Patent GB-1028304-A (GB)
Title: Improvements in or relating to facsimile systems
IPC: H04N1/36
Filed: 1963-02-07
Abstract: 1,028,304. Facsimile apparatus; automatic gain control; semi-conductor circuits. DICK CO. A. B. Feb. 7, 1963 [Feb. 26, 1962]. No. 5040/63. Headings H3Q, H3T and H4F. Facsimile transmitter.-Copy 12, Fig. 1, mounted on a rotating drum 10 is scanned lineby-line by a photo-multiplier 20 operated in conjunction with a mirror drum 16 to produce signals, Fig. 3, which are applied through a D.C. amplifier 30. In accordance with a first feature of the invention, the effects of variations in the reflectivity of the copy are compensated by an automatic gain-control circuit 36 which responds to the negative excursions of the signal which represent white and applies a control signal to a dynode of the photo-multiplier. In accordance with a second feature of the invention an automatic bias circuit 34 responds when positive excursions of the signal, which represent black, extend beyond a predetermined level and corrects the signal level by applying a correcting bias to the input of the amplifier 30. The resulting signal which is thereby constrained to a maximum (black) level 55 and maximum (white) level 57 is then applied to a clipping circuit which establishes (1) a fixed clipping level 59 to define the " black " level of the signal, and (2) a dynamic clipping level 61 which varies with instantaneous signal 53 and defines the white level. The operation of the clipping circuit is such that the dynamic level follows signal variations rapidly in the white direction and slowly in the black direction. The output of the clipping circuit controls a Schmitt trigger circuit 44 to produce a signal as shown in Fig. 2. A synchronizing pulse of amplitude exceeding the copy signal is generated at the start of each line by a mirror 26. The pulse is separated from the output of amplifier 30 by a circuit 38 and employed to trigger a one-shot multivibrator 40. The resulting pulse is then added to the copy signal in mixer 42. The trigger time of multivibrator 40 is controlled by a switch 22 which responds to the presence of copy. The effect is to cause the production of wide synchronizing pulses when copy is being scanned and narrow synchronizing pulse 47 when the copy is not in the field of the scanner. Facsimile receiver, Fig. 5.-Received signals after demodulation from a transmission carrier are D.C. clamped at the black level in circuit 160 and then applied to a sync. pulse separator 162 and a clipping circuit 164 which limits the maximum negative excursions. The sync. pulse separator 162 is controlled by an inhibit circuit 172 so as to be responsive only during a synchronizing interval preceding the copy signal. Circuit 172 is controlled by a multivibrator 168, which determines the response interval, and a multivibrator 174 which determines the blanking-off interval. The separated sync. pulses are applied to an integrator circuit 166 which responds to only the wide pulses to produce an output which triggers multivibrator 176. This actuates a relay 180 which controls the feeding of recording paper 186 and is effective to maintain a paper feed throughout the reception of wide sync. pulses. At the end of a complete copy signal, when only narrow pulses appear, the paper feed is stopped and a knife 88 is operated to sever the record. Recording is effected by a cathode-ray electrostatic printing tube 190, the copy signal from clip circuit 164 being applied via circuits 212, 214, 216 and 218 to modulate the tube beam intensity and the sync. pulses being applied through a delay multivibrator 170 to control a push-pull scanning system 192, 194, 196, 198, 208 so as to cause the tube beam to be scanned transverse the recording paper in synchronism with the line-by-line scanning of the copy at the transmitter. The effect of the delay multivibrator 170 together with control from multivibrator 174 to circuits 192, 164 and 206 is to prevent the generation of a scan until just before the leading edge of the copy signal in each line. Correction of defocusing of tube 190 is effected by a mixer circuit 202 which detects the more negative of the sweep drive signals from circuits 196 and 200 to obtain a triangular wave which is applied through amplifier 204 to the focus coil. Detailed transistor circuits. Transmitter, Figs. 4A, 4B.-The signal from the last dynode 20A of photo-multiplier 20 is applied through D.C. coupled amplifier 52, 54 to an emitter follower stage 56. The signal at the output is as shown in Fig. 3. Transistor 36, which has an emitter bias determined by potentiometer 70, 72, conducts when the negative (white) excursions exceed the bias level and causes a negative-going change in potential at the tapping of a potentiometer 80 connected in the supply circuit for dynode 20D. The time-constant of this gaincontrol circuit is determined by capacitor 74 and is such that the circuit responds to slow variations due to background reflectivity and not to variations representing copy. Transistor 58 has an emitter bias established by a circuit including Zener diode 34 and conducts when the positive (black) signal excursions exceed the bias level to produce a potential variation across capacitor 62 which is applied as a bias to the base of the first transistor amplifier 52. The resulting output signal, which is constrained to a maximum (white) level 57, Fig. 3, by the automatic gain-control circuit and to a maximum (black) level 55 by the automatic bias circuit, is applied through a high-frequency peaking circuit 86, 88 to a transistor amplifier 84 which acts as a clipping circuit and has the clipping level set by the circuit including transistor 92. The circuit receives the amplifier output signal via a D.C. level regulating circuit 100, 102 and includes a capacitor 108 which charges rapidly in the negative (white) direction via rectifier 106 and discharges slowly in the position (black) direction via resistor 105. A diode 110 limits maximum excursion in the black direction, whilst a Zener diode 112 limits excursions in the white direction due to the sync. pulses. The clipped signal triggers a Schmitt circuit 96, 98 to produce a signal as in Fig. 2 and this is applied via sync. signal mixing circuit 42 to output emitter followers 150, 152. The sync. signals 51 with the signal at the output from emitter follower 56 are separated by biased diode 128 and applied via transistor 120 and transformer 130 to trigger a one-shot multivibrator 132, 134. The multivibrator normally generates narrow pulses but has its time constant increased by operation of relay 136 so as to generate wide pulses whenever copy actuates switch 22, Fig. 1. The pulses from the multivibrator are applied to mixer 42 via rectifier 146. Receiver, Fig. 6.- The demodulated and clamped signal is applied to the base of transistor 230 which is biased by network 236, 238, 242 so as to conduct only in response to sync. pulses. In addition to the fixed bias a dynamic bias is developed across resistor 244 and capacitor 246 in response to the separate pulses. The separated pulses at the collector are applied via rectifier 250 to transistor 248, a connection from the base of 248 extending through a rectifier 252 to unblocking multivibrator 174 (see Fig. 5). Except during the synchronizing interval, rectifier 252 prevents the base of transistor 248 from going positive and so effectively inhibits the operation of the sync. separator to prevent false action due to interference. The output from transistor 248 is integrated in circuit 270, 272, only wide sync. pulses being sufficient to render transistor 260 conductive to produce an emitter output to trigger multivibrator 176. A suitable sweep generator circuit, Fig. 7, comprises a capacitor 280 which is charged through a transistor 282 provided with fixed base bias and discharged via a transistor 300 when it is rendered conducting by a trigger pulse applied to the base.