Group coded recording

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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. The first, used in Template:Val magnetic tape since 1973,[1][2] is an error-correcting code combined with a run-length limited (RLL) encoding scheme, belonging into the group of modulation codes.[3] The others are similar encoding methods used in mainframe hard disks or microcomputer floppy disks until the late 1980s. GCR is a modified form of a NRZI code, but necessarily with a higher transition density.[3]

Script error: No such module "anchor".Magnetic tape

Group coded recording was first used for magnetic-tape data storage on 9-track reel-to-reel tape.[3] The term was coined during the development of the IBM 3420 Model 4/6/8 Magnetic Tape Unit[1] and the corresponding 3803 Model 2 Tape Control Unit,[4][1] both introduced in 1973.[1][5] IBM referred to the error correcting code itself as "group coded recording". However, GCR has come to refer to the recording format of Template:Val (250 bits/mm[3]) tape as a whole, and later to formats which use similar RLL codes without the error correction code.

In order to reliably read and write to magnetic tape, several constraints on the signal to be written must be followed. The first is that two adjacent flux reversals must be separated by a certain distance on the media, defined by the magnetic properties of the media itself. The second is that there must be a reversal often enough to keep the reader's clock in phase with the written signal; that is, the signal must be self-clocking and most importantly to keep the playback output high enough as this is proportional to the density of flux transitions. Prior to Template:Val tapes, Template:Val tapes satisfied these constraints using a technique called phase encoding (PE), which was only 50% efficient. For Template:Val GCR tapes, a (0, 2) RLL code is used, or more specifically a Template:Sfrac (0, 2) block code[3] sometimes also referred to as GCR (4B-5B) encoding.[6] This code requires five bits to be written for every four bits of data.[3] The code is structured so that no more than two zero bits (which are represented by lack of a flux reversal) can occur in a row,[3] either within a code or between codes, no matter what the data was. This RLL code is applied independently to the data going to each of the nine tracks.

Of the 32 five-bit patterns, eight begin with two consecutive zero bits, six others end with two consecutive zero bits, and one more (10001) contains three consecutive zero bits. Removing the all-ones pattern (11111) from the remainder leaves 16 suitable code words.

The Template:Val GCR RLL code:Template:R

4-bit value GCR codeTemplate:R
hex bin bin hex
0x0 0000 1.1001 0x19
0x1 0001 1.1011 0x1B
0x2 0010 1.0010 0x12
0x3 0011 1.0011 0x13
0x4 0100 1.1101 0x1D
0x5 0101 1.0101 0x15
0x6 0110 1.0110 0x16
0x7 0111 1.0111 0x17
4-bit value GCR codeTemplate:R
hex bin bin hex
0x8 1000 1.1010 0x1A
0x9 1001 0.1001 0x09
0xA 1010 0.1010 0x0A
0xB 1011 0.1011 0x0B
0xC 1100 1.1110 0x1E
0xD 1101 0.1101 0x0D
0xE 1110 0.1110 0x0E
0xF 1111 0.1111 0x0F

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 aabcd. The other five values are assigned codes beginning with 11. Nibbles of the form ab00 have codes 11baa, i.e. the bit reverse of the code for ab11. The code 0001 is assigned the remaining value 11011.

Because the all-ones code is not used in normal data, at most 8 one-bits can appear in a row. Sequences of 9 or more one-bits (in practice 14 all-ones codes, or 70 one-bits, were used) are used as a synchronization pattern.

Because of the extremely high density (for the time) of Template:Val tape, the RLL code is not sufficient to ensure reliable data storage. On top of the RLL code, an error-correcting code called the Optimal Rectangular Code (ORC) is applied.[7] This code is a combination of a parity track and polynomial code similar to a CRC, but structured for error correction rather than error detection. For every seven bytes written to the tape (before RLL encoding), an eighth check byte is calculated and written to the tape. When reading, the parity is calculated on each byte and exclusive-ORed with the contents of the parity track, and the polynomial check code calculated and exclusive-ORed with the received check code, resulting in two 8-bit syndrome words. If these are both zero, the data is error free. Otherwise, error-correction logic in the tape controller corrects the data before it is forwarded to the host. The error correcting code is able to correct any number of errors in any single track, or in any two tracks if the erroneous tracks can be identified by other means.

