Group coded recording

From Wikipedia, the free encyclopedia
  (Redirected from Group Code Recording)
Jump to: navigation, search

In computer science, group coded recording or group code recording (GCR) refers to several distinct but related encoding methods for magnetic media. 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.[1] The others are different mainframe hard disk as well as floppy disk encoding methods used in some microcomputers until the late 1980s. GCR is a modified form of a NRZI code, but necessarily with a higher transition density.[1]

GCR for magnetic tape[edit]

Group coded recording was first used for magnetic tape data storage on 9-track reel-to-reel tape.[1] The term was coined during the development of the IBM 3420 Model 4/6/8 Magnetic Tape Unit[2] and the corresponding 3803 Model 2 Tape Control Unit,[3][2] both introduced in 1973.[2][4] IBM referred to the error correcting code itself as "group coded recording". However, GCR has come to refer to the recording format of 6250 bpi (250 bits/mm[1]) 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. The second is that there must be a flux reversal often enough to keep the reader's clock in phase with the written signal; that is, the signal must be self-clocking. Prior to 6250 bpi tapes, 1600 bpi tapes satisfied these constraints using a technique called phase encoding (PE), which was only 50% efficient. For 6250 bpi GCR tapes, a (0, 2) RLL code is used, or more specifically a 4/5 (0, 2) block code.[1] This code requires five bits to be written for every four bits of data.[1] 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,[1] 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 6250 bpi GCR RLL code:[5][6]

4-bit value GCR code[5][6]
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 code[5][6]
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 of the extremely high density of 6250 bpi 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 24000 bpi, 4/5 (0, 2) GCR was replaced by a more efficient 8/9 (0, 3) modulation code, mapping eight bits to nine bits.[1]

GCR for hard disks[edit]

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

GCR for floppy disks[edit]

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

Micropolis[edit]

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[9] on 5¼-inch 100 tpi 77-track diskette drives to store twelve 512-byte sectors per track since 1977 or 1978.[10][11][12][13]

Micro Peripherals[edit]

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.[14][15]

Durango[edit]

The Durango Systems F-85 (introduced in September 1978[16][17]) 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,[13] with 77-track Micropolis drives.[18] In later models such as the Durango 800[19] series this was expanded to a double-sided option for 960 KB (946 KB formatted[19][nb 1]) per diskette.[17][20][18][10]

Apple[edit]

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.

