Buyer's Guides

Digitising Tablets

Publication Details:
Publication:“PC Plus”, UK
Date:December 1993

‘PC Plus’ Buyer’s Guide Price Watch:

Digitising Tablets

Digitising Tablets have been overlooked by the mass computer market for many years, although they’ve always been popular with professional designers. The new generation of Pen-driven portable computers has re-kindled interest in this well-established technology. TIM BATY describes the pros and cons.

When asked to select the ideal hardware with which to control graphics software, the device which springs to most users’ minds is invariably the ubiquitous Mouse. Digitising Tablets are largely ignored in the mass market, yet they are well-known in high-end graphics work such as professional CAD (Computer-Aided Design). Although more expensive than mice, Tablets offer several advantages for graphics software users.

The main difference between mice and tablets concerns how the screen cursor behaves when the hardware pointer is moved. Mice move the on-screen cursor relative to the screen boundaries. There’s no fixed link between the mouse’s position on the desk and the screen cursor’s location – if you lift a mouse up and put it somewhere else, the screen cursor stays put. In contrast, a tablet tells the host PC where the hardware pointer is in the form of an absolute co-ordinate on the tablet’s surface – if you pick up a tablet cursor and put it down elsewhere on the tablet surface, the screen cursor jumps to the new position. Although this difference in operation might at first seem trivial, the resulting improvement in control and accuracy cannot be over-emphasised. The ability of tablets to detect the pointers’ absolute position also makes them much more versatile. As well as conventional screen-cursor control, areas on a tablet’s surface can be used for other functions such as command-selection boxes or for ‘tracing’ paper drawings into a graphics program.

There are three application areas in which tablets are popular, the best-known of which is CAD. Professional-standard CAD programs have traditionally had a much larger set of commands in constant use than would be the case with, for example, word-processing programs. Tablets are popular here because of the ability to use parts of the tablet surface for command-selection, and the precision with which screen cursors can be controlled. Ergonomics and comfort are important issues here, so tablets for CAD are usually quite small, with A4 (12″x12″) the most popular size. Another business area where tablets are popular is in GIS (Geographical Information Systems) and Cartography. Tablets are used here for digitising of survey information recorded on paper maps, and consequently large tablet sizes are essential – often up to A0. The third field of use for tablets is in graphics and design. Here the tablet becomes an electronic substitute for an artists’ pencil or paintbrush. As with CAD, comfort is important, and tablets for graphics work are seldom larger than about A3 size. More important still, though, is for the tablet to behave as similarly as possible to traditional media, so cordless and pressure-sensing pen-shaped cursors are popular.

All tablets consist of two major components, namely the active area and the hardware cursor. The underlying technology in the vast majority of tablets is based on Electro-Magnetism. The tablet surface contains two layers of fine parallel wires, set at right-angles to each other – each layer represents one axis of a 2-dimensional plane. Electronics look at which conductors have detected the magnetic field from a small electro-magnet in the cursor tip, and translate this into an X-Y co-ordinate within the tablet’s active area. Electro-Magnetic technology is robust, well-proven and accurate – resolutions of over 1000 dots per inch are common. A small number of devices use Infra-Red or Ultrasonic reflection, instead of electro-magnetism – accuracy is indifferent, but they can cover large areas at moderate costs.

Tablets are available in all sizes, from A5 (9″x6″) to A0 (45″x32″) and larger. Tablet size figures normally refer to the usable or active area – physical dimensions are typically 2″ or 3″ larger. Choosing the correct size of tablet is a balancing act between having a large area available for menu overlays and digitising, and the discomfort of using a large tablet for long periods. The most popular sizes are A4 (12″x12″) and A3 (18″x12″) – these represent a good compromise between cost, ergonomics and versatility. The tablet surface is usually set into a slim plastic case which houses both the active surface and the electronics. Many provide transparent covers to protect paper command-menu overlays. Some tablets embed the active surface in a thin, flexible plastic laminate, with the electronics in a box along the top edge. Others have an active surface which is completely transparent, allowing the tablet to be placed over the item to be traced.

