Patent GB-647892-A (GB)
Title: Improvements in television transmitters
IPC: H04N7/16
Filed: 1947-06-30
Abstract: 647,892. Television transmission systems. BURGESS, A. F. (Zenith Radio Corporation). June 30, 1947, No. 17180. [Class 40 (iii)] A television transmission system has provision for intermittently varying the video signal during synchronizing retrace intervals and for sending simultaneously a key signal correcting the video signal by line conductor to a subscriber so that payment of a toll charge for proper viewing is ensured. The video signal may be at random intervals coincident with a frame pulse reversed in phase to give a negative photograph effect, or the picture may be turned upside down or reversed and the time of line or frame scanning varied at intervals determined by a camera switch, coincident with a frame pulse, so that the picture oscillates up and down or laterally on a receiver screen. The key signal may be sent by telephone or power line on request from a central switchboard which meters the time of supply of the key signal, or an on-off switch of the receiver may automatically connect the key signal. With colour television the colour sequence may be varied. The tone frequency may be modulated on a carrier to avoid interference with normal telephone use. The television signal may include phase reversal at intervals of the video signals and frame synchronizing pedestals, producing a "negative photo " or overall grey, using a randomoperated switch (Fig. 2) generating also a key tone for line transmission. The switch comprises a multivibrator 92 producing pulses acting only at coincidence intervals with frame gate pulses applied at 78 to actuate a trigger circuit 98, 99 (with one stable state) operating to one of two stable states an electronic switch 103, 104, controlling the alternate unblocking of tone generators 115, 122, and a phase inverter switch 175, 177, Fig. 3. On input valves 178, 179 of the latter being unblocked alternately from the switch 103, 104, valves 175, 177 are alternately unblocked, the latter creating phase reversal a,t its anode of the video signals and frame pedestals applied at 66, the former not, being driven through a previous phase-inverting valve 174. Valves 174, 175 have in conjunction the same gain as the valve 177. Line and frame synchronization pulses are added after the phase reversal of the video, the inverted frame synchronizing signals serving for D.C. restoring. Thus, (Fig. 1, not shown), a pulse f is continuously generated at 62 and impressed at 69 within the frame pedestal, producing no effect on the normal signal but providing during the reversed phase a frame synchronizing pedestal 87. At its trailing edge the line synchronizing pulse 88 is added. The part 86 representing the inverted frame pulse serves for D.C. restoring. The two tones may be used for party line calling, only one being necessary as the key signal. Alternatively the programme signal may introduce reversal at intervals of the direction of vertical and/or horizontal scanning of the image and also alternate advance and return of the time of line or frame scanning so that the picture is alternately turned upside down or reversed and moves forward and back across the receiving screen, or oscillates up and down. To this end each time the 'camera is moved to left or right a snap switch on the camera pivot operates so that either a ground or negative potential on line 225 is applied to a valve relay 239, Fig. 4, operating or not a tone oscillator 241, the oscillations of which are sent to line 244, and similarly this polarity is altered when the camera switch is operated to change cameras. When a tone is generated, amplified and rectified in part at 247, and passed to a gate 249, frame synchronizing pulses are released to pass to a valve relay 253 operating a relay 265 to reverse the vertical scanning, which thus occurs only at the end of a frame. Further, the frame pulses pass to a .frequency divider 252 to produce different D.C. output signals at a sub-multiple of the frame frequency which pass to an inverter. switch 255. The line synchronizing pulses pass from the generator 221 to a blocking oscillator 257 producing a controllable width pulse with leading edge coinciding with that of the line synchronizing pulses, which is reversed in polarity or not by the switch 255, depending on the switch input from the frequency divider, and the edges of which are differentiated by the input circuit of a horizontal sweep blocking oscillator 260. Since the latter is tripped and locked only by a positive differentiated pulse the output pulse is delayed and retarded alternately at the frequency of the inverter switch 255 by an amount depending on the controlled width of the blocking oscillator 257 pulse. In Fig. 5 (not shown), showing the vertical (frame) pulse gate and tone generator switch (239), when camera lead 225 is at negative potential the valve 271 is cut off at all times irrespective of the frame pulses from lead 240, valves 289, 299 are unblocked, and the tone generator 305 is operated. When, however, lead 225 is at zero potential the frame pulses block valve 289 which remains blocked during negative going trailing edges of the pulses due to its forming with a further valve 284 a flip-flop with period almost equal to that between frame pulses so that it is continuously blocked by frame pulses and the tone generator ceases. In Fig. 6 (not shown), the frame pulses from 250 are gated to pass to the relay switch and frequency switch only when the tone is generated and exceeds a minimum amplitude. Fig. 7 shows the frequency divider comprising two stages of thermionic relays, each having two states of stability alternated by negative input pulses, reducing to one quarter the input vertical pulse frequency. Fig. 8 shows the relay swtch. On an input at 251 of negative frame synchronizing pulses the valve 389 is blocked and remains so due to the time constant of its input condenser resistance coupling 392, 400 until just before the next pulse. The output leads 264 therefore pass only a small current to the scanning reversal relay (Fig. 9) which remains inoperative until the frame pulses are blocked by the tone generator ceasing. The relay switch also rectifies the pulses so that on the pulses ceasing an output 385 is produced biassing the last valve of the frequency divider to a desired conducting state. Figs. 10, 11 show the blocking oscillators and inverter switch. When positive pulses from the adjustable blocking oscillator are fed at 258 to the first of the triode valve pair (Fig. 11) comprising the inverter switch, having common cathode coupling, the series output at 259 of the resistance connected anodes is positive in the absence of any input at 254 from the frequency divider to the grid of the second valve. But when a negative going pulse is fed to the second valve from the frequency divider the valve is cut off and remains so due to the large time constant of its input and the blocking oscillator input pulses are inverted in polarity. The second oscillator as described produces alternately advanced and retarded sweep synchronizing pulses due to the differentiating circuit 451, 452. Specification 647,893 is referred to.