MODEL RELEASED. Compact disc test. Technician checks a newly-produced compact disc (CD) as part of quality control. He is using electronic equip- ment which measures and evaluates the quality of the data on the disc. The data (such as music, text or images) is stored as a series of fine dep- ressions on the disc's plastic surface, encoding a digitised signal that can be read by a laser. To reflect the laser light, the coded layer is coated with a fine film of metal which follows the depressions exactly. This is then covered by ano- ther layer of transparent plastic which prevents dust and scratches from affecting the readout. Audio compact discs have high sound quality.