Patent Publication Number: US-8120560-B2

Title: Image displaying method, image displaying device, and contrast-adjusting circuit for use therewith

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is a Continuation Application of U.S. application Ser. No. 11/751,624, filed May 21, 2007, now U.S. Pat. No. 7,719,551 which is a Continuation Application of U.S. application Ser. No. 10/411,791, filed Apr. 10, 2003, now U.S. Pat. No. 7,227,559 which in turn is related to and claims priority from Japanese Patent Application No. 2002-241579, filed Aug. 22, 2002, which are incorporated by reference herein in their entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an image displaying technique that converts an analog video signal to a digital video signal to display an image. 
     Image displaying devices, which use a fixed pixel device such as a plasma display panel (PDP) or a liquid crystal display panel (LCD), generally have low contrast compared to image displaying devices that use a cathode-ray tube. Conventional measures to improve contrast in PDPs include at least a technique for increasing the light-emitting efficiency of phosphor and a technique for improving control of the panel. They are described in detail, for example, in Japanese Patent Application Laid-Open No. Hei 10-208637 and Japanese Patent Application Laid-Open No. Hei 8-138558. An example of a technique for adjusting video contrast in a television receiver includes the technique described in Japanese Patent Application Laid-Open No. Hei 4-10784. Japanese Patent Application Laid-Open No. Hei 4-10784 describes a technique in which the maximum value, the minimum value, and the mean of a digital signal is converted from a video signal before storing the values. Based on the result of the detection and calculation, amplification of the video signal is performed to improve contrast. 
     BRIEF SUMMARY OF THE INVENTION 
     For image displaying devices that use a fixed pixel devices such as a PDP or an LCD, higher contrast is required. The present invention is particularly devised to obtain stable high contrast even in an area of intense brightness. To improve the contrast, the present invention provides a technique for displaying an image. Based on information about the average brightness level of a digital luminance signal, for a corresponding analog luminance signal or a digital luminance signal, so-called black-correction processing is performed to decrease the brightness level. This is performed according to a predetermined quantity of correction in response to the average brightness level. In addition processing that increases contrast gain within the range of a margin of a dynamic range is performed; thereby improving video contrast where the average brightness level is comparatively high. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, objects and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a basic configuration diagram illustrating a first embodiment according to the present invention; 
         FIG. 2  is an explanatory diagram illustrating a contrast adjusting operation for the configuration shown in  FIG. 1 ; 
         FIG. 3  is an explanatory diagram illustrating the relationship between an average brightness level and a black-correction level in contrast adjustment; 
         FIG. 4  is an explanatory diagram illustrating the relationship between a black-correction level and a contrast gain in a contrast adjustment operation; 
         FIG. 5  is a diagram illustrating a specific example of the configuration shown in  FIG. 1 ; 
         FIG. 6  is a basic configuration diagram illustrating another embodiment according to the present invention; 
         FIG. 7  is a diagram illustrating a specific example of the configuration shown in  FIG. 6 ; and 
         FIG. 8  is an explanatory diagram illustrating color correction in the configuration shown in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although we have shown and described several embodiments in accordance with our invention, it should be understood that disclosed embodiments can be changed or modified without departing from the scope of the invention. Therefore, the present invention is not bound by the details shown and described herein but should be understood to cover all such changes and modifications that fall within the scope of the appended claims. Embodiments of the present invention are described below with reference to the drawings. 
       FIGS. 1 through 5  are explanatory diagrams illustrating a first embodiment of the present invention.  FIG. 1  is a basic configuration diagram illustrating an image displaying device which mainly comprises a contrast-adjusting circuit.  FIG. 2  illustrates a contrast-adjusting operation within a dynamic range.  FIG. 3  illustrates the relationship between an average brightness level and a black-correction level.  FIG. 4  illustrates the relationship between a black-correction level and a contrast gain.  FIG. 5  is a diagram of the configuration of the embodiment shown in  FIG. 1 . This embodiment is an example of a circuit configuration in which a digital luminance signal is offset within a dynamic range to decrease the brightness level; that is, black-correction processing is performed before increasing a contrast gain to improve contrast. 
       FIG. 1  shows a contrast-adjusting circuit unit  1 , a display unit  2  for displaying an image by a contrast-adjusted signal, an A/D converter  3  for converting an inputted analog luminance signal into a digital signal, a signal-level detecting circuit  5  for detecting the average brightness level of a digital luminance signal obtained within a given period, a variable-brightness circuit  6  that offsets a digital luminance signal to change the brightness level, a variable-contrast-gain circuit  7  for changing the contrast gain of a digital luminance signal (the brightness level of which has been changed), and a microcomputer  8  as a control circuit to control signal-level detecting circuit  5 , variable-brightness circuit  6 , and variable-contrast-gain circuit  7  based on information about the detected average brightness level. 
