Patent GB-947391-A (GB)
Title: Improvements in apparatus for producing color printers
IPC: H04N1/60
Filed: 1960-04-21
Abstract: 947.391. Copying in colour by scanning. NEWSPAPER ENTERPRISE ASSOCIATION Inc. April 21, 1960 [April 29, 1959], No. 14018/60. Heading H4F. General.-The invention relates to image reproduction apparatus in which an original coloured transparency mounted on a drum subjected to rotational and axial displacement, so as to effect point-by-point scanning, is analysed photo-electrically in terms of its blue, green and red components and the resulting signals employed to control glow tubes which expose yellow, magenta and cyan photographic printers carried by three drums displaced synchronously with that carrying the original. The invention is characterized by the use in each colour channel of a logarithmic circuit following the photoelectric cell so as to produce a signal proportional to the optical density of the original, and " colour masking " means (termed " undercolour removal " means) by which the lesser of the optical density signals in the other two channels is subtracted from the channel signal. Provision is also made for selecting the smallest of the three colour signals at each instant and employing it to produce a black printer by modulation of a further glow tube exposed to a printer on a fourth drum. Colour channels.-The three channels are identical and each is constructed as shown in Fig. 2. The channel comprises a multiplier type of photo-electric cell 34 whose final dynode is connected directly to the grids of twin triode 43 arranged to operate in a positive grid voltage region where its grid voltage is logarithmically related to grid current. As a consequence the valve delivers across its anode load 47 a signal proportional to the optical density of the original. The signal is applied to a one grid of balanced cathode-follower circuit 53, 54, the other grid of which receives a steady reference potential. The circuit is set up so that in response to a maximum colour signal no difference in potential appears between cathodes 66 and 67 whereas cathode 66 becomes increasingly positive with respect to cathode 67 as the colour signal reduces to zero. The potential between the cathodes constitutes the signal for controlling a glow tube 8C to expose the colour printer. The signal is applied firstly to a further balanced cathode follower circuit 55, 56 through a C.R. network 104, 105 which serves to enhance edge detail and a diode 107 which compensates for " highlight " detail, and then to yet a further balanced cathode-follower circuit 57, 58 which is used for colour masking (see below). The potential between cathodes 70, 71 is taken over leads 134, 135 to a balanced modulator stage 144, 145 where it constitutes the anode-cathode potential of the stage. The colour signal is thereby caused to modulate a carrier (8000 c/s.) from source 140. The modulator is followed by an amplifier 158, 159 whose output is detected in stage 174, 175 to furnish an amplified version of the original colour signal. The signal controls the glow tube 8c through a valve 185, a reversing switch 179 being included in the grid input connection to permit the production of either photographic positives or negatives. A non-linear resistor 199 in the grid circuit compensates for non- linearity in the glow tube characteristic. The circuit includes voltmeters 96, 215 and 227 for measuring potentials at important points, and the Specification describes in detail the procedure for adjusting the apparatus for correct operation. Colour masking (undercolour removal).-Fig. 2 shows the cyan channel. Signals from the magenta and yellow channels are received over leads 13MC and 13YC, and selected portions of the signals are derived from potentiometers 123, 124 and applied to one end of potentiometers 127, 130 in opposition to the cyan signal which is applied to the other ends over lead 112 from the second balanced cathode-follower circuit. Portions of the difference signals are then applied to grid 116 of triode 65 via rectifiers 131, 132 which select the smaller of the two signals. By these means the channel signal appearing between cathodes 70 and 71 is reduced or " masked " in accordance with two other twochannel signal to an extent determined by the settings of potentiometers 123, 124, 127, 130. Signals from the cyan channel for masking in the magenta and yellow channels are supplied over leads 13CM and 13CY respectively. The cyan signal is also supplied over lead 15CB k , together with reference potential over lead 16, to the black printer channel (see below). Black printer channel.-The channel is identical in all respects with the colour channels except that only a single balanced cathodefollower circuit is provided arranged as shown in Fig. 3. One grid of the cathode follower receives the channel signals over leads 15YB k , 15CB k , 15MB k  and the other receives a reference potential over lead 16 which may be obtained conveniently from one of the colour channels, the cyan channel as described. The channel signals are applied through rectifiers 254-256 which select the smallest at any instant. The cathode difference potential is applied over leads 265, 266 to control the associated modulator (not shown).