The idea of a double density writable and rewritable CD media is not new. In the early 1990s a number of companies experimented with extending the White Book and Red Book standards to store large amounts of video. However, these technologies were quickly dismissed because of standards issues. When the DVD format was subsequently accepted it appeared that the prospect of high density CDs had disappeared.

Not so. With the battle between the competing DVD standards still unresolved by the start of the new millennium, a number of drive manufacturers revisited the idea of trying to extend the life of the CD. Sony – one of the co-inventors of the CD in the early 1980s – was one of these, and in mid-2000 the company announced the Purple Book standard, and its plans to adapt the ISO 9660 format to double the density of a standard CD, to 1.3GB. It’s important to note that the new format is designed to store data only; there are no plans for a 140+ minute Red Book audio CD standard.

The new standard achieves the increase in capacity by means of a few simple modifications to the conventional CD format. The physical track pitch was narrowed from 1.6 to 1.1 micron and the minimum pit length shortened from 0.833 to 0.623 micron. In addition, a parameter in the error-correction scheme (CIRC) has been changed – to produce a new type of error correction which Sony refers to as CIRC7 – and the address format (ATIP) has been expanded. The format will also include a copy control scheme to meet the increasing demands for secure content protection.

Not only are the resulting double density media are much like existing CD-R and CD-RW media, but the drives that read them have not had to be changed much either. They use the same laser wavelength, but the scanning velocity has been slowed from 1.2 – 1.4ms to about 0.9ms to read the higher density discs.

Within a year of announcing the new format the first Sony product to handle the 1.3MB media – referred to as DD-R and DD-RW – had reached the market. Boasting an 8MB buffer memory for secure writing at high speeds, the Spressa CRX200E-A1 drive is capable of 12x (1,800KBps) recording for DD-R/CD-R, 8x (1,200KBps) re-writing of DD-RW/CD-RW media and a 32x (4,800KBps) reading speed. However, the new format is not without compatibility issues of its own. Whilst DD-R/DD-RW drives will be able to read and write CD-R and CD-RW discs, existing CD-ROM and CD-RW drives won’t be able to play DD-R and DD-RW media.

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