gray scale techniques for liquid crystal displays t.n. ruckmongathan february 2003
TRANSCRIPT
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