Patent GB-1147603-A (GB)
Title: Improvements in or relating to television transmission systems
Inventor: JACKSON RICHARD NORMAN
IPC: H04N7/00
Filed: 1965-06-15
Abstract: 1,147,603. Educational appliances. MULLARD Ltd. 10 May, 1966 [15 June, 1965; 11 March, 1966], Nos. 25240/65 and 20661/66. Heading G5G. [Also in Division H4] In an educational television system arranged for programmed instruction the frames of information which make up the complete programme are recorded on a film or video tape, which is formed into a loop and run through the transmitter continuously so that the complete programme is repeated at regular intervals, e.g. every three seconds where there are 75 frames of information and each is transmitted during one television frame scanning period. To permit each information frame to be identified identification signals are picked up from the film or tape, either by a separate pick-up device P2 or the same device P1 that generates the television picture signals, and after translation in circuit T are added to the video waveform in adder A. In one arrangement, Fig. 2 (b) (not shown), the signals take the form of a group of pulses which identify the frame in accordance with a nine unit binary code, The pulses are arranged to occur in the field blanking intervals preceding either one or both of the fields forming the television frame, the group of pulses being repeated in each of six lines of the blanking interval to ensure positive operation of identification circuits (see below). In an alternative arrangement (not described in detail) one or more points in the programme are identified by signals and particular frames are identified by counts initiated by the identification signals. At a receiving station the whole programme is available but at any one time only a single frame is selected for display by means of a gate V which opens for the period of the desired frame and passes the signal to a store S from which it may be read out repetitively to the display D. Operation of the gate is controlled by a coincidence recognition circuit C which receives all the identification signals as they arrive and compares them with the identification of the desired information frame which is chosen by operation of a keyboard K. Change from one frame to another, either in a regular sequence or in accordance with "branching" programmes, is carried out by the student in the customary manner by answering questions, included in the frames of information, by the act of operating specified keys on the keyboard and thereby establishing the identification of the next frame. A delay in obtaining a desired frame occurs since it is necessary to wait until the frame appears in the complete sequence which comprises the programme. Where a "non-branching" programme is used the delay may be avoided by the use of a second store which holds available the next frame in sequence of frames. The arrangement of a receiving station is described more fully with reference to Fig. 3 (not shown). The identification signals are separated from the composite television signal by means of a gate (G) which is opened at an appropriate time in the starting interval under the control of field gating pulses, the pulses which form the signal being directed into separate inputs of a register (R2) under the control of a sequence switch (X1) operated by a pulse generator (Q). The identification of the desired frame as determined by operation of keyboard (K) is set up on register (R1) and the settings of the two registers are compared by coincidence recognition circuit (C). To prevent a student losing his position in the programme in the event of an erroneous operation of the keyboard the immediately preceding identification is held in a register (R3). Operation of a switch X2 then causes a return to the frame which was displayed when the false operation was made. As an alternative the keyboard may store a fixed identification which can be utilized to select a frame which gives a plan of the programme. To prevent noise from interfering with the identification signals, the pulses may comprise "bursts" of sinusoidal signal of a predetermined frequency which can be distinguished from noise by a filter (F). The system may be arranged for broadcast or closed-circuit operation. In the latter case the recognition, and possibly also the storage, portions of the receiving apparatus may be located in a central unit with the transmitting apparatus.