     Microcomputer  8  identifies a brightness area corresponding to the detected average brightness level, then generates and outputs a control signal corresponding to the result. An inputted analog luminance signal is converted to a digital luminance signal by A/D converter  3 . The digital luminance signal is then inputted into signal-level detecting circuit  5 . Signal-level detecting circuit  5  detects the average brightness level of the digital luminance signal obtained during a video period, for example, in one field or in one frame. Information (a signal) about the detected average brightness level is supplied to microcomputer  8 . Microcomputer  8  identifies a brightness area corresponding to the average brightness level based on the received information about the average brightness level, then generates and outputs a control signal based on the result. The control signal is provided to signal-level detecting circuit  5 , variable-brightness circuit  6 , and variable-contrast-gain circuit  7 . The control signal controls the range of detection by signal-level detecting circuit  5 . In variable-brightness circuit  6 , in this example, the control signal controls black correction for a digital luminance signal within the range of an average brightness level greater than or equal to a given value. More specifically, the control signal controls a digital luminance signal, the average brightness level of which is greater than or equal to the given value, so that the digital luminance signal is offset to the minus side. In addition, for variable-contrast-gain circuit  7 , the control signal is associated with a level of black correction in variable-brightness circuit  6 , and is used to control the contrast gain of a digital luminance signal within the range of an average brightness level greater than or equal to a given values so that the contrast gain is increased within a dynamic range. 
     Variable-brightness circuit  6  and variable-contrast-gain circuit  7  are controlled by a feedforward method. As described above, performing black-correction processing for a digital luminance signal within the range of an average brightness level greater than or equal to the given value, and increasing a contrast gain within a dynamic range according to a level of the black correction, cause video contrast, particularly contrast on the bright video side, to increase. An increased-contrast video signal is transmitted to display unit  2  where the increased-contrast image having increased contrast is displayed. Note that in this embodiment a control signal is separately output from microcomputer  8  to the color matrix circuit, which converts a digital luminance signal and a digital color-difference signal into digital video signals of red (R), green (G), and blue (B). The color matrix circuit performs color correction (control of the depth of color). 
       FIG. 2  illustrates a contrast-adjusting operation within a dynamic range in the configuration shown in  FIG. 1 . In  FIG. 2 , “a” is a waveform obtained when black-correction processing is performed for a digital luminance signal; and “b” is a waveform obtained when black-correction processing and contrast-control processing (contrast-gain increasing processing) are performed. In this example, the A/D converter  3  in  FIG. 1  has a dynamic range in which, for example, the highest gray-scale level  255  when it is expressed by 8-bit data is an upper limit of the maximum brightness level, and the lowest gray scale level 0 is the minimum brightness level. In this case, the upper limit “ 255 ” of the dynamic range is a white level, and the lower limit “0” is a black level. Within the range of an average brightness level greater than or equal to the given value, black-correction processing offsets a digital luminance signal to the minus level side to decrease brightness, which permits a white level within a dynamic range to have a given margin (waveform a). For the first embodiment, the quantity of offset is the quantity that corresponds to the average brightness level value. In contrast-control processing (contrast-gain increasing processing), it is associated with the brightness level decreased by black-correction processing, that is, a black-correction level. In other words, in the first embodiment, contrast gain is increased within a dynamic range to eliminate the margin (waveform b). 
       FIG. 3  illustrates the quantity of offset to the minus level side of a luminance signal corresponding to the average brightness level value (APL value). Thus  FIG. 3  illustrates the relationship between a black-correction level and the APL value. In  FIG. 3 , the black correction (offset to the minus side) is performed within a range of an average brightness level value (APL value) greater than or equal to a given value APL 0 . If the APL value is APL 0 , correction of the black-correction level (the quantity of offset to the minus side) B 0  is performed. Then, the black-correction level is increased as the APL value increases in the following manner: if the APL value is APL 1 , the black-correction level is increased to B 1 ; if the APL value is APL 2 , the black-correction level is increased to B 2 ; if the APL value is APL 3 , the black-correction level is increased to B 3 ; and if the APL value is APL 4 , at which the average brightness level value becomes a white level, the black-correction level is increased to B 4 , the highest black-correction level. In  FIG. 1 , microcomputer  8  performs black-correction processing by controlling variable-brightness circuit  6  based on information about the average brightness level. 
