PCAV vs. ZCLV Recording Technlogies
2. World Of Error Correction
Review Pages
2. World Of Error Correction
3. Ricoh 74min 24x Test Results
4. Ricoh 74min 24x Test Results - Acer CRW2010A
5. Ricoh 74min 24x Test Results - LiteOn LTR-24102B
6. Ricoh 74min 24x Test Results - Plextor PX-W2410A
7. Ricoh 74min 24x Test Results - Ricoh MP7200A
8. Ricoh 74min 24x Test Results - Ricoh MP9200A
9. Ricoh 74min 24x Test Results - Sanyo CRD-BP1500P
10. Ricoh 74min 24x Test Results - Yamaha CRW2200E
11. Conclusion
Partial-CAV versus Zone-CLV - Page 2
World Of Error Correction
- What is CIRC?
CIRC (Cross Interleaved Reed-Solomon Code) is the error detection and correction technique used on a CD (and on DVD for that matter). It is a variant of the more general BNC codes. These codes are actually Bytes added during premastering or recording to the normal data bytes for achieving error free reading of a disc. The CIRC bytes are present in all CD modes (audio and data alike). The whole error correction method that makes use of these added CIRC bytes is commonly referred to as the CIRC based algorithm. While the laser head reads the bits out of the disk surface introducing one erroneous bit out of 10^6, the basic level of error correction provided for the Audio CD by CIRC results to only one unrecoverable bit out of every 10^9 bits read! CD-ROM provides additional protection for data (ECC/EDC), the so-called layer 3 (L3) error correction, reducing this error rate to just one bit out of 10^12 bits!
Let's see how this is important. One data bit corresponds to 3.0625 so-called "channel bits". These are the ones that are actually recorded onto a disc surface. A whole disk is 333,000 sectors long. Each sector contains 2,048 data bytes. By performing the necessary simple arithmetic calculations, we can easily see that every data disk would be unreadable without the implementation of both the above error correcting codes.
- How CIRC works?
CIRC uses two distinct techniques in order to achieve this remarkable
ability to detect and correct erroneous bytes. The first is redundancy. This
means that extra data is added, which offers an extra chance for reading 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 cleverly
used to record information about the original data, from which it is possible
the missing information to be deduced.
(For simplicity, in our explanation throughout this presentation, we silently
ignore the 8 to 14, EFM, modulation. In a rigorous discussion of any CD related
subject, this should be seriously taken into account.)
The other technique used during the implementation of CIRC is interleaving. This means that the data is distributed over a relatively large physical area of the disk. If the data were recorded sequentially, a small defect could easily wipe out an entire word (byte). With CIRC, the bits are interleaved before recording, and de-interleaved during playback. One data block (frame) of 24 data bytes is distributed over 109 adjacent blocks. To destroy one byte, you would have to destroy these other bytes. With scratches, dust, fingerprints, and even holes in the disc, there is usually enough data left to reconstruct any data bytes that have been damaged or caused the disk to become unreadable.
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 (hopefully) 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.
Moreover, the Reed-Solomon codes used for both levels of error correction are specifically designed for cutting-off "burst" errors, not just those systematic ones that usually come in the form of random errors, the so called Gaussian or white noise.
- What are C1/C2 errors?
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 the reading unit can correct two bad symbols per block in the first stage (C1) and up to four bad symbols per block in the second stage (C2). The C1 level of CIRC is meant to correct small, random errors. The C2 level corrects larger errors and burst errors.
- How high/low those values must be?
The lowest as possible! C1 errors cannot be ever zero (0), due to the various media asymmetries, physical defects, fingerprints, dust, etc. C2 errors also must be zero (0), or else you will have reading problems in most CD-ROMs.
The "Red Book" specification (IEC 908) calls for a maximum of 220 C1 errors per second averaged over ten seconds. Discs with higher C1 values are more likely to produce uncorrectable errors. Nowadays, the best discs have an average measurable value of C1 below 10. A low C1 value shows that the system as a whole is performing well, and the pit geometry is good. A peak of 100 bad data blocks per second is acceptable for a CD-ROM, but an average C1 value of 50 per second over the entire disc is a good cutoff point to ensure data integrity.
- Are C1/C2 errors a good indicator about the written media condition?
Yes. We would say that by more than a 80% chance, a disc with low measured C1/C2 error values will have great compatibility among various CD/DVD-ROMS/CD-RW drives.
- What can cause C1 error?
C1 errors can be caused due to physical problems (disk/drive asymmetries, dust, fingerprints) and the writing technology/strategy of each specific recorder. Each manufacturer has it's own writing strategy for achieving the maximum writing speed along with an acceptable writing quality (low error rate). Each drive has "prepackaged" in its firmware a set or orders on how to perform a specific writing strategy with each specific media. For this reason all recordable media are categorized. Each drive reads these codes and adjusts its recording strategy accordingly. Many times the writing quality can be improved with a newer firmware update. C1 errors can also be produced when you don't proper store your burned media.
- What is Writing Strategy?
Proper phase-change recording (that is re-writing), as well as amorphous recording, demands a sophisticated write-strategy, that accounts for the physical properties of the disc (e.g. thermal conductivity). The essential part of every pulse-width-recording method is the use of a multi-pulse write-strategy. All this means that not all 0's and 1's are of equal length when recorded on a disk's surface! The writing strategy takes (or should take) into account all three parameters: drive, disk and data, and perform an optimized laser power adjustment according to predetermined empirical evidence.
- How i can prevent C1/C2 errors?
You must keep your media closed in a jewel case in order to prevent dust and physical damage. Also there is the possibility that your disc can be scratched during insertion/ejection from a drive. This might cause C1 or, even worse, C2 errors.
And, of course, always choose best quality media! (Don't forget,
cheaper is not always worse, nor the reverse of this statements holds in all
cases.)
- What are the possible causes for a C2 error?
C2 errors are caused either from large media defects, a not properly defined writing strategy, or a soon-to-die laser diode. Overburning is another cause of C2 errors. As you know overburning is not officially supported from any CD standard. Therefore both media and drives are not manufactured to support overburning and this fact alone can cause C2 errors. Where? In the overburned part of a disk of course. The media quality at this point is not as well prepared to perform well as in the rest of the disc, and the drive probably will not be able to burn it properly either. Scratches can also occur when you are NOT carefully placing a burned CD in the tray or throw it from some height, especially over a rough surface.
Review Pages
2. World Of Error Correction
3. Ricoh 74min 24x Test Results
4. Ricoh 74min 24x Test Results - Acer CRW2010A
5. Ricoh 74min 24x Test Results - LiteOn LTR-24102B
6. Ricoh 74min 24x Test Results - Plextor PX-W2410A
7. Ricoh 74min 24x Test Results - Ricoh MP7200A
8. Ricoh 74min 24x Test Results - Ricoh MP9200A
9. Ricoh 74min 24x Test Results - Sanyo CRD-BP1500P
10. Ricoh 74min 24x Test Results - Yamaha CRW2200E
11. Conclusion