DISPLAYS: Improvements brighten future of passive displays

June 1, 1997
Color-supertwist-nematic (CSTN) passive displays have consistently suffered from inferior visual performance when compared to thin-film transistor (TFT) displays.

Color-supertwist-nematic (CSTN) passive displays have consistently suffered from inferior visual performance when compared to thin-film transistor (TFT) displays. At half the price, however, their performance has been good enough to capture several large-volume applications. Recent advances on various fronts are now pushing CSTN performance into traditional TFT territory. In fact, according to Joel Pollack, marketing manager at Sharp Microelectronics Group (Camas, WA), "The improvements in CSTN performance are so significant that they will capture a much larger share of emerging applications for large-diagonal displays, such as deskto¥monitor replacement, than is commonly thought."

Although many suppliers of CSTN displays have performance improvements available now or later this year, market leader Sharp is probably ahead of the pack in rolling out the new CSTNs. Its enhancements have been encapsulated into three technology packages designated Sharp Addressing (SA), High-Contrast Sharp Addressing (HCSA), and High-Contrast Addressing (HCA).

Improving size and resolution

Until recently, the size of CSTN displays was stuck at about 10-11 in. (diagonal) and resolution at VGA (640 × 480 pixels). In trying to move beyond that level, contrast, brightness, and speed of response all fell off, and shadowing (crosstalk) increased. The Sharp Addressing technology has allowed this barrier to be pierced. The company can now supply CSTN displays up to 17.7 in. (diagonal) at XGA (1024 × 768 pixels) resolution (see figure on p. 40).

From an electrical standpoint, CSTN displays form a large RC (resistor-capacitor) network. Consequently, the waveforms of the pulses that drive the displays become elongated, rounded, and reduced in amplitude as they travel the length of the row electrode. To counter this problem, the conventional approach has been to turn up the overall voltage bias level, but this tends to trigger adjacent pixels to switch on, producing crosstalk.

Crosstalk can cause up to a 25% "blurring" of the edges of image objects. Sharp Addressing, however, reduces crosstalk to less than 5%. This was achieved with the development of new driver chips and peripheral circuitry that effectively compensate for waveform distortion created by the RC network. In addition, new electrode-deposition methods have reduced resistance and improved uniformity, particularly at the upper and lower edges.

Boosting contrast

The High-Contrast Sharp Addressing technology package doubles contrast in CSTNs to about 50:1. It does this with a combination of thinner cell gap (thickness of liquid-crystal layer), new liquid crystals, and other material parameter changes. The CSTNs work like phase retarders. "At just the right liquid-crystal thickness, one-quarter wavelength, and with sufficient voltage, the incoming linearly polarized light is rotated 90 and emerges linearly polarized again," explains Pollack. "The light can then easily pass the second polarizer. If the thickness is not right or the voltage insufficient, the light emerges elliptically polarized, thereby reducing contrast." By shrinking the cell gap, the liquid-crystal material experiences a higher electrical field, without raising the overall bias level, thereby improving the efficiency of the light-switching process and hence the contrast.

Improving response speed

High-Contrast Addressing is a multiline addressing technology that improves speed of response and contrast simultaneously. The technology improves contrast from 25:1 to 40:1 and response speed from 300-350 ms to 150 msby the end of the year, the response speed should be down to 100 ms. By comparison, TFTs run at 80 ms and are good enough to be used for some video applications.

Instead of addressing the display in a row-sequential manner, HCA first divides the display into four sections. Data frames are then analyzed in memory to determine which pixels need updating faster than others. This selective approach to pixel updating is what allows the faster overall response speed and contrast.

Sharp`s High-Contrast Addressing package adds about a 30% price premium over conventional CSTN but is still much less than other multiline approaches and TFTs. As the technology is developed, there is a good chance that designers will select this over TFTs, especially for price-sensitive applications such as desktop monitors, point-of-sale displays, or automatic teller machines. "Improved CSTNs may be the only way to reach the magic price points needed to generate any kind of volume in the early stages of this market," concludes Pollack. He might be right.

About the Author

Chris Chinnock

Chris Chinnock is president and owner of Insight Media in Norwalk, CT. He is a 30-year display industry veteran with experiences in displays, broadcast, cinema, ProAV, and consumer electronics.

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