Most optical drives today are combo CD/DVD drives that support all the DVD and CD formats. The speeds of the drives are rated by their DVD-ROM and CD-ROM transfer rates. The original DVD drive (1x) transferred data at 1.39MB per second, sufficient for movies. By doubling the spindle RPMs, the data transfer rate increased to 2.77MB/sec (2x) and so on.
Nine Times the CD-ROM Rating
Each DVD X is equivalent to roughly nine X's of CD speed because a DVD 1x is 1.39MB and a CD-ROM 1x is 150KB. See DVD player
and CD-ROM drives
DVD-ROM Transfer Rate
X Rating (MBytes/sec)
A Lot of X's
This is the label from a Memorex combo CD/DVD drive. Each "X" number means X times 1.39MB for DVDs and X times 150KB for CDs.