Audio compression was developed some time before the formation of MPEG and its Eureka 147 project. Eureka 147 developed the MUSICAM (Masking pattern adapted Universal Sub-band Integrated Coding and Multiplexing) jointly with some European manufacturers. MUSICAM was designed for digital audio broadcasting ( DAB). In parallel with this, another system was being developed called ASPEC (Adaptive Spectral Perceptual Entropy Coding) jointly by AT&T Bell Labs, Thomson and others. ASPEC included high compression for audio transmission on ISDN lines. Following a comprehensive subjective testing on both systems by the Swedish Broadcasting Corporation, MPEG audio group combined both systems into a common standard with three levels known as layers: I, II and II. The three layers differ in coding complexity and performance in terms of bit rate reduction. Layer I is a simplified version of MUSICAM which provides low compression rates at low cost. Layer II employs MUSICAM technology in full; it provides high compression rates and is generally employed in DAB and digital television (DTV). Layer III (commonly known as MP3) combines the best features of both techniques, providing extremely high rates of compression. It is mainly employed in music download and telecommunication applications where high compression ratios are necessary. Further development has produced advanced audio coding ( AAC) with increased rates of compression making multi-channel surround sound a practical possibility for home users.