Value Code[21]
hex bin bin hex
0x00 0000.0000 1010.1010 1010.1010 0xAA 0xAA
0x01 0000.0001 1010.1010 1010.1011 0xAA 0xAB
0x02 0000.0010 1010.1011 1010.1010 0xAB 0xAA
0x03 0000.0011 1010.1011 1010.1011 0xAB 0xAB
0x04 0000.0100 1010.1010 1010.1110 0xAA 0xAE
0x05 0000.0101 1010.1010 1010.1111 0xAA 0xAF
0x06 0000.0110 1010.1011 1010.1110 0xAB 0xAE
0x07 0000.0111 1010.1011 1010.1111 0xAB 0xAF
0x08 0000.1000 1010.1110 1010.1010 0xAE 0xAA
0x09 0000.1001 1010.1110 1010.1011 0xAE 0xAB
0x0A 0000.1010 1010.1111 1010.1010 0xAF 0xAA
0x0B 0000.1011 1010.1111 1010.1011 0xAF 0xAB
0x0C 0000.1100 1010.1110 1010.1110 0xAE 0xAE
0x0D 0000.1101 1010.1110 1010.1111 0xAE 0xAF
0x0E 0000.1110 1010.1111 1010.1110 0xAF 0xAE
0x0F 0000.1111 1010.1111 1010.1111 0xAF 0xAF
0x10 0001.0000 1010.1010 1011.1010 0xAA 0xBA
0x11 0001.0001 1010.1010 1011.1011 0xAA 0xBB
0x12 0001.0010 1010.1011 1011.1010 0xAB 0xBA
0x13 0001.0011 1010.1011 1011.1011 0xAB 0xBB
0x14 0001.0100 1010.1010 1011.1110 0xAA 0xBE
0x15 0001.0101 1010.1010 1011.1111 0xAA 0xBF
0x16 0001.0110 1010.1011 1011.1110 0xAB 0xBE
0x17 0001.0111 1010.1011 1011.1111 0xAB 0xBF
0x18 0001.1000 1010.1110 1011.1010 0xAE 0xBA
0x19 0001.1001 1010.1110 1011.1011 0xAE 0xBB
0x1A 0001.1010 1010.1111 1011.1010 0xAF 0xBA
0x1B 0001.1011 1010.1111 1011.1011 0xAF 0xBB
0x1C 0001.1100 1010.1110 1011.1110 0xAE 0xBE
0x1D 0001.1101 1010.1110 1011.1111 0xAE 0xBF
0x1E 0001.1110 1010.1111 1011.1110 0xAF 0xBE
0x1F 0001.1111 1010.1111 1011.1111 0xAF 0xBF
0x20 0010.0000 1011.1010 1010.1010 0xBA 0xAA
0x21 0010.0001 1011.1010 1010.1011 0xBA 0xAB
0x22 0010.0010 1011.1011 1010.1010 0xBB 0xAA
0x23 0010.0011 1011.1011 1010.1011 0xBB 0xAB
0x24 0010.0100 1011.1010 1010.1110 0xBA 0xAE
0x25 0010.0101 1011.1010 1010.1111 0xBA 0xAF
0x26 0010.0110 1011.1011 1010.1110 0xBB 0xAE
0x27 0010.0111 1011.1011 1010.1111 0xBB 0xAF
0x28 0010.1000 1011.1110 1010.1010 0xBE 0xAA
0x29 0010.1001 1011.1110 1010.1011 0xBE 0xAB
0x2A 0010.1010 1011.1111 1010.1010 0xBF 0xAA
0x2B 0010.1011 1011.1111 1010.1011 0xBF 0xAB
0x2C 0010.1100 1011.1110 1010.1110 0xBE 0xAE
0x2D 0010.1101 1011.1110 1010.1111 0xBE 0xAF
0x2E 0010.1110 1011.1111 1010.1110 0xBF 0xAE
0x2F 0010.1111 1011.1111 1010.1111 0xBF 0xAF
0x30 0011.0000 1011.1010 1011.1010 0xBA 0xBA
0x31 0011.0001 1011.1010 1011.1011 0xBA 0xBB
0x32 0011.0010 1011.1011 1011.1010 0xBB 0xBA
0x33 0011.0011 1011.1011 1011.1011 0xBB 0xBB
0x34 0011.0100 1011.1010 1011.1110 0xBA 0xBE
0x35 0011.0101 1011.1010 1011.1111 0xBA 0xBF
0x36 0011.0110 1011.1011 1011.1110 0xBB 0xBE
0x37 0011.0111 1011.1011 1011.1111 0xBB 0xBF
0x38 0011.1000 1011.1110 1011.1010 0xBE 0xBA
0x39 0011.1001 1011.1110 1011.1011 0xBE 0xBB
0x3A 0011.1010 1011.1111 1011.1010 0xBF 0xBA
0x3B 0011.1011 1011.1111 1011.1011 0xBF 0xBB
0x3C 0011.1100 1011.1110 1011.1110 0xBE 0xBE
0x3D 0011.1101 1011.1110 1011.1111 0xBE 0xBF
0x3E 0011.1110 1011.1111 1011.1110 0xBF 0xBE
0x3F 0011.1111 1011.1111 1011.1111 0xBF 0xBF
Value Code[21]
hex bin bin hex
0x40 0100.0000 1010.1010 1110.1010 0xAA 0xEA
0x41 0100.0001 1010.1010 1110.1011 0xAA 0xEB
0x42 0100.0010 1010.1011 1110.1010 0xAB 0xEA
0x43 0100.0011 1010.1011 1110.1011 0xAB 0xEB
0x44 0100.0100 1010.1010 1110.1110 0xAA 0xEE
0x45 0100.0101 1010.1010 1110.1111 0xAA 0xEF
0x46 0100.0110 1010.1011 1110.1110 0xAB 0xEE
0x47 0100.0111 1010.1011 1110.1111 0xAB 0xEF
0x48 0100.1000 1010.1110 1110.1010 0xAE 0xEA
0x49 0100.1001 1010.1110 1110.1011 0xAE 0xEB
0x4A 0100.1010 1010.1111 1110.1010 0xAF 0xEA
0x4B 0100.1011 1010.1111 1110.1011 0xAF 0xEB
0x4C 0100.1100 1010.1110 1110.1110 0xAE 0xEE
0x4D 0100.1101 1010.1110 1110.1111 0xAE 0xEF
0x4E 0100.1110 1010.1111 1110.1110 0xAF 0xEE
0x4F 0100.1111 1010.1111 1110.1111 0xAF 0xEF
0x50 0101.0000 1010.1010 1111.1010 0xAA 0xFA
0x51 0101.0001 1010.1010 1111.1011 0xAA 0xFB
0x52 0101.0010 1010.1011 1111.1010 0xAB 0xFA
0x53 0101.0011 1010.1011 1111.1011 0xAB 0xFB
0x54 0101.0100 1010.1010 1111.1110 0xAA 0xFE
0x55 0101.0101 1010.1010 1111.1111 0xAA 0xFF
0x56 0101.0110 1010.1011 1111.1110 0xAB 0xFE
0x57 0101.0111 1010.1011 1111.1111 0xAB 0xFF
0x58 0101.1000 1010.1110 1111.1010 0xAE 0xFA
0x59 0101.1001 1010.1110 1111.1011 0xAE 0xFB
0x5A 0101.1010 1010.1111 1111.1010 0xAF 0xFA
0x5B 0101.1011 1010.1111 1111.1011 0xAF 0xFB
0x5C 0101.1100 1010.1110 1111.1110 0xAE 0xFE
0x5D 0101.1101 1010.1110 1111.1111 0xAE 0xFF
0x5E 0101.1110 1010.1111 1111.1110 0xAF 0xFE
0x5F 0101.1111 1010.1111 1111.1111 0xAF 0xFF
0x60 0110.0000 1011.1010 1110.1010 0xBA 0xEA
0x61 0110.0001 1011.1010 1110.1011 0xBA 0xEB
0x62 0110.0010 1011.1011 1110.1010 0xBB 0xEA
0x63 0110.0011 1011.1011 1110.1011 0xBB 0xEB
0x64 0110.0100 1011.1010 1110.1110 0xBA 0xEE
0x65 0110.0101 1011.1010 1110.1111 0xBA 0xEF
0x66 0110.0110 1011.1011 1110.1110 0xBB 0xEE
0x67 0110.0111 1011.1011 1110.1111 0xBB 0xEF
0x68 0110.1000 1011.1110 1110.1010 0xBE 0xEA
0x69 0110.1001 1011.1110 1110.1011 0xBE 0xEB
0x6A 0110.1010 1011.1111 1110.1010 0xBF 0xEA
0x6B 0110.1011 1011.1111 1110.1011 0xBF 0xEB
0x6C 0110.1100 1011.1110 1110.1110 0xBE 0xEE
0x6D 0110.1101 1011.1110 1110.1111 0xBE 0xEF
0x6E 0110.1110 1011.1111 1110.1110 0xBF 0xEE
0x6F 0110.1111 1011.1111 1110.1111 0xBF 0xEF
0x70 0111.0000 1011.1010 1111.1010 0xBA 0xFA
0x71 0111.0001 1011.1010 1111.1011 0xBA 0xFB
0x72 0111.0010 1011.1011 1111.1010 0xBB 0xFA
0x73 0111.0011 1011.1011 1111.1011 0xBB 0xFB
0x74 0111.0100 1011.1010 1111.1110 0xBA 0xFE
0x75 0111.0101 1011.1010 1111.1111 0xBA 0xFF
0x76 0111.0110 1011.1011 1111.1110 0xBB 0xFE
0x77 0111.0111 1011.1011 1111.1111 0xBB 0xFF
0x78 0111.1000 1011.1110 1111.1010 0xBE 0xFA
0x79 0111.1001 1011.1110 1111.1011 0xBE 0xFB
0x7A 0111.1010 1011.1111 1111.1010 0xBF 0xFA
0x7B 0111.1011 1011.1111 1111.1011 0xBF 0xFB
0x7C 0111.1100 1011.1110 1111.1110 0xBE 0xFE
0x7D 0111.1101 1011.1110 1111.1111 0xBE 0xFF
0x7E 0111.1110 1011.1111 1111.1110 0xBF 0xFE
0x7F 0111.1111 1011.1111 1111.1111 0xBF 0xFF
