Group coded recording

GCRgroup-coded recordingGCR (4/5)4/5 group coded encoding5-and-3 encoding5-bit6 and 2 encoding6-and-2 encoding6-bitGroup Coded Recording (GCR)
In computer science, group coded recording or group code recording (GCR) refers to several distinct but related encoding methods for magnetic media.wikipedia
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Floppy disk

floppy disk drivefloppy drivediskette
The others are different mainframe hard disk as well as floppy disk encoding methods used in some microcomputers until the late 1980s.
There were competing floppy disk formats, with hard- and soft-sector versions and encoding schemes such as FM, MFM, M 2 FM and GCR.

9 track tape

vacuum column9-track tapevacuum columns
Group coded recording was first used for magnetic tape data storage on 9-track reel-to-reel tape.
Various recording methods were employed during its lifetime as tape speed and data density increased, including PE (phase encoding), GCR (group coded recording) and NRZI (non-return-to-zero, inverted, sometimes pronounced "nur-zee").

Magnetic tape data storage

magnetic tapetapeCassette
Group coded recording was first used for magnetic tape data storage on 9-track reel-to-reel tape.
Nine-track tapes had densities of 800 (using NRZI), then 1600 (using PE), and finally 6250 (using GCR).

Non-return-to-zero

NRZINRZnon-return-to-zero, inverted
GCR is a modified form of a NRZI code, but necessarily with a higher transition density.
Synchronized NRZI (NRZI-S, SNRZI) and group-coded recording (GCR) are modified forms of NRZI.

Western Digital FD1771

WD1770FD17811793
The machine was using a Western Digital FD1781 floppy disk controller, designed by a former Sperry ISS engineer, with 77-track Micropolis drives.

Floppy-disk controller

floppy disk controllerFDCcontroller
Offering GCR-compatible diskette drives and floppy disk controllers (like the 100163-51-8 and 100163-52-6), Micropolis endorsed data encoding with group coded recording on 5¼-inch 100 tpi 77-track diskette drives to store twelve 512-byte sectors per track since 1977 or 1978.

Disk II

Disk IIc5.25" DrivesApple disk drives
Following the Shugart controller's manual, Wozniak attempted to develop an FM-type controller with 10 sector per track storage, but realized that Group Coded Recording could fit 13 sectors per track.

Run-length limited

RLLrun length limitedMFM
The first, used in 6250 bpi magnetic tape since 1973, is an error-correcting code combined with a run length limited (RLL) encoding scheme, belonging into the group of modulation codes.
This is the original IBM group coded recording variant.

Nibble

nybblehalf-byte4 bits
11 of the nibbles (other than xx00 and 0001) have their code formed by prepending the complement of the most significant bit; i.e. abcd is encoded as abcd.
In the Apple II microcomputer line, much of the disk drive control and group-coded recording was implemented in software.

Durango F-85

Durango Systems CorporationDurango 800Durango Systems
The Durango Systems F-85 (introduced in September 1978 ) used single-sided 5¼-inch 100 tpi diskette drives providing 480 KB utilizing a proprietary high-density 4/5 group coded encoding.
The F-85 used single-sided 5¼-inch 100 tpi diskette drives providing 480 KB utilizing a high-density 4/5 group coded encoding.

Floppy disk variants

flippy disksuperfloppiessuperfloppy
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.
Both took turnable diskettes named CE-1650F with a total capacity of 2×64 KB (128 KB) at 62464 bytes per side (512 byte sectors, 8 sectors/track, 16 tracks (00..15), 48 tpi, 250 kbit/s, 270 rpm with GCR (4/5) recording).

Micropolis (company)

MicropolisMicropolis CorporationMicropolis MDOS
Offering GCR-compatible diskette drives and floppy disk controllers (like the 100163-51-8 and 100163-52-6), Micropolis endorsed data encoding with group coded recording on 5¼-inch 100 tpi 77-track diskette drives to store twelve 512-byte sectors per track since 1977 or 1978.

Apple FileWare

FileWareApple "Twiggy" FileWareTwiggy drive
This scheme is known as 6-and-2 encoding, and was used on Apple Pascal, Apple DOS 3.3 and ProDOS, and later with Apple FileWare drives in the Apple Lisa and the 400K and 800K 3½-inch disks on the Macintosh and Apple II.
The disk format uses group coded recording (GCR) in a manner very similar to that of the Disk II.

Steve Wozniak

WozniakStephen WozniakWoz Way

Commodore 4040

404020403040
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 Group Coded Recording (GCR) scheme of binary encoding is used to store data on the magnetic disk medium.

Manchester code

Manchester encodingManchesterManchester coding
Prior to 6250 bpi tapes, 1600 bpi tapes satisfied these constraints using a technique called phase encoding (PE), which was only 50% efficient.
Manchester code was widely used for magnetic recording on 1600 bpi computer tapes before the introduction of 6250 bpi tapes which used the more efficient group-coded recording.

Modified frequency modulation

MFMFMDelay encoding
Apple did not originally call this scheme "GCR", but the term was later applied to it to distinguish it from IBM PC floppies which used the MFM encoding scheme.

Sirius Systems Technology

Victor 9000Sirius 1Victor
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 ten-bit GCR and constant bit-density recording by gradually decreasing a drive's rotational speed for the outer tracks in nine zones to achieve formatted capacities of 606 kB (single sided) / 1188 kB (double-sided) on 96 tpi media.
This, combined with group-coded recording (GCR), allowed standard floppy disks to hold more data than others at the time, 600 kB on single- and 1.2 MB on double-sided floppies compared with 140-160 kB per side of other machines such as the Apple II and early IBM PC, but disks made at constant bit density were not compatible with machines with standard drives.

4B5B

Other 4-to-5-bit codes have been used for magnetic recording and are known as group coded recording (GCR), but those are (0,2) run-length limited codes, with at most two consecutive zeros.

Integrated Woz Machine

Disk ControllerIWM
Instead of storing 8–10 sectors (each holding 256 bytes of data) per track on a 5.25-inch floppy disk — something standard at that time, Wozniak utilized group-coded recording (GCR), and with 5-and-3 encoding he managed to squeeze as many as 13 sectors on each track using the same mechanics and the same storage medium.

Chuck Peddle

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 ten-bit GCR and constant bit-density recording by gradually decreasing a drive's rotational speed for the outer tracks in nine zones to achieve formatted capacities of 606 kB (single sided) / 1188 kB (double-sided) on 96 tpi media.
*Group coded recording

Computer science

computer scientistcomputer sciencescomputer scientists
In computer science, group coded recording or group code recording (GCR) refers to several distinct but related encoding methods for magnetic media.

Magnetic storage

magnetic recordingmagnetic mediamagnetic disk
In computer science, group coded recording or group code recording (GCR) refers to several distinct but related encoding methods for magnetic media.

Magnetic tape

tapetapesanalog tape
The first, used in 6250 bpi magnetic tape since 1973, is an error-correcting code combined with a run length limited (RLL) encoding scheme, belonging into the group of modulation codes.