Patent GB-735541-A (GB)
Title: Improvements in coding arrangement for a television transmitter or receiver
IPC: H04N7/169
Filed: 1953-03-31
Abstract: 735,541. Television. ZENITH RADIO CORPORATION. March 31, 1953 [April 9, 1952], No. 8904/53. Class 40 (3). Television signals are coded at a transmitter and decoded at a receiver by means responsive to the code pattern of a control signal comprising a series of N components individually and selectively representing one of at least two coding conditions and collectively representing a code pattern. The coding is effected either: (a) by introducing a plurality of different delays in the line scanning of the camera, or (b) by introducing a plurality of different delays in the relationship of the camera video signal to the transmitted synchronizing signals. Conventional synchronizing signals are transmitted in each case together with a "key" signal which is employed, at the receiver, to effect decoding. Fig. 1 shows one embodiment in which coding is effected by variation of the camera line scanning. In this arrangement the coding device 19 which has two operating conditions, i.e. "delay" and no delay," is connected between the camera line scanning generator 20 and a source 17 of line synchronizing pulses and is effective in response to the presence or absence of signals from a multivibrator 44 during successive frame retrace intervals to operate in one or. other of these conditions, the multivibrator 44 which has two two stable states, being triggered back to its initial condition at the end of each frame scanning period by means of a pulse derived from the leading edge of the next frame synchronizing signal in circuit 45. The code signals which are supplied to 44 via amplifier 46 are derived as follows: a pulse train F (Fig. 2), comprising a number, e.g. six, of the line synchronizing pulses 54 which occur at the end of a frame synchronizing interval 50 is derived from the composite video and synchronising signals in circuit 13 (forming part of the chain between the camera amplifier 11 and the transmitter 14) by means of a gate circuit 26 which is triggered by means of a pulse D produced in a multivibrator 24 under the control of a pulse C produced in a further multivibrator 23 controlled by a pulse B derived from the synchronizing and blanking signal generator 17. Pulse train F is supplied to a " random " divider 30 the output from which (e.g. pulse train G) controls a multivibrator 31 producing a pulse train H which is applied to the beam intensity control electrodes 32 of. a cathode-ray commutating device 29, the beam of which is scanned over the commutating elements 37 in time coincidence with the occurrence of the pulses F. In this manner the " random " pulse train G is transferred to appropriate ones of the elements 37 and this (code) signal is further coded in accordance with the positions of switches 1 to 6 of a transposition mechanism 38 to provide (for the particular " random " division and setting of switches 1 to 6 shown) the final code signal J which is supplied to multivibrator 44 via amplifier 46. In order to allow a television signal coded in this manner to be decoded at a receiver a " key " signal comprising (a) a " burst " of trequency f1 developed in circuit 33 in time coincidence with and for the duration of each pulse of train E (which controls the scanning of cathode-ray device 29), and (b) a " burst " of frequency f2 developed in circuit 35 in time coincidence with and for the duration of the pulses of train H is combined with the transmitted composite signal in mixer 12 a delay line 34 ensuring that the " bursts " occur in the spaces between the line synchronizing pulses 54. At a receiver (Fig. 3), the " key signal " bursts " are selected from the output of the second detector 65 by means of filters 75 and 76 respectively, the former controlling, via sweep generator 76<SP>1</SP>, the beam scanning of a cathode-ray commutating device 78 (similar to device 29 of Fig. 1), the beam intensity of which is controlled by the output of filter 76. In this manner the (code) signal is transferred to the commutating elements 79 and when the switches of the transposition mechanism 81 are set to correspond to the settings of the switches of transposition mechanism 38 (Fig. 1), a pulse train is fed to multivibrator 80 (via amplifier 84) which corresponds to that supplied to multivibrator 44 (Fig. 1). The output from 80 then controls the decoder 71 in similar manner to the control of coder 19 (Fig. 1), and this in turn controls the line scanning generator 72. At the commencement of each field synchronizing period multivibrator 80 is returned to its initial state by means of a pulse derived in 83 from the leading edge of each field synchronizing pulse. The commutating elements of devices 29 (Fig. 1) or 79 (Fig. 2) may be replaced by fluorescent patches or by an apertured marked fluorescent screen co-operating with photo-electric cells having adjustable shutters in their optical paths (the shutters corresponding to the switches 1 to 6). Alternatively the receiver picture reproducing tube may also perform the functions of the commutating device by providing commutating elements 79 (Fig. 4A), in the top portion of the screen in positions corresponding to the initial line traces of each field. With this arrangement the coding/decoding interval occurs during the commencement of each field trace instead of during the retrace interval and consequently the transmitter must be modified so that the pulses fed to the " random " divides are selected from the line synchronizing pulses following the termination of each field re-trace interval (Fig. 4B, not shown). In a modification of Fig. 1 a second commutating device 29<SP>1</SP> (Fig. 5), operating in parallel in the device 29 (Fig. 1), controls a series of " key-signal " generators 100 to 105 such that a " hunt " of distinctive frequency is fed (over line 106) to the mixer 12 (Fig. 1) for each pulse derived from multivibrator 31 (Figs. 1 and 5). For use in this arrangement the receiver of Fig. 3 is modified as shown in Fig. 6 where the output of the second detector 65 is connected to filter units 100-115, each selective to a single one of the " key-signal " frequencies and the output of each of which is connected via switches 1-6 of transposition mechanism 116 to the decooler 71 via amplifier 84 and multivibrator 80 (Figs. 3 and 6). In this embodiment the cathode-ray commutating device 78 (Fig. 3) is not employed. Fig. 7 shows an alternative coding arrangement in which the signal output of 31 (Figs. 1 and 7), e.g. curve H (Fig. 2), is combined in a mixer 125 with a signal comprising a code combination of mark-space pulses derived via a pick-up lead 131 from a magnetic record 127 rotated synchronously via devices 128-130 at the field frequency of the system. The output of mixer 125 is supplied via a limiting device 126, the clipping level of which is such that only coincidence between pulses from 31 and 131 are passed to amplifier and multivibrator 46, 44 respectively (Figs. 1 and 7). A receiver cooperating with this transmitting arrangement would employ a similar arrangement for decoding. In the arrangements so far described the coding apparatus is actuated between two (different) modes only, i.e. " delay " and " no delay ". In an alternative embodiment, Fig. 8 (not shown), however, which is an example of the type (b), i.e. that in which coding is effected by introducing a plurality of different delays in the relationship of the camera video signal to the transmitted synchronizing signals, four (different) modes (including zero delay) are provided. This is effected by the selective introduction of delay lines into the video channel of the transmitter by means of electronic switching devices controlled by the pulse train J (Fig. 2).