Patent GB-969014-A (GB)
Title: Computer
Inventor: HERRMANN RICHARD C
IPC: H04N7/167
Filed: 1961-01-16
Abstract: 969,014. Secret transmission. ZENITH RADIO CORPORATION. Jan. 16, 1961, No. 1792/61. Heading H4R. In a television transmission system in which the audio signals are coded by inverting their phase from time to time, the times at which the inversions take place are chosen so as to minimize the D.C. component of the resulting coded signals. During each field trace period phase inversions and restorations occur at intervals of sixteen line pulses, the first inversion taking place on the occurrence of a selected one of the first thirty-two line synchronization pulses as determined by an encoding signal produced during the preceding field retrace interval. In order to determine when this first inversion shall take place, the audio signals of the field period concerned are coded simultaneously in all the thirty-two possible ways and from the resulting coded signals that introducing the minimum D.C. component is chosen. The corresponding coding signal is then used to encode the audio concerned at the transmitter and to decode it at the receivers, the audio signals being delayed at the transmitter while the choice is made. Coding mechanism at transmitter and receivers (Fig. 1).-This includes a control mechanism 34, comprising five bi-stable multi-vibrators B1 to B5, which is driven by the line-drive pulses of the television system to develop a square wave having changes of amplitude every sixteen line-trace intervals. Each bi-stable circuit B1 to B5 has a " common " input, a negative pulse on which triggers the circuit concerned from its instantaneous state to its opposite state; and a " right " input, a negative pulse on which drives the circuit to its "0" state, if it is not already there. In addition, B4 and B5 each have a " left " input, a negative pulse on which drives the circuit to its " 1 " state, if it is not already there. Linedrive pulses are applied to the " common " input of B1, and the output of each bi-stable circuit is connected to the " common " input of the next, so that successive line-drive pulses cause the control circuit to advance step-bystep from its condition 1 to condition 32 and to repeat this cycle. As during each field-trace interval line-drive pulses are applied to the B1 " common " input only, the amplitude of the B5 output changes once per sixteen line trace intervals. This output controls the phase inverting encoding device 33. During each fieldretrace interval code bursts of frequencies f1 to f6 are supplied to filter and rectifier units 42 which selectively close the gates 36 to 44 to gate line-drive pulses to the leads f1 to f6. Frequency f6 controls gate 44 and hence gate 43, to effect re-setting of B1, B2 and B3 if B5 is in the " 1 " state. The switching mechanism 35 is preset (a sample setting being shown in dashed lines) to deliver the fine-drive pulses gated into it, to selected inputs of B4 and B5. In this way the control mechanism 34 is set to the desired starting state before each field-trace interval, the Specification discussing the possible states into which it can be set from its instantaneous state by means of a maximum of 10 successive code bursts. As shown in Fig. 2, the code bursts 45, each consisting of one of the frequencies f1 to f6, are combined with the vision signal as downward or inward modulation of the vertical blanking pedestal during ten slots ( 1 to 10) occurring between the line synchronizing pulses subsequent to the second series of equalizing pulses. Producing the required coding signals.-The apparatus for doing this, which is described with reference to Figs. 5 to 18 (not shown), performs the following functions:- (1) Simultaneously codes the undelayed audio for a field-trace interval under control of the thirty-two differently phased square waves available. (2) Integrates the thirty-two coded audio signals thus produced to determine their respective D.C. components. (3) Determines the " past history D.C. component (resulting from the previously transmitted audio) and registers it in thirty-two threshold detectors. (4) Determines which of the thirty-two operating states it is impossible to reach, during the ten slot period preceding the next fieldtrace intervals, from that state in which the control mechanism 34 will be after reception of the line synchronizing pulse immediately preceding slot 1, and registers this information in the threshold detectors; (5) Selects by means of the threshold detectors that one of the coded signals, the D.C. component of which is of opposite sign to and nearest in magnitude to the " past history " D.C. component, and selects the corresponding state of control mechanism 34 as that in which it is to be established immediately following slot 10. (6) Stores this state in the corresponding one of a set of thirty-two binary stores termed the A binaries. (7) Determines the state in which the control mechanism 34 will be set in response to the line synchronizing pulse immediately preceding slot 1, and registers this in the C binaries and then in the D binaries (similar to A binaries). (8) Registers the required destination state in the B binaries (similar to A binaries) and then determines and stores in the B binaries those states in which the mechanism 34 may be established responsive to the line sync. pulse immediately prior to slot 2 in order that it may reach the destination state immediately following slot 10. (9) Compares the states which may be reached from the starting state with those stored in the B binaries, selects those common to both and stores them in the B binaries. (10) Selects in the C binaries one only of these states. (11) Selects an action to effect the required transition of mechanism 34 from the starting state to the selected state to be established in response to the line sync. pulse immediately preceding slot 2. (12) Selects that frequency of f1 to f6 which will achieve this action, and stores this information. (13) Repeats steps (8) to (12) to determine the code burst frequencies for slots 2 to 10. (14) Generates the signal bursts of the frequencies selected for use in coding at the transmitter and decoding at the receivers, the audio signals having been delayed while the above functions are performed. Specifications 784,108 and 820,374 are referred to.