The present invention relates to a digital-signal-processing circuit and a display apparatus and a liquid-crystal projector that employ the digital-signal-processing circuit. More particularly, the present invention relates to a digital-signal-processing circuit carrying out gamma correction on a digital video signal for driving a display device exhibiting a non-linear optical response characteristic and to a display apparatus and a liquid-crystal projector that employ the digital-signal-processing circuit.
A display apparatus exhibits a device-peculiar non-linear optical response to an input voltage. An example of such a display device is a liquid-crystal display apparatus employing liquid-crystal cells distributed among pixels to serve as electro-optical devices. An example of the optical-response characteristic of the display apparatus is a characteristic representing a relation between the transmittance and the applied voltage, as shown in FIG. 5. Such a characteristic is referred to as a V-T characteristic.
From a gradation-recognition characteristic of a human being, on the other hand, a desirable characteristic of a picture display apparatus display luminance, such as the transmittance, is an exponential function with respect to the level of an input voltage, as shown in FIG. 6. In order to obtain the desirable characteristic shown in FIG. 6 from a picture display apparatus with the V-T characteristic shown in FIG. 5, the voltage to be applied to the liquid-crystal device must be generated appropriately from the level of an input signal, as indicated by the non-linear relation shown in FIG. 7. The non-linear relation shown in FIG. 7 is referred to as a gamma-correction curve for an input digital video signal.
Generally known examples of the gamma correction include multi-break-point correction and correction based on an LUT (Look-up Table). These corrections entail the use of an analog or digital circuit. The LUT-based correction using a digital circuit has the problem of a large circuit scale. With a higher degree of IC integration achieved in the recent years, however, the limitation imposed by the large scale of the circuit is reduced. In addition, offering the merit of a high degree of precision, the LUT-based correction using a digital circuit has been becoming most popular.
In accordance with a related-art technology for implementing the LUT-based digital gamma correction, a digital gamma-correction circuit is used for carrying out the gamma correction as well as the transformation N≧n+2, where notation n denotes the bit count of the input digital video signal supplied to the digital gamma-correction circuit and notation N denotes the bit count of the digital video signal output by the digital gamma-correction circuit. For details of the digital gamma-correction circuit, refer to documents such as Japanese Patent Laid-open No. 2000-20037. This related-art technology is intended for avoiding gradation deterioration in a gray zone.
To put it in detail, the gamma-correction curve representing a relation between the voltage to be applied to the liquid-crystal device and the level of an input signal has large gradients in a black-side zone and smaller ones in the gray zone, as shown in FIG. 7. These gradients indicate that, with the output-bit count set at the same value as the input-bit count, the gradation in the gray zone cannot be kept at the same degree of precision as the input gradation. The related-art technology described above carries out the gamma correction as well as the transformation N≧n+2 in order to avoid gradation deterioration in the gray zone, where notation n denotes the number of bits input to the LUT and notation N denotes the number of bits output from the LUT.
With the related-art technology described above, however, the number of output bits rises with an increase in input-bit count. There is thus raised the problem that the output-pin count of a signal-processing IC for carrying out the gamma correction also increases. In addition, the input-pin count of a D/A converter provided at a later stage rises as well. As a result, the circuit scales of the ICs also increase and the power consumption rises as well, raising the problem of an increased amount of unnecessary radiation. Furthermore, the related-art technology described above is no more than a technology for avoiding gradation deterioration in a gray zone where the gradient of the gamma correction curve is small.