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CD Storage capacity
Scanning velocity: 1.21.4 m/s (constant linear velocity) - Equivalent to about 500 rpm at the inside of the disc, or about 200 rpm at the outside edge.
Track pitch: 1.6 µm.
Disc diameter 120 mm.
Disc thickness: 1.2 mm.
Inner radius program area: 25 mm.
Outer radius program area: 58 mm.
The program area is 86.05 cm², so that the length of the recordable spiral is 86.05/1.6 = 5.38 km. With a scanning speed of 1.2 m/s, the playing time is 74 minutes, or around 650 MB of data on a CD-ROM. If the disc diameter were 115 mm, the maximum playing time would have been 68 minutes, i.e., six minutes less. A disc with data appearing slightly more densely is allowable. Using a linear velocity of 1.2 m/s and a track pitch of 1.5 micrometre leads to a playing time of 80 minutes, or a capacity of 700 MB. This is the limit for most conventional audio CDs today.
Another technique to increase the capacity of a disc is store data in the lead out groove that is normally used to indicate the end of a disk, and an extra minute or two of recording is often possible. However, these discs can cause problems in playback when the end of the disc is reached.
The 74-minute playing time of a CD, being more than that of most long-playing vinyl albums, was often used to the format's advantage during the early years when CDs and LPs vied for commerical sales. CDs would often be released with one or more bonus tracks, enticing consumers to buy the CD for the extra material. However, attempts to combine double LPs onto one CD occasionally resulted in an opposing situation in which the CD would actually offer fewer tracks than the LP equivalent.
CD capacities are always given in binary units. A "700 MB" (or "80 minute") CD has a nominal capacity of about 700 MiB. But DVD capacities are given in decimal units. A "4.7 GB" DVD has a nominal capacity of about 4.38 GiB.
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