Display apparatus

Abstract: Manually handling keys, etc. are provided to supply color temperature coefficients to contrast and brightness control units, so that an adjusted ratio of the color temperature coefficients is generated independently of contrast and brightness control signals. Consequently, a color temperature which is once determined is maintained, even if contrast and brightness are changed.

Description: FIELD OF THE INVENTION This invention relates to a display apparatus such as a CRT display, a liquid crystal display, etc., and more particularly to, a display apparatus in which the quality of color images is improved. BACKGROUND OF THE INVENTION A conventional CRT display which is used in a television set, a personal computer, etc. is provided with knobs for controlling contrast and brightness of images on a screen thereof, so that the quality of the images is improved in accordance with the adjustment of contrast and brightness by use of the knobs. In recent years, a display apparatus in which the color tone (determined by the color temperature at the state of all white pattern display, and defined "color temperature" hereinafter) is adjusted in addition to the contrast and brightness is required to be put into practical use. In such a display apparatus, a desired color temperature is obtained to provide reddish white, bluish white, etc. Such demand is high in a display apparatus for an information processing apparatus in which still images are displayed on a screen thereof, although the demand is relatively low in a television set in which motion images are displayed on a screen thereof, so that the color temperature is observed difficult to be due to the motion thereof. In addition, the demand of controlling the color temperature is high, for instance, in a display apparatus of a computer graphics system, an apparel design system, etc., in which the reproducibility of colors is important, because colors are required to be displayed precisely on a screen thereof. In the conventional display apparatus, the color temperature is adjusted in a manufacturing factory by use of sub-contrast and sub-brightness knobs which are provided in the interior of the display apparatus, so that these knobs are not handled by a user. However, the conventional display apparatus has disadvantages in that specialized operation and a considerably long time is required in the adjustment of the color temperature in the manufacturing factory. For this reason, even if a color temperature knob which is handled by a user is provided at the exterior of the display apparatus, this is not a practical structure. In addition, it is considered that the color temperature must be adjusted for plural times, because the color temperature tends to change in a case where contrast and brightness are adjusted, after the color temperature is once adjusted. In other words, the so-called color tracking, in which the color temperature is not changed before and after the adjustment of contrast and brightness, when the color temperature is once adjusted, is required to be realized in a display apparatus. In patent abstracts of Japan Vol. 4, No. 56 (E-8) (538) there is disclosed a standard color display device in which either one of the brightness and color switches is selected via selectors and through a brightness selection switch, to select several units of brightness, and a switch to set the set white of the color TV and the adjustment allowance range out of the set white. The purpose therein is to obtain a device which can be used as the standard of the white balance adjuster covering from the low to high brightness. In WO-A-86 03087 there is disclosed a circuit arrangement for color television receivers, for regulating the contrast and color saturation, in which an amplifier is inserted in each of the color signal paths, the degree of amplification of which corresponds to the product of the amplification factor for color saturation and brightness contrast. This arrangement has the advantage that the parallelity control of three separate amplification factors can be dispensed with. Summary of the invention Accordingly, it is an object of the invention to provide a display apparatus in which the color temperature is adjusted independently to enhance the handling property. It is a further object of the invention to provide a display apparatus in which the color tracking is automatically obtained. The invention is described in claim 1. Further embodiments are described in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS This invention will be explained in more detail in conjunction with appended drawings, wherein: Fig. 1 is a block diagram showing a display apparatus of a first preferred embodiment according to the invention; Figs. 2A to 2D and 3A to 3D are explanatory diagrams explaining principle of the color tracking in the first preferred embodiment; Fig. 4 is a block diagram showing a circuit for generating color temperature coefficients in a display apparatus of a second preferred embodiment according to the invention; Fig. 5 is a flow chart explaining the generation of the color temperature coefficients in the second preferred embodiment; Fig. 6 is a block diagram showing a