Value Code[21]
hex bin bin hex
0x80 1000.0000 1110.1010 1010.1010 0xEA 0xAA
0x81 1000.0001 1110.1010 1010.1011 0xEA 0xAB
0x82 1000.0010 1110.1011 1010.1010 0xEB 0xAA
0x83 1000.0011 1110.1011 1010.1011 0xEB 0xAB
0x84 1000.0100 1110.1010 1010.1110 0xEA 0xAE
0x85 1000.0101 1110.1010 1010.1111 0xEA 0xAF
0x86 1000.0110 1110.1011 1010.1110 0xEB 0xAE
0x87 1000.0111 1110.1011 1010.1111 0xEB 0xAF
0x88 1000.1000 1110.1110 1010.1010 0xEE 0xAA
0x89 1000.1001 1110.1110 1010.1011 0xEE 0xAB
0x8A 1000.1010 1110.1111 1010.1010 0xEF 0xAA
0x8B 1000.1011 1110.1111 1010.1011 0xEF 0xAB
0x8C 1000.1100 1110.1110 1010.1110 0xEE 0xAE
0x8D 1000.1101 1110.1110 1010.1111 0xEE 0xAF
0x8E 1000.1110 1110.1111 1010.1110 0xEF 0xAE
0x8F 1000.1111 1110.1111 1010.1111 0xEF 0xAF
0x90 1001.0000 1110.1010 1011.1010 0xEA 0xBA
0x91 1001.0001 1110.1010 1011.1011 0xEA 0xBB
0x92 1001.0010 1110.1011 1011.1010 0xEB 0xBA
0x93 1001.0011 1110.1011 1011.1011 0xEB 0xBB
0x94 1001.0100 1110.1010 1011.1110 0xEA 0xBE
0x95 1001.0101 1110.1010 1011.1111 0xEA 0xBF
0x96 1001.0110 1110.1011 1011.1110 0xEB 0xBE
0x97 1001.0111 1110.1011 1011.1111 0xEB 0xBF
0x98 1001.1000 1110.1110 1011.1010 0xEE 0xBA
0x99 1001.1001 1110.1110 1011.1011 0xEE 0xBB
0x9A 1001.1010 1110.1111 1011.1010 0xEF 0xBA
0x9B 1001.1011 1110.1111 1011.1011 0xEF 0xBB
0x9C 1001.1100 1110.1110 1011.1110 0xEE 0xBE
0x9D 1001.1101 1110.1110 1011.1111 0xEE 0xBF
0x9E 1001.1110 1110.1111 1011.1110 0xEF 0xBE
0x9F 1001.1111 1110.1111 1011.1111 0xEF 0xBF
0xA0 1010.0000 1111.1010 1010.1010 0xFA 0xAA
0xA1 1010.0001 1111.1010 1010.1011 0xFA 0xAB
0xA2 1010.0010 1111.1011 1010.1010 0xFB 0xAA
0xA3 1010.0011 1111.1011 1010.1011 0xFB 0xAB
0xA4 1010.0100 1111.1010 1010.1110 0xFA 0xAE
0xA5 1010.0101 1111.1010 1010.1111 0xFA 0xAF
0xA6 1010.0110 1111.1011 1010.1110 0xFB 0xAE
0xA7 1010.0111 1111.1011 1010.1111 0xFB 0xAF
0xA8 1010.1000 1111.1110 1010.1010 0xFE 0xAA
0xA9 1010.1001 1111.1110 1010.1011 0xFE 0xAB
0xAA 1010.1010 1111.1111 1010.1010 0xFF 0xAA
0xAB 1010.1011 1111.1111 1010.1011 0xFF 0xAB
0xAC 1010.1100 1111.1110 1010.1110 0xFE 0xAE
0xAD 1010.1101 1111.1110 1010.1111 0xFE 0xAF
0xAE 1010.1110 1111.1111 1010.1110 0xFF 0xAE
0xAF 1010.1111 1111.1111 1010.1111 0xFF 0xAF
0xB0 1011.0000 1111.1010 1011.1010 0xFA 0xBA
0xB1 1011.0001 1111.1010 1011.1011 0xFA 0xBB
0xB2 1011.0010 1111.1011 1011.1010 0xFB 0xBA
0xB3 1011.0011 1111.1011 1011.1011 0xFB 0xBB
0xB4 1011.0100 1111.1010 1011.1110 0xFA 0xBE
0xB5 1011.0101 1111.1010 1011.1111 0xFA 0xBF
0xB6 1011.0110 1111.1011 1011.1110 0xFB 0xBE
0xB7 1011.0111 1111.1011 1011.1111 0xFB 0xBF
0xB8 1011.1000 1111.1110 1011.1010 0xFE 0xBA
0xB9 1011.1001 1111.1110 1011.1011 0xFE 0xBB
0xBA 1011.1010 1111.1111 1011.1010 0xFF 0xBA
0xBB 1011.1011 1111.1111 1011.1011 0xFF 0xBB
0xBC 1011.1100 1111.1110 1011.1110 0xFE 0xBE
0xBD 1011.1101 1111.1110 1011.1111 0xFE 0xBF
0xBE 1011.1110 1111.1111 1011.1110 0xFF 0xBE
0xBF 1011.1111 1111.1111 1011.1111 0xFF 0xBF
Value Code[21]
hex bin bin hex
0xC0 1100.0000 1110.1010 1110.1010 0xEA 0xEA
0xC1 1100.0001 1110.1010 1110.1011 0xEA 0xEB
0xC2 1100.0010 1110.1011 1110.1010 0xEB 0xEA
0xC3 1100.0011 1110.1011 1110.1011 0xEB 0xEB
0xC4 1100.0100 1110.1010 1110.1110 0xEA 0xEE
0xC5 1100.0101 1110.1010 1110.1111 0xEA 0xEF
0xC6 1100.0110 1110.1011 1110.1110 0xEB 0xEE
0xC7 1100.0111 1110.1011 1110.1111 0xEB 0xEF
0xC8 1100.1000 1110.1110 1110.1010 0xEE 0xEA
0xC9 1100.1001 1110.1110 1110.1011 0xEE 0xEB
0xCA 1100.1010 1110.1111 1110.1010 0xEF 0xEA
0xCB 1100.1011 1110.1111 1110.1011 0xEF 0xEB
0xCC 1100.1100 1110.1110 1110.1110 0xEE 0xEE
0xCD 1100.1101 1110.1110 1110.1111 0xEE 0xEF
0xCE 1100.1110 1110.1111 1110.1110 0xEF 0xEE
0xCF 1100.1111 1110.1111 1110.1111 0xEF 0xEF
0xD0 1101.0000 1110.1010 1111.1010 0xEA 0xFA
0xD1 1101.0001 1110.1010 1111.1011 0xEA 0xFB
0xD2 1101.0010 1110.1011 1111.1010 0xEB 0xFA
0xD3 1101.0011 1110.1011 1111.1011 0xEB 0xFB
0xD4 1101.0100 1110.1010 1111.1110 0xEA 0xFE
0xD5 1101.0101 1110.1010 1111.1111 0xEA 0xFF
0xD6 1101.0110 1110.1011 1111.1110 0xEB 0xFE
0xD7 1101.0111 1110.1011 1111.1111 0xEB 0xFF
0xD8 1101.1000 1110.1110 1111.1010 0xEE 0xFA
0xD9 1101.1001 1110.1110 1111.1011 0xEE 0xFB
0xDA 1101.1010 1110.1111 1111.1010 0xEF 0xFA
0xDB 1101.1011 1110.1111 1111.1011 0xEF 0xFB
0xDC 1101.1100 1110.1110 1111.1110 0xEE 0xFE
0xDD 1101.1101 1110.1110 1111.1111 0xEE 0xFF
0xDE 1101.1110 1110.1111 1111.1110 0xEF 0xFE
0xDF 1101.1111 1110.1111 1111.1111 0xEF 0xFF
0xE0 1110.0000 1111.1010 1110.1010 0xFA 0xEA
0xE1 1110.0001 1111.1010 1110.1011 0xFA 0xEB
0xE2 1110.0010 1111.1011 1110.1010 0xFB 0xEA
0xE3 1110.0011 1111.1011 1110.1011 0xFB 0xEB
0xE4 1110.0100 1111.1010 1110.1110 0xFA 0xEE
0xE5 1110.0101 1111.1010 1110.1111 0xFA 0xEF
0xE6 1110.0110 1111.1011 1110.1110 0xFB 0xEE
0xE7 1110.0111 1111.1011 1110.1111 0xFB 0xEF
0xE8 1110.1000 1111.1110 1110.1010 0xFE 0xEA
0xE9 1110.1001 1111.1110 1110.1011 0xFE 0xEB
0xEA 1110.1010 1111.1111 1110.1010 0xFF 0xEA
0xEB 1110.1011 1111.1111 1110.1011 0xFF 0xEB
0xEC 1110.1100 1111.1110 1110.1110 0xFE 0xEE
0xED 1110.1101 1111.1110 1110.1111 0xFE 0xEF
0xEE 1110.1110 1111.1111 1110.1110 0xFF 0xEE
0xEF 1110.1111 1111.1111 1110.1111 0xFF 0xEF
0xF0 1111.0000 1111.1010 1111.1010 0xFA 0xFA
0xF1 1111.0001 1111.1010 1111.1011 0xFA 0xFB
0xF2 1111.0010 1111.1011 1111.1010 0xFB 0xFA
0xF3 1111.0011 1111.1011 1111.1011 0xFB 0xFB
0xF4 1111.0100 1111.1010 1111.1110 0xFA 0xFE
0xF5 1111.0101 1111.1010 1111.1111 0xFA 0xFF
0xF6 1111.0110 1111.1011 1111.1110 0xFB 0xFE
0xF7 1111.0111 1111.1011 1111.1111 0xFB 0xFF
0xF8 1111.1000 1111.1110 1111.1010 0xFE 0xFA
0xF9 1111.1001 1111.1110 1111.1011 0xFE 0xFB
0xFA 1111.1010 1111.1111 1111.1010 0xFF 0xFA
0xFB 1111.1011 1111.1111 1111.1011 0xFF 0xFB
0xFC 1111.1100 1111.1110 1111.1110 0xFE 0xFE
0xFD 1111.1101 1111.1110 1111.1111 0xFE 0xFF
0xFE 1111.1110 1111.1111 1111.1110 0xFF 0xFE
0xFF 1111.1111 1111.1111 1111.1111 0xFF 0xFF

