Synchronization of Television ^ 



By H. M. STOLLER and E. R. MORTON 



Synopsis: Synchronization of Television is the problem of holding two 

 scanning disks so that their phase displacement is always less than four and 

 one third minutes of arc. A 240-pole synchronous motor of the variable 

 reluctance type is used as a basis. Coupled to it a direct current motor 

 carries the steady component of the load. Hunting is eliminated by a 

 condenser in series with the two synchronous motors whose capacitance is 

 slightly less than that required to tune the circuit. 



As the motor might lock into step in any of 120 possible angular positions, 

 only one of which would give the proper phase relations, a two-pole motor, 

 with only one locking position, was provided by tapping the armature of 

 the direct current motor at two points and bringing out the leads to slip rings. 

 This was used for synchronizing while the 240-pole motor, connected 

 subsequently, held the close synchronism required. The disks rotate at 

 1062.5 r.p.m. which gives 17.7 cycles on the two-pole and 2125 cycles on 

 the 240-pole motor. • 



For transmission the synchronizing current is attenuated to a level of 

 .6 milliwatt and amplified at the receiving end. The 17.7-cycle current is 

 an undesirably low frequency for transmission over telephone cables and 

 so is used to modulate a 760-cycle current through a polarized relay. This 

 is demodulated at the receiving end, where a polarized relay by interrupting 

 a local battery current gives a rectangular wave which acts through vacuum 

 tubes on the field of the direct-current motor. 



THE problem of synchronization involved in television transmitting 

 and receiving equipment is similar in principle to any synchronous 

 motor problem but the requirements are of such a special nature that 

 it is necessary to employ unusual features of motor design and control 

 circuits to secure the required results. 



General Requirements 



At the transmitting end a scanning disk is employed containing 50 

 holes spirally spaced around the periphery of the disk rotating at a 

 speed of 1060 r.p.m.^ It is desired to rotate a similar scanning disk 

 at the receiving end so that the hole through which the observer is 

 looking at a neon lamp will be in a position corresponding to the hole 

 which is transmitting light at the same instant at the transmitting 

 end. Since there are 50 holes in each disk, the holes will be spaced 

 apart 7.2 degrees, thus 7.2 degrees of arc correspond at the receiving 

 end to the width of the picture. Since the horizontal resolving power 

 is approximately the same as the vertical (0.02 of the picture dimen- 

 sion), the arc occupied by a picture element is 0.02 X 7.2 or 0.144 

 degree. In order not to appreciably impair the quality of the picture, 



1 Presented at the Summer Convention of the A. I. E. E., Detroit, Mich., June 

 20-24, 1927. 



2 This speed was determined by transmission considerations and is discussed in 

 the companion paper by Messrs. Gannett and Green. 



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