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     This story was printed from CdrInfo.com,

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Appeared on: Monday, June 17, 2002
Writing Quality

1. Introduction

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- Introduction

We have all heard the familiar clicks and skips from our CD Audio Player that result from a scratched, damaged, or dirty CD disc. Sometimes the effects of disc defects can be subtle and result in audio distortion instead of obvious click and skips. This distortion results from the inability of standard CD player's error correction codes (ECC) to correct for the ablation of the underlying audio information by a scratch defect or electrical noise.

In fact, when the CD ECC fails, many CD drives have special circuitry called "error concealment" circuitry to conceal the errors that ECC was unable to fix. The error concealment circuitry introduces distortion while it tries to interpolate through the errors. Many types if error concealment circuity will even insert audio noise when activated to mask the errors. This is hardly what lovers of high fidelity audio reproduction seek.

In order to complete this article we used information from:

  1. Calimetrics (http://www.calimetrics.com)
  2. ECMA (http://www.ecma.ch/)
  3. Kelin Kuhn's paper (http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio/95x6.htm)
  4. Clover Systems (http://www.cloversystems.com)
  5. YAMAHA Japan (http://www.yamaha.co.jp/computer)
  6. Plextor Europe (http://www.plextor.be)
  7. Pioneer Japan (http://www.pioneer.co.jp)
  8. Quantized (http://www.quantized.com)
  9. MS Science (http://www.mscience.com)
  10. Philips (http://www.licensing.philips.com)
  11. Advanced Surface Microscopy (http://www.asmicro.com/)
  12. Eclipse Data (http://www.eclipsedata.com)
  13. MMC Draft Standard (http://www.t10.org)
  14. JVC Japan (http://www.victor.co.jp/products/others/ENCK2.html)
2. Pits and Lands

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- General information about Pits and Lands

An enlarged view on top of a CD shows a picture like this.


Pit structure

In a CD-player, a laser beam with a specific wavelength detects the digital code by determining the lengths of the pits and the lands. Therefore, it is important that the shape of the pits as well as the intervals meet the necessary preconditions. In the pit structure the next basic parameters can be recognized:


Typical pit parameters

Each pit is approximately 0.5 microns wide and 0.83 microns to 3.56 microns long. (Remember that the wavelength of green light is approximately 0.5 micron) Each track is separated from the next track by 1.6 microns. The leading and trailing edge of the pits represent ones and the length of the pits represents the number of zeros. The space between the pits, called lands are also of varying lengths representing only zeros.

The CD laser 'reads' the pit-information by processing the reflected wave signals. The reflection is caused by the aluminium layer of the CD. The laser beam that is focussed on the pit track 'recognizes' the transition between pits and lands.

Thus not the pits or lands itself but the pit edges are responsible for data information. The pits are encoded with Eight-to-Fourteen Modulation (EFM) for greater storage density and Cross-Interleave Reed-Solomon Code (CIRC) for error correction.

3. Error Correction - Page 1

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Error Correction - Page 1

- Introduction

With analogue audio, there is no opportunity for error correction. With digital audio, the nature of binary data lends itself to recovery in the event of damage. When audio data is stored, it can be specially coded and accompanied by redundancy. This enables the reproduced data to be checked for error. Error correction is an opportunity to preserve data integrity, and it is absolutely necessary to ensure the success of digital audio storage. With proper design, CD and DVD can be reduced to an error rate of 10^-12, that is, less than one uncorrectable error in 10^12 bits.

- Sources of Errors

Optical media can affected by pit asymmetry, bubbles or defects in substrate, and coating defects. The most significant cause of error in digital media is dropouts, essentially a defect in the media that causes a momentary drop in signal strength. Dropouts can be traced to two causes: a manufactured defect in the media or a defect introduced during use. A loss of data or invalid data can provoke a click or pop. An error in the least significant bit of PCM word might pass unnoticed, but an error in the most significant bit word would create a drastic change in amplitude.

- Seperation of Errors

Errors that have no relation to each other are called random-bit errors. A burst error is a large error, disrupting perhaps thousands of bits. An important characteristic of any error-correction system is the burst length, that is, the maximum number of adjacent erroneous bits that can be corrected.

Error-correction design is influenced by the kind of modulation code used to convey the data. With normal wear and tear, oxide particles coming form the backing and other foreign particles, such as dust, dirt, and oil from fingerprints, can contribute to the number of dropouts. The scratches perpendicular to tracks are easier to correct, but a scratch along one track could be impossible to correct.

- Objectives of errors correction

Redundancy alone will not ensure accuracy of the recovered information; appropriate error detection and correction coding must be used. An error-correction system comprises three operations :

  1. Error detection uses redundancy to permit data to be checked for validity.
  2. Error correction uses redundancy to replace erroneous data with newly calculated valid data.
  3. In the event of large errors or insufficient data for correction, error concealment techniques substitute approximately correct data for invalid data.
4. Error Correction - Page 2

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Error Correction - Page 2

- Errors Detection - Single bit parity

All error-detection and correction techniques are based on the redundancy of data. Practical error detection uses technique in which redundant data is coded so it can be used to efficiently check for errors. The technique of casting out 9s can be used to cast out any number, and forms the basis for a binary error detection method called parity. Given a binary number, a residue bit can be formed by casting out 2s. This extra bit, known as a parity bit, permits error detection, but not correction.

An even parity bit is formed with a simple rule : if the number of 1s in the data word is even, the parity bit is a 0; if the number of 1s in the word is odd, the parity bit is a 1. An 8-bit data word, made into a 9-bit word with an even parity bit, will always have an even number of 1s.

- Cyclic Redundancy Check Code

The cyclic redundancy check code (CRCC) is an error detection method preferred in audio applications. The CRCC is cyclic block code that generates a parity check word. In 1011011010, the six binary 1s are added together to form binary 0110 (6), and this check word is appended to the data word to form the code word for storage.

- Error Correction Codes

With the use of redundant data, it is possible to correct errors that occur during storage or transmission. However, there are many types of codes, different in their designs and functions. The field of error-correction codes is highly mathematical one. In general, two approaches are used: block codes using algebraic methods, and convolution codes using probabilistic methods. In some cases, algorithms use a block code in a convolution structure known as a cross-interleave code. Such codes are used in the CD format.

- Interleaving

Error correction depends on an algorithm's ability to efficiently use redundant data to reconstruct invalid data. When the error is sustained, as in the case of a burst
error, both the data and the redundant data are lost, and correction becomes difficult or impossible. Data is interleaved or dispersed through the data stream
prior to storage or transmission. With interleaving, the largest error that can occur in any block is limited to the size of the interleaved section. Interleaving greatly increases burst error correctability of the block codes.

- Cross interleaving

Although the burst is scattered, the random errors add additional errors in a given word, perhaps overloading the correction algorithm. One solution is to generate two correction codes, separated by an interleave and delay. When block codes are arranged in rows and columns two dimensionally, the code is called a product code (in DVD). When two block codes are separated by both interleaving and delay, cross-interleaving results. A cross-interleaved code comprises two (or more) block codes assembled with a convolutional structure.

The above picture shows a Cross Interleaving encoder as used in the CD format. Syndromes from the first block are used as error pointers in the second block. In the CD format, k2=24, n2=28, k1=28 and n1=32; the C1 and C2 codes are Reed-Solomon codes.

5. Error Correction - Page 3

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Error Correction - Page 3

- Error Concealment

A practical error-correction method allows severe errors to remain uncorrected. However, subsequent processing - an error-concealment system - compensates for those errors and ensures that they are not audible.

There are two kinds of uncorrectable errors :

- Interpolation

Following de-interleaving, most errors are interspersed with valid data word. It is thus reasonable to use techniques in which surrounding valid data is used to
calculate new data to replace the missing or incorrect data.

In many digital audio systems, a combination of zero- and first-order interpolation is used. Other higher-order interpolation is sometimes used.

- Muting

Muting is the simple process of setting the value of missing or uncorrected words to zero. Muting might be used in the case of uncorrected errors, which would otherwise cause an audible click at the output. Also in the case of severe data damage or player malfunction, it is preferable to mute the data output.

To minimize audibility of a mute, muting algorithms gradually attenuate the output signal's amplitude prior to a mute, and then gradually restore the amplitude afterward. Such muting for 1 to 4 ms durations cannot be perceived by the human ear.

6. CIRC - Page 1

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CIRC - Page 1

- Reed-Solomon Codes

For compact discs, Reed-Solomon codes are used and the algorithm is known as the Cross-Interleave Reed-Solomon code (CIRC). Reed-Solomon codes exclusively use polynomials derived using finite field mathematic known as Galois Field to encode and decode block data. Either multiplication or addition can be used to combine elements, and the result of adding or multiplying two elements is always a third element contained in the field. In addition, there exists at least one element called a primitive such that every other element can be expressed as a power of this element.

The size of the Galois Field, which determine the number of symbols in the code, is based on the number of bits comprising a symbol; 8-bit symbols are commonly used. A primitive polynomial often used in GF (28) systems is x^8+x^4+x^3+x^2+1.

The code can use the input word to generate two types of parity, P and Q. The P parity can be a modulo 2 sum of the symbols. The Q parity multiplies each input word by a different power of the GF primitive element. If one symbol is erroneous, the P parity gives a nonzero syndrome S1. The Q parity yields a syndrome S2. By checking this relationship between S1 and S2, the RS code can locate the error. Correction is performed by adding S1 to the designated location.

Reed-Solomon codes are used for error correction in the compact disc, DAT, DVD, direct broadcast satellite, digital radio and digital television applications. Read more about Reed-Solomon Code over here.

- Cross-Interleave Reed Solomon Code

The particular error correction code used in the CD system is called the Cross Interleave Reed-Solomon Code, or CIRC, followed by EFM.

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.

CIRC corrects error bursts up to 3,500 bits (2.4 mm in length) and compensates for error bursts up to 12,000 bits (8.5 mm) such as caused by minor scratches.

The CIRC is designed such that the C1 code corrects most of the random errors and C2 code corrects most of the burst errors. The interleaver between the C1 and C2 codes is performed to make it easier for the C2 Reed-Solomon code to correct burst errors. Interleaving also protects against burst errors appearing in the Audio steam: A long burst due to a failed C2 creates short audio errors.

For a detailed explanation of the CIRC you can read the ECMA 130 standard.

7. CIRC - Page 2

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CIRC - Page 2

- 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.

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.

8. CD Decoding system

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CD Decoding system

The following pictures show the CD decoding system in detail.

The RF signal is sent to the EFM decoder where the EFM channel bit stream is decoded and passes to the first stage of the CIRC decoder. After the C1 decoder, the data interleave is removed and passed to the final stage of the CIRC.

If the C2 decoder data can be decoded, the data is then de-interleavered and driven to the Error concealment and after the D/A to speakers. In case of CD-ROMs the data is de-scrambled (additional interleave stage) before driven to the Error concealment circuit.

If the C2 decoder fails, it flags the errors for the error concealment circuitry where the remaining errors are concealed before being passed to the digital to analog converter and finally to your speakers. Only rarely (about once every 125 hours), is the CIRC decoder in unable to identify and flag an error for the concealment circuitry. When this happens, an audible click is often heard.

The digital audio stream is error-free unless the CIRC determines that a C2 codeword is contaminated by an error pattern that it cannot correct. If the 24 bytes were flagged wrong, then it would turn 6 samples wrong at once, on both channels. If all the audio data that lie adjacent to the audio samples in the uncorrectable C2 codeword are reliably recovered from the disc, then the values of the un-reliable audio samples are interpolated.

In case of Data discs, CD-ROM chipsets use the C2 error flag to mark data going into ECC layer 3 so that they can use erasure correction for improved performance, similar to the C1 flag being used to mark erasures for C2.

A more detailed explanation of the various strategies used in the C1/C2 decoders follows (source: Digital-Inn):

"…Assuming there are no input flags from the EFM decoder (the normal situation), C1 can detect and correct up to 2 symbols (bytes). However the exact performance is determined by the strategy employed. The two basic strategies are (a) correct 1 symbol or (b) correct two symbols.