Close to a month prior to the shipment of the disk drive in spring 1978,[22] 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):[23]

5-bit value GCR code[23][24]
hex bin bin hex
0x00 0.0000 1010.1011 0xAB
0x01 0.0001 1010.1101 0xAD
0x02 0.0010 1010.1110 0xAE
0x03 0.0011 1010.1111 0xAF
0x04 0.0100 1011.0101 0xB5
0x05 0.0101 1011.0110 0xB6
0x06 0.0110 1011.0111 0xB7
0x07 0.0111 1011.1010 0xBA
0x08 0.1000 1011.1011 0xBB
0x09 0.1001 1011.1101 0xBD
0x0A 0.1010 1011.1110 0xBE
0x0B 0.1011 1011.1111 0xBF
0x0C 0.1100 1101.0110 0xD6
0x0D 0.1101 1101.0111 0xD7
0x0E 0.1110 1101.1010 0xDA
0x0F 0.1111 1101.1011 0xDB
5-bit value GCR code[23][24]
hex bin bin hex
0x10 1.0000 1101.1101 0xDD
0x11 1.0001 1101.1110 0xDE
0x12 1.0010 1101.1111 0xDF
0x13 1.0011 1110.1010 0xEA
0x14 1.0100 1110.1011 0xEB
0x15 1.0101 1110.1101 0xED
0x16 1.0110 1110.1110 0xEE
0x17 1.0111 1110.1111 0xEF
0x18 1.1000 1111.0101 0xF5
0x19 1.1001 1111.0110 0xF6
0x1A 1.1010 1111.0111 0xF7
0x1B 1.1011 1111.1010 0xFA
0x1C 1.1100 1111.1011 0xFB
0x1D 1.1101 1111.1101 0xFD
0x1E 1.1110 1111.1110 0xFE
0x1F 1.1111 1111.1111 0xFF

