Patent GB-1222591-A (GB)
Title: Television message system apparatus
Inventor: DORSEY DENIS PETER
IPC: H04N1/00
Filed: 1968-05-17
Abstract: 1,222,591. Multiplex radio signalling. R.C.A. CORPORATION. 17 May, 1968 [18 May, 1967], No. 23703/68. Heading H4L. In a system for receiving special message images which are transmitted together with regular television programmes, means are provided for preventing the operation of the message receiver if the desired message has already been started and for permitting the operation when the special transmissions start again. In the system described the special message signals are inserted during the vertical blanking interval at a rate of one line scan signal per message per field time interval of the television images and the start of each message signal is denoted by a burst of a distinctive code frequency so that desired signals may be selected. At the receiver the desired message signal is selected by switches 174, 176 and assuming that this message is already being transmitted when the message receiver is switched on the transistor 102 of multivibrator 102, 104 will be cut off, due to resistor 106, and its collector voltage will rise slowly, due to the time constant network 98. This prevents the operation of gate 170 at this stage since it requires an abrupt transition to enable it and the logic circuits 172 controlling the message recorder will be inhibited. A bias control circuit 132 supplies a positive voltage to the base of transistor 116 which saturates and shorts the capacitor 130 so that the unijunction transistor 114 is non-conducting and a capacitor 122 is charged. These conditions remain unchanged throughout the remainder of the message signal because the positive going signal pulses from the detector 140 also drive the transistor 116 into saturation. When it is desired to record the message associated with the setting of switches 174, 176 the switch 160 is closed to discharge capacitor 158 and the resulting negative pulse turns off transistor 104 so that transistor 102 conducts. The resulting negativegoing signal at the collector of transistor 102 switches off the transistor 116 via diode 138 so that capacitor 130 is charged. Resistor 126 and capacitor 130 have a time constant which is much greater than the time of one television field so that the capacitor will charge to a voltage less than the threshold voltage of the unijunction transistor 114, the capacitor 130 being discharged each time a signal pulse is supplied from the detector 140. When the message is completed, positive-going pulses are no longer supplied to the transistor 116 which is kept non-conductive by the voltage at the collector of transistor 102 which is "on". The capacitor 130 charges until the transistor 114 is triggered (after about ten television fields) to discharge capacitor 122 and produce a negativegoing signal to switch of transistor 102 and thereby switch on transistor 104. The resulting abrupt signal transistor enables the gate 170 so that the next signal pulses from the detector 140 are supplied to the circuit 172. Circuit 172 responds to an end of message signal to shut down the display tube and recording means until activated again by receipt of a selected code signal.