contrast control unit in the second preferred embodiment; Fig. 7 is a block diagram showing a brightness control unit in the second preferred embodiment; and Fig. 8 is a block diagram showing a display apparatus of a third preferred embodiment according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a display apparatus of the first preferred embodiment which comprises a contrast control unit 1, a brightness control unit 2, a contrast adjusting unit 3, a gain control circuit 4, a color temperature adjusting unit 5, a brightness adjusting unit 6, and a shift amount control circuit 7. In the contrast control unit 1, primary color signals Rin, Gin and Bin are supplied to linear amplifiers 1R, 1G and 1B to be amplified therein. In the brightness control unit 2, the color signals which are amplified in the linear amplifiers 1R, 1G and 1B are supplied to direct current (DC) shift circuits 2R, 2G and 2B, in which DC levels of the primary color signals Rin, Gin and Bin are shifted to provide DC shifted primary color signals Rout, Gout and Bout which are then supplied to cathodes of respective primary color signals for a cathode ray tube. As described later, the adjustment of contrast is carried out by changing gains of the linear amplifiers 1R, 1G and 1B, and the adjustment of brightness is carried out by changing DC shift amounts of the DC shift circuits 2R, 2G and 2B. The contrast is adjusted independently of the color temperature in accordance with the provision of the contrast adjusting unit 3, the gain control circuit 4, and the color temperature adjusting unit 5. In the contrast adjusting unit 3, a knob is handled to provide contrast information by an operator, so that a contrast control signal V CONT which is common to the primary color signals is generated to be supplied to the gain control circuit 4. In the color temperature adjusting unit 5, a knob is handled to provide a color temperature by an operator, so that color temperature coefficients K R , K G and K B for the respective primary color signals are generated to be supplied to the gain control circuit 4, from which the gain control signals G R (K R · V CONT ), G G (K G · V CONT ) and G B (K B · V CONT ) are supplied to the linear amplifiers 1R, 1G and 1B, respectively. Consequently, the original color signals Rin, Gin and Bin are amplified in the linear amplifiers 1R, 1G and 1B by gains which are determined by products of the gain control signals G R , G G and G B and an inherent gain A, that is, "A.K R .V CONT ", "A.K G .V CONT ", and "A.K B .V CONT ", respectively. In addition, the brightness is adjusted independently of the color temperature in accordance with the provision of the color temperature adjusting unit 5, the brightness adjusting unit 6, and the shift amount control circuit 7. In the brightness adjusting unit 6, a knob is handled to provide brightness information by an operator, so that a brightness control signal V BRI which is common to the primary color signals is generated to be supplied to the shift amount control circuit 7. In the color temperature adjusting unit 5, a knob is handled to provide a color temperature by an operator, so that color temperature coefficients K R , K G and K B for the respective primary color signals are generated to be supplied to the shift amount control circuit 7, from which the shift amount control signals S R (K R · V BRI ), S G (K G · V BRI ), and S B (K B · V BRI ) are supplied to the DC shift circuit 2R, 2G and 2B, respectively. Consequently, shift operation is carried out in the DC shift circuits 2R, 2G and 2B, so that black levels are set to be "V RCO - K R · BRI ", "V GCO - K G · BRI ", and "V BCO - K B · BRI ", respectively, where V RCO , V GCO and V BCO are cut-off levels of the cathode ray tube. In this first preferred embodiment, the color tracking is automatically obtained even at the state that contrast and/or brightness are changed. This will be explained in Figs. 2A to 2D and 3A to 3D. First, voltages applied to cathodes of a CRT will be explained in relation to contrast, brightness and color temperature, respectively. Fig. 2A shows exampled relations of cut-off levels V RCO , V GCO and V BCO to black levels V RW , V GW and V BW , respectively, for the primary color signals R, G and B. In the relations, the brightness is determined by a difference between a corresponding one of the cut-off levels V RCO , V GCO and V BCO and a corresponding one of the black levels V RB , V GG and V BB . On the other hand, the color temperature is determined by a ratio among intensities V R , V G and V B which are differences between the levels V RCO and V RW , V GCO and V GW , and V BCO and V RB . As understood in Fig. 2A, the intensities V R , V G and V B are sums of the brightness components and the contrast components for the respective original color signals R, G and B. Accordingly, even if the contrast and the brightness are changed, the color temperature is not changed, as long as a ratio K R :K G :K B is constant among the intensities V R , V G and V B for the primary color signals R, G, and B. In Fig. 2A, the color temperature is shown in a case where the ratio K R :K G :K B is 1:1:1 which is determined by the intensities V R , V G and V B . Hereinafter K R , K G and K B are defined as color temperature coefficients. In Fig. 2B, there is shown a case in which the contrast is decreased from 50V to 25V by 50%, as compared to that of Fig. 2A, while the color temperature is maintained by keeping the intensity ratio 1:1:1 (35V:35V:35V) unchanged. In Fig. 2C, there is shown a case in which the brightness is decreased from 10V to 5V by 50%, as compared to that of Fig. 2A, while the color temperature is maintained by keeping the intensity ratio 1:1:1 (55V:55V:55V) unchanged. In Fig. 2D, there is shown a case in which the contrast and the brightness are decreased from 50V to 25V and from 10V to 5V, respectively, by 50%, as compared to those of Fig. 2A, while the color temperature is maintained by keeping the intensity ratio 1:1:1 (30V:30V:30V) unchanged. In Fig. 3A, there is shown a case in which the color temperature is changed by changing the intensity ratio from 1:1:1 (60V:60V:60V) to 0.8:1:1 (48V:60V:60V), as compared to that of Fig. 2A. In this case, the maximum values of the contrast and the brightness have a ratio of 0.8 in the red signal R relative to those in the green and blue signals G and B. In this manner, it is preferable in regard to the adjustment of the contrast and the brightness that both are changed by the same proportion, but only one of the contrast and the brightness is not changed. In Fig. 3B, there is shown a case in which the contrast is decreased from 40V to 20V in the red signal R and from 50V to 25V in the green and blue signals G and B, respectively, by 50%, as compared to that of Fig. 3A, while the color temperature is maintained by keeping the intensity ratio 0.8:1:1 (28V:35V:35V) unchanged. In Fig. 3C, there is shown a case in which the brightness is decreased from 8V to 4V in the red signal R and from 10V to 5V in the green and blue signals G and B, respectively, by 50%, as compared to that of Fig. 3A, while the color temperature is maintained by keeping the intensity ratio 0.8:1:1 (44V:55V:55V) unchanged. In Fig. 3D, there is shown a case in which the contrast is decreased from 40V to 20V in the red signal R and from 50V to 25V in the green and blue signals G and B, respectively, by 50%, as compared to that of Fig. 3A, and the brightness is also decreased from 8V to 4V in the red signal R and from 10V to 5V in the green and blue signals G and B, respectively, by 50%, while the color temperature is maintained by keeping the intensity ratio 0.8:1:1 (24V:30V:30V) unchanged. Changing view points, the cases of Fig. 3B to 3D are regarded that the color temperature is changed, as compared to the cases of Fig. 2B to 2D. As explained above, each of the intensities V R , V G and V B is determined by a sum of a corresponding contrast and a corresponding brightness, and the ratio K R :K G :K B of the intensities V R , V G and V B is well maintained by equalizing a ratio of the contrast values for the color signals R, G and B and a ratio of the brightness values for the color signals R, G and B to the intensity ratio. As explained in Fig. 1, when the gains of the linear amplifiers 1R, 1G and 1B are set to be "A · K R · V CONT ", "A · K G · V CONT ", and "A · K B · V CONT ", respectively, the color temperature is maintained in accordance with the unchanged intensity ratio K R :K G :K B , even if the contrast is changed. In addition, when the shift amounts of the DC shift circuits 2R, 2G and 2B are set to be "K R · V BRI ", "K G · V BRI ", and "K B · V BRI ", respectively, the color temperature is maintained in accordance with the unchanged intensity ratio K R :K G :K B , even if the brightness is changed. As clearly understood from the above explanation, the gains of the linear amplifiers 1R, 1G and 1B define a difference of the black and white levels, and the shift amounts of the DC shift circuits 2R, 2G and 2B define a difference of the cut-off and black levels. In this first preferred embodiment, the intensities V R , V G and V B of the primary color signals R, G and B are defined by the equations (1), (2) and (3). V R = K R (V CONT · A + V BRI ) V G = K G (V CONT · A + V BRI ) V B = K B (V CONT · A + V BRI ) As apparent from the equations (1), (2) and (3), the color temperature is maintained in accordance with the intensity ratio K R :K G :K B independently of the contrast and brightness control signals V CONT and V BRIT . At the same time, the color temperature is changed by use of the color temperature adjusting unit 5. As a matter of course, the color temperature is not necessary to be re-adjusted, even after the contrast and the brightness are changed. Therefore, a desired color temperature is obtained in a short time by simple operation. Next, a display apparatus of the second preferred embodiment according to the invention will be explained. Even in this second preferred embodiment, the color tracking is realized in accordance with the aforementioned equations (1) to (3). Fig. 4 shows a hardware structure for generating color temperature coefficients K R , K G and K B which is used in the second preferred embodiment. This hardware structure comprises a color temperature coefficient generating unit 10 including a microprocessor and a memory, up-keys 11RU, 11GU and 11BU for increasing