The Purple Book is the informal name for Philips and Sony's specification document for the double-density compact disk (DDCD) format. By narrowing the track pitch (to 1.1 micron from 1.6 micron), and shortening the minimum pit length (to 0.623 micron from 0.833 micron), the Purple Book enables a CD to hold 1.3 gigabytes, roughly twice the capacity of a standard CD. Other Purple Book specifications include a new type of error correction (known as CIRC7), an adaptation of the ISO 9660 file format, and a scanning velocity of 0.9 meters per second.
The Purple Book-defined products are expected to be released in recordable and rewritable formats, rather than read-only. Because of the specific requirements for reading DDCD, they cannot be read by other drives currently on the market. Sony has plans to manufacture DDCD-R/RW drives that can read any type of CD, record CD-R, rewrite CD-RW, and perform all three tasks on DDCD. Some upcoming CD-R and CD-RW drives, such as Adaptec's new version of Easy CD Creator, are being designed to be DDCD-compatible.