Group code recording

Group Coded Recording ( group -coded recording, in short: GCR ) denotes a recording method for magnetic storage media such as magnetic tapes or diskettes. In this method, a data byte having ten or more bits is coded so that there is never more than two consecutive zero bits. One bits are represented by a change in polarity of the magnetic disk surface, zero bits by any change in polarity within a certain time period. The occurrence of one bits after two zero bits is a constant synchronization of the read electronics. This is necessary because at long sequences of zero bits (ie, more than two consecutive zeros) the exact number of existing zero bits could not be determined with certainty because of the ever-present slight variations in the rotational speed.

For use came the GCR method, for example, in 5 ¼ " floppy disk drive Commodore (see also CBM floppy drives ) and Apple, and there are several slightly different variations of the GCR method. Therefore Commodore and Apple disks are not mutually interchangeable.

Commodore GCR

At Commodore, there was the additional condition that no more than nine one bits may follow each other, since such a juxtaposition as a sector - start mark (sync) would be misinterpreted. In each case, a nibble ( four bits) are encoded in five bits. It follows that four bytes must be encoded in five bytes. The table for Commodore GCR coding looks like this (left the payload, to the right of it written on the disk code):

This coding is ensured that never occur consecutively for more than two zero bits or one bits longer than nine.

Apple GCR

Apple, there was no restriction of the number of one bits, which may follow each other, as there other sector start marks were used. There were other additional conditions: Each byte must start with a one bit, and in the remaining seven bits have at least a pair of consecutive one bits and no more than a pair of consecutive zero bits occur ( in an early code variant that was used with Apple only in the operating systems Apple DOS 3.1 and 3.2 from 1978 to 1980, no consecutive zero bits were allowed). Ultimately, Apple GCR were ( in the 1980 version, introduced by Apple DOS 3.3 and used on the Macintosh until well into the 1990s) six bits recoded into eight bits, so the code was a total of about six percent less efficient than Commodore GCR, but the hardware cost was also significantly lower. The 1978 variant encoded only five bits to eight bits and was again significantly less efficient. The code table for Apple GCR in the 1980 version looks like this:

Other systems

At IBM PCs instead Modified Frequency Modulation ( MFM) was used. The MFM encoding and decoding, is implemented in PCs by hardware, the floppy controller of the chipset and therefore can not be bypassed easily. With the help of special controllers like the Catweasel controller or KryoFlux it is also PC floppy drives possible to read GCR encoded Commodore and Apple disks, and also to write.

However, there are also software (eg " Disk2FDI "), which at least allows the reading of GCR floppy drives and other non- PC formats into a standard PC through sneaky means of two coupled disk drives, unless because the mainboard of the PC also two floppy disk drives are supported ( which is no longer the case with the vast majority of newer cards ). Writing GCR floppy is not possible according to current knowledge, for PC drives to ordinary PC floppy controllers with the most amazing tricks.

• Digital storage medium
• Memory technology
• Line code