CD Response to Defects

The signal from a compact disc is relatively insensitive to the presence of small defects such as dust or fine scratches on the bottom surface of the CD because the laser beam is fairly large at that point, about 0.8 mm. As illustrated below, typical dust particles are much smaller than that. As the laser is further focused down to about 1.7 micrometers at the depth of the pits, any shadow from the small defects is blurred and indistinct and does not cause a read error. Larger defects are handled by error-correcting codes in the handling of the digital data.

Error-Correction of CD Signals

The data on a compact disc is encoded in such a way that some well- developed error-correction schemes can be used. A sophisticated error- correction code known as CIRC (cross interleave Reed-Solomon code) is used to deal with both burst errors from dirt and scratches and random errors from inaccurate cutting of the disc. The data on the disc are formatted in frames which contain 408 bits of audio data and another 180 bits of data which include parity and sync bits and a subcode. A given frame can contain information from other frames and the correlation between frames can be used to minimize errors. Errors on the disc could lead to some output frequencies above 22kHz (half the sampling frequency of 44.1 kHz) which could cause serious problems by "aliasing" down to audible frequencies. A technique called oversampling is used to reduce such noise. Using a digital filter to sample four times and average provides a 6-decibel improvement in signal-to-noise ratio. For more details, see the references.

Data Encoding on Compact Discs

Detection of Compact Disc Data