Patent GB-1236567-A (GB)
Title: Improvements in subscription television and like systems
Inventor: HAMBURGER GERHART LOTHAR
IPC: H04N7/173
Filed: 1967-08-24
Abstract: 1,236,567. Distributing music &c. SANGAMO WESTON Ltd. 26 Aug., 1968 [24 Aug., 1967], No. 39116/67. Heading H4R. In a wired television or radio distribution system in which a number of groups of subscribers are fed via slave stations from a central station and in which the slave stations have means for determining the number of active subscribers and transmitting the resulting data to the central station, as described in Specification 1,002,298, each slave station has a timing clock to control the interrogation of subscribers and transmission of data all of which are started in unison by a common control signal from the central station and each of which has its own characteristic timing period for the transmission of data so that the signals from the slave stations to the central station are transmitted in time multiplex. Fig. 1 shows a block diagram of one slave station which receives a common interrogating pulse of one second duration over line 10 which is connected via " AND " gate G1 to " AND " gate G2 so that pulses at 50 Hz are gated through to a counter PWC for the duration of the interrogation pulse. An interrogation pulse exceeding 1 second in duration causes an output on line 15 to switch trigger circuit M5 into a condition where it provides an output to " AND " gate G3 so that pulses at 1 Hz are fed to timing counter CC1 which produces, via matrix DM1, output pulses on respective lines after 2, 13, 20 &c., to 147 seconds. The pulse occurring after 2 seconds sets trigger M1 which, via " OR " gate B1, initiates a charging operation on the subscribers' lines and the capacitors associated therewith at all the active subscriber stations. The pulse occurring after 13 sees., resets M1 to terminate the charging operation and, by its application to trigger M2, to initiate the discharge of the subscribers' lines and capacitors through an integrator, e.g. of the type of Specification 1,049,851, in the circuit ILC. The pulse at 20 seconds resets M2 to terminate the discharge and sets trigger M3 enabling pulse generator CPG1 to send pulses at 10 KHz to reset the integrator to zero, the number of pulses required to do this being recorded, via a sealer SCR on a counter CC2, as a measure of the number of active subscribers, the reset of the integrator to zero causing a pulse P which is used to reset trigger M3, thereby inhibiting the pulse generator CPG2. With the counter CC2 set to a value corresponding to the number of active subscribers, a pulse at 25 seconds converts the state of the counter to the complement of this number, and a pulse at 26 seconds sets trigger M6 which enables pulse generator CPG2 to feed pulses to the counter CC2 and to the reply transmitter, to transmit the number of active subscribers back to the central station, until the counter CC2 reaches its maximum count when AND" " gate G5 resets trigger M6 to inhibit pulse generator CPG2. The pulse at 26 seconds is also applied to set trigger M4 into the condition to inhibit gate G1 and enable gate G4 so that subsequent pricing signals received from the distribution line 10 operate a Schmitt trigger STC to control charging and discharging of the subscriber's network to operate the metering devices at the subscribers' stations. While in the station shown the transmission of active subscriber data commences on the production of a pulse at 26 seconds, other slave stations serving other groups of subscribers will start transmission of data at intervals after this time, e.g. after 31, 36, 41 &c. seconds. Range changing.-In order to provide maximum accuracy of the assessment of the number of active subscribers, the sensitivity of the integrator in the unit ILC is changed in accordance with the number of active subscribers counted by counter CC2. For example, the integrator of the type of Specification 1,049,851 may have shunts provided to give full scale deflection with 500, 1000, 2000, or 4000 subscribers and the most appropriate range for each interrogation is selected in accordance with the information derived during the previous interrogation. When the integrator sensitivity is changed corresponding changes are made to the current/time characteristic of the resetting pulses, from generator CPG1, so that the same number of pulses are required to reset the integrator to zero. Recording active subscribers is carried out by recording the pulses from generator CPG2 which are required to reset counter CC2 on tape recorder TRM. In order to identify the particular programme associated with a particular measurement, the length of the control pulse fed over line 10 is varied and the number of pulses from " AND " gate G2, in excess of 50, are also recorded on the recorder TRM via AND gate G6 and " OR " gate B4.