GCR (group coded recording)
An encoding that packs data bits into longer code groups so the tape or floppy never loses its clock — the home computer's way around FM.
GCR translates groups of four or five data bits into longer code words chosen so that no run of zeros gets too long, which keeps the read circuit's clock in sync without the wasteful clock transitions FM demanded. The Apple II used 5-and-3 (DOS 3.2) and then 6-and-2 (DOS 3.3); the Commodore 1541 built its entire disk format on GCR combined with zoned recording. On magnetic tape, GCR at 6250 cpi replaced the older phase encoding (PE) in 1973.
Where MFM and RLL won the fight on the hard disk, GCR ruled the floppy drives of the 8-bit home computers — same core idea, different camp.