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Home > Essays > Optical Storage

Monday, June 17, 2002
Writing Quality

3. Error Correction - Page 1

Writing Quality - Page 3

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.

  • The bit-error rate (BER) is the number of bits received in error divided by the total number of bit received. An optical disc system can obtain error-correction algorithms able to handle a BER of 10^-5 to 10^-4.
  • The block error rate (BLER) measures the number of blocks or frames of data per second that have at least one occurrence of uncorrected data.
  • The burst-error length (BERL) counts the number of consecutive blocks in error.

- 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.

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