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CD-Rs are optical storage media. Saving data on CD-Rs requires a CD-Recorder, which also has a read function. It is worth noting that a reader alone cannot record information. A CD-R has a capacity of around 650Mb - roughly equivalent to 450 floppy disks. Recording on and reading from a CD or CD-R involves a laser beam traveling across the surface of the disc in the same way a needle crosses a vinyl disc. The pick-up head is attached to a moving unit which follows the variations in position of the height of the CD's surface.
Two types of rotation are possible :  Constant linear speed (CLS)Constant angular speed (CAS)With constant angular velocity, the rotation