     Thus, the microcomputer controls a black-correction level predetermined according to an APL value, that is, the variable magnitude of brightness. As a result, black correction which is more stable and provides an excellent image, is realized. 
       FIG. 4  illustrates the relationship between a black-correction level in black-correction processing and a contrast gain in the contrast gain control. In  FIG. 4 , (1) is an example of properties observed in the following control operation. Although the black-correction level, that is, the quantity of offset to the minus side of a luminance signal, does not reach a given level (starting level of contrast control), the contrast gain is kept to zero. As soon as the black-correction level reaches the given level (the starting level of contrast control), a contrast gain of a given value is generated; and within the range of the black-correction level that is greater than or equal to the given level, the contrast gain increases as the black-correction level increases. Microcomputer  8  controls the contrast gain according to this example of properties. As for the properties in  FIG. 3 , when the APL value becomes APL 2  and the black-correction level reaches B 2 , for example, the increase in contrast gain starts from black-correction level B 2 , which is the starting level of contrast control. In addition, (2) is an example of properties observed in the following control: irrespective of the value of a black-correction level, even though the quantity of offset to the minus side of a luminance signal is low enough not to reach a given level, a contrast gain of a given value is generated, and the contrast gain increases as the black-correction level increases. As for the properties in  FIG. 3 , when the APL value becomes APL 0  and consequently enters a black-correction level, an increase in contrast gain is started. In examples (1) and (2), when the black-correction level is at the maximum level, contrast gain is also maximized. Although the contrast gain is rectilinearly changed relative to the black-correction level in the examples of properties (1) and (2), the present invention is not limited to the above. 
       FIG. 5  illustrates an embodiment of the configuration shown in  FIG. 1 .  FIG. 5  shows a contrast-adjusting circuit  1 , a display unit  2  comprising a PDP or a liquid crystal panel which display an image, an input terminal T 1  for inputting an analog luminance signal Ya, an A/D converter  12  for converting inputted analog luminance signal Ya into a digital luminance signal Yd, a scan converter  13  for converting timing of an input signal into timing by which display unit  2  can display the signal, a variable-brightness circuit  31  which offsets digital luminance signal Yd to change its brightness level (equivalent to reference numeral  6  in  FIG. 1 ), and a color matrix circuit  32  that converts digital luminance signal Yd and digital color (color difference) signals Cbd, Crd into digital video signals Rd, Gd, Bd for red (R), green (G), and blue (B), respectively. Color matrix circuit  32  includes variable-contrast-gain circuit  7  shown in  FIG. 1 . T 2  and T 3  are input terminals of analog color (color difference) signals Cb, Cr. An A/D converter  14  converts the analog color (color difference) signals Cb, Cr into digital color (color difference) signals Cbd, Crd. Noise-removing LPF  15  is a low-pass filter for removing noise from the digital luminance signal Yd obtained by A/D converter  12 . An average-brightness detecting circuit  16  detects the average brightness level of an output signal (digital luminance signal) output from noise-removing LPF  15  during a given period, for example, in one frame or in one field. An average-brightness-determining unit  17  inputs information (signals) about the average brightness level detected by average-brightness detecting circuit  16  to find an area of brightness corresponding to the average brightness level. A gain controller  18  generates and outputs a control signal for controlling variable-brightness circuit  31  and color matrix circuit  32  based on information about an area of brightness corresponding to the average brightness level. Gain controller  18  performs the following control: variable-brightness circuit  31  is controlled by the control signal to perform black-correction control in variable-brightness circuit  31 , more specifically, to decrease the brightness level by offsetting a digital luminance signal to the minus side so that a margin is provided between the decreased brightness level and the upper limit of a dynamic range as shown in  FIG. 2 . In association with the brightness level decreased by black-correction processing, that is, the black-correction level, color matrix circuit  32  is controlled to increase the contrast gain of a digital luminance signal within a dynamic range in a manner such that the margin is eliminated, thereby increasing contrast. Among the above-mentioned units, the average-brightness-determining unit  17  and the gain controller  18  are configured as microcomputer  8  in  FIG. 1 . A/D converters  12 ,  14 , scan converter  13 , noise-removing LPF  15 , average-brightness detecting circuit  16 , variable-brightness circuit  31 , and color matrix circuit  32  can be embodied in a large-scale integrated circuit. Note that noise-removing LPF  15  is not required. 
