Frequently Asked Questions About Compact Discs

Please contact Media Sciences if your questions are not answered on these pages.

Why do low BLER discs fail in some user drives?

CD discs contain Cross-Interleaved Reed-Solomon codes (CIRC) that are used by the read drive for error correction. All CD information is organized into frames, each containing sync, subcode, twenty-four bytes of user information, and eight associated CIRC parity bytes.

After deinterleaving, small errors are corrected by the read drive at the C1 level. P-parity can correct one or two bad bytes in one C1 frame. BLER measures the average rate of erroneous frames. A small, 3 mm long tangential defect can obliterate 16 sequential frames, creating uncorrectable errors, but BLER for such a defect would only be 16 when averaged over one second, or 1.6 when averaged over the standard ten seconds. BLER as high as 220 per sec. is allowed by standards, although 50 is often used as an upper limit.

After another stage of deinterleaving, large errors are corrected at the C2 level by Q-parity that can correct one or two bad bytes in a C2 frame (some drives can correct up to four bad bytes). Errors from the small 3 mm tangential scratch generate five erroneous bytes in C2 frames and are thus uncorrectable, even though BLER is low. Such defects must be detected by E22, E32, and BURST error tests.

A 2002 study by Media Sciences of 100 CD-R field failures at cdrfail.html showed that only 19% of the faulty discs failed BLER tests alone, while 40% failed full error testing. Sixty-four percent of the discs failed electrical evaluation of all parameters and errors, while 64% failed visual inspection. These tests are important because many read errors are not embedded in the disc but are soft errors that are generated when the disc is read.

Read drive servos utilize information in the reflected laser beam to maintain proper beam focus, radial position, and disc linear velocity. Read errors can result from improper disc parameters or visual defects that diminish or distort the beam and adversely affect these servomechanisms. In the absence of standards for drives, such discs may be readable in some drives but not in others.

Discs can also fail because of logical format problems, even though their physical characteristics are acceptable. Incorrect post-gaps, improper descriptors, and wrong session structures were observed by Media Sciences on 57% of the field failures that were analyzed.

Although BLER is one of many useful quality indicators, predictable readability can only be assured when full electrical parameter and error evaluation, mechanical and visual inspection, and logical format tests are utilized to assure full compliance with all requirements of ISO/IEC 10149, color book, and ISO 9660 standards for interchange.