Group Code Recording
GCR typically uses a group of five bits of code to represent four bits of data. The encoding ensures no more than two or three zeros occur in a row, and no more than eight or so ones occur in a row, where zeros represent an absense of magnetic change.
GCR is also used on Commodore Business Machines diskette drives; the 4040, 8050, 154x, 157x and 158x series of 5.25" and 3.5" low and high density diskette drives used with 8-bit home computers circa 1977 to 1992. It was also supported on Amiga internal and external drives but only used for reading non-Amiga disks.