74 minute CD-R discs. Careful investigation shows that data written on 63 minute CD-R discs may be of significantly higher quality than data on 74 minute discs. 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. This eliminates the need for an expensive laser mounted on an ultra precise radial mechanism in the CD-R writer. It is the effects of different spatial periods that distinguish 74 and 63 minute CD-R discs. 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.