Patent GB-770885-A (GB)
Title: Improvements in or relating to systems employing gaseous discharge lamps
IPC: H04N1/40
Filed: 1955-05-26
Abstract: 770,885. Valve circuits for discharge lamps. TIME, Inc. May 26, 1955 [June 1, 1954], No. 15190/55. Class 40 (3). In apparatus in which a gaseous discharge lamp is excited by an electric signal supplied by an electronic channel, means are provided to generate a correcting signal which varies in a manner representative of the time drift in intensity of the light produced by the lamp consequent upon a transient change in the average level of the electric signal, the correcting signal being applied to modify the amplitude of the signal part-way through the channel so as to offset the drift and maintain the characteristic of lamp intensity against amplitude of electric signal the same in the presence of a transient change in average amplitude as it is in the absence of such a change. The drift in intensity, which is considered to be due to changes in thermal conditions in the lamp, consists of an exponential increase or decrease following respectively an increase or decrease in the average level of current through the lamp, the magnitude of the drift being proportional to the change in average level. The invention is described as applied to a colour facsimile system, Fig. 1, as described in Specification 765,443, in which a scanner 10 generates yellow, magenta and cyan signals which are applied through identical channels 12, 121, 12<SP>11</SP> to energize discharge lamps 14, 141, 14<SP>11</SP>. In each channel the signal is modulated on to a carrier signal in unit 20 so as to be capable of amplification in a tuned amplifier 22, the signal then passing through a further amplifier 40 to a detector 41 whence an amplified version of the original signal is derived and applied to the grid of a pentode 44 controlling the current in the discharge lamp. In accordance with the invention, a potential proportional to the output signal is developed across the pentode cathode resistor 51 and applied to a resistor-capacitor network 71, 72 to develop at each change in average level an exponentially varying potential at point 80. This potential constitutes the correcting signal and is applied to control the current through three thyrite resistors 81, 82, 83, of which the third 83 is effectively in shunt with the tuned anode load 37 of amplifier 22. By these means, the gain of the amplifier is varied exponentially, being decreased following an increase in the average level of the signal, and being increased following a decrease in average level. Adjustment of the degree of gain control is effected by resistor 90. The time-constant of network 71, 72 is of the order of sixteen seconds. In the arrangement shown in Fig. 1, the correcting arrangement has assumed identical exponential drifts for the increase and decrease. This is not strictly correct, the drift in intensity following a decrease in average level occupying a longer period than that following an increase. In a modification, Fig. 3, account is taken of this. The correction here is effected by applying the correcting signal to control the potential of the suppressor of the output pentode 44. The potential across cathode resistor 51 is applied to resistor-capacitor circuit 102, 103, 104, the circuit including also a diode 105. On an increase in average signal level, the diode conducts so that only resistor 103 is included in the capacitor changing path, whereas when a decrease in level occurs diode 105 is rendered non-conducting and resistor 104 is included in the circuit. The exponential correcting voltage on lead 84 is accordingly of longer duration for a decrease.