Writing Quality - Page 9

C1/C2 Errors

- Measuring C1/C2 errors

For measuring C1 and C2, industry has established a testing methodology. Those two measurements can be placed in the "Data Channel" tests, according to the CD standard measuring methods. Data Channel tests are concerned with the integrity of the decoded data from the disc in terms of the amount of and severity of errors on the disc. This is a good overall indication of disc quality, however, when there are underlying problems causing high error rate the root of the problem can be found by looking at other tests.

In CD terminology, errors are usually mentioned as Exy, where x denotes the number of bytes containing an error and y denotes the decoder stage (1 or 2).

- C1 Decoder

At the lowest level, a CD-ROM drive reads EFM frames from the CD. An EFM frame consists of 24 user data bytes, 1 Subcode and 8 P&Q parity bytes. When reading data from a CD, the EFM data is de-modulated and the 24-bytes of user data are passed through the CD drive's C1 and C2 decoders:

• C1 is a (32, 28) code producing four P parity symbols. P parity is designed to correct single-symbol errors and detect and flag double and triple errors for Q correction.
• C2 is a (28, 24) code, that is the encoder input 24 symbols, and outputs 28 symbols, including 4 symbols of Q parity. Q parity is designed to correct one erroneous symbol, or up to four erasures in one word.
  

Using four P parity symbols, the C1 decoder corrects random errors and detect burst. The C1 decoder can correct up to four symbols if the error location is known, and two symbols can be corrected if the location is not known. Three error counts (E11, E21 and E31) are measured at the output of the C1 decoder.

1. E11: the frequency of occurrence of single symbol (correctable) errors per second in the C1 decoder.
2. E21: the frequency of occurrence of double symbol (correctable) errors in the C1 decoder.
3. E31: the frequency of triple symbol, or more, (un-correctable) errors in the C1 decoder. This block is uncorrectable at the C1 stage, and is passed to the C2 stage.
   

C1 is defined as the sum of E11+E21+E31 per second within the inspection range. The unit for this measurement is [number]. The block error rate (BLER) equals with the sum of E11 + E21 + E31 per second averaged over ten seconds.

- C2 Decoder

Given pre-corrected data, and help from de-interleave, C2 can correct burst errors as well as random errors that C1 was unable to correct. When C2 cannot accomplish correction, the 24 data symbols are flagged as an E32 error and passed on for interpolation.

In the case of audio data, the E32 error is also referred to as a CU. When CU errors occur on audio, the 24-bytes are passed on to a concealment circuit on the CD drive. This circuit uses different methods to conceal the error so that it does not cause audible effects such as a pop or a click. If too many bytes are corrupt or if there are many CUs in a row, it is possible that even the concealment circuit will not be able to conceal the error and a pop or click may be heard during playback.

There are three error count ( E12, E22, and E32 ) at the C2 decoder.

1. E12 count indicates the frequency of occurrence of a single symbol (correctable) error in the C2 decoder. A high E12 is not problematic because one E31 error can generate up to 28 E12 errors due to interleaving. Each C1 byte is sent in a different C2 frame, therefore never affecting more than one byte in any C2 frame
2. E22 count indicates the frequency of double symbol (correctable) error in the C2 decoder. E22 errors are the worst correctable errors.
3. E32 count indicates triple-symbol, or more, (un-correctable) errors in the C2 decoder. E32 should never occur on a disc.

C2 is defined either as the total of

• E32 per second within the inspection range for some manufacturers
• E12+E22 per second within the inspection range for other manufacturers
  

The unit for this measurement is [number]. The E21 and E22 signals can be combined to form a burst error (BST) count. This count is often tabulated as a total number over an entire disc.

Based upon Exy errors, we can define new measurements like BEGL (Burst Error Greater than Length). The Red Book specifies that the number of successive C1 uncorrectable frames must be less than seven. Therefore, the threshold for BEGL is set to 7 frames and any instance where the Burst Error Length is 7 or more frames, counts as a BEGL error. Good discs should not generate BEGL errors, because error bursts of this length could produce E32 errors.

External Links: There is an interesting article that explains how to calculate the internal audio error correction ability of a CD ROM drive over here.

- Comparison with DVD

In the DVD format, the Block Error Rate, Burst Error Length, and E-flags, E11-E32, are replaced by simple counts of parity errors, PI and PO. The error detection / correction scheme used in the DVD system is still a Reed-Solomon code but it is conceptually much simpler. There is only one error detection / correction code resulting in only one specification for inner parity errors, Pl< 280/(8 ECC Blocks).