Color temperature control circuit using saturation level detector

A color television receiver is disclosed, which includes a circuit for detecting a color saturation level of a color from red, green and blue primary color signals or from color difference signals and a color temperature control circuit for increasing at least a blue color component on a screen of a color cathode ray tube in accordance with an increase of the color saturation level, the color temperature control circuit being controlled by the output of the color saturation level detecting circuit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention generally relates to color television receivers and, more 
particularly, to a color temperature control or correction for a color 
television receiver. 
2. Description of the Prior Art 
In an NTSC (national television standard committee) system, chromaticity 
close to the blackbody radiation at a color temperature of 6740.degree. K. 
is selected to be a standard white color at the image transmitter side. 
Accordingly, the reference white color upon adjusting the white balance of 
a color picture tube has to be selected at this color temperature 
fundamentally. 
However, in practice, the white color on the picture screen of the color 
cathode ray tube which is adjusted as described above is perceived as 
remarkably "reddish white" under, for example, the illumination of a 
fluorescent lamp having high color temperature. The reason for this is 
that human eyes follow illumination light and recognize "bluish white" 
having high color temperature around the color cathode ray tube as "pure 
white". 
As described above, goodness or badness of the white on the picture screen 
of the color cathode ray tube greatly depends on the observation condition 
and also on the preference of each viewer. Generally, it is said that 
white color having color temperature higher than that of the illumination 
light is preferred. 
While, if the color temperature is set high regardless of the content of a 
picture image, there arises a problem that a chromatic color, particularly 
flesh color, becomes bluish and hence unnatural color. 
Therefore, in the art it has been proposed to control or correct the color 
temperature in response to the brightness of a color video signal. 
FIG. 1 is a block diagram showing an example of the prior art. In FIG. 1, a 
composite color video signal is applied to an input terminal 1 and then 
fed to a Y/C separating circuit 2 where it is separated to a luminance 
signal Y and a chrominance signal C. The chrominance signal C is 
color-demodulated by a color demodulating circuit 3 which then produces 
red, green and blue color difference signals R-Y, G-Y and B-Y, 
respectively. These color difference signals R-Y, G-Y and B-Y are supplied 
to a matrix circuit 4. The matrix circuit 4 is also supplied with the 
luminance signal Y from the Y/C separating circuit 2 through a contrast 
adjusting variable resistor 5. Thus, the matrix circuit 4 produces red, 
green and blue primary color signals R, G and B. These red, green and blue 
primary color signals R, G and B are, respectively, supplied through gain 
control circuits 6R, 6G and 6B to the cathodes of a cathode ray tube 7. 
The input vs. output characteristics of the gain control circuits 6R, 6G 
and 6B are respectively shown in FIGS. 2A, 2B and 2C. 
That is, the red primary color signal R is amplified linearly relative to 
the input. However, although the green primary color signal G and the blue 
primary color signal B are amplified linearly when the inputs are lower 
than a predetermined level, when the inputs are higher than the 
predetermined level, the green primary color signal G and the blue primary 
color signal B are respectively amplified so as to emphasize green and 
blue colors. In this case, the blue primary color signal B is emphasized 
much, while the green primary color signal G may be emphasized a little. 
In this case, the above predetermined level is selected to correspond to a 
portion of high brightness, for example, the telop portion of a white 
character on a picture screen. The telop portion refers to a character 
display superimposed upon the television picture on the screen, for 
example, to display advertising information, announcements, stock market 
prices and the like. 
Thus the telop portion of the white character and the like are made bluish, 
which is equivalent to the case that the color temperature is raised, 
white color being reproduced without being reddish. In addition, in the 
portion of the brightness less than the predetermined level, under the 
standard color temperature, for example, 9300.degree. K., each primary 
color signal is supplied to the cathode ray tube so that a flesh color or 
the like can be prevented from being made bluish. 
