COMPACT DISCS

The compact disc is a digital audio medium. The disc stores stereo 16-bit audio sampled at 44.1 KHz. As an industry specification developed when CDs were invented, CDs can record 74 minutes 33 seconds of audio at this quality, although many newer CDs store up to 80 minutes. In terns of the amount of digital data that can be stored on a CD, this translates to 650 Megabytes (MBs) of information (700 MB for newer CDs).

Prerecorded audio CDs contain audio items such as songs organised into tracks, with each track identified by a track number. However, CDs can also contain digital data of other kinds, in which case they may be seen as files, much as you can see them on a computer's hard disk. If the CD contain audio in other digital audio formats like mp3 or wav, this audio data will not be seen as tracks, but as data files.

A CD is only 120mm (millimetres) in diameter and about 1.2 mm thick. Most of a CD consists of a piece of clear polycarbonate plastic. During manufacturing, this plastic is impressed with microscopic bumps, arranged as a single, continuous, extremely long spiral track of data. These bumps mark the binary numbers, that is, the 1's and 0's (bits), which make up the digital audio on the disc. In the case of a data CD that is used in a computer, the bumps mark the bits that make up the data files stored on the CD. In all other respects it works in exactly the same way.

Once the clear piece of polycarbonate is formed, it is covered with a thin, reflective aluminium layer. A thin acrylic layer is then sprayed over the aluminium to protect it. The label and artwork is then printed on the acrylic.

The data track on the CD spirals outwards from the centre to the edge. The data tracks are extremely small and are measured in microns. A micron is a millionth of a metre. The data track is about 0.5 microns wide, and the gap separating one track from the next is only 1.6 microns wide. The bumps that make up the track are each only 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high. (A nanometer is a billionth of a meter.) The microscopic sizes of the bumps make the spiral track on a CD extremely long. If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and over 5.5km long!

Reading the CD data
 
To read something this small requires an extremely accurate disc-reading mechanism. A CD player uses a low power laser to read the data track from the disc. A laser (Light Amplification by Stimulated Emission of Radiation) is a beam of intense light of a specific wavelength which can be focused and controlled very precisely. A motor unit spins the disc in the player and a sensitive tracking system keeps the laser beam focused on the spiral data track. The laser shines through the polycarbonate layer onto the aluminium backed data track. The shiny aluminium layer reflects the laser light back off the surface of the disc. The light is either reflected straight off the aluminium, or is deflected off a bump in the data track. The CD player detects these differences. It registers the light deflected off a bump as a 1, and light that is not deflected (the reflected light) as a 0. In this way, the CD player creates the stream of digital information that can be converted into an audio signal. The digital information on the CD is coded to ensure that as long as the CD is well looked after, it will work perfectly.

 Memorex 700MB/80-Minute 52x Data CD-R Media (50-Pack Spindle)


The CD also contains other digital data, called sub-code. The sub-code contains a Table of Contents that organises the data on the CD. The subcode also contains information about the tracks on the CD, such as the number of tracks and their duration. The sub-code makes it possible to find different tracks on the CD easily, and allows the CD player to display things such as the elapsed and remaining playback time of a track.



Writable CDs

 
CD-Rs (compact disc-recordable) and CD-RWs (compact disc-rewritable) are increasing finding their way into many radio studios as the technology is becoming more affordable and easier to use. Some community stations have CD-Writers, and many are using them to distribute programming. This is especially important where CR stations share programming.

 
CD-Rs - writable CDs that can only be written once - can be read by almost any conventional CD player. CD-RWs - rewritable CDs that can be erased and reused several times - may not be read by some CD players (though this is changing rapidly). This is because CD-Rs are often "finalised" when they are written - that is, a Table of Contents is written into the sub-code when it is written. It may be worth investigating if your choice of CD player can play "unfinalised" CD-Rs, as this feature may become more useful in the future.

Caring for CDs

The CD is a very delicate system, using the deflections of light off microscopic bumps to read audio. The CD is coded with a variety of error-correcting techniques to make it more reliable, but unless you look after your CDs, and handle them properly, they will be damaged and start "skipping" or not play at all. Being an optical based system, the usual problem is scratches and chips on the clear polycarbonate layer which prevent the laser from reading the data properly.

• To take a disc from its case, press down on the centre of the case and lift the disc out, holding it carefully by the edges.

• Fingermarks and dust should be carefully wiped off the disc's recorded surface (the silvered side) with a soft cloth. CDs do not have any grooves that can collect dirt and dust, so gently wiping the disc with a soft cloth should remove most dirt. To clean the disc, wipe in a straight line from the centre to the edge. Wiping in a circular motion will scratch a large part of the spiral data track and will cause the disc to "skip".

• Never use chemicals such as record sprays, anti-static sprays, benzene, methylated spirits or thinners to clean CD's. Such chemicals can do irreparable damage to the disc's plastic surface.

• Return discs to their cases after playing to avoid scratches that could cause the disc to "skip".

• Do not bend the disc.

• Do not leave the discs in the sun, or in areas of high temperature or humidity. Long periods in high temperatures can cause the disc to warp.

• Do not stick labels on the label side of the CD. If you need to write on the label side of the CD, write very gently with a permanent felt-tipped or other soft-tipped marker. Do not use a hard-tipped pencil or ballpoint pen. Never mark or write on the recorded side of the disc.