Clear blue circuit for use with computer monitors and the like

Abstract: A control signal is produced by first arithmetic circuit based on the difference between the blue signal component and one of the red and green components. A second analog arithmetic circuit increases the level of one of the R or the G signal component in accordance with the control signal.

Description: The present invention relates generally to a display for use with a computer or the like type of device and more specifically to a so called clear blue circuit for use with such a display or monitor which improves the clarity and visibility of blue characters. It is widely accepted that characters which are displayed in blue monochromatic color exhibit reduced visibility as compared with those displayed in green. In an attempt to overcome this problem a prior art arrangement shown in Fig. 3 has been proposed. In this arrangement the red (R), green (G) and blue (B) digital signals which have been supplied from the computer to the display interface, are subject to simple digital-analog conversion by D/A converters 101R, 101G and 101B to form analog output signals R, G and B. In this instance each of these circuits includes a resistor dividing circuit. A logic circuit 102 determines the presence of a blue signal when a blue signal appears in the absence of red and green signals. This circuit improves the visibility of the blue characters by inducing the D/A converters 101R and 101G to output red and green signal in a predetermined ratio with respect to the blue signal. In other words, the circuit mixes a little red and green into the blue to improve clarity. However, this circuit is incapable of processing analog RGB input. Further, the digital RGB signals can have either 8 or 16 color levels. On the other hand, analog RGB signal can have essentially an infinite number of levels or graduations. Accordingly, the above type of circuit is such that the logic circuit is incapable of distinguishing between pure blue and a blue which contains some red or green. It is an object of the present invention to provide a so called "clear blue" circuit which improves the clarity with which blue or bluish color characters appear on the display of a computer or the like type of monitor. In brief, the above object is achieved by an arrangement wherein a control signal is produced by a first arithmetic circuit based on the difference between the blue signal component and the sum of the larger of the red and green components and a constant. A second analog arithmetic circuit increases the level of one of the R or the G signal component in accordance with the control signal. In accordance with a first aspect of the present invention there is provided a method of controlling a display apparatus comprising the steps of: comparing the levels of first and second colour signals to determine which is the larger; determining the sum of the larger of the first and second signals and a predetermined value; subtracting the sum from the value of third colour signal to determine the level of a control signal; and modifying the level of one of the first and second colour signals in accordance with the control signal. In accordance with a second aspect of the present invention there is provided a control circuit for use in a display apparatus, comprising: means for comparing the levels of first and second colour signals to determine which is the larger; means for determining the sum of the larger of the first and second signals and a predetermined value; means for subtracting the sum from the value of third colour signal to determine the level of a control signal; and means for modifying the level of one of the first and second colour signals in accordance with the control signal. The present invention will be further described hereinafter with reference to the following description of exemplary embodiments and the accompanying drawings, in which: Fig. 1 is a block diagram showing the arrangement according to an embodiment of the present invention; Fig. 2 is a circuit diagram showing an actual example of the circuit which can used in the arrangement shown in Fig. 1; and Fig. 3 is a block diagram showing the arrangement of the previously proposed clear blue circuit which was discussed in the opening paragraphs of the instant disclosure. Fig. 1 shows in block diagram form the arrangement which characterizes an embodiment of the present invention. In this embodiment the ratio of green in a blue character display in increased. Clamping circuits 1R, 1G and 1B clamp each of the respective R, G and B signals of an analog RGB signal at a 0 level via feedback operation. A first analog arithmetic unit 2 receives each of the R, G and B from the clamping circuits and produces a control voltage a which increases in this example in accordance with the difference between the largest of the three signals and the sum of a predetermined constant and the larger of the other two signals. A second analog arithmetic unit 3 is arranged to receive inputs from the blue and green signal clamping circuits 1B and 1G along with the control voltage from the first analog arithmetic unit 2. R′, G′ and B′ signals appear on the output terminals of the circuit. In the first analog arithmetic unit 2 the control voltage "a" is developed in accordance with the following elation: a = {B - (G ∨ R) - C } wherein (G ∨ R) indicates the greater signal component between the G and R signals, B denotes the blue signal component, and C denotes a predetermined value in the form of a fixed number. In accordance with this, when B is larger than the sum of C and the larger of G and R, the control voltage "a" which increases as same as a difference of B-[(G ∨ R) + C], is obtained. In the second analog arithmetic unit 3 a calculation in accordance with the following equation is executed. G′ = (1 - ak1) G + ak1B where 0 ≦ ak1 < 1; and k1 is a predetermined constant. In accordance with this calculation, the original value of G is modified by the control voltage "a" which varies with the relative difference between the R and B signal levels. In this instance the higher the original G signal component is the lower the control voltage becomes. In this instance it is considered that an upper interface level be set a level of 0.7 Volt (for example). In the case red is mixed into the blue the following relationship is utilized: R′ = (1 - ak2)R + ak2B where 0 ≦ ak2 < 1; and k2 is a predetermined constant. The operation of the above described circuit is such that the first analog arithmetic unit 2 produces a control signal of the nature which varies in accordance with the degree by which the B signal component is greater than the R or G signals. The second analog arithmetic unit 3 responds to the signal "a" and increases the amount of green which is produced and which mixes into the B signal by dynamically varying the ratio defined between the two. This enables a clear blue function to be realized. Of course it is within the scope of the present invention to use the red signal in place of the green and/or use both if so desired and/or deemed appropriate. However, in actual practice, the B signal component is not mixed with the R component under the conditions k2 = 0 in elation (3) and if only the G signal is mixed with the blue signal the blue or bluish color characters become more clear and apparent. The reason for this is deemed to be the fact that the human eye perceives green to be more visible than other colors. As mentioned above, the present invention is not limited to mixing in only one color. When green is mixed into the blue, the blue color tends to shift toward cyan while when red is mixed in the resulting color tends to shift in the magenta direction. These colors tend to be brighter and provide improved visibility. Fig. 2 shows an actual circuit arrangement which can be used in accordance with the first embodiment. In this circuit clamping circuit 1R is comprised of a transistor Q1, a MOS transistor Q4 and an operational amplifier OP1. Similarly, the clamping circuit 1G is comprised of transistor Q5, a MOS transistor Q7 and an operational amplifier 0P2. Clamping circuit 1B includes transistor Q15, a MOS transistor Q18 and an operational amplifier 0P3. These clamping circuits 1R, 1G and 1B are responsive to a clamp pulse P in a manner wherein transistors Q4, Q7 and Q18 are rendered conductive (ON) and maintain the feedback clamp on. In the instant circuit arrangement the first and second analog arithmetic units 2 and 3 are difficult to distinguish from one another. However, the first unit may be deemed to include transistors Q3, Q6, Q9, Q10, Q16 and Q17. In this circuit, transistors Q3 and Q6 perform the (G ∨ R) calculation. The differential calculation is performed by transistors Q9 and Q10 and receive the arithmetic results "a" in accordance with equation (1) to produce a control voltage ak1. In this instance, the constant k1 is determined by the values of resistors R1, R2 and R3. The second arithmetic unit 3 can be deemed to consist essentially of transistors Q8, Q11, Q12, Q13 and Q14. The circuit including transistors Q9 and Q10 perform the gain control of the G signal component (viz., the (1-ak1)G portion of equation (2) via transistor Q8. The circuit including transistors Q11 and Q12 perform the gain control for the B signal component or the ak1B operation in accordance with equation (2). Transistor Q3 add the current flowing through the collectors of the commonly connected transistors Q10 and Q12 to derive the G′ signal in accordance with said equation (2). As will be readily apparent the desired effect can be obtained even if the ratio of R and G signal components to the B signal one is nearly constant. Accordingly, the present invention is not limited to the above described circuit arrangement and various modifications and/or changes can be made in the manner in which the RGB signal are treated.

