Writing Quality - Page 10

C1/C2 Errors - Page 2

- E32 errors

There are some drives that can correct even up to four bad symbols at the second stage. However for the majority of the tests, we consider E32 an uncorrectable error, even though some drives may be able to correct it. The quality of our recorded media will decrease with regular use and aging. Discs with already E22 or E32 will not leave adequate margin for future degradation.

Uncorrectable errors may not be a problem on audio discs, since the player's interpolation circuitry will hide these errors. Some players can interpolate over up to eight consecutive bad samples.

Uncorrectable errors do not necessarily make a CD-ROM unusable either. Errors that are uncorrectable in the main CIRC correction stage may still be corrected by the EDC/ECC sector level error correction used on CD-ROM's. Therefore, the data may still be recoverable, and can still verify if you are comparing to the original. Of course a disc like this has no tolerance for additional degradation, such as scratches and fingerprints, so access time will increase and it will soon fail.

- Testing Speeds

Some error rates may be higher at faster speeds, and others will be higher at lower speeds. In general, error rates on good discs will be about the same at higher speeds as at 1X. Small errors such as E11 & E21 will not be affected much. Burst errors, on the other hand, will be greatly affected. Most burst errors (E22 & E32) are caused by disturbances to the servo systems, rather than missing data. This effect is greatly magnified at high speed.

Error on a disc is not a physical thing. It is a manifestation of how well the total system (disc + player) is working. The disc itself does not have an error rate; playing the disc produces errors. Ideally, what you want is a disc that will play back on ALL players with a low error rate. Unfortunately, there are no standards for players, only for the discs. Therefore, each type of player will give different results.

Most testing equipment manufacturers believe that "…1X testing: This is still the best way to isolate the disc characteristics from the player influences. When we test discs at 1X, we can judge the disc, rather than the player…"

- Limits of Errors

The Data Channel Error rate must be as lower as possible.

The "Red Book" specification (IEC 908) calls for a maximum BLER of 220 per second averaged over ten seconds. The CD block rate is 7350 blocks per second; hence 220 BLER errors per second shows that 3% of frames contains a defect.

Discs with higher BLER are likely to produce uncorrectable errors. Nowadays, the best discs have average BLER below 10. A low BLER shows that the system as a whole is performing well, and the pit geometry is good. BLER only tells you how many errors were generated per second; it doesn't tell you anything about the severity of those errors. Therefore, it is important to look at all the different types of errors generated. Just because a disc has a low BLER, doesn't mean the disc is good.

For instance, it is quite possible for a disc to have a low BLER, but have many uncorrectable errors due to local defects. The smaller errors that are correctable in the C1 decoder are considered random errors. Larger errors like E22 and E32 are considered burst errors and are generally caused by local defects. As you might imagine, the sequence E11, E21, E31, E12, E22, E32 represents errors of increasing severity.

A Burst Error is defined as seven consecutive blocks in which the C1 decoding stage has detected an error. This usually indicates a larger scale defect on the disc, such as a scratch. In general, new discs which have not been handled on the read surface should not exhibit any burst errors. A Burst Error constitutes a disc failure. That's why many testing equipment offers a "Digital Error Mapping" for quick viewing C1, C2 and CU errors: