Digital Imaging Systems – Part 2

by D.R.Hunt

The Optical Disks.

The final stage of the process is to save the compressed image and that’s usually done on optical disks. I’ve seen and heard more confusion and mis-information on this subject than any other, so perhaps you’ll forgive my ‘going off at length.

Firstly there’s the type of optical disks which may be used. Currently there are four diameters of optical disks, 5.25″, 8″, 12″ and 14″, of which 5.25″ and 12″ are the most common. Now some people mix up the type of optical disk with the information which is stored on it; and some confuse the fact that optical disks fall into two types, those which may be written to, that is information placed on an otherwise blank disk; and those which are manufactured with information already on the disk, and which can not be written to. Some people manage to confuse all types together.

The first type are available in-all sizes and are commonly known as WORMs (WORM meaning ‘Write Once Read Many’). This is a feature of the technology of the disks. As we shall see later, once an area on the disk has been written to, it is not (yet, they’ve just-crept outside the laboratory – Tomorrow’s World – BBC TV — 25th February 1988) possible to write to the same area again. On the other hand, once written to, it is possible for the computer to read the information as many times as‘it likes – hence Write Once, Read Many.

Another aside – the use of acronyms. Many acronyms like WORM are used in the computer world. They just sort of grow up. But there is a tendency for people to invent acronyms simply because they sound ‘technical’. For instance these new Erasable Optical Disks – they haven’t found an acceptable acronym yet and EOD sounds distinctly odd. An acronym for these won’t take long to fall into common usage. But beware the person who talks in acronyms – ‘Computerese’, more often than not these people are simply trying to sound clever and don’t even know what the acronyms they use actually mean half the time. Never be afraid to ask. If these people really know what they are talking about, they won’t mind speaking plain English, and if they don’t know what they’re talking about, the English will come across as rubbish and you’ll be able to recognise an idiot when you see him !!

The second type of optical disk have the information fixed in them during manufacture and are most commonly of the 5.25″ size although there are some 12″