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CD Replication: Physical Characteristics & Tech Info 2

The following text is an edited extract from original article at http://en.wikipedia.org/wiki/Compact_Disc

Main physical parameters

The main parameters of the CD (taken from the September 1983 issue of the audio CD specification) are as follows:

  • Scanning velocity: 1.2–1.4 m/s (constant linear velocity) – equivalent to approximately 500 rpm at the inside of the disc, and approximately 200 rpm at the outside edge. (A disc played from beginning to end slows down during playback.)
  • Track pitch: 1.6 μm
  • Disc diameter 120 mm
  • Disc thickness: 1.2 mm
  • Inner radius program area: 25 mm
  • Outer radius program area: 58 mm
  • Centre spindle hole diameter: 15 mm

The program area is 86.05 cm² and the length of the recordable spiral is 86.05 cm² / 1.6 μm = 5.38 km. With a scanning speed of 1.2 m/s, the playing time is 74 minutes, or around 650 MB of data on a CD-ROM. If the disc diameter were only 115 mm, the maximum playing time would have been 68 minutes, i.e., six minutes less. A disc with data packed slightly more densely is tolerated by most players (though some old ones fail). Using a linear velocity of 1.2 m/s and a track pitch of 1.5 μm leads to a playing time of 80 minutes, or a capacity of 700 MB. Even higher capacities on non-standard discs (up to 99 minutes) are available at least as recordables, but generally the tighter the tracks are squeezed the worse the compatibility.

Data structure

The smallest entity in a CD is called a frame. A frame consists of 33 bytes and contains six complete 16-bit stereo samples (2 bytes × 2 channels × six samples equals 24 bytes). The other nine bytes consist of eight Cross-Interleaved Reed-Solomon Coding error correction bytes and one subcode byte, used for control and display. Each byte is translated into a 14-bit word using Eight-to-Fourteen Modulation, which alternates with 3-bit merging words. In total there are 33 × (14 + 3) = 561 bits. A 27-bit unique synchronization word is added, so that the number of bits in a frame totals 588 (of which only 192 bits are music).

These 588-bit frames are in turn grouped into sectors. Each sector contains 98 frames, totalling 98 × 24 = 2352 bytes of music. The CD is played at a speed of 75 sectors per second, which results in 176,400 bytes per second. Divided by 2 channels and 2 bytes per sample, this results in a sample rate of 44,100 samples per second.

For CD-ROM data discs, the physical frame and sector sizes are the same. Since error concealment cannot be applied to non-audio data in case the CIRC error correction fails to recover the user data, a third layer of error correction is defined, reducing the payload to 2048 bytes per sector for the Mode-1 CD-ROM format. To increase the data-rate for Video CD, Mode-2 CD-ROM, the third layer has been omitted, increasing the payload to 2336 user-available bytes per sector, only 16 bytes (for synchronisation and header data) less than available in Red-Book audio.