Hardware Reference
In-Depth Information
acceptable for data storage.
In the case of an audio CD, missing data can be interpolated—that is, the information follows a
predictable pattern that enables the drive to guess the missing values. For example, if three values are
stored on an audio disc (say, 10, 13, and 20 appearing in a series), and the middle value is missing—
because of damage or dirt on the CD's surface—you could interpolate a middle value of 15, which is
midway between 10 and 20. Although this might not be exactly correct, in the case of audio recording,
it probably won't be noticeable to the listener. If those same three values appear on a data CD in an
executable program, there is no way to guess at the correct value for the middle sample. Interpolation
can't work because executable program instructions or data must be exact; otherwise, the program
will crash or improperly read data needed for a calculation. Using the previous example with a data
CD running an executable program, guessing 15 is not merely slightly off—it is completely wrong.
In a CD on which data is stored instead of audio information, additional information is added to each
sector to detect and correct errors as well as to identify the location of data sectors more accurately.
To accomplish this, 304 bytes are taken from the 2,352 that originally were used for audio data and
are instead used for sync (synchronizing bits), ID (identification bits), ECC, and EDC information.
This leaves 2,048 bytes for actual user data in each sector. Just as when reading an audio CD, on a 1x
(standard speed) CD, sectors are read at a constant speed of 75 per second. This results in a standard
CD transfer rate of 153,600 bytes per second (2,048×75), which is expressed as either 153.6KBps or
150KiBps.
Note
Some of the copy-protection schemes used on audio CDs intentionally interfere with the audio
data and CIRC information in such a way as to make the disc appear to play correctly, but
copies of the audio files or of the entire disc will be filled with noise. Copy protection for both
audio and data CDs is discussed later in this chapter.
CD Capacity
Each second of a CD contains 75 blocks of data containing 2,048 bytes per block. From this
information, you can calculate the absolute maximum storage capacity of an 80-minute or 74-minute
CD, as shown in
Table 11.3
.
The table also shows the structure and layout of each sector on a CD on
which data is stored.
Table 11.3. CD Sector Information and Capacity
