Your Spectrum
Issue 5, July 1984 - Hardware Review
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So what's needed is an input device which can translate the lines and curves of the drawing or design into digital information that a computer can then recreate as a display. In the professional computer market graphics tablets have been around for almost as long as computers, but low-cost designs for the home user have only recently started to become available. High precision graphics tablets, also known as digitisers because they reduce analogue shapes to digital information, use a wide variety of techniques to produce the required information. Among the most accurate are the magnetic and capacitive systems; a capacitive system can offer accuracies of around 1/100th of an inch - sufficient for engineers and draughtsmen - while the magnetic method is potentially even better.
The cheapest and simplest digitisers are the various pantographs. Based on the same principle as the old-fashioned drawing aid, they use jointed arms and coordinate geometry to give a direct measure of the position of the stylus. Variable resistors (potentiometers) are mounted at the two joints that provide a varying voltage to the computer. This can be interfaced through a suitable analogue port; a joystick socket may be enough, given suitable software. The resolution of the pantograph is limited by the accuracy of both the variable resistors and the mechanical linkages; typically it's around five per cent. However, sophisticated pantographs based on optical measurement of the rotation of the joints can offer much better results, although they still fall short of the sort of accuracies that can be pulled from the best systems. While the pantograph is probably the simplest type of digitiser there is, it's often quite adequate for home computer use.


One stage more advanced than the pantograph (though still lacking the sophistication of a magnetic or acoustic digitiser) is the pressure sensitive digitiser. Often referred to as a 'bit pad' (although that's actually the trade name of a specific device) it can take one of two forms. The simpler of the two is based on a resistive sheet, often just

Discussing the artistic potential of various micro add-ons, Henry Budgett concentrates his critical powers on British Micro's Grafpad.

One of the most powerful selling points of any of today's home computers must be the type of graphics facilities they have to offer. With just a few simple lines of Basic, great sweeping designs can be made to appear and disappear ... colours change, new patterns emerge. Unfortunately, for the really creative user, all this requires expert programming knowledge. The concept can't be created on paper first and then loaded into the computer as a completed work of art. While there are a number of on-screen drawing programs which allow the creation and manipulation of an image, they cannot be used to copy a picture from a sheet of paper. Designers, technical draughtsmen and architects, as well as interior decorators, landscape gardeners and fashion designers (to name but a few) can all benefit from the facilities that a computer graphics system can provide. Once the design is safely stored away on tape or disk, it can be quickly loaded back into the computer's memory. Now, additions and alterations can be tried without wasting valuable raw materials or creating a product that isn't quite what the customer expected. Yet all these potential users of computer systems are limited by the fact that the computer cannot be interfaced to a piece of paper!


heavy paper with a graphite coating. If an AC voltage is passed across the sheet, a stylus will be able to detect the potential at any point. Using an electronic sampling method which is locked to the same frequency as the supply voltage, it's possible to derive a direct relationship between the voltage detected
pulsed high frequency signal. The signal detected by the grids is compared with a reference signal and provides a direct measure of the x,y position of the stylus. The capacitive system works the other way round in that the stylus is used to detect a series of coded pulses fed into a two-layer grid.
The data produced by a graphics tablet must be converted into information suitable for display on the screen and, to this end, most of the commercial products come with all the necessary
misprint between friends? Well, nothing really, except that they also reckon it comes with a light pen and ... it doesn't. But then it does have a really nice stylus to make up for it!
Also included in the packing is a multi-way cable to link the tablet to the Spectrum's edge connector, a cassette containing the software, a keyboard overlay and a wad of A5-sized photo-copied paper that claims to be a manual. The tablet is well-made and its surface is ruled with a grid corresponding to the Spectrum's graphics resolution of 176 by 256 pixels; it also includes a 32-box menu panel which can, in theory at least, be utilised by users to create their own software. The grid is protected from damage by a sheet of clear acrylic over which the stylus can be moved with ease - although it does scratch.
The first move is to connect the tablet to the Spectrum - an easy task you might think, that simply involves plugging a cable between the Spectrum's edge connector and an identical connector on the tablet? No such luck! On my Spectrum at least, the aluminium heatsink prevented the plug from being properly inserted. Out with the toolkit, some minor surgery and the cable now connects perfectly - although it's still too short to allow the tablet to go anywhere other than on the right, or directly behind the Spectrum. That's fine if you're right-handed, but then, not everyone is.
With the cable connected all that remains is to plug the stylus into the socket at the side of the tablet, load the program and start drawing. To make life easier for the user, a keyboard overlay is supplied which carries the various options. All of the Spectrum keys are used by the program - most with two functions, a fact that is fairly strange given the presence of the menu panel on the tablet. To keep the user aware of what's going on, the screen display includes a highlighted bar detailing the current screen mode, foreground and background colour selected, and the drawing and dot modes. A neat feature is that the bar always moves out of the way when you get near it, hopping from the top to the bottom of the screen and vice versa.
To activate the stylus, you press down on the tip and a microswitch makes contact; the on-screen cursor then follows the motions of the stylus around the grid. To use any selected function, press the Enter key and the function will operate as long as the stylus is depressed; releasing it cancels the function.