In (a), the decoder with detect and correct a frame with one errored symbol. It will accurately detect frames with 2 or 3 errored symbols. It will mostly but not always detect frames with 4 or more errored symbols (see later). For frames with one errored symbol the flags will all be set to definitely correct. For frames with 2 or 3 errored symbols, there are two possible strategies; either marks the detected errors as possibly incorrect or mark the whole frame all possibly incorrect. The second possibility is usually employed mainly due to accuracy; basically it would be as inaccurate as the 2 symbol scheme without the increased correction capability. For frames with 4 or more errors, all symbols in the frame will be flagged as possibly incorrect as there is no way of differentiating the correct from the incorrect symbols.

In (b), the decoder will reliably detect and correct frames with one or two errored symbols. Frames with 3 errored symbols will either be correctly detected (generally resulting in the whole frame being marked as unreliable) or will be misdetected as having 2 errors and will be incorrectly corrected resulting in 5 unflagged errors being passed to C2. For 4 errors, there is a slightly possibility (2^-19) that C1 will misinterpret this as 1 error again resulting in 5 undetected errors being passed to C2 (the same as the 1 correction strategy), otherwise 4 or more errors will be detected and the whole frame will be marked as unreliable.

C2 uses the original information and the flags to try to detect all errors undetected or miscorrected by C1; here the strategies become more divergent. Unfortunately the manufacturers are not giving this information away.

There are other strategies (including multiple pass ones) and it is also possible to apply many strategies at once and then decide which to use based on the results…"

9. C1/C2 Errors - Page 1

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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:

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

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).

10. C1/C2 Errors - Page 2

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C1/C2 Errors - Page 2

- E32 errors

There are some drives that can correct even up to four bad symbols at the second stage. However for the majority of the tests, we consider E32 an uncorrectable error, even though some drives may be able to correct it. The quality of our recorded media will decrease with regular use and aging. Discs with already E22 or E32 will not leave adequate margin for future degradation.

Uncorrectable errors may not be a problem on audio discs, since the player's interpolation circuitry will hide these errors. Some players can interpolate over up to eight consecutive bad samples.

Uncorrectable errors do not necessarily make a CD-ROM unusable either. Errors that are uncorrectable in the main CIRC correction stage may still be corrected by the EDC/ECC sector level error correction used on CD-ROM's. Therefore, the data may still be recoverable, and can still verify if you are comparing to the original. Of course a disc like this has no tolerance for additional degradation, such as scratches and fingerprints, so access time will increase and it will soon fail.

- Testing Speeds

Some error rates may be higher at faster speeds, and others will be higher at lower speeds. In general, error rates on good discs will be about the same at higher speeds as at 1X. Small errors such as E11 & E21 will not be affected much. Burst errors, on the other hand, will be greatly affected. Most burst errors (E22 & E32) are caused by disturbances to the servo systems, rather than missing data. This effect is greatly magnified at high speed.

Error on a disc is not a physical thing. It is a manifestation of how well the total system (disc + player) is working. The disc itself does not have an error rate; playing the disc produces errors. Ideally, what you want is a disc that will play back on ALL players with a low error rate. Unfortunately, there are no standards for players, only for the discs. Therefore, each type of player will give different results.

Most testing equipment manufacturers believe that "…1X testing: This is still the best way to isolate the disc characteristics from the player influences. When we test discs at 1X, we can judge the disc, rather than the player…"

- Limits of Errors

The Data Channel Error rate must be as lower as possible.

The "Red Book" specification (IEC 908) calls for a maximum BLER of 220 per second averaged over ten seconds. The CD block rate is 7350 blocks per second; hence 220 BLER errors per second shows that 3% of frames contains a defect.

Discs with higher BLER are likely to produce uncorrectable errors. Nowadays, the best discs have average BLER below 10. A low BLER shows that the system as a whole is performing well, and the pit geometry is good. BLER only tells you how many errors were generated per second; it doesn't tell you anything about the severity of those errors. Therefore, it is important to look at all the different types of errors generated. Just because a disc has a low BLER, doesn't mean the disc is good.

For instance, it is quite possible for a disc to have a low BLER, but have many uncorrectable errors due to local defects. The smaller errors that are correctable in the C1 decoder are considered random errors. Larger errors like E22 and E32 are considered burst errors and are generally caused by local defects. As you might imagine, the sequence E11, E21, E31, E12, E22, E32 represents errors of increasing severity.

A Burst Error is defined as seven consecutive blocks in which the C1 decoding stage has detected an error. This usually indicates a larger scale defect on the disc, such as a scratch. In general, new discs which have not been handled on the read surface should not exhibit any burst errors. A Burst Error constitutes a disc failure. That's why many testing equipment offers a "Digital Error Mapping" for quick viewing C1, C2 and CU errors:

11. EFM - Page 1

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EFM - Page 1

The data stream must undergo CIRC error correction and EFM modulation to reduce the possibilities of creating an error.

- Whats is EFM?

EFM (Eight to Fourteen Modulation) is a method of encoding source data for CD formats into a form that is easy to master, replicate and playback reliably. EFM modulation gives the bit stream specific patterns of 1s and 0s, thus defining the lengths of pits and lands. EFM permits a high number of channel bit transitions for arbitrary pit and land lengths. The merging bits ensure that pit & land lengths are not less than 3 and no more than 11 channel bits. This reduces the effect of jitter, distortions on the error rate, increases data density and helps facilitate control of the spindle motor speed.

- How EFM is performed?

Block of 8 data bits are translated into blocks of 14 channel bits. The 8-bit symbols required 2^8=256 unique patterns, and of the possible 2^14=16,384 patterns in the 14-bit system, 267 meet the pattern requirements; therefore, 256 are used and 11 discarded.

Blocks of 14 channel bits are linked by the three merging bits to maintain the proper run length between words, as well as suppress dc content, and aid clock synchronization. The digital sum value (DSV) is used to monitor the accumulating dc offset. The ratio of bits before and after modulation is 8:17.

The resulting channel stream produces pits and lands that are at least two but no more than ten successive 0s long. 3T, 4T…11T where T is one channel bit period. The pit / land lengths vary from 0.833 to 3.054 µm at a track velocity of 1.2 m/s, and from 0.972 to 3.56 µm at a track velocity of 1.4 m/s.

- Why using 14-bit system?

