Patent GB-1051931-A (GB)
IPC: H04N3/00
Abstract: 1,051,931. Television; pulse modulation systems; transistor circuits; cathode-ray tube operating circuits. STANDARD TELEPHONE & CABLES Ltd. April 2, 1964, No. 13603/64. Headings H3T, H4F, H4L and H4T. Television.-In a system for transmitting a still picture over a narrow band, e.g. telephone, circuit the target of a storage pickup tube 44, Fig. 2, is exposed to an image of the-picture for a predetermined interval 86, curve A, Fig. 3, determined by operation of a shutter 47, and during this period there is transmitted a control signal, curve E, Fig. 3, in the form of a 400 c/s. tone. Following the period, the image is evaluated at a scanning rate, appropriate to the circuit bandwidth, to produce a video signal, curve H, Fig. 3. At a receiver, Fig. 11, there is utilized a direct viewing, storage-type, cathode-ray tube 29. The control signal is separated from the received signal by a filter 109 and employed to operate a control relay 112 which causes a previously recorded picture to be erased from the tube. Thereafter the write gun 65 is supplied with the video signal for one frame only to cause the new picture to be written into the tube. Operation at the transmitter is initiated by actuation of a switch 59 which triggers a control relay 84 associated with a hold delay 85. The relay actuates shutter 47 and connects tone generator 93 to output modulator 90. Scanning of the pick-up tube proceeds continuously under the control of timing circuits 72, 73, 75, 76, which are synchronized by 60 c/s. source 74. Operation of relay 84 however causes the production of a reset pulse, curve D, Fig. 3, by circuit 91 which triggers frame timer 73 to interrupt the line and frame sequence and initiates a new sequence as shown in curve A, Fig. 3. The exposure period 86 is shorter than frame blanking. The scanning may be adjusted so that one frame occupies 10, 20 or 40 seconds. A bi-stable circuit 87 controls " transmit ready " and " transmit on " lamps to indicate to an operator when a new picture may be introduced, the circuit being triggered from control relay 84 and reset after one frame by pulse 105, curve E, Fig. 3, from frame timer 88. The lamps are thus operated as shown in curves F and G, Fig. 3. The video signal channel includes a circuit 96 to compensate for variation in signal level due to pictures of differing background colour. A circuit 97 introduces blanking. Filter restricts the bandwidth to 1À75 kc./s. The signal is transmitted as modulation of a 2À2 k.c./s. carrier, curve I, Fig. 3, provided by a square wave generator 89, and is restricted to the lower side-band. The operation of the erase control relay 112 at the receiver, Fig. 11, in response to the control signal is delayed by circuit 123 so that erasure occupies almost one frame, curve B, Fig. 16. Erasure is effected by raising the potential of the storage target 113 and removing the voltage on the display screen 114, curves D and E, Fig. 16. The write gun 65 is controlled by a circuit 119 which receives a signal from a trigger circuit 118 which is triggered on by a pulse from relay 112 in response to the control signal and remains on until the completion of the complete frame during which the video signal is transmitted as indicated in curve C, Fig. 16. The circuit is triggered off at the end of the following frame pulse via hold-off circuit 120. The cathode-ray tube is scanned in response to synchronizing pulses separated by circuit 115 which also supplies blanking signals to circuit 119. The writing and flood guns are pulsed on and off alternately by 10 kc./s. signals from generator 122. The cathode-ray tube will hold a picture for a period of 6-8 minutes, but a new picture may be introduced, as indicated to the operator by lamps 57, 60, at any time following the period of the one frame required for picture transmission. A construction of the transmitter having means for illuminating a picture to be transmitted is described with reference to Fig. 1 (not shown). It is stated that transmitter may be made portable and constructed so as to permit " snapshots " to be taken of moving scenes. Detailed circuits for carrying out the invention are described. The principal features of interest are as follows:- Pick-up tube circuit. Fig. 4 (not shown).-The pick-up tube comprises a vidicon with a selenium target so as to increase the storage time to a value appropriate to the slow scanning rates. The high frequency response of the circuit is preserved by in-phase feedback from cathodefollower 130 to the shield 136 of the target output lead 53. Line and frame scan timing circuits. Fig. 6 (not shown).-Flip-flops 183-185 form a counting chain which is driven from 60 c./s. source 74 and which is reset at a desired count in accordance with a selected scanning rate by feedback via diodes 192-194, transistor 198 and inverter 210. The output pulses on line 209 trigger frame counter 214 which functions in a similar manner with an adjustable count by feedback via diodes 215, transistor 217 and inverter 228. Scanning and blanking signals are extracted as indicated. Resetting of the operating cycle (see above) is effected by pulse 99 via transistor 229. Transmitter scanning circuits. Fig. 10 (not shown).