Patent GB-617872-A (GB)
Title: Automatic control system suitable for use in the focusing of television pick-up tubes
IPC: H04N5/228
Filed: 1945-04-25
Abstract: 617,872. Automatic control systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. April 25, 1945, No. 10449. Convention date, April 25, 1944. [Class 38 (iv)] [Also in Group XL (b)] A control system providing automatic focusing of a television camera comprises means for deriving signals representative of a selected portion of a scene to be televised and means for utilizing the derived signals to automatically control the focusing means of the camera by comparison of energies respectively dependent upon the position of the focusing means at two successive time intervals. An image of an object 38 is focused on the screen 11 of an iconoscope type tube 10 and also on a half-silvered glass screen 42 by the optical systems 36, 41 respectively (Fig. 1). Both optical systems are carried by a rack 46 driven by means of a pinion 47 and reversible motor 49. A source SG supplies horizontal and vertical beam deflection energy and synchronizing and blanking signals for the tube 10 and for a cathode-ray tube CR. The source SG also provides controlling signals for two oscillators OSW<SP>1</SP> and OSW<SP>11</SP>, Fig. 2, the former operating at line frequency and the latter of a frequency of 10 c.p.s. (approx.). The outputs of both of these oscillators are fed to phase-shifters PS<SP>1</SP> and PS<SP>11</SP>, having a phase-shift range of about 180 degrees, and thence through rectifying and shaping stages which produce output pulse waveforms 62 and 72 respectively. These pulses are combined in the conductor 47 to provide operating bias for the control element 70 of CR and for the amplifier SSA (Fig. 1) which amplifies the image signal output of PA. The operating biasses of both CR and SSA are such that the pulses 62, 72 must be made to coincide in time, by adjustment of PS<SP>1</SP> and PS<SP>11</SP>, to cause the former (i.e. CR and SSA) to become operative. Thus, any area of the image of 38 on VF may be selected by the beam 78 by this adjustment and in this condition SSA passes only the portion of the image signal which represents this selected area. When this area is correctly focussed, the output of the tube 10 is a maximum and, after amplification by PA and SSA, the H.F. component of this signal is applied to the control grids of V1 and V2 over conductor 90 (Fig. 3). These valves also receive square wave biassing pulses controlled by the oscillator OSW<SP>111</SP>, operating at about 10 c.p.s., so that they operate alternately to amplifying the signal applied over conductor 90. RV1 and RV2 rectify the outputs of V1 and V2 respectively so that the point 110 assumes positive or negative polarity according to which of the valves V1, V2 is operating, this being determined by the relative amplitudes of the signals fed over conductor 90 between successive cycles of OSW<SP>111</SP>, and these changes in polarity are applied over 19 to polarized relay 82 which operates contacts 128, 129 to reverse the direction of current in the motor field coil 122. When the signal output of 10 changes to a lower value, due to 38 going out of focus, the motor 49 operates to counteract this movement and consequently to increase the output of 10. When switched on initially, if the desired area of 38 is completely out of focus, the motor may continue to drive the lens system to still further decrease the desired signal output until either of the projections 143, 144 on the rack 46 operate the limit switches 141, 142 in the relay circuit 147. This relay energizes the stepping magnet 149 which is provided with a pawl-and-ratchet 151, 152 and intermittently drives switch arm 153 of rotary switch 154, alternate contact studs 156 being connected to a source of negative polarity and intermediate studs 159 to a source of positive polarity so that the polarized relay 124 which operates the motor reversal contacts 136, 137 is fed with a voltage of the required polarity. Instead of allowing the rack to be driven to the extremity of its travel, a reversal of the motor may be effected by the manuallyoperated switch 165 which operates in the same manner as the limit switches. The Specification as open to inspection under Sect. 91 describes a modification in which the driving motor and associated control apparatus and the apparatus of Fig. 3 are omitted and the camera is focussed by any usual manual control, the output of amplifier HF being rectified and applied to the cathode-ray tube CR so that the spot produced on the screen thereof is brightest when the selected portion of the object 38 is correctly focussed. Alternatively, a meter either at the camera or at a remote point may be used to indicate when the camera is in focus on the selected area (Fig. 4, Cancelled, not shown). This subject-matter does not appear in the Specification as accepted.