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Home > Hardware Reviews > Optical Storage

Wednesday, January 12, 2011
Sony BDX-S500U

3. Reading damaged CDs

This series of tests checks the drive's ability to correct/conceal possible erroneous data after reading artificially scratched / defective audio discs.

Using a CD-R in best shape to do the DAE test is generally not a safe way to test the drive's error correction capabilities. If your drive would not read audio CDs error free from an error free disc, you would probably bring the drive back to the vendor. It is far more interesting to see how a drive is behaving under critical conditions (which will also tell something about the DAE quality on CDs that have manipulated C2 error information on purpose). For that a special test CD like the ABEX discs from ALMEDIO can be used, that can be used to do a comparison between different drives. The ABEX test disc is actually an AudioCD that has artificial scratches and other physical disc error patterns on its surface.

Using a special software, we compare two audio files using FFT analysis. The first audio file has been extracted by a normal audio disc without physical error patterns on it . The second one is the result of the extraction of the ABEX test discs which hold the same audio tracks, but it also has specific defects on its surface. The similarity factor of the the two tracks unveils the error correction capabilities of the drive.

The differences between the two compared tracks are translated to a signal (noise) illustrated in the following graphs. Each graph tells a lot about the abilities of the drive. The quality of the optical system (and/or of the error correction capabilities of the firmware) is shown in at which time index the error start. The error hiding qualities are shown when the wedge gets bigger. The X position of a grid line is always a start of a new minute position on the CD (in play time, up to 74 min). The Y axis shows the dB(A) value of the error in the extracted file. The 0 dB(A) baseline at the top is marked slightly different. So the graph shows a range of 6 dB(A) down to -120.0 dB(A). Each line represents 6 dB(A) of volume (6 dB(A) louder means that the sound is double as loud).

- ABEX TCD-721R

 

Errors total Num: 827283
Errors (Loudness) Num: 46273 Avg: -74.9 dB(A) Max: -34.9 dB(A)
Error Muting Num: 2669 Avg: 1,0 Samples Max: 7 Samples
Skips Num: 1 Avg: 6.0 Samples Max:6 Samples
Results C2 Accuracy: 99.6 %
73.4 points (out of 100.0 maximum)

The drive's error correction is good here, starting at the point where the defect is starting to grow, but t error hiding mechanisms are average, since there are some wedges that went beyond the -60 db(A) level, especially a the 32~34 min areas where the scratch on the surface of the disc becomes bigger. The drive also muted many samples as it couldn't "hide" them and also skipped once. This means that sync was lost and the drive had to reposition again (and started on a slightly different position).

- ABEX TCD-726R

Errors total Num: 608
Errors (Loudness) Num: 23 Avg: -69.3 dB(A) Max:-57.4 dB(A)
Error Muting Num: 1 Avg: 1.0 Samples Max:1 Samples
Skips Num: 0 Avg: 0.0 Samples Max: 0 Samples
Results C2 accuracy: 100 %
92.6 points (out of 100.0 maximum)

This time the errors started earlier than expected and before the drive reach the defected area of the disc. This has to do with the quality of the optical system (and/or of the error correction capabilities of the firmware). .

- CD-Check Audio Test Disc

CD players have built-in D/A converters that turn the digital data on a CD into analog signal - what we hear as music. Ideally, all the digital data should be converted to the analog format. In reality, many factors cause digital data to be lost and sound reproduction to detoriate.

CD players handle this data loss using a sophisticated error correction system that allows them to recover it. However, when the data loss is greater than a system's recovery ability, some of the signal is lost. It is then that the CD player uses compensation methods such as interpolation, data substitution or signal muting to make this loss as anaudible as possible. However, this results in altered and often distorted sound.

The level of sound distortion depends on the amount of data loss. Initially, music may sound brittle and there may be subtle problems with stereo imaging or dynamics. Over time, disc skipping, clicks, pops in the signal or audible signal muting may result. CD-CHECK contains a special signal (tone), designed for early detection of the most subtle forms of distortion. The disc offers a signal combination with disc error patterns to rate the drive's abilities to read music and reproduce it completely. Five tracks on the disc contain a sequence of progressively more difficult tests. These tracks are referred to as Check Level-1 through Check Level-5.

The tracks are reproduced through a software multimedia player (e.g. Windows Media Player). Each level is considered as passed, if the tone is smooth, continuous without interruptions, skipping or looping. The higher the Check Level passed, the more reliable the sound reproduction of the tested drive.

Error Level
1
2
3
4
5
Sony BDX-S500U
5/5
5/5
5/5
5/5
5/5

A good performance for the Sony drive , as it successfully played all the tracks.




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