Patent GB-1241852-A (GB)
Title: Improvements relating to image reproduction equipment
Inventor: PUGSLEY PETER CHARLES
IPC: H04N1/60
Filed: 1968-11-15
Abstract: 1,241,852. Function generators for colour printers. CROSFIELD ELECTRONICS Ltd. 12 Nov., 1969 [15 Nov., 1968: 26 Nov., 1968], Nos. 54387/68 and 56088/68. Heading G4G. [Also in Division H4] Apparatus for preparing colour printers comprises means for scanning different colour components of an original and deriving uncorrected colour printer signals, means for deriving difference signals each representing the excess of one of a pair of uncorrected signals over the other of the pair, means for combining weighted difference signals to form correction signals each for a restricted colour range, and means for selecting a correction signal only when it represents an increase in the density of the respective restricted colour range in the original, and means for applying the correction signals to the uncorrected colour printer signals. The corrected signals are used to modulate a source which scans and exposes a sensitive surface. As shown (Fig. 1), uncorrected yellow, magenta and cyan (y, m, c) signals are fed to function generators 11, 21, 31 which generate magenta, violet, cyan, green, yellow and red (m, v, c, g, y, r) correction signals. Signals y and m are fed to a differential amplifier 10 and the difference (y - m) fed to diodes 12, 14 which detect the positive P(y - m). and negative N(y - m) parts of the signal both of which are fed to adding network 15. A signal P(c - y) from function generator 31 is also fed to network 15 the output of which may be represented as (y - m) - N(y - c) using the fact that P(x) = - N(- x). This signal is inverted by amplifier 17 and the positive part detected by diode 18a, the resulting signal being repersented by P[α(m -y) + #N(y - c)]. The summing networks incorporate resistors which generate the weighting factors α and #. This is the magneta (m) correction signal. A violet (v) correction signal P[α(w - y) + #N(c - m)] is derived in like manner in adding network 16 and inverter 19. The other correction signals c, g, y, r are similarly derived and have similar formulµ with the exception of green (g) which is derived in adding network 26 from three difference signals and has the formula P[α(c - m) + #N(y - c) - yP(m - c)]. Considering only the yellow colour printer channel (the other two are similar), thesix correction signals are fed through six hand-controlled potentiometers (Fig. 2, not shown) arranged in two groups assigned respectively to c, v, m and r, y, g. Clockwise rotation of the c, v, m group produces a decrease in yellow in corresponding colour areas and clockwise rotation of the r, y, g group produces an increase in yellow in corresponding colour areas. The outputs of two sets of potentiometers are combined to give a yellow decrease signal (y<SP>11-</SP>) and a yellow increase signal (y<SP>11</SP>+). The six potentiometers are arranged in parallel with six fixed resistors having values equal to the mean values of the respective potentiometers, whereby the mean value of a correction signal can be instantly recovered by switching the corresponding fixed resistor into circuit. The corrected yellow signal y<SP>1</SP> = (y<SP>11</SP> +) + (y<SP>11</SP> -) is derived by the network of Fig. 3 which also comrpises similar networks for deriving the corrected magenta (m<SP>1</SP>) and cyan (c<SP>1</SP>) signals. The signals (y<SP>11</SP> + ) and (y<SP>11</SP> -) are fed to the inverting inputs of respective operational amplifiers and the outputs summed to provide y<SP>1</SP>. The potentiometer 80 is provided for the purpose of balancing the amplifiers. In order to permit variation in the correcting effect between pastel and solid colours, the output of diodes 12, 14, Fig. 1, may be modified to produce signals: and where k may be equal, e.g. to 0.1. The signal m is fed to the inverting input of amplifier 10 over resistances 100, 101 of aggregate value 10k, and the signal y is fed to the non-inverting input over resistances 102 and 103 respectively of value 10K and 100K. The positive and negative parts of the output pass through respective diodes 105, 107 and separate feedback paths for the two outputs comprise respectively a resistance 108 of value 10K and parallel resistances 109, 110 respectively of value 10K and 100K. The values of resistances 103, 110 may be expressed as R/k where R = 10K and k = o.1. The non-inverting input is connected to the collector of a transistor 115 over a resistance 116 of value and a bias is applied to the base of the transistor such that, with earth potential at the output of the amplifier, the diode is partly switched. It is stated that switching of the transistor places the resistance 116 in and out of cricuit and, by altering the gain of the non-inverting part of the differential amplifier 10 produces the required output.