1. Field of the Invention
The invention relates to a driving method of a plasma display panel (hereinafter, simply referred to as a xe2x80x9cPDPxe2x80x9d) of a matrix display type.
2. Description of Related Art
As one of the display panels of the matrix display type, a PDP of AC (alternating current discharge) type is known.
The AC type PDP has a plurality of column electrodes (address electrodes) and a plurality of row electrode pairs which are arranged so as to perpendicularly intersect the column electrodes and in which one scanning line is formed by one pair. Each of the row electrode pairs and the column electrodes are covered by a dielectric layer against a discharge space and a discharge cell corresponding to one pixel is formed at an intersection point of the row electrode pair and the column electrode.
As a method of allowing the PDP to embody a halftone display, there is a so called a subfield method wherein one field period is divided into N subfields each emits the light only for a time corresponding to a weight of each bit digit of pixel data of N bits and the data is displayed. The method is disclosed, for example, in Japanese Patent Kokai No. 4-195087.
FIG. 1 is a diagram showing a format of the light-emission driving in one field period according to the subfield method.
In the example shown in FIG. 1, in which it is presumed that the pixel data being supplied consists of six bits, the period of time of one field is divided into six subfields SF1, SF2, . . . , and SF6, to perform the light-emission driving. By executing the light emission through the six subfields once, an expression of 64 gradations can be performed for an image of one field.
Each subfield is constructed by an all-resetting step Rc, a pixel data writing step Wc, and a sustain light emitting step Ic. In the all-resetting step Rc, by discharge-exciting (reset discharge) all of the discharge cells of the PDP in a lump, wall charges are uniformly formed in all of the discharge cells. In the next pixel data writing step Wc, a selective erasure discharge according to the pixel data is excited every discharge cell. The wall charges in the discharge cell in which the erasure discharge has been performed are extinguished and this discharge cell becomes a xe2x80x9cnon-light emission cellxe2x80x9d. The discharge cell in which the erasure discharge is not performed becomes a xe2x80x9clight emission cellxe2x80x9d because the wall charges remain there. In the sustain light emitting step Ic, a discharge light emitting state is continued only for the light emission cell only for a time corresponding to the weight of each subfield. In each of the subfields SF1 to SF6, consequently, a sustain light emission is sequentially performed with weights of light emitting period ratios of 1:2:4:8:16:32.
For example, if an image such that a flat object moves is displayed by the driving method, however, there is a problem such that a fringe-like pseudo outline like a video image as if the gradations were lost is observed at points near the positions where the luminance gradation level transverses the boundary of n-th power of 2, such as xe2x80x9c32xe2x80x9d or xe2x80x9c16xe2x80x9d.
The above problem occurs because in the case where the luminance gradation level is xe2x80x9c32xe2x80x9d, the light emission is performed only in the subfield SF6 during the 1-field period as shown in FIG. 1 and, when the luminance gradation level is equal to xe2x80x9c31xe2x80x9d, the light emission is not performed in the subfield SF6, and the light emission is performed in the subfields SF1 to SF5. That is, for a period of time during which the discharge cell which should perform the light emission at the luminance gradation level xe2x80x9c32xe2x80x9d is lit on, the discharge cell which should perform the light emission at the luminance gradation level xe2x80x9c31xe2x80x9d is certainly in a light-off state, so that the fringe-like outline that is not concerned with the image is confirmed on the boundary of the discharge cells.
To suppress the pseudo outline and improve a display quality, therefore, a driving method whereby a subfield of a relatively long light emitting period is further divided into a plurality of subfields and they are distributed and arranged in the 1-field period has been proposed. According to the driving method, as a light emission pattern in the 1-field period is uniformed by increasing the number of subfields, a suppressing effect of the pseudo outline rises.
There is, however, a problem such that as the number of subfields increases, the number of bits of drive data that is formed in correspondence to each subfield has to be also increased in accordance with it and a scale of the apparatus increases with the increase in number of bits.
The invention is made to solve the problems and it is an object of the invention to provide a driving method of a plasma display panel, in which a scale of an apparatus can be reduced by suppressing the number of bits of drive data while maintaining an image display of a high quality in which a pseudo outline has been suppressed.
According to the invention, there is provided a driving method of a plasma display panel in which a display period of one field is divided into a plurality of subfields and a light emitting state in each of the subfields is set in accordance with each bit of pixel data, to perform a halftone display, wherein the light emitting state in a subfield of a relatively long light emitting period is also set in accordance with the bit of the pixel data which sets the light emitting state in a subfield of a relatively short light emitting period among the subfields.