Device for regulating the contrast of video images

Abstract: The invention relates to a device enabling the luminance signal (Y) and the chrominance signals (CB, CR) of a video image to be converted into red (R), green (G) and blue (B) colour signals. The device comprises a conversion matrix and means enabling the amplitude of the luminance signal (Y) and of the blue colour difference chrominance signal (CB) and red colour difference chrominance signal (CR) to be modified by means of a user adjustment control. The device comprises additional means preventing the colour signal which has the highest value at the output of the conversion matrix from being able to exceed a saturation value equal to x% of the white level when the colour signal reaches the saturation value. Preferably, the percentage of x% is equal to 70%. The invention applies to television sets.

Description: The invention relates to a device enabling the contrast level of a video image to be regulated. It is known to adjust the colour and the contrast of video images. For example, a colour conversion device is disclosed in EP-A-0 677 972 and a device for controlling the contrast of a video image is disclosed in EP-A-0 589 513. As is known from those skilled in the art, a highly contrasted video image area prevents both the details of which it is composed and the details which surround it from being seen clearly. It is then necessary to decrease the contrast level so as to make the details reappear. In the case, for example, of the displaying of a video image on a television screen, the viewer has available a contrast-level adjustment. In the case in which one or more areas of the image are too highly contrasted compared to the rest of the image, the viewer can therefore act on the contrast control so as to decrease the contrast. The contrast control is applied by means of an adjustment parameter acting on circuits enabling the level of the luminance signal (Y), the level of the blue colour difference chrominance signal (CB) and the level of the red colour difference chrominance signal (CR) to be simultaneously adjusted. As a result of simultaneous application of the user adjustment control, the darkest areas of the image are then no longer contrasted enough. It is the details of which these darkest areas are composed which are no longer visible. The invention does not have these drawbacks. The invention relates to a device as set out in claim 1. According to the preferred embodiment of the invention, the colour information which has the highest value at the output of the conversion matrix cannot exceed 70% of the value of the white level. However, other values are possible according to other embodiments. Advantageously, according to the invention, the limitation in contrast for high values of the luminance or red or blue colour difference signals makes it possible for the user to avoid making frequent contrast adjustments. Other characteristics and advantages of the invention will appear on reading a preferred embodiment, given with reference to the figures appended hereto, among which: Figure 1 represents a first embodiment of the regulating device according to the invention; Figure 2 represents a second embodiment of the regulating device according to the invention; Figure 3 represents the response curves of a device according to the prior art and of a device according to the invention. In all the figures, the same references denote the same elements. Figure 1 represents a first embodiment of the regulating device according to the invention. According to the invention, the regulating device makes it possible to adjust the level of the luminance component Y and of the blue CB and red CR colour difference chrominance components based on the detection of the maximum value assumed by one of the colour information items R, G, B coming from the matrix 7 whose function is to convert the Y, CB, CR components into R, G, B components. The maximum value of R, G, B, denoted hereafter by M(R, G, B), is detected using an OR function, referenced 8, having three inputs and one output. For this purpose, each of the R, G, B colour information items is applied at a different input of the OR function. The OR function 8 is produced by any known means. It may, for example, be a circuit produced using three diodes. The output of the OR function is connected to the first input of a comparator 9. The value M(R, G, B) is therefore applied to the first input of the comparator 9, a second input of which receives a voltage VT2 from a multiplier 10. The voltage VT2 is equal to k times the voltage VT1, k being the user contrast-adjustment parameter and VT1 being a reference voltage whose value is preferably set in the factory. According to the invention, k is preferably between 0.25 and 1. The variations in the output signal of the comparator 9 are applied via a diode D to the terminals of an integrating capacitor C. A resistor A is mounted in parallel with the capacitor C so as to ensure that the capacitor C discharges. The values of C and A are chosen so that the voltage S at the terminals of the capacitor C are substantially constant throughout the duration of at least one video image frame. By way of non-limiting example, the capacitor C may have a value of 500 nF and the resistor A a value of 680 kohms. The output voltage S received at the terminals of the integrating capacitor C is applied to a voltage/coefficient conversion circuit 11 so as to generate a regulating coefficient ks. The voltage/coefficient conversion circuit 11 is produced in a manner known per se. There is therefore no point in recalling this here. The regulating coefficient ks is applied simultaneously to the three control inputs of three multipliers 4, 5, 6, which receive on their respective inputs the signals k×Y, k×CB and k×CR from the three respective multipliers 1, 2 and 3, where k is the user contrast-adjustment parameter mentioned hereinabove. It follows that the luminance, blue colour difference and red colour difference signals applied at the input of the conversion matrix are respectively equal to: ks × k × Y; ks × k × CB and ks × k × CR According to the invention, the coefficient ks preferably has a value of between 0.5 and 1. Advantageously, the regulating operation is performed on the basis of any one of the three signals R, G or B. Figure 2 shows a second embodiment of the regulating device according to the invention. According to this second embodiment, the reference voltage VT1 is directly connected to the second input of the comparator and the output of the OR function 8 is connected to the input of a divider 12 whose output is connected to the first input of the comparator. The divider 12 is controlled by the user contrast-adjustment parameter k. It follows that the output voltage of the divider 12 is equal to the input voltage of the divider 12 divided by the quantity k. All the other elements of the circuit in Figure 2 are identical to those described in Figure 1. It follows that the operation of the circuit described in Figure 2 is in principle identical to that described in Figure 1. However, in terms of production, the circuit described in Figure 1 has the advantage of not requiring a divider to be produced. According to the circuit described in Figure 1, the user adjustment coefficient k is applied to four multipliers 1, 2, 3 and 10. Preferably, these four multipliers may then be identical, which avoids complexifying the circuit. Advantageously, the action of k, on the one hand, on the signals Y, CB, CR and, on the other hand, on the signal M(R, G, B) is thus matched under optimal conditions. Figure 3 represents the response curves of a device according to the prior art and of a device according to the invention. Figure 3 comprises 3 curves, C1, C2 and C3. Curves C1 and C2 are response curves according to the prior art. Curve C1 shows the phenomenon of saturation of the quantity M(R, G, B) when there is no adjustment of the contrast level. The contrast variations shown by the change in curve C1 over time correspond to an example of video image reception. It is clear that any other type of contrast variations, corresponding to other examples of video image reception, is possible. According to the example shown by curve C1, the quantity M(R, G, B) goes from the black level VB at time t1 to the white level VW at time t2, remains at the white level between times t2 and t3 and goes from the white level VW at time t3 to the black level VB at time t4. Curve C2 shows, by way of example, the saturation curve for the quantity M(R, G, B) between the black level VB and a saturation level VS fixed by the user. The contrast variations shown by the change in curve C2 over time correspond to video image reception conditions identical to those for curve C1. The saturation level VS is adjusted by the user by means of the regulating parameter k. If the image has highly contrasted areas, the user is obliged to reduce the contrast significantly. It follows that the level VS may be a level which greatly reduces the quantity M(R, G, B). When the image no longer has these highly contrasted areas, the user is then obliged to modify his adjustment again, so as to restore the contrast. For images whose contrast varies greatly, it may therefore be necessary for the user to have to modify the contrast a great many times. Curve C3 shows, according to the invention, by way of example, the saturation curve for the quantity M(R, G, B) between the black level VB and the saturation level VT2. The contrast variations shown by the change in curve C3 over time, correspond, by way of example, to video image reception conditions identical to those for curves C1 and C2. Whatever the adjustment by the user, the quantity M(R, G, B) does not reach the value of the white level VW. Advantageously, the device according to the invention comprises means, such as those described in Figures 1 or 2, for automatic saturation of the quantity M(R, G, B) to be implemented so as to prevent the white level from being reached. This automatic saturation value is equal to VT2. As was mentioned previously, the value of VT2 corresponds to 70% of the white level VW. Advantageously, according to the device of the invention, it is then not necessary for the user to have to make as many adjustments as according to the prior art in order to obtain images whose contrast variations are not disagreeable to the eye. The above description is made with reference to a colour video image comprising the various colour information items R, G, B For those skilled in the art, it is obvious that the words "luminance information" may replace the words "colour information" in the case where the invention applies to the processing of a black-and-white image and no longer of a colour image.

