CD-ROMs somewhat obsolete now days, look similar to an audio CD, but contain software applications, encyclopedias, glossaries, presentations and other multimedia programs. CDs were originally designed to hold 74 minutes of high-quality digital audio but can hold up to 650MB of data.

cd rom disc

The basic CD ROM disk is 120mm in diameter and is basically a 1.2mm sandwich of three coatings: a back layer of clear polycarbonate plastic, a thin sheet of aluminium and a lacquer coating to protect the disc from external scratches and dust.

CD ROM Cross section

During the traditional mastering process, a polycarbonate plastic blank is stamped with millions of tiny indentations called pits – moulded in a spiral from the centre of the disc outwards – which is then coated with a thin layer of aluminium, giving the disc its characteristic silver colour. The pits are typically 0.5 microns wide, 0.83 to 3 microns long and 0.15 microns deep. The space between the tracks – the pitch – is just 1.6 microns.

CD track pitch

Track density is over 16,000 tpi, compared to 96tpi of a floppy disk and the average 400 of a hard disk. Unravelled and laid in a straight line the spiral of data would stretch for four miles.

Mechanically, the CD is less vulnerable than an analogue record, but that does not mean it should not be treated with care. It is not generally realised that it is the top printed layer of a CD that is the more vulnerable surface. Though the aluminium layer is protected from corrosion and damage by a lacquer on which a disc label can be printed, this protective layer is very thin – only 0.002mm. Careless treatment or granular dust can cause small scratches or hairline cracks, enabling air to penetrate and evaporate the aluminium coating and cause oxidising – rendering the disk unusable.

On the other surface, the laser focuses on a layer within the clear base. If the laser hits a land (shown below left), the laser is reflected back to the photo cell and is registered as a 1. If the laser hits a pit (shown below right), the light is deflected away and is registered as a 0.

Reading a CD with a laser

Read page 52 of the textbook “Exploring Computer Hardware”.