A NEW TELEPHOTOGRAPH SYSTEM 567 



of the circuit. Since these sudden variations in net loss produce 

 noticeable changes in shade of the received picture, means similar to 

 those employed with the earlier Bell System telephotograph equipment 

 have been made available to prevent these relays from operating 

 while a picture is being transmitted. Simple types of control units 

 actuated by signals transmitted over a control circuit are connected 

 to each regulating repeater associated with the picture circuit. This 

 control circuit consists of two one-way d-c. channels obtained by 

 compositing the telephotograph circuit and extended to each tele- 

 photograph station over simplexcd loop arrangements. The control 

 circuit is also arranged to perform other functions such as effecting 

 one-way transmission of the 2-wire circuits during a picture trans- 

 mission. The operation of the control circuit normally is performed 

 automatically at the sending telephotograph station. 



Inasmuch as the transmission requirements for this control circuit 

 are very lenient compared with those for telegraphy, it has been 

 possible to employ simple types of d-c. repeaters as illustrated in 

 Fig. 12. A signal from the subscriber's sending equipment operates 

 the receiving relay of the station repeater, which in turn places a 

 ground on the M lead and thus transmits the signal to all line repeaters 

 which may be associated with this junction. Only one direction of 

 operation at a time is possible so that when a sending telephotograph 

 station takes control at the beginning of a picture transmission the 

 control circuit is operated and remains in this condition until released 

 automatically at the end of the transmission. A slow release circuit 

 is provided in the d-c. repeater used at regulating network points on 

 the cable circuits and also in another type of repeater, not shown but 

 used on open-wire circuits to obviate false operation of the repeaters 

 as the result of interruptions of less than two seconds duration. 



Delay Equalization 



Delay equalization " of telephotograph circuits is not new, but was 

 applied in 1925-26 to certain medium-heavy loaded toll cable circuits 

 between New York and Boston which w^ere used in the early Bell 

 System telephotograph service. (This application was discussed in 

 reference 14 relative to delay distortion, and examples of transmitted 

 printed matter were reproduced.) However, because of the increased 

 speed of transmission of the new telephotograph equipment and the 

 demand for longer circuits for picture transmission it has been necessary 

 to make further application of delay equalization to some of the more 

 common types of circuits used for this purpose. 



