CA1073095A - Color-difference signal modifying apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A color-difference signal modifying apparatus for modifying a color-difference signal in a color television receiver. The apparatus gives to a color-difference signal a non-linear characteristic which approximates a theoretical characteristic for completely eliminating both luminance and chromaticity errors of a reproduced color. The apparatus is constituted by a threshold signal producing means which is supplied with a luminance signal to produce a threshold signal proportional to the luminance signal, and a gain reduction means which is supplied with the color-difference signal to reduce the gain to remove a specific portion of the color-difference signal which exceeds the threshold signal level.

Description

1073~95 This invention relates to a color television receiver, and more particularly to a color-difference signal modifying apparatus for reducing both the luminance errors and chromaticity errors of a reproduced color.
Chromaticities of phosphors applied to recent color picture tubes are much different from those of the primary color picture tubes defined in the NTSC television system. Chromaticity errors of a reproduced color caused by this difference are almost corrected by appropriate demodulatin~ angles and gains of color-difference signal demodulators, as taught by N.W. Parker in a paper entitled "An Analysis of the Necessary Decoder Correction for Color Receiver Operation with Non-standard Receiver Primaries"
in the U.S. magazine IEEE Transactions on Broadcast and Television Receivers, April 1966. However, Parker's linear ---- .
correction method is essentially ineffective for reducing ~ -luminance errors of a reproduced color. In order to compensate for the defect of linear correction, several means have been proposed to decrease a portion of an extremely ~ ;
20 large color-difference signal which does not contribute to ~ -the chromaticity fidelity but increases luminance errors.
Therefore, these means have no effect on unsaturated colors.
It is an object of this invention to provide a ;~ color-difference signal modifying apparatus for modifying a color-difference signal in a color television receiver, ~; which apparatus eliminates luminance errors of a reproduced color.
Another object of this invention is to provide a color-difference signal modifying apparatus which decreases a luminance noise transmitted through the chrominance channel.
A further object of this invention is to provide a color-difference signal modifying apparatus which improves ., ~ '~

- 2 -.. , :. - .

~073~95 the chromaticity of a reproduced color.
These and other objects are achieved according to this invention by providing a color-difference signal modifying apparatus for modifying a color difference signal in a color tele-vision receiver, the apparatus comprising a threshold signal pro-ducing means for being supplied with a luminance signal and for producing a threshold signal which is proportional to said lumi-nance signal; a color difference signal demodulator for producing a color difference signal; and a gain reduction means coupled to said threshold signal producing means and to said color differ-ence signal demodulator for reducing the gain of the color dif-ference signal by at least a portion of the amount of the dif-- ference between said color difference signal and said threshold , signal only when said color difference signal exceeds said thres-hold signal.
~; The apparatus can further comprise a switch connected ! between said threshold signal producing means and ground for preventing the reduction of said color difference signal when said switch is in the "OFF" state.
- 20 The threshold signal producing means can be a thres-hold limiting means for eliminating a portion of said threshold ' signal lower than a stationary level.
The apparatus can further comprise a ~C voltage pro-ducing means for receiving the outputs of three color difference signal sources, and for producing a reference DC voltage equal ~`
,~ to a stationary voltage of said color difference signal sources ,,, by adding the three outputs, and a signal reduction means for receiving at least one of the color difference signals other than the red color difference signal and coupled to said DC volt-age producing means and receiving the reference DC voltage for attenuating said other color difference signal by a portion of - the amount of the difference between said other color difference '"~ .1.
~ - 3 -... .. . .
: : . . i ;
. :-signal and said reference DC voltage.
The apparatus can further comprise a reference signal producing means for being supplied with the red and blue color difference signals and for producing a reference signal by add-ing said two color difference signals, and a signal reduction means for receiving said blue color difference signal and coupled to said reference signal producing means for attenuating said blue color difference signal by a portion of the amount of the differ-ence between said blue color difference signal and said reference 10 signal. ~ -`, The apparatus can alternatively further comprise a ~ -~ DC voltage producing means for producing a reference DC voltage .;~ .
equal to a stationary voltage of the color difference signal sources, and a signal reduction means coupled to said DC voltage ; producing means for receiving at least one of the color differ-ence signals and for attenuating the said one color difference ,~ signal by a portion of the amount of the difference between said ~ color difference signal and said reference DC voltage.
: ~ .
~; Details of this invention will be apparent from the following description taken in connection with the accompanying :, , .
: drawings, in which:
Fig. 1 is a schematic block diagram o~ a conventional color television receiver;

. ~ .
Fig. 2 is a CIE 1960 USC chromaticity diagram :.
showing chromaticity errors of colors reproduced by systems employing color-difference signal demodulators according to .:
~ the NTSC standard, recent phosphors and reference white of A .
9300K+27MPCD;

~` Fig. 3 is a chromaticity diagram of colors re-produced by conventional demodulators having linear correction added thereto in receivers having the recent phosphors and ~.,, , , ~ ~ - 3a -.... . .

