Gray Scale Techniques for Liquid Crystal Displays

T.N. Ruckmongathan

February 2003

Introduction

Electro-optic response of LCDs

Line by line addressing

Frame modulation

Pulse width modulation

Amplitude modulation

Row pulse height modulation

Successive approximation

Conclusion

Electro-optic response of LCDs

Matrix Displays

Line by line addressing

Alt P M and Pleshko P, IEEE Trans. ED-21, pp 146-155, (1974)

Line by line addressing

N1)V(N)V(V

V2c

2cr

RMS

1N

1NatioSelectionR

Maximum selection ratio

Ensures high contrast

Line by line addressing

H Kawakami, et.al., BIENNIAL Display Conference, pp 50-53, (1976).

Hardware complexity

A matrix display with N rows and M columns needs (N+M) drivers Typical driver consists of shift

register to accept serial data, latch to hold the data during row select time and multiplexer to apply a voltage

corresponding to the data Hardware complexity increases with number of voltage levels in the row

and column waveforms

Schematic of a typical LCD driver

Frame modulation

Y Suzuki et.al., SID 83 Digest, pp 32-33, (1983).

Frame modulation

Simple technique

Low hardware complexity

Flicker- a few gray shades

Not useful for TV displays

Popular with STN

Pulse width modulation

H Kawakami et.al., SID 80 Digest, pp28-29, (1980).

Pulse width modulation

(w+1) gray shades with w duty cycles.

Limited number of gray shades

Poor brightness uniformity for large number of gray shades

Amplitude modulationTwo time intervals

c2 )Vg1(g c

2 )Vg1(g

T N Ruckmongathan, Japan Display’92, pp 77-80, (1992)

Amplitude modulation

NNVVgVV

V ccrrRMS

22 2

N2

V)g1(g)g1(g1)(NV)g1(gVV)g1(gV 2c

22222

c2

r

2

c2

r

Amplitude modulation

A R Conner and T J Scheffer, Japan Display’92, pp. 69-72, (1992).

Amplitude modulation

No limit on number of gray shades

Can be extended to multi-line addressing

Large number of voltage levels in the column waveforms

High cost of drivers

Amplitude modulation

A 16x16 matrix LCD using amplitude modulation for displaying 8 shades

B.E Project, (2000-2001)

Row pulse height modulation

H Mano et.al., SID 93 Digest, pp. 93-96, (1993)

Row pulse height modulation

Amplitude of row select pulse is varied in each frame based on the gray shade bit used as data for scanning the

display 2m gray shades with m time intervals More gray shades with less flicker Selection ratio is less than the

maximum

Nm

NmVV2V2V2

V

1m

0i

2cc

ir

2r

1m

0i

2i

RMS

m

i

m

iccr

i

RMSN

)V)N()VV((V

Row pulse height modulation

Successive approximation

Similar to row pulse height modulation Amplitudes of both row and column

waveforms are varied in successive frames

Amplitude is increased by a factor 2½ for each bit from LSB to MSB

Selection ratio is maximum 2m gray shades with m time intervals

Successive frames

B.E Project, (1999-2000)

Successive pulses

B.E Project, (2000-2001)

Successive approximation

1m

0i

1m

0i

1m

0ic

ic

ir

i

RMS

N

V)2(1)(NV)2(V)2(

V

22

mN)NVVV21)(V(2

V2ccr

2r

m

RMS

Successive approximation

A 16x16 matrix LCD displaying 8 gray shades using Successive approximation

B.E Project, (2000-2001)

Successive approximation andMulti-line addressing

Panikumar and Ruckmongathan, ASID’02, pp 229-232, (2002)

Successive approximation with Multi-line addressing

A 32x32 matrix LCD displaying 16 gray shades 3 rows are selected at a time

ASID’02, (2002)

Gray shades based on Wavelets

K.G. Pani Kumar

M. Govind

Sanjay,Santhosh,Santhosh and Raghavendra

Anusha,Anitha and Suman

B.P.Neena

A.R. Shashidhara

Summary

Frame modulation -- Limited gray shades

Pulse width modulation -- Limited gray shades

Amplitude modulation -- No limit on gray shades Large number of levels

in column waveform

Row pulse height modulation -- More gray shades Low selection ratio

Conclusion

More gray shades than frame modulation

Less hardware complexity as compared to amplitude modulation

Maximum selection ratio and hence good contrast in the display

Successive approximation is a better choice for displaying gray shades in passive matrix LCDs.

Acknowledgments

K.G. Pani Kumar

M. Govind

Sanjay,Santhosh,Santhosh and Raghavendra

Anusha,Anitha and Suman

B.P.Neena

A.R. Shashidhara

Thank you

for your attention

Acknowledgments

K.G. Pani Kumar

M. Govind

Sanjay,Santhosh,Santhosh and Raghavendra

Anusha,Anitha and Suman

B.P.Neena

A.R. Shashidhara