 The Dolby SR-D marking refers to both analog Dolby SR and digital Dolby Digital soundtracks on one print.Dolby "S" was introduced in 1989.It was intended that Dolby "S" would become standard on commercial pre-recorded music cassettes in much the same way that Dolby "B" had in the 1970s, but it came to market when the Compact Cassette was being replaced by the Compact Disc as the dominant mass market music format.