Reserved GCR-codes: 0xAA and 0xD5.[23]

Wozniak called the system "my most incredible experience at Apple and the finest job I did".[22]

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,[23] and was used on Apple Pascal, Apple DOS 3.3[23] and ProDOS,[25] and later with Apple FileWare drives in the Apple Lisa and the 400K and 800K 3½-inch disks on the Macintosh and Apple II.[26][27] Apple did not originally call this scheme "GCR", but the term was later applied to it[27] to distinguish it from IBM PC floppies which used the MFM encoding scheme.

6-bit value GCR code[26][23][25][24][21]
hex bin bin hex
0x00 00.0000 1001.0110 0x96
0x01 00.0001 1001.0111 0x97
0x02 00.0010 1001.1010 0x9A
0x03 00.0011 1001.1011 0x9B
0x04 00.0100 1001.1101 0x9D
0x05 00.0101 1001.1110 0x9E
0x06 00.0110 1001.1111 0x9F
0x07 00.0111 1010.0110 0xA6
0x08 00.1000 1010.0111 0xA7
0x09 00.1001 1010.1011 0xAB
0x0A 00.1010 1010.1100 0xAC
0x0B 00.1011 1010.1101 0xAD
0x0C 00.1100 1010.1110 0xAE
0x0D 00.1101 1010.1111 0xAF
0x0E 00.1110 1011.0010 0xB2
0x0F 00.1111 1011.0011 0xB3
6-bit value GCR code[26][23][25][24][21]
hex bin bin hex
0x10 01.0000 1011.0100 0xB4
0x11 01.0001 1011.0101 0xB5
0x12 01.0010 1011.0110 0xB6
0x13 01.0011 1011.0111 0xB7
0x14 01.0100 1011.1001 0xB9
0x15 01.0101 1011.1010 0xBA
0x16 01.0110 1011.1011 0xBB
0x17 01.0111 1011.1100 0xBC
0x18 01.1000 1011.1101 0xBD
0x19 01.1001 1011.1110 0xBE
0x1A 01.1010 1011.1111 0xBF
0x1B 01.1011 1100.1011 0xCB
0x1C 01.1100 1100.1101 0xCD
0x1D 01.1101 1100.1110 0xCE
0x1E 01.1110 1100.1111 0xCF
0x1F 01.1111 1101.0011 0xD3
6-bit value GCR code[26][23][25][24][21]
hex bin bin hex
0x20 10.0000 1101.0110 0xD6
0x21 10.0001 1101.0111 0xD7
0x22 10.0010 1101.1001 0xD9
0x23 10.0011 1101.1010 0xDA
0x24 10.0100 1101.1011 0xDB
0x25 10.0101 1101.1100 0xDC
0x26 10.0110 1101.1101 0xDD
0x27 10.0111 1101.1110 0xDE
0x28 10.1000 1101.1111 0xDF
0x29 10.1001 1110.0101 0xE5
0x2A 10.1010 1110.0110 0xE6
0x2B 10.1011 1110.0111 0xE7
0x2C 10.1100 1110.1001 0xE9
0x2D 10.1101 1110.1010 0xEA
0x2E 10.1110 1110.1011 0xEB
0x2F 10.1111 1110.1100 0xEC
6-bit value GCR code[26][23][25][24][21]
hex bin bin hex
0x30 11.0000 1110.1101 0xED
0x31 11.0001 1110.1110 0xEE
0x32 11.0010 1110.1111 0xEF
0x33 11.0011 1111.0010 0xF2
0x34 11.0100 1111.0011 0xF3
0x35 11.0101 1111.0100 0xF4
0x36 11.0110 1111.0101 0xF5
0x37 11.0111 1111.0110 0xF6
0x38 11.1000 1111.0111 0xF7
0x39 11.1001 1111.1001 0xF9
0x3A 11.1010 1111.1010 0xFA
0x3B 11.1011 1111.1011 0xFB
0x3C 11.1100 1111.1100 0xFC
0x3D 11.1101 1111.1101 0xFD
0x3E 11.1110 1111.1110 0xFE
0x3F 11.1111 1111.1111 0xFF

Reserved GCR-codes: 0xAA and 0xD5.[23][25]

Commodore[edit]

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, and no more than eight one bits 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, according to the following table:

4-bit value GCR code[28]
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[28]
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

Inhere, no code starts with two zero bits, nor ends with two zero bits. This ensures that regardless of the input data, the encoded data will never contain more than two zero bits in a row. With this encoding not more than eight one bits in a row are possible. Therefore, Commodore used sequences of ten or more one bits in a row as synchronization mark.