Claims: A method of controlling a display apparatus comprising the steps of: comparing the levels of first and second colour signals to determine which is the larger; determining the sum of the larger of the first and second signals and a predetermined value; subtracting the sum from the value of third colour signal to determine the level of a control signal; and modifying the level of one of the first and second colour signals in accordance with the control signal. A control circuit for use in a display apparatus, comprising means (2) for comparing the levels of first and second colour signals to determine which is the larger; means (2) for determining the sum of the larger of the first and second signals and a predetermined value; means (2) for subtracting the sum from the value of third colour signal to determine the level of a control signal; and means (3) for modifying the level of one of the first and second colour signals in accordance with the control signal. A circuit according to claim 2 further comprising: first, second and third clamp circuits, (1 R ,1 B ,1 G ), said first, second and third clamp circuits being arranged to receive first, second and third colour signals respectively, said first second and third clamp circuits being responsive to a pulse signal; first and second analog arithmetic units (2,3), said first analog arithmetic unit (2) being operatively connected with said first, second and third clamp circuits in such a manner as to receive outputs therefrom, said second analog arithmetic unit (3) being arranged to receive the output of said second and third clamp circuits and the output of said first analog arithmetic unit, and wherein said first analog arithmetic unit includes: said means for comparing; said means for determining; and said means for subtracting; and said second analog arithmetic unit includes: said means for modifying.

Inventors: YAMAKAWA YOSHIFUMI C O SONY CO, YAMAKAWA, YOSHIFUMI, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: H04N9
IPC codes: G09G5, H04N9, G09G1