Using 14-bit symbols allows up to 2^14, or 16384, 14-bit combinations. This provides bit patterns that have a low number of transitions between 0 and 1. Using 8-bit symbols would require too many pits due to the large number of transitions. The 14-bit symbols used in EFM are taken from lookup tables, which are defined by the Red Book specifications. However, only patterns in which more than two but less than ten 0s appear continuously are used. If the size of the data gets 17/8 times bigger, the number of bits per pit is at least 3 times bigger. Thus at the end, more data can be recorded on the CD in spite of the 17 bits used instead of 8.

Below are some examples of 8-bit data symbols and their 14-bit equivalent.

Once the 14-bit symbols are put together, it is possible that the bits between the two symbols will create an illegal bit pattern. For example, joining the last and first bit patterns in the examples above would create an illegal bit pattern since a minimum of two 0s must be recorded continuously.

In such cases, Merge Bits must be used between the two 14-bit symbols in order to satisfy the specifications. The following picture shows a bit pattern and the pits that it would produce. The areas between the pits are called "lands".

If the source binary data were recorded without encoding in this way, the disc would frequently need to represent a single '1' or '0' requiring mastering and replication to reproduce very small artifacts on the disc. EFM encoding ensures that the smallest artifacts on the disc is three units long and the average artifacts is seven units long.

Within the EFM lookup table, it is possible for the 14 bit code to start or end with a '1'. In order to prevent a situation in which one 14 bit code ends and the next one both start with a '1', three merging '0' bits are added between each 14 bit code. So in reality, EFM is eight-to-seventeen modulation.

12. EFM - Page 2

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EFM - Page 2

- RF Signal

The RF signal from the disc contains all data, and is also used to maintain proper CLV rotation velocity of the disc. 3T describes a 720 KHz signal, and 11T describes a 196 KHz signal at 1.2 m/s. T refers to the period of 1 channel bit which is 464 ns. A collection of EFM waveforms is called the "eye pattern". Whenever a player is tracking data, the quality of the signal can be observed from the pattern.

Below we can see a picture of showing the various stages from the original PCM signal up the eye pattern. The modulated EFM signal is read from the disc as a RF signal. The RF signal can be monitored through an eye pattern by simultaneously displaying successive waveform transitions.

- Demodulation

During the demodulation, the "eye pattern" from disc is converted to NRZI (Non Return to Zero Inverted) data coding, then is converted to NRZ then to EFM data and finally to the Audio data. Following demodulation, data is sent to a Cross-Interleave Reed-Solomon code circuit for error detection and correction.

Eye Pattern

The eight-to-fourteen (EFM) modulation scheme used produces just nine different possible lengths of pits and lands. Therefore, the resulting HF consists of square waves of nine different durations. The signal appears sinusoidal on the 'scope because of the limited frequency response of the optics. It is common to display the photo detector output on a scope with a conventional trigger.

This results in a display where the nine possible frequencies (3T to 11T) all add up on top of each other. This type of display is termed an "eye pattern" and provides valuable information about the various alignment parameters of the CD player. Notice that the relationship between size and wavelength is very distinct in the eye pattern.

The RF output is converted to a square wave, and then phase locks a clock with the period T. Each of the nine pit/land lengths are exact multiples of one fundamental length, called 1T.

The T represents the data clock period, which is 231 nanoseconds at 1X read speed. The particular run length is referenced by nT where n is an integer multiple of the clock period. The nine possible lengths of pits and lands are 3T, 4T, 5T, 6T, 7T, 8T, 9T, 10T, and 11T. The waveforms generated by these pits & lands are called I3, I4, I5, I6, I7, I8, I9, I10 and I11 respectively. I3 represents the shortest pit / land, and I11 represents the longest pit or land.

13. Jitter - Page 1

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Jitter - Page 1

- Introduction

Along the 5km of track in a typical CD, there are bright and dark marks ranging in size from about 0.8mm to 3.1im in length. Without any defects or noise, these sequences of bright and dark marks contain all the information necessary to play back the original recording.

To read these sequences, a laser is focused on to the track and a photo detector (PD) measures the amount of light reflected back from the track. The RF signal from the PD represents the reflectivity of a small region of track. An example of the marks in the track and the RF signal are shown in picture below. To determine the pattern of marks on the discs, a clock is generated off the data and a threshold is set for the RF signal.

If the RF signal crosses the threshold, a "1" is entered for that time bin, otherwise a zero is entered. This binary bit stream represents the "EFM" code, the first of the three primary codes used in the CD system (EFM, C1, and C2). If the RF signal contains noise, the time of the threshold crossing can be too early or too late. The average amount of time there threshold crossings move around with respect the time bin is called "Jitter".

If the "Jitter" is large enough, the threshold crossings will be assigned to incorrect time bins and result in errors introduced into the EFM bit stream. While the next two codes in the CD system protect against these types of errors, it is possible for jitter to appear at your speaks as distorted audio if these errors go uncorrected.

- History Of Jitter

Jitter first appeared as a required measurement for optical disc in the Orange book. Subsequently it was amended to the Red Book. The Red Book stipulated that the measurements specified must be performed on a single beam drive. This relates to the method of tracking and was the early standard for consumer players. Early tester equipment was based on the Philips CDM1 or CDM4 mechanisms, both single beam. However, by the time the jitter amendment to the Red Book was ready, most drives were triple beam.

Research revealed that there was low correlation between single and triple beam measurements of jitter. The standard for jitter measurement became another Philips drive, the CDM12, which was triple beam.

Data to data jitter had a drive dependency due to the method of measuring pit and land lengths in time. A drive contributed variations from its electronics and optics. This was one of the main reasons that drive selection was important to CD test equipment manufacturers: they had to try to constrain the range of variation by selecting the drives they used. When a consumer drive looks at a disc its job is to get the best out of that disc regardless of what the disc is like. Hence optimising focus, tracking or slicing is all part of the strategy.

14. Jitter - Page 2

Writing Quality - Page 14

Jitter - Page 2

- What causes Jitter?

