Optical Storage - Upgrading and Repairing PCs - page 531

Note
Current versions of Windows Media Player (WMP) do not natively support CD TEXT for
playback or during the creation of music CDs. However, a free plug-in, called WMPCDTex, is
available to add CD TEXT support to WMP. (The plug-in supports Windows Media Player
version 11 and above.) Other media players, such as Winamp ( www.winamp.com ), support
CD TEXT natively. Popular CD-burning programs with support for CD TEXT include Nero
( www.nero.com ) , Roxio Creator ( www.roxio.com ) , and the free ImgBurn program
( www.imgburn.com ) .
Handling Read Errors
Handling errors when reading a disc 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 parity 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 learn more about the concepts behind parity and error correction, which were originally used
to guard against errors in memory and modem communications, see Chapter 6 , “ Memory , ” p. 325 ,
and Chapter 16 , “ Internet Connectivity , ” p. 775 .
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 CDs, 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 data
CD, 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 data CD 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. Data CDs, however, can't tolerate
imprecision. Each bit of data must be read accurately. For this reason, data CDs 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