In newer IBM half-inch 18-track tape drives recording at Template:Val, Template:Sfrac (0, 2) GCR was replaced by a more efficient Template:Sfrac (0, 3) modulation code, mapping eight bits to nine bits.[3]

Script error: No such module "anchor".Hard disks

In the mid-1970s, Sperry Univac, ISS Division was working on large hard drives for the mainframe business using group coding.[8]

Script error: No such module "anchor".Floppy disks

Like magnetic tape drives, floppy disk drives have physical limits on the spacing of flux reversals (also called transitions, represented by one-bits).

Micropolis

Offering GCR-compatible diskette drives and floppy disk controllers (like the 100163-51-8 and 100163-52-6[9]), Micropolis endorsed data encoding with group coded recording[10] on 5¼-inch 100 tpi 77-track diskette drives to store twelve 512-byte sectors per track since 1977 or 1978.[11][12][13][14]

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.[15][16]

Durango

The Durango Systems F-85 (introduced in September 1978[17][18]) 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,[14] with 77-track Micropolis drives.[19] In later models such as the Durango 800[20] series this was expanded to a double-sided option for 960 KB (946 KB formatted[20][nb 1]) per diskette.[18][21][19][11]

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.

Script error: No such module "anchor".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.

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Script error: No such module "anchor".Close to a month prior to the shipment of the disk drive in spring 1978,[23] 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 Template:Ill:[24]

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Reserved GCR-codes: 0xAA and 0xD5.[24]

Wozniak called the system "my most incredible experience at Apple and the finest job I did".[23]

Script error: No such module "anchor".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,[24] and was used on Apple Pascal, Apple DOS 3.3[24] and ProDOS,[26] and later with Apple FileWare drives in the Apple Lisa and the 400K and 800K 3½-inch disks on the Macintosh and Apple II.[27][28] Apple did not originally call this scheme "GCR", but the term was later applied to it[28] to distinguish it from IBM PC floppies which used the MFM encoding scheme.

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Reserved GCR-codes: 0xAA and 0xD5.[24][26]

Script error: No such module "anchor".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 Template:Val 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:

4-bit value GCR code[29]
hex bin bin hex
0x0 0000 0.1010 0x0A
0x1 0001 0.1011 0x0B
0x2 0010 1.0010 0x12
0x3 0011 1.0011 0x13
0x4 0100 0.1110 0x0E
0x5 0101 0.1111 0x0F
0x6 0110 1.0110 0x16
0x7 0111 1.0111 0x17
4-bit value GCR code[29]
hex bin bin hex
0x8 1000 0.1001 0x09
0x9 1001 1.1001 0x19
0xA 1010 1.1010 0x1A
0xB 1011 1.1011 0x1B
0xC 1100 0.1101 0x0D
0xD 1101 1.1101 0x1D
0xE 1110 1.1110 0x1E
0xF 1111 1.0101 0x15

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 Template:Val on a standard single-sided single-density 5.25-inch floppy, where Apple fit Template:Val (with 6-and-2 encoding) or Template:Val (with 5-and-3 encoding) and an FM-encoded floppy held only Template:Val.

Script error: No such module "anchor".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[30] to achieve formatted capacities of Template:Val (single sided) / Template:Val (double-sided) on 96 tpi media.Template:R The GCR code is identical to the Commodore one.[31]

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 Template:Ill 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.[14][32][33][34] Reportedly, prototypes were already shown at the Internationale Funkausstellung 1979 (IFA) in Berlin.

