Hardware Reference
In-Depth Information
added,resultingin33bytestotalforeachframe.NotethatthePandQparitybytesarenot
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.
345
, and
Chapter 16
,
“
Internet Connectivity
,”
p.
780
.
Tominimizetheeffectsofascratchorphysicaldefectthatwoulddamageadjacentframes,
several interleaves are added before the frames are actually written. Parts of 109 frames
arecross-interleaved(storedindifferentframesandsectors)usingdelaylines.Thisscram-
bling 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
(throughinterpolation)errorsupto13,282bitslong(8.9mmintracklength).
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
betweenaudioCDplayersanddataCDdrives.AudioCDsconvertthedigitalinformation
stored on the disc into analog signals for a stereo amplifier to process. In this scheme,
someimprecisionisacceptablebecauseitwouldbevirtuallyimpossibletohearinthemu-
sic. Data CDs, however, can't tolerate imprecision. Each bit of data must be read accur-
ately. 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 acceptable for data
storage.
In the case of an audio CD, missing data can be interpolated—that is, the information fol-
lowsapredictable pattern that enables the drive toguessthe missing values. Forexample,
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
interpolateamiddlevalueof15,whichismidwaybetween10and20.Althoughthismight
notbeexactlycorrect,inthecaseofaudiorecording,itprobablywon'tbenoticeabletothe
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 be-
cause executable program instructions or data must be exact; otherwise, the program will
