The bit rate of a single channel of high-quality digital audio may now be reduced by up to 90% using so-called -perceptual coding systems- with minimal effects on sound quality. Such systems form the basis of proposals for digital audio broadcasting (DAB), this being the subject of the Eureka 147 programme, and they offer the possibility for extending the storage times available from digital recording products. Perceptual coders rely for their success on studies of psycho-acoustic masking, which suggest that a considerable amount of information in most audio signals is redundant as far as the hearing mechanism is concerned.