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Appeared on: Monday, January 13, 2003
Testing C2 information


1. Introduction

Testing C2 Error Information - Page 1

Introduction

When a drive can report C2 error pointers with high accuracy, it can be used to extract Audio tracks accurately, since the software knows where the error occurred and corrected or not. The software can re-read the specific sector many times until it gets the correct extracted data or use special algorithms to correct the un-corrected data. Software like PlexTools and EAC supports such extraction mode.

- C2 pointers vs. C1/C2 measurement

We have to make a note between C2 error pointers from the C1/C2 error measurement.

In general, the readers usually can correct errors with data discs. C2 errors in most times are not "real" errors (scratches) but warnings that the condition of our disc is not in the best shape. A good cleaning of our disc could improve the readability. If too many C2 errors occur, the reader will report a read error.

When more than two bytes are corrupt at the C2 encoder, then the data is passed on to a concealment circuit on the CD drive. This circuit uses different methods to conceal the error. Most new drives will pass the data through a concealment circuit, usually only drives made before 1998 do not so...

All drives can read C1/C2 errors from a disc, when using the proper software commands. When reading C1/C2 errors, the data is not passed through the concealment circuit, so we can measure the C1/C2 errors as reported from the drive.

A drive that has low C2 % error accuracy doesn't mean it's not useful for reading C1/C2 errors, but it's not the best solution for extracting Audio tracks. A drive with high C2% error accuracy would be ideal, since its results would be more absolute comparable. As you may understand, the C2 errors reported from various drives with the same disc will vary between them. The different optics, board design and different firmware affect the C2 error results.

- Retrieving C2 Error information

There are two methods can get C2 error data:

According to the MMC standard, a bit is associated with each of the 2352 bytes of main channel where: 0 = No C2 error and 1 = C2 error. The resulting bit field is ordered exactly as the main channel bytes. Each 8-bit boundary defines a byte of flag bits. The below C2 Errors code provides for the inclusion of fabricated information based upon the results of C2 error correction (on main channel). For Read CD command, the return 294 byte C2 flag data each bit indicates 2352 byte each byte C2 error. That means 294 X 8 = 2352 bit C2 error status.

C2 Errors Code
Numbers of bytes
Definition
Description
00b
0
None
No error information will be included in the data stream
01b
294
C2 Error Flag data
The C2 Error Flag (pointer) bits (2352 bits or 294 bytes) will be included in the data stream. When the C2 Error pointer bits are included in the data stream, there will be one bit for each byte in error in the sector (2352 total). The bit ordering is from the most significant bit to the least significant bit in each byte. The first bytes in the sector will be the first bits/bytes in the data stream.
10b
296
C2 & Error Flag data
Both the C2 Error Flags (2352 bits or 294bytes) and the Block Error Byte will be included in the data stream. The Block Error byte is the OR of all the C2 Error flag bytes. So that the data stream will always be an even number of bytes, the Block error byte will be padded with a byte (undefined). The Block Error byte will be first in the data stream followed by the pad byte
11b
Reserved
Reserved for future enhancement

If a drive doesn't report C2 errors according to the MMC, but does support C2 error information, both software will give a 0 score. In that cases we cannot test the C2 error information since special commands are needed to retrieve (and test) the C2 error information level.

The second is by using each vendor unique command(s). For vendor unique command, each command return data include C1 error and C2 error in 75 Blocks (1 Second) data.

That means 1 frame include 1bit C1 and 1 bit C2 parity result. So, the C1 maximum error is 75blocks X 98 frame = 7350 C1 error. The C2 maximum error is 75blocks X 98 frame = 7350 C2 error.

The C1, C2 error output is selectable, include before correct or after correct, based on the chipset design. That explains the differences between the various drive designs since one manufacturer may return C2 errors as the sum of E22+E32, while others as the E22 or E32 values. The same issue occurs at the C1 decoder (E31, or E11+E21+E31 errors). The C1/C2 errors counted/measured by one drive will not be likely the same with other, especially if the chipset design is different.

- Testing software

There are mainly two test utilities that can be used to determine if a drive support C2 error information:

  1. EAC
  2. CDSpeed

Both software works only with specific test discs that contain artificial scratches/defects. For our tests we are using EAC CD-R, ABEX TCD-721R and ABEX TCD-726 test discs. The results are not the same with each disc so user should look carefully the differences.


2. EAC

Testing C2 error information - Page 2

EAC

- Introduction

EAC author have written a utility called "DAE Quality" that can test the DAE quality of any optical storage device. The package is not included in the EAC software and you have to download it from EAC website. The package contains a C2 error pointer checker (analyse.exe) and two DOS extractor, one for drives that supporting C2 error information (c2extract.exe) and one for those who doesn't (extract.exe).

After using the c2extract.exe software, a .wav and a .c2 file are created that will be used from the analyser. After the analyser finishes, a bitmap graph and also a .dat file containing all graph data are created, in order to enable the user to create higher resolution graphs.

The bitmap is splitted in three graph views. The marks on the X-axis are again measured in minutes of audio data. The Y-axis for each graph is a logarithmic scale (important!) of the number of C2 errors. So each line on the Y-axis represents approximately that two times as much C2 errors occurred as on the previous line on the Y-axis.

As EAC author mentions "…This third one is the most interesting figure, it will show how much blocks are going unreported by C2 error correction...The 16 bit blocks (instead of 8-bit, the native format of C2) was because of some drives interpolate a (mono) sample, etc. but only flag the part with the read error on it (but changed both parts). Thus only 16 bit blocks are checked for consistency. It is enough if one of the two bits is flagged to accept that any of the two bytes are wrong..."

- Use

The ANALYSE.EXE program has a new parameter for C2 analysis, called '-c2flip'. The position of the arguments is important, it always must be in this order :

ANALYSE WAV-File [-c2flip] [X-sizeofbitmap Y-sizeofbitmap]

The -c2flip switch will change MSB and LSB coding of the C2 error pointers. The standard defining the C2 error pointer was at the beginning very weak, it was not always defined which bit represents what byte. Thus some (few) drives uses a code exactly the other way round:

7 6 5 4 3 2 1 0

,while other do it this way

0 1 2 3 4 5 6 7

Usually some TEAC and drives using RICOH's chipset needs this additional command.

- Results

Below is a C2 error graph using SONY CRX-220E1 recorder with ABEX TCD-721R test disc. Many C2 errors are reported at the damaged area and are 100% reported from the drive. The blue line is flat! The drive gets a 100% accuracy score!

Below is another example. This time the drive (TEAC DW-224E) reports many C2 errors but misses to report few...The drive gets a 99.4% accuracy score.

Lastly, below is another example of a drive that may be able to return C2 error information but its accuracy is very low. When this kind of result occurs, users should first try the -c2flip command. Without it, 25% of the C2 reports may be misplaced. An accuracy score of 67% is far from acceptable.


3. Nero CD Speed

Testing C2 error information - Page 3

Nero CD Speed

The Nero CD Speed software compares on-the-fly, byte-to-byte, and the extracted data of the test disc with an image. The software measures three types of C2 errors:

According to how many C2 errors are reported/missed each drive gets a % accuracy score. The CDSpeed test also includes a "reverse" switch for some drives (usually with RICOH chipsets), as also found with the EAC analyser software.

- Results

Below is the CDSpeed graph of the SONY CRX-220E1. As we have seen earlier the drive got 100% score with EAC and also gets 100% with CDSpeed!

Below is another example of the EAC and CDSpeed similar scores. With EAC the drive got a 99.4%, while with CDSpeed 97.4%.

Lastly, with some drives the EAC and CDSpeed results will not agree. EAC gives a 67% score, while CDSpeed 26.38%.

A possible explanation is that when so many sync error occur its nearly impossible to calculate with accuracy the C2 error report information. Remember that if we need drives with at least 99% accuracy to be used for perfect DAE extraction.

Note: In most cases the EAC and CDSpeed C2 percentages would be similar. Both are linear (that means x per 100 c2 errors are not reported). However, due to different calculation method, there have been cases, where a drive got much different percentages in CDSpeed and EAC tests. Never less, a drive should have more than 95% score to be used for checking the writing quality of a disc...


4. LiteOn results

Testing C2 error information - Page 4

Results from LiteOn drives

Drive
C2 Errors Reported
C2 Errors Missed
C2 Errors Total
C2 Accuracy%
2268476
201
2268677
99.98
1853082
45
1853127
99.99
1957576
114
1957690
99.99
1982076
45
1982121
99.99
2608720
0
2608720
100

The results are really interesting…Three different generations of MediaTek chipsets and SANYO pickups produced totally different results. The latest model (LTR-52246S) had 100% accuracy and reported the higher total C2 Errors with the inserted disc. The LTR-32123S series reported the lower total C2 errors. The results showing the improvement done as time passes and manufacturers improve their reference board design and tweak the optics.



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