-The line and frame circuits are identical apart from choice of circuit values. The trigger pulse from the timing circuit is applied via complementary pair 237, 238 and transistor 248 to turn on transistor 239 and discharge the sweep capacitor 244 to the potential at the emitter of transistor 239 which may be adjusted by potentiometer 263 for scan position control. Upon the termination of the pulse the capacitor charges via resistors 254- 259, selected in accordance with the desired scan rate, to the potential of line 260 which is set by Zener diode 265. D.C. feedback to the line via transistors 241-243 maintains a constant charging potential on the capacitor (bootstrap action). The receiver scanning circuits, Figs. 18 and 19 (not shown) are of somewhat similar configuration but include cross connection from the frame circuit to the line circuit to apply the frame sawtooth to the charging resistors and a scan amplifier to compensate for the keystone distortion which arises due to the off-axis writing gun. Video signal channel, Fig. 7 (not shown).- The signal from the camera is applied through an amplitude adjuster 10 preset in accordance with the scanning rate to an amplifier 287 and thence to a circuit 96 which compensates for signal variations due to pictures (particularly documents) formed on papers of different colour. A diode 303 clamps the most positive signal excursion (i.e. " white " due to the picture or document background) to earth whilst triode 300 clips the negative " black " excursion at - 1 volt. The resulting signal then passes through cathode follower 308 to output stage 333, 334 which produces a signal with " white " clipped at - 3 volts. D.C. restoration is effected on the grid of the cathode follower by clamp diodes 320, 319, which are driven respectively by line blanking pulses obtained directly and inverted line blanking pulse obtained via inverter 325. The output of the cathode follower is short-circuited during both line and frame blanking by transistor 313 which is supplied on its base with both sets of blanking pulses. Composite camera blanking is effected by way of transistors 342, 350, 353 and 354. Modulator circuit, Fig. 8 (not shown).-The blanked video signal and 400 c./s. control signal are applied through 0-1750 c./s. filter 98 to amplifier 362, 361 and thence via resistor 374 to transistor 376. Diodes 378, 379 perform the modulation in response to a 2À2 kc./s. square wave signal from multivibrator 383, 384. The diodes function to short-circuit the bottom of resistor 374 to earth, whereby the video signal applied to transistor 376 is chopped into pulses at the desired carrier frequency. Cancellation of the modulating signal component from the output is effected by feeding the original signal in antiphase and at half amplitude to the emitter of transistor 376 via transistor 405. The final signal is applied via double emitter follower 412, 422 to filter 92, which removes the upper side-band, thence via amplifier 425 and emitter follower 432 to the transmission channel 32. Receiver automatic gain control circuit, Fig. 12 (not shown).-The received carrier signal is applied through emitter follower 438 to a pair of transistors 443, 444 which function as a variable current divider. Output to a demodulator (see below) is taken from transistor 444 via double emitter follower 482, 485 and automatic control of gain is effected by using the demodulated voltage to control the base potential of transistor 443. For this purpose the demodulated signal is applied through amplifier 448 and double emitter follower 457, 458 to a peak detector 464, 465. The resulting potential is applied to transistor 443 via a further double emitter follower 450, 452. Transistors 446 and 447 comprise a similar current dividing stage to transistors 443, 444 and are cross-connected therewith for the purpose of direct current balancing. Receiver demodulator, Fig. 13 (not shown).- The carrier signal is applied through double emitter follower 502, 503 to full-wave rectifier 510, 512 and the demodulated output is applied through double emitter follower 519, 520 to band-pass filter 524 which eliminates the carrier component doubled in frequency by the demodulation process. Circuits for separating synchronizing from vision component and frame from composite lines and frame signal, Fig. 14 (not shown).-The positive going synchronizing signals turn transistor 530 off and transistor 533 on. The resulting positive collector excursion causes diode 543 to conduct and this in turn causes transistor 546 to be turned off. In this way negative going synchronizing components are derived from the collector of the transistor. The positive excursion at th