What's the difference between an audio CD I have burnt on a CD‑R and one that is produced 'professionally' at a mastering plant? Are the home‑produced ones burnt by a different method to 'professional' ones?
Gary Shepherd
Assistant Editor Sam Inglis replies: The answer is 'yes'. Commercial CDs of the type you buy in record shops are made by a physical pressing process analogous to the pressing of vinyl LPs. This process creates small physical indentations or 'pits' in the CD, which have a different reflectance from the unpitted areas. The manufacturing process is quite complex and uses expensive special equipment. The first stage in commercial CD manufacture is the creation of a 'glass master': this is a flat glass disc covered with a photosensitive chemical. Pits are made by using a laser to soften areas of this chemical. These soft areas are then dissolved using a solvent, leaving holes in the chemical layer.
The glass master itself, however, is too delicate to be used to press CD blanks. More durable metal masters are used to do the actual pressing: these are made by spraying the glass master with metal film to make it conductive, then electroplating it. The resulting layer of metal, when removed from the glass master, is a 'mirror image' of the CD's surface (ie. with bumps where the CD would have pits) and can, therefore, be used to stamp the pits into the CD blanks.
In a CD‑R, on the other hand, there are no physical pits: areas of different reflectance are created by firing a laser at a photo‑sensitive chemical layer. The consequence is that the difference in reflectivity between 'high' and 'low' areas on a CD‑R is lower than that on a standard pressed CD. CD‑ROM drives are designed to cope with this lower difference in reflectivity, but some audio CD players, particularly older models, are only designed to read audio CDs, where there is a big difference in reflectivity between the 'pitted' and flat areas. Some audio CD players can, therefore, struggle with CD‑Rs, or even fail to play them.