Patent GB-935972-A (GB)
Title: Electronic previewer for televised color pictures
IPC: H04N1/60
Filed: 1959-11-23
Abstract: 935,972. Colour television. HAZELTINE CORPORATION. Nov. 23, 1959 [Dec. 2, 1958], No. 39642/59. Class 40 (3). The invention relates to an electronic previewer for simulating the photographic and electric signal processing steps of a colour reproduction system in which a picture is derived from an original by a photo-chemical process and is then scanned and reproduced by a television system. The previewer includes controls which may be adjusted to produce a final television picture of optimum quality. The settings of the controls provide information from which may be determined the relative proportions of the colour components of the printing light which should be used in the photochemical process to produce a picture which, when employed as the original image in the actual television system which the previewer simulates, will result in a final reproduced picture of corresponding optimum quality. The previewer includes circuits which simulate colour masking in the television transmitter. In a first embodiment the operation is not subject to adjustment, but in a second embodiment the masking operations may be adjusted to further improve the picture quality after the printing light adjustments as described above have been completed. The masking adjustments may then be used in a suitably constructed transmitter, the adjustments determined by the previewer being recorded, e.g. in binary form on a punched or magnetic tape, and employed subsequently automatically to control the actual transmitter. First embodiment, Figs. 1 and 1a. An original image 28, e.g. a negative colour transparency, is scanned by a cathode-ray tube flying-spot scanner 30 and the transmitted light directed to red, green, and blue responsive photo-cells 32R, 32G, 32B whose responses are adjusted by means of filters 35R, 35G, 35B in accordance with the sensitivities of the three emulsions of a typical positive colour film. The resulting cell signals pass through linear amplifiers to potentiometers 38R, 38G, 38B and are then applied to non-linear amplifiers 40R, 40G, 40B whose characteristics simulate the dye density versus exposure characteristic of the film emulsions. The signals appearing at terminals 40d, 40e, 40f are thus proportional to the respective dye densities. The signals are applied to a cross-coupling or matrix network 103 which adds predetermined portions of each signal to the other to take account of the overlapping spectral absorption characteristics of the film dyes and also the effects of such overlapping in relation to the spectral responses of the " taking characteristics " of the film scanner employed in the transmitter of the television system. At terminals 103R, 103G, 103B there appear signals representative of the dye densities as seen by the transmitter and these are converted to corresponding light value signals by passage through amplifiers 109R, 109G, 109B proportioned in accordance with the negative exponential relationship between density and light transmission. The signals then enter a unit 110 which simulates the signal translating characteristics of the transmitter of the television system. Amplifiers 111R, 111G, 111B introduce gamma correction in accordance with the reciprocal of the exponential relationship between the light output and input signal of a colour display tube used in a receiver. Matrix 112 introduces colour masking by cross-coupling between the colour channels. In accordance with typical transmitter practice, amplifiers 111 are employed to introduce an excess gamma correction in the interest of improved colour reproduction in dark regions and matrix 112 is arranged to compensate for resulting colour desaturation. The signals appearing finally at terminals 113B, 113G, 113B correspond to the signals issuing from the simulated television transmitter and are applied to a cathode-ray tube 115 to reproduce a picture corresponding to that reproduced by a television receiver. It is assumed that a linear relationship exists in the transmission circuits between the transmitter and receiver, thus the signals from matrix 112 are passed to tube 115 via linear amplifiers 114R, 114G, 114B which are merely proportioned in accordance with the respective drive requirements of the three tube guns. Unit 112 has invariable characteristics and simulates a transmitter in which the corresponding circuits are invariable. In operation of the previewer, potentiometers 38R, 38G, 38B are adjusted to produce the best possible reproduced picture, and the potentiometer settings are then employed to give the settings of the colour component controls of the printing light in the photo-chemical process. The part of the previewer shown in Fig. 1 corresponds in part to that disclosed in Specification 841,165. In a modified arrangement, Fig. 4 (not shown), amplifiers 109 and 111 in each colour channel are combined in a single amplifier, and the three combined amplifiers are proportioned to have relative gains corresponding to amplifiers 114 in Fig. la whereby amplifiers 114 are dispensed with. Second embodiment. This differs from Figs. 1 and 1a only in that matrix 112 is replaced by an adjustable matrix circuit arranged as shown in Fig. 5 so as to simulate a transmitter with a corresponding or related adjustable matrix. The circuit comprises first and second fixed matrices 503, 504 coupled by an adjustable circuit. The second matrix 504 is designed so that it requires input signal Y<SP>111</SP>, I<SP>111</SP>, and Q<SP>111</SP> corresponding to the luminance and chrominance component signals in the N.T.S.C. type of system. Thus the Y<SP>111</SP> signal, in the absence of the I and Q signals, produces a black-andwhite final image, the I<SP>111</SP> signal controls the production of flesh colour on the orange to cyan colour axis and the Q<SP>111</SP> signal controls the production of non-flesh colour on the yellowishgreen to magenta axis. The first matrix 503 is designed to produce Y, I and Q signals from the gamma corrected signals from amplifiers 111. The adjustable coupling circuit between the matrix has a " zero " setting at which the combined effect of matrices 503 and 504 is the same as matrix 112 in Fig. 1a. In the coupling circuit, the I and Q channels include main paths terminating in potentiometers 509, 510, and subsidiary paths comprising phase splitters 511 and 512 feeding centre-tapped potentiometer 513, 514 and 518, 516 whereby components of either polarity may be added to the other channel and the Y channel. The Y channel similarly includes a main path and a shunt path comprised of a square law circuit 519 and a phase-splitter 520 feeding a centretapped potentiometer 521 whereby portions of a Y<SP>2</SP> signal may be added or subtracted from the Y signal. In operation, potentiometer 509 controls saturation of flesh colours, potentiometer 510 controls saturation of non-flesh colours, potentiometer 514 controls the hue of the flesh colours, potentiometer 516 controls the hue of non-flesh colours, potentiometer 513 controls the differential brightness of flesh versus sky colours, potentiometer 515 controls the differential brightness of green versus magenta colours and potentiometer 521 controls expansion and compression of picture contrast. In using the previewer including the matrix described above, the matrix is first set to its " zero " position and potentiometers 38 (Fig. 1) adjusted to obtain the best picture, the potentiometer settings being employed in the photo-chemical process. Thereafter the matrix is adjusted to further improve the picture and its settings recorded, e.g. in binary form on punched or magnetic tape, to permit the actual transmitter subsequently to be controlled automatically in a corresponding manner. The adjustable matrix circuit shown in Fig. 5 forms the subject of Specification 935,973.