Patent GB-850824-A (GB)
Title: Improvements in or relating to television systems
Inventor: WESTBROOK MICHAEL HEREWARD
IPC: H04N7/167
Filed: 1957-07-02
Abstract: 850,824. Television. RANK ORGANISATION Ltd. July 1, 1958 [July 2, 1957], No. 20863/57. Class 40(3). In a television system in which unauthorized reception is prevented by periodically varying the mode of transmission of the signals the mode determining device is controlled by a sequence device which in response to a train of similar signals assumes in turn a number of stable states in which signals are developed of respective ones of a like number of output points these latter points being connected in predetermined order to the mode determining device to control the operation thereof. The sequence device may comprise a ring- counter circuit but preferably comprises a gas-filled sequence discharge tube as shown at 3 in Fig. 1. In this arrangement the discharge between the anode 4 and the cathode 5 - 14 is initiated by pulses from a driving circuit 2 to which signals at the frame securing frequency of the system are supplied at terminal 1 the potential developed at the cathodes being supplied to the (parallel connected) fixed contacts of switches 28 - 32 and thence via selected contacts to a common input connection 34 of a bi-stable trigger circuit 35 the different states of which determine the mode in which the television signals are transmitted or, in the case of a receiver, the mode in which they are supplied to the picture reproducer. To synchronize the operation at a receiver with that of a transmitter a manually operated push-button 36 which steps the discharge "on" to the next cathode irrespective of the signals at terminal 1 is operated one or more times until an unscrambled picture appears or, alternatively, an additional manually operated switch allowing the connections between the cathodes 5 - 14 and the contacts of switches 28 - 32 to be varied may be employed. In another arrangement for effecting synchronous operation a positively going reset signal is supplied via terminal 26 to a selected one of the cathodes (in this case cathode 5) to restore the discharge to this cathode the reset signal being also supplied to the circuit 35 to cause this to assume a desired condition. The reset signals, the periodicity of which may vary in a random manner are, modulated on to a sub-carrier wave transmitted with the television signals and at a receiver are separated out and employed to control the resetting of the mode determining device. The trigger 35 may be replaced by a ring-counter device each of the stable states of which controls a respective mode of operation or the potential on line 34 may be used directly to control the mode determining device. When separate mode change control signals are transmitted the trigger 35 may be employed to control a gate circuit which passes such control signals to a further mode changing trigger circuit. In a second embodiment (Fig. 4) the cathodes 5-14 are connected via individual two-way manually operated switches 75 - 78 such that only conduction of the cathodes connected to resistor 84 produce effective control signals. Such signals open a gate 88 which passes signals from a source 90 (if any are present) to trip the bi-stable trigger circuit 35 which in addition to controlling the mode determining device via terminal 94 also supplies reset signals via amplifier 91 to cathode 14. The source 90 may be a random generator in the case of the transmitter or a filter which selects such random signals in the case of a receiver. If separate reset signals are employed these (at the receiver) may be selected by a filter 93 and utilized to restore trigger 35 and tube 3 to a predetermined condition. A description of the utilization of the embodiments of Figs. 1 and 4 in both transmitters and receivers is given with reference to Figs. 2, 3, 5 and 6 (not shown).