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 kB on a standard single-sided single-density 5.25-inch floppy, where Apple fit 140 kB (with 6-and-2 encoding) or 114 kB (with 5-and-3 encoding) and an FM-encoded floppy held only 88 kB.

Sirius/Victor[edit]

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.[29][30][31]

Brother[edit]

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.[13][32][33][34] Reportedly, prototypes were already shown at the Internationale Funkausstellung 1979 (IFA) in Berlin.

Sharp[edit]

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 kbits/s, 270 rpm) with GCR (4/5) recording.[35][36]

Other uses[edit]

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[edit]

Notes[edit]

  1. ^ The product flyer for the Durango 800 series documents a formatted "on-line capacity" of 1.892 MB for the diskette drives. The system, however, was equipped with two 5¼-inch Micropolis 100 tpi 77-track floppy drives by default, and 1.892 MB is about twice as large as the physical drive capacity documented in various other sources (480 KB per side), therefore, by "on-line capacity" they must have meant the available storage capacity available to users for the combination of two drives.
  2. ^ The sources give slightly contradicting parameters regarding the Brother diskette formats. 12 sectors á 256 bytes would give 120 KB per side on a 40-track drive, but one source claims the drives were 38-track only.
  3. ^ a b The following Brother models are known to support a 120 KB diskette format (incomplete list): WP-1 (1985/1987), WP-5 (1987/1989), WP-6 (1989), WP-55 (1987/1989), WP-500 (1987/1989). The following models are known to support a 240 KB format (incomplete list): WP-70, WP-75 (1989), WP-80 (1985/1989), WP-3400, WP-3410, WP-3550, WP-3650D, WP-760D, WP-760D+, LW-1 (1989), LW-20, LW-30, LW-100, LW-400.

References[edit]

