Plextor University - Capacity of a Compact Disc

Capacity of a Compact Disc (CD)

In general, the term "capacity" refers to the capacity of a CD, in MegaBytes (MB) of data. The capacity of a CD can be determined by multiplying the number of "Bytes per sector", times 75 "sectors per second", times the "total recorded-time" on the disc. Furthermore, the actual total (in MegaBytes) will depend on the definition of MegaByte. Most vendors of computer hard drives, for example, use a decimal value (where 1,000,000 equals 1 MB), when describing the capacity of their drives, i.e., 9.1 GB (GigaBytes, representing 9,100,000,000 Bytes). However, data is stored in a binary format (as in 2^x). The same hard drive capacity will actually be displayed in the operating system as 8.7 GB when the binary MegaByte, 2²⁰, or 1,048,576, is used to determine the true capacity.

Optical discs require substantial overhead for their encoding, to work effectively and to provide the integrity required for computer data. For any CD, in percentages, the overhead includes: the Bytes used for the required eight-to-fourteen modulation (34%), the merging bits (17%), the error detection and correction codes (11%), and synchronization and subcodes (5%). This leaves about 33% net space for user data (a 74 minute CD will contain about 680 MegaBytes of usable data). Currently, CDs can hold 63, 74, or 80 minutes of data. Using 2²⁰, or 1,048,576, we can calculate 527 MegaBytes for a 60 minute CD. Obviously, this figure will be much higher for an 80-minute CD. Moreover, with multimedia CDs, all capacity figures have to take into consideration the format of the data. For instance, a Mode 2 data format allows more space for user data per sector (2336 user Bytes) than a Mode 1 format (2048 user Bytes). It is therefore possible to produce a 74-minute disc in Mode 2, with about 778 million Bytes, or 741 MegaBytes of user data--and still remain within the ISO 9660 specifications.

Another factor emerges when CD-RW or CD-R discs are formatted for writing with the UDF file structure used to "drag-and-drop" data. When using a UDF application software such as Roxio's DirectCD, formatting the disc changes the logical data structure from the CD-R "sector" format to the UDF "block" format. This can use 150 MegaBytes of additional space for overhead alone, reducing the capacity of the disc even further. The versatility gained by formatting the disc for use with "drag-and-drop" is partially offset by the reduced capacity of the disc. Users must take these variables into account when discussing CD capacities.

Many people feel that current media sizes (74 and 80-minute CDs) are not of sufficient capacity for them. Recently 90 and 99-minute CDs have appeared in the market, mostly from offshore producers. The news has made some users happy, since they believe they can now burn much more data onto the disc. However, most users have not been able to capitalize on this "new" media because many CD-RW drives are NOT able to use this 99-minute media. Why? The "disc length" that CD recorders read from the disc, is stored in a pre-made area of the disc known as ATIP ("absolute time in pre-groove", literally). In ATIP, according to the Orange Book standard, the maximum value of the ATIP is 79:59:74. This means that it is not possible for "normal" CD writers to know when a 90 or 99-minute disc is inserted. The "standard" calculation for converting between sector numbers and MSFs (minutes, seconds, and frames) causes problems with these "over-capacity" discs. In the original CD-ROM standard, it was defined that MSF periods of "90 minutes and above" were to be considered "negative". That is, 99:59:74 (sector -151) comes immediately BEFORE 00:00:00 (sector -150). This was done to give MSF addresses to sectors in the lead-in area, an area that exists BEFORE the data on a disc. However, this can cause problems now because it creates 2 different places on the disc with the same address.

90:00:00 can be either understood as a negative address, such as the reserved lead-in area described above, or 90 minutes into the data portion of the disc. When a drive sees 90:00:00, it won't know how to deal with it. How have vendors managed to fit 99-minutes into one CD and keep the original size? They did it in the same way as they did for the 80-minute discs---by moving the tracks of the Helix closer together and doing an overburn beyond the "stated" capacity of the disc, which boosts the full capacity of the media beyond the given specifications, adding an additional 2 to 4 minutes. The drive uses the pre-groove (the manufactured groove in a blank CD-R/RW disc) to create tracking signals to accurately position the laser to read and write. By increasing the pitch of the helix to create a 99-minute CD, this forces the laser to track a "tighter" spiral that is less tolerant of drift, and reduces the surface area between the helix onto which the laser 'burns' the data.

This "stretching the envelope" of CD-R technology actually causes the 99-minute discs to deviate from the accepted book standards, and fail in many CD-RW drives. Any minute physical variation in the disc itself, such as thickness variations of the polycarbonate or reflective alloy, or flutter caused by an eccentric (i.e, out-of-balance) disc, are magnified at the outer edge and may result in excessive jitter, or be beyond the capability of the CD-R mechanism to read with a minimum error rate.