Group Coded Recording

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(May 2016)

Group Coded Recording (group coded recording or group swap, abbreviated: GCR ) describes a recording process for magnetic data carriers such as magnetic tapes or floppy disks . With this method, a data byte with ten or more bits is encoded in such a way that never more than two zero bits follow one another. One bits are represented by a polarity change on the magnetic data carrier surface, zero bits by no polarity change within a certain period of time. The occurrence of one-bits after two zero-bits at the latest results in constant synchronization of the reading electronics. This is necessary because with longer sequences of zero bits (i.e., with more than two consecutive zeros), the exact number of zero bits present could no longer be reliably determined because of the slight fluctuations in the rotational speed that are always present.

The GCR process was used for B. in 5¼ "floppy disk drives from Commodore (see also CBM floppy disk drives ) and Apple , whereby there are various slightly different variants of the GCR process. Therefore, Commodore and Apple floppy disks are not interchangeable.

Commodore GCR

With Commodore there was the additional condition that no more than nine one-bits may follow one another, since such a sequence would be misinterpreted as a sector start mark (sync). One nibble (four bits) was encoded in five bits. This means that four bytes must be encoded in five bytes. The table for Commodore's GCR coding looks like this (on the left the user data, on the right the code written for it on the diskette):

Data code 0000 01010 0001 01011 0010 10010 0011 10011 0100 01110 0101 01111 0110 10110 0111 10111

Data code 1000 01001 1001 11001 1010 11010 1011 11011 1100 01101 1101 11101 1110 11110 1111 10101

This coding ensures that never more than two zero bits or more than nine one bits occur in succession.

Apple GCR

At Apple there was no restriction on the number of one-bits that can follow one another, since different sector start marks were used there. There were other additional conditions for this: Each byte must begin with a one bit, and the remaining seven bits must contain at least one pair of consecutive one bits and no more than one pair of consecutive zero bits (in an early code variant, the was only used at Apple in the operating systems Apple DOS 3.1 and 3.2 from 1978 to 1980, no consecutive zero bits were allowed). Ultimately, Apple-GCR (in the 1980s version, introduced with Apple DOS 3.3 and used on the Macintosh until the 1990s) each recoded six bits into eight bits, making the code around six percent less efficient than Commodore -GCR, but at the same time the hardware effort was significantly lower. The 1978 variant only re-encoded five bits into eight bits and was therefore significantly less efficient. The code table for Apple GCR in the 1980 variant looks like this:

Data code 000000 10010110 000001 10010111 000010 10011010 000011 10011011 000100 10011101 000101 10011110 000110 10011111 000111 10100110

Data code 001000 10100111 001001 10101011 001010 10101100 001011 10101101 001100 10101110 001101 10101111 001110 10110010 001111 10110011

Data code 010000 10110100 010001 10110101 010010 10110110 010011 10110111 010100 10111001 010101 10111010 010110 10111011 010111 10111100

Data code 011000 10111101 011001 10111110 011010 10111111 011011 11001011 011100 11001101 011101 11001110 011110 11001111 011111 11010011

Data code 100,000 11010110 100001 11010111 100010 11011001 100011 11011010 100100 11011011 100101 11011100 100110 11011101 100111 11011110

Data code 101000 11011111 101001 11100101 101010 11100110 101011 11100111 101100 11101001 101101 11101010 101110 11101011 101111 11101100

Data code 110000 11101101 110001 11101110 110010 11101111 110011 11110010 110100 11110011 110101 11110100 110110 11110101 110111 11110110

Data code 111000 11110111 111001 11111001 111010 11111010 111011 11111011 111100 11111100 111101 11111101 111110 11111110 111111 11111111

Other systems

In IBM PCs instead was Modified Frequency Modulation (MFM) used. The MFM coding and decoding is implemented in PCs by hardware in the floppy controller of the chipset and can therefore not be bypassed easily. With the help of special controllers such as the Catweasel controller or KryoFlux , PC floppy drives can also read and write GCR-coded Commodore and Apple floppy disks.

However, there is also software (e.g. "Disk2FDI") that at least enables reading of GCR disks and other non-PC formats in normal PCs through sophisticated tricks using two coupled disk drives, provided that the PC's mainboard also has two Floppy drives supported (which is no longer the case with the vast majority of newer boards). According to the current state of knowledge, writing GCR disks for PC drives on conventional PC disk controllers is not possible even with tricks.

Individual evidence

1. ↑ https://www.degruyter.com/view/j/itit.1976.18.issue-1-6/itit.1976.18.16.302/itit.1976.18.16.302.xml
2. ↑ http://www.online.uni-marburg.de/hrz/chronik/quellen/sperry-univac-magnetbandsystem-uniservo-30-1983.pdf