However, according to such prior art color temperature control or 
correction system, only the portion of high brightness level such as a 
portion of a white character and the like can be controlled or corrected 
and a white color having low brightness level can not be corrected. The 
reason for this is that this prior art system carries out the color 
temperature control or correction not for the color saturation level but 
for the brightness. 
Further, since the green and blue primary color signals are passed through 
the gain control circuits 6G and 6B which are non-linear circuits, there 
is then a defect that a non-linear characteristic causes color shading. 
In addition, there is a further defect that the color temperature is 
changed by the adjustment of contrast. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide an improved color 
television receiver, 
It is another object of this invention to provide a color television 
receiver capable of carrying out color temperature control or correction 
so as to produce a chromatic color as natural color and to produce a 
beautiful white color on the picture screen of a color cathode ray tube. 
It is a further object of this invention to provide a color television 
receiver having a simplified construction. 
According to one aspect of this invention, there is provided a color 
television receiver comprising: 
means for detecting a color saturation level of a color from red, green and 
blue primary color signals or from color difference signals; and 
color temperature control means for increasing at least a blue color 
component on a screen of a color cathode ray tube in accordance with 
increase of said color saturation level, said color temperature control 
means being controlled by the output of said color saturation level 
detecting means. 
According to another aspect of this invention, there is provided a color 
television receiver comprising: 
a matrix circuit for producing red, green and blue primary color signals at 
three output terminals respectively, said matrix circuit being supplied 
with a luminance signal and three color difference signals; 
means for detecting a color saturation level of a color from said red, 
green and blue primary color signals which are supplied thereto; and 
means provided on green and blue primary color signal lines for increasing 
the level of said blue and green primary color signals, said level 
increasing means being controlled by the output of said color saturation 
level detecting means. 
According to a further aspect of this invention, there is provided a color 
television receiver comprising: 
a matrix circuit for producing red, green and blue primary color signals in 
negative polarity at three output terminals, respectively, said matrix 
circuit being supplied with luminance signal and color difference signals; 
first, second and third transistors whose bases are connected to said three 
output terminals, respectively, whose emitters are connected in common to 
the ground, and whose collectors are connected in common to a power supply 
source; and 
subtracting means respectively provided on a blue primary color signal line 
and a green primary color signal line for subtracting the output of said 
transistors from said blue primary color signal and green primary color 
signal, respectively. 
According to a yet further aspect of this invention, there is provided a 
color television receiver comprising: 
a color demodulator supplied with a chroma signal and for producing three 
color difference signals; 
a matrix circuit for producing red, green and blue primary color signals in 
negative polarity at three output terminals, respectively, said matrix 
circuit being supplied with a luminance signal and said three color 
difference signals from said color demodulator; 
first, second and third transistors whose bases are supplied with said 
three color difference signals, respectively, whose emitters are connected 
in common to the ground, and whose collectors are connected in common to a 
power supply source; and 
subtracting means respectively provided on a blue primary color signal line 
and a green primary color signal line for subtracting the output of said 
transistors from said blue and green primary color signals, respectively. 
The other objects, features and advantages of the present invention will 
become apparent from the following description taken in conjunction with 
the accompanying drawings through which the like references designate the 
same elements and parts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, the present invention will hereinafter be described in detail with 
reference to the drawings. 
FIG. 3 is a block diagram showing a principle embodiment of a color 
television receiver according to this invention. In FIG. 3, like parts 
corresponding to those in FIG. 1 are marked with the same references and 
will not be described in detail. 
First, the principle of this invention will be described and this principle 
is based on the following consideration. 
That is, on the chromaticity diagram as shown in FIG. 4, the color 
temperature control or correction is inherently carried out such that the 
control or correction is not performed for each primary color but the 
controlling or correcting amount thereof must be increased as approaching 
the white W (namely, in accordance with the arrow directions in FIG. 4). 
The most accurate method for obtaining such control or correction signal 
is to detect a color saturation level. The color temperature can be 
controlled or corrected by the signal thus detected. Because, the color 
saturation level indicates a ratio in which each color is lightened by 
white color. 
