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on 12/11/06 9:02 AM, Christie Peterson at [log in to unmask] wrote:

CDs use two levels of error correction combined with data interleaving which
can correct both single bit and burst errors. The first level is called C-1
and can correct single-bit errors, the second, called C-2, corrects burst
errors. Any errors which are too large for the C-2 circuit to correct are
passed on to the error concealment algorithm which will interpolate the
missing data. Errors are labeled as follows: E11, E21, E31 are one, two and
three or more errors per block at the C-1 level, and E12, E22, E32 are one,
two, and three or more errors per block at the C-2 level. Data interleaving
is a method of reducing the effect of many contiguous errors. Simply put,
the data is mixed up and recorded to the disc out of order. When the data is
retrieved from disc, it is returned to the proper order. This has the effect
of breaking contiguous errors caused by the recording layer into smaller bit
errors after the data has been de-interleaved.

Data is retrieved from the disc, is assembled into blocks and passed on to
the C-1 circuit. block error rate, or "BLER" errors are counted at this
level. E11 and E21 errors are corrected, E31 errors are flagged and passed
onto the C-2 circuit. At the C-2 circuit, "BRST" or burst errors are
counted. The data is de-interleaved at this point dispersing the flagged E31
errors over many blocks, E12 and E22 errors are corrected and E32 errors are
passed onto the interpolation circuit. It should be noted that most CD
ripping programs skip the interpolation step, they do the C-1 and C-2
correction, but E32 errors will show up as clicks and pops in the ripped
audio. Also, CD-ROMs have a third level of error correction which can
correct many E32 errors.

-- 
Konrad Strauss
Director of Recording Arts
Associate Professor of Music
Jacobs School of Music
Indiana University
http://php.indiana.edu/~kstrauss
http://www.music.indiana.edu/department/audio/