GCR is a modified form of a NRZI code, but necessarily with a higher transition density.
In order to reliably read and write to magnetic tape, several constraints on the signal to be written must be followed.
Because of the extremely high density (for the time) of 6250 bpi tape, the RLL code is not sufficient to ensure reliable data storage.
[3] In the mid-1970s, Sperry Univac, ISS Division was working on large hard drives for the mainframe business using group coding.
[18][19] The Durango Systems F-85 (introduced in September 1978[20][21]) used single-sided 5¼-inch 100 tpi diskette drives providing 480 KB utilizing a proprietary high-density 4/5 group coded encoding.
The machine was using a Western Digital FD1781 floppy disk controller, designed by a former Sperry ISS engineer,[17] with 77-track Micropolis drives.
[21][24][22][14] For the Apple II floppy drive, Steve Wozniak invented a floppy controller which (along with the Disk II drive itself) imposed two constraints: The simplest scheme to ensure compliance with these limits is to record an extra "clock" transition before each data bit according to differential Manchester encoding or (digital) FM (frequency modulation).
Known as 4-and-4 encoding, the resulting Apple implementation allowed only ten 256-byte sectors per track to be recorded on a single-density 5¼-inch floppy.
[26] Later, the design of the floppy drive controller was modified to allow a byte on disk to contain up to one pair of zero bits in a row.
Similar, the 5.25-inch floppy drives of the Victor 9000 aka Sirius 1, designed by Chuck Peddle in 1981/1982, used a combination of GCR and zone bit recording by gradually decreasing a drive's rotational speed for the outer tracks in nine zones while increasing the number of sectors per track[33] to achieve formatted capacities of 606 KiB (single sided) / 1188 KiB (double-sided) on 96 tpi media.
[38] Starting around 1985, Brother introduced a family of dedicated word processor typewriters with integrated 3.5-inch 38-track[nb 2] diskette drive.
Early models of the WP and LW series [de] used a Brother-specific group-coded recording scheme with twelve 256-byte sectors to store up to 120 KB[nb 3] on single-sided and up to 240 KB[nb 3] on double-sided double-density (DD) diskettes.
In 1986, Sharp introduced a turnable 2.5-inch pocket disk drive solution (drives: CE-1600F, CE-140F; internally based on the FDU-250 chassis; media: CE-1650F) for their series of pocket computers with a formatted capacity of 62464 bytes per side (2× 64 kB nominal, 16 tracks, 8 sectors/track, 512 bytes per sector, 48 tpi, 250 kbit/s, 270 rpm) with GCR (4/5) recording.