DE1294451B - Correction circuit for color television signals derived from color films - Google Patents

Description

Cannot be played back on television. io "black stretching" means an effect that the

It is assumed that one or more "white compressions" are similar to this, but more strongly im

the following reasons exist: lower part of the brightness range than in the upper

(a) Films are shown in the cinema without any substantial space.

lighting shown; The errors listed under 1 and 2 are very great

(b) the optical projection as it takes place in the cinema - probably an incorrect exposure either seems to attribute the overall gamma of the displayed in the camera or in the stage at degrade th image; which the negative in the contact process to the

(c) A color-positive material was transmitted for the color film in the cinema. The bugs below loyal norm set up, which is not very high, points 3, 4 and 5 are very likely one while in particular with regard to the 20 errors in film handling to be attributed to the Novelty television viewers in terms of either the negative or the positive stage Color fidelity can be more critical; may have taken place. A major source of error

(d) the overall gamma of a color television system is due to one of the five aforementioned errors usually greater than one; can arise lies in the incorrect choice of

(e) Color television will likely use a 25 filter designed to produce a positive copy of Looking at room lighting, the color temperature of which is optimally balanced from a negative from that of the white point of the dar- that used during the copying process position is different. is assessed visually. The visual assessment of

It has been found that the gradation ratio of negatives is done by a trained overall contrast ratio of a color television person called a "grader" and viewing system including the picture tube - although there are many occasions when viewing under conditions low room lighting
such is that an adequate balance of the three
Color separation channels over a contrast of
at least 30: 1 is obtained. In the case of the lightest 35, the entire roll of film seems to persist, instead of areas and the darkest tones leading to deviations from changing from picture to picture,
From the article in "Proceedings of the IRE," etc. on color errors, so that for some time for January 1954, p. 186, a correction circuit was considered desirable, the contrast of in color television signals, as described at the outset to restrict television-transmitted scenes in such a way, 40 has become known, in which a luminance compendation of the capabilities of the television system is not over-sated by automatic gain control. The investigation of many samples of subsequent normal gamma correction by showing in the cinema certain color films has been carried out. The known circuit will, however, show that the color copies of the same suffer from the reproduction of color films for color television similar distortions, if the image is used and the object, which requires a very high contrast in color photography, should intensify a luminance component that is frequently weakened.

Grader can make an incorrect assessment, this does not appear to be a major source of error, since the most prominent errors are about a

shows. Furthermore, the greatest distortion can be determined at the ends of the brightness range, and in addition, many films also have an overall color cast that is particularly annoying.

Many samples of test color films contain a normal gray wedge in their object, which is extends over the entire brightness range of the film material and interacts with the rest of the film material.

ken. The compensation takes place automatically and is dependent on the color of the one just scanned Part of the image pixel for pixel applied. The invention has a different object provided, namely a manual adjustment option for the color balance and light transmission properties for each of the three color signals derived when scanning the film, so that un-

acts is. An examination of the gray wedges in 55 desired and unforeseeable errors in these film samples has shown that the errors in the examination of the image generated when the film is scanned can be compensated for with the following categories,
can be: According to the invention this object is thereby

(1) Excessive average density of the film; solved that the three devices interact with each other

(2) an inadequate average density of the 60 clutch and that are manually adjustable and that one Films; maximum gain in each channel at one

(3) excessive density in one or two of the three emulsion ingredients with respect to the average density of the film;

(4) a "black compression" in one or two of the three emulsions;

(5) a "white compression" in one or two of the three emulsions. With the "white compression"

Another setting is achieved in which the gain in the other channels is a corresponding one has lesser value.

This relatively simple circuit makes it possible to compensate for the errors listed above.

An embodiment of the invention will now be described with reference to the drawings, namely shows

F i g. Figure 1 graphs the signal voltage at each of a number of levels in a gray scale display a color film, with a graphic representation given for each primary color,

F i g. 2 of FIG. 1 corresponding graphical representations for the same signals according to the invention Correction,

F i g. 3 is a circuit diagram of a preferred embodiment of the invention,

F i g. 4 dje each time the control organs in F i g are set. 3 corrections made and

5 shows a graph of the signal output voltage versus the signal input voltage with a linear relationship and with "black stretching" and "black compression".

