A report on Group coded recording and Sirius Systems Technology

This, combined with group-coded recording (GCR), allowed standard floppy disks to hold more data than others at the time, 600 KB on single- and 1.2 MB on double-sided floppies compared with 140–160 KB per side of other machines such as the Apple II and early IBM PC, but disks made at constant bit density were not compatible with machines with standard drives.

- Sirius Systems Technology

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 (a form of zoned constant linear velocity (ZCLV)) while increasing the number of sectors per track (a variant of zone bit recording (ZBR)) to achieve formatted capacities of 606 kB (single sided) / 1,188 kB (double-sided) on 96 tpi media.

- Group coded recording

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Overall
Physical layout of sectors in a zone-bit disc: As distance from the centre increases, the number of sectors in a given angle increases from one (red) to two (green) to four (grey).

Zone bit recording

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Method used by disk drives to optimise the tracks for increased data capacity.

Method used by disk drives to optimise the tracks for increased data capacity.

Physical layout of sectors in a zone-bit disc: As distance from the centre increases, the number of sectors in a given angle increases from one (red) to two (green) to four (grey).

Commodore 1541 floppy disk (combined ZBR, ZCAV and GCR for 17–21 sectors á 256 bytes in 4 writing speed zones)

Sirius 1/Victor 9000 floppy disk (combined ZBR, ZCLV and GCR for 11–19 sectors á 512 bytes in 9 rotation speed zones)