Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-09-05
2003-03-18
Saras, Steven (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S087000, C345S100000
Reexamination Certificate
active
06535195
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to the problem of addressing large-area liquid-crystal displays that do not require the costly processing steps: needed to incorporate a thin-film transistor, or some other switching device, into each subpixel.
BACKGROUND OF THE INVENTION
The liquid-crystal displays currently used on most portable computers are classifiable as active-matrix LCDs. An active, i.e. switching, device incorporated into each subpixel of these displays transfers and holds a specified charge on a transparent electrode. These switches are typically field-effect transistors formed in thin films of amorphous silicon, or sometimes polycrystalline silicon. However, the capital investment required for the equipment needed to deposit silicon and form thin-film transistors (TFTs) with the necessary electronic switching behavior is high. Furthermore, this large investment must be repeated for each new generation of processing equipment capable of handling larger glass substrates. It appears now that the next (fourth) generation TFT-LCD fabs will use 800-mm by 950-mm substrates. The capital cost of the processing equipment per substrate divided by the number of 17.x-inch displays in the array (
6
), taking the yield of individual displays into account, should be attractive for desktop computing in the next few years.
Making large TFT-LCD backplanes one at a time in a 4th-generation fab would be inappropriate for a cost-sensitive application like consumer television. In the first place, the capital expense would be concentrated 6-fold compared to the 17.x-inch desktop display scenario. Furthermore, the yield for such large backplanes would be lower, raising the cost even higher. Finally, the maximum size at the desirable 16 by 9 aspect ratio would only be about 42 inches in diagonal, which is of marginal interest for standard definition (480-line) digital TV and clearly too small for high-definition (720-line or better) TV in the US market. In order to make LCDs at larger sizes and lower costs, it seems to be necessary to forego the advantages associated with creating an active element to control each sub-pixel.
On several occasions over the past quarter of a century, LCD technologists overcame what turned out to be only apparent limits to the information content that can be displayed without using active switching elements.
FIG. 1
illustrates the first approach to this problem, which was described by Allan R. Kmetz in “Liquid-Crystal Display Prospects in Perspective,” IEEE Transactions on Electron Devices, Vol. ED-20 (1973) pp. 954-961. The pixels in a row are addressed together by applying a “select” pulse to one row at a time while “data” voltages are presented to the columns. All other (i.e. unselected) rows are kept at ground potential. The display is scanned, which means repeating this process for each row until all: the rows are addressed in what is called a frame. Therefore, addressing voltage waveforms must be repeated at some acceptable frame frequency. This frame rate is typically chosen to be at least 30 Hz so that the illusion of continuous motion can be created as the pixel patterns change from one frame to the next. However, it should be noted that the response time of the LCD may be different from the frame time.
The voltage applied to a pixel in
FIG. 1
is equal to the difference between the select and data-voltage waveforms on the two electrodes defining that pixel. A larger voltage appears across the pixel if the row and column voltages have opposite sign. Now, it was known experimentally that nematic liquid crystals respond to the RMS (root mean-square or AC average) of the voltage applied to them. Choosing the data waveform to be ±V
0
makes the undesirable contribution to the RMS voltage (applied when other rows are being addressed) independent of the image. Furthermore, choosing, the select-pulse voltage V
S
to be exactly twice the magnitude, of the data voltage causes the voltage applied to an off pixel while it is being addressed to have this same value also. Thus the RMS voltage applied to an off pixel is V
0
independent of the rest of the image. The voltage applied to turn a pixel on is 3V
0
while the pixel is being addressed and ±V
0
the rest of the time. If there are N rows to be addressed, the ratio of the RMS voltages V
ON
/V
OFF
turns out to be
V
ON
V
OFF
=
(
1
+
8
N
)
1
/
2
.
(
1
)
This ratio is not very large for N=240, which is appropriate for VGA or SDTV in the dual-scan configuration where the columns are split in the middle and independently driven from the top and the bottom. Although the rows are also independently driven, the two halves of a dual-scan display can be synchronized if necessary. Now the value of V
ON
/V
OFF
that will be required to address a display depends on the details of the liquid-crystal composition and alignment. Turning the relationship around, we can see that this 3:1 method of addressing limits the number of addressable rows to:
N
=
8
(
V
ON
V
OFF
)
2
-
1
,
(
2
)
when the on/off ratio is specified. For example, if the minimum ratio available is 1.2, only about 18 rows can be addressed. A dual-scanned display could have twice as many rows, but the total would still be discouragingly low for general purposes. For the more interesting case of N
MAX
=240, which allows a VGA or SDTV format with dual scanning, the on/off voltage ratio available is about 1.0165. This ratio was inadequate to switch known liquid-crystal configurations.
The 3:1 addressing scheme was predicated on the assumption that the RMS values of the unselected and off voltages should be the same. Whatever benefits that might have, it is not the same thing as maximizing the on/off ratio for a given number of rows. Paul M. Alt and Peter Pleshko showed this in “Scanning Limitations of Liquid-Crystal Displays,” which appeared in IEEE Transactions on Electron Devices, Vol. ED-21 (1974) on pp. 146-155. In fact, a bigger on/off ratio can be obtained by increasing the select voltage relative to the data voltage as shown by the dashed pulse waveform in FIG.
1
. The optimum ratio according to Alt and Pleshko is given by:
V
S
V
0
=
N
1
/
2
.
(
3
)
On the other hand, for a given value of the ratio of the RMS values of the on and off voltages, the maximum number of rows that can be addressed is given by:
N
MAX
=
(
(
V
ON
V
OFF
)
2
+
1
(
V
ON
V
OFF
)
2
-
1
)
2
.
(
4
)
For the interesting case of N=240, the on/off ratio that is needed turns out to be about 1.067. That was a big improvement over 3:1 addressing, but to be generally useful, a liquid-crystal configuration that could be switched by such a small ratio was still needed.
In 1985, Scheffer et al. reported their work on a liquid-crystal configuration that does respond to a reduced on/off voltage ratio in “24×80 Character LCD Panel Using the Supertwisted Birefringence Effect,” 1985 SID Symposium Digest of Papers, Vol. 16, pp. 120-123. This was the beginning of a new generation of what are now called supertwist or STN displays because the nematic liquid crystal twists by more than 90 degrees between the front and back substrates. STN displays with 240:1 multiplexing can be made to switch within the narrow voltage range available in Alt-Pleshko addressing. Dual-scan VGA STN displays have been used successfully for notebook computers but they were originally not practical at video rates. Of course, faster response can be obtained in active-matrix LCDs, where a thin-film switch is provided to control each subpixel. Unfortunately, AMLCDs are prohibitively expensive in the large sizes desirable for family entertainment. Furthermore, they have motion artifacts caused by the memory of the pixels between successive address times. This is because the human visual system tries to track an element moving quickly across a display, and the excessive persistence of the pixels smears the perceived image.
STN LCDs can be made with a fast-responding liquid-crystal material, but pixels relax between successive selections of the rows defining them when c
Nelson Alecia D.
Saras Steven
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