REALIZATION OF VIDEO-RATE STN DISPLAY ...

REALIZATION OF VIDEO-RATE STN DISPLAY USING ROW VOLTAGE MODULATIQN METHOD Seung-Woo Lee, Sung-Hoon Park', Hi-Deok Lee', Choong-Ki Kim, and Chul-Hi Han Centerfor Elecfro-Optics Dept. of Elec. Eng., Korea Advanced Institute of Science and Technology, Taejon, 305-701, Korea I also with LG Semicon Co., Ltd. Abstract - A driving method which makes it possible to use conventional digital driver IC's and to achieve multi-gray displays with simple computation is proposed. Low cost video-rate 320x240 8-gray STN-LCD has been realized using the proposed method which adopts multi-line selection scheme.

I. INTRODUCTION Active Addressing(AA) method has made video-rate

S T N displays possible [l][2]. However, multi-gray, active addressed STN displays need high cost analog dnver IC and consequently, redundant digital-analog conversion circuitry [11[2]. Furthermore, complex computation for column signal is indispensable. If active addressed multithe gray display uses pulse height modulation additive virtual signal computation (square, square root and summation) [3] is necessary. Therefore, it is difticult to realize video-rate STN-LCD's with simple configurationand low cost. The objectives of this work are realization of videorate STN-LCD with simple circuit configuration and utilization of digital driver IC's to cut cost and to remove redundant conversion circuitry. The row voltage amplitude modulation (RVAM) method has shown the possibility of gray shades display using digital dnver IC's[4]. It has, however, two problems. One is low selection ratio which is directly related to contrast ratio and the other is high driving voltage. In this paper, the driving method that can achieve higher selection ratio and lower dnving voltage than RVAM is described. We describe the prototype 320x240, 8-gray, video-rate STN display with simple circuit configuration using the 8 level digital driver IC, TMS57206 .

waveforms of the method in case of 16 gray display. 16 gray image can be expressed by 4 bit digital data. The time interval is subdivided into unequal bit time intervals as shown in Fig. 1. In fig. 1 (a), the lst, the 2nd and the 3rd bit time interval have same width, which shows amplitude modulation (AM). The 3rd and the 4th bit time interval of 1:2 width ratio shows width modulation (WM). Hence, the ratio of amplitudes of the lst, the 2nd and the 3rd bit time interval is 1:2:4. On the other hand, the widths of the lst, the 2nd and 3rd are equal and the amplitudes for the 3rd and the 4th are equal with each other. Figure 1 (b) shows the AM of row voltage among upper two bits and lower two bits and WM between the 1st and the 2nd, also between the 3rd and the 4th bit time interval.

1;1-1

o,

II. ROW VOLT-4GE MODULATION METHOD

The row voltage modulation method uses not only amplitude modulationbut also width modulation of the row voltage. Figure 1 shows row selection pulse

unit time interval. t

.,

unit time interval,t .......

...... ;.. ........

I...: I

.

. t l

.................................

0. ....../........... i.............L ..............................

L:

s: t

j

tl

tn2 L

t

(a) (b) Figure 1. The selection pulse waveforms at the case of 16gray display using the proposed method. t k is the kth bit time interval during the nth unit time interval. The voltage, G,(t&, applied to jth column at time t k is written as

a,

where s = J is the total number of the unit time intervals per frame period, K is the number of selection Pulses Per frame period for any row, N is the total number of rows, Iuk 1s thc kth bit of the Pixel information on row 1 Of Column J, is the digital expression of the ith row signal during the nth unit time

0-7803-2624-5/95/$4.00 @ 1995 IEEE

464

interval ("1" for positive and "0" for negative pulse), L is the number of selected rows at a time and "0"means the XOR operation. The equation (1) shows that the RVM gives much simpler column signal computation than P H M in order to achieve multi-gray shades. Just XOR and summation are sufficient for multi-gray. The maximum row voltage can be lowered in the proposed method in comparison with the RVAM. The comparison of the selection ratio and the driving voltage with the RVAM is shown in Table 1. As in Table 1, the selection ratio is much higher and the driving voltage is much lower than those for the RVAM. By WM, both selection ratio and driving voltage level are improved. The proposed method takes advantages of RVAM and WM.

I

personal analog video signal

Fj+z,

Figure 4. The schematic diagram of the controller for 320x 240, 8-gray STN display.

The prototype STN-LCD has shown moving mouse without residual image and has demonstrated 8-gray shades. But the column crosstalk has been appeared when high frequency image, that is, complex picture has been displayed. However, the improved selection ratio and driving voltage have made it possible to implement low cost LCD module. Furthermore, the simple computation, the use of conventional digital driver IC's (TMS57206's, the 8 level driver IC's) have lowered the cost of the STN-LCD. Therefore portable TV's or portable phones can be realized with low cost using the proposed method.

Table 1. Comparison of selection ratio and m a x i " driving voltage of the proposed method with the RVAM.

RVAM I 1 OS86 Fig. 1 (a) 1.0629 Fig. 1 (b) 1.0647 c.f)h.l[ax. selection ratio = 1.0664 (N=240), Vth =: 2 V ( Vth is the threshold voltage crystal to transmit the light. )

followed by storing them in the buffer memory. The digital data are read and then XORed by digital row data, R,(tJ The sum of the XORed results, digital column value, is temporarily stored in the mid memory. Finally, the column data are sent to digital driver IC's.

28.0 v 24.9 V 20.5 V applied to liquid

III. 320~240,8-GRAYVIDEO-RATE STN DISPLAY In order to achieve video-rate STN display, we adopt the multi-line selection (MLS) scheme. Six lines are selected at a time. Each row is selected 8 times per frame. One example of a row voltage waveform is shown in Fig. 2. The sdected pulse waveform must be modulated as in Fig. 3 to achieve 8-gray display.

IV.SUMMARY The 320x240 8-gray video-rate STN display has been implemented by using the row voltage modulation method and adopting MLS (L=6). This display has higher selection ratio than the RVAM and can show the full video-rate image.

V. ACKNOWLEDGMENT The authors greatly appreciate the cooperation of S. J. Choi and C. W. Park of Samsung Display Device Co., Ltd. for preparing the STN panel and would like to thank S . H. Hur for his helpful discussions.

Figure 2. The row driving voltage waveform. Each row is selected 8 times by the selection pulses which are equally distributed throughout the frame period.

rl. I -+

unit time lnterval

tnl

tn2

VI. REFERENCES

tn3

Figure 3. The selection pulse waveform in order to achieve 8gray display. The number of unit time intervals is 320 when N=240 and L=6.

The diagram of the column generation controller is shown in Fig. 4. The analog image information data coming from a computer are converted to the digital data,

[l]T.J.Scheffer and B.Clifton, "Active Addressing Method For High-Contrast Video-Rate STN Displays", pp. 228-231, SID '93 Digest. [2] B.Clifton and D.Prince, "Hardware Architectures For' Video-Rate, Active Addressed STN Displays", pp. 503506, Japan Display '92. [3] A.R.Conner and T.J.Scheffer, "Pulse-Height Modulation (PHM) Gray Shading Methods for Passive Matrix LCD's", pp. 69-72, Japan Display '92. [4] H.Mano, et.al, "An Eight-Gray-Level Method For FastResponding STN-LCD's", pp. 93-96, SID '93 Digest.

465