Patent GB-1433383-A (GB)
Title: Electronic bidirectional communication system for commercial and supplementary video and digital signaling
IPC: H04N7/16
Filed: 1973-04-06
Abstract: 1433383 Selective signalling COMPUTER TELEVISION Inc 6 April 1973 [10 April 1972] 16598/73 Heading G4H [Also in Division H4]  In a video distribution system also affording data interchange between a central and outer stations, the data is in TDM format, a central clock and synchronized outer clock indicate which outer station is connected to the central station and each outer clock additionally enables a decoder which signals to its station that it is so connected. The system is employed in an hotel &c. in which each room has a TV set to receive normal broadcasts and two restricted pay channels, and includes provision for an alarm call and acknowledgement, an indication that the room is or is not ready for occupation, and includes an operational/security check via the decoder such that the central station receives no response if the system is faulty or the TV set has been removed.  Outer stations.-(Figs. 1A, 1B) incoming video information is filtered 16 and according to the position of switch 39 passes directly to a TV set for normal broadcasts or is hererodyned with an appropriate local oscillator 62, 64 to enable either of two restricted channels A, B distributed at mid-band frequencies to be viewed. The digital data passes through a hybrid 20 to an FSK detector 26 and flip-flop 88. The first part of data transmission to all outer stations consists of eight digital "1's" followed by a "0" as a synchronizing signal. Each station recognizes the pattern in a circuit 72 containing NAND gates 74, 76 fed with pulses from a cyclic 0-8 counter 84 receiving mains-derived 120 Hz clock pulses. Pattern recognition sets F/F 80 to reset counter 86 which begins counting out until it reaches the configuration set by a switch or jumper array 87, unique to each station, to provide an output to AND gate 89 preparing NAND gate 118 and AND gates 142, 144. The Q output of F/F 80 and the Q output from F/F 112 controlled by counter output inverter 79 complete enabling of gate 118 to energize the local oscillator 38 in the detector 26. Reception mode.-If the digital data includes a "1" in the 14th and 15th time slot, which it does in alternate scans, this completes enabling of gate 114 to set F/F 116 to the RECEIVE mode which completes enabling of AND gate 142 to energize a LISTEN bus 106. This situation obtains whilst the counter 84 performs a complete eight count cycle and provides outputs to lines 90 to enable AND gates 94-102 in turn. Gate 92 is reserved for a security check in the transmission mode (see below). These gates energize appropriate buses 107-110 to prepare NAND gates 176-184 (listen bus 106 already energized) at the time slots during which any data from F/F 88 will appear at the gates. A binary 1 in time slot 3 or 4 enables gates 178, 180 to reset F/Fs 190, 192 thereby disabling AND gates 150, 154 to prevent transistors 152, 156 energizing the appropriate heterodyning oscillator LO A, B. Conversely a binary 0 enables the gate 150, 154 selected by switch 39 (which is energized by mains current detector 125) to allow the selected programme to be viewed, and to prepare NAND gates 166, 168 to indicate to the central station during a TRANSMIT mode that payment programmes are being received. Similarly, binary 1s in slots 5, 6 operate alarm 188 and message light 161 respectively whilst a binary 1 in slot 7 resets F/F 148 indicating that the room is available. Transmit mode.-The absence of binary 1s in the 14th and 15th time slots enables gate 144 to energize a TALK bus 105 and prepare gate 92 which is fully enabled in the first two time slots of counter 84 output to enable OR gate 172 to energize oscillator 174 feeding modulator 66 by which AM data is transmitted to the central station. Operation of "alarm acknowledgement", "maid in room" and "room available" switches 187, 151, 149 respectively transmit binary 1s in the appropriate time slots in a similar manner to the enabled payment TV gates 166, 168. Central station.-(Fig. 2, not shown). After generation of the synchronizing code by a counter (204) and NAND gates (296, 208), a counter (230) begins counting in synchronization with those 82 at the outer stations. A 1-out-of-8 time slot decoder (246) enables in turn circuits (240) which provide a single digit representing the state of switches (242) corresponding to the outer stations thereby building up an eight-bit message (with 1st, 2nd and 8th vacant) for transmission via frequency shift modulator (212) to the outer stations, together with TV broadcasts. The incoming information is processed (220- 224) to recover the digital data which is fed to circuits (260) the inverse of circuits (240) to light lamps (270) appropriate to the outer station and to the function (e.g.: room available), and to feed a totalizer to evaluate the charge for viewing the restricted TV channels. The totalizer is fed via a divide-by-three circuit (282) so that charging is implemented when the TV set has been detected in three successive TDM scans to be receiving a restricted TV programme. The lamps provide indication of the number of viewers.