  1. ^ a b c d e f g h Patel, Arvind Motibhai (1988). "5. Signal and Error-Control Coding". In Mee, C. Denis; Daniel, Eric D. Magnetic Recording. II: Computer Data Storage (1st ed.). McGraw-Hill Book Company. ISBN 0-07-041272-3. 
  2. ^ a b c CW staff (1973-03-14). "6,250 Byte/In. Density - IBM 3420 Storage More Than Tripled". Computerworld. White Plains, New York, USA. VII (11): 1–2. Archived from the original on 2017-03-23. Retrieved 2017-03-23. IBM added three new models to the 3420 magnetic tape system than can record data at the "densest recording capability yet offered", according to the company. Using a new method called Group Coded Recording (GCR), the IBM drives can handle tapes containing a data density of 6,250 byte/in. compared with 1,600 byte/in. on earlier models of the 3420. […] An upgraded control unit was also announced - the 3803 Model 2 - which operates with both the earlier and latest 3420 tape units. The Model 2 includes the capability of correcting errors in one or two tracks "simultaneously while the tape is in motion", IBM said. […] The GCR method segments data written on tape into groups of characters to which a special coding character is added. And the higher density is based on a combination of a modified coding scheme, a smaller interrecord gap (called an interblock gap) and modified electronics and electromechanical components, IBM said. Installed 3803/3420 tape systems can be converted to the higher densities in the field. […] 
  3. ^ "The Gallery of Old Iron". 2004. Archived from the original on 2008-12-25. […] I moved to the lab at Poughkeepsie in 1958 […] I later was Lead designer and architect for the 2802 Tape Control Unit and a few years after that, Lead Designer and Architect of the 3803 which was a very large modification based on the 2802. Three of us shared a Corporate Award for the 3803 and I, along with Planner Charlie Von Reyn, came up with the name "Group Coded Recording (GCR)" as the name of the recording method. […]  (NB. An anonymous comment by one of the developers on the origin of the name "Group Coded Recording".)
  4. ^ Harris, John P.; Phillips, William B.; Wells, Jack F.; Winger, Wayne D. (September 1981). "Innovations in the Design of Magnetic Tape Subsystems". IBM Journal of Research and Development. International Business Machines Corporation. 25 (5): 691–700. Archived from the original on 2017-05-07. Retrieved 2017-03-23. 
  5. ^ a b c Keong, Kwoh Chee, Computer Peripherals (PDF), School of Computer Engineering, Nanyang Technological University, Singapore, Chapter 7. Magnetic Recording Fundamentals, archived (PDF) from the original on 2017-03-23, retrieved 2017-03-23 
  6. ^ a b c Watkinson, John (1990). "3.4. Group codes". Coding for Digital Recording. Stoneham, MA, USA: Focal Press. pp. 51–61. ISBN 0-240-51293-6. ISBN 978-0-240-51293-8. 
  7. ^ Patel, Arvind Motibhai; Hong, Se June (1974). "Optimal Rectangular Code for High Density Magnetic Tapes". IBM Journal of Research and Development. 18 (6): 579–588. 
  8. ^ Jacoby, George V. (2003-01-06) [September 1977]. "A new look-ahead code for increased data density". IEEE Transactions on Magnetics. Sperry Univac, ISS Division, Cupertino, CA, USA: IEEE. 13 (5): 1202–1204. doi:10.1109/TMAG.1977.1059670.  (NB. This article about the 3PM code was also presented at the Intermag 1977 in June 1977.)
  9. ^ US 4261019, McClelland, S. Barry, "Compatible Digital Magnetic Recording System", published 1981-04-07, assigned to Micropolis Corporation  [1] (NB. Application Number: US 06/098381)
  10. ^ a b "NCC Preview: OEMs at NCC - Micropolis Corp.". Computerworld. CW Communications, Inc. XII (22): P/50. 1978-05-28. Retrieved 2017-06-12. […] Micropolis has extended the capacity of 5.25-in. floppy disk subsystems via double-sided models with formatted file storage of up to nearly 2 million bytes […] The Megafloppy series also features an intelligent controller that facilitates interconnection of four subsystems to a common host interface for a total on-line storage capacity of more than 15M bytes […] Double-sided versions of the product line will be implemented first in two OEM series - Model 1015 and Model 1055 […] The Model 1015 is an unpackaged drive designed for the manufacturer who integrates floppy disk storage into his own system enclosure. A range of storage capacities from 143,000 to 630,000 bytes per drive is available […] Model 1015 customers have the option of using the Micropolis intelligent controller and Group Code Recording (GCR) method to further expand file space up to 946,000 bytes […] Offering GCR and a microprocessor-based controller as standard features, the Model 1055 5.25-in. floppy has four soft-sectored formats for each of its 77 tracks, yielding a maximum capacity of 1,892,000 bytes of file space on its double-sided version […] An add-on module available for the 1055 is comprised of two read/write heads and two drives, sharing a common controller. The subsystem capacity (formatted) with the module is 3,784,000 bytes […] Up to four 1055s, each with an add-on module, can be daisy-chained[disambiguation needed] to a common host for a maximum on-line storage capacity of more than 15M bytes […] 
  11. ^ Micropolis Maintenance Manual Floppy Disk Subsystem (PDF) (revision 1, 1st ed.). Micropolis Corporation. February 1979. 1082-04. Archived (PDF) from the original on 2017-06-12. Retrieved 2017-06-12.  (NB. Micropolis 100163-51-8 and 100163-52-6 are GCR-based.)
  12. ^ "InfoNews/Hardware: Hardware/Briefs". InfoWorld. 2 (2): 19. 1980-03-03. Retrieved 2017-06-12. […] Four new 96 tracks-per-inch products have been added to Micropolis' current line of 100 tpi single-sided and double-sided floppy disks. The 96 tpi drives offer 70 tracks-per-side, as opposed to the 77 offered by the MegaFloppy line. The four models are: 1) The 1015-V: 436 KB, unformatted, FM/MFM recording […] 2) The 1016-V: 532 KB unformatted, Group Coded Recording (GCR) […] 3) The 1015-VI: a two-head version of the MFM drive, 872 KB […] 4) The 1016-VI: also a two-head drive, 1.064 MB GCR encoding […] 
  13. ^ a b c Guzis, Charles P. (2015-09-20). "Multi-platform distribution format". Sydex. Archived from the original on 2017-06-14. Retrieved 2017-06-14. […] At the same time Micropolis was working a 5.25" drive that could hold about as much as an 8", using some tricks. The Micropolis drive was 100 tpi, 77 track and, by using GCR, could hold 12 512-byte sectors per track. That's 462 KiB. This was about 1977-78. […] The […] drive and controller implementation (ours was done by a guy we'd recruited from Sperry ISS) was […] complex and expensive […] Brother WP disks […] are 38 track, single-sided, Brother-encoded GCR that hold […] 120K on 2D floppies. […] 
