Group Code Recording
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Group Code Recording (GCR) is a floppy disk data encoding format used by the Apple II and Commodore Business Machines in the 5¼" disk drives for their 8-bit computers (the best-known drives being the Disk II for the Apple II family and the Commodore 1541, used with the Commodore 64 computer). It was also used on the Apple Macintosh.
The purpose of GCR is to avoid too many consecutive zeroes (i.e. absence of transitions), because the ones (transitions) synchronize the read clock. GCR permits consecutive ones.
GCR is more efficient than FM coding, but less efficient than MFM. Because MFM does not permit consecutive transitions, it runs at twice the clock rate of GCR.