CD ROM Capacity
In general, the term capacity refers to the capacity of a standard 12cm CD, in megabytes of user data. Currently, there are CD-ROM media that can hold 74 or 80 minutes of data (80 min. is the maximum designed capacity). Originally, the maximum was only 74 minutes (of 640 MB of data), which is approximately the length of Beethoven’s Ninth Symphony. The user data capacity of the CD-ROM is the number of user bytes per sector, times 75 sectors (user data) per second, times the total time recorded in the disc. Furthermore, the total, in Megabytes, will depend on the definition of Megabyte (2 ^20, or Million bytes). Therefore, for a 60 min. CD-ROM, we arrive to the figure of true 527 Megabytes. Obviously, this figure will be much higher for a 80-minute CD-ROM. Moreover, with multimedia CD-ROMs, capacities have the ability to record in “Mode 2”, which allows more space for user data (2336 bytes/sector). It is therefore possible to produce a 74- minute disc, in Mode 2, with about 741 true Megabytes of user data in it--within the ISO 9660 specifications. Using millions to mean megabytes (or billions to mean Gigabytes), the figures will obviously be slightly larger--and this practice has become more common. Users must take these variables into account when discussing CD capacities.
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