The single spool cassette has a rectangular housing with top (1a) and bottom plates (1b) coupled by a spool chamber. The spool has a cylindrical core (12) with a drive ring (20) set into it. The drive ring is normally axially loaded by a braking disc (21) that is subjected to a spring force (22) and maintains position when not in use. When activated the drive engages serrations on the underside (26).