the proportion of the color temperature coefficients K R , K G and K B , and down keys 11RD, 11GD and 11BD for decreasing the proportion of the color temperature coefficients K R , K G and K B . The color temperature coefficient generating unit 10 generates the color temperature coefficients K R , K G and K B in accordance with the processing as shown in Fig. 5, when any one of the up keys 11RU, 11GU and 11BU, and the down keys 11RD, 11GD and 11BD is pressed. In operation, it is determined which key is pressed, and to which primary color the key is related among the primary color signals R, G and B, when any one of the up keys 11RU, 11GU and 11BU, and the down keys 11RD, 11GD and 11BD is pressed, the processing steps are then carried out for an primary color signal selected from the primary color signals R, G and B (step "SP1"). In the following description, the processing of the red color signal R will be explained, and this will be applied to the processing of the green and blue color signals G and B. When it is determined that a key for the red color signal R is pressed, it is then determined whether the pressed key is an up or down key (step "SP2R"). When the up key 11RU is pressed, it is determined whether a parameter "αR" is zero or not (step "SR3R"). Here, the parameter "αR" is used to determine a color temperature coefficient K R by subtracting the parameter "αR" from the maximum value, for instance, 1000 of the color temperature coefficient K R . Therefore, it is said that the decision of the step "SP3R" is a decision as to whether the color temperature coefficient K R is the maximum value or not at the very moment. If the parameter "αR" is not zero, the parameter "αR" is decremented to provide a parameter "αR-1" by one (step "SR4R"). On the other hand, the parameter "αR" is zero, the color temperature coefficient K G or K B is decreased, because the color temperature coefficient K R can not be increased any more (step "SP9R" to "SP10R"), so that a red color component of the color temperature is increased relatively. That is, if both parameters "αG" and "αB" for green and blue colors are not the maximum value, the parameters "αG" and "αB" are incremented to provide parameters "αG+1" and "αB+1", respectively, by one, and the step "SP17" and following steps are then carried out. On the other hand, if only one of the parameters "αG" and "αB" is not the maximum value, the parameter "αG" or "αB" having no maximum value is incremented by one, and the step "SP17" and the following steps are carried out. Otherwise, if both the parameters "αG" and "αB" are the maximum value, the output of the color temperature coefficients K R , K G and K BR is not carried out, and the processing is returned to a main routine of the flow, because the red component can not be increased relatively. On the contrary, when the pressed key is determined to be the down key 11RD (step "SP2R"), it is determined whether the parameter "αR" is zero or not (step "SP11R"). If the parameter "αR" is not zero, it is determined whether the parameter "αR" is the maximum value or not (step "SP12R"). When the parameter "αR" is the maximum value, the processing is returned to the main routine of the flow, because a red color component of the color temperature can not be decreased. On the other hand, when the parameter "αR" is not the maximum value, the parameter "R" is increased by one, and the processing is shifted to the step "SP13R". When it is determined that the parameter "αR" is zero, it is determined whether at least one of the parameters "αG" and "αB" is zero or not (steps "SP14R" and "SP15R"). In this decision, "YES" is met, the parameter "αR" is incremented by one, and the processing is shifted to the step "SP17" (step "SP13R"). On the other hand, when both the parameters "αG" and "αB" are not zero, the parameters "αG" and "αB" are decremented by one, and the processing is shifted to the step "SP17" (step "SP16R"). The up processing of the red color component (steps "SP3R" to "SP10R") and the down processing thereof (steps "SP11R" to "SP16R") are not symmetrical, because it is assumed that at least one of the color temperature coefficients K R , K G and K B is the maximum value. When at least one of the color temperature coefficients K R , K G and K B is set to be the maximum value, the maximum number of color temperatures are defined, and the most bright screen is obtained under a state a color temperature is designated. As explained later, each of a contrast control unit includes a multiplication type digital to analog converter, so that the color temperature coefficients K R , K G and K BR is the maximum value. When at least one of the color temperature coefficients K R , K G and K BR is set to be the maximum value, the maximum number of color temperatures are defined, and the most bright screen is obtained under a state a color temperature is designated. As explained later, each of a contrast control unit and a brightness control unit includes a multiplication type digital to analog converter, so that the color temperature coefficients K R , K G and K B are supplied from the color temperature coefficient generating unit 10 to the contrast and brightness control units so as to