The in F i g. The graphs given in Figure 1 show the results obtained with a test film which contains a gray wedge in its subject. The signals from each of the three Channels red, green and blue should of course be identical «o but in most cases it is not. As can be seen, there are differences in the peak values of the signals obtained from the red, green and blue channels. More important, however, is the fact that the ninth and darkest level, as can be seen, significantly different signal levels in the three Channels results.

This failure of the film to give three color separation television signals which have identical waveforms if the object lies entirely on a neutral (color-free) gray scale is called an equilibrium error and leads to the fact that the image becomes one Has a color cast, d. that is, certain areas of the image have an erroneous tendency towards a particular one Colour. As can be seen, for example, the white level (# 1) would appear a little orange while the lowest level (No. 9) would appear cyan, with slight deviations from neutral gray in many of the stages occur.

Like F i g. 3 shows, color separation signals are generated in a manner known per se by scanning a Color film are derived, input terminals 10, 11 and 12 fed, which through separate channels are connected to output terminals 13, 14 and 15 respectively. Blanking signals from a source not shown are fed to an input terminal 16 and synchronized with the blanking intervals.

In Fig. 3, the same elements in all three channels have the same reference numbers, preceded by the letters R, G or B to identify the red, green or blue channel. The description is mainly limited to the red channel as the other channels work in the same way.

The blanking pulses, which are shown as negative, are fed from 16 to a circuit D which supplies positive and negative pulses at its two outputs, which are fed to the ends of a potentiometer 17. The circuit of D and 17 forms a main black control with the aid of which the direct current component of all color separation signals can be changed uniformly. The size and the meaning of the pulse from 17 are set by the slide of the potentiometer and this pulse is added to or subtracted from the signals in the adding diodes RC, GC and BC during the horizontal blanking intervals.

In each channel there is a main gain control RA, GA, BA, and the three controls are coupled to one another. Each of these controls comprises an adjustable attenuator followed by an amplifier with a fixed gain factor.

The errors 1 and 2 can be compensated for by setting the main black controller 17 and the main gain controller RA and so on.

The change in the gain of the three channels, used to compensate for color casts that are primarily visible in the lower density parts of the film (ie, the lightest areas of the image), occurs in the "Color Gain" section. Two adjustment devices are provided for this purpose. One of these devices is a switch RSW1 with twelve switch positions, which is coupled to corresponding switches in the other channels and selects a color channel or a suitable combination of two color channels and thus an effective color axis along which an increased gain can be applied. A separate gain controller RR1, which is coupled to corresponding controllers in the other channels, is used to determine the amount of such a correction. The twelve positions of the switch RS Wl represent vectors that are located on a Maxwell's color triangle at intervals of 30 ° from one another, on which the three primary colors red, green and blue are 120 ° apart. The color axes chosen in the example are as follows:

1. Red

2. Red / Magenta

3. Magenta

4. Magenta / blue

5. Blue

6. Blue / Cyan

7. Cyan

8. Cyan / Green

9. Green

10. Green / yellow

11. Yellow

12. Yellow / Red

F i g. 4 shows the color axis assigned to each switch position. It also shows the coefficients which determine the contributions of the primary colors to each of the twelve colors, the values of these coefficients being such that twelve vectors are obtained, the lengths of which are equal in this particular diagram. The changes in the gain in the color separation signal channels correspond to the coefficients shown in Fig. 4, are determined by resistors RR3, RR4, RR5 (in the red channel), which are assigned to the switch RSWl with twelve switching positions, and the size of the correction is determined by the triple potentiometer RR1 regulated. The mode of operation of the correction circuit is only described below in connection with the red channel. If the slide of the potentiometer i? I? L is connected to the input terminal 10, a maximum correction is applied. RB is an amplifier with a low input impedance, so a change in the effective value of .Ri? 2 results in a change in the signal current flowing into the amplifier when a correction is applied. The effective change in RR2 takes place with the help of a switch level of the switch RSW1 and the fixed resistors RR 3, RR 4 and RRS. The following table shows the effect achieved in each of the twelve switch positions.

position Resistances in
Parallel connection
■ πι ~ t> ό * y Additional signal
electricity at the entrance
from AB (in arbitrary TO KK 2 common units) 1 RR5 + RR3 1.00 2 RRS 1.16 3 RR5 + RR3 1.00 4th RR5 + RR3 + RR4 0.58 5 open circuit 0.00 6th open circuit 0.00 7th open circuit 0.00 8th open circuit 0.00 9 open circuit 0.00 10 RR5 + RR3 + RR4 0.58 11 RR5 + RR3 1.00 12th RRS 1.16

formation of a black stretch on the red channel together with a slight cyan gain achieved.

