Patent GB-1406754-A (GB)
Title: Cable television system
IPC: H04N7/173
Filed: 1973-04-26
Abstract: 1406754 Cable television distribution HUGHES AIRCRAFT CO 26 April 1973 [26 April 1972] 19949/73 Heading H4R In a cable television and radio programme distribution system having a distribution network 19, 21, Fig. 1, and facilities for transmitting both TV and digital signals in each direction between a central station 13 and a plurality of subscriber stations STS apparatus 29, 39, 57, 87, responsive to command signals specifically addressed thereto from the central station is provided at a number of positions in the network to effect control of switches 31, 41 for blocking, when required, all upstream signals applied thereto. The central station 13 includes a local processing centre 16 comprising a computer 17 and equipment 12 A -12 N  and 15 for processing video transmissions. The computer may respond to local noise or level measurements made on upstream signals in equipment 25 or may respond to signals from monitoring equipment 29, 39 at downstream stations 27, 38 to send digital switching and control signals. A section in which noise is induced by an external source is located by the LPC 16 blocking sequentially the path through downstream stations, and the noise is overcome, after location, by stepping up the gain of the signal in an amplifier preceding the noisy section and introducing an attenuation in the upstream section following the noisy section, i.e. between the noisy section and the central station. Frequency band allocations for the two directions are described with reference to Fig. 2 (not shown) and include TV bands in groups of nine in various regions in the downstream band, a downstream pilot band for monitoring and control purposes and spaces for digital data channels over which control information is transmitted. Frequency shift keying may be used for downstream digital communication and phase shift keying for upstream, and as a result of the control none, some or all of the two upstream TV channel transmissions or the upstream digital transmission may be passed by switches 111, 113, 115, Fig. 3, to gain controlled amplifier 137 via selectively switched attenuators 129-132. Suitable transistor switches are described with reference to Fig. 4 (not shown). The system may also include parallel connected fixed gain amplifiers such as 53, 61, 63, for amplification of the signals in the two directions, or since less amplification is necessary for the lower frequencies a downstream amplifier may be by-passed with an upstream bandpass filter. The subscribers may be constantly and sequentially interrogated by the computer for requests, programme selection or alarm. Downstream signals are separated at a subscriber control circuit by a tapping circuit. The digital signal from the central station includes a start bit, address bits, command information bits indicating what is to be controlled, and command function bits indicating the degree of control. A signal may include two lots of command information. The address bits are compared with stored bits at the station and when identity is established the command information bits develop a signal to activate the controller of the circuit to be controlled to enable the function bits to be read in. The outputs from the command function receivers are fed to effect gain via d/a converters, and directly to actuate switches. A number of ancillary controls are listed, and include remote meter reading and control of noise measuring apparatus at the subscribers' circuits. The gain control may include the control of gain as a function of frequency. Specification 1,406,753 is referred to.