be adapted to the contrast and brightness control units. At the step "SP17", a flag is set to designate a color temperature coefficient which is supplied to the contrast and brightness control units. Then, one of the parameters aR, aG and aB relating to the flag is read out, and a corresponding color temperature coefficient (one of K R , K G and K BR ) is calculated by subtracting the parameter from the maximum value thereof (step "SP18" and "SP19"). Thereafter, the output of channel data is carried out, and it is made clear to which primary color signal the color temperature coefficient relates. At the same time, a strobe signal becomes significant, and each bit of the color temperature coefficient is supplied in serial to the contrast control unit by a clock signal (step "SP20" to "SP24"). When the output of all bits of the color temperature coefficient finishes, the strobe signal becomes insignificant. In this manner, when the output of the color temperature coefficient finishes for one primary color, it is determined whether the output of the color temperature coefficients K R , K G and K BR finishes or not for the primary color signals R, G and B. When this output operation does not finish completely, the processing is returned to the step "SP17", so that the output of the color temperature coefficient continues for the remaining primary color signal. Finally, when the output of the color temperature coefficients finishes for all the primary color signals R, G and B, the processing is returned to the main routine of the flow (step "SP26"). Next, Fig. 6 shows the contrast control unit used in the display apparatus of the second preferred embodiment. In the contrast control unit, the color temperature coefficients K R , K G and K BR , each of which is classified to designate one of the primary color signals R, G and B by the channel data, are supplied to a multiplication type digital to analog converter 20 which includes a data receiving unit 21, and three multipliers 22R, 22G and 22B. The data receiving unit 21 determines a primary color signal to which a serially received color temperature coefficient relates and converts the color temperature coefficient to a parallel color temperature coefficient which is supplied to a corresponding one of the multipliers 22R, 22G and 22B. The contrast control unit further comprises a volume VR 1 for supplying a contrast control signal through a voltage follower circuit including an operational amplifier OP 1 , a transistor Q 1 , and a resistance R 1 commonly to the multipliers 22R, 22G and 22B. Thus, the multipliers 22R, 22G and 22B supply gain control signals "K R · V CONT ", "K G · V CONT " and "K B · V CONT " to amplifying stages for the primary color signals R, G and B. The amplifying stages include linear attenuators 23R, 23G and 23B, pre-amplifiers 24R, 24G and 24B, and main amplifiers 25R, 25G and 25B, wherein the attenuation factor is changed for the linear attenuators 24R, 23G and 23B by the gain control signals "K R · V CONT ", "K G · V CONT " and "K B · V CONT ", and gains of the pre-amplifiers 24R, 24G and 24B, and the main amplifiers 25R, 25G and 25B are fixed, so that input signals Rin, Gin and Bin for the primary color signals R, G and B supplied to the linear attenuators 24R, 23G and 23B are amplified in the amplifying stages to provide amplified output signals by gains of "A · K R · V CONT ", "A · K G · V CONT ", and "A · K B · V CONT ". Practically, it is hard to adjust a gain linearly at a video signal band by use of a single amplifier which is controlled by a gain control signal. Therefore, an amplifier having a fixed gain is combined with a linear attenuator, so that a linear amplifier is obtained as a whole. Further, Fig. 7 shows the brightness control unit which is used in the display apparatus of the second preferred embodiment. In this brightness control unit, a multiplication type digital to analog converter 30 comprises a data receiving units 31, and three multipliers 32R, 32G and 32B, wherein the data receiving unit 31 determines an primary color signal to which a serially received color temperature coefficient relates, and converts the color temperature coefficient to a parallel color temperature which is supplied to a corresponding one of the multipliers 32R, 32G and 32B. The brightness control unit further comprises a brightness volume VR 2 for supplying a brightness control signal through a voltage follower circuit of an operational amplifier OP 2 commonly to the multipliers 32R, 32G and 32B. The multipliers 32R, 32G and 32B supply voltages "K R (V BRI - V E ) + V E ", "K G (V BRI - V E ) + V E ", and "K B (V BRI - V E ) + V E " to DC clamp stages, wherein the voltage V E which is obtained in a division of a power supply voltage by resistances R 2 and R 3 is applied through a voltage follower circuit of an operational amplifier OP 3 to a reference voltage input terminal of the multiplication type digital to analog converter 30. The voltage V E is not equal to a cut-off level, but corresponds to the cut-off level. In this brightness control unit, the primary color signals which have been already controlled in contrast are passed through capacitances C 1R , C 1G , C 1B provided on output