The CD-ROM industry is using a different measurement standard for rating data transfer speed. Instead of using bits or bytes per second, they are indicating the transfer speed by the quantity of X, which indicates the number of X transmitted in a given amount of time, in this case per second. In reality bits and bytes are still in use since 1 X is equivalent to 0.15 MB/sec or 1.2288 Mbps. The use of the X definition leaves out some confusion in comparison to the other terms.

Apart from the CD-ROM industry the X definition is used by the memory card industry for transfer speed rating, but it is not as widely used as the MB/sec which is the most common used definition. Some manufactures are indicating transfer speed only in MB/sec and some only in one of them or just in X. Since transfer speeds are getting really fast the Mbps definition is not that common used in the memory card industry, it makes more sense to use MB/sec.

Comparing different definitions with each other can be confusing. A comparable table over the different transfer speed definitions and more information about transfer speed can be found at data transfer speed and at buying guide.

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