A report on Group coded recording

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

- Group coded recording

13 related topics with Alpha

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8-inch, 5¼-inch, and 3½-inch floppy disks

Floppy disk

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Obsolete type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a fabric that removes dust particles from the spinning disk.

Obsolete type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a fabric that removes dust particles from the spinning disk.

8-inch, 5¼-inch, and 3½-inch floppy disks
8-inch, 5¼-inch (full height), and 3½-inch drives
A 3½-inch floppy disk removed from its housing
8-inch floppy disk, inserted in drive, (3½-inch floppy diskette, in front, shown for scale)
3½-inch, high-density floppy diskettes with adhesive labels affixed
Imation USB floppy drive, model 01946: an external drive that accepts high-density disks
Front and rear of a retail 3½-inch and 5¼-inch floppy disk cleaning kit, as sold in Australia at retailer Big W, circa early 1990s
Different data storage media (Examples include: Flash drive, CD, Tape drive, and CompactFlash)
A floppy hardware emulator, same size as a 3½-inch drive, provides a USB interface to the user
Screenshot depicting a floppy disk as "save" icon
Inside the 8-inch floppy disk
Disk notcher converts single-sided 5¼-inch diskettes to double-sided.
Rear side of a 3½-inch floppy disk in a transparent case, showing its internal parts
The spindle motor from a 3½‑inch unit
A read-write head from a 3½‑inch unit
How the read-write head is applied on the floppy
Visualization of magnetic information on floppy disk (image recorded with CMOS-MagView)
8-inch floppy disk
A 3 1⁄2-inch floppy disk drive
A box of about 80 floppy disks together with one USB memory stick. The stick is capable of holding over 130 times as much data as the entire box of disks put together.

There were competing floppy disk formats, with hard- and soft-sector versions and encoding schemes such as differential Manchester encoding (DM), modified frequency modulation (MFM), M2FM and group coded recording (GCR).

IBM 2401 System/360 tape drives that introduced the 9-track format

9-track tape

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Now generally known as 9-track tape.

Now generally known as 9-track tape.

IBM 2401 System/360 tape drives that introduced the 9-track format
Full-size reel of 9-track tape
9 Track tape drive used with DEC minicomputers
Inside a 9 Track tape drive. The vacuum columns are the two gray rectangles on the left.
Full size 1/2" tape reel in protective case
Two small 1/2" tapes, front and back
Aluminum foil strips mark the start and end of tape
The write protection ring prevents the tape from being written when removed
A typical library of half-inch magnetic tape
3M 777 High Grade 6250 CPI - Security Computer Tape

Various recording methods have been 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").

10+1/2 in diameter reel of 9-track tape

Magnetic-tape data storage

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System for storing digital information on magnetic tape using digital recording.

System for storing digital information on magnetic tape using digital recording.

10+1/2 in diameter reel of 9-track tape
Quarter-inch cartridges
An IBM 3590 data cartridge can hold up to 10GiB uncompressed.
Linear
Linear serpentine
Helical
IBM 729V

Common 7-track densities started at 200 characters per inch (CPI), then 556, and finally 800; 9-track tapes had densities of 800 (using NRZI), then 1600 (using PE), and finally 6250 (using GCR).

An example of Manchester encoding showing both conventions for representation of data

Manchester code

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Line code in which the encoding of each data bit is either low then high, or high then low, for equal time.

Line code in which the encoding of each data bit is either low then high, or high then low, for equal time.

An example of Manchester encoding showing both conventions for representation of data

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.

Physical layout of sectors in a zone-bit disc: As distance from the centre increases, the number of sectors in a given angle increases from one (red) to two (green) to four (grey).

Zone bit recording

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Method used by disk drives to optimise the tracks for increased data capacity.

Method used by disk drives to optimise the tracks for increased data capacity.

Physical layout of sectors in a zone-bit disc: As distance from the centre increases, the number of sectors in a given angle increases from one (red) to two (green) to four (grey).

Commodore 1541 floppy disk (combined ZBR, ZCAV and GCR for 17–21 sectors á 256 bytes in 4 writing speed zones)

The binary signal is encoded using rectangular pulse-amplitude modulation with polar NRZ(L), or polar non-return-to-zero-level code

Non-return-to-zero

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Binary code in which ones are represented by one significant condition, usually a positive voltage, while zeros are represented by some other significant condition, usually a negative voltage, with no other neutral or rest condition.

Binary code in which ones are represented by one significant condition, usually a positive voltage, while zeros are represented by some other significant condition, usually a negative voltage, with no other neutral or rest condition.

The binary signal is encoded using rectangular pulse-amplitude modulation with polar NRZ(L), or polar non-return-to-zero-level code
Unipolar NRZ(L), or unipolar non-return-to-zero level
Non-return-to-zero space
Encoder for NRZS, toggle on zero
An example of the NRZI encoding, transition on 1
The opposite convention, transition on 0
Encoder for NRZI, toggle on one

Synchronized NRZI (NRZI-S, SNRZI) and group-coded recording (GCR) are modified forms of NRZI.

IBM DemiDiskette media and Model 341 FDD

Floppy disk variants

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Data storage and transfer medium that was ubiquitous from the mid-1970s well into the 2000s.

Data storage and transfer medium that was ubiquitous from the mid-1970s well into the 2000s.

IBM DemiDiskette media and Model 341 FDD
Dysan 3¼" Flex Diskettes (P/N 802950)
A Maxell-branded 3-inch Compact Floppy Disk
A 3" floppy disk by Amstrad. This format was used by their CPC and Spectrum lines and in some systems by other manufacturers.
An Amstrad 3-inch floppy drive
3-inch diskette of Nintendo Famicom Disk Systems
3-inch diskette from Smith Corona labelled 2.8-inch for the diameter of the magnetic disc itself
2-inch video floppy from Canon
2-inch LT-1 floppy disk from Fuji
A write-notch puncher for 5 1⁄4-inch disks
The pictured chip, codenamed Paula, controls floppy access on all revisions of the Commodore Amiga as one of its many functions

Both took turnable diskettes named CE-1650F with a total capacity of 2×64 KB (128 KB) at 62,464 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).

Zilog Z765A

Floppy-disk controller

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Special-purpose integrated circuit (IC or "chip") and associated disk controller circuitry that directs and controls reading from and writing to a computer's floppy disk drive (FDD).

Special-purpose integrated circuit (IC or "chip") and associated disk controller circuitry that directs and controls reading from and writing to a computer's floppy disk drive (FDD).

Zilog Z765A
5-1/4 Diskette Drive Adapter found on the IBM PC. This card is based on the NEC D765AC, the large chip at the top of the image.
Block diagram showing FDC communication with the CPU and the FDD.
A setup disk of Japanese Microsoft Office 4.3, provided with 3.5" 1.2 MB and 1440 KB formats.

Translate data bits into FM, MFM, M²FM, or GCR format to be able to record them

Durango F-85

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Early personal computer introduced in September 1978 by Durango Systems Corporation, a company started in 1977 by George E. Comstock, John M. Scandalios and Charles L. Waggoner, all formerly of Diablo Systems.

Early personal computer introduced in September 1978 by Durango Systems Corporation, a company started in 1977 by George E. Comstock, John M. Scandalios and Charles L. Waggoner, all formerly of Diablo Systems.

The F-85 used single-sided 5¼-inch 100 tpi diskette drives providing 480 KB utilizing a high-density 4/5 group coded encoding.

Sirius Systems Technology

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Personal computer manufacturer in Scotts Valley, California.

Personal computer manufacturer in Scotts Valley, California.

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.