lines L R , L G , and L R which are connected to cathode terminals of a CRT, so that direct current components of the primary color signals to be applied to the cathode terminals are removed. The DC clamp stage for the red signal will be explained. The output voltage "K R (V BRI - V E ) + V E " of the multiplier 32R is applied through a voltage follower circuit of an operational amplifier OP 4R to a modal point aR. Resistances R 4R and R 5R , a collector and an emitter of an NPN transistor Q 2R , a resistance R 6R and a variable resistance R 7R are connected between the nodal point aR and a power supply voltage line +B. The transistor Q 2R provides a voltage follower circuit together with an operational amplifier OP 5 common to the primary color signals R, G and B, and a potential of the emitter of the transistor Q 2R is the divided voltage V E of the resistances R 2 and R 3 at a nodal point bR which is connected through a resistance R 8R and a variable resistance R 9R to ground. Further, a nodal point cR between the resistances R 4R and R 5R is connected through a charge and discharge capacitance C 2R to ground. A potential of the capacitance C 2R defines an intensity V R which is determined by a collector current of the transistor Q 2R . This collector current is a sum of a current I RR flowing through a resistance R 8R and a variable resistance R 9R and a current I R flowing through a resistance R 6R and a variable resistance R 7R . The current I RR is a fixed value, after the variable resistance R 9R is adjusted to be fixed, because the potential V E is fixed at the nodal point bR. That is, the resistance R 8R and the variable resistance R 9R is used for determining the fixed level having a connection with the intensity V R , so that a cut-off level is adjusted. The current I R is determined by a voltage K R · (V BRI - V E ), after the variable resistance R 7R is adjusted to be fixed. This variable resistance R 7R functions as a sub-brightness adjusting resistance. Accordingly, the collector current "I RR + I R " and the intensity V R are changed only by the output voltage "K R (V BRI - V E ) + V E " of the multiplier 32R, and these changes are proportional to the voltage change "K R (V BRI - V E )". Here, the change of the intensity V R which is carried out without the change of a contrast component is identical to the change of brightness. Thus, the direct current shift can be carried out in accordance with the color temperature coefficient K R and the brightness control signal V BRI . In addition, there are provided a resistance R 10R connected between the output line L R and the power supply voltage line +B, a PNP transistor Q 3R connected at a base to the nodal point CR, a diode D 1R connected at an anode to the output line L R and at a cathode to an emitter of the transistor Q 3R , and a parallel connection of a resistance R 11R and a capacitance C 3R positioned between a collector of the transistor Q 3R and ground. Even in this second preferred embodiment, the color temperature can be varied by use of a handling structure for directly designating the color temperature. Even more, the color temperature is maintained under a state that the contrast and/or the brightness are adjusted, so that the re-adjustment of the color temperature becomes unnecessary. Therefore, a desired color temperature is obtained in a short time by simple operation. A display apparatus of the third preferred embodiment according to the invention will be explained. Fig. 8 shows a contrast control unit used in the display apparatus of the third preferred embodiment. In the contrast control unit, color temperature coefficients K R , K G and K B of digital signals are generated in a color temperature coefficient generating unit (not shown) in accordance with signals from handling knobs (not shown) for varying the color temperature, and the color temperature coefficients K R , K G and K B are converted from digital data to analog data in a digital to analog converter to be supplied to analog multipliers 40R, 40G and 40B. A contrast control signal V CONT which is supplied from a contrast volume (not shown) including usually an impedance conversion circuit is also supplied commonly to the analog multipliers 40R, 40G and 40B. As a result, multiplied signals "K R · V CONT ", "K G · V CONT ", and "K B · V CONT " are obtained to be supplied to linear amplifiers 41R, 41G and 41B, in which input primary color signals Rin, Gin and Bin are amplified by using the multiplied signals as gain control signals. The linear amplifiers 41R, 41G and 41B are connected to clamp pulse injecting stages comprising resistances R 20R , R 20G and R 20B , NPN transistors Q 10R , Q 10G and Q 10B , resistances R 21R , R 21G and R 21B , and switches SW R , SW G and SW B , correspondingly, positioned between a power supply voltage line and ground. In these clamp pulse injecting stages, when the switches SW R , SW G and SW B are turned on, amplified output signals of the linear amplifiers 41R, 41G and 41B are supplied to bases of the transistors Q 10R , Q 10G and Q 10B , so that the output signals are inverted to be amplified therein and supplied from collectors thereof to following stages. On the other hand, when