Patent GB-718123-A (GB)
Title: Synchronizing apparatus, particularly for color television
IPC: H04N9/44
Filed: 1951-12-27
Abstract: 718,123. Synchronous movements. COLUMBIA BROADCASTING SYSTEM, Inc. Dec. 27, 1951 [Jan. 6, 1951], No. 30216/51. Class 40 (3). [Also in Group XXXV] In apparatus for synchronizing a non-synchronous motor with an incoming synchronizing signal which comprises a generator coupled to the motor and producing a local signal of periodicity varying with the motor speed, and wherein one of said signals, preferably the synchronizing signal, comprises a train of relatively short pulses while the other signal, preferably the local signal, comprises periodic sloping portions, and the two signals are applied to phase-comparing means which delivers a control signal which is used to effect retro-active control on the motor to maintain the pulses in a predetermined phase relationship with the sloping portions of the other signal and thus maintain the motor in synchronism with the incoming signals means are provided for rendering the phase-comparing means inoperative during the intervals when the pulses do not coincide with the sloping portions of the other signals, so that the control signal is maintained, during these intervals, at the value it last had when the phase-comparing means was operative. By this means the control effected upon the motor is such as to tend to bring it into synchronism with the incoming signals even when the pulses coincide with portions of the other signal which slope oppositely from the previously mentioned sloping portions. The invention is particu'arly applicable to the control of a rotating colour disc in a colour-television receiver in accordance with received synchronizing signals. The colour disc 12 is driven by a two-phase capacity motor 27 and is coupled by a shaft to a generator 28 which delivers a sinusoidal output at the colourcycle frequency. This output is applied over lead 34 to a phase-comparing circuit 35 comprising a pair of valves, the anode of one being connected to the cathode of the other, and vice versa. This circuit is rendered conductive for short intervals at the colour cycle frequency by means of incoming synchronizing pulses 18, 19 so that the storage condenser 37 successively assumes the values of the generator output during these short intervals. The storage condenser 37 is connected to a motor-control valve 49 the anode circuit of which includes the primary of a saturable-core reactor 57, the secondary of the reactor 57 being connected in series with the power supply to the motor 27. The motor speed thus increases with increase of potential on the condenser 37 and the sloping portions of the generator output over which control is effected by comparison with incoming pulse signals are so chosen that the control is such as to maintain the co!our disc 12 in synchronism with the incoming pulses. A connection is taken as shown from the screen-grid circuit of the valve 49 to the circuit of the generator 28 ; this is stated to reduce hunting. In the arrangement illustrated, the pulses 18 which are applied in parallel to the two grids of the valves of circuit 35 are frame synchronizing pulses and thus tend to render the valves conducting at three times the colour-cycle rate. In addition pulses 19 coinciding with every third frame synchronizing pulse are applied to valve 44 which normally short-circuits resistor 41 across which the pulses 18 are app'ied to circuit 35 and thus periodically cut valve 44 off, thus removing the short-circuit and allowing every third frame-synchronizing pulse 18 to render circuit 35 conducting. In a modification, Fig 7 (not shown), the generator coupled to the colour disc delivers an output at the frame frequency and this is compared in circuit 35 with pulses 18 only, the correct colour phasing being achieved by means of a pushbutton which is connected across the secondary of the reactor 57. By depressing the push-button momentary loss of synchronism is obtained and this is repeated until correct colour phasing is achieved. In addition to the apparatus already described a valve 86 connected across the resistor 41 is provided. The sinusoidal output from generator 28 is applied to the control grid of the valve 86 through a 90 degree phase-advance circuit 87. This signal applied to valve 86 is of large amplitude so that the valve is rendered alternately conducting and non-conducting for approximately equal intervals, thus periodically short-circuiting the input 18 to the circuit 35. This is arranged to take place during those portions of the sine-wave output from generator 28 which slope in such a direction that were it possible to effect a comparison in circuit 35, the control signal derived would be such as to increase the error. Thus during these intervals the potential on condenser 37 remains constant and may be shown to be of such a value as to tend to reduce the error between the disc rotation and the synchronizing pulses whether the motor speed is too fast or too slow, the potential upon the condenser 37 during these intervals depending upon the direction from which the non-operative portions of the sine-wave signal are approached. In. a modification a non-linear resistance element 97 is connected across the output of generator 28 in order to increase the steepness of the sloping portions of the output waveform. In the modifications of Fig. 7 (not shown) the generator output comprises sinusoidal portions spaced by steady-amplitude portions, the signal applied to valve 86 being derived 'rom a phase-changing network. In another modification, an irregular waveform, Fig. 10 (not shown) having a short wavesloping portion is employed, the signal applied to valve 86 being obtained by differentiation. Reference has been directed by the Comptroller to Specification 656,890.