Frequently Asked Questions About CD-R and CD-RW Discs

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I always buy high capacity, 74 minute CD-R discs. Why would anyone use discs with only 63 minute capacities?

More is not always better! Careful investigation shows that data written on 63 minute CD-R discs may be of significantly higher quality than data on 74 minute discs. Also, 63 minute discs may be compatible with more recorders. Distinctions between the two types are subtle. The recording area is not smaller on a 63 minute disc. Areas are actually the same on both types.

Major differences do exist in pregrooves and in the resulting bit spacings. Both 63 and 74 minute discs contain pregrooves that cover the full information area, from the inner buffer zone at a diameter of 45 mm to the outer buffer zone at 118 mm diameter. This spiral pregroove defines the exact radial location of the recording laser beam, resulting in data that is written exactly on-track. Normal CD-ROM mastering and molding processes create the pregroove before the CD-R dye, metal layer, and protective coating are applied. This eliminates the need for an expensive laser mounted on an ultra precise radial mechanism in the CD-R writer.

All pregrooves are "wobbled" by a very small radial distance that is only two percent of the spacing between tracks. The period, or peak-to-peak spacing, of this pregroove "wobble" is always at a precise frequency of 22.05 kHz at 1X, but is allowed to vary spatially between 54.4 and 63.5 micrometers in the direction of the track, depending on the intended disc capacity. It is the effects of different spatial periods that distinguish 74 and 63 minute CD-R discs.

High capacity, 74 minute discs use the 54.4 micron spacing. This causes the unrecorded CD-R disc to rotate in the writer at a constant linear velocity of exactly 1.2 meters per second at 1X in order to accurately lock to the 22.05 kHz frequency. Optical data marks can now be recorded as close together as 0.83 micrometers at this speed. Such data spacings are very close to the wavelength of the laser beam that writes and reads the disc. Minor quality problems during recording or reading can result in high error rates or even read failure. The write strategy of the recorder should always be carefully matched to the dye used in the CD-R disc in order to obtain a usable recorded disc, but the requirements are more demanding when 74 minute discs are written.

The larger 63.5 micrometer "wobble" spacing is used for 63 minute discs. Recording drives must then speed up to a constant linear velocity of 1.4 meters per second at 1X in order to precisely lock to the 22.05 kHz frequency. Since data is always written at a maximum 3T rate of 1.4406 mHz, optical marks are now written at a minimum spacing of 0.97 micrometers. This 17% increase in the minimum spacing results in higher quality recording, and also achieves greater tolerance to the recorder write strategy.

Generally accepted capacities are 650 MB for 74 minute discs and 550 MB for 63 minute discs. High quality recording should utilize only 550 MB or less of a 74 minute disc, because performance may rapidly degrade at large diameters. Consequently, usable capacities of 74 and 63 minute discs are nearly the same.

How often do you record 500 MB, 400 MB, or even 300 MB on your CD-R discs? Most finalized discs use much less than the full capacity of a 74 minute disc, or even that of a 63 minute disc. In such circumstances, recording on 74 minute discs invites problems while offering no advantages.

Use 63 minute CD-R discs for all but the highest capacity applications. Benefit from higher quality and the improved probability of successful interchange. When 74 minute capacities are required, carefully conduct quality tests to ensure the proper match between the write strategy of your recorder and the dye used in your CD-R discs. Make intelligent choices. Do not mistakenly assume that more is always better, especially concerning 80 minute and higher capacity discs that have values of track pitch and scanning velocity that do not conform to specifications.