Cross-Interleaved Reed Solomon Coding

In the Compact Disc system, error correction and detection is provided by CIRC (Cross-Interleaved Reed-Solomon Code). CIRC adds to every three data bytes one redundant 'parity' byte. Reed-Solomon codes are specifically useful in combating mixtures of random and burst errors. 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) that may be caused by minor scratches.

Notable characteristics of CIRC are

  • high random error correctability
  • long burst error correctability
  • in case burst correction capability is exceeded, we still have good concealment possibility
  • simple decoder strategy possibility with reasonable sized external random access memory
  • high efficiency (rate)
  • possibility for future introduction of four audio channels without major changes in the format.

The errors found in the CD system are a combination of a random and bursty character, and in order to alleviate the load on the error control code some form of interleaving is required. The interleaving scheme is tailored to the specific requirements of the Compact Disc system. In particular, the adopted 'cross'-interleaving technique will make it possible to effectively mask errors if correction is found to be impossible. Depending on the magnitude of the error to be concealed, this can be done by interpolating or by muting the audio signal. If a large error has occurred and a single audio sample cannot be reconstituted by the error control circuitry, it is possible to obtain an approximation to it by interpolating the neighboring audio samples. The concealment will make errors almost inaudible, and as a result, it offers a graceful degradation of the sound quality. Specifically, sharp temporary degradation of the audio signal, "clicks", are avoided. The judicious positioning of the left and right stereo channel as well as the audio samples on even- or odd-number instants within the interleaving scheme are key parameters to the success of the concealment strategy. There are a number of interleave structures used in the CD, each of which makes it possible to correct and detect errors with a minimum of redundancy.

The error control code used in the CD system employs not one but two Reed-Solomon codes, which are interleaved cross-wise. In particular, the synergy of the two RS codes gives excellent results.



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