Hi, I've found a very good paper on the topic, please see: http://www.cdpage.com/CD-R/testing.html . I quote extensively from it here (please pardon me). If you skip to the last paragraph at least, you will notice that I don't know what I'm talking about. I've been saying the BLER spec is "220 bits per second", it's 220 *blocks* with one or more bad symbols per second. Rick Cross Interleaved Reed-Solomon Code (CIRC) All CDs incorporate an error detection and correction scheme known as Cross Interleave Reed-Solomon Code (CIRC). It would be impossible to make a usable CD without this error correction, since many errors are generated playing even the best discs under ideal conditions. CIRC is a powerful error detection code that can detect and completely correct all errors on a reasonably good disc. It relies on two principles for its operation. One is redundancy. Extra parity bits are added to the data stream to facilitate error detection and correction. These extra bits reduce the available capacity of the CD by about 25%. The other principle is interleaving. The data is not recorded n the disc in its natural order as it would be on tape or magnetic disk. The data are organized into blocks of 24 bytes (called "symbols" in CIRC terminology.) These are the blocks referred to in the Block Error Rule measure. Four parity bytes are added to each block of 24 symbols (bytes), making the block 32 bytes long. The data symbols belonging to one block are then distributed over a fairly large area of the disc by "interleaving" them with symbols from other data blocks. The 24 symbols of one data block end up distributed over 109 data blocks. The advantage of this technique is that physical defects on the disc do not eliminate complete data blocks, but instead, parts of many blocks. These partially bad blocks can then be reconstructed using the parity information. CIRC error correction is done in two stages referred to as C1 and C2, with deinterleaving of the data taking place between the stages. The C1 stage is used to recover from random errors caused by noise in the signal; the C2 stage is used to recover from larger errors caused by physical defects such as scratches and dirt. The error correction chip typically can correct two bad symbols per block in the first stage, and two bad symbols per block in the second stage. Some chips can correct four bad symbols in the second stage. Layered Error Detection Code and Error Correction Code CD-ROMs include an extra layer of error detection and correction, usually called Layered Error Detection Code and Error Correction Code (for brevity's sake, typically referred to as Layered ECC) which works similarly to CIRC by adding parity information to each data block. The extra error correction capability can correct errors that are nor correctable by the CIRC because it adds additional parity bytes and additional scrambling of the data. If a CD-ROM drive encounters an error which is uncorrectable even by the layered ECC, it will try again to read the sector. this is the failure mode most familiar to the CD-ROM user. Obviously, having to re-try slows down access to the data, but sometimes a second try is successful. If the drive cannot recover the data within a certain number of retries, the disc is unusable. Types of Errors Disc testing equipment documents the errors that are being detected ( and usually corrected) by CIRC. The errors reported during disc testing are classified into a variety of categories. An error of type E11 means that one bad symbol was corrected in the C1 stage of CIRC. E21 means two bad symbols were corrected in the C1 state; E31 means that three or more errors were found at the C1 stage. If there are more errors are found at the C1 stage, the block is uncorrectable at the C1 stage, and is passed to the C2 stage. Because of the deinterleaving of the data between the stages, those three (or more) bad symbols are now in separate blocks, and so can be corrected by the C2 stage. Similarly, E12 means that one bad symbol was corrected in the C2 stage, and E22 means that two bad symbols were corrected in the C2 stage. E32 means that three or more bad symbols were found in one block at the C2 stage, and therefore this error is not correctable. Block Error Rate (BLER) Block error Rate (BLER) is defined as the number of data blocks per second that have any bad symbols. BLER is the most general and useful measurement of the quality of a disc. The Red Book specification (IEC 908) calls for a maximum BLER of 220 per second averaged over ten seconds. Discs with higher BLERs are likely to produce uncorectable errors. Presently, the best discs have average BLERs below 10. A low BLER shows that the system as a whole is performing well, and that the pit geometry is good. Relying on the BLER alone is not advisable, however, although the BLER provides information on the number of bad blocks per second, it does not indicate the severity of the errors. In principle, a disc with an average BLER of five can be unusable, if all the those errors are uncorrectable! The error codes described above provide details that indicate the severity of the errors and distinguish between correctable and uncorrectable errors. ----------------------------------------------------------------- To stop getting this list send a message containing just the word "unsubscribe" to [EMAIL PROTECTED]
