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CD-R
iasa Journal no23 - July 2004
CD
Writing on the CD-R is done by a laser beam of the same wave length as that used for reading, except that it is approximately ten times more powerful (4-8 mW).The recording layer is photosensitive and is therefore heated to 250-300°C by the recording laser beam. The exposed area melts and is deformed, which changes the reflection characteristics for reading. The result is similar to pits on CDs. The recording layer or dye is made of organic material and is a highly sensitive and vulnerable part of the CD-R. Usual dye types are cyanine, AZO and phthalocyanine. Their life expectancy under good storage conditions is believed to be about 100-200 years. However, manufacturers experiment a lot, especially with dyes and reflective layers, in order do decrease manufacturing costs and to increase writing speed.To them, CD-R stability and lifespan are becoming less important.
Digital Recordings on CDs and CD-Rs
Recordings on CDs and CD-Rs, which are defined in the Red and Orange Book standards, are rather complicated; the procedure is not merely simple coding of audio data represented by a binary form of '0' and' I '. Data is grouped in blocks, 24 bytes per block. Blocks are then further joined in frames, 98 blocks per frame.A block has 192 audio bits, but because of the additional synchronisation, code, parity etc, the total number of bits per block is 588. Seventy-five frames are read per second; in other words, 7 350 blocks per second. [2]
Owing to high data density and large quantities of bits read each second, error frequency in reading the data is high. That is why special algorithms are included in the writing and reading process, which enable detection of errors and attempts to correct them. Error detection and correction are done in different ways, such as adding parity bits, displacing data before recording (interleaving), Eight to Fourteen Modulation (EFM) and interpolation.
An error detection and correction system can successfully detect and correct a certain number of errors per second without any audible consequences.
When there are more errors than that, or when they are grouped (burst errors), the system can no longer correct them successfully and the data is lost for reading. They can be replaced, or covered up by interpolation, a procedure based on mathematical guessing that predicts the value of the missing data. It does not, however, correct the errors; it merely guesses what the missing parts could be. Consequently, the result is no longer the original recording but its more or less satisfactory approximation.
Error correction is done at various levels and degrees. Usually, three parameters are observed at the first level (E I I, E21 , E3 I) in order to establish how many blocks containing one, two, or three errors were detected and corrected. The parameters at the second level (E 12, E22, E32) are similar, with the exception that three errors in the same block cannot be corrected.
Therefore, the E32 error type is uncorrectable and should not be present on the CD, if the replay of its contents are to be intact.
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iasa Journal no23 - July 2004
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CD recording quality can be estimated from the number of blocks containing errors of any kind. The estimate is called the block error rate (BLER) which is the sum total of E I I, E21 and E3 I per second.According to the Red Book standard, the error detection and correction system is successful if the number of blocks containing errors is less than three per cent. With 7 350 blocks per second, this means BLER should be less than 220. The same standard tolerates up to seven consecutive blocks containing errors (burst error length, BERL <7) and no uncorrectable errors (E32 = 0). When these values are exceeded, the sound carrier is corrupted and problems in replaying it are expected. Furthermore, the proposed parameter limits for carriers used for archival purposes are still lower.
The Susceptibility to Sunlight Experiment1
It was the aim of the experiment to establish how various CD-Rs are affected by their exposure to sunlight. Several samples of identical CD-Rs, which were of different makes and price ranges, were bought from different manufacturers. CD-Rs contained two principal dye types, cyanine and phthalocyanine, and had different reflective layers. After purchase, all the samples were recorded on the same professional stand-alone CD burner in the CD-DA format, in accordance with the Orange Book standard, which can be read by the Red Book-compliant player. The experiment was first conducted in July 2000 and repeated (and updated) in 200 I. In 200 I, some brands of discs were exposed to sunlight again and some new CD-Rs were added.
After the CD-Rs had been recorded, they were marked on the plastic inner (unrecorded) hub, and measured by the CD Errormonitor.A CD Errormonitor is specialized error collection software running on a standard PC and hardware connection that is connected to a CD audio player with a Sony error correction chip. This measures Block Error Rate (BLER), Burst Error Length (BERL), uncorrectable errors (E32) and several other error parameters, enabling reliable monitoring of a CD's error condition. Although it does not allow error analysis, it does allow detection of those errors.A professional studio CD player was used as a CD player with the built-in hardware connection for CD Errormonitor.
Once measured, the CD-Rs were then stored in different locations (each copy of the same kind of disc was placed in a different location), and exposed to different conditions. In the paper, some of the most typical brands of CD-R used in the experiment are discussed. For easier understanding, the discs are labelled A, B, C, D and E.
!?L~cs A are inexpensive, multipurpose CD-Rs with cyanine dye and a silver reflective layer. Owing to poor manufacturing, or perhaps to insufficient compatibility with our CD burner, even the very first measuring revealed a comparatively high average
1 see also [3]
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-iasa Journal no23 - July 2004
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BLER (around 30.0). Nevertheless, peak BLER was within the Red Book fail level standard of 220. The first measuring did not detect any E32 errors, nor was it possible to detect any audible errors during the playing.