     In the configuration shown in  FIG. 5 , an analog luminance signal Ya from input terminal T 1  is converted into a digital luminance signal Yd by A/D converter  12  before digital luminance signal Yd is provided to scan converter  13  and noise-removing LPF  15 . Noise-removing LPF  15  removes noise from digital luminance signal Yd. Then, digital luminance signal Yd is sent to average-brightness detecting circuit  16  where the average brightness level during a given period is detected. The signal of the detected average brightness level is inputted into average-brightness-determining unit  17  where the area of brightness corresponding to the detected average brightness level is verified. This area of brightness is either a high average area of brightness (high APL area), a middle average area of brightness (middle APL area), a low average area of brightness (low APL area), or an extremely low average area of brightness (extremely low APL area), for example. Information about the area of brightness which has been identified is inputted into gain controller  18 . 
     In addition, information about the average brightness level used for finding the area of brightness is also provided from average-brightness-determining unit  17  to gain controller  18  together with information about the area of brightness. Based on the information about the area of brightness and the information about the average brightness level, gain controller  18  generates a control signal which controls variable-brightness circuit  31  and color matrix circuit  32 . On the other hand, analog color (color difference) signals Cb, Cr from input terminals T 2 , T 3  are also converted into digital (color difference) signals Cbd, Crd by A/D converter  14 . After that, digital signals Cbd, Crd are inputted into scan converter  13  where the signals are subjected to pixel conversion. In color matrix circuit  32 , digital luminance signal Yd and digital color (color difference) signals Cbd, Crd output from scan converter  13  are converted into digital video signals Rd, Gd, Bd of red (R), green (G), and blue (B) before digital video signals Rd, Gd, Bd are output. The outputted digital video signals Rd, Gd, Bd are then inputted into display unit  2  where digital video signals Rd, Gd, Bd are displayed as an image. 
     In the configuration of the first embodiment, the black-correction processing for a digital luminance signal is performed within a range of an average brightness level greater than or equal to a given value. However, the present invention is not limited to the above. Black correction may also be performed for an analog luminance signal before A/D conversion, or black-correction processing also may be performed without limiting the range of an average brightness level. According to the above, effectively using a dynamic range of a digital luminance signal enables a stable improvement in contrast. 
       FIGS. 6 through 8  illustrate other embodiments of the present invention.  FIG. 6  shows an image displaying device mainly comprising a contrast-adjusting circuit.  FIG. 7  illustrates a configuration of the embodiment. This embodiment has a configuration in which the contrast-adjusting circuit expects a brightness level decreased by offsetting the level to the minus side as a result of black-correction processing for a digital luminance signal, and contrast gain is increased in association therewith. Accordingly unlike the first embodiment, the variable-contrast-gain circuit is set before the variable-brightness circuit is set. 
     The embodiment of  FIG. 6 , like that of  FIG. 1 , includes a contrast-adjusting circuit  1 , a display unit  2 , an A/D converter  3 , a signal-level detecting circuit  5  for detecting an average brightness level of a digital luminance signal obtained during a given period, a variable-brightness circuit  6  that offsets a digital luminance signal to change its brightness level, a variable-contrast-gain circuit  7  that changes a contrast gain of a digital luminance signal by expecting the brightness level to be changed, a microcomputer  8  as a control circuit that controls signal-level detecting circuit  5 , variable-brightness circuit  6 , and variable-contrast-gain circuit  7  based on information about the detected average brightness level. As in  FIG. 1 , an initial analog luminance signal is converted into a digital luminance signal by A/D converter  3  and inputted into signal-level detecting circuit  5 . Signal-level detecting circuit  5  detects an average brightness level of the digital luminance signal obtained during a video period, for example, in one field or in one frame. Information (a signal) about the detected average brightness level is inputted into microcomputer  8 . Microcomputer  8  identifies an area of brightness corresponding to the average brightness level based on information about the inputted average brightness level, then generates and outputs a control signal based on the result. The control signal is inputted into signal-level detecting circuit  5 , variable-brightness circuit  6 , and variable-contrast-gain circuit  7 . For signal-level detecting circuit  5 , the control signal is used to control the range of detection. 
     Variable-contrast-gain circuit  7  expects a level of black correction in variable-brightness circuit  6 , specifically, the offset quantity of a digital luminance signal to the minus side. According to this expectation, variable-contrast-gain circuit  7  is controlled so that the contrast gain of a digital luminance signal is increased within a dynamic range. 
