Section 5. Advanced Features and PlexTools Professional
76 PX-716 Installation and Users Manual
What the Measurements Mean
CDs are written via a non-contact optical technology, resulting in errors
on the written disc. Error correction is therefore an important component
of drive quality.
Audio written to a disc contains no major error correction. Missing
audio bytes are interpolated (replaced by an approximate value) in the
drive, and the reproduced output is generally very close to the original.
(Differences are usually detectable only by an audiophile.) If there are
large numbers of missing bytes due to poor media, scratches,
fingerprints, etc., these are reproduced as pops or crackles in the audio
output, but the media will still play.
Data (non-audio) content is much more sensitive to errors. The data
blocks on the recorded disc contain extensive embedded error correction
as part of the data stream. This error correction is derived from the
actual content of the original computer data, and is used to reconstruct
the original data, if needed, when the disc is read.
Every CD has two layers of error correction, called C1 and C2:
• C1 is error correction for the block error rate (BLER), which consists
of bit errors at the lowest level. BLER is normally given in errors per
second. The typical maximum BLER for quality recording is 220
errors per second.
• C2 error correction applies to bytes in a frame (24 bytes per frame,
98 frames per block) and is an indication of the drive’s attempt to
use extended error correction to recover the data. Even a few C2
errors can be an indication of poor media quality or a drive’s
inability to write or read correctly.
• CU error correction applies to uncorrectable errors, or errors that are
present after C2 level correction. No CU errors are allowed in a
recorded disc. CU errors are usually a result of damage to a disc and
represent unrecoverable data. Discs with CU errors quite often
cannot be read at all.