the switches SW R , SW G and SW B are turned off, output signals of the power supply voltage are supplied to the following stages for the off duration. In this manner, clamp pluses are injected along color signal lines to be used in direct current clamp circuits (to be explained later). Here, the injection of the clamp pulses is carried out during horizontal and vertical blanking periods. In this third preferred embodiment, the input primary color signals Rin, Gin and Bin are inverted in the clamp pulse injecting stages, so that the white level is higher in polarity than the black level. Referring to the aforementioned equations (1) to (3), total gains of the linear amplifiers 41R, 41G and 41B and the clamp pulse injecting stages are required to be equal to "A · K R · V CONT ", "A · K G · V CONT " and "A · K B · V CONT ", respectively. As explained above, the contrast control unit is composed of the multipliers 40R, 40G and 40B, the linear amplifiers 41R, 41G and 41B, and the clamp pulse injecting stages. Output signals of the clamp pulse injecting stages are passed through direct current removing capacitances C 10R , C 10G and C 10B to be transmitted along output lines L R1 , L G1 and L B1 connected to cathodes of a CRT. Direct current clamp circuits (direct current shift circuits) including diodes D 10R , D 10G and D 10B and capacitances C 11R , C 11G and C 11B are positioned between a corresponding one of the output lines L R1 , L G1 and L B1 and ground to clamp direct current in accordance with charge voltages of the capacitances C 11R , C 11G and C 11B . The capacitances C 11R , C 11G and C 11B are charged in a following manner. The analog color temperature coefficients K R , K G and K B are supplied to analog multipliers 42R, 42G and 42B, correspondingly. At the same time, a brightness control signal V BRI supplied from a brightness volume (not shown) usually including an impedance conversion circuit is also supplied commonly to the analog multipliers 42R, 42G and 42B. As a result, multiplied signals "K R · V BRI ", "K G · V BRI " and "K B · V BRI " are obtained in the multipliers 42R, 42G and 42B to be supplied to corresponding differential amplifying circuits which comprise operational amplifiers OP 10R , OP 10G and OP 10B , input resistances R 22R and R 23R , R 22G and R 23R , and R 22B and R 23B , negative feedback resistances R 24R , R 24G and R 24B , ground resistances, R 25R , R 25G and R 25B , respectively, which are connected to each other as shown in Fig. 8. The operational amplifiers OP 10R , OP 10G and OP 10B receive the multiplied signals "K R · V BRI ", "K G · V BRI " and "K B · V BRI " at inverted inputs, and CRT cut-off level signals V RCO , V GCO and V BCO specified for the primary color signals R, G and B at non-inverted inputs. The above described direct current clamp circuits, multipliers and differential amplifying circuits provide a brightness control unit. In this brightness control unit, charge voltages of the capacitances C 11R , C 11G and C 11B are levels "V RCO - K R · V BRI ", "V GCO - K G · V BRI " and "V BCO -K B · V BR1 ", which are black levels for output signals Rout, Gout and Bout of the primary color signals R, G and B. It should be noted that the aforementioned equations (1) to (3) are met even in this third preferred embodiment. Therefore, the advantages set out in the first and second preferred embodiments are obtained in this third preferred embodiment. In the first to third preferred embodiments, the invention is applied to a CRT display apparatus. In the same manner, the invention is also applied to a liquid crystal display apparatus, in which contrast and brightness are controlled in accordance with the adjustment of bias voltage and transmission coefficient by control of voltage. The invention is most effective in a display apparatus which is used as a periphery apparatus of an information processing apparatus, and in which static images are displayed on a screen thereof. However, the invention is also applied to a CRT display apparatus of a television set, in which motion images are displayed on a screen thereof. In the first to third preferred embodiments, the color tracking is realized in regard to contrast and brightness. However, it may be realized in regard to either contrast or brightness. In the first to third preferred embodiments, contrast and brightness are controlled commonly for the primary color signals R, G and B, and, in the second preferred embodiment, sub-brightness is controlled in accordance with the adjustment of keys, etc. provided in the interior of an apparatus by an inspector, etc. However, handling knobs, etc. may be provided at the exterior of an apparatus to control contrast and/or brightness separately for the primary colors R, G and B. For instance, the color temperature coefficients supplied to the multiplication type digital to analog converter in the brightness control unit may be changed in accordance with a designated value of sub-brightness, and, in the third preferred embodiment, analog multipliers may be further provided to carry out the multiplication of a sub-brightness value between each of the analog multipliers 42R, 42G and 42B and each of the differential amplifiers.

