1. Field of the Invention
The present invention relates to a semiconductor display device for displaying images by a time gray scale method. In addition, the invention relates to an electronic appliance using the semiconductor display device.
2. Description of the Related Art
As a driving method of a light-emitting device which is one of the semiconductor display devices, there is a time gray scale method for displaying gray scales by controlling the emission period of pixels using a binary voltage value of a digital video signal. Generally, electroluminscent materials exhibit high response in comparison with liquid crystals; therefore, the light-emitting device is more suitable for the time gray scale method. Specifically, in the case of displaying images by the time gray scale method, one frame period is divided into a plurality of sub-frame periods. In accordance with video signals, pixels are controlled to emit light or no light in each sub-frame period. According to such a structure, the total length of periods in which pixels emit light within one frame period can be controlled by the video signals, thereby gray scales can be displayed.
Display-Period-Separated (DPS) method is one of the time gray scale methods. The following Non-patent Document 1 discloses a specific driving method of the DPS method. [Non-patent Document 1]
Tatsuo Uchida, Heiju Uchiike, Yukio Shimura, Flat-panel Display Comprehensive Dictionary, first edition, Japan, published by Kogyo Chosakai Publishing Inc. in Dec. 25, 2001 (p. 456).
In the DPS method, each sub-frame period includes a period for writing video signals to pixels (writing period), and a period for controlling the luminance of light-emitting elements according to the video signals (display period). Gray scales of the pixels are determined by the total length of display periods, in which the light-emitting elements emit light, in the whole display periods within one frame period.
In the case of the DPS method, each sub-frame period is required to be longer than the writing period. Therefore, in the DPS method, when the number of sub-frame periods is increased in order to increase the number of gray scales, the writing period is required to be shortened by increasing the drive frequency of a driver circuit, or the frame frequency is required to be decreased. However, taking into account the reliability of the driver circuit, it is not preferable to increase the drive frequency much. Further, the time gray scale method including the DPS method has a problem in that more pseudo-contours tend to be displayed in the pixel portion if the frame frequency is lower.
The pseudo-contours are unnatural contour lines which are often perceived when a middle-level gray scale is displayed using the time gray scale method, which are supposedly caused by variations of perceived luminance as a peculiar characteristic of the human vision. It is known that when the sub-frame period is further divided into a plurality of periods, generation of pseudo-contours can be suppressed even when the display device is driven with the same frame frequency. In the case of dividing the sub-frame period, however, each divided sub-frame period is required to be longer than the writing period. Accordingly, the drive frequency of the driver circuit is undesirably increased as in the case where the number of gray scales is increased.