BOOT CAMP 251 (12/11/02)




In just a couple of years CD-writers have evolved from an expensive luxury to an essential ingredient of most new multimedia PCs and laptops. However, judging by the number of emails and letters we receive on the subject it’s all happened rather quickly and caught a lot of people on the hop. To the uninitiated the technology can appear baffling, many manufacturers seem to take it for granted that every PC users knows instinctively how to use it and there’s lots of new jargon words to learn.


A new generation of PCs fitted with DVD-writers is only going to complicate matters so the next few episodes of Boot Camp are devoted to those shiny discs, in all their many and various incarnations, what they do, how they do it what they can do for you. This week a general overview, in subsequent episodes we’ll be looking at CD-writer software, backing up PC files creating your own music and picture discs and recordable DVD.


First the basics, a standard 12cm CD/CD-ROM disc can hold between 650 and 700Mb of digital data, which can be in the form of music, video, or plain old computer data. Think of DVDs as high capacity CDs that can hold between 4.7 and 15.9 gigabytes of data, depending on how they are constructed and configured.


All optical discs, and that includes audio CDs, CD-ROMs, DVDs, Laserdiscs, MiniDiscs and so on are made up from several layers of plastic, bonded together to protect the all important reflective layer in the middle, which holds the digital data. Seen under a under a microscope the reflective layer appears as a spiral of shiny dots or ‘pits’, each one representing a binary digit or ‘bit’ of digital information. As the disc spins a low power laser tracks across the surface of the disc, the reflections from the ‘pits’ are picked up by an optical sensor and converted into a stream of digital data.


On pre-recorded CDs, CD-ROMs and DVDs the pits are stamped into the plastic using a powerful press and then coated with a reflective layer of aluminium -- just a few atoms thick -- inside a vacuum chamber. Clearly this process would be a little difficult to replicate within the confines of a PC so CD-writers use a different technique to record data onto blank discs.


There are two types of recordable disc: CD-Rs, which can be recorded once, and rewritable CD-RWs, which can be erased and reused many times. The DVD variants are DVD-R and DVD-RW (in fact there are three types of rewritable DVD discs, which we’ll look at in more detail in part four). CD-Rs are cheap and cost only a few pence each when bought in bulk, inevitably CD-RW discs are dearer costing a couple of pounds each. Blank DVDs are still quite expensive (three or four pounds each for DVD-Rs and seven or eight pounds apiece for rewritable discs) but there’s every reason to suppose prices will fall reasonably quickly when the economies of scale start to take effect.  


On a record-once disc the shiny reflective layer – preformatted with a spiral guide track -- is coated with a photosensitive organic dye (the characteristic green or blue tint) that turns opaque when ‘burned’ with a high-powered laser beam. During the recording process the laser beam, controlled by the data being recorded, switches on and off as it tracks across the surface of the disc, the unexposed areas of the disc allow the reflective layer to show through as a spiral of reflective dots.


CD-RW discs use a different process, known as ‘phase change’. The reflective layer is made up of a compound that changes between reflective crystalline and non-reflective amorphous states when heated by a high power laser beam. However, unlike the dyes used on CD-R discs the process is reversible and if the compound is exposed to a slightly lower power laser it effectively ‘melts’ and becomes reflective once again, erasing the recording. The reflective dots on a CD-RW are not as shiny or sharply defined as the pits on a factory produced discs or CD-R disc and some older CD players and CD-ROM drives may refuse to read CD-RW discs.


Incidentally, the MiniDisc format employs a yet another variation of the theme, called magneto-optical phase change, which uses a combination of laser light and magnetism to switch the state of the disc’s reflective layer.


These processes highlight some of the important differences between CD-R/RW disc technologies and the more familiar hard and floppy magnetic disc storage systems. The recording process is a lot slower than writing data to a hard disc and it’s not possible to selectively erase and rewrite data. Well, that’s not strictly true, some CD-writers allow data to be added to a recordable disc in separate sessions using ‘packet’ writing software, but in order for the disc to be readable on another CD player or CD-ROM drive the session has to be ‘closed’ or ‘finalised’, which fixes the disc’s table of contents (TOC). Before a disc is finalised data cannot be erased but references to it can be removed from the TOC, so it won’t be readable by normal means. However the ‘erased’ data is still on the disc and the space it occupies cannot be re-used. There are a few ifs and buts and specialist software and CD-writers that can get around some of these limitations but they often compromise compatibility or reliability.


CD-R/RW is not an alternative to hard disc storage but it is a convenient means of storing large volumes of data, discs are cheap, robust, portable and easily read on other PCs and once you’ve got the hang of using a CD-writer you’ll wonder how you ever managed without it…


Next week – Part 2, CD-Writer tecnospeak tutorial





The process of recording a CD in a CD-writer



Motion Picture Experts Group audio layer 3 -- digital audio compression system commonly used to send files containing audio and music over the Internet



Table of contents, the main directory on a CD-ROM, listing all of the files it contains



Blank CD-Rs are very cheap nowadays and can be bought in bulk, on ‘spindles’ of 100 for less than £20 by mail order and from computer fairs, but there’s a lot of unbranded rubbish on the market, including reject, faulty and out of spec discs masquerading as top quality product. It’s definitely worth paying a little extra for blanks from a well-known manufacturer, especially if you are planning to use them for backing up valuable data.


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