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

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Why does my recording software indicate different capacities for CD-R discs from various manufacturers?

There are at least three reasons for different capacities of both 63 minute and 74 minute CD-R discs. These reasons result in 74 minute capacities that vary between 73:50 and 74:50 for discs from different suppliers. User data capacity of a nominal CD-R is 74:12, or about 652 MB of mode 1 user data. Capacities of 63 minute discs vary between 63:02 and 64:02, with a nominal value of 63:18, or about 556 MB of mode 1 user data.

The first reason for different capacities is that disc manufacturers have certain tolerances in disc design. Two of these tolerances are the exact positions of start of lead-in and the last possible start of lead-out. The exact values chosen by the manufacturer are encoded as ATIP in the lead-in section of the wobbled pre-groove.

Standards only require that lead-in ends at a diameter of 49.6 mm to 50.0 mm, and that lead-out ends at a maximum diameter of 117 mm. Different choices by disc manufacturers can each satisfy the Standards, resulting in slightly different capacities. Although track spacing is fixed at 1.6 micrometers in the Standards, the 0.1 micrometer track space tolerance allows design variations that also can result in different capacities.

Recording software is a second source of variation. Various programs may indicate different capacities for the same disc. User data capacity may be evaluated differently based upon assumptions about lead-in, post-gap, lead-out, and other elements of a recorded track. Lead-in usually occupies about 2.5 seconds but can vary by a few tenths of a second. Post-gap may be several seconds shorter or longer than the correct value of 2 seconds. Nominal 2 second lead-out regions may be a short as 1.5 seconds or can be much longer. Complex directory trees can consume many seconds of capacity, because sub-directories are recorded in the user data region together with data files. Large numbers of sub-directories will also lengthen the system area because root directory and path tables become larger.

Some software may report a smaller capacity for an unrecorded disc but will actually write larger amounts of information. Or the program may simply report a default capacity of 74 minutes, not the true capacity encoded by the disc manufacturer into the pre-groove of every disc. Certain software may report only available space for files, while other programs might include gaps and the system area in available capacity.

Conversion of recording time to bytes is the third source of variation. Sectors are always 2352 bytes long and repeat at a rate of 75 per second at 1X. User data contents of a sector can vary from 2048 bytes for Mode 1 and for XA Mode 2/Form 1 up to 2324 bytes for XA Mode 2/Form 2 and to 2352 bytes for CD-DA. This results in further capacity variations based upon format, even if maximum ATime does not vary. For example, the capacity of a 650 MB Mode 1 disc would be 737 MB for XA Mode 2/Form 2 and 746 MB for CD-DA.

Capacity variations are normal, and a slightly higher reported capacity does not mean that the disc is better. Discs having lower capacity may actually result in better recorded quality. Although some CD-R manufacturing methods can produce discs with uniformly high quality, other discs may have poor quality near the outer rim because of injection molding and spin-coating limitations. Heating effects during a long recording may also degrade quality at high values of ATime. It is always best to avoid recording computer files during the last four-to-eight minutes unless detailed test results for BLER, asymmetry, error rates, and other quality characteristics indicate that it is safe to record at maximum capacity. Only 35 MB to 70 MB of capacity will be unused, while quality may be significantly improved.

Another capacity risk is the appeal of 80 minute discs. Limited availability coupled with premium prices create a false impression of superiority. Avoid super-high capacity discs having capacities in excess of 74 minutes. These capacities can only be achieved by violating one or more requirements of the CD-R and CD-ROM Standards, such as track pitch. Such discs can be recorded in many drives, but not in all. More important, they will not be readable in all drives.