Hardware cursors are available in one of two forms. Pen-shaped cursors are popular for artistic applications, due to their similarities with traditional manual techniques. They are normally provided with a switch in the tip for command selection or point picking, with a second switch on the barrel on some models. Cordless designs are also available, and some types can also detect the amount of pressure on the tablet’s surface. Cursors are also available in a mouse or ‘puck’ shape. This type is particularly good for digitising – the electro-magnetic element is usually set in clear plastic around a cross-hair, which can be aligned precisely over the point required. Pucks normally have 3 or 4 buttons, which can be programmed to execute the most frequently-used commands.

All tablets (except those for Apple Macintoshes) connect to the host PC by means of an RS-232 Serial interface. As with many other types of PC peripherals, a de-facto data protocol standard has evolved over the years, and in the case of tablets this is Summagraphics’ MM1201/Summasketch format. The majority of tablets can emulate this, and most DOS-based graphics programs provide drivers for it – curiously, however, Windows does not support it directly. Most tablets also provide their own hardware data standards, which offer improvements over MM1201 format to a greater or lesser degree. In addition to different hardware interface standards, virtually all tablets provide a number of software drivers – the commonest are for Microsoft Mouse emulation, Windows and AutoCAD ADI. One feature of tablets which is often overlooked is their suitability for use with Pen-driven user interfaces, and a small number of tablets are now providing Pen Extensions for Windows 3.1.

Prices of tablets vary widely, depending upon the features on offer and the brand-name. It’s worth pointing out that tablets contain more components than mice, and will therefore always be more expensive than their rodent cousins. However, entry-level tablets are available at prices which are only a little more than a premium-quality mouse. £90 represents the bottom end of the market, and will buy a basic tablet with an active area of A5 and a pen-shaped cursor – tablets of this size from the major manufacturers will be closer to £200. The popular A4 and A3 sizes typically start at about £190 for a budget A4 model, and rise to prices approaching £500 for a top-quality A3 variety. Above A3, tablets enter the realms of niche markets, and command correspondingly high prices.

Although tablets are well-established in the world of professional graphics and design as the ideal pointing device, they have been unjustly neglected in the mass market. The base technology is mature, having been refined over many years to the point where accuracy, robustness and reliability are beyond question. They are far more versatile than mice, being suitable for tracing paper drawings and designating areas of the active surface for command selection. Their naturalness of operation has made them a favourite of people who draw and design on computers for their living. This factor, combined with ease of technical implementation, has brought tablets to the fore as a fundamental part of the new generation of Pen-driven portable computers.

Tim Baty

Pen Computers

Pen computers combine LCDs (Liquid Crystal Displays) and Digitising Tablets. The need for transparent surfaces and low power consumption has produced interesting variations on electro-magnetic tablet technology. Older Pen computers used ‘corded’ metal pens, which made contact with microscopic conductors etched on the LCD’s surface. Others use patterns of inductive coils, which detect the presence of special cordless pens. A simpler method uses a transparent overlay on the LCD – the overlay’s resistance changes where the pen is pressing. Power consumption is particularly low with this design, and it can be operated with any sharp implement, including fingernails.

Character-Recognition software is vital to all Pen computers. However, some machines are evolutions of conventional keyboard-driven systems, with general-purpose Pen operating systems sitting on top. Operating systems like this, such as Go Corporation’s PenPoint and Microsoft’s Pen Windows, use special pen ‘Gestures’ to replace common keystrokes and mouse actions. Other machines, such as Amstrad’s Pen Pad and the Apple Newton, have been designed from the ground up as Pen systems. Systems of this type can include any icons or buttons required for the embedded applications, and consequently do not need to rely so heavily on general-purpose pen-strokes to activate features.