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ComputerAnswers Column 8


COMPUTER ANSWERS AUGUST 1985



Copyright 1984-1989 Simon N Goodwin

INSURANCE AND MAINTENANCE

I would be grateful if you would let me know the names and addresses of any insurance companies which offer policies covering maintenance etc, for micro-computers. My equipment is not a essential part of my business and therefore a repair time of up to seven days would be acceptable.
Glyn Taylor, South Kyme, Lincoln.

The best choice of insurance depends to a great extent on the value of the equipment and it's intended use. There are really two questions here - one about insurance policies, which offer you money if your computer is lost or damaged, and one about maintenance - prompt repair of a faulty computer.

If you want to ensure a small system for personal use you may be able to include it under your house contents or personal possessions insurance. In such a policy, the computer is treated like any other 'consumer durable' - washing-machines, TV sets, record players and so on. Some policies require that you list such items, with their serial numbers; an additional premium may be required if the computer is worth more than a certain amount, which varies from one policy to the next.

Many 'contents' policies will also cover your computer in transit to and from temporary sites, such as computer-clubs. All reputable computer clubs should have their own insurance; the Association of Computer Clubs offer a range of schemes to member clubs, at low prices. For more information contact the ACC boss, Rupert Steele, on 01 370 0601.

You should check the small print of 'contents' policies carefully to make sure that your computer is covered. Some policies specifically exclude cover on software, or equipment used for business rather than leisure; you may also have trouble arranging insurance cover for recorded data. In such cases you have to rely upon the broker to suggest an alternative.

High-street insurance brokers are only slowly becoming aware of the home computer market; make sure they understand exactly what sort of cover you require before you buy a policy, and get the brokers to confirm the cover with the company that actually issues the policy if there seems to be any doubt. Most brokers will 'phone and check at once if they are not sure whether a policy is appropriate.

As with car insurance, you get cover for different contingencies depending upon the amount you pay. 'All risks' cover generally provides insurance against accidental damage or breakdown, but cheaper policies may only cover fire and theft. There are always a few exclusions: for instance, it seems impossible to insure computers against the effects of Electromagnetic Pulse - the chip-destroying shock that follows an atomic explosion - but presumably you'd have more pressing worries than computer repair in such circumstances.

Computers used in offices may be covered under existing policies for typewriters, desks, coffee-machines and so on, but you should not assume that this is the case. Again, serial numbers and details of valuable items will be required by the insurers. Commercial Union Insurance and Eagle Star Engineering offer computer-specific policies for equipment worth more than a few hundred pounds. Halsey and Company describe themselves as 'the Insurance Brokers for Computer People' - contact them on 0272 503716 for details of their 'Repaircover' and 'Datacover' policies.

Of course insurance may not fill your needs. As one cynic put it, 'insurance is fine until you need to make a claim.' It can take a while to get your money after equipment fails or is stolen; the insurance company will need to make enquiries before it pays out, and that could leave you computer-less for quite a while.

You should insure your computer as a matter of course, in case it is stolen; but there are a number of other actions which you might take if your policy does not provide for a prompt replacement.

Probably the best idea is to come to some agreement with your supplier - many big manufacturers (such as IBM, ACT and DEC) offer maintenance agreements which cover personal computers, although some of these are quite expensive - perhaps 20 per cent of the value of the equipment, per year. As usual, you get what you pay for - the most expensive schemes offer same-day replacement of faulty machines, while others may only guarantee repair or replacement within a few days.

Most of the more professional computer dealers offer similar services for computers which they have supplied. Such cover can be convenient and cost-effective, especially if you are based close to the dealer's offices, but it is up to you to satisfy yourself that the dealer can be relied upon.

If you use a small computer it can be cost effective to buy more than one system, keeping one as a 'back up', especially since the value of software and data soon outstrips that of hardware, when a computer is in serious use. Some firms (including Mancomp, 061 224 1888, and Video Vault, 04574 66555) offer a 24-hour turnaround on small computer repairs (BBC micro and 'downwards'). If you live near one of them, or can afford postal delays, you may be able to get by without a maintenance agreement.

Repairs to obscure machines, printers and disk drives, tend to be slower and it is often hard to find firms with the required specialist expertise. Your dealer may be able to advise you - local help is very valuable in such circumstances.

