Various discs were tested using a Clover CD Analyzer.
The tests are not intended to be comprehensive, but cover only Block
Error Rates. This is nonetheless a good indicator of whether or not
the disc is good.
Block errors on a disc are not a physical thing. They are a manifestation of how a disc interacts with a player. So different players can produce different error-rates from the same disc. Although there are rigid specifications that define what a CD should be, there are no such specifications for players. Therfore, to ensure wide compatibility, discs should have low errors. Additionally, a disc that is unreadable on one player, may seem to perform well on another.
The CIRC error correction used in CD players uses two stages of error correction called C1 and C2, with de-interleaving of the data between the stages. The error correction chip can correct two bad symbols per block in the first stage and up to four bad symbols in the second stage.
BLER, or Block Error Rate, is the number of data blocks per second that contain detectable errors at the input of the C1 decoder. The "Red Book" specification allows BLER up to 220 per second averaged over 10 seconds. These days, with high speed readers commonplace, the generally accepted maximum is 50.
An E11 error means one bad symbol was corrected at the C1 stage. An E21 error means two bad symbols corrected at the C1 stage. E31 means three or more bad symbols at the C1 stage, and is uncorrectable at C1 and so is passed on to the C2 stage.
Because the data is de-interleaved between the stages, each of the bad symbols is now in separate blocks, and so they can be handled by the C2 stage. As a result of the interleaving, one uncorrectable symbol at C1 can become up to 28 bad symbols at C2, which is why E12 is often much higher than E31.
An E12 error means that one bad symbol was corrected at the C2 stage. E22 is two bad symbols, and E32 is three or more bad symbols at C2 and therefore cannot be corrected. It is theoretically possible for C2 to correct four bad symbols, but not all players can do so. To allow for high compatibility, we consider E32 to be uncorrectable, even though some drives may be able to correct it.
For more information on testing discs, check the Web site for Media Sciences.