You've probably never heard of Bruce Swedien, yet you've heard many of the songs he's worked on. Swedien is considered the best sound engineer in the history of the music industry. In the early 1980s, together with Michael Jackson, they began a project that would forever alter the way pop music is recorded. The result was the album Thriller (the first of a collaboration between Jackson and the technician that would last over 30 years). Marcos Cabotá (director nominated for two Goya's) travelled to L.A. to talk with Swedien (and several other sound engineers) and find out how music history was revolutionised.