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 , (1974)

Line by line addressing  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 modulation Two time intervals T N Ruckmongathan, Japan Display’92, pp 77-80, (1992)

Amplitude modulation

A R Conner and T J Scheffer, Japan Display’92, pp , (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, ( )

Row pulse height modulation H Mano et.al., SID 93 Digest, pp , (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  2 m gray shades with m time intervals  More gray shades with less flicker  Selection ratio is less than the maximum

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 2 ½ for each bit from LSB to MSB  Selection ratio is maximum  2 m gray shades with m time intervals

Successive frames B.E Project, ( )

Successive pulses B.E Project, ( )

Successive approximation

A 16x16 matrix LCD displaying 8 gray shades using Successive approximation B.E Project, ( )

Successive approximation and Multi-line addressing Panikumar and Ruckmongathan, ASID’02, pp , (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