CD-ROM drives, which today typically means a CD-RW drive that is a combo CD-ROM, CD-R and CD-RW drive, come in a variety of speeds. The original drive (1x) transferred data at 150KB per second. By doubling the spindle speed (RPMs) for 2x drives, the transfer rate increased to 300KB/sec and has been increasing ever since. See CD-ROM and DVD drives.
X Rating Data Transfer Rate
1x 150 Kilobytes/sec
2x 300 Kilobytes/sec
4x 600 Kilobytes/sec
8x 1.2 Megabytes/sec
10x 1.5 Megabytes/sec
12x 1.8 Megabytes/sec
20x 3.0 Megabytes/sec
36x 5.4 Megabytes/sec
40x 6.0 Megabytes/sec
48x 7.2 Megabytes/sec
50x 7.5 Megabytes/sec
52x 7.8 Megabytes/sec
What Do 40/12/48 or 40x/12x/48x Mean?
CD-RW drives have three speeds that are designated with three numbers or sometimes three "X" numbers; for example, 40/12/48 or 40x/12x/48x." The numbers are write/rewrite/read speeds.
First X / Second X / Third X
CD-R write / CD-RW write / CD-ROM read
This is the label from a Memorex combo CD and DVD drive in 2005. Each "X" number means the speed for that format is X times 150KB for CDs and X times 1.35MB for DVDs.
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