476 BELL SYSTEM TECHNICAL JOURNAL 



Mr. W. A. Marrison in his paper "A High Precision Standard of 

 Frequency," Proceedings I. R. E. July, 1929, described a crystal 

 controlled oscillator which would maintain a precision as to frequency 

 of 1 part in 10,000,000. This oscillator employs a quartz crystal as 

 its primary means of control and by means of secondary circuits the 

 natural period of the crystal, which is approximately 100,000 cycles, 

 may be stepped down to lower frequencies which are more convenient 

 for such purposes as motor control. 



By this means, a frequency of the desired value may be obtained 

 with a precision so great that the speed of the scanning discs under 

 control of the above described circuit will be so nearly perfect that no 

 synchronization channel at all is required. For example, if the period 

 of observation of the television image is 5 minutes, the scanning disc 

 will make 5300 revolutions when operating at a speed of 1060 r. p.m. 

 Assuming a precision of control of 1 part in 10,000,000, the maximum 

 error during the 5 minute interval will be 5300 divided by 10,000,000 

 or about 1/2000 of 1 revolution. Expressed in degrees on the periphery 

 of the disc, this is equivalent to approximately 1/6 of 1 degree or since 

 the width of the television image with 72 holes in the scanning disc is 

 5 degrees, the image will drift 1/30 of a frame width during the 5 

 minute interval. If the speed of the scanning disc at the other end 

 drifts an equal amount in the opposite direction, the displacement of 

 the television image will be only 1/15 of a frame width, which is a 

 tolerable amount of drift. 



From a practical viewpoint, however, it is apparent that the addi- 

 tional cost of very precise independent oscillators would be greater than 

 the cost of providing the synchronization channel, except possibly for 

 transmission over long distances. 



Framing 



Referring to Fig. 2, it will be noted that a phase shifter is provided 

 between the oscillator and the input terminals to the control circuit. 

 This phase shifter is designed with a split phase primary member 

 producing a rotating magnetic field. The secondary member is sin- 

 gle phase and is mounted on a shaft provided with a handle. By 

 rotating the handle of the phase shifter in the desired direction, the 

 frequency delivered from the phase shifter will be the algebraic sum 

 of the frequencies of the oscillator plus the frequency of rotation of the 

 armature of the phase shifter. It is, therefore, a simple matter for 

 the operator at the receiving end to momentarily increase or decrease 

 the control frequency and thus bring the picture into frame. 



