194 BELL SYSTEM lECIISICAL JOLRXAL 



possible to have two forks at \viclel>' separated points running at 

 exactly the same speed, the problem of synchronizing would be 

 immediately solved. ActualK' this is not practical, since variations 

 of speed with temperature and other causes prevent the two forks 

 from operating closely enough together for this purpose. If the two 

 cylinders are operated on separate forks, even though each end of 

 the apparatus runs at a uniform rate, the received picture will, in 

 general, l)e skewed with respect to the original. The method by 

 which this difficulty has been overcome in the present instance is due 

 to Mr. M. B. Long. Fundamentally the problem is solved by con- 

 trolling the phonic wheel motors at each end by the same fork. For 

 this purpose it has been found desirable to transmit to the receiving 

 station impulses controlled by the fork at the sending end. The prob- 

 lem of transmitting both the fork impulses and the picture current 

 sinuiltaneoush- could be solved by the use of two separate circuits. 

 If this were done the currents going over the two lines would be 

 substantially as shown in Fig. 6, where the upper curve represents 

 the modulated picture carrier for two successive re\olutions of the 

 picture cylinder, and the lower curve shows the synchronizing carrier 

 current modulated by the fork impulses. 



It would iioi, liowcwT, lie economical to use two separate circuits 

 for the picture and s\nchronizing channels, consequently the two 

 currents are sent on the same circuit. In order to accomplish this, 

 the picture is sent on the higher frequency carrier, approximately 

 \,'M)() cycles per second, and the synchronizing pulses are sent on 

 the lower frequency carrier, approximately 400 cycles per second, 

 both l>ing in the range of frequencies readily transmitted by any 

 telephone circuit. These carrier frequencies are obtained from 

 two vacuum tube oscillators.^ The two currents are kept separ- 

 ate from each other by a system of electrical filters at the sending 

 and receiving ends, so that while the current on the line consists 

 of a mixture of two modulated frecjuencies, the appropriate parts of 

 the receiving apparatus receive only one carrier frecjuency each." 



' The vacuum tube oscillator as a source of carrier current is described in Colpitis 

 and Blackwell, Loc. Cit. .\ general discussion of the vacuum tube oscillator is 

 given in the ".Xudion Oscillator," Heising, J(yur. A. /. E. E. .April and May, 1920. 

 .\ discussion of the arrangement of the particular oscillator used with the picture 

 transmission equipment is given in "Vacuum Tube (Jscillalor," Morton, Belt System 

 Tech. Jour. July, 1924, \'ol. 3, .\o. \ pp. 508-.S24. 



•The application of wave filters to niulli-channel communication systems is 

 discussed in Colpitis and Blackwell, Loc. Cit. More complete discussions are to 

 be found in: "Physical Theory of Electric Wave Kilters," Campliell, Bell Sysleni 

 Tech. Jour. Nov., 1922, Vol. 1, No. 2, pp. 1-32. 



