the basic configuration of a plasma display panel driven according to a method of controlling the luminance of the display panel equivalent to this embodiment is similar to that of the plasma display panel described in the prior art and as shown in fig1 one discharge cell 12 is provided to the intersection of one scanning electrode 2 , one sustain electrode 3 and one data electrode 5 perpendicular to these . the driving waveform is also similar to that of the conventional type plasma display and gradation is represented by varying the quantity of emission in a sustain period and combining subfields each of which includes a priming period , a selective operational period , the sustain period and a sustain elimination period . next , the weighting of the luminance of each subfield for gradation representation according to the invention will be described . table 2 shows relation between the number of sustain cycles of each subfield according to an average luminance level ( apl ) of an image and the total number of sustain cycles in one field . next , the operation of a circuit for converting a video signal to a signal for driving the plasma display panel will be described . fig6 is a block diagram showing the operation of the circuit used in this embodiment for converting a video signal . as a video signal for a cathode - ray tube is transmitted in a state having gamma characteristics , an output level is first controlled in a gamma control circuit for the plasma display panel having no gamma characteristic as an output characteristic . next , the luminance level of each color of rgb is converted to a digital signal by an a / d converter . in this embodiment , it is converted to a 10 - bit binary code . afterward , the luminance level of each pixel is added by an apl determination device and apl is determined . next , the conversion of a gradation according to each apl value is made in a gradation conversion circuit . in the case of weighting based upon the number of sustain cycles as shown in table 2 , when apl is high , the gradation representation of 256 gradations ( 8 bits ) is enabled at apl 3 for example as heretofore . in the meantime , when apl is low , the number of representable gradations increases . for example , at apl 2 , representation in 512 gradations ( 9 bits ) is enabled , at apl 1 , representation in 768 gradations is enabled and further , at apl 0 , representation in 1024 gradations ( 10 bits ) is enabled . therefore , in the gradation conversion circuit , the number of gradations is adjusted based upon apl . in the case of apl 0 , a 10 - bit luminance signal is output as it is . in case apl is high , the number of gradations is reduced according to the respective numbers of gradations . concretely , in case apl is 1 , the least - significant bit of original 10 bits is deleted and 9 - bit data has only to be output . luminance data output from the gradation conversion circuit is sent to a subfield coding circuit and there , the selective state of a subfield according to each apl is determined . actually , the above operation is realized by having lut for coding every apl . the effect of the improvement of image representation by the above operation will be described below . according to this embodiment , the luminance of sf1 which is at the lowest luminance level is always fixed without depending upon apl and is the minimum luminance level representable on the plasma display panel . therefore , a luminance step for gradation representation of an image at apl 0 on a relatively dark screen though peak luminance is high can be also reduced . [ 0141 ] fig7 is a graph showing relation between the luminance levels of images at apl 0 and at apl 3 and actually output luminance . in fig7 only a part of low luminance is displayed . the smoothness of gradation in a particularly dark image is improved by reducing a step of luminance as described above and a more natural image can be represented . further , an effect upon a false contour of an animation will be described . generally , the more the number of subfields is for the number of bits for gradations , that is , the higher redundancy is , the more a false contour of an animation is improved . therefore , in case the number of gradations is increased as in this embodiment , performance related to a false contour of an animation may be deteriorated . however , a false contour of an animation is originally remarkably seen in a relatively light image having smooth gradation . such an image can be said an image the apl of which is relatively high . in the meantime , in a dark image as a whole in a part of which a light part exists and the apl of which is low , a false contour of an animation is hardly recognized . therefore , as subfield coding the redundancy of which is high is applied to an image the apl of which is high and a false contour of an animation of which often appears and subfield coding the redundancy of which is low is applied to an image the apl of which is low and a false contour of an animation of which hardly appears if the luminance control method equivalent to this embodiment is used , the whole performance against a false contour of an animation is hardly deteriorated . in this embodiment , after a / d conversion , the circuit for determining apl is used , however , a circuit for determining apl based upon an analog signal and converting from analog to digital may be also used . a method of complementing a gradation after 8 bits are converted from analog to digital , executing temporal and spatial multi - gradation processing such as the error diffusion in the case of apl 3 and executing multi - gradation processing in which luminance at an intermediate level and processing such as the error diffusion are combined at apl 2 or less can be also applied . however , even if apl is low , performance against a false contour of an animation may be deteriorated when the redundancy of subfield coding is reduced depending upon an image . therefore , an embodiment in case the further improvement of performance against a false contour is required will be described below . a plasma display panel driven in this embodiment and the basic driving waveform are also similar to those in the first embodiment . the number of subfields for gradation representation is also similarly 12 . in the meantime , values shown in table 3 are used for the weighting of the luminance of each subfield for gradation representation . table 3 shows relation between the number of sustain cycles of each subfield according to an average luminance level ( apl ) of an image and the total number of