Patent GB-932399-A (GB)
Title: Subscription receiver
IPC: H04N7/167
Filed: 1960-06-23
Abstract: 932,399. Secret television. ZENITH RADIO CORPORATION. June 23, 1960 [June 29, 1959], No. 21967/60. Drawings to Specification. Class 40 (3). In a subscription television receiver in which decoding apparatus having a plurality of adjustable elements has to be adjusted in accordance with the code schedule of a received signal, means are provided for comparing the instantaneous state of adjustment of the decoding apparatus with the code schedule and for producing a control signal indicating the degree of correlation. The invention is described as applied in a system as described in Specification 742,140 where the code schedule is determined at the transmitter by means of a generator which develops bursts of frequencies in a random order. The bursts are applied through a multiple switch unit to control signal coding and are also communicated (during vertical blanking) to the receiver to control decoding, the receiver including a multiple switch unit corresponding to that at the transmitter which must be adjusted identically if the resulting decoding signal is to be identical with the coding signal at the transmitter. The coding and decoding signals are square-wave signals with transitions recurring at times in accordance with the code schedule which are employed to introduce delays between the video and synchronizing components. In carrying out the invention, one of the frequencies is employed for determining correlation and is arranged at the transmitter to be transmitted-when it occurs, only if the coding signal is of a predetermined polarity. The presence of a burst of that frequency thus indicates the instantaneous condition of the coding signal. At the receiver a channel separates bursts received at the selected frequency and thereby produces a signal indicating the code schedule. This signal is then compared with the generated decoding signal. If the multiple switch unit is correctly adjusted the bursts always occur when the signal is of the same predetermined polarity. If the switch unit is incorrectly adjusted the bursts and decoding signal only agree occasionally. A switch circuit is arranged to transmit the bursts to a flip-flop circuit with one polarity when the decoding signal is of the correct polarity and with the reverse polarity when the decoding signal is of incorrect polarity. Therefore when the multiple switch unit is correctly adjusted, the flip-flop circuit remains set in one state, whereas incorrect adjustment results in the flipflop being triggered back-and-forth. In one arrangement the flip-flop controls a decoding circuit in the associated audio signal channel with the effect that the audio signal (which may have only simple coding, such as displacing the sound in the frequency spectrum by hetrodyning) is decoded only provided the adjustments for the more complex video decoding are made correctly. In other arrangements it may be arranged that the video decoder is operated only when the flip-flop circuit indicates correct adjustment. This is stated to render it difficult to determine the correct adjustment of the multiple switch unit by observing the reproduced image whilst manipulating the switches. In this respect there is also described an arrangement involving a delay circuit to prevent the decoder operating for incorrect adjustment during brief periods when the bursts and decoding signal occasionally agree. An arrangement is described in which the received bursts of the selected frequency pass through the multiple switch unit so as to be dependent on its adjustment. Reference is made to the use of the flip-flop circuit to control a charge meter so that a recording is made only when the multiple switch unit is correctly adjusted. Specifications 735,541, 784,108 and U.S.A. Specifications 2,758,153 and 2,852,598 also are feferred to.