types in use. These are most definitely ‘Read Only’ disks and are often referred to
as ‘CD-ROMs’, CD-ROM meaning Compact Disk Read Only Memory.
CD-ROMs are easily mass produced, using a photographic technique, where the
digital patterns to be reproduced are etched on to a plastic disk and the disk is then
coated with a very thin film of aluminium to make it reflective. The final touch is to
coat the surface with a tough plastic to protect the mirror finish of the disk.
Where the confusion arises, is that the information contained on an optical disk is
not defined by the disk itself. Digital information is digital information, and although
there are different types of digital information, computer programs, digitised music,
digitised pictures, digitised television – and many others; it must be understood
that the optical disk is simply a means of storing digital information, it doesn’t imply
what the information is.
Let’s get that across by an analogy. Take a glass bottle – milk bottle, beer bottle —
any old bottle. Now the one thing we know about a bottle is that it will hold most
fluids. The shape of the bottle might imply what sort of fluid is in the bottle, but
doesn’t guarantee it. Take those sad stories a few years back of thoughtless
gardeners storing Paraquat in cola bottles and the like. The fact that the
unsuspecting thought that the bottles contained Coke didn’t mean that the bottle
must contain Coke.
It’s the same with optical disks. They are simply a means of storing digital
information, they don’t imply what the information is.
The real confusion is about the 5.25" disks, CD-ROMS. These are small, cheap to
manufacture and have enormous capacity, and because of this have been adopted
by the publishing world for the new technology of ‘Micro-Publishing’. Here the
CD-ROMS contain a mixture of text and pictures, in other words, books. The thing
is that the books are not stored as images of pages, they are stored in two separate
ways on the disk, text is stored letter for letter, and pictures are stored as patterns
of dots. With Digital Imaging Systems, as we’ve already discussed, the information
is stored as images only, regardless of whether the image is of text or of a picture. I
might add that 5.25" CD-ROMs are identical to those you buy as Compact Disks,
and there the information stored is music. Can you see how each is different, despite
the disks being identical ? Another fallacy. Measure a Compact Disk, you’ll find it’s
120mm across, now that’s as near as damn it, 4.75" – so why 5.25" disks ? That’s
The same confusion goes for 12" CD-ROMs (which actually are 12" in diameter);
they are an offshoot of the ill-fated TV disks which appeared about 8 years‘ago. A
TV disk contains digital information (although the system used was not truly digital,
but a form of frequency modulation encoding). These days 12" CD-ROMs are most