Group Coded Recording

Group Coded Recording (recording group-coded, briefly: GCR) designates a recording method for magnetic data media such as magnetic tapes or disks. With this procedure a data byte with 10 is coded in such a way or more bits that never more than 2 zero-bits sequences. Thus takes placea constant synchronization of read electronics: With zero-bits this no signal keeps, so that with longer consequences by zero-bits due to the always existing easy fluctuations of the rotating speed the exact number of existing zero-bits could not be determined any longer surely.

The GCR procedure was e.g. used. inFloppy disk drives of Commodore and Apple, whereby there are various easily different variants of the GCR procedure. Therefore Commodore and Apple disks are not among themselves exchangeable. With Commodore a Nibble in five bits were coded in each case. From it it results that four bytes in five bytes must be coded.The table for the GCR coding looks as follows:

Nibble code
0000 01010
0001 01011
0010 10010
0011 10011
0100 01110
0101 01111
0110 10110
0111 10111
Nibble code
1000 01001
1001 11001
1010 11010
1011 11011
1100 01101
1101 11101
1110 11110
1111 11111

With this coding it is guaranteed that never more than two zero-bits arise one behind the other.

In the PC instead Modified frequency modulation (MFM) uses. Only with the help of special CONTROLLERs like the Catweasel CONTROLLER it is possible for PC floppy disk drives to read GCR noted Commodore - and Apple - disks. It is howeveralso the software “Disk2FDI” admits, which makes possible this by means of two of coupled floppy disk drives. The letter from GCR disks is however definitv in PC drive assemblies at normal PC-diskettecontrol-learns not possible.


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