Group coded recording

Micropolis

Offering GCR-compatible diskette drives and floppy disk controllers (like the 100163-51-8 and 100163-52-6^[12]), Micropolis endorsed data encoding with group coded recording^[13] on 5¼-inch 100 tpi 77-track diskette drives to store twelve 512-byte sectors per track since 1977 or 1978.^{[14][15][16][17]}

Micro Peripherals

Micro Peripherals, Inc. (MPI) marketed double-density 5¼-inch disk drives (like the single-sided B51 and double-sided B52 drives) and a controller solution implementing GCR since early 1978.^[18][19]

Durango

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.^[22] In later models such as the Durango 800^[23] series this was expanded to a double-sided option for 960 KB (946 KB formatted^{[23][nb 1]}) per diskette.^{[21][24][22][14]}

Apple

For the Apple II floppy drive, Steve Wozniak invented a floppy controller which (along with the Disk II drive itself) imposed two constraints:

• Between any two one bits, there may be a maximum of one zero bit.
• Each 8-bit byte must start with a one bit.

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. It uses two bytes for each byte.

Close to a month prior to the shipment of the disk drive in spring 1978,^[26] Wozniak realized that a more complex encoding scheme would allow each eight-bit byte on disk to hold five bits of useful data rather than four bits. This is because there are 34 bytes which have the top bit set and no two zero bits in a row. This encoding scheme became known as 5-and-3 encoding, and allowed 13 sectors per track; it was used for Apple DOS 3.1, 3.2, and 3.2.1, as well as for the earliest version of Apple CP/M [de]:^[27]

Reserved GCR-codes: 0xAA and 0xD5.^[27]

Wozniak called the system "my most incredible experience at Apple and the finest job I did".^[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. This allowed each eight-bit byte to hold six bits of useful data, and allowed 16 sectors per track. This scheme is known as 6-and-2 encoding,^[27] and was used on Apple Pascal, Apple DOS 3.3^[27] and ProDOS,^[29] and later with Apple FileWare drives in the Apple Lisa and the 400K and 800K 3½-inch disks on the Macintosh and Apple II.^[30][31] Apple did not originally call this scheme "GCR", but the term was later applied to it^[31] to distinguish it from IBM PC floppies which used the MFM encoding scheme.

Reserved GCR-codes: 0xAA and 0xD5.^[27][29]

Commodore

Independently, Commodore Business Machines (CBM) created a group coded recording scheme for their Commodore 2040 floppy disk drive (launched in the spring of 1979). The relevant constraints on the 2040 drive were that no more than two zero bits could occur in a row; the drive imposed no special constraint on the first bit in a byte. This allowed the use of a scheme similar to that used in 6250 bpi tape drives. Every four bits of data are translated into five bits on disk, using the same 5-bit codes as IBM to ensure there are never more than two zero bits in a row, but in a different order:

Like the IBM code, at most eight one bits in a row are possible, so Commodore used sequences of ten or more one bits in a row as a synchronization sequence.

This more efficient GCR scheme, combined with an approach at constant bit-density recording by gradually increasing the clock rate (zone constant angular velocity, ZCAV) and storing more physical sectors on the outer tracks than on the inner ones (zone bit recording, ZBR), enabled Commodore to fit 170 KiB on a standard single-sided single-density 5.25-inch floppy, where Apple fit 140 KiB (with 6-and-2 encoding) or 114 KiB (with 5-and-3 encoding) and an FM-encoded floppy held only 88 KiB.

Sirius/Victor

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.^{[34][35][36][37]} The GCR code is identical to the Commodore one.^[38]

Brother

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.^{[17][39][40][41]} Reportedly, prototypes were already shown at the Internationale Funkausstellung 1979 (IFA) in Berlin.

Sharp

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.^[42][43]