Displays and Batteries are Glaring Weak Spots in Pen-Based Systems

David Mentley, an expert in the field of computer and video display technology, sees some glaring deficiencies in early versions of pen-based computers. The weak spots are display and battery technology.

Pen computing has recently been raised to a new level of visibility. At first glance, it appears that all of the hardware components are falling into place in order to fuel the 1 to 2 million unit markets widely predicted by 1995. Microprocessors seem to be fast and powerful enough to handle the interface requirements. Rotating and solid state memory are compact enough to provide the basic utility for most applications. Communications links using cellular modems or RF may complete the network. But there are two vital hardware technologies which are not yet up to the minimum performance level needed for a broad based horizontal pen computer. The two weak components are display technology and battery technology. The problems are linked.

All of the pen computers under development or on the market employ supertwisted nematic (STN) type LCDs. The performance of these displays is adequate for many of the current embodiments of the computers, but future improvements may be troublesome. STN LCDs are not yet capable of video rate performance. Even with rapid scrolling it is difficult to maintain an image on the screen. More troubling is the fact that as temperatures decrease below room temperature, the response time slows considerably. In addition, digitizers are currently simply layered on top of the LCD, creating optical interface problems between the LCD and the digitizer. Pen-based system vendors will have to work with Japanese display vendors to improve the digitizer/LCD interface.

Backlighting is the most troublesome issue, however. An efficient backlight for a monochrome display uses around 4 watts of power. Pen computers can be operated without a backlight (in a reflective mode) if there is enough ambient light. Most laptop and portable computer vendors have found, however, that a backlit display is far preferable to a reflective mode display in terms of user acceptability.

The problem is that the power to run a backlight completely dwarfs the power consumption of all other sub-systems. NiCad battery technology stores an average of around 40 Watt-hours per kilogram, implying that a one kilogram (2.2 pound) battery is needed to support this backlight for 10 hours. Pen computers are targeting 3 to 4 pounds total weight. Clearly some improvement is in order. The newly developed nickel-metal hydride battery technology offers hope with about 1.8 times the energy density and 500 charge-discharge cycles.

This power problem is multiplied by about four or five times when a color display is considered. There is no color liquid crystal display existing or known to be under development that can be operated without a backlight. Since the color filters used in most color displays filter out selected bands from white light, the technique is inherently inefficient. Transistors and bus lines in active matrix displays block out even more light. As a result, only 2 to 3% of light is used. This translates to a power usage of between 15 and 25 watts— not very compatible with the portable pen computer concept.

Then there is the operating temperature problem. The first few years of pen computing applications are predicted to be vertical market applications. These systems will be used in a wide range of ambient environments ranging from sub-zero cold to desert heat. This range is difficult for the current generation of LCDs to span. At 0 degrees centigrade, many STN LCDs have a response time on the order of several seconds. The claims adjuster who relies on the pen computer will not have the luxury to tell the client to wait until the temperature hits 15 degrees centigrade. Even more commonplace will be the pen computer which overheats on the car seat as the sun makes the liquid crystal material go isotropic (it gets black until it cools down). These are severe issues that need to be resolved before the predicted market growth is realized. Note that these are not insurmountable problems, they are just current obstacles which have not been adequately addressed in the rush to bring pen computers to the market.

Stanford Resources is a market and technology research firm located in San Jose, CA, focusing on electronic information displays. The firm has a wide assortment of multi-client reports covering all flat panel displays and also provides custom research for clients with particular market and technology evaluation requirements.