CD-R’s have a pre-formed spiral track, with each sector address hard-coded into the media itself. The capacity of the most widely available CD format is expressed either as 74 minutes or 650MB. Each second of playing time occupies 75 CD sectors, meaning a complete CD has a capacity of 74x60x75 = 333,000 sectors. The actual storage capacity of those 333,000 sectors depends on what’s recorded on the CD, audio or data. This is because audio imposes less of an error correction overhead than data, the capacity of a sector being 2353 bytes for the former, compared with 2048 for the latter. Consequently, a 74-minute disc has a capacity of 783,216,000 bytes (746MB) for audio but only 681,984,000 bytes (650MB) for data.

In the late-1990s CD-R media began to emerge with more capacity than the 74-minute maximum allowed by the Red Book audio CD or Yellow Book CD-ROM standards. The additional capacity was achieved by reducing the track pitch and scanning velocity specification tolerances to a minimum. The margin of error between drive and media is reduced as a consequence which, in turn, leads to potential compatibility issues, especially with respect to older CD drives and players.

The first of these higher capacity formats had a play time of 80 minutes and 360,000 sectors instead of the usual 333,000. In terms of data capacity this meant 703MB compared with a standard 74-minute CD’s 650MB. Not long into the new millennium even higher capacity CD media appeared in the shape of 90-minute and 99-minute formats, boasting capacities of around 791MB and 870MB respectively. It’s interesting to note that since CD time stamps are encoded as a pair of binary-coded decimal digits, it won’t be possible to push the capacity of a CD beyond 99 minutes!

It was easy to see the attraction of these high-capacity CD formats:

  • the confusion surrounding rewritable DVD and its various competing formats, was severely hampering the technology’s adoption
  • CD-RW technology was by now ubiquitous, with even entry-level PCs being equipped with CD-RW drives
  • CD-R and CD-RW media was unbelievably cheap compared with its DVD counterparts
  • the huge popularity of the VCD and SVCD formats meant that CD capacity was never enough for many users in the far east and China.

The fashion for CD overburning is a further manifestation of consumers’ desire to extract as much as they possibly can from CD-RW technology.

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