The single most difficult thing I found when using the Grafpad was to remember
bow tie A
For a night on the town, why not dress yourself up with a Grafpad? First, it's on with the red bow tie ...
bow tie B
... and then the white shirt (a tricky item to draw, especially with the Grafpad's 'disappearing cursor' feature).
bow tie C
Be careful when adding colour ... if you position the yellow dots too near the bow tie, they'll come out red instead.
bow tie D
One last touch, blue stripes on the shirt - now, where's my top hat and tails?
and the position of the stylus on the sheet.
The second type of pressure sensitive digitiser uses two sheets of resistive material separated by a cellular membrane. The pressure of the stylus causes the two conductive surfaces to make contact, a principle that was used in the Sinclair Research ZX81 keyboard. A high frequency signal is fed into the two layers, each signal (normally) either opposite in phase or connected at right angles to the other. The signal detected by the stylus when it makes an electrical connection between the two sheets provides a measure of its position. Typical problems encountered with both these systems include changes in the surface resistance due to damage, or the pressure of a hand at another point on the surface.
The best types of digitiser available to the home user are based on either magnetic or capacitive effects. Both systems rely on a series of wire grids embedded in the baseboard of the tablet. In the magnetic system the grids are used to detect the position of the stylus which, in this case, is a coil radiating a
software. However, just entering the data isn't the end of the graphics tablet's usefulness. Once the information is stored in the computer the tablet can be used as an editing tool, allowing colour to be added or changed and shapes to be modified. The surface of the tablet can also be programmed to act as a menu that selects standard options from the program, so that the keyboard need only be used for selecting major functions or entering text.


So much for theory - let's look now at a version for the Spectrum user. One such device called the Grafpad system from British Micro (previously only offered for the BBC Micro) has recently been adapted to work with the Spectrum and Commodore 64 systems. The unit comes extremely well-packed in polystyrene - not a bad thing considering it costs a fairly staggering £143.75 - and consists mainly of a 25mm by 355mm by 260mm black plastic box. Already that's a 'slight' discrepancy from the advertising blurb ... they reckon it's only 55mm wide but what's a simple


whether l was drawing on the screen or mapping the attributes. The current screen mode is displayed in the centre of the information bar, so it should be simple to keep track of what's going on; however, dropping into Attribute mode and spraying colour across pixel boundaries can be very frustrating. The accuracy of the tablet is properly matched to the Spectrum - the grid inside the tablet's surface consisting of a wire mesh 176 by 256; the movement of the stylus on the screen corresponds exactly to its motion on the surface of the Grafpad. Don't expect too much at once because drawing freehand requires practice and traced pictures are never really exact.
Fortunately the software makes allowances for this and it's generally quicker to knock up an approximate likeness and then use the editing features to tidy up. Of these the most useful is Magnify which blows up the selected area by up to eight times. Here the individual pixels are about the size of characters and you can Set and Reset them at will. The movement of the stylus is also scaled to the current magnification, so the picture
doesn't leap about all over the place.
Among the other built-in features of note are the Line, Circle and Box drawing routines, all rubber-banded from any given starting point. Quite what the Vertical and Horizontal Line facilities are for is a mystery; they appear completely redundant. While almost all the routines work very fast indeed, the Circle is amazingly slow; any suggestions as to why would be gratefully received - there seems to be no obvious answer. To assist in matching your image to others the program provides two scaling grids, but neither really proved very effective. The highlight grid which produces alternate light and dark background character cells doesn't work with dark backgrounds and while the second grid is better, it reduces all the colours to magenta or white, making it almost useless for anything other than outlining.
Against these dubious features Grafpad does possess several very useful facilities. You can invert an image, flip up/down or left/right, and store up to two pictures, either elsewhere in memory or permanently on tape. Areas can be painted-in or you can edit the Attribute screen on a pixel-by-pixel basis (in Magnified mode if you really want to be accurate). If you want to protect areas of your picture from accidental damage, a window facility is provided and only
the area inside can be edited while the window is active. About the only other interesting facility promised by the manual is that you can pick up UDGs, but it fails to suggest from where!
The manual actually fails to mention quite a lot and I only hope that the one I got is a prototype and not what the paying public will receive! Quite apart from the mis-spellings and shabby presentation it hardly tells you anything at all. It's almost as though it was translated from Esperanto by an artificial intelligence program suffering from dyslexia. (Author's note: all spelling mistakes in this review are the responsibility of the editor! [No they're not, Ed.]) All the commands are dealt with but seemingly in as few words as possible - it's more a quick reference guide than a piece of documentation.


If you want to create works of art on your Spectrum's screen then you can certainly do it with the Grafpad. But then again you can also do it with the various screen-based editors like Melbourne Draw for substantially less than £143.75. The only thing the Grafpad has to offer that the screen-based packages don't is the facility to use the pen to trace an existing drawing on paper into the computer's memory; even this is limited by the accuracy of the tablet. Check it out yourself and draw your own conclusions ...
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