Script error: No such module "anchor".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 Template:Val 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.[35][36]

Other uses

GCR was also evaluated for a possible use in bar code encoding schemes (packing efficiency, timing tolerances, amount of storage bytes for timing information, and DC output level).[37]

See also

Notes

Template:Reflist

References

Template:Reflist

Further reading

  • ANSI INCITS 40-1993 (R2003) Unrecorded Magnetic Tape for Information Interchange (9-track, 800 bpi, NRZI; Template:Val, PE; and Template:Val, GCR)
  • ANSI INCITS 54-1986 (R2002) Recorded Magnetic Tape for Information Interchange (Template:Val, GCR)
  • Script error: No such module "Citation/CS1".
  • Script error: No such module "Citation/CS1".
  • Script error: No such module "Citation/CS1".
  • Script error: No such module "Citation/CS1". (NB. Mentions the 5/4 RLL code used on Template:Val tape drives.)
  • Script error: No such module "citation/CS1". (NB. Additional detail on the GCR tape format.)
  • Template:Cite patent (NB. Application No: US Template:Val. See also: CA993998A, CA993998A1, DE2142428A1)
  • Template:Cite patent (NB. Application Number: US 5/904420)
  • Template:Cite patent (NB. Application Number: US 06/559210. See also: CA1208794A, CA1208794A1, DE3443272A1, DE3443272C2)
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  • Script error: No such module "citation/CS1".
  • Script error: No such module "Citation/CS1".
  • Script error: No such module "citation/CS1".
  • Script error: No such module "citation/CS1".
  • Script error: No such module "citation/CS1".
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External links

  • Script error: No such module "citation/CS1"., Script error: No such module "citation/CS1"., Script error: No such module "citation/CS1".
  1. ↑ a b c d Cite error: Invalid <ref> tag; no text was provided for refs named CW_1973_IBM
  2. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named X3.54
  3. ↑ a b c d e f g h Cite error: Invalid <ref> tag; no text was provided for refs named Patel_1988_MR
  4. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named OldIron_2004_IBM3803
  5. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Harris_1981_IBM
  6. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Geoffroy_2002
  7. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Patel_1974_ORC
  8. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Jacoby_1977_3PM
  9. ↑ Script error: No such module "citation/CS1".)
  10. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named McClelland_1979_GCR
  11. ↑ a b Cite error: Invalid <ref> tag; no text was provided for refs named Micropolis_1978
  12. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Micropolis_1979_DiskMaint
  13. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named IW_1980_Micropolis
  14. ↑ a b c Cite error: Invalid <ref> tag; no text was provided for refs named Guzis_2015_Multi
  15. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Allen_1978_Byte
  16. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named CW_1979_MPI
  17. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Schultz_1978_Durango
  18. ↑ a b Cite error: Invalid <ref> tag; no text was provided for refs named Hendrie_2003_Comstock
  19. ↑ a b Cite error: Invalid <ref> tag; no text was provided for refs named Guzis_2009_Micropolis
  20. ↑ a b Cite error: Invalid <ref> tag; no text was provided for refs named Durango_800
  21. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Guzis_2006_F85
  22. ↑ a b c d e f g h Cite error: Invalid <ref> tag; no text was provided for refs named CP_1989_CopyII
  23. ↑ a b Cite error: Invalid <ref> tag; no text was provided for refs named Williams_1985_Byte
  24. ↑ a b c d e f g h i j k Cite error: Invalid <ref> tag; no text was provided for refs named Lechner_1982_Beneath_Apple_DOS
  25. ↑ a b c d e f Cite error: Invalid <ref> tag; no text was provided for refs named Sather_1983_Understanding_Apple_II
  26. ↑ a b c d e f Cite error: Invalid <ref> tag; no text was provided for refs named Lechner_1985_Beneath_Apple_ProDOS
  27. ↑ a b c d e Cite error: Invalid <ref> tag; no text was provided for refs named Feichtinger_1987
  28. ↑ a b Cite error: Invalid <ref> tag; no text was provided for refs named IWM_1982
  29. ↑ a b Cite error: Invalid <ref> tag; no text was provided for refs named Hildon_1985_Anthology
  30. ↑ Script error: No such module "citation/CS1".
  31. ↑ Script error: No such module "citation/CS1".
  32. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Gieseke_2003_Brother
  33. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Mick_2002_Brother_WP-6
  34. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Cotgrove_2009_Brother
  35. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Sharp_1986_CE1600F
  36. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Sharp_1986_CE140F
  37. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Moseley_1979_Byte


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