There are three possible causes of Jitter:

a) The recorded pits are not perfectly accurate in terms of size. The cause of that are the laser noise and the recording strategy of the recorder. Even if the pits were perfectly recorded and replicated, there would still be jitter. This is because of the limited resolution of the pickup in the player.

b) There is the influence of other pits nearby in the same track. The readout spot is broad enough that when the centre of the spot reaches the beginning of a short pit, the end of the pit lies within the fringes of the spot. So the apparent position of the one pit end is slightly dependent on where the other end is. The same applies to short lands. This is called inter-symbol interference. The jitter which arises from this is not truly random, but is associated with the pattern of recorded pit and land lengths.

c) There is crosstalk between pits in adjacent tracks, because the readout spot does not fall wholly on one track. It is a largely random contribution. It is worse at lower recorded velocities, because the highest frequency components of the readout signal in the wanted track, with which the crosstalk is competing, are weaker.

- Why Measure Jitter?

Jitter is important because the CD information is carried in the edges of the pits. A transition between pit and land (either pit to land or land to pit) is a one, and everything else is a zero. Since the data is self-clocked at a constant rate, if the edges of the pits are not in the correct places, errors will be generated.

If a transition between pit and land comes as little as 115 ns early or late, an error will result. A standard deviation of less than 35 ns will result in only about 1% probability of the effect lengths varying by more than 115 ns. In addition, any types of disc defects will affect the jitter, making this a sensitive test of disc quality. Pit distortion, crosstalk from adjacent tracks, intersymbol interference, pit wall steepness, low signal-to-noise ratio, and LBR (or CD-R writer) instability can all cause jitter on the disc.

15. Jitter - Page 3
Writing Quality - Page 4

Jitter - Page 3

- Measuring Jitter

Pit geometry and Jitter can be measured by looking at the HF signal coming from the pickup. The HF (High Frequency) signal coming from the pickup represents the light intensity of the beam reflected back from the disc surface. A higher voltage represents greater light intensity, and a lower voltage represents less light. The signal is rapidly changing between light and dark as the beam passes over the pits. When the beam is over a pit, the light intensity is reduced. When the beam is between pits (over "land"), the light intensity is higher. Data is encoded in the transitions between pits and lands.

The waveform shown below shows "Jitter" as regenerated from an optical disk. "Jitter" values and limits are specified in CD-ROM and DVD published industry standards. Jitter is made up primarily of two phenomena. One is the measurement of the time difference between the data and clock and the other is the measurement of pulse width (pulse cycle measurement for Magnetic Optical drives). The pulse width measurement is used to evaluate disk media.

Because the oscilloscope is triggered by the signal itself, we can see the jitter in the different pit lengths (such as 3T, 4T…11T) separately. The other thing we can see is the "deviation" for each pit length. This is a measure of how different a given pit length appears to be from what it should be. So, for example, the usual tendency is for the shortest pits (the 3T pits) to appear a little shorter than 3T. Jitter and deviation are two aspects of the same thing: jitter is the random variation, and deviation is the average error, in the apparent length.

16. Oscilloscope

Writing Quality - Page 15

Oscilloscope

Generally, HF measurements are easier to make on an analog oscilloscope. This is because an analog 'scope shows a multitude of sweeps superimposed, whereas a digital 'scope shows only a "snapshot" of one brief sample. The "Eye Pattern" is what you see on an analog scope. You cannot see this eye pattern on a digital scope. Instead, you see just a sampling of a few pits and lands. A digital 'scope is good for close inspection of individual pits, but what we are interested in is the average values over many pits.

Jitter is a statistical measurement of the variation in pit or land length around the mean value for each run length. For each pit and land length a large number of lengths are measured and the standard deviation is reported.

If the standard deviation is high (greater than 35ns) this can cause instability in the clock and data decoding circuitry on target players and hence reduce the readability of the disc. Once again this parameter is more important when considering CD-ROM discs as these discs are likely to be read in high-speed players.

17. Jitter at DVD

Writing Quality - Page 17

Jitter at DVD

For the CD format Philips maintains industry's Reference Standard measurement system, "RODAN," or "Read-Only Disc Analyzer." RODAN is kept in Paris. In the DVD industry, there has been no appointment of a "DVD Paris Meter." Instead, a collection, or sub-group, of fundamentally standard Pulstec DDU-1000 analyzers has been used to produce the multi-point calibration disc sets whose values are based on the sub-group averages for the key parameters. Both approaches, if executed properly, can be effective. Most testing equipment vendors using Pulstec SDP-1000 drive for DVD Jitter measurements.

The DVD specification includes a standard for jitter, which is not found in the CD specification. The logical format specifies disc mechanical and optical characteristics, but specifications for sources of degradation, like inter-symbol interference introduced in the disc cutting step, are not included in the current CD specification.

There are also items which cannot be expressed by parameters listed in the current CD specification, such as degradation introduced by the mastering machine or unevenness in pit replication. These effects can't be ignored in DVDs with their higher recording density, and thus it is necessary to specify such factors. There was an error rate specification in the CD specification, but this is impossible to measure unless there are defects or degradation due to the playback device.

The disc specification for Pioneer's Karaoke System specified an error rate using a tilted pickup, but this was a difficult measurement to make, and certainly not efficient. In the DVD specification it was decided to add a jitter specification, as jitter is a parameter where degradation can be measured numerically. Jitter is measured in the absence of tilt, which is an ideal disc specification, but it isn't practical to compensate for tilt at all points. Since the effect of jitter due to the varying component of surface tilt is small, it was decided to measure across one full revolution and measures the average value of tilt variation. As a result, it is only necessary to compensate for the average radial tilt when taking measurements.

18. Technologies for Reducing Jitter

Writing Quality - Page 18

Technologies for Reducing Jitter

Because of the effects of jitter can make it to your speakers, there are growing number of jitter reduction technologies of the creation of recordable CD audio discs including Yamaha's "Audio Master Quality Recording" and Plextor's "VariRec" for CD recordable discs, as well as JVC's "K2 Laser Beam Recorder" for mastering CD-ROMS.

Yamaha's Audio Master Quality Recording technology increases the window margin by lengthening the EFM marks along the track. While the actual jitter of the RF threshold crossings as measured in nanoseconds does not change, lengthening the EFM marks has the effect of lowering the relative jitter as measured as a percent of the channel clock period. The reduction of relative jitter by this technique results in a lower capacity disc (63mins vs. 74mins.), but reduces the need for activation of the error correction concealment circuitry and ultimately results in less audio distortion.

