 <unk> words are unintelligible noise when played forward , but when played backwards are clear speech . Listening to backmasked audio with most turntables requires disengaging the drive and rotating the album by hand in reverse ( though some can play records backwards ) . With magnetic tape , the tape must be reversed and spliced back in to the cassette . Compact discs were difficult to reverse when first introduced , but digital audio editors , which were first introduced in the late 1980s and became popular during the next decade , allow easy reversal of audio from digital sources . 