This invention is based on the above consideration and so, in the 
embodiment as shown in FIG. 3, red, green and blue primary color signals 
R, G and B delivered from the matrix circuit 4 are all supplied to a color 
saturation level detecting circuit 11 which detects the color saturation 
level of each color signal. The detected output is supplied through level 
adjusting circuits 12G and 12B to multiplying ciccuits 13G and 13B. The 
multiplying circuit 13G is also supplied with the green primary color 
signal G and the multiplying circuit 13B is also supplied with the blue 
primary color signal B. Then, such a control or correction is carried out 
that in response to the color saturation level, or as the color saturation 
level becomes high, the levels of the green and blue primary color signals 
G and B become high. In this case, the red primary color signal R is not 
corrected or controlled. Further, the correction or control amounts are 
adjusted by the level adjusting circuits 12G and 12B such that the control 
or correction amount of the green primary color signal G is made smaller 
than that of the blue primary color signal B. In addition, it may be 
possible that the green primary color signal G is not corrected but only 
the blue primary color signal B is corrected. 
Thus, since the blue and green color signal components are corrected to 
become large as the color saturation levels thereof become high, the 
correction is performed such that the color temperature is just made high. 
As a result, white color is beautifully reproduced on the picture screen 
of the color cathode ray tube. While the color saturation level is low and 
the nearer the color approaches the primary color, the lower the 
correcting amount becomes so that the color temperature becomes close to 
the ordinary standard color temperature (for example, 9300.degree. K.), 
thus a chromatic portion never being made bluish but being presented with 
a natural color. 
By the way, if a circuit capable of accurately detecting the color 
saturation level is used as the color saturation level detecting circuit 
11, the circuit arrangement becomes very complicated. Therefore, a 
description will be given on an example in which the color saturation 
level can be easily detected and the color temperature can be controlled 
or corrected simply. 
The principle of this example will be described first. 
As is clear from the chromaticity diagram as shown in FIG. 5, all colors 
can be presented by three primary colors or red, green and blue colors. In 
other words, every color within the triangular region encircled by red, 
green and blue colors on the chromaticity diagram can be represented by 
the three primary colors. Accordingly, on the basis of similar 
consideration, a desired color can be represented by white color component 
W and two primary colors of three primary colors of red, green and blue. 
By way of example, a certain color C.sub.1 on the chromaticity diagram of 
FIG. 5 can be presented by the white color component W and red and green 
primary colors R and G. In this case, the white color component W is the 
amount of white color contained in a certain color, namely, the amount 
corresponding to the color saturation level thereof. 
While the principle of this example is described with reference to the 
chromaticity diagram of FIG. 5 as above, this principle will also be 
described with reference to voltage amplitudes E.sub.R, E.sub.G and 
E.sub.B of the respective primary color signals as follows. 
That is, a desired color can be presented by three primary color signal 
voltages E.sub.R, E.sub.G and E.sub.B as shown in FIG. 6. Here, let us 
consider the minimum value of three voltage amplitudes E.sub.R, E.sub.G 
and E.sub.B, namely, E.sub.W =Min (E.sub.R, E.sub.G, E.sub.B). This 
minimum value is considered to be the voltage amplitude corresponding to 
the white color component of the desired color and the desired color can 
be presented by three of amplitudes E.sub.W, E.sub.1 =E.sub.R and E.sub.2 
=E.sub.G. 
As described above, this voltage amplitude E.sub.W indicates the color 
saturation level so that the color saturation level can be detected by 
detecting the voltage amplitude E.sub.W. 
FIG. 7 shows an embodiment of the color television receiver according to 
this invention which takes the above principle into consideration. 
In this embodiment, each primary color signal derived from the matrix 
circuit 4 has the negative polarity. 