~073095 reference white of 9300K+27MPCD;

Fig. 4 is a diagram showing luminance errors caused by the linear correction as shown in Fig. 3;

Figs. 5a-5c are graphs illustrating demodulation characteristics required for color-difference signal de-modulators to reproduce an accurate color with respect to both chromaticity and luminance;

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' -1''-''' :, Fig. 6 is a graph showing non-linear characteristics of demodulation obtained by using an embodiment of a color-difference signal modifying apparatus according to this invention which approximates the ideal characteristics shown in Figs. 5a-5c;
Fig. 7 is a schematic block diagram showing a color- -difference signal modifying apparatus according to this -invention;
Fig. 8 is a schematic circuit diagram, partially 10 in block form, of a system using an example of a color- ` ~
difference signal modifying apparatus of this invention for ~ ~ -a receiver having the recent phosphors and a higher color-temperature reference white;
-Fig. 9 is a chromaticity diagram showing the chromaticity errors of colors reproduced by the system shown in Fig. 8;
Fig. 10 is a diagram showing the luminance errors of colors reproduced by the system shown in Fig. 8;
Fig. 11 is a graph illustrating demodulation characteristics required for the color-difference signal demodulators to reproduce an accurate color with respect to both chromaticity and luminance in a receiver having the recent phosphors and the illuminant C as a reference white;
Fig. 12 is a schematic circuit diagram, partially :, in block form, of a system using another embodiment of a color-difference signal modifying apparatus according to this invention, the demodulation characteristics af which approximate the function shown in Fig. 11; and Fig. 13 is a schematic circuit diagram, partially in block form, of a system using still another embodiment of a color-difference signal modifying apparatus according to this invention whiFh further improves a chromaticity .", . .
` ~' ' ' ` . ' .

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accuracy of a color around bluish green.
Fig. 1 is a block diagram of a conventional color television recsiver for decoding NTSC television signals.
Referring to Fig. 1, a video detector 1 detects from an intermediate-frequency signal a composite video signal which consists of a luminance signal and a carrier chrominance signal. The luminance signal is amplified by a video amplifier 3 and fed to each of three (red, green and blue) matrix circuits 7, 8 and 9. The carrier chrominance signal is amplified by a chrominance signal amplifier 2, demodulated by red, green and blue color-difference signal demodulators 4, 5 and 6, and fed in the form of color-difference signals to the matrix circuits 7, 8 and 9. Each of the matrix circuits 7, 8 and 9 adds the corresponding color-difference signal to the luminance signal and produces a primary color signal.
The primary color signals R, G and B are supplied to a red cathode 11, a green cathode 12, and a blue cathode 13, ~-respectively, so as to energize a picture tube 10. The picture tube 10 converts the primary color signals to light outputs.
In the NTSC television system, the chromaticities of the three primaries and the reference white are defined as shown in the following table.

; Phosphor X Y

R 0.67 0.31 G 0.21 0.71 B 0.14 0.08 white 0.310 0.316 (illuminant C) where X and Y are coordinates on the ~IE 1931 (x,y~ chromaticity diagram.

:, A ~:

.- ~ , . ,` . . .
.- .
:~. . . .`. ~ `

In this system, the demodulating angle and gain of each color-difference signal demodulator are also defined, as shown in the following table.

angle gain R-Y 90 1.147 G-Y 235.67 0.706 B-Y 0 2.046 A color television receiver provided with NTSC specified phosphors and white and NTSC specified demodulators is capable of accurate color reproduction.
~ owever, improvement of the luminous efficiency of phosphors which has been made in recent years has brought a chromaticity change toward less saturated colors. Moreover, the reference white in almost all recent receivers has a higher color-temperature than 6774K, which is the color-temperature of the illuminant C. Typical chromatiaities of phosphors and a higher color-temperature reference white in a recent television receiver are shown in the following table.

