the 

 test signals is, of course, of prime importance because the measurements 

 are generally made on a straightaway basis and it is not practicable 

 to make correction for accidental distortion in the test signals at the 

 sending end of the circuit. On high-grade circuits where the distortion 

 is generally less than about 5 per cent, distortion in excess of a few 

 per cent in the test signals is very undesirable. 



Undistorted Test Signals 



Substantially perfect test signals are usually supplied from motor- 

 driven commutators. One type supplies telegraph reversals. In this 

 case an accurately governed motor drives two brush arms, each of 

 which is associated with two rings of segments so that four sources 

 of signals are provided by each machine. These reversals or dot 

 signals are used in a number of ways, their principal advantage being 

 that since the average of the marking and spacing intervals is zero, a 

 simple integrating meter circuit may be used for measuring bias. 



When it is desired to employ miscellaneous or recurring teletype- 

 writer characters machines known as transmitter-distributors are used. 

 Such a device, arranged to send a standard test sentence, is illustrated 

 in Fig. 16. This consists of a motor-driven commutator with a con- 

 tinuously rotating brush arm and a direct-coupled cam transmitter 

 (on the left) which changes the connections to the segments of the 

 commutator in accordance with the code which is cut on the cams. 

 In another form of this device a tape transmitter ^ is used; the tape is 

 usually of parchment although metal tapes and wheels drilled with the 

 code combinations have been used. 



When a number of sources of undistorted signals is required in a 

 repeater station a device called a "multiple-sender" is used. This 

 employs the distributor of Fig. 16 to operate a number of relays. 

 The transmitting contacts of these relays are connected to jacks at 

 convenient locations in the telegraph test board. The standard test 

 sentence supplied by this device contains desirable signal combinations 

 for testing transmission over lines and also for testing the operation 

 of teletypewriters. 



The signals from the commutator face traverse two groups of relay 

 windings, a marking group and a spacing group, the circuit being as 

 indicated in Fig. 17. This circuit effectively provides polar operation 

 of the relays, the transmitter closing the circuit through one group of 

 windings to operate the relays to their marking contacts and through 



