Frequently Asked Questions About Compact Discs

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

Is it true that BLER is the best measure of CD quality?

Son of clip level is alive! BLER is definitely not the most important CD quality measurement. The most important measurement is always the one you do not perform. Hidden problems are much more likely to adversely affect quality than issues you monitor.

Amplitude parameters such as I3, I11, asymmetry, push-pull, radial noise, crosstalk, and reflectivity are indispensable. Length deviation and jitter are important timing measurements. Error rate measurements such as E11, E21, E31, E12, E22, E32, and BURST provide vital information. Important mechanical properties include center hole and outer rim diameters, thickness, weight, unbalance, eccentricity, deviation, and deflection. Logical volume and file structures are another important quality requirement. CD quality testers should measure most of the above. Manufacturers use many tools to regularly check every one of these plus others such as birefringence, track pitch, scanning velocity, and radial location of key areas such lead-in, program area, and lead-out.

BLER is an abbreviation of BLock Error Rate, but really represents frame error rate. One frame represents the smallest integral data package, and contains 24 bytes of data along with sync, subcode, Q parity, and P parity. Data is read from a CD at the rate of 7350 frames per second in a 1X player. After alternate bytes are delayed by one frame, BLER measures the rate of bad frames that contain one or more read errors. If one percent of the frames contain errors, then BLER will be 73.5 per second at 1X. ISO 10149 requires a frame error rate less than 3%, or a 1X BLER of 220 per second. High quality discs have much lower frame error rates than required by the ISO Standard.

BLER is a good quality indicator if all other properties of the disc indicate high quality. The weakness of BLER is that it counts error frames, not bad bits or bytes. One error frame might contain only one bad bit or the entire contents of the frame might be erroneous. Both cases would contribute only one count to BLER. C1 error correction using P parity can easily correct one bad bit, but would be ineffective if the entire frame was bad. It could only flag the bad bytes for C2 error correction that applies Q parity to deinterleaved frames. This means that the bad bytes of a C1 error frame are redistributed, one byte at a time, over 109 frames containing bytes from other C1 frames. Although the resultant error patterns are usually detected and corrected at the C2 level, this strains system capabilities and performance approaches an uncorrectable state.

Seven tests exist for probing the severity of errors. E11 measures the rate at which C1 is required to detect and correct only one error per frame. E21 evaluates the rate at which C2 corrects exactly two errors per frame. E31 is the rate at which C1 declares the frame to be uncorrectable. E12 measures the rate at which C2 corrects one error in the deinterleaved frame, E22 the rate of two corrections in a frame, and E32 the rate of uncorrectables at the C2 level. A BURST error results if seven or more contiguous C1 frames contain E21 or E31 errors. High quality CD-ROM discs should have no E22, E32, or BURST errors. BLER values cannot provide this information.

Another BLER issue is averaging time. ISO 10149 requires a 10 second average. Severe, highly localized defects generate high error spikes that average to much lower values during the 10 second period. Good test equipment usually provides a one second average BLER that can reveal the presence of dangerous, localized errors. Modern CD-ROM testers simultaneously measure many quality indicators during one pass. Use all of the available information to predict successful interchange. Avoid reliance on just one indicator such as BLER.