I believe these will require more chips:
Mobile Phones to Display 65,536 Colors
Mobile phone models with color TFT-LCD screens are entering the market in competition with inexpensive STN and mid-range TFD screens.
A new type of cellular telephoneis stirring up a storm among liquid crystal display (LCD) panel manufacturers: the J-SH05 (Fig 1) marketed by the J-Phone Group, a major carrier in Japan, in December 2000. It has a thin-film transistor (TFT) color LCD to provide a high image quality not found in competing models, and is manufactured by Sharp Corp. LCD panel manufacturers agree that it marks a major change in the market, and is sure to trigger a succession of models equipped with high-performance LCD panels for video display.
Color Has Taken Off
The communications carriers began to sell cellular telephones with color LCD panels in earnest in December 1999. This was when NTT DoCoMo, Inc of Japan released the Digital Mova F502i, a second-generation i-Mode cell phone, manufactured by Fujitsu Ltd. The J-Phone Group also released the J-SH02, manufactured by Sharp.
A host of other cell phone models with color LCD panels followed. In fact, of the 60 models released during 2000 by Japanese cell phone operators, 31 had color displays. In the second half of 2000, 25 of 33 models were color. According to Epson Corp of Japan, a major manufacturer of LCD panels for the cell phone market, "All of the requests we received in 2000 were for color panels."
The majority of these new phones had super twisted nematic (STN) LCD panels, with only 256 colors. Until the fall of 2000, models that had just a color panel were sufficient to convince users to switch to a new model, and the difference from the old models was obvious.
A number of new manufacturers entered the field, and there was a sharp rise in the price of STN-LCD panels as well as a shortage of parts.
With the appearance of the J-SH05, however, shipment schedules were accelerated for successive models then under development. Naosuke Itoh, general manager of ED Marketing Department, Seiko Epson Electronic Devices & Components Marketing Division, revealed, "Once TFT-LCD models became available, there was no choice but to compete head-on. We moved our shipment schedules up by about one generation, jumping to new panels."
Java Applications Leverage
Application developers are also beginning to demand improved display performance. The key driving force for improved performance at present, however, is not International Mobile Telecommunications 2000 (IMT-2000), as was expected a few years ago, but rather Java, which became available for the cell phone in 2001. Cell phones capable of running Java were marketed by NTT DoCoMo in January 2001. As with personal computers (PC), users could download Java applications via the cell phone network for a fee. The first applications that have become available seem to be mostly games (Fig 2).
Firms that develop games for the cell phone are demanding LCDs that offer improved display performance. For example, a spokesman for Bandai Networks said, "If we had an LCD panel with a more attractive display and less of an after-image problem, it would be a lot easier for users to play games."
J-Phone and the KDDI Group plan to market Java-capable models to catch up with NTT DoCoMo. One forecast claims that over 20 million Java-capable cellular telephones will be shipped in 2001 (Fig 3). LCD manufacturers are aiming at this market of 20 million, which is emerging as the successor to the existing cell phone market.
NTT DoCoMo plans to launch its IMT-2000 service in May 2001, and is expecting some 150,000 service contracts for the year ending March 31, 2002. Even after adding service contracts with J-Phone, scheduled to begin service in the fall of 2001, the total cell phone sales volume for fiscal 2001 would only be several hundred thousand units.
Three Contenders
Until now, LCDs for use in cell phones only had to offer low power consumption and low cost. Now, image display performance has become important; for example, to enable moving pictures to be displayed.
As a result, the TFT-LCD and thin film diode (TFD) LCDs, which offer better images than possible with present STN panels, have joined the market. The performance of the STN-LCD is also being steadily improved, and the three-way competition seems likely to continue for some time to come.
Right now, all three types of panel have their drawbacks. The TFT-LCD offers superior display performance, with an excellent contrast ratio and quick response, but has excessive power dissipation and is relatively costly. The STN-LCD panel holds the advantage in cost and power requirement, but image performance is inferior to the TFT-LCD.
It is said that TFD-LCD panels, on the other hand, combine the low cost and power consumption of the STN-LCD, while having the excellent imaging of the TFT-LCD. However, all of the specifications are roughly the same, and not especially superior, making it difficult to sell users on the new designs.
First TFT Model
While the J-SH05 boasted the high image quality of the TFT-LCD panel, it does seem to have been released a bit undercooked. One reason why this seems so is that there were no applications that especially needed the quick response of the TFT-LCD panel, a key characteristic promoted at its release. While the response time of STN-LCD panels in most cell phones now is about 300ms, for the TFT-LCD it is only 50 to 60ms. Video display becomes possible at about 20 to 30 frames/s, and the first J-SH05 could only download still images. It did not support Java, and in fact there was very little need to ever use moving pictures.
Furthermore, the TFT-LCD drew a considerable amount of power. With the 75mW of the LCD panel module and the 150mW of the backlight, the display system alone drew over 200mW. The capacity of cell phone batteries currently is about 2,000mW (600mAh, +3.3V drive), which means that the battery will run out within ten hours, assuming the backlight is left on continuously (Fig 4). This means the phone will have to be recharged more often, making it more difficult to sell.
Still, this is a very special case. As the number of Java applications increases, they will be able to make better use of the TFT-LCD's performance. And a number of new TFT-LCDs that offer lower power consumption are on the verge of release.
NEC Corp, for example, has developed a color amorphous Si TFT-LCD that has a power consumption of only 5mW for both moving and still images. The drive integrated circuit (IC) integrates random access memory (RAM) to store data, making the lower power possible (Fig 5). Compared to loading data from external memory, there is no loss in the interface circuit driver. This technique is already extensively used in LCD panel modules for cell phones, but the J-SH05 consumed a large amount of power by applying the method used by PCs with external RAM.
In the second half of 2001, products with built-in RAM designed specifically for cell phones are expected from many firms in addition to NEC.
TFD Uses Less Power
According to Shoichi Iino, senior general manager of the Display Development Center at Epson, the TFD-LCD combines "the low power consumption and cost of the STN-LCD panel, with the imaging performance of the TFT-LCD." The TFD-LCD uses the same liquid crystal as the TFT-LCD, and the same drive method as the STN-LCD, combining the advantages of both.
Epson is the only firm that offers commercial TFD-LCDs at present. With regard to image quality, Epson's Iino commented, "For a screen about two inches in size, for a cell phone, it's about the same as a TFT-LCD panel."
Epson offers a semi-transparent design, having the priority of higher reflectivity, because the screen is viewed without a backlight. By optimizing LCD materials, reflective layer and film, Epson expects the reflectivity to be about 20% higher than that of existing products. The reflectivity of newspaper is said to be about 60%, and Epson has set that as their target. The color LCD panels in cellular telephones now have a reflectivity of only about 20-30%.
More Display Colors
Epson is not only boosting the number of gray levels per pixel, but also combining image processing technology to handle more colors. Normally when the number of colors increases, the data transfer frequency rises, and the onboard memory capacity must be boosted in parallel. Epson Color Modulation (ECM) software-implemented image-processing technology is applied instead to improve imaging performance (Fig 6).
To reduce power consumption, the drive circuit has been changed to a design with built-in RAM, the panel drive voltage reduced and the power supply simplified, achieving a low 2.5mW power consumption. The firm plans to continue improvements, targeting 1mW in 2002. In conventional STN-LCD panels, 1mW is already widely available.
The firm also believes that the TFD-LCD panel is superior in terms of manufacturing cost: "The number of masks used in the manufacturing process, which is a dominant factor in total cost, is only three or four for TFD. An amorphous Si TFT-LCD might use anywhere from nine to about 12."
Return of STN-LCDs
In 2001, moving to counter the in-roads made by TFT and TFD-LCD panels, the STN-LCD will improve display performance and attempt to lock in the market. Sharp, which achieved considerable success with color STN-LCD panels in 2000, is developing new varieties and releasing them to the market.
The first improvement will be in color filters, providing a major improvement in STN-LCD panel image quality. Color filters are manufactured in liquid crystal cells on a glass substrate, and have a major affect on a panel's vividness and brightness. Generally when filters are made thinner, the screen becomes brighter, but colors fade. Thicker filters increase color density, and darken the screen.
Sharp has a design priority on the appearance of the transparent type with a backlight. In reflective types, the light passes through the color filters twice, requiring them to be as thin as possible. With the transparent design, light passes through only once, and it is easier to provide color depth via thicker filters. By changing the color filter material and thickness, Sharp expects to be able to roughly double color performance by mid-2001.
And while details have not yet been announced, the firm says it has cleared the technical barriers to achieving a five-fold improvement by the end of the year, through major changes in LCD material and panel construction.
The number of colors displayed will be increased from the current 256 to 65,536. "It is pretty hard for the naked eye to tell the difference between 256 and 4,096 colors," said Yukihiro Inoue, division general manager of Duty LCD Development Center at Sharp Duty Liquid Crystal Display Group. "That's why we have targeted 65,536 colors instead of 4,096." The designers believe that merely boosting the number of displayed colors without a similar improvement in color rendition performance is meaningless, and are working to achieve both at once.
Low-Temp Poly-Si
Around the end of 2001, low-temperature polycrystalline Si TFT-LCD panels will appear. Each pixel is provided with built-in memory, reducing power consumption by generating control signals for off-chip memory.
A product developed by Toshiba Corp places static random access memory (SRAM) at each pixel, holding one bit of data each for red, green and blue (RGB) pixels. This data makes eight color displays possible. Toshiba's tests show that this design runs at about 1.3mW. Sanyo Electric Co, Ltd expects to use the same method to achieve a power consumption of under 1mW.
Low-temperature poly-Si TFT-LCD panels are only available in reflective types. Because of the internal memory, pixels have reduced aperture ratios, and transparent designs are too dark. With a reflective type, a front light would be needed for use in a dark environment.
The major problem is the quality of the light guide used with the front light. Because it is located on the user side, surface scratches, dirt and dust become obvious, and as a result production yield drops.
The front light design also increases the cost of the low-temperature poly-Si display, so it will probably be used mostly in high-end models. Sanyo predicts it will be positioned as the LCD panel for IMT-2000.
by Takahiro Kikuchi
Websites:
Bandai Networks: bandai-net.com
DDI (KDDI) : kddi.com
Fujitsu: fujitsu.co.jp
J-Phone: j-phone.com
NEC: nec.com
Nikkei Market Access: ma.nikkeibp.co.jp
NTT DoCoMo: nttdocomo.com
Sanyo Electric Co: sanyo.co.jp
Seiko Epson: epson.co.jp
Sharp: sharp-world.com
Toshiba: toshiba.co.jp
(May 2001 Issue, Nikkei Electronics Asia) |