The best option for many business users will be leasing. Rather than buy your computer outright you hire it from a specialist firm. If it breaks down you are supplied with a replacement while the original is repaired. The arrangement is just like TV rental, except that the suppliers are kept rather more busy since computers are much less reliable than TV sets! Leasing can seem fairly expensive but it is convenient and often has tax advantages compared with the outright purchase of equipment. Micro leasing firms exist in most big cities - check Yellow Pages or magazine small ads for details.



STRUCTURED BASIC

In the April issue of PCW Brian Heywood pointed out that rational program control can be maintained by the judicious use of procedures and functions, and the risk of data corruption can be reduced by the use of local variables.

Could you publish a short list of computers that have such a Basic so that those of us who are self-taught may take the opportunity to advance to bigger (and better?) systems?
B. Nesbit, Chopwell, Newcastle upon Tyne.

The best 'structured Basic' is called Comal, which stands for 'COMmon Agorithmic Language'. Comal is available to run on most CBM computers, including the Commodore 64.

BBC Basic, on the Acorn BBC micro, Electron, and the Tatung Einstein, has many structured features but some annoying limitations - you can't return parameter values from a procedure, for instance (even functions only allow one value to be returned). Selection facilities are poor - IF THEN ELSE cannot be nested properly and there is no CASE statement to resolve choice between more than two alternatives.

The Basic interpreter of the Enterprise micro is better in this respect, although rather verbose; SuperBASIC on the Sinclair QL is excellent, although it has a few bugs. RML Basic, for Research Machines computers, is worth examining, as is Apple's wonderful (but costly) Macintosh Basic.

As a last resort, you can buy programs which add a smattering of structured features to a built-in Basic. Popular choices are Simon's Basic, for the Commodore 64, and MegaBasic, for the Spectrum.

All structured Basic's have a common weakness - they may allow you to structure your program, but they offer little in the way of facilities to represent data in a structured way. The Records, Sets, Pointers and Subrange types in a full implementation of Pascal are very valuable if you really want to develop good programming habits, though Pascal has irritating features like every other language.



WOT PRICE EDUCATION?

I am considering setting up a company to programme and sell educational software for infant and primary schools. Could you please advise wether you consider there to be scope for such a venture, and what competition there is at present, to your knowledge? We'd be very grateful for any advise of any kind.
D. Turley, Kingswinford, Dudley.

I would not advise anyone capable of making three spelling errors in as many sentences to sell, let alone write, educational software. But, putting that aside...

The market for educational software of any kind is small and fragmented; primary schools have few computers and very little money to spend on software. They are also a very sophisticated audience, wary of 'automatic teaching' after the debacle of the rudimentary 'teaching machines' introduced in the seventies. You would be well-advised to employ a working teacher to assess your products.

If you want to sell more than a few dozen copies, you should aim your product at parents, rather than teachers, for 'educational' use at home. Existing educational software publishers fall mainly into two camps - 'cottage industries', usually run by teachers, producing a mixed bag of cheap products, and 'heavyweights' - generally text- book publishers - producing glossier, more expensive packages generally aimed at the home user.



A DISPLAY OF PRECISION

How does one quantitively relate the resolving capability of a domestic TV with aerial input, a domestic TV with RGB input, and computer monitors of 'Medium', 'High' and 'Low' resolution? I presume one counts the number of pixels across and down the screen. Perhaps you could expand on this point as it appears that there is little available to provide meaningful comparison between the above. Some figures (in pixels?) for the above types of set would be useful.
D.S Gladwell, Camborne, Cornwall.

The resolution of a display is not easily expressed as a single statistic, which is probably why you can't find the comparative figures you describe. The resolution varies with all sorts of factors, including the brightness setting of the display (and thus, the ambient light level), the relative compatibility of the computer and display, the source of the signal and the information being shown.

All that can be said with confidence is that a monitor should out-perform a TV set, and a 'high' resolution display should give greater clarity than a low-resolution one. An RGB or 'composite video' signal should give better resolution than a signal conveyed through a TV aerial socket, since less processing (and concomitant degredation) is needed to convert the signal into a form which can drive the display 'tube'.