, : .
~ 20Phosphor X Y
.- ~:
R 0.631 0.347 G 0.268 0.585 B 0.150 0.071 ;~ white 0.281 0.311 . . .
(9300K+27MPCD) If the specified demodulators are used for color reproduction, the recent phosphors and a higher color-temperature white will cause large chromaticity errors of the reproduced '~ colors on the CIE 1969 Uniform-Chromaticity-Scale ~UCS) 30 diagram, as shown in Fig. 2. But these errors can be `decreased by redesign of the color-difference signal h :
..

demodulators with proper demodulating angles and gains. The specifications for the recent color-difference signal demodulators for the recent phosphors and a higher color-temperature white are set forth in the following table.

angle gain R-Y100 2.5 G-Y237 0.75 B-Y 0 2.65 .. . . .
where R-Y designates the red color-difference signal, G-Y
the green one, and B-Y the blue one.
Fig. 3 shows the chromaticity errors for several colors reproduced by the recent phosphors, the higher color-~ temperature white and the recent color-difference signal ; demodulators. Referring to Fig. 3, it will be noticed that chromaticity errors are slight, as compared with those in Fig. 2. This means that the recent demodulators are com-petent for correcting chromaticities of reproduced colors.
However, they have a serious defect in that they produce a `~ large amount of luminance error, especially in the red color-difference signal as shown in Fig. 4. In Fig. 4, the position of the arrow indicates the ahromaticity of a reproduced color and the length of the arrow represents the luminance error of the color in percent. A luminance error makes a reproduced color much brighter and less natural than the .; .

, originally televised one. Moreover, a luminance error causes , a deterioration in the sharpness of a reproduced image, because the luminance error, passing through the chrominance channel having a narrow frequency band of about 500 KHz, ~` covers up the detail of a luminance signal having a wide frequency spectra of more than 3MHz. Moreover, a luminance :. .
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error causes noise or cross-color components to carry a large amount of luminance component through the chrominance channel and to appear in an image as a luminance noise, not just as a chromaticity noise.
This luminance error described above cannot be eliminated by the known linear correcting matrix circuits.
In other words, an accurate color reproduction with respect to both chromaticity and luminance is very difficult to achieve by the linear correction provided for the color-difference signal demodulators.
The inventors have considered the characteristicsrequired to reproduce an accurate color without chromaticity and luminance errors, and the theoretical results thereof are set forth in the following description. (These characteristics are essentially non-linear, and the non-linearity of the recent phosphor system is apparent from the following description.) First of all, there are the colors reproduced by the NTSC specified phosphors and the NTSC specified demodulators, which colors are thought to be the exact ones being televised. In this case, the red, green and blue light outputs, RN, GN, and BN, can be expressed by:
RN=EyY{l+eAR cos (~-~R)}~ ;
~/ GN=EyY{l+eAG cos (~-~G)}Y _____~____-~--(1) ;
BN=Ey~{l+eAB cos (~-aB)}Y
where Ey is a luminance signal, e is the amplitude of a carrier chrominance signal normalized b~ the luminance signal,~ is the phase of the carrier chrominance signal, AR, AG and AB are demodulating gains and ~R~ ~G and ~ B are demodulating phases of the NTSC specified demodulators.
The symbol y is the gamma value~of a picture tube, The tristimulus values X, Y and Z for the CIE 1931 standard ~. , -` 107309S
observer are related to the light outputs by the following equation:

~ 3~ 2) where (TN) is a 3x3 matrix for transforming (X, Y, Z) to (RN, GN, BN), which is defined by the chromaticity coordinates of the three primaries and the reference white. The trans-formation matrix (T) for the recent phosphors and the higher color-temperature white can be obtained in the same manner as (TN), but differs from (TN) because of the difference in the chromaticities of the phosphors and the reference white between the two systems.
In order to reproduce a color having the same tristimulus values (X, Y, Z) as in equation (2), the recent phosphor and the higher color temperature white require the following red, green and blue light outputs, R, G and B.

~: ( G ~ = l T¦ ~ Y ~ ------------ -______ :
Let is be assumed that the three functions FR(e,~), FG(e,~), FB(e,~) are the characteristics required for the color-difference signal demodulators to produce the light outputs R, G and B from the luminance and carrier chrominance signals. Then the relationships between the light outputs , ~
~ and these functions are:
A

...... . . . . .
: ...... . . . . ~
.
. .