In this embodiment, as the color saturation level detecting circuit 11, 
there are provided three transistors 11R, 11G and 11B. The collectors of 
these transistors 11R, 11G and 11B are connected common and the emitters 
thereof are respectively grounded. The commonly connected collectors of 
the transistors 11R, 11G and 11B are connected through a resistor 14 to a 
power source terminal 15. Then, the red, green and blue primary color 
signals R, G and B from the matrix circuit 4 are respectively supplied to 
the bases of the transistors 11R, 11G and 11B and the output is derived 
from the common connection point of the collectors of these transistors 
11R, 11G and 11B. The output therefrom is supplied through the level 
adjusting circuits 12G and 12B to a subtracting circuit 16G for the green 
primary color signal G and a subtracting circuit 16B for the blue primary 
color signal B, respectively. 
In this case, while appropriate control or correction amounts are 
determined by the level adjusting circuits 12G and 12B, the correction 
amount for the green primary color signal may be very small unlike that 
for the blue primary color signal. 
As mentioned before, since the signals derived from the matrix circuit 4 
are the voltage signals of negative polarity (the white level has lower 
voltage), the base voltage of the transistor to which the minimum 
amplitude (the amplitude E.sub.W) of the amplitudes E.sub.R, E.sub.G and 
E.sub.B is supplied becomes highest and the output produced at the common 
connection point among the collectors of the transistors 11R, 11G and 11B 
is substantially dominated by the collector voltage of the transistor to 
which this voltage amplitude E.sub.W is supplied. Then, this output 
voltage becomes low when the voltage amplitude E.sub.W is small, while 
this output voltage becomes high when the voltage amplitude E.sub.W 
becomes large. Since this output voltage is supplied through the level 
adjusting circuits 12G and 12B to the subtracting circuits 16G and 16B, 
the control or correction voltage corresponding to the magntiude of the 
voltage amplitude E.sub.W is added to the green and blue primary color 
signals G and B with negative polarity, namely, so as to make the 
amplitudes thereof larger in the subtracting circuits 16G and 16B. In 
other words, when the amplitudes E.sub.W of the white color component 
contained in the chrominance signal C, or the color saturation level 
thereof is low (near the primary color), the correction voltages added in 
the subtracting circuits 16G and 16B with negative polarity are low, while 
when the amplitude E.sub.W becomes large and hence the color saturation 
level becomes higher (near the white color), the correction voltage to be 
added becomes high. That is, the higher the color saturation level 
becomes, the more the blue and green color components are increased. As a 
result, such correction is performed that the color temperature is 
increased. 
In order to detect the color saturation level, the color difference signals 
R-Y, G-Y and B-Y may be used as shown in FIG. 8. 
That is, this reason is as follows: 
EQU Min (E.sub.R, E.sub.G, E.sub.B) 
EQU =Min (E.sub.R-Y +E.sub.Y, E.sub.G-Y +E.sub.Y, E.sub.B-Y +E.sub.Y) 
EQU =Min (E.sub.R-Y, E.sub.G-Y, E.sub.B-Y)+E.sub.Y 
is established and calculating the minimum value of the primary color 
signal amplitude E.sub.R, E.sub.G and E.sub.B is equivalent to calculating 
the minimum value of the color difference signal amplitudes E.sub.R-Y, 
E.sub.G-Y and E.sub.B-Y. In this case, since the luminance component 
E.sub.Y is not contributable to the color saturation level, it is not 
necessary to take the luminance component E.sub.Y into consideration. 
In the above embodiments, it is possible that the green primary color 
signal is not corrected but only the blue primary color signal is 
corrected. 
According to this invention, since the color temperature correction or 
control is carried out in accordance with the color saturation level, the 
white color can be presented more beautifully and the chromatic colors are 
not made bluish and reproduced as natural color on the picture screen of 
the color cathode ray tube. 
Especially when the method for detecting the minimum value of the primary 
color signal amplitudes or the color difference signal amplitudes is used 
as the method for detecting the color saturation level, the circuit 
arrangement can be made very simple. 
The above description is given on the preferred embodiments of the 
invention, but it will be apparent that many modifications and variations 
could be effected by one skilled in the art without departing from the 
spirits or scope of the novel concepts of the invention, so that the scope 
of the invention should be determined by the appended claims only.