Patent GB-1266897-A (GB)
IPC: B41C1/14
Filed: 1969-03-11
Abstract: 1,266,897. Facsimile apparatus. ZEUTHEN & AAGAARD A.S. 11 March, 1969 [14 March, 1968], No. 12832/69. Heading H4F. In a device for producing a perforated laminar printing forme 3, a control signal derived from scanning an original 2, using a light source L and receiver PHC, is fed to a summation amplifier AMPL preceding a trigger TR which connects and disconnects a voltage from an oscillator OSC to a perforation electrode PE, and the device is characterized in that the amplifier is also supplied with a threshold reference voltage from a circuit S, adjustable to determine e.g. the separation of those parts of the original which are to be regarded and reproduced as wholly black and those parts which are to be subject to grey tone modulation, a sawtooth modulation voltage on a constant D.C. baseline voltage, from a circuit G, the amplitude of the sawtooth voltage being adjustable to determine e.g. the separation of those parts of the original to be regarded as wholly white and those parts subject to grey tone modulation, and an automatically controlled calibration voltage from a circuit REG to compensate for changes in operating conditions such as mains voltage variation, and changes in the optical system. The forme 2 and original 3 are mounted on a rotatable and displaceable drum 1 together with a reference original GB, and a permanent magnet PM on the drum induces a signal in a coil PV when the reference original GB is to be scanned, so that a switch F, shown as mechanical but in practice an electronic switch, disconnects circuits S and G from the amplifier and connects the output of trigger TR to the circuit REG to adjust the calibration voltage output for the next scan of the original, as described more particularly in Specification 1,194,717. The light receiver PHC is arranged to give a voltage output with a first component whose variation for a given variation in light intensity is practically constant even if the total voltage output varies e.g. due to a second, temperature dependent, component, so that the second component is automatically compensated by the circuit REG and a fixed relationship between the amplitude of the modulation signal and the first, light sensitive, component, is established. Thus the receiver PHC may be a photo-diode, a photo-semi-conductor having a linear sensitivity characteristic, or a photosensitive component in series with semi-conductor diodes e.g. of silicon. Fig. 7 (not shown) shows a photoconductive component which does not give the desired dependence between the light intensity and voltage, in series with two diodes, across which is developed the desired output voltage. In the circuit shown in Fig. 1, the oscillator OSC feeds the electrode PE via an adjusting circuit V, an electronic switch SW 1  controlled by the trigger TR and an amplifier PA, and also energizes the lamp L. According to the adjustment of an inverting circuit P/N, the control signal is adapted for positive or negative reproduction. In a modification, Fig. 2 (not shown), the output of the amplifier AMPL is fed back to the regulating circuit REG during scanning of the reference original, and the trigger TR acts directly on the amplifier PA.