552 BELL SYSTEM TECHNICAL JOURNAL 



transmission of one inch of picture per minute, measured along the 

 axis of the picture cylinder. This rate of scanning produces essential 

 signal frequencies extending approximately from zero to 1,000 cycles 

 per second and is more than double the speed of transmission used in 

 the earlier equipment. However, by employing the single-side-band 

 method of transmission it has been possible to use this speed of trans- 

 mission over telephone circuit facilities of normal band width but 

 specially modified as described in a later section. 



Synchronism 



Operation of the earlier Bell System telephotograph equipment over 

 long telephone circuits indicated the desirability of providing improved 

 means for synchronizing the sending and receiving equipment. Ac- 

 cordingly, development work was undertaken, and local frequency 

 sources of the required stability were made available to permit inde- 

 pendent speed control without transmitting synchronizing signals. 

 Experimental oscillator units were installed for tests at three tele- 

 photograph stations about two years after the opening of the public 

 telephotograph service in 1925. Experience gained from the use of 

 these oscillators, which were vacuum tube driven tuning forks main- 

 tained within close temperature limits, indicated that this method was 

 practicable, although the particular arrangements employed at that 

 time could be advantageously improved. 



A new design of tuning fork controlled oscillator has been provided 

 in the new equipment whose frequency can readily be adjusted and 

 maintained constant to within a few parts in a million. This difference 

 in speed between sending and receiving machines is so slight that 

 skewing of the received picture is not noticeable. 



Starting and Phasing 



The simultaneous starting of all machines participating in the 

 transmission and reception of a picture is effected by means of a signal 

 sent over the line by the transmitting machine. Phasing of the 

 machines is automatic, since all are started simultaneously from the 

 same angular position by a positive action clutch. This requirement 

 is similar to that met by the earlier equipment, but more difficult to 

 fulfill because of the use of a much larger picture cylinder. It required 

 the development of a new type of clutch which would permit a gradual 

 increase in velocity of the cylinder and yet be positive in action. 

 The fulfillment of this requirement is important as it assures accurate 

 phasing without consuming valuable circuit time, irrespective of the 

 number of machines involved in a transmission. 



