Differences between CDs

Differences between CD-R/CD-RW discs and standard discs

The main physical difference between these two disc types and the standard prerecorded CD (audio or CD-ROM) is that the latter has no recording layer; the information is permanently stamped in the aluminium reflecting layer.

There is also a difference in terms of the data areas on the disc. Compared with standard CDs, the CD-R and CD-RW discs have an additional Cd-R/CD-RW area located in front of the lead-in area. This additional area is used to store data specific to the recording process, and is divided into two parts:

  1. Program Memory Area (PMA), which contains the track numbers of the recorded titles and their respective start and stop points.
  2. Program Calibration Area (PCA), which is used by the CDR 870 to calibrate the required laser energy by means of a brief trial recording (Optimum Power Calibration) each time a disc is loaded. This calibration is necessary to allow for production tolerances between individual discs, temperature variations etc. As well as this initial OPC, the required laser energy is constantly optimized during recording by running OPCs, which deal with dust, scratches and other possible variations across the disc surface. See figures 1, 2 and 3.

The CD-R and CD-RW discs Both CD-R and CD-RW discs have the same basic structure but with significant detail differences. The CD-R disc has a dye-based recording layer, with a reflectivity of 40 - 70 %, while the CD-RW disc has a phase-change recording layer with a reflectivity of 15 - 25 %. Both discs have an additional reflecting layer: golden for the CD-R, which accounts for that disc's distinctive appearance, and silver (aluminium) for the CD-RW.

Both disc types have a track spiral which is preformed during manufacture, onto which the audio data is written during the recording process. This track ensures that the recorder follows the same spiral pattern as a conventional CD, and has the same width of 0.6 mm and pitch of 1.6 mm as a conventional CD. In addition to the spiral pattern, the track has a slight superimposed sinusoidal excursion of ñ 0.3 mm at a frequency of 22.05 kHz. See figure 4.

The frequency of the sinusoidal excursion is used by the recorder to control the speed of rotation. The frequency read-out from the disc is constantly monitored, and the speed is adjusted as needed to maintain the frequency at exactly 22.05 kHz. An additional ñ 1 kHz frequency modulation is applied to provide the recorder with an absolute time reference. See figure 5.

The writing process: CD-R Digital information is written to the disc by burning (forming) pits in the recording layer. The energy of the laser beam - in the range 4 to 11 mW - causes limited heating of the substrate and recording layer to approximately 250 C. At this temperature the recording layer melts, reducing its volume, while the substrate expands into the space that becomes available. By constant switching between writing and reading power, a pit pattern corresponding to that of a conventional CD is produced.

The write pulse initially has a higher power to produce the required heating of the dye. Subsequently, the power is reduced to a level that is sufficient to maintain the dye temperature at the desired level.

The writing process: CD-RW Recording In the CD-RW disc, the recording layer is made of an alloy of silver, indium, antimony and tellurium. In its original state, this layer has a polycrystalline structure. During the recording process, the laser selectively heats tiny areas of the recording track to a temperature above the layer's melting point (500 - 700 C). For CD-RW writing, the laser power used is in the range 8 to 14 mW.

The pulsed energy delivered by the laser beam melts the crystals in the heated areas into a non-crystalline amorphous phase (`pits'), which has a much lower reflectance than the remaining crystalline areas (`lands'). This difference in reflectance allows the recorded data to be read-out, producing a signal similar to that obtained from a standard CD. The physical characteristics of the amorphous phase are `frozen-in' during cooling, making the recording just as permanent as any standard CD. See figure 7.

Erasing Erasing of a CD-RW disc is performed by returning the material in the recording layer which has been changed to the amorphous state back to the crystalline state. This is done by an annealing process, consisting of heating the layer to a temperature of about 200 C (i.e. less than the melting point) and maintaining that temperature for an extended period (in practice, this takes some 37 minutes for a complete disc). The disc is then returned to its original, completely unrecorded state.

A much faster `on the fly' erasing facility is also available, allowing the last recorded track to be erased simply by erasing the subcode reference to that track while leaving the recorded data in place in the recording layer. See figure

Overwriting A direct overwrite strategy is obtained by combining the write and erase techniques. In this case, new pits are written in the recording layer using the same pulsed laser beam energy as in the standard writing strategy. However, in the areas between the newly recorded pits, a lower-energy, non-pulsed laser beam is used to write new crystalline lands. The laser beam is repeatedly switched to the lower- energy erase level between the new pits, resulting in complete erasure of the audio data that was formerly contained in these areas.

As in the writing of a CD-R disc, a higher energy level is used initially to create the temperature increased required to melt the recording layer. Between the pits, the temperature is reduced to the annealing (erase) level. This provides a higher starting temperature, so less energy is subsequently needed each time the melting temperature has to be reached. See figure 9.