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Let's take a look at the unique hardware components needed by tablet devices in order to handle pen input. (And it's good to be able to answer hardware questions from curious clients.)
| by Andrew Binstock |
July 15, 2004 | | |
How does the Tablet PC know where the stylus is? How can it tell when the user presses down harder? Why can't you check off a box with a pencil instead of a stylus? Use a Tablet PC for just a few hours and these questions will naturally come to mind. Because the technology tends to be unfamiliar, there is no widely understood body of knowledge to lean on nor, as it turns out, are there many engineers who understand how these devices work. This article, based on interviews with several manufacturers of Tablet PCs, explains how these devices deliver their magic. |
For purposes of this discussion, I will focus on convertible models. These are the devices that look and behave like notebooks, but when the screen is bent back over itself, become Tablet devices. This illustration from my review of the Fujitsu LifeBook shows just such a notebook converting into a Tablet PC. The other kind of Tablet, called a Slate, lacks the notebook aspect. It resembles an Etch-a-Sketch pad and is always in Tablet PC mode (though consumer versions usually have some form of keyboard available). It is frequently employed in niche markets such as delivery--the devices used by the drivers of UPS and Federal Express, for example, are customized Slates. I will refer to these as Slates whenever my explanations are unique to them.
Digitizers
The technology that figures out where the input point is at any given moment is called the digitizer. There are several ways in which digitizers know where the cursor is, but they mostly break down into two categories: active and passive. All Tablet PCs use active digitizers. However, to make clear how active digitizers work, it's helpful to start our discussion with the passive designs.
Passive Digitizers
The passive (sometimes termed resistive) digitizer is used in PDAs and on touch-screen devices. On PDAs, these are implemented by two transparent layers of material placed above the display area. The top layer carries a slight voltage and it is separated by spacer dots from the bottom layer. When the user applies pressure to the top layer (with a stylus, for example), it is pushed down so that it makes contact with the bottom layer. When this contact occurs, voltage streams toward the four corners of the digitizer layer in amounts proportional to the distance. From the strength of the current received at the four corners, the digitizer can compute the location where the contact occurred. A typical passive digitizer can take roughly 40 samples per second and resolve location to within a quarter millimeter.
Active Digitizers
Active digitizers are located behind the screen/writing area. They generate an electromagnetic field around the writing area and use a series of sensors (typically 48 of them) around the display perimeter to monitor disturbances in the field. These disturbances are created by the stylus which contains either a small replaceable battery or, more commonly, a ferrite core magnet wrapped in a coil. In the latter case, the magnet itself causes the disturbance and hence no battery is necessary. When the stylus enters into the field (which extends up about 4 inches from the writing surface, the sensors detect the presence of the stylus. By comparing the interference generated by the stylus, they can among themselves determine the coordinates of the point over which the stylus is hovering. This location is communicated to the software. Typical active digitizers today can sample the stylus location at well over 100 samples/second, with resolution ten times as fine as a passive digitizer.
To measure pressure, there is a pressure-sensitive switch in the stylus. Its electrical properties change as greater pressure is applied to the stylus point. In turn, it changes the interference that is picked up by the sensors around the digitizer. In theory, the hardware can detect up to 255 increments of pressure. However, neither I nor anyone I've asked has ever seen evidence of more than a few (certainly less than 10) pressure levels being recognized by software.
Because active digitizers respond only to a stylus with specific electrical properties, simple pressure on the writing surface will have no effect: using a finger, a pencil, or even a plastic stylus from a PDA will have no effect. The digitizer responds only to a stylus model for which it is designed This is a distinct benefit for users who write notes on a Tablet PC. Consider that when right-handed users write near the left edge of the writing area, their fingers are likely to touch the screen. If the active digitizer were sensitive to pressure, this contact would create an ink streak on the display. However, since only the stylus can cause ink to flow, the incidental contact from the palm or fingers of the user has no effect.
At one time, there were two principal vendors of active digitizers for Tablets: Arizona-based Finepoint, which uses battery-powered stylii and once made the digitizer used in HP Tablets; and Wacom, a firm founded in Japan, which makes digitizers that respond to magnet-based stylii. Today, all vendors of Tablet PCs used Wacom's active digitizers.
Other Hardware Aspects
There are a few other hardware features that distinguish Tablet PCs. The first is a switch that is triggered when the convertible's display is switched back so that the device shifts from notebook to Tablet PC. This switch signals the software, which does several things to accommodate the new interface. Most visible is changing the display from landscape mode (as used in the notebook) to portrait mode so that the user can take notes as if writing on an 8-1/2"x11" pad of paper.
A second difference in Tablets is the nature of the display. Tablets are much more likely to be used outdoors, so the display must be able to brighten and darken according to the ambient light, to facilitate reading. Most Tablets have hardware and software controls for the display brightness to satisfy the needs of outdoor users.
The remaining hardware considerations relate to design. On regular notebooks, the keyboard area is a major surface for the release of heat. Components are placed near the keyboard for this purpose. On Slates, this design is not possible, as the "keyboard" is the actively used writing area and because the display and digitizer are themselves generating heat. To lower heat, slate designs tend to focus on reducing power consumption and so they use ultra-low voltage (ULV) processors to accomplish this. These chips are less powerful than their mainstream brethren, and for this reason, Slates typically have less horsepower than convertable Tablets.
Weighting Slates correctly is also an important design function. These devices are often held by the user in the same way the Statue of Liberty holds her book: with one arm wrapped around the back and with the bottom edge resting against the hip, while the other hand is used for writing. Such a device must have its center of gravity close to the bottom edge, otherwise the Tablet will handle awkwardly. To attain the right weight distribution, certain hardware elements (such as the batteries) must be jammed together at the bottom without causing too much heat to occur from the component density. As can be seen, Slates have a whole series of hardware constraints that are not immediately visible nor present in other devices.
Choosing Hardware
I spoke with engineers from Viewsonic and Fujitsu for this article and asked them both how their devices differ in ways that matter to users? Curiously, both engineers answered in roughly the same way. Focusing primarily on the convertible models, they pointed out that all manufacturers use roughly the same components--they all use the Wacom digitizer, their choices for vendors of LCD screens are limited, and only a handful of vendors supply memory, processor, and disk-drive components suitable for use in notebooks. Consequently, differentiating has to be done on other criteria other than the quality of hardware components—nearly all innards are pretty much the same.
System design—what a vendor has chosen to focus on—does differentiate offerings. Viewsonic, for example, has a lip with button controls that juts out from the notebook base so as to be accessed by users in Tablet mode (see Figure 1). This makes certain shortcuts easier, even possible. Viewsonic, being an acclaimed vendor of monitors, also focuses on the display, writing drivers that facilitate use in outdoor and indoor settings. Fujitsu points out that it has been in the Tablet PC market longer than nearly all other vendors and so has perfected the design for the vertical niches it sells to. Certainly, the company has figured out how to reduce weight, with a convertible notebook that weighs merely 4 lbs. 2.5 oz. Fujitsu models also focus on extended battery life—a key criterion in Slates, especially, since the device is nearly useless if it has to be plugged in frequently. (For more on Fujitsu notebooks, see my previously referred to review.)
This quick tour has primarily focused on showing how the Tablet PC's unique hardware works. This information should be useful not only in choosing the right model, but in understanding what is happening below the covers to your code. And it should have clarified how operations that appear similar on Tablet PC's and PDA's are brought about by entirely different technologies.
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