Patent GB-1424739-A (GB)
Title: Subscriber television system
IPC: H04N7/16
Filed: 1973-08-03
Abstract: 1424739 TV subscriber systems OAK INDUSTRIES Inc 3 Aug 1973 [3 Aug 1972] 37020/73 Heading G4H  In a TV subscriber system TV signals are switched between different channels and enabling signals are sent from the transmitting centre to authorized subscriber locations to synchronize switching at the centre and the locations so as to provide useful TV signals for authorized subscribers. Authorized subscribers are firstly placed in an "enabled " condition, then channel switching signals are transmitted during the vertical blanking intervals. When a flip-flop 10, Fig. 1, receives a signal 12 it inhibits gate 14 to prevent channel switching. The signal also triggers a counter 16 which sequentially feeds the subscriber addresses to a computer 20. For each received address, computer 20 generates a signal to enable gate 24 or gate 34 according as the subscriber is to be " enabled " or not: a "right or " wrong " enable signal P 0 -P 3  is fed from a store 26 or 37 to a register 30 which also receives the address P 4 -P 17  from counter 16. The signal to gate 24 or 34 also passes via a gate 38 to provide a signal P 19  and to open a gate 40 so that the register contents can be shifted to a modulator 46; the signal and clock pulses modulate a RF carrier and are fed to a mixer 48. At each location the signals are received at 56, Fig. 2, and passed to a register 61. When the appropriate address is received, gate 62 causes latches 64-70 to store the enable code for that location. The program material may be supplied by two sources 72, 74, Fig. 1; 72 always provides a useful signal (it is the program normally received) and is connected to a separator 82. When the inhibit signal is removed, each vertical sync signal from 82 passes to a counter 84 connected to generators 86, 88 which provide a channel switch pulse P 18  every 4 vertical sync pulses. In this " channel switching mode, " register 30 stores an enable code P 0 -P 3 , a channel switching command P 18  and an execute signal P 19 . The signals from 84 and 40 alternately set and reset a flip-flop 96 to change the state of a switch 76: the carrier frequency of source 72, (74) then alternates between channels Q and R (R and Q), and this continues until the arrival of a new signal 12. At each location, a gate 104, Fig. 2, provides an output to gates 108 or 106 according as the enable code Q 0 -Q 3  sent with the channel switching command Q18 matches the code stored in 64-70; the gates cause a switch 110 to change to the appropriate channel Q or R (the tuning voltage 114 being applied to a CATV converter 100 in the case of a cable system), switching between channels in the receiver being in synchronism with channel switching at the transmitter only if the " right enable code " was originally stored in latches 64-70. Instead of an incorrect enable code, unauthorized subscribers may receive no address, and the enable code may be changed during a program. Means may be provided for transmitting messages back to the centre (e.g. authorization requests, audience polling replies, emergency alarms) when each location has recognized its address.