hereinafter , an implementation of this document will be described in detail referring to the attached drawings . fig2 is a schematic block diagram illustrating an organic light emitting display device according to a first embodiment of the present invention . referring to fig2 , the organic light emitting display device comprises a display unit 10 and a driver 20 that drives the display unit 10 upon receiving rgb data from the exterior . the display unit 10 comprises a plurality of sub - pixels positioned at crossings of scan lines and data lines . a sub - pixel 11 may comprise a diode that comprises a first electrode , an organic light emission layer , and a second electrode . in the display device according to the first embodiment of the present invention , the display unit 10 comprises unit pixels of 240 × 320 . fig3 is an equivalent circuit diagram of the sub - pixel in fig2 . the sub - pixel 11 comprises first and second thin film transistors ( tfts ) m 1 and m 2 , p - type mos transistors , which are turned on when a negative polarity signal is applied to their gate electrodes , and a capacitor cst . the first tft m 1 transfers a data signal inputted through a data line dm according to a scan signal applied through a scan line sn . the capacitor cst stores the inputted data . the second tft m 2 is turned on or off according to the inputted data signal to supply current to an oled . the operation of the sub - pixel 11 of the organic light emitting display device constructed as described above will now be explained in detail . when the first tft m 1 is turned on by a select signal applied to its gate electrode , the data signal is transferred through the data line dm . the data signal is stored in the capacitor cst and the stored data signal is applied to the gate electrode of the second tft m 2 . the second tft m 2 generates current corresponding to a difference between the data signal applied to its gate electrode and voltage connected with its source electrode and provides the current to the oled . then , the oled generates a corresponding current . fig4 is a schematic block diagram illustrating a driver of the organic light emitting display device according to the first embodiment of the present invention referring to fig4 , the driver 20 of the organic light emitting display device according to the first embodiment of the present invention comprises a host memory unit 22 of 1 . 38 mbit , a gamma correction unit 24 , a half - tone unit 26 , a sub - field memory unit 28 , and a digital - to - analog converter ( dac ) ( not shown ). the host memory unit 22 receives rgb data from a main body interface and transmits image data to the gamma correction unit 24 . in this case , in the driver 20 , each unit pixel of 240 × 320 comprises rgb sub - pixels and a single sub - pixel comprises six sub - fields , so the main memory unit 22 comprises 1 . 38 mbit (= 240 × 320 × 3 ( rgb )× 6 ). the gamma correction unit 24 gamma - corrects the image data . the gamma correction unit 22 provides a gamma value according to luminance characteristics of the organic light emitting display device to a data driver . the half - tone unit 26 half - tones the gamma - corrected image data by using one of truncation , a random e / d , a normal e / d and a dither . as mentioned above with respect to the related art , in the digital driving method , gray levels that may be represented may differ according to the number of sub - fields , and currently , six sub - fields are used at 60 hz . the number of real gray levels that may be represented by using the six sub - fields is 2 6 = 64 . a sub - field may be expressed by a mapping therefore 64 mappings of the binary number exist . in this respect , however , due to a problem of a false contour , all the gray levels cannot be used but only some gray levels are selectively used and other gray levels are generated by the half - tone unit 26 . the half - tone unit 26 creates multiple in - between gray levels with the selected gray levels by using one of the truncation , the random e / d , the normal e / d , and the dither , and uses them . in other words , a certain number of mappings that allow changes of optical axes to continue the most smoothly are selected to use as real gray levels , and the half - tone unit 26 generates multiple in - between gray levels as required with the selected gray levels through half - toning . the sub - field memory unit 28 comprises a sub - field memory a 30 and a sub - field memory b 32 each having 7 . 37 mbit (= 240 × 320 × 3 ( rgb )× 16 ( sub - fields )). in order to remove the false contour and improve gray linearity , respective sub - fields should be disposed to have a step as small as possible therebetween . thus , for this purpose , the sub - field memory a 30 and the sub - field memory b 32 use 16 sub - fields . ( the number of sub - fields that are used may exceed 16 .) the sub - field memory a 30 selects a sufficient number of real gray levels . the sub - field memory a 30 calculates a gravity center to create a sub - field code . this basic mapping code is called a mapping table a and used as a sub - field code applied for a still image . because the sub - field memory a 30 has the sufficient number of real gray levels , it may well express a still image with a half - tone noise . the sub - field memory b 32 creates a second mapping table in which all the remaining sub - fields except for a maximum ( the highest ) sub - field of the mapping table a , namely , which are lower than the maximum sub - field , are turned on , and uses it as a false contour buffering mapping table b . in this case , because the sub - field memory 32 uses the mapping table b in which the sub - fields lower than the maximum sub - field are turned on as the buffering table , gray levels of a finally created mapping code are slightly degraded compared with the previous one . in case of a moving image ( or moving picture , video ), the sub - field memory unit 28 may use an ab alternating method , namely , both mapping codes of the sub - field memory a 30 and the sub - field memory b 32 , to increase the probability that the sub - fields generating the false contour are turned on to distributively cancel out the positions of the false contour , thus obtaining the effects of restraining the false contour . in the first embodiment , because the mapping with multiple real gray levels is used , a sharp still image may be obtained . in case of the moving image , because the probability of illumination of the sub - fields increases by using the buffering sub - fields , the false contour may be reduced . if the false contour is intended to be restrained as much as possible , another mapping table may be additionally created in which the buffering sub - fields are larger than the previous one . by using the two buffering sub - fields , mutual values of the false contour may be canceled out to create the required gray levels . the thusly modulated data is transferred to the d / a converter , which is converted into an analog signal by the d / a converter , which is then supplied to the display unit . the display unit displays a corresponding image . fig5 is a schematic block diagram illustrating a driver of the organic light emitting display device according to a second embodiment of the present invention . referring to fig5 , a driver 40 of the organic light emitting display device according to the second embodiment of the present invention has the same structure as that of the first embodiment of the present invention as discussed above in that the driver 40 comprises a host memory unit 42 of 1 . 