Patent GB-292597-A (GB)
Title: Improvements in or relating to synchronising for use in television systems
IPC: H04N5/067
Filed: 1928-06-13
Abstract: 292,597. Electrical Research Products, Inc., (Assignees of Stoller, H. M., and Morton, E. R.). June 23, 1927, [Convention date]. Television; synchronous movements. - The receiving and transmitting apparatus of a television system are synchronized coarsely under the control of a low-frequency current, and finely under the control of a high frequency current. In the system illustrated the transmitter B, Fig. 3, is controlled as regards synchronism by the receiver A, Fig. 1, the television signals being sent over the line 115. Maintaining constant speed. The controlling motor 10, Fig. 1, is driven from the mains 38 by a series field winding 11, shunt field winding 12, and an additional shunt field winding 13 which regulates the speed under the control of a tuned circuit 31, 32. The regulating winding 13 is included in the plate circuit of valves 35, 36, whose high tension comprises a d.c. component derived from the mains 38 and a low frequency a.c. component of, say, 17.7 cycles derived via the transformer 37 from slip rings 14. The strength of this regulating current depends on the grid voltage of the valves 35, 36, and therefore on the voltage drop in the resistance 34. This resistance together with the inductance 32 lies in the plate circuit of the further valve 33, and the inductance 32 forms part of the tuned circuit 19, 31, 32, which is energized by the high-frequency alternator 16 coupled to the motor 10. The normal frequency of the alternator 16 lies on the falling part of the resonance curve of the circuit 19, 31, 32, so that when the speed of the motor increases, for instance, the voltage drop in the inductance 32 decreases; hence the plate current in the valve 33 and therefore the voltage drop in the resistance 34 decrease. The grid bias of the valves 35, 36 consequently becomes more positive, the current in the regulating winding 13 increases, and the rise in speed of the motor is thus checked. The control is rendered more precise by the circumstance that increase in the plate current of the valves 35, 36 involves an increased voltage drop in the resistance 40 and consequently a lowering of the grid potential of the valve 33. This action is delayed, however, by the condenser 43 for a period not less than the natural period of the system. Control of speed at a distance. The control currents of high frequency (such as 2125 cycles) are sent to line through the leads 45, Fig. 1, and a filter circuit (Fig. 2, not shown). Those of low frequency (such as 17.7 cycles) pass over leads 46 to a contact-making relay (Fig. 2, not shown) which modulates a carrier current of intermediate frequency (such as 760 cycles). Both signals pass over the same line and after being demodulated and separated are applied to the controlled motor 20, Fig. 3. This motor is driven from 110-volt mains 51. 52 by the series winding 21, shunt winding 22, and regulating shunt winding 23. The latter is included in the plate circuit of the back-to-back valves 47, 48, whose grids are energized by the low-frequency synchronizing currents received over the lines' 121, while their plate circuits are energized in part by the d.c. voltage from mains 51, 52, and in part by a.c. voltage derived through the transformer 58 from slip-rings 24 on the controlled motor 20, and the reaction is such as to maintain coincidence of phase between the motor and the received impulses. The power factor is corrected by a condenser 60. Synchronism is thus coarsely obtained by the low-frequency currents. Fine synchronism is secured by the high frequency currents arriving over the leads 202. These are amplified by the valves 49, 50, whose high tension is derived from the 750 volt mains 51, 53, and amplified currents are applied to a winding 28 on the high frequency alternator 26 which is coupled to the controlled motor 20. The circuit 28, 61 62, is tuned to a frequency slightly above the working frequency, to prevent hunting. Synchronism when established may be maintained by the high-frequency currents alone. For the sake of stability the inertia of the controlling motor may be high and that of the controlled motor low; thus the controlling motor may be that at the receiver, where the scanning disc is larger than at the transmitter. Manual phase - adjustment. the television picture may be framed by turning the carcase of the receiver motor manually by worm-gear.