Patent GB-1019023-A (GB)
Title: Subscription television system
IPC: H04N7/167
Filed: 1962-11-08
Abstract: 1,019,023. Radio signalling. BRITISH TELEMETER HOME VIEWING Ltd. Nov. 8, 1962 [Nov. 9, 1961], No. 42300/62. Heading H4L. In a subscription television system in which the audio programme can only be received by authorized subscribers, the programme audio signals are frequency modulated on a carrier having a frequency other than that normally used for the sound signals, and barker audio signals (giving programme details, charges &c.) are frequency modulated on the carrier which would be used for the sound signals in the normal non-subscription television system. In addition, the barker audio signals are frequency modulated on the already frequency modulated audio programme carrier. At the subscriber's receiver an attachment is provided whereby upon proper authorization the barker audio is removed from the programme audio modulated signals which are thereafter processed in such manner that the audio programme can be reproduced on a conventional receiver. In the example described the video carrier is centred at 1À25 Mc/s. within the 6 Mc/s. band, the audio carrier is centred at 0À25 Mc/s., and the barker carrier is centred at 5À75 Mc/s. In one embodiment, Fig. 2 (not shown) the barker audio signal frequency modulates the sound programme carrier identically in direction and magnitude with the frequency modulation of its own carrier. At the receiver, Fig. 3, the received signals after frequency-changing are supplied to a wideband I.F. amplifier 54, a narrow band I.F. amplifier 56 for the barker signals, a narrow band I.F. amplifier 58 for the sound programme, and a narrow band amplifier 60 centred about the video carrier frequency. A wavetrap 62 attenuates the sound programme signals on 46À75 Mc/s. and a second trap 64 removes the barker signals on 41À25 Mc/s. when switch 65 is operated. The outputs from amplifiers 56, 58 are supplied to an intercarrier detector 66 providing a difference frequency in which the barker audio signal is cancelled and the frequency modulated sound programme signal remains. This signal is mixed at 68 with an input from a 1 Mcs. oscillator 70 to provide a 4À5 Mc/s. sound signal which is supplied to an intercarrier detector 72 together with the video carrier (45À75 Mc/s.) from the amplifier 60. When the coin box 84 is operated, switches 65 and 74 are closed and the 41À25 Mc/s. sound programme I.F. carrier is combined with the video I.F. signals in an adder 76 so that it now appears in its proper position in relation to the video signals. The output of the adder is supplied to a mixer 78 and an oscillator 80 feeds the mixer with a frequency such that the output of the mixer 78 is at the frequency of one of the allotted television channels and is received by a conventional television receiver 82. If desired, the sound signal could be obtained from the intercarrier detector 66 by following it with an I.F. circuit and a frequency discriminator. In an alternative transmitting system, Fig. 4 (not shown) the usual barker frequency modulated carrier is provided and the sound programme carrier is frequency modulated as before with the sound programme but is frequency modulated with the barker signal in opposite sense to that of the barker carrier. At the receiver, Fig. 5 (not shown), the outputs of the barker I.F. amplifier and sound programme I.F. amplifier are supplied to an intercarrier detector as before but the sum signal is taken as the output in order to balance out the barker signal from the programme signal. It is stated that the two systems may be combined by changing from one to the other in a random manner.