GCR encoding
From Just Solve the File Format Problem
GCR encoding (Group code recording) is a name for a group of schemes for encoding data on magnetic media such as magnetic tape or floppy disks. In GCR encoding, a sequence of data bits is represented by a particular other sequence of bits which "translates" to the data bits in a table of encoded values. The use of different sets of bits instead of the raw bits of the data is necessary due to constraints on consecutive bits with no signal transition, as well as (in some formats) providing error-correction.
Varieties of GCR encoding
- 9-track reel-to-reel tape used an error-correcting GCR encoding
- 5 and 3 encoding: invented by Steve Wozniak and used in Apple DOS 3.1 and 3.2 on Apple II 13 sector disks
- 6 and 2 encoding: an update to 5 and 3 used on Apple II 16 sector disks and later Apple disk formats
- Commodore's encoding scheme used first on PET disks (Commodore 2040) and continued on later Commodore formats
Floppy disk formats using GCR encoding
- Apple II 13 sector disk (Apple DOS 3.2)
- Apple II 16 sector disk (Apple DOS 3.3, ProDOS, Apple III SOS)
- Apple double-density 3 1/2" disk
- Commodore 1541 disk
- Commodore 1571 disk
- Twiggy floppy