Claims (11)

Claims: 10 If this table is compared with Fig. 4, it can be seen that the required changes in the gain are brought about in the red channel. The changes in the gain in the separating signal channels for green and blue take place in a corresponding manner. As mentioned, the correction applied in each channel is changed with the help of RR1. If the slider of RR1 is grounded, no correction is applied and there is no overall gain change because the input impedance of amplifier RB is low compared to RRS. The triple switch RSWl etc. with twelve switch positions and the triple potentiometer RRl etc. enable the correction of the white value of the film. The compensation for "black stretching" or "black compression" is applied in a similar way as described above for the different control of the gain in the three channels. In applying a non-linear correction to the signals derived from the film, black stretching is used to balance the three signals when the film generally exhibits black compression type distortion, so that when the film generally exhibits black stretch distortion the black compression state is selected will. The non-linear compensation devices are shown in FIG. 3 and are labeled "color gamma". The amplifier RB feeds two non-linear circuits 18 and 19 with black compression and black expansion properties, respectively. These properties are shown in FIG. 5 can be seen. The two non-linear signals can be achieved at each end of a potentiometer RR 6, and the movement of the slide of this potentiometer enables a smooth transition from maximum black expansion to maximum black compression via a middle position in which the effective transmission characteristic of the complete circle is approximately linear. As in the case of the gain control, the triple switch RSW 2 with twelve switch positions and the resistors RR1, RR8 and RR9 select the color axis along which the non-linear compensation is applied. F i g. 2 shows the color separation signals after correction. As can be seen, the errors shown in FIG. 1 are largely avoided. In this particular example, the correction was made by the user 1. A circuit for correcting errors which occur in color television signals derived from color films, each with a device for changing the gain in each of the three color channels, these devices each having characteristic curves adapted to the color of their assigned color channel, characterized in that the three devices (RSWl, GSWl, BSWl) are coupled to one another and adjustable by hand and that a maximum gain is achieved in each channel with a different setting in which the gain in the other channels has a correspondingly lower value. 2. A circuit according to claim 1, characterized in that the gain in each of the three channels are changed according to the same function, the beginning of which, however, over the entire setting range shifted from one channel to the other by a third of the setting range is. 3. A circuit according to claim 1 or 2 with an amplifier in each channel, characterized in that that the device for changing the gain in each case the input resistance change to the assigned amplifier. 4. A circuit according to one or more of claims 1 to 3, characterized in that the devices for changing the gain (RSWl, GSWl, BSWl) work practically linearly. 5. Circuit according to one or more of claims 1 to 3, characterized in that the devices for changing the gain (RSW2, GSW2, BSW2) do not work linearly. 6. A circuit according to one or more of claims 1 to 3, characterized in that two devices for changing the gain (z. B. RSWl and RSW 2) are provided for each color channel, one of which (RSWl) and the practically linear other (RSW2) does not work linearly. 7. A circuit according to one or more of the preceding claims, characterized in that the adjustment of the devices takes place in stages, each adjustment stage of all three devices (RSWl, GSWl, BSWl or RSW2, GSW2, BSW2) by an equally long vector of Maxwell's Color triangle is characterized in which the three primary colors red, green and blue are offset from one another by under 120 ° and the vectors of all setting levels are separated from one another by the same angle. 8. Circuit according to one or more of the preceding claims, characterized in that that the devices for changing the gain by three levels of a rotary Switches with several contacts are represented, via the appropriately sized resistors can be switched on. 9. Circuit according to the preceding claims, characterized by a main black control (D), by means of which the direct current component of all three color separation signals can be changed simultaneously. 10. Switching according to the preceding speak, characterized by an adjustable main gain control (RA, GA, BA) in each signal channel, the maximum gain in each such main gain control being separately adjustable. 11. A circuit according to claim 10, characterized in that the main gain controls are coupled to one another for simultaneous actuation. 1 sheet of drawings 909519/349