TRANSMISSION SYSTEM FOR TELETYPEWRITER 541 



link. Correspondingly, a connection iiuohing 5 links would permit 

 a coefficient of only 1.6 per link. It happens, however, that the 

 transmission capabilities of the teletypewriter circuits generally in use 

 are such that none of the circuits has a coefficient of less than 2.0 and 

 that single sections of circuit may lie in the range of about 2.0 to 5.0. 

 Practically, the availability of the higher grade circuits is limited by 

 reasons of economy since, for example, the multi-channel carrier 

 telegraph facilities which have coefficients of 2.0 to 2.6 would be too 

 expensive to use for routes where only a few circuits are required or 

 for the shorter links. It is apparent, therefore, that an over-all 

 coefficient of 7.0 to 8.0 cannot be realized on either 4 or 5 link connec- 

 tions without some means for overcoming the over-all distortion. 

 For these cases the operators are provided with connections to re- 

 generative repeaters, which are inserted in series with the circuit and 

 retransmit the teletypewriter signals exactly as they were originally 



HIGH FREOUENCV VOICE FRE- 

 CARRIER ON QUENCY CAR-" 

 OPEN WIRE :RIER in CABLE 



Fig. 5 — Diagram of topical teletypewriter exchange service connection requiring 



regenerative repeater. 

 Numbers are transmission coefficients. 



transmitted into the circuit at the sending end, provided they have 

 not been distorted beyond the point where they can be correctly 

 interpreted by the regenerative repeater. The latter has about the 

 same signal distortion tolerance that a teletypewriter would have if 

 the circuit terminated at that point. Thus the regenerative repeater 

 wipes out the distortion of the preceding toll links and subscriber 

 line so that the coefficient at its output will again be zero. The circuit 

 layout for an actual connection is shown in Fig. 5 illustrating the use 

 of a regenerative repeater. 



For the purpose of the teletypewriter exchange circuit layout, it is 

 assumed that regenerative repeaters are av'ailable at the switchboards 

 of all regional centers so they may be used to handle 4 and 5-link con- 

 nections. They are required occasionally at routing outlets to provide 

 satisfactory over-all results on 3-link connections. In the case of 

 2-link connections the use of regenerative repeaters is ordinarily 

 avoided by limiting the coefficient of the subscriber line, switchboard, 

 and first toll circuit to 5.0. For subscriber lines on which it is not 



