Patent GB-1343862-A (GB)
Title: Electronic communication apparatus employing encripted signal distribution
IPC: H04N7/167
Filed: 1972-02-09
Abstract: 1343862 Secret transmission; distribution systems; automatic gain control WARNERTVC CORP 9 Feb 1972 [10 March 1971] 6074/72 Heading H4R In a transmission system wherein a signal source is connected to different ones of a plurality of transmission channels according to a timed sequence and a pilot signal is concurrently transmitted so as to synchronize a receiver, the latter includes a decoder for following the transitions of information from channel to channel and also incorporates an a.g.c. circuit which compensates for the different propagation characteristics of the different channels. The channels may comprise "unused" frequency bands of a wired TV distribution system wherein normal commercial TV programmes occupy some of the bands and special programmes are switched around the other bands so that they may only be received by subscribers who have paid an additional fee. Transmitter, Figs. 1A, 1.-TV signals received on a community antenna 15 are applied in a conventional manner to selected frequency bands and are transmitted over cable amplifier system 38, 39 to the subscribers. Additional video programmes from sources 10 1 ... 10 4  are applied to the cable in encripted format. Each source is paired, e.g. 10 1  and 10 2 , and is connected alternately via gate, e.g. 20, to modulators 30A and 30B which are supplied with respective carriers from oscillators 40A, 40B. The modulator outputs are linearly combined with the normal programmes and with a pilot signal at 35 prior to transmission over the cable. The pilot is derived by detecting, as at 52, the vertical synch, pulses of one of the additional signal sources (it is assumed that the sources are themselves synchronized) and gating the so derived pulse train through AND gate 58 which is controlled in a periodic or random manner by oscillators 62 or 63. Bistable 70 is therefore set and reset in a corresponding manner so as to alternately enable gates 72, 74 and thereby inject pilot signals having one or the other of two frequencies into the transmission cable. (One of the pilot oscillators may be dispensed with, in which case the absence of a pilot signal constitutes the alternate switching criterion). The bistable is also effective via flip-flop 75 to control the alternate switching of the gate 20 1 . The delay circuits shown ensure that there is no overrunning of the various circuits which could occur more particularly in the receiver (see below) due to the multiplicity of signal circuit branches. Receiver, Figs. 3A, 3.-For normal reception ganged switches 100, 103 are in their full line position and channel selection is achieved in the receiver proper. For special reception, the receiver's channel selector is set to a predetermined channel and the ganged switches are set in the dotted line position. The pilot frequency is recovered at 115, 117 and its alternation is converted into a square wave 118 which is used to control pairs of gates 130. The particular pair controlled depends on the setting of switches 135, 123, 124 which is effected by the subscriber in choosing his programme Considering programme source 10, switch 135 is set to position 136 thus applying an operating potential to local oscillators 140A, 140B, and switch 123, 124 is set to its lower position so that gates 130A, 130B are opened alternately by the pilot signal. The outputs of oscillators 140A, 140B are thus mixed alternately with the incoming signal in mixer 107 so as to derive the wanted carrier which now occupies the predetermined channel to which the conventional receiver is set. Gain control.-In view of the different propagating characteristics of the two channels over which a particular programme is alternately sent it is preferable to adjust the gain of signal amplifier 111 immediately after switchover in a more rapid manner than could be effected using signal strength alone in order to avoid visual flicker of the received signal. This is achieved by utilizing two capacitors 166, 168 which are alternately switched into the negative feedback loop around the amplifier under control of the pilot. The latter turns off one of transistors 162, 164 so as to isolate the associated capacitor 166, 168 from the loop whereby the capacitor stores a.g.c. information for one channel during the time that programme is being received over the other channel. The mixer 148 and filter 150 select a heterodyne output corresponding to the picture carrier and its adjacent (in frequency) modulation products. Since the signal voltage of the vertical synch. pulse is the largest signal of the total TV signal this is used for a.g.c. purposes, its peak level being picked off by transistor 155 and CR network 157, 166 or 157, 168 depending on which transistor 162 or 164 is turned on by the pilot.