Claims: Device for regulating the contrast of video images comprising : a conversion matrix (7) for converting luminance signal (Y), blue colour difference chrominance signal (CB), and red colour difference chrominance signal (CR) of a video image into red (R), green (G) and blue (B) colour signals; means (8) for detecting the maximum value M(R,G,B) of said red (R), green (G) and blue (B) colour signals outputted by said conversion matrix (7); characterized in that it further comprises : first means (1, 2, 3) for adjusting the amplitude of said luminance signal (Y), of said blue colour difference chrominance signal (CB), and of said red colour difference chrominance signal (CR) at the input of said conversion matrix, in response to a user adjustment parameter (k) ; and second means (4, 5, 6) for adjusting the amplitude of said luminance signal (Y), of said blue colour difference chrominance signal (CB), and of said red colour difference chrominance signal (CR) at the input of said conversion matrix, in response to a regulating coefficient (ks) in order to prevent said maximum value M(R,G,B) of red, green and blue signals from exceeding a saturation value less than the white level, said regulating coefficient (ks) being generated by a sub-assembly (9, 10, D, C, A, 11, 12), controlled by said user adjustment parameter (k), in response to the output of said maximum value M(R,G,B) detecting means (8) and to a reference voltage (VT1). Device according to claim 1, wherein said sub-assembly for generating the regulating coefficient (ks) comprises a comparator (9) having a first input, a second input and an output, the first input of said comparator being connected to the output of said maximum value M(R,G,B) detecting means (8), and the second input of said comparator being connected to the output of a multiplier (10) which is controlled by said user adjustment parameter (k) and the input of which is connected to said reference voltage (VT1). Device according to claim 1, wherein said sub-assembly for generating the regulating coefficient (ks) comprises a comparator (9) having a first input, a second input and an output, the first input of said comparator being connected to the output of a divider controlled by said user adjustment parameter (k), the input of said divider being connected to the output of said maximum value M(R,G,B) detecting means (8) and the second input of said comparator being connected to said reference voltage (VT1). Device according to claim 1, 2 or 3, wherein said means (8) for detecting the maximum value M(R,G,B) of red (R), green (G) and blue (B) colour signals comprise an OR function having three inputs and one output, each of said inputs being connected to a different colour signal (R, G, B) and the output being connected to said sub-assembly. Device according to any one of the preceding claims, wherein said first (1, 2, 3) and second (4, 5, 6) means for adjusting the amplitude of said luminance signal (Y), of said blue colour difference chrominance signal (CB), and of said red colour difference chrominance signal (CR) are multipliers. Device according to any one of the preceding claims, wherein said user adjustment parameter (k) has a value of between 0.25 and 1. Device according to any one of the preceding claims, wherein said regulating coefficient (ks) has a value of between 0.5 and 1. Device according to claim 1, wherein said saturation value corresponds to 70% of the value of the white level.

Inventors: HAUSER SIGRID, LENZ KUNO, HAUSER, SIGRID, LENZ, KUNO
Applicants: THOMSON MULTIMEDIA SA, THOMSON MULTIMEDIA
Application year: 1997
Main classification: H04N5
IPC codes: H04N5, H04N9