sustain cycles in one field . if the weighting of luminance shown in table 3 is used , the representation of 256 gradations is enabled at apl 3 as in the first embodiment , the representation of 384 gradations is enabled at apl 1 and the representation of 512 gradations is enabled at apl 2 and apl 0 . when such weighting is used , the redundancy of subfield coding is deteriorated by one bit . as the redundancy is deteriorated by two bits in the first embodiment , performance against a false contour is more improved than that in the first embodiment . in the meantime , as for the gradation representation of an image the apl of which is low , the minimum luminance level is 2 at apl 0 , the gradation representation is improved because the luminance step is reduced to a half or less compared with that in the conventional type driving method . though half luminance is not enough for independent gradation representation , it is very effective in case a halftone is temporally and spatially represented by the combination with the error diffusion and others and is effective as a method of holding performance against a false contour of an animation and improving gradation representation . in this embodiment , a method of inhibiting the deterioration of a false contour of an animation by another method will be described . a plasma display panel driven in this embodiment and the basic driving waveform are also similar to those in the first embodiment . the number of subfields for gradation representation is also similarly 12 . further , values for the weighting of the luminance of each subfield for apl are also similar to the values shown in table 2 as in the first embodiment . next , table 4 shows relation between a luminance level of an image acquired by an a / d converter and an output luminance level in the case of apl 0 . the luminance levels are all represented by 10 bits ( 0 to 1023 ). as shown in table 4 , when a luminance level of an image is 0 to 255 , a luminance step of output is 1 , when it is 256 to 511 , a luminance step of output is 2 and further , when it is 512 or more , a luminance step of output is 4 . in other words , at a luminance level exceeding intermediate luminance , a luminance step is equivalent to 8 bits . a human visual sense is very sensitive to the change of luminance at low luminance , however , when the absolute value of luminance is large , it is difficult to discriminate small difference in luminance . therefore , in case a minute luminance step can be represented in a low luminance region as described above , the deterioration of gradation is hardly recognized even if a step of the change of luminance becomes large in a high luminance region . if the luminance step is 4 as in this embodiment , it is similar to the luminance step at apl 0 in the conventional type luminance control method and it can be said that there is no deterioration of gradation . next , a method of inhibiting the deterioration of performance against a false contour of an animation will be described . a false contour of an animation is caused because the selection / the non - selection of a subfield having large weight changes in case an image having smooth gradation moves or a sight is moved . in the concrete , in the case of an image the gradation of which changes from the 760th gradation to the 780th gradation for example in case a luminance step is 1 at all gradations at apl 0 , the selection of s subfield at the 766 gradation is represented as ( 111111111110 ) and the selection of a subfield at the 767th gradation is represented as ( 100110111101 ) ( in this case , an emitted subfield is represented as 1 and an unemitted subfield is represented as 0 ). at the 766th gradation , the center of the gravity of emission is located near to a front half of a field and at the 767th gradation , the center of the gravity of emission is located relatively near to a rear half because largely weighted sf12 is selected . therefore , the maximum false contour of an animation occurs in parts having the 766th gradation and the 767th gradation . as described above , a false contour of an animation is caused by the temporal movement of the center of the gravity of emission by the change of the selection / the non - selection of a subfield at a high luminance level . according to this embodiment , at a luminance level exceeding the 512th gradation , a luminance step is 4 . therefore , an output luminance level is like 759 , 763 , 767 and 771 . the selection of a subfield at the 763th gradation is represented as ( 001111111110 ) and the center of the gravity of emission moves on the rear side of a field , compared with the 766th gradation . therefore , the movement of the center of the gravity of emission is less in the arrangement of the 763th gradation and the 767th gradation , compared with that in the arrangement of the 766th gradation and the 767th gradation , and a false contour of an animation is greatly improved . in this embodiment , a luminance step in a high luminance region is 4 equivalent to 8 bits , however , in actual display , in consideration of correlation with a visual sense characteristic , a luminance step can be also set to a further large luminance step . in each luminance region , a luminance step is not required to be set to an equal interval and a method of arbitrarily selecting a used gradation except a gradation at which a false contour of an animation often occurs can be also applied . as described above , according to the method of controlling the luminance of the display panel according to the invention , smooth gradation representation is enabled independent of the change of an average luminance level ( apl ) particularly , gradation characteristics in a relatively dark image the apl of which is low are improved . this reason is that a luminance level of a subfield represented as a luminance step is fixed without depending upon apl or the whole change of luminance is inhibited up to smaller change and in an image the apl of which is low , the number of display gradations is increased . the deterioration of a false contour of an animation is hardly caused tough the number of display gradations is increased and the redundancy of subfield coding is deteriorated . this reason is that in only an image the apl of which is low , the number of gradations is increased . further , the deterioration of performance against a false contour of an animation can be inhibited by combining method of inhibiting the deterioration of redundancy and a method of disusing a gradation at which a false contour of an animation often occurs for display .