dont know exactly, but I'm a passionate music fan, and I've heard Dylan and U2 search for De Lanois and Springsteen thank his Landau for taking out a special 'sound' for their albums. For me the producer, as a responsible of the overall result of the album, has to be expert in arranging, recording, sound engineering and mixing (and songwriting too) - for he has to take the final decision over the work of all those people (if it is good and sounds good). I believe it is and it should be so, like a coach for a football team.