The present invention relates to a gamma correction circuit for correcting a digital video signal, which has been produced by A/D conversion, in accordance with a light-emitting characteristic of a display device. 
In General, a relationship between amplitude of a video signal inputted into the display device and brightness of the displayed image is referred to as gamma characteristic.  A television signal system such as an NTSC system is designed for a CRT-type display device.  Therefore, if the display device is of LCD (liquid crystal display) type, a reverse gamma correction normally using a gamma value xcex3 of 2.2 must be performed on a video signal that is to be inputted into the LCD device.  In other words, since a gamma characteristic of an LCD is different from that of a CRT, both a reverse gamma correction using a gamma value of the CRT and a gamma correction using a gamma value of the LCD must be performed on the video signal that is to be inputted into the LCD device in order to improve the gradation expression in the LCD device.  Furthermore, in a display device that is capable of adjusting the color temperature, which is defined by a ratio of intensities of RGB colors in a white image, in accordance with light-emitting characteristics of the RGB colors, user""s preferences or other conditions, the gamma characteristics in the respective colors must be adjusted so as to avoid a change of the color temperature that may occur when the brightness of the displayed image varies. 
A gamma correction circuit disclosed in the Japanese Patent Kokai Publication No.  10-79956 (date of publication: Mar. 24, 1998) and shown in FIG.  10 has a memory 41 that stores a plurality of different look-up tables in connection with the gamma correction data in order to enable the above-mentioned adjustment.  In the gamma correction circuit, the controller 42 selects from among the look-up tables for the respective colors.  The input signals R, G, B are converted into the gamma-corrected signals Rxcex3, Gxcex3, Bxcex3 of desired levels.  The color correction circuit 43 converts the gamma-corrected signals Rxcex3, Gxcex3, Bxcex3 into color-corrected signals Rc, Gc, Bc, using predetermined conversion coefficients. 
However, in the prior-art gamma correction circuit described above, a great number of different look-up tables must be loaded into the memory 41 in advance in accordance with the display types, user""s preferences or other conditions.  Since such gamma correction circuit needs a memory with extremely large capacity, it is difficult to make the gamma correction circuit at low cost. 
It is an object of the present invention to provide a gamma correction circuit that can cause a video signal to acquire a desired gamma characteristic with a simple configuration. 
According to the present invention, a gamma correction circuit for correcting an input signal to produce a gamma-corrected output signal, comprises a data group generator which stores first to n-th gamma correction tables, receives first to n-th gains, and adjusts a level of the input signal using the first to n-th gamma correction tables and the first to n-th gains, thereby producing a gamma-corrected data group including first to n-th gamma-corrected data, n being an integer not less than two; a gamma characteristic controller which loads the first to n-th gains into the data group generator; a signal level discriminator which outputs a discrimination signal based on a level of the input signal, the first to n-th gamma correction tables, and the first to n-th gains; and an output selector which selects from among the first to n-th gamma-corrected data on the basis of a value of the discrimination signal, thereby producing the gamma-corrected output signal. 
According to another aspect of the present invention, a gamma correction circuit for correcting an input signal to produce an output signal, comprises a data group generator which stores first to n-th gamma correction tables, receives first to n-th gains, and adjusts a level of the input signal using the first to n-th gamma correction tables and the first to n-th gains, thereby producing a gamma-corrected data group including first to n-th gamma-corrected data, n being an integer not less than two; a gamma characteristic controller which inputs the first to n-th gains to the data group generator; a signal level discriminator which outputs a discrimination signal based on levels of the first to n-th gamma-corrected data; and an output selector which selects from among the first to n-th gamma-corrected data on the basis of a value of the discrimination signal, thereby producing the gamma-corrected output signal.