Claims: A display apparatus comprising a first input terminal receiving a red color input signal (Rin), a second input terminal receiving a green color input signal (Gin), a third input terminal receiving a blue color input signal (Bin), each of said red, green and blue color input signals (R in , Gin, Bin) having a voltage amplitude and a d.c. voltage, a first output terminal outputting a red color output signal (Rout), a second output terminal outputting a green color output signal (Gout), a third output terminal outputting a blue color output signal (Bout), each of said red, green and blue color output signals having a contrast voltage component and a brightness voltage component, a sum of said brightness and contrast voltage components representing a voltage intensity of an associated one of said red, green and blue output signals, a contrast control unit (1) coupled between each of said first, second and third input terminals and each of said first, second and third output terminals, said contrast control unit (1) being adapted to control the respective voltage amplitudes of said red, green and blue color input signals to thereby produce the respective contrast voltage components of said red, green and blue color output signals, a brightness control unit (2) coupled between each of said first, second and third input terminals and each of said first, second and third output terminals, said brightness control unit (2) being adapted to control the respective d.c. voltages of said red, green and blue color input signals to thereby produce the respective brightness voltage components of said red, green and blue color output signals, a color temperature adjusting unit (5) producing a red color temperature coefficient, a green color temperature coefficient and a blue color temperature coefficient, first means (3) for generating contrast control information (V CONT ), second means (6) for generating brightness control information (V BRI ), and third means (4, 7) coupled between said color temperature adjusting unit (5) and at least one of said contrast control unit and said brightness control unit for causing said at least one of said contrast control unit (1) and said brightness control unit (2) to control at least one of the voltage amplitude and the d.c. voltage of said red color input signal (R in ) in accordance with a multiplication between said red color temperature coefficient and an associated one of said contrast control information (V CONT ) and said brightness control information (V BRI ), at least one of the voltage amplitude and the d.c. voltage of said green color input signal (G in ) in accordance with a multiplication between said green color temperature coefficient and an associated one of said contrast control information (V CONT ) and said brightness control information (V BRI ), and at least one of the voltage amplitude and the d.c. voltage of the blue color input signal (B in ) in response to both said blue color temperature coefficient and an associated one of said contrast control information (V CONT ) and said brightness control information (V BRI ), the respective voltage intensities of said red, green and blue color output signals (R out , B out , G out ) being thereby produced. A display apparatus according to claim 1, wherein the input signals have a primary black level that represents primary brightness information by a voltage difference thereof from a reference voltage level, the output signals have a controlled black level that represents output brightness information by a voltage difference thereof from said reference voltage level, and the means (4, 7) coupled between said color temperature adjusting unit (5) and at least one of said contrast control unit (1) and said brightness control unit (2) comprise a control circuit (7) coupled to said second means (6) and responding to said brightness control information to produce first, second and third brightness control signals for respectively controlling the black levels of said first, second and third input signals, the color temperature adjusting unit (5) comprises means for supplying said first, second and third color temperature coefficience to said control circuit (7) to cause said control circuit to modify said first, second and third brightness control signals in response thereto, respectively, said control circuit (7) thereby producing a first modified brightness control signal, a second modified brightness control signal and a third modified brightness control signal, and wherein the brightness control unit (2) controls the respective primary black levels of said first, second and third input signals in response respectively to said first, second and third modified brightness control signals to thereby produce said first, second and third output signals having the respective black levels. A display apparatus according to claim 1, wherein the input signals have a primary voltage amplitude defined by a black level and a white level, the output signals have a control voltage amplitude to represent a contrast among said output red color, said output green color and said output blue color, and the means (4, 7) coupled between said color temperature adjusting unit (5) and at least one of said contrast control unit (1) and said brightness control unit (2) comprise a control circuit (4) coupled to said first means (3) and responding to said contrast control information to produce a first contrast control signal, a second contrast control signal and a third contrast control signal for respectively controlling the voltage amplitudes of said first, second and third input signals, wherein the color temperature adjusting unit (5) comprises means for supplying said first, second and third color temperature coefficients to said control circuit to cause said control circuit to modify said first, second and third contrast control signals in response respectively to said first, second and third color temperature coefficients, said control circuit thereby producing a first modified contrast control signal, a second modified contrast control signal and a third modified contrast control signal, and wherein the contrast control unit (4) controls the amplitudes of said first, second and third input signals in response respectively to said first, second and third modified contrast control signals to thereby produce said first, second and third output signals having the respective controlled voltage amplitudes. A display apparatus, according to claim 1, wherein said contrast control unit comprises : three linearly amplifying means each comprising a serial connection of an attenuator, and pre-and main amplifiers. A display apparatus, according to claim 3, wherein: said contrast adjusting unit is adapted for manually handling the contrast volume. A display apparatus, according to claim 1 further comprising : means for designating a color temperature; means for generating color temperature coefficients to define a ratio of intensities of three primary colors, said ratio being determined by said color temperature; and wherein said control caused by said third means depends on multiplications of said color temperature coefficients by a brightness control signal. A display apparatus, according to claim 6, wherein: said designating means is to be handled manually by keys for adjusting a ratio of said color temperature coefficients, and said generating means generates said color temperature coefficients of said ratio.

Inventors: KUMAKI SHINGO, KUMAKI, SHINGO, Kumaki, Shingo, c/o NEC Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1991
Main classification: H04N9
IPC codes: H04N9