Patent GB-795331-A (GB)
Title: Improvements in or relating to facsimile reproduction systems
IPC: H04N1/60
Filed: 1956-05-30
Abstract: 795,331. Copying by scanning. TIME, Inc. May 30, 1956 [June 14, 1955], No. 16751/56. Class 40 (3). A facsimile reproduction system having an electro-optical scanner which develops an electrical information signal whose amplitude varies in accordance with the intensity values of the scanned elemental areas of a visual subject includes a mixing network by which an electrical correction signal of constant amplitude is added to the information signal to produce a modified information signal which is utilized to produce a corrected information signal. In the arrangement shown for a three-colour reproduction system including a black printer, the scanning arrangement 11 generates blue, green and red signals fed to modulators 12, 12<SP>1</SP>, 12<SP>11</SP> of the type described in Specification 630,888, [Group XL (c)], which produce amplitude modulations on a frequency of 150 kc. from a modulating frequency of 75 kc. derived from an oscillator 17, frequency divider 15 and filter 16. The colourmodulated carrier frequency is fed, as indicated for the blue channel, to the grid 50 of a pentode 51, having its screen grid 60 coupled to the cathode 52 and having signals of appropriate, constant amplitude at 150 kc. and brought into phase agreement with the amplified modulated output of the valve, applied over conductor 71 and resistor 70 to the anode circuit. The modified signal from the combining circuit 20 is passed to an exponential compressor circuit 21, Fig. 2, comprising a network formed by non- linear thyrite resistors 30, 31, 32 and groups of linear resistors 33a ... 33d, 34a ... 34d and 35a ... 35d selectively included by ganged switches 36, 37, 38. The output of a compressor is fed to a circuit 22 which includes further operations, such as rectification and provides a feed-back signal over conductor 42, resistor 43 and tuned circuit 44 to increase the effectiveness of the compressor circuit. The outputs of the circuits 22, 22<SP>1</SP>, 22<SP>11</SP> are fed individually to yellow, magenta and cyan glowlamps (23, 23<SP>1</SP>, 23<SP>11</SP>) to produce the corresponding subtractive colour negatives, and simultaneously to a black channel circuit 24 controlling a glow lamp 25 to produce the negative for the black printer. A principal advantage of the arrangement resides in a more accurate control of the amount of ink derived from the negative in response to the originating signals of small amplitude since the effect of the added signal is greater in relation to these signals which result in greater deposit of ink from the negative, and also by increasing the effective amplitude of the resultant signals with increased density of the negative the over-deposit of the ink is, to some extent, limited. In other words, for the range of small amplitude scanning signals which would result in the printing of black the addition of the signals of constant amplitude, provide a modified amplitude range which, after subjection to the compression process, results in corrected signals giving rise to tone variations in the area which would normally be reproduced as black by the unmodified signals.