CHAPTER 13

698 Chapter 13 Optical Storage
Handling Errors
Handling errors was a big part of the original Red Book CD standard. CDs use parity and interleaving
techniques called cross-interleave Reed-Solomon code (CIRC) to minimize the effects of errors on the disk.
This works at the frame level. When being stored, the 24 data bytes in each frame are first run
through a Reed-Solomon encoder to produce a 4-byte parity code called “Q” parity, which then is
added to the 24 data bytes. The resulting 28 bytes are then run though another encoder that uses a
different scheme to produce an additional 4-byte partity value called “P” parity. These are added to
the 28 bytes from the previous encoding, resulting in 32 bytes (24 of the original data plus the Q and
P parity bytes). An additional byte of subcode (tracking) information is then added, resulting in 33
bytes total for each frame. Note that the P and Q parity bytes are not related to the P and Q subcodes
mentioned earlier.
To minimize the effects of a scratch or physical defect that would damage adjacent frames, several
interleaves are added before the frames are actually written. Parts of 109 frames are cross-interleaved
(stored in different frames and sectors) using delay lines. This scrambling decreases the likelihood of a
scratch or defect affecting adjacent data because the data is actually written out of sequence.
With audio CDs and CD-ROMs, the CIRC scheme can correct errors up to 3,874 bits long (which
would be 2.6mm in track length). In addition, for audio CDs, only the CIRC can also conceal
(through interpolation) errors up to 13,282 bits long (8.9mm in track length). Interpolation is the
process in which the data is estimated or averaged to restore what is missing. That would of course be
unacceptable on a CD-ROM data disc, so this applies only to audio discs. The Red Book CD standard
defines the block error rate (BLER) as the number of frames (98 per sector) per second that have any
bad bits (averaged over 10 seconds) and requires that this be less than 220. This allows a maximum of
up to about 3% of the frames to have errors, and yet the disc will still be functional.
An additional layer of error detection and correction circuitry is the key difference between audio CD
players and CD-ROM drives. Audio CDs convert the digital information stored on the disc into analog
signals for a stereo amplifier to process. In this scheme, some imprecision is acceptable because it
would be virtually impossible to hear in the music. CD-ROMs, however, can’t tolerate any imprecision.
Each bit of data must be read accurately. For this reason, CD-ROM discs have a great deal of
additional ECC information written to the disc along with the actual stored information. The ECC
can detect and correct most minor errors, improving the reliability and precision to levels that are
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 will not be noticeable to the listener. If those same three values appear on a CD-ROM 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 CD-
ROM running an executable program, to guess 15 is not merely slightly off, it is completely wrong.
In a CD-ROM 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-ROM, sectors are read at a constant speed of 75 per second. This results in a
standard CD-ROM transfer rate of 2,048 × 75 = 153,600 bytes per second, which is expressed as either
153.6KB/sec or 150KiB/sec.