     In this case, for example, to prevent a digital luminance signal from exceeding the dynamic range of variable-contrast-gain circuit  7  and variable-brightness circuit  6  as a result of the increase in contrast gain, the number of gray-scale bits of a digital luminance signal may be made higher than that of A/D converter  3 , which is set at a level before those circuits. Black-correction control of a digital luminance signal is performed to control variable brightness circuit  6 . Specifically, variable-brightness circuit  6  is controlled so that a digital luminance signal is offset to the minus side. Control of variable-brightness circuit  6  and variable-contrast-gain circuit  7  is by a feedforward method, and is performed within a range of an average brightness level greater than or equal to a given value. This causes video contrast, particularly contrast on the bright video side, to increase. A video signal whose contrast gain has been increased in the contrast-adjusting circuit  1 , is transmitted to display unit  2  where the image having increased contrast is displayed. Note that in this embodiment, a control signal is separately output from microcomputer  8  to the color matrix circuits which converts a digital luminance signal and a digital color (color-difference) signal into digital video signals of red (R), green (G), and blue (B). The color matrix circuit corrects color (controls depth of color). 
       FIG. 7  illustrates an embodiment of the above-mentioned configuration shown in  FIG. 6 .  FIG. 7  shows a variable-contrast-gain circuit  30  for changing the contrast gain of a digital luminance signal Yd (and is equivalent to element  7  in  FIG. 6 ), a variable-brightness circuit  31  which offsets digital luminance signal Yd to change its brightness level (equivalent to element  6  in  FIG. 6 ), and a gain controller  18 ′ for generating a control signal to control variable-contrast-gain circuit  30  and variable-brightness circuit  31 , based on information about the area of brightness corresponding to the average brightness level. Gain controller  18 ′ controls variable-contrast-gain circuit  30  by a control signal; more specifically, gain controller  18 ′ expects the brightness level to be decreased by offsetting it to the minus side by black-correction processing, and increases contrast gain within a dynamic range in association with the expectation. As described in  FIG. 6 , for example, to prevent a digital luminance signal from exceeding the dynamic range of variable-contrast-gain circuit  30  and variable-brightness circuit  31  as a result of the increase in contrast gain, the number of gray-scale bits of a digital luminance signal may be made higher than that of the A/D converter, which is set at a level before those circuits. In addition, gain controller  18 ′ controls variable-brightness circuit  31  performing black-correction control in the variable-brightness circuit, more specifically, offsetting the digital luminance signal to the minus side, so that the brightness level is decreased. Video contrast is increased by a combination of increase in contrast gain of the digital luminance signal and offset of the digital luminance signal to the minus side. In this connection, color control  33 , a noise-removing LPF  151 , a maximum-brightness detecting circuit  161 , and a maximum-brightness-determining unit  171  are provided as additional elements, but can also be omitted. Therefore, they will be described later. The other elements are similar to those in the first embodiment shown in  FIG. 5 . 
     In the configuration shown in  FIG. 7 , average-brightness-determining unit  17  and gain controller  18 ′ are configured as microcomputer  8  in  FIG. 6 ; and A/D converters  12 ,  14 , scan converter  13 , noise-removing LPF  15 , average-brightness detecting circuit  16 , variable-contrast-gain circuit  30 , variable-brightness circuit  31 , and color matrix circuit  32  are configured as, for example, an LSI circuit. 
     In the embodiment described above black-correction processing and contrast-gain increasing processing for the digital luminance signal are performed within the range of an average brightness level greater than or equal to a given value. However, the present invention is not limited to the above. Black-correction also may be performed for an analog luminance signal before the A/D conversion, or it may be performed without limiting the range of an average brightness level. Effectively using the dynamic range of a digital luminance signal with the above-mentioned configuration makes stable video contrast improvement possible. 
     Next, element  33 , which performs additional color correction, is described. Element  33  is a color control circuit that corrects the color of digital (color difference) signals Cbd, Crd output from scan converter  13 . More specifically, based on information about the average brightness level detected by the average-brightness-detecting circuit and information about the area of brightness corresponding to the average brightness level, gain controller  18 ′ controls variable-contrast-gain circuit  30  and variable-brightness circuit  31  to increase contrast, and also controls color control circuit  33  to perform the color correction. Color control circuit  33  is also configured as, for example, an LSI (large-scale integration). 
