1.  Field of the Invention
The present invention relates to a processing technique regarding video signals.  More specifically, the present invention relates to a video signal processing circuit, a video display device, and a video signal processing method for executing conversion processing on video signals inputted from outside. 
2.  Description of the Related Art
In display devices whose thickness has been reduced more and more in accordance with the recent technical innovation, it is being tried to lower the power consumption by employing LED for the backlight (B/L), for example.  However, even with the thin-type display device employing the LED and the like, the proportion of the power consumed by the backlight among the total power consumption is still large.  Therefore, the techniques and the like for achieving low power consumption by controlling the luminance of the backlight in accordance with the video signals are being continuously studied and developed. 
When trying to reduce the power consumption of the entire thin-type display device, a large effect cannot be acquired unless not only the power consumption of the backlight but also the power consumption of the circuits (IC: Integrated Circuits) for driving and controlling the backlight is reduced as well.  That is, even when the power consumption of the backlight is reduced, the reduced effect of the entire device is lightened unless the reduction of the power consumption of the drive and control circuits is done at the same time.  Thus, low power consumption of the drive and control circuits is also an important factor for acquiring a more significant reduction effect. 
In the technical field of the liquid crystal display devices and the like used while the backlight is lighted up at all times, there is known a method which controls the luminance of the backlight in accordance with the inputted video signals.  As an example of such method, there is a method which decreases the luminance of the backlight and performs corresponding gamma correction at the same time when a video that seems dark as a whole is inputted, for example.  Thereby, the influence imposed upon the visibility of the displayed image is reduced, and the power consumption of the backlight is decreased. 
Such method is also called CABC (Content Adaptive Brightness Control) (generally referred to as CABC hereinafter).  More specifically, it is the technical content which achieves the low power consumption of the backlight through increasing the decrease amount of the luminance of the backlight (luminance decrease amount) and increasing the gradation conversion amount (degree of converting low gradation to high gradation) to increase the transmittance of the panel when the inputted video signals are constituted with dark gradations (low gradations) as a whole. 
Further, when the inputted video signals are constituted with bright gradations (high gradations) as a whole, employed in CABC is a method which keeps the visibility of the original displayed image to be inputted through decreasing the luminance decrease amount as well as the gradation conversion amount. 
When video signals of low gradation as a whole, for example, are inputted to a control circuit (CABC control circuit) which employs CABC and it is judged inside the circuit that the luminance decrease amount is 50%, it is ideal if it is possible to deal with such case with the processing which increase the transmittance to twice as high by executing gradation conversion. 
However, when there is a part of region with a high gradation (high gradation region) even in a case where the gradation of the inputted video signals is low as a whole, gradation expression of the high gradation region cannot be achieved due to gradation collapse (phenomenon where the gradations of the high gradation region are all converted to the maximum gradation by the gradation conversion to be in the same gradation) and the like. 
The gradation collapse causes striking image quality deterioration in the inputted original video signals.  Thus, when a high gradation region is contained in a part of the video signals of low gradation as a whole, it is necessary to have a gradation difference at least between the high gradation region and other low gradation regions. 
Therefore, in order to suppress the image quality deterioration and to increase the luminance decrease amount of the backlight still more in the CABC control circuit, it is necessary to determine in advance a threshold value (border point) for setting the border for the video signals, to divide the gradation at least to two or more regions based on the threshold value, and to execute gradation conversion suited for each of the regions. 
That is, it is required to section the regions for converting the gradation into two or more based on the threshold value, to perform the gradation conversion corresponding to the luminance decrease amount up to the region of the threshold value and to perform not the gradation conversion corresponding to the luminance decrease amount but the gradation conversion by another method determined in advance after the gradation of the threshold value. 
As a method for achieving such gradual gradation conversion, there is known a method which stores gradation conversion information by using LUT (lookup table), for example, in a memory in advance and outputs a single gradation corresponding to the inputted gradation by referring to the LUT (e.g., Japanese Patent Application Publication 2008-117784 (Patent Document 1) or Japanese Patent Application Publication 2009-081602 (Patent Document 2)). 