38 mbit (= 240 × 320 × 3 ( rgb )× 6 ), a gamma correction unit 44 that gamma - corrects image data , and a half - tone unit 46 that half - tones the gamma - corrected image data by using one of truncation , a random e / d , a normal e / d , and a dither , so its detailed description will be omitted . a sub - field memory unit 48 is different from the sub - field memory unit 28 of the first embodiment of the present invention in that the sub - field memory unit 48 comprises a sub - field memory a 50 and a sub - field memory b 52 each having 240 × 320 × 3 ( rgb )× 6 ( sub - fields ) while the sub - field memory unit 28 comprises the sub - field memories of 240 × 320 × 3 ( rgb )× 16 ( sub - fields ). consequently , the sub - field memory unit 48 has a memory capacity reduced to ⅓ of that of the sub - field memory unit 28 in the first embodiment of the present invention , while having the same picture quality . the reduction in the memory capacity may lead to a reduction of a size of a die of wafer , so the production yield may be enhanced . the sub - field memory a 50 may express a still image by selecting a sufficient number of real gray levels while the sub - field memory b 52 uses the false contour buffering mapping table b in which the sub - fields lower than the maximum sub - field , excluding the maximum sub - field , are turned on , likewise as in the first embodiment of the present invention . thus , in case of a moving image , the sub - field memory unit 48 uses the ab alternating method , namely , alternately uses the both mapping codes of the sub - field memory a 50 and the sub - field memory b 52 . in addition , in order to maximize the effects of restraining the false contour , another mapping table may be additionally created in which the buffering sub - fields are larger than those of the related art , and used . fig6 is a conceptual view of a driving method of the driver of the organic light emitting display device according to the second embodiment of the present invention . referring to fig6 , the driver of the organic light emitting display device according to the second embodiment of the present invention employs a time - to - bit mapping method in order to obtain the same picture quality as that of the organic light emitting display device according to the first embodiment of the present invention , while reducing the memory capacity of the memory unit 48 by one - third . this driving method is a method extending from a bit mapping split method , whereby the false contour problematic in the digital driving may be reduced . of the sub - field memory unit 48 , the sub - field memory a 50 and the sub field memory b 52 , each having 240 × 320 × 3 ( rgb )× 6 ( sub - field ), use 6 bits by dividing them into 16 sub - fields , so when each sub - field is called , bit data is called from each memory . as shown in fig6 , in the sub - field memory a 50 and the sub - field memory b 52 , sub - field # 1 is allocated to 1 bit , sub - fields # 2 and # 3 are allocated to 2 bits , . . . , and sub - fields # 13 , # 14 , # 15 , and # 16 are allocated to 6 bits , so as to be used . in this driving method , which sub - fields and how many sub - fields are allocated to bits of the sub - field memory a 50 and the sub - field memory b 52 may be arbitrarily varied depending on picture quality . fig7 is a view illustrating the structure of sub - fields of the organic light emitting display device according to the second embodiment of the present invention . referring to fig7 , in the organic light emitting display device according to the second embodiment of the present invention , sub - fields are divided into two groups and their bit values are called according to the order of the sub - fields . the reason for dividing the sub - fields into two groups is to prevent flickering of the sub - fields . the groups of sub - fields may be arbitrarily divided in mapping them . t 16 is fixedly used for an sp ( scan pulse ). t 16 comprises an addressing time and a display time . the sp proceeds in the display time of the t 16 . the display time is formed in consideration of a delay time of the sp . as stated above referring to fig6 and 7 , the driver of the organic light emitting display device according to the second embodiment of the present invention has the advantages that the sub - field memory capacity may be reduced and the sub - fields may arbitrarily extend and be varied in disposition . in addition , this driving method does not need such a sub - field mapping table . the embodiments of the present invention have been described referring to the drawings but the present invention is not limited thereto . the above - described embodiments exemplarily describes the organic light emitting display device , but without being limited thereto , the present invention may be also applied for any display device such as an inorganic light emitting display device or the like . in the organic light emitting display device according to the above - described embodiments , the sub - pixels comprise two transistors and one capacitor , but without being limited thereto , the sub - pixels may comprise a different number of transistors and capacitors . in the above - described embodiments , the driver comprises the gamma correction unit and the half - tone unit , but if the driver may be driven without them , the gamma correction unit and the half - tone unit may be omitted . in the above - described embodiments , one frame is divided into 6 sub - fields and 16 sub - fields , but the number of sub - fields may be arbitrarily determined in consideration of resolution and luminance . the present invention constructed as described above has the following advantages . first , the picture quality may be improved by employing the time - to - bit mapping method , while reducing the sub - field memory capacity and freely extending the sub - fields . second , the sub - fields may be arbitrarily ( freely ) disposed , and because the sub - fields may be divided by two or more groups , generation of false contour or flicking may be prevented . the foregoing description of the preferred embodiments of the present invention has been presented for the purpose of illustration and description , and it is not intended to be exhaustive or to limit the invention to the precise form disclosed , and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention . thus , it is to be understood that the invention is not limited to the disclosed embodiments , but , on the contrary , is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims .