Block Error Rates:
A short explanation of BLER and what constitutes the final figure, or average BLER, may help in interpreting the numbers we obtained in our tests.

CDs use, at their base level of error correction, CIRC, or Cross Interleaved Reed Solomon Code. CIRC applies two levels of error correction, known as C1 and C2. The errors E11, E21, E31, E12, E22, and E32 derive from these two levels of error correction. CIRC employs two principles to detect and correct errors: redundancy and interleaving. CIRC uses about 25 percent data redundancy. The data is laid out in a way that will allow errors to be corrected, even though the data is not 100% redundant; it uses a parity-checking algorithm to reproduce data that is unreadable.

In addition, the data on a disc is interleaved. Interleaving means that the data is distributed over a relatively large physical-disc area. The data bytes are interleaved during recording and de-interleaved during playback. One data block (frame) of 24 data bytes is distributed over 109 blocks. To destroy one byte, you would have to destroy these other bytes. With scratches, dust, fingerprints, and even holes in the disc, there is usually enough data left to reconstruct any that has been damaged or rendered unreadable. The C1 level of CIRC is meant to correct small, random errors. The C2 level corrects larger errors and burst errors.

The E Errors, Level 1
E11, E21, and E31 errors occur at the first stage of error correction. An E11 error means that one bad symbol (byte) was corrected at the C1 stage. An E21 error means two bad bytes, and E31 signifies that three bad bytes were detected. E31 errors are uncorrectable at the first stage, and must be passed on to the second stage of correction. The first number is always the number of errors, and the second number is always the decoder level.

The E Errors, Level 2
In the second stage, C2, E12 means one bad symbol (byte) at decoder level C2; E22 means two bad symbols; and E32, three or more bad symbols. For CD-ROM, any E32 errors are unacceptable. First, an uncorrectable error in computer data can have a devastating effect. Second, the equipment used by mastering facilities will abort when an uncorrectable error on input media occurs, and the process must be restarted, which can result in additional costs and delays in your replication run. Some, but not all, CD-ROM drives use error correction and detection to such an extent that even an E32 error can be recovered, but there is no guarantee that even the best CD-ROM drive will be able to recover every E32 error. E32s on a pressed audio or ROM disc, or a CD-R, indicate that the user is dangerously close to losing stored data with the next scratch or fingerprint.

BLER
BLER stands for Block Error Rate. It indicates the number of data blocks that have any bad symbols (bytes) at the C1 error-correction stage. The reason that BLER is a good indicator of overall disc quality is that it essentially indicates the number of errors of all types, since errors that are uncorrectable at C1 get passed to C2. The Red Book specifies a maximum BLER of 220 per second, averaged over 10 seconds. Top-quality discs have an average BLER of below 10. A peak of 100 bad-data blocks per second is acceptable for CD-ROM, but an average BLER of 50 per second over the entire disc is a good cut-off point to ensure data integrity. CD-Audio discs can be acceptable with high BLER, but high BLER indicates that their longevity may be limited, and that there may be some problems in playing reliably on all types of readers.