Patent GB-751909-A (GB)
Title: Automatic-gain-control system for photoelectric engraving machines
IPC: H04N1/407
Filed: 1954-07-27
Abstract: 751,909. Copying by scanning. FAIRCHILD CAMERA & INSTRUMENT CORPORATION July 27,1954 [Aug. 31,1953], No. 21901/54. Class 40 (3). In a photo-electric engraving machine for producing on a plate a screened relief pattern for image reproduction by printing processes and including photoelectric pick-means for scanning the image sheet to develop an image signal, an automatic gain control system comprises a controllable image signal repeater, means for generating during a small fraction of the scanning period a periodic reference signal of an amplitude corresponding to a predetermined shade value of an image being scanned, circuit means for developing a unidirectional control potential representative of the amplitude of said reference signal and a circuit for applying said control potential to said repeater automatically to compensate for variations in the sensitivity of said pick-up means. A scanning lamp 16 is controlled from an amplifier 27 fed with alternating potential from a rotor 23 associated with a stator 24 as in the arrangement shown in Specification 692,349. The output of the photo-cell 18 in response to light reflected from the image sheet 11 is fed via potentiometer 9 to a preamplifier 28, the output of which is applied to an automatic gain control amplifier 32. The output of the amplifier 32 is fed via an adjustable potentiometer 37, 38 or a fixed potentiometer 39, 40 to a driver amplifier 41 controlling an engraving head 20 operating a stylus 21. Whilst the output of the system is being standardized the driving clutch 59 is disengaged and switch 57 is closed so that the output of the generator 23, 24 is applied via potentiometer 25, 26, a bias amplifier 45 and diodes 48, 49 to a capacitor 50 connected to the control circuit of the amplifier 32 so that its gain is locked to a preselected value. The cylinder 12 is moved so that light is reflected from the strip 29 of predetermined shade and the output from the potentiometer 9 is adjusted so that via potentiometer 39, 40 rendered operative by the switch 42 in its lower position, the predetermined output is indicated by a meter 60. The drum 12 is then rotated so that a portion of the same reference shade is opposite the scanning beam and with switch 42 in its upper position the tapping point 38a is adjusted so that the same output is maintained and indicated by the meter 60. When both drums 12, 14 are driven, a switch 30 operated by a cam 31 closes contact 30b initially energizing relay 54b to close contact 54a discharging the capacitor 50. When relay 54b releases, the capacitor 50 is thereupon recharged by the signal, corresponding to the strip 29, passing via resistor 44 and bias amplifier 45. When the switch 30 returns to its upper position, the input to the amplifier 45 is short-circuited and the amplifier 32 is operated according to the reference signal as stored on the capacitor 50. If in subsequent operation the signal from the reference strip 29 has decreased, the decreased negative voltage derived from the capacitor 50 increases the gain from the amplifier 32 so as to maintain approximately constant the output from the amplifier 41 to the engraving head 20. Specification 692,349 is referred to.