As usual, there are some useful rules of thumb. TV engineers measure resolution in terms of 'bandwidth'. This is a measure of the number of changes which can be processed in a second. A respectable TV will have a bandwidth of at least 6 MHz. Monitors range in bandwidth from about 5 MHz to 15 MHz and beyond. The greater the bandwidth, the greater the precision. You can't make absolute statements about pixels - display dots - since different computers use different proportions of the screen area for a single pixel. The Sinclair QL, for instance, uses twice the area used by the Spectrum, so it's pixels are not much smaller even though it can display two and a half times as many. There is no standard size (or even shape) of pixel; as a very rough guide, a display with a bandwidth of 6 MHz will blurr information if it is asked to display more than about 300 pixels across the screen. You generally need a bandwidth of 10 MHz or more to display 80 column text crisply.

When you examine a display you should check that the 'white' display is even, with a rectangular border, and make sure that characters near either edge of the display are not unacceptably distorted (or missing altogether!). It is possible to adjust sets to reduce this type of distortion, but you should get the supplier to do this - there are potentially lethal voltages inside a TV set or monitor.

So far we've only talked about black and white displays. When colour is added, the picture becomes even less clear (sic). The format of television signals was decided before colour TV appeared, so an ingenious trick called 'PAL' was invented to add colour information to a black and white signal. In America and Europe similar but incompatible tricks were used, called NTSC and SECAM respectively. NTSC is named after the National Television Standards Committee, though some have claimed that the letters stand for Never Twice Same Colour.

All of the colour encoding systems used in broadcasting suffer from weaknesses, because they try to cram the 'colour' information into a small bandwidth. Electronics in the receiver extracts the information, but this process is imprecise. You have probably seen 'fringes' of colour to one side of graphics on many micro displays. These are caused by inaccurate synchronisation between the brightness or 'luminance' information and the colour or 'chrominance' signal.

This problem occurs on all computers and TVs to some extent - it also crops up on video monitors connected via a 'composite video' lead. Its severity depends upon the relative adjustment of the display and computer; the only way to avoid it is to try before you buy. There is a lot of variation in the performance of micros and displays - especially cheap models - so it is important to take your computer to the shop and walk out with the particular display you found satisfactory. Some shops, especially discount chains, are not co-operative in this regard; if they won't let you try your own computer with their display - maybe by prior arrangement - you're strongly advised to go somewhere else.

It is possible to find a second-hand colour TV that will perform almost as well as a monitor if you shop around carefully. You are unlikely to find a TV that performs as well as an RGB monitor. Colour TV pictures are built up from dots of three colours - Red, Green and Blue, hence the name RGB. These displays require a separate feed to each of the 'guns' which produce coloured dots. The tubes are independently controlled, reducing interference problems and - in theory - giving the best possible display.

There is another trap for the unwary, in that there are two different types of RGB display - the 'analogue' and 'digital' variety. Digital displays only allow one level of control over each gun - it can either be on or off. This simplifies the electronics, but it restricts you to eight colours - the eight permutations of three binary values. With an analogue monitor the intensity of each gun can be set to any value, so you can use a potentially infinite variety of hues - in practice your computer will restrict you to a certain palette, but you will generally be able to use many more than eight colours.

Of course, an analogue display is only useful if the computer has an analogue RGB output. You should check with your supplier if you are in any doubt about this.

Any colour display can suffer from alignment problems. In such a case the relative positions of dots in each primary colour is skewed, so that coloured shapes have borders in other, unwanted colours (like the pictures on a badly-printed cereal-packet) and white displays will appear to be off-colour. These effects can be concealed if you only test a display with text in primary colours on a black background, so you should also look at a range of colours on a white background. If the guns are mis-aligned the 'white' may look off-colour and there may be coloured fringes around some shapes.



DRAGON PORT

Please could you tell me the POKE address of the Dragon 32's user port as I want to build a robot.
James Good, Exeter, Devon.

The Dragon hardware contains two Peripheral Interface chips - one handles printer output, the keyboard and the joysticks; the other provides signals from the printer, and a variety of other facilities that you might find useful, including control over the video chip, sound, cartridge port, cassette relay, and a digital to analogue converter.

Output to the printer is sent via address $FF02 (decimal 65282). All eight bits are used. The printer's 'strobe' signal, which you might find useful, is conveyed via bit 1 of the port at address $FF20 (65312).

You can write any value to the printer port with a simple PRINT #-2,CHR$(N); statement - the binary pattern corresponding to N will be output at the printer port. Remember to type the semicolon, or a carriage-return (binary 00001101) will also be sent. Of course, POKE 65282,N would work as well, although it would not drive the printer correctly since the normal 'handshaking' signals would be absent.


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