In Patent Document 1, disclosed is a technical content in which each structure within a mobile phone terminal having various kinds of tables that store values and the like for correcting the luminance level of the backlight performs luminance setting and gamma correction of the backlight by referring to those tables. 
In Patent Document 2, disclosed is a method which reduces the power consumption of the backlight by decreasing the luminance of the backlight and increasing the transmittance of the liquid crystal panel through performing gradation conversion.  Disclosed therein is a technical content in which gradation conversion is performed by applying a constant gain (amplification rate) when the gradation is smaller than the border gradation set in advance and gradation conversion is performed by applying a gain that is decreased as the gradation is larger when the gradation is larger than the border gradation,
Further, as the technical document which discloses gradation conversion executed based on numerical expressions defined in advance, following Japanese Unexamined Patent Publication 2007-310097 (Patent Document 3) or Japanese Unexamined Patent Publication 2005-249891 (Patent Document 4) is known, for example. 
In Patent Document 3, disclosed is a display device which executes a correction calculation for generating output gradation data from input gradation data of a target frame image not based on the LUT described above but based on a calculation expression.  Disclosed therein is a technical content in which a plurality of correction point data corresponding to different gamma values are calculated and gamma correction is executed by using the calculated data. 
In Patent Document 4, disclosed is a method which decreases the luminance of the backlight and increases the transmittance of the panel by gradation conversion to keep the contrast visibility of the liquid crystal display.  This method performs correction by executing gradation conversion corresponding to an ideal gamma curve (γ curve).  More specifically, it is a technical content in which the decrease rate of the backlight luminance is determined based on the maximum gradation value called the peak luminance or the average value of the gradation to make it close to the ideal gamma curve. 
However, with the above-described CABC, the difference between the originally inputted video signal and the video signals on which the conversion processing is done becomes significant when the luminance decrease amount and the gradation conversion amount are increased.  This causes deterioration in the image quality. 
Further, when the luminance change amount changes, the gradation conversion amount also changes accordingly.  Thus, it is required with the conventional CABC control circuit to have an LUT of the resolution of the luminance change amount in advance, so that the circuit scale becomes massive.  In addition, the contents disclosed in Patent Documents 1 and 2 described above are the techniques designed on assumption that an LUT is used.  Thus, there is also an issue with those techniques that the circuit scale is increased. 
With the display device disclosed in Patent Document 2 in particular, the gain is decreased when a gradation larger than the border gradation is inputted.  Thus, in a case where there are a plurality of pixel regions with high gradation in a part of an image that is dark as a whole and the gradations of each pixel in the high gradation part are close to each other, the gradation difference between each of those pixels becomes small.  This causes gradation collapse and a sense of discomfort in the image quality. 
Further, the gradation conversion calculation expression disclosed in Patent Document 3 is in a structure which picks up only the data of some specific points from an ideal gamma curve and executes approximation.  Thus, errors thereof become large, so that the image quality deterioration in the low gradation region in particular becomes conspicuous. 
Further, with the method disclosed in Patent Document 4, the gradation conversion amount called a gain is standardized only to a single expression that is reciprocal to the luminance level.  Moreover, the disclosure therein simply shows on a graph that the gain is reciprocal to the luminance level, and there is no depiction regarding specific gradation conversion numerical expressions and the like.  That is, there is no disclosure mentioned for overcoming the issues such as the image quality deterioration caused due to the above-described gradation collapse and the like. 
It is an exemplary object of the present invention to improve the inconveniences of the above described related techniques.  More specifically, it is an exemplary object of the present invention to provide a video signal processing circuit, a video display device, and a video signal processing method, which can achieve suppression of the image quality deterioration and low power consumption with a small circuit scale when performing the conversion processing on the video signals.