~073095 R = EyY{l+FR(e~)}~
G = Eyy{l+FG(e/~)}y ---------------(4) B = EyY{l~FB(e~a)}~

Analyzing equations (1), (2), (3) and (4), the three functions FR, FG and FB can be expressed as the functions of the amplitude e and phase ~ of the chrominance signal. Each function will hereinafter be called a "demodulation function".
Figs. 5a, 5b and 5c, respectively, show the red, green and blue demodulation functions for the recent phosphors and the higher color-temperature white. In Figs. 5a-5c, the abscissa indicates the amplitude of the carrier chrominance signal e normalized by the luminance signal Ey~ and the ordinate indicates the value of the demodulation function.
At e=0, each demodulation function has a certain value which corresponds to the shift of the reference white from 9300K+27MPCD
to the illuminant C. These demodulation functions indicate that the color-difference signal demodulators should have -~
certain kinds of non-linear characteristics for decreasing both luminance and chromaticity errors. ~
The characteristic of the recent R-Y demodulators, --shown by a dotted straight line in Fig. 5a, greatly exceeds the red demodulation function at a large value of e, although ~ ~t coincides with the function at the flesh tone typically ;~ reprqsented by e=0.133 and 0-123. The characteristic of the recent B-Y demodulator also exceeds the demodulation function in much the same manner as for R-Y. It will be noticed that the excess amplitude of the color-difference --signal causes the luminance errors.
This invention is based on the analytical findings as set forth above. One of the practical ways to approximate ; the demodulation functions is illustrated in Fig. 6. In ~, - 10 -. *, - . - . ... . . . .

--~ Fig. 6, curve A shows the characteristics of ~n R-Y de~odulator containing two different gains, which change at the amplitude for producing the flesh tone, and curves B and C, respectively, indicate the characteristics of G-Y and B-Y demodulators having a larger gain for a negative color-difference signal than for a positive one.
Fig. 7 is a schematic block diagram of a color-difference signal demodulation system using an embodiment of a color-difference signal modifying apparatus according 10 to this invention. A conventional color-difference signal ; demodulator 4 demodulates the carrier chrominance signal to obtain a color-difference signal, which is fed to a gain reduction means 16. A threshold signal producing means 15 is supplied with the luminance signal, and produces a threshold signal which is proportional to the luminance signal, and ; feeds the threshold signal to the gain reduction means 16.
The gain reduction means 16 reduces the grain so as to decrease a specific portion of the excessive color-difference signal above the threshold signal level to achieve character-20 istic A, as shown in Fig. 6.
Fig. 8 is a schematic circuit diagram, partially in block form, of a system using an embodiment of a color-difference signal modifying apparatus of this invention for the recent phosphors and the higher color-temperature white. The dotted block 14 is a DC voltage producing means.
The color-difference signals obtained at the outputs of the conventional color-difference signal demodulators 4, 5 and 6 are added to each other through resistors 20, 21 and 22.
If the resistances rR, rG, and rB of the resistors 20, 21 and 22 satisfy the following equation, the color-difference signals are eliminated at the base of a transistor 25.

~. , . ~. , -, ;,~ , ~ :

(rB/BB)sin(~B-~G) =(rG/BG)sin(~R-~B) =
(rg/Bg)Sin(~G~~R) where BR, BG and BB are the demodulating gains, and ~R~ ~G and BB are the demodulating angles of the conventional demodulators 4, S and 6. Conse~uently, a reference DC voltage which is always equal to the stationary voltage of the demodulators is obtained in spite of the existence of the color-difference signals. The reference DC voltage is applied to the base of the transistor 25. A capacitor 24 connected between the base of the transistor 25 and ground provides a bypass for any transient pulse which otherwise might be applied to transistor 25 due to an abrupt change of a color-difference signal. The transistor 25 is an emitter follower amplifier with an emitter load resistor 26. The voltage at the emitter of the transistor 25 is almost equal to the reference DC ~-voltage. ~ -A diode 37 in the gain reduction means 16 is connected in series with a resistor 40 between the transistor 25 and - an input of the blue matrix circuit 9. The cathode of the diode 37 is connected to the emitter of the transistor 25.
The output voltage of the B-Y demodulator 6 is applied . .
~, through a resistor 43 to the input of the blue matrix circuit 9. When the B-Y signal is positive, the diode 37 is in the "ON" state, because the cathode of the diode 37 is held at the reference DC voltage. In this case, the positive B-Y signal is divided by the resistors 43 and 40, and fed to the blue matrix circuit 8. The same operation as for the ~-Y signal is performed for the G-Y signal by a diode 38, a resistor 41, the green matrix circuit 8, the G-Y demodulator 5 and a resistor 44.

