About Audio CDs


As with other digital data, the music or audio signals on a CD is stored digitally (in 1's and 0's). However, digial music are encoded in a format that allows them to be stored on the CD. So, if the bits of information on the CD player are read off without decoding it you would not get music.

Encoding is necessary because of algorithms that are incorporated in the music at recording time. Also, because of how the laser detects ones and zeroes on the CD, there cannot be consecutive ones when storing the digital information. The solution for this problem is called eight-to-fourteen modulation (EFM). At the final preparation of storing the encoded music on the CD, eight bit chunks of information are transformed to fourteen bits. Also, three merging bits of zeros are tacked on to each fourteen bit chunk. This final process ensures that no consecutive ones are present before storage.

The encoding of digital audio on CD player is governed by IEC 908. CD players use parity and interleaving techniques to minimize the effects of an error on the disk. In theory, the combination of parity and interleaving in a CD player can detect and correct a burst error of up to 4000 bad bits -- or a physical defect 2.47 mm long. Interpolation can conceal errors up to 13,700 or physical defects up to 8.5 mm long.

Here are the steps that are involved in the encoding process of digital music:

  • Analog music is converted to a digital signal via an analog-to-digital converter.
  • The left and right channels are combined to one digital data string via a multiplexor.
  • The digital data string is encoded with error detection and correction algorithms. The algorithms used is Cross Interleave Read-Solomon Code (CIRC) which accomplishes two things: first it provides error detection and correction for defects on the CD, second it spreads out data (interleave) so that different portions of a frame of music is physically located at different places on the CD. This helps in the correction of data that are damaged due to scratches and fingerprints.
  • The encoded data string now passes through the EFM circuits to prepare it for storage on the CD. At this point, the encoded and modulated music is a string of ones and zeros which contains no consecutive ones.

The CD player can be called a decoder because it basically reverses the encoding process as data read from the CD must be:

  • demodulated
  • decoded
  • demultiplexed and;
  • converted from digital to analog using a digital-to-analog converter
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