Data Modulation & Error Correction
It is not possible to manufacture CDs where every pit is intact. Small defects in manufacture are permissible and even minor scratches which can occur with use do not usually affect the disc's playability. Therefore the CD specification includes two levels of error correction to compensate for these defects. These are CIRC error correction and EFM modulation.

CIRC - A CIRC (Cross Interleaved Read-Solomon Code) encoder adds two dimensional parity information, to correct errors, and also interleaves the data on the disc to protect from burst errors. CIRC corrects error bursts up to 3,500 bits (2.4 mm in length) and compensates for error bursts up to 12,000 bits (8.5 mm) such as caused by minor scratches.

EFM Modulation - The EFM (Eight to Fourteen) modulation scheme encodes each 8-bit symbol as 14 bits plus 3 merging bits. The EFM data is then used to define the pits on the disc. The merging bits ensure that pit & land lengths are not less than 3 and no more than 11 channel bits. This reduces the effect of jitter and other distortions on the error rate.

CD-ROM discs generally include a third level of protection.

- posted by J @ 3:50 PM