36
DVX
Instruction
Manual
INTERPRETING THE RESULTS
INTERPRETING ERROR RATES
Any serious disc defects will cause an increase in the error rates. Therefore, measuring the number and severity of
errors gives a good indication of disc quality. High error rates are generally caused by local physical defects or poor pit
geometry.
Although in some sense any disc that plays without uncorrectable errors is “perfect,” there are other considerations.
For one thing, we may wish to know how close is it to getting uncorrectable errors. Obviously, a disc with very low
error rates has more tolerance for dirt, scratches, and aging before it will produce an uncorrectable error. Other discs,
although they may not produce uncorrectable errors, may be on the verge of doing so. In addition, older first
generation players may produce many uncorrectable errors on such a disc because they use a less effective error
correction algorithm than newer players do. Because the time code used to search to a location does not have CIRC
error correction, disc access times can rise dramatically with error rates, even though the data is fully recoverable.
CDs and DVDs could not work without a highly effective error detection and correction scheme. Because the pits are
so small, it is impossible to read the disc without errors. Keep in mind that the width of the pits is less than the
wavelength of light used to read them! Therefore, it is the error detection and correction codes that really make this
technology feasible. The error detection and correction code used on CDs and DVDs is known as Cross Interleave
Reed-Solomon Code (CIRC) or Reed-Solomon Product Code.
How CIRC Error Correction Works
This scheme uses two principles to achieve a remarkable ability to detect and correct errors. The first is redundancy.
This means that extra information is added, which gives you an extra chance to read it. For instance, if all data were
recorded twice, you would have twice as good a chance of recovering the correct data. The CIRC has a redundancy of
about 25%; that is, it adds about 25% additional data. This extra data is not just a repeat of the data, but is cleverly used
to record information about the original data, which provides the ability to deduce what the missing information must
have been.
The other principle used is interleaving. This means that the data is distributed over a relatively large physical area. If
the data were recorded sequentially, a small defect could easily wipe out an entire word. With CIRC, the bits are
interleaved before recording, and de-interleaved on playback. What happens is that the bits of individual words are
mixed up and distributed over many words. Now, to completely obliterate a single byte, you have to wipe out a large
area. Using this scheme, local defects destroy only small parts of many words, and there is always enough left of each
sample to reconstruct it. To completely wipe out a data block would require a hole in the disc of about 2 mm in
diameter.
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 in this unit can correct two bad symbols per block
in the first stage and up to four bad symbols per block in the second stage.
Types of Errors
The error type E11 means one bad symbol (byte) was corrected in the C1 stage. E21 means two bad symbols were
corrected in the C1 stage. E31 means that there were three or more bad symbols at the C1 stage. This block is
uncorrectable at the C1 stage, and is passed to the C2 stage. Because of the de-interleaving 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. Because
of the interleaving, one uncorrectable symbol at the C1 stage can be turned into as much as 28 bad symbols at the C2
stage. This is why E12 is typically much larger than E31.
E12 means one bad symbol was corrected in the C2 stage and E22 means two bad symbols were corrected in the C2
stage. E32 means that there were three or more bad symbols in one block at the C2 stage, and therefore this error in not
correctable.
BLER (Block Error Rate) is defined as the number of data blocks per second that contain detectable errors, at the input
of the C1 decoder. This is the most general 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 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.