~.~
~5i ~ 1073095 -The dotted block 15 is the threshold signal producing means. A black-positive luminance signal is fed to a terminal 34 and applied through a resistor 33 to the base of a transistor 29. The black-negative luminance signal appears across a collector load resistor 28. The bias circuits of the transistor 29 are comprised of the resistors 30, 31 and 32. A diode 27 between the emitter of the trans-ist~r 25 and the collector of the transistor 29 is a threshold limiting means which eliminates blanking pulses contained in the signal at the collector of the transistor 29. The signal at the collector of the transistor 29 is proportional to the luminance signal, and is fed through the emitter follower, which comprises a transistor 35 and a resistor 36 to the cathode of a diode 39 in the gain reduction means 16. The signal appearing at the cathode of the diode 39 becomes the threshold signal for the red color-difference signal R-Y. If the R-Y signal exceeds the threshold signal level which is proportional to the luminance signal, the diode 39 turns "ON" and the excessive color-difference signal is decreased by resistors 42 and 45 to achieve the non-linear characteristic A as shown in Fig. 6.
If the blanking pulses were not removed by the diode 27, the voltage of the cathode of the diode 39 during a blanking period would be much lower than the stationary voltage of the color-difference signal demodulator 4.
Therefore, the diode 39 would turn "ON", and the output `
voltage of the color-difference signal demodulator 4 would be lowered during a blanking period. Usually, in the color-difference signal demodulator, the output voltage during a blanking period is compared with a standard voltage and controlled so as to stay at the stationary voltage. If the output voltage during a blanking period were decreased by -.. ...
. ~ , ~ . .

the blanking pulse, the control circuit in the color-difference signal demodulator would not work correctly and would harm the white balance of an image seriously.
Although this example is capable of accurate chromaticity and luminance reproduction, optimum color ren-dition sometimes depends on viewers' preferences. Therefore, this example has a switch 46 which is able to prohibit the reduction of color-difference signals. When the switch 46 is turned "OFF", the cathode voltages of the diodes 37, 38 and 39 ar~e almost equal to the voltage +VB so as to keep them in the "OFF" state. The same function would be obtained by disconnecting the diode 37 and the resistor 40, the diode 38 and the resistor 41, and the diode 39 and the resistor 42, but it would need a more complicated switch. Moreover, the ; switch would be located far from the circuit board, and it would need long wires to connect them. The long wires would be very likely to pick up interferences due to the impedances of the resistors 40, 41 and 42. The apparatus of this example is stable, since the switch 46 is in the ground line. Moreover, the switch is a simple switch 46 having only two terminals to control three color-difference signals.
Fig. 9 shows the chromaticity errors of colors reproduced in the apparatus of this example. It will be seen that the chromaticity errors are smaller than those of the ~
conve~tional demodulator as shown in Fig. 3. In addition, - --Fig. 10 indicates that the luminance errors of reproduced colors in this example are very much smaller than those from the conventional demodulator shown in Fig. 4.
.
Thus far the description has been limited to the case in which the temperature of the reference white is rather high, 9300K~27MPCD. If the illuminant C is used as the reference white, the proportion constant K shown in ``" 1073095 Fig~ 6 bec~mes zero and the threshold signal producing means 15 in Fig. 7 can be replaced by other DC voltage producing means. This is a particular case among examples of this invention. With almost the same method as previously des-cribed, the demodulation functions are obtained for a com-bination system of the recent phosphors and the illuminant C as the reference white. Solid lines A, B and C in Fig. ll respectively show the red, green and blue demodulation functions for this system. Since these functions are zero at e=0,-they can be approximated by dotted lines in Fig. 11, the gains of which change at the origin.
A schematic circuit diagram, partially in block form, of a system using an example of the apparatus of this invention having the characteristics described above is shown in Fig. 12, where the identical blocks and components shown in Fig. 8 are denoted by the same numerals. The threshold signal which is a reference level to decrease the R-Y signal does not vary with the luminance signal, but stays at the stationary level of the R-Y signal demodulator 4. This means that the use of the illuminant C as the ref-.~ erence white makes the proportion constant of the threshold signal equal to zero and simplifies the circuitry of the apparatus of this invention.
The examples of the apparatus of this inventionwhich are shown in Figs. 8 and 12 make it possible to achieve accurate color reproduction with respect to chromaticity and luminance. However, there remains a small chromaticity error around the color bluish-green, for instance, green grass and leaves. Fig. 13 shows another example of the apparatus of this invention which further improves color reproduction, especially the chromaticity errors around bluish-green. The dotted block 50 is a reference signal .
h -15 -.

`` 1073095 producing means. The signals from the R-Y and B-Y demodulators 4 and 6 are added to each other to obtain a reference signal which is proportional to an inverted G-Y signal.
This signal is fed to the base of a transistor 62 which is part of an emitter follower amplifier together with a load resistor 63. The diode 37 and the resistors 40 and 43 are identical with those designated by the same numerals in Figs. 8 and 12. Therefore, the B-Y signal is decreased when it exceeds the cathode voltage of the diode 37 which is proportiona~ to the inverted G-Y signal. If this clrcuit is included in the embodiment shown in Fig. 8 and/or Fig. 12, the color around bluish green is also improved.
As described hereinbefore in detail, this invention is based on the demodulation functions which are the ideal characteristics for color-difference signal demodulators.
The ideal characteristics produce a color without chromaticity and luminance errors. The apparatus shown hereinbefore is designed so that the characteristics thereof are such as to achieve approximately the ideal demodulation functions by 20 the use of non-linear operation. ~ ~ -Although the foregoing description is concerned with the NTSC system, the apparatus can be adopted to other color television systems, such as the PAL and SECAM systems.
Having described this invention as related to the embodiments shown in the accompanying drawings, it is intended that this invention should not be limited to the details of description, unless otherwise specified, but should rather be construed broadly within its spirit and scope as set out in the following claims.

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Claims (6)

1. Claim 1. A color difference signal modifying ap-paratus for modifying a color difference signal in a color television receiver, the apparatus comprising a threshold signal producing means for being supplied with a luminance signal and for producing a threshold signal which is propor-tional to said luminance signal; a color difference signal demodulator for producing a color difference signal; and a gain reduction means coupled to said threshold signal produc-ing means and to said color difference signal demodulator for reducing the gain of the color difference signal by at least a portion of the amount of the difference between said color difference signal and said threshold signal only when said color difference signal exceeds said threshold signal.
2. Claim 2. A color difference signal modifying ap-paratus as claimed in claim 1 which further comprises a switch connected between said threshold signal producing means and ground for preventing the reduction of said color difference signal when said switch is in the "OFF" state.
3. Claim 3. A color difference signal modifying ap-paratus as claimed in claim 1 wherein said threshold signal producing means comprises a threshold limiting means for eliminating a portion of said threshold signal lower than a stationary level.
4. Claim 4. A color difference signal modifying ap-paratus as claimed in claim 1 which further comprises a DC
   voltage producing means for receiving the outputs of three color difference signal sources, and for producing a reference DC voltage equal to a stationary voltage of said color difference signal sources by adding the three outputs, and a signal reduction means for receiving at least one of the color difference signals other than the red color dif-ference signal and coupled to said DC voltage producing means and receiving the reference DC voltage for attenuating said other color difference signal by a portion of the amount of the difference between said other color difference signal and said reference DC voltage.
5. Claim 5. A color difference signal modifying ap-paratus as claimed in claim 1 which further comprises a refer-ence signal producing means for being supplied with the red and blue color difference signals and for producing a refer-ence signal by adding said two color difference signals, and a signal reduction means for receiving said blue color differ-ence signal and coupled to said reference signal producing means for attenuating said blue color difference signal by a portion of the amount of the difference between said blue color difference signal and said reference signal.
6. Claim 6. A color difference signal modifying ap-paratus as claimed in claim 1 which further comprises a DC
   voltage producing means for producing a reference DC voltage equal to a stationary voltage of the color difference signal sources, and a signal reduction means coupled to said DC vol-tage producing means for receiving at least one of the color difference signals and for attenuating the said one color difference signal by a portion of the amount of the difference between said color difference signal and said reference DC
   voltage.