Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-04-27
2003-07-29
Shankar, Vijay (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S004000
Reexamination Certificate
active
06600467
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the field of display structures and display architecture. In particular, this invention relates to the architecture of passive liquid crystal display (LCD) flat panels.
BACKGROUND OF THE INVENTION
The number of applications for display technologies is rapidly increasing. However, display technologies have been generally employed in relatively small devices, such as, notebooks, computers, cameras, telephones, projection displays and direct-view flat panel televisions. For example, recent digital cameras employ flat panels as viewfinders and play back displays. Commonly, super twisted nematic (STN) displays are employed in such applications. On the other hand, more expensive thin film transistor (TFT) LCDs have applications in notebook computers and flat panel monitors.
The television industry is beginning to use flat panel displays and supplant older unwieldy cathode ray tube (CRT) technologies. For example, LCDs are becoming common in hand-held televisions. LCDs also provide some digital performance enhancements over older cathode ray tube (CRT) technologies.
The television market has been dominated by the CRT for the past few decades. As a result, the mature CRT manufacturing industry has been able to significantly lower the production cost of CRTs. Nevertheless, the market price for smaller televisions has eroded so that the cost to build a small CRT-based television is very close to the selling price. Therefore, there is little margin in the manufacturing cost to permit lowering the price to compete with lower cost alternatives. Consequently, a cost-driven market window for TV displays is apparent. Yet, current LCD architectures are too costly to exploit this potential opportunity and suffer from acceptance-limiting performance shortcomings. For example, it is too costly to build television size LCD displays, and the contrast ratio of most LCD displays has yet to rival CRTs.
Non-CRT based displays generally have limited viewing angle, poor contrast, low brightness, and high manufacturing costs. Numerous attempts have been made to overcome these drawbacks by devising different architectures and employing various materals.
For example, the electro-luminescent display showed early promise as a competitor to the CRT, but these displays were never able to match the color requirements or price goals for the television market. Currently, plasma displays may be the best contender to replace CRTs in the television market. Other than cost, plasma displays have all the attributes required for the television industry. Because of price, plasma displays are, targeted toward the commercial market and the high definition television consumer. Facilities to build these devices are very specialized and expensive.
Field emission displays have also been tried in television applications. Field emission displays are essentially flat CRTs that replace a single electron emitter (gun) with millions of tiny emitters. Theoretically, these displays should have exactly the same performance as CRT's. However, manufactured field emission displays have fallen short of expected performance and cost goals. Today, they have limited commercial and military sales in applications requiring ruggedness and low color definition such as instrumentation.
Currently, the premiere LCD flat panel is the thin film transistor (TFT) display. TFT displays employ one transistor (switch) for each sub pixel of the display (three sub-pixels red, green and blue make one pixel). Consequently, TFT displays are relatively expensive. A typical XGA flat panel has, for example, 2,359,296 transistors. In addition, each transistor must be functional. Development of production equipment and technology to reach significant yields has resulted in high fabrication costs. The typical factory investment is greater than several hundred million dollars. Additionally, millions of dollars every year are spent in research and development, increasing the costs by a significant amount. TFT displays are used in notebook computers, small hand-held televisions and monitor displays, and other similar devises.
In contrast to TFT displays, the super twisted nematic (STN) display is a much simpler device and the lowest cost LCD display technology available today. In a STN display, the pixels are formed by an orthogonal grid of transparent conductors placed on adjacent plates or substrates. The performance of these displays does not equal TFT's, but the cost is significantly less. Passive displays are used in common devices such as watches, gauges and games.
STN LCDs suffer from some performance shortcomings. For example, on existing STN LCD's, two polarizers are employed on each display. The polarizers reduce the light passing through the LCD by approximately 50%. Additionally, the liquid crystal display media must twist a certain amount to align the light between the polarizers. Consequently, the spacing between the top and bottom plates or substrates, the cell gap, is extremely critical. To maintain uniform spacing, the top and bottom plates are polished. Thousands of precision spacers are then sprayed onto one of the plates. These spacers maintain exact separation of the plates so that the liquid crystals twist no more than required to align the top and bottom polarizers. If the gap is too large or too small, the crystal “over twists” or “under twists.” Any variation in thickness or twist causes distorted images.
The viewing angle of a typical LCD is affected by the orientation of the light passing through the polarizers. Optimum viewing is obtained if the orientation of the light is toward the observer. The result is a brighter display with a wider viewing angle. The light is oriented at a specific angle based on the polarizer orientation. To control light orientation, LCD manufacturers purposely modify the display elements (pixels) by a method referred to as “rubbing.” Rubbing causes the LCD material to polarize or orient in the direction of rubbing. To widen the viewing angle, some manufactures modify the LCD material in the display element with a special rubbing technique by rubbing one half of the display element to spread some of the light out the right side of the display element and rubbing the left side of the display element to have some of the light spread out of the left side of the display element. In this way, the light is directed both left and right toward the observer. Premium designs divide the display element into four differentially rubbed sections to direct the light vertically out the top and the bottom of the display element as well as left and right. Such LCDs have greatly improved viewing angles, but at the cost of less overall brightness due to light spreading in four directions.
Viewing angle can also be improved by making the display thinner. This reduces parallax effects by shortening the distance from the light source to the “lens” (i.e., the LCD element). LCDs take considerable time to turn completely ON or OFF. Consequently, in a television application where relatively high frame rates are required, LCDs generally do not have sufficient time to turn completely ON or OFF. This is manifested in low contrast. Contrast is a major problem for non-emissive devices such as LCD's which control a light passing from back to front. Therefore, because of addressing time limitations, a compromise is made between light OFF and light ON.
Generally, matrix address displays have activating drivers that provide data on one set of data electrodes and another set of drivers for scanning electrodes. In such displays, the electrical connections to the scanning electrodes remain connected to the driver output even while the scanning electrode is not selected (not driven). Thus, display elements associated with the data electrode may pick up charges from addressed display elements, but not the addressed row. This charge spill-over, sometimes called “crosstalk,” happens when the data electrode provides unintended charge to the display elements associated with adjacent selector e
Patel Nitin
Shankar Vijay
Sun Raymond
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