A report on Zone bit recording, Sirius Systems Technology and Group coded recording

One striking difference between it and other machines on the market at the time was the fact that the disc utilized a form of zoned constant linear velocity (ZCLV) (using 9 different speed-zones selected out of 15 supported by the hardware) with a variant of zone bit recording (ZBR) (11 to 19 sectors depending on zone) to spun at different speeds according to where the data was stored, running slower towards the outer edge of the disc in such a way that bit density (bits per cm passing the head), rather than rotational speed, was approximately constant.

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.

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)

This more efficient GCR scheme, combined with an approach at constant bit-density recording by gradually increasing the clock rate (zone constant angular velocity, ZCAV) and storing more physical sectors on the outer tracks than on the inner ones (zone bit recording, ZBR), enabled Commodore to fit 170 kB on a standard single-sided single-density 5.25-inch floppy, where Apple fit 140 kB (with 6-and-2 encoding) or 114 kB (with 5-and-3 encoding) and an FM-encoded floppy held only 88 kB.

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.