  14. ^ Allen, David (February 1978). "A Minifloppy Interface" (PDF). BYTE. Kansas City, USA. 3 (2): 114, 116–118, 120, 122, 134–125. Archived from the original (PDF) on 2017-06-14. Retrieved 2017-06-14. […] Of the alternative codes used to achieve double density, GCR (Group Coded Recording) looks quite attractive. Micro Peripherals Inc has implemented double density using GCR in a full size floppy disk and controller system currently being marketed. […] GCR is nothing more than the old standby NRZ with its attendant advantages, but, since ordinary NRZ has no clocking information and a potentially high DC content during long strings of ones or zeros, the data is reformatted to eliminate the long strings. The reformatting converts each four bit group of original data into five bits of group coded data; the five bits in the encoded version will always have a mix of ones and zeros, even if the real data is all in one state. Reformatting in GCR can be accomplished in software, as opposed to MFM, etc, which almost unavoidably must be encoded and decoded in hardware. Thus, GCR has good possibilities as a low cost, high reliability scheme for achieving double density. […]  [2]
  15. ^ "Floppies Claim Improved Performance". Computerworld. CW Communications, Inc. XIII (7): 90. 1979-02-12. Retrieved 2017-06-14. 
  16. ^ Schultz, Brad (1978-10-02). "Business Mini Weighs 65 Pound - What is Durango?". Computerworld. CW Communications, Inc. XII (40): 1, 4. Retrieved 2017-06-13. 
  17. ^ a b Comstock, George E. (2003-08-13). "Oral History of George Comstock" (PDF). Interview with Hendrie, Gardner. Mountain View, California, USA: Computer History Museum. CHM X2727.2004. Archived (PDF) from the original on 2017-03-23. Retrieved 2017-03-23. 
  18. ^ a b Guzis, Charles P. (2009-09-13). "Durango GCR". Sydex. Retrieved 2017-03-25. 
  19. ^ a b "800 Technical Summary - 800 Series Business Computer System" (PDF). San Jose, CA, USA: Durango Systems, Inc. Archived (PDF) from the original on 2017-03-23. Retrieved 2017-03-23. 
  20. ^ Guzis, Charles P. (October 2006). "The Durango F-85 Computer". Sydex. Archived from the original on 2017-03-23. Retrieved 2017-03-23. 
  21. ^ a b c d e f g h Copy II Plus Version 9 - ProDOS/DOS Utilities - Data Recovery, File Management, Protected Software Backup (PDF). 9.0. Central Point Software, Inc. 1989-10-31 [1982]. Retrieved 2017-03-21. 
  22. ^ a b Williams, Gregg; Moore, Rob (January 1985). "The Apple Story / Part 2: More History and the Apple III". BYTE (interview): 166. Archived from the original on 2012-02-12. Retrieved 2013-10-26.  [3] (NB. Interview with Steve Wozniak, where he describes creating the Apple version of GCR.)
  23. ^ a b c d e f g h i j k Worth, Don D.; Lechner, Pieter M. (May 1982) [1981]. Beneath Apple DOS (4th printing ed.). Reseda, CA, USA: Quality Software. Retrieved 2017-03-21.  [4][5][6]
  24. ^ a b c d e f Sather, James Fielding (1983). Understanding the Apple II - A Learning Guide and Hardware Manual for the Apple II Computer (1st ed.). Chatsworth, CA, USA: Quality Software. pp. 9–26, 9–27. ISBN 0-912985-01-1. Archived from the original on 2016-05-27. Retrieved 2017-03-21. 
  25. ^ a b c d e f Worth, Don D.; Lechner, Pieter M. (March 1985) [1984]. Beneath Apple ProDOS - For Users of Apple II Plus, Apple IIe and Apple IIc Computers (PDF) (2nd printing ed.). Chatsworth, CA, USA: Quality Software. ISBN 0-912985-05-4. LCCN 84-61383. Archived (PDF) from the original on 2017-03-21. Retrieved 2017-03-21.  [7]
  26. ^ a b c d e Feichtinger, Herwig (1987). Arbeitsbuch Mikrocomputer (in German) (2 ed.). Munich, Germany: Franzis-Verlag GmbH. pp. 223–224. ISBN 3-7723-8022-0. 
  27. ^ a b Apple Computer, Inc. (February 1982) [1978]. Integrated Woz Machine (IWM) Specification (PDF) (19 ed.). DigiBam Computer Museum. Archived (PDF) from the original on 2016-08-06. Retrieved 2016-08-06. 
  28. ^ a b Hildon, Karl J. H. (March 1985). "GCR codes". The Complete Commodore Inner Space Anthology (PDF). Milton, Ontario, Canada: Transactor Publishing Incorporated. p. 49. ISBN 0-9692086-0-X. Archived (PDF) from the original on 2017-03-23. Retrieved 2017-03-23.  [8] (NB. Commodore GCR codes—but this reference erroneously claims that a 1-bit indicates a lack of a transition.)
  29. ^ "Victor 9000/Sirius 1 Specification" (PDF). commodore.ca. Archived (PDF) from the original on 2017-03-23. Retrieved 2017-03-23. 
  30. ^ "Supplemental Technical Reference Material". Revision 0 (1st printing ed.). Scotts Valley, CA, USA: Victor Publications. 1983-03-23. Application Note: 002. […] Single-sided floppy drive offers 80 tracks at 96 TPI […] Double-sided floppy drive offers 160 tracks at 96 TPI […] Floppy drives have 512 byte sectors; utilising a GCR, 10-bit recording technique. […] Although the Victor 9000 uses 5 1/4-inch minifloppies of a similar type to those used in other computers, the floppy disks themselves are not readable on other machines, nor can the Victor 9000 read a disk from another manufacturers machine. The Victor 9000 uses a unique recording method to allow the data to be packed as densely as 600 kbytes on a single-sided single-density minifloppy; this recording method involves the regulation of the speed at which the floppy rotates, explaining the fact that the noise from the drive sometimes changes frequency. 
  31. ^ "Chapter 7. Disk Drive Assembly". Victor 9000 Technical Reference Manual (PDF). Victor Business Products, Inc. June 1982. pp. 7–1..7–9. 710620. Archived (PDF) from the original on 2017-03-23. Retrieved 2017-03-23. […] Track density is 96 tracks per inch, and recording density is maintained at approximately 8000 bits per inch on all tracks. […] The VICTOR 9000 uses an encoding technique called group code recording (GCR) to convert the data from internal represenation to an acceptable form. GCR converts each (4-bit) nibble into a 5-bit code that guarantees a recording pattern that never has more than two zeros together. Then data is recorded on the disk by causing a flux reversal for each "one" bit and no flux reversal for each "zero" bit. […] 
  32. ^ Gieseke, Hans-Werner (2003-08-27). "Brother WP-1" (in German). Archived from the original on 2017-06-14. Retrieved 2017-06-14.  (NB. Reportedly, the Brother WP-1 technical data was derived from page 109 of the user manual.)
  33. ^ French, Mick (2002-09-13). "Brother WP-6". Archived from the original on 2017-06-14. Retrieved 2017-06-14. […] The 3.5" 240Kb disk drive is a single head Brother part no.13194989 and is connected with a 15 pin ribbon. […] it initializes (formats) the disk to a capacity of 236.5Kb. […] 
  34. ^ Cotgrove, Michael S. (2009-02-26). "archaic floppy disc format". Retrieved 2017-06-14. […] There were several 3.5" Brother disks that are completely nonstandard. […] One had 1296 byte sectors and another had 12 x 256 byte GCR sectors […] 
  35. ^ "Model CE-1600F". Sharp PC-1600 Service Manual (PDF). Yamatokoriyama, Japan: Sharp Corporation, Information Systems Group, Quality & Reliability Control Center. July 1986. pp. 98–104. Archived (PDF) from the original on 2017-05-07. Retrieved 2017-03-23. GCR is an abbreviation of Group Coded Recording. A single byte, 8 bits, data are divided into two 4-bit data which is also converted onto a 5-bit data. Thus, a single byte (8 bits) is recorded on the media as a 10-bit data. 
  36. ^ Sharp Service Manual Model CE-140F Pocket Disk Drive (PDF). Sharp Corporation. 00ZCE140F/SME. Archived (PDF) from the original on 2017-03-11. Retrieved 2017-03-11. 
  37. ^ Moseley, Robin C. (April 1979). "Technical Forum: A Comparison of Bar Code Encoding Schemes" (PDF). BYTE. Andover, MA, USA. 4 (4): 50, 52. Retrieved 2017-06-14. 

Further reading[edit]

External links[edit]