     When adjusting contrast, a gain is increased only for a luminance signal. Accordingly, the depth of video color decreases as a contrast gain associated with the black-correction level increases. In this embodiment, color correction is performed as a preventive measure. More specifically, the depth of video color is increased according to the increase in contrast gain associated with a black-correction level. The color correction is controlled by microcomputer  8  according to, for example, properties (1) or (2) in  FIG. 8 . Properties (1) are used in the following control process: color correction is not performed until a black-correction level reaches a given color-correction starting level; within a range allowed after the black-correction level reaches the color-correction starting level; the color-correction gain is substantially increased in proportion to the black-correction-level value; and the highest color gain is provided at the highest black-correction level. Properties (2) are used in the following control process: the given color-correction starting level is not provided as a black-correction level; the color-correction gain is substantially increased in proportion to the black-correction level value; and the highest color gain is provided at the highest black-correction level. This can prevent the depth of color from decreasing when adjusting contrast. Although the gain of color correction is rectilinearly changed relative to the black-correction level in the examples of properties (1) and (2), the present invention is not limited to the above. 
     According to the configuration in the embodiment, video contrast can be improved by effectively using the dynamic range of a digital luminance signal, and it is also possible to prevent the depth of color from decreasing when improving the contrast. 
     Additional elements  151 ,  161 ,  171  are now described.  FIG. 7  shows a noise-removing LPF that is one of low-pass filters for removing noise from digital luminance signal Yd obtained by A/D converter  12 ; a maximum-brightness detecting circuit for detecting the maximum brightness level of an output signal (digital luminance signal) of noise-removing LPF  151  during a given period of time, for example, in one frame or in one field; and a maximum-brightness-determining unit that inputs information (a signal) about the maximum brightness level detected by maximum-brightness detecting circuit  161  to identify a bright area corresponding to the maximum brightness level. A gain controller  18 ′ generates and outputs a control signal which controls variable-contrast-gain circuit  30 , variable-brightness circuit  31 , and color control circuit  33 , based on information about the area of brightness corresponding to the maximum brightness level, information about the area of brightness corresponding to the average brightness level, and information about the average brightness level. 
     In the above-mentioned configuration, an analog luminance signal Ya from input terminal T 1  is converted to digital luminance signal Yd by A/D converter  12 . Digital luminance signal Yd is inputted into scan converter  13  and also into noise-removing LPFs  15 ,  151 . After the noise-removing LPFs  15 ,  151  remove noise, digital luminance signal Yd is inputted into average-brightness detecting circuit  16  and maximum-brightness detecting circuit  161 . In average-brightness detecting circuit  16 , the average brightness level during a given period is detected. In maximum-brightness detecting circuit  161 , the maximum brightness level is detected. The pieces of information about the average brightness level and the information about the maximum brightness level, which have been detected, are inputted into average-brightness-determining unit  17  and maximum-brightness-determining unit  171 , respectively. Average-brightness-determining unit  17  identifies an area of brightness corresponding to the detected average brightness level. Maximum-brightness-determining unit  171  identifies an area of brightness corresponding to the detected maximum brightness level. More specifically, an average brightness area corresponding to the detected average brightness level is identified. This average brightness area is, for example, one of four average brightness areas: a high average-brightness area (high APL area), a middle average-brightness area (middle APL area), a low average-brightness area (low APL area), and an extremely low average-brightness area (extremely low APL area). In addition, an area corresponding to the detected maximum brightness level is also identified. This area is, for example, one of three maximum areas of brightness: a saturation brightness area (saturation MAX area), a high brightness area (high MAX area), and a low brightness area (low MAX area). The information about the area of brightness corresponding to the average brightness level and the information about the area of brightness corresponding to the maximum brightness level, which have been identified, are supplied to gain controller  18 ′. In addition, the average brightness level used to identify the area is also provided together with information from average-brightness-determining unit  17 . Based on information about the area of brightness and information about the average brightness level, gain controller  18 ′ generates a control signal which controls variable-contrast-gain circuit  30 , variable-brightness circuit  31 , and color control circuit  33 . 
     According to the configuration in the embodiment, it is possible to obtain stable high contrast, and a decrease in the depth of color can be prevented. In this connection, in each configuration of the embodiments, within a range of an average brightness level greater than or equal to a given value, black-correction processing and contrast-gain-increasing processing are performed for a digital luminance signal after the A/D conversion. However, the present invention is not limited to the above. Either or both of black-correction processing and contrast-gain-increasing processing also may be carried out on an analog luminance signal before the A/D conversion. Further processing may be performed without limiting the range of an average brightness level. 
     This invention provides stable high contrast by detecting an average brightness level to control the contrast gain of a luminance signal, and by black correction using a predetermined quantity of correction according to the average brightness level. The depth of video color can also be improved.