Similar EFM mark lengthening and associated relative jitter reduction can be obtained using a disc such as Kodak's 63minute Photo-CD (perhaps without also giving up recording speed, multiple sessions and "SafeBurn". There 63minute discs are made by increasing the physical length of one cycle of the wobble (the wobbled track provides the reference for spindle speed control). This lower relative wobbles frequency results in the drive spinning the disc faster which makes the marks longer.

19. JVC ENC K2

Writing Quality - Page 19

JVC "ENC K2"

The following information is taken directly from JVC's website. Since this technology is used only in pressed discs is presented as was originally posted. For more more information visit JVC's Japanese website.

Victor Company of Japan, Limited (JVC) and Victor Entertainment, Inc., at 6th of November 2002 announced the co-development of "Encode K2" (ENC K2) technology that according to JVC brings dramatic improvements in sound quality to formats such as CD and DVD Audio during the manufacturing process. The development of ENC K2 has been made possible by the application of JVC's proprietary "K2 Technology" during the CD format encoding process.

JVC will employ the technology on CCCDs to be released on November 13, 2002, and display the logo marks on the left as a guarantee of a musical presence virtually identical to that of the original master tapes (http://www.jvcmusic.co.jp/cccd).

In the quest to convey "the truth of the music" accurately, the two companies have worked together for years to develop technologies that improve the sound quality of disc media, aiming to achieve faithful reproduction on CD of the musical quality of original master tapes.

Based on conventional K2 Technology, ENC K2 reproduces a musical presence virtually identical to that achieved on the original master tapes. ENC K2 technology eliminates any artifacts that might alter or degrade sound quality from the digital signal transmission and occur during the CD format encoding process, such as distortion and noise.

- Background information

Theoretically, the quality of digital audio should not change as long as codes remain constant. In fact, however, it is modified during digital signal transmission.
The CD cutting process used when manufacturing CDs usually includes three processes: "Reproduction of Master Tapes," "CD Format Encoding Process," and "Laser Cutting Process." Most recently, PC software and copy control functions have been introduced as a part of the "CD format encoding" process.

ENC K2 was developed to eliminate the artifacts that alter sound quality from digital signal transmission from input to output during the "CD format encoding" process; a process which is becoming increasingly multi-functional. (See Chart 1 on the next pages.)

The two companies have jointly developed and introduced technologies to improve music quality based on their proprietary K2 technology, including "Digital K2" which eradicates alterations in the audio quality of master tapes and "K2 Laser Cutting," (K2 LC) which improves the precision and purity of laser cutting during the CD cutting process.

With the development of ENC K2, the companies have established the Full Code Transfer System at all processes during the cutting process in CD manufacturing. (See Chart 2 on the next pages.)

- "K2 Technology" and "ENC K2"

Theoretically, the quality of digital audio should not change as long as codes remain constant. In fact, however, it is modified during digital signal transmission. This occurs due non-code components, such as "ripple" and "jitter," which exist in digital signals. "K2 Interface," the first version of the two companies' proprietary K2 technology, was developed in 1987. This technology instantly reads only "1" and "0," logical codes from all digital signals including those that change the music quality, then recreates new digital signals. With this technology, the companies succeeded in eliminating any quality modifying factors from digital signal transmission. JVC has successfully inserted "ENC K2" in the CD format encoding process by adapting "K2 Technology" to EFM and controlling data synchronization.

The companies realized the "Master Direct" concept, in which every CD faithfully reproduces the music quality of master tapes, by establishing the "Full Code Transfer System." This system removes the factors that alter music quality during the three most important CD cutting processes: "Reproduction of Master Tapes," "CD Format Encoding Process," and "Laser Cutting Process."

20. AudioMASTER

Writing Quality - Page 20

AudioMASTER

The theory behind this entire "new" recording mode is relatively simple. According to the original red-book standard, the linear velocity of the laser beam, at 1x reading speed, over the CD's surface is allowed to vary between 1.2 and 1.4 meters per second. Anything between these extreme values is acceptable and all CD players, even the most archaic ones, are expected to be compliant when a disk is recorded at such a speed. This speed might even be non-constant for a single disk, as long as its variation is limited within certain bounds prescribed by the above standard.

As some of the older among this audience might remember the first recordable disks where of a lower capacity, in the area of 63 minutes. Remember this, as we will move on to our technical explanation below.

Consequently, when recording takes place at a higher linear speed, the length of the pits is proportionally greater. In particular, at 1.4m/s, this length is about 15% greater than the length of the same pit at 1.2m/s.

In real life, the pit length depends on several factors and matches the theoretical figure within a certain amount of uncertainty (error). During a disk reproduction, this type of errors is reported as "jitter". In "digital" terms, these errors are reported as C1/C2 errors.

Using a photo detector on the analog signal that passes through the laser diode on the pickup mechanism of a drive, increased jitter is seen as more "blurring" in the following pictures taken from a Yamaha white paper:

Audio Master Quality Recording
Conventional Recording

It has been found by laboratory experiments that this uncertainty is mostly irrelevant to the pit length. So a 15% longer pit contains about 15% less pit-length uncertainty! This is the crucial point in all of our discussion here. That is: a 15% recording/reading speed increase results to about 15% less errors in the pit-length.

A somewhat simplistic argument might suggest an analogous decrease in jitter. In fact, the Yamaha marketing department seems to take into account the uncertainty on both sides of a pit and claims (an erroneous according to our opinion) 30% jitter decrease when recording at a 15% increased linear speed. Overall, this new Yamaha "discovery" seems to make use of an old recording mode that was presumably in use several years ago during the initial market introduction of commercial recorders.

21. VariREC

Writing Quality - Page 20

VariRec

comes from the "Variable Recording" words and allows the change of the laser power when writing CD-DA or CD-R at 4X. (TAO or DAO). Users are allowed to make slight adjustments to the default value (0). This will change the quality of the sound and will also increase the playability or compatibility (or in extreme cases in-compatibility) with the existing players. Plextor says that the default setting (0) already reflects to the optimized laser power with the lowest jitter.

