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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 2x). 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.
Most CD-RW drives that are capable of overburn
require a firmware upgrade to support that feature.
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