VariRec can change the value of the laser power. The resulting effects are:

Most audio professionals have a personal preference for higher/lower laser power (some even say they can hear the difference between a recording at high speed and at 1X). When VariRec is used to the most extreme settings, there is a chance that the playback device cannot read the disc properly. In this case, use the default setting or switch VariRec off.

Users may compare Plextor's "VariRec" towards Yamaha's "AudioMASTER" system. We asked Plextor the same question and the answer was that "...VariRec is not an answer to any technology of any other manufacturer. The write quality of Plextor recorders is already much better, several tests have proven this. However, the idea that VariRec is more an option to 'tune according to personal taste'..." doesn't seem to un-veil the whole truth. Both "AudioMASTER" and "VariRec" uses 4x-recording speed, and both technologies promise a reduction of jitter and better AudioCDs.

A good question here is whether if we can listen to such slight changes of the sound, since the sound digital signal is processed through many DA/AD processes and circuits before the analogue audio playback. Some people claim that they can hear to such changes, probably with high-end systems…The addition of such specialized technologies for AudioCD recording is reasonable for user who may needed them. I think most of you have forgotten how many minutes it takes to write a full CD at 4x ;-)

22. TEAC Boost Function

Writing Quality - Page 21

TEAC's Boost function

TEAC offers a similar technology to VariRec that "boosts" the laser power in order to reduce Jitter. As TEAC explains "…The write state will be changed by varying the laser power and its radiation duration of the drive. As a result, the sound quality or read performance may fluctuate depending on the congeniality of the media and the drive. The change may work better or worse. It is impossible for TEAC to make verification as it depends on the congeniality of the media and the drive. Changing the settings is your own responsibility…"

The CD-W540E tuning tool offers that option, under the "CD-R(4X) Strategy" function. These settings function only for the CD-R media and only when the write setting is 4X speed inside the CD-R software. Below are explained the two 4X writing modes:

* Normal is selected in the default settings.
* Note in advance that some effects may not be noticeable or a side effect such as increasing errors may result when Boost is selected.

23. Testing Equipement - Page 1

Writing Quality - Page 23

Testing Equipement - Page 1

The official testing equipment that sold can be divided in two major categories.

Normal modified drives

The low end testing equipment is using either a CD-ROM or a CD-R/RW drive. The most used CD-ROM comes from Plextor (PX-40TS). This kind of equipment only can measure recorded CDs, while testing equipment based upon CD-RW drives can measure also blank CDs. Some manufacturers are using Plextor's PX-W1210S or TEAC CD-W512S CD-RW drives. The modification is not exactly known (and kept secret for obvious reasons); probably an extra circuit is added to intercept signals from the optical drive (E11, E21, E31, E12, E22, and E32).

Each manufacturer has developed software that calculates the average/maximum of BLER, C1, C2, Jitter and other measurements and draws graphs of each measurement:

High-end drives

High-end solutions are custom build-in drives with special components. This ensures maximum readability but also maximum price. Most high-end drives are based upon Philips (and Pulstec) optical pickups. There are three types of Philips optical pickups:

a) CDM24
b) CDM4, single-beam, linear polarized
c) CDM12 three-beam pickup

For DVD format, manufacturers seem to select either normal modified DVD±R/RW drives or drives based upon Pulstec pickups. Currently there are drives based upon Pioneer DVR-201 (Authoring 635nM) or Pioneer DVR-A03 (General 650nM) and at RICOH DVD+RW5125A (for the DVD+R/+RW format).

Especially for Jitter measurements, the optical response of the pickup must be up to a uniform standard. The Red Book does specify the optical characteristics (wavelength, numerical aperture, the illumination of the aperture, polarisation, and wavefront quality) of the pickup for jitter measuring purposes - although the polarisation specified is different from the one used for other Red Book measurements! - But it does not say anything about ordinary players.

Measuring systems do in practice use the same types of pickup as domestic players, but some care must be taken in choosing them, or the results could be pessimistic. Also it must be remembered that when we test discs against the given specification, we are really supposed to be testing the jitter attributable to the disc alone, after eliminating any contributions due to imperfections of the players.

24. Testing Equipement - Page 2

Writing Quality - Page 24

Testing Equipement - Page 2

- Categories

Testing equipment can be found for:

- CD (pressed media)
- CD-R/RW media
- CD Stamper media
- Disc Balance Checker
- DVD-ROM media
- DVD-RAM media
- DVD±R/RW media

- Manufacturers

AudioDev (http://www.audiodev.com/)
Audio Precision (http://www.audioprecision.com)
Aeco (http://www.aecogroup.com/)
CD Associates (http://www.cdassociates.com/)
Clover Systems (http://www.cloversystems.com)
Cube Tec (http://www.cube-tec.com)
DaTARIUS (http://www.datarius.com/)
Dr. Schenk (http://www.drschenk.com/)
Eclipse (http://www.eclipsedata.com/)
Pulstec (http://www.pulstec.co.jp/Epulstec/)
Reflekt Technology (http://www.reflekt.com/)
Quantized Systems (http://www.quantized.com/)
Sony Precision Technology (http://www.sonypt.com/)
Stagetech (http://www.stagetech.se/)

- Testing services

AudioDev (http://www.audiodev.com/)
MS Science (http://www.mscience.com)
Philips (http://www.licensing.philips.com)
Sony (http://www.sony.co.jp/en/Products/Verification/)

- Testing software

Eclipse Data (http://www.eclipsedata.com)

- Testing Standards

Philips has a testing methodology for measuring CDs called "CD Reference Measuring Methods". A description of the way to measure the physical parameters of a CD in a standardized way. Its price is $200. More information about measuring writing quality can be found at: Philips/Sony Book Standards, Orange/Red/Yellow/Blue/White Books, ISO/IEC 908, ISO/IEC 10149, and ECMA 130

25. Calibration media

Writing Quality - Page 25

Calibration media

In order to ensure perfect performance and reliable test results, test players have to be regularly calibrated. Over time players get less accurate because of several influences and effects on electronics and other components.

Some examples:

Philips test disc SBC444A

This disc has built-in defects that can be used for checking error rates. In addition to providing known errors, it tests the player under maximum stress.

On disc SBC 444A, certain results may not be perfectly repeatable because the disc stresses the player to its limits. For instance, the number of E32 errors generated by the Black Spots in particular may vary. This is because these errors are "soft errors." That is, they are caused by disturbance to the player's servo systems, rather than loss of data. Each time the disc is played, the disturbance is slightly different, and the results cannot be predicted.

Disc SBC 444A provides two kinds of defects: Missing information, and black spots. The tracks with missing information should provide fairly repeatable results since these errors are encoded into the data. The sections with Black Spots have the information in tact, but obscured by the black spots. In this case, not only is there information lost, but the servomechanisms are stressed. For example, when the readout beam encounters the black spot, focus, track following, and clock recovery servo signals disappear. After the beam has passed the black spot and the signal is restored, the pickup is out of focus, off track, and the bit clock is at the wrong frequency. This causes many additional errors to be generated in an unpredictable way.

Test Signal Disc 5B.3

The HF Test Signal Disc 5B3 is a calibration disc designed for CD disc and player manufacturers and manufacturers of CD test equipment. The disc costs $100.

Multi-point Calibration CD Disc set

The discs are intended to be used for calibration of CD test equipment. The Multi-point Calibration CD Set contains three (3) discs:

Disc 1: Predefined BLER, HF measurements.
Disc 2: Predefined radial noise, predefined radial acceleration.
Disc 3: Predefined jitter and effect length.

This three disc Philips Multi-Point Calibration Set is available from Philips Consumer Electronics, Co-ordination Office Optical and Magnetic Media Systems, Building SWA-112, PO Box 80002, 5600 JB Eindhoven, The Netherlands (fax +31 40 2732113). Cost of the complete set is US$750, Disc 1 only is available for US$275, Disc 2 costs US$325, and the price of Disc 3 is US$300, plus shipping charges. Each disc contains multiple test points recorded on a dedicated track. Tracks are separated by gaps.

26. Tests before recording

Writing Quality - Page 26

Tests before recording

Not all testing equipment can measure un-written blank media. In general there are three types of tests. For detailed explanation visit Quantized (http://www.quantized.com).

Pre-Groove Integrity Tests

Blank CD-R Servo Parameters

Blank CD-R HF Tests Parameters

Below are listed the initial and the explanation of various measurement tests for blank CD-R/RW media:

27. Tests after recording

Writing Quality - Page 27

Tests after recording

In general there are three types of tests. For detailed explanation visit Quantized (http://www.quantized.com).

Data Channel tests

Those 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 that category there is only the BLER measurement.

Servo tests

Those tests are generally indicating the trackability of the target disc. This indicates whether there are any problems with the overall track geometry which are likely to cause playability problems. In that category we can find the following measurements:

High Frequency tests

Those tests examine the read signals form the test player laser pickup. The nature of these signals indicates the overall pit structure on the disc is usually a good indicator of disc playability. In that category we can find the following measurements:

Below are listed the initial and the explanation of various measurement tests for written CD-R/RW media. Note that not all measurements are described in the CD specifications. Several testing equipment manufacturers, using the standard standards, created new measurements that describe in a better way, according to them, the condition of a written disc:

28. Atomic Force Microscopy

Writing Quality - Page 28

Atomic Force Microscopy

Despite the fact that the industry checks the writing quality of stampers with the use of CD/DVD analyzers, there are alternative ways with the use of Atomic Force Microscopy! This kind of technology is mostly used not for written media, but mostly during the development of dye layer or for testing the stamper quality.

Pit fidelity is a determining factor for the playability of a disc and the error rate of the disc. The final CDs produced for sale are not free of defects. Defects in the pit surface may be caused by mastering errors, stamper defects, and manufacturing (molding) defects. An elaborate error correction scheme with data redundancy is utilized to allow the compact disc to offer robust performance in the presence of some errors. Atomic Force Microscopy (AFM) is a tool for manufacturers to identify defects and their causes on both the stamper and disc surfaces.

Optical disc manufacturers continue to push for faster cycle times to increase capacity while maintaining disc quality. This creates the need for improved methods to analyze the quality of the disc and stamper surfaces. AFMs are ideally suited to the characterization of nanometer sized pit and bump structures in CD and DVD manufacturing.

AFM provides quantitative, three dimensional imaging of the disc or stamper surface within minutes. Similar quantitative information is possible using SEM or TEM, but these techniques are destructive, time consuming, measure in only two dimensions, and provide a limited field of view. Another major advantage of AFM over other techniques is that once an image is captured, cross sections can be obtained in seconds to provide pit depth, pit width, pit side-wall angle, and track pitch anywhere in the data set - and without physically damaging the disc.

For pit characterization with AFM, the discs may be examined after molding and before metallization. Representative three-dimensional AFM images of CD and DVD stamped discs (replicas) are shown below:

Both images were obtained at the same lateral magnification; the increased pit density and reduced track pitch for DVD are readily apparent. The types of measurements which AFM can provide for CD and DVD characterization are listed in the below table:

Pits (disc)
Bumps (stamper)
Tracks
Depth
Height
Pitch
Width
Width
Left/right sidewall angle
Left/right sidewall angle
Roughness of pit floor
Roughness of bump surface

CD and DVD measurements readily accessible by AFM. Note that the position at which the width is measured will have to be determined depending on the ability of the tip to measure sidewall angles accurately.

The corresponding images below show changes in the CD pit structure which accompany both the observable (by eye) staining and BLER measurements.

In Figure a, the pit structure is uniform and replicates the stamper accurately.

In Figure b, the pits are locally deformed with polycarbonate piled up or smeared toward the perimeter of the disk. This deformation is manifested as visible staining and can lead to high block error rates depending on degree.

In Figure c, the polymer surface is severely distorted and some of the pits are barely recognizable.

In the lands (surfaces surrounding pits), the polymer appears to have stuck to the stamper during disc ejection leading to severe deformation. Note that the molding conditions used here to demonstrate severe staining are atypical; area [C] in particular represents surface deformations which are rarely seen in actual disc production.

- Manufacturers

- Digital Instruments (http://www.di.com and http://www.veeco.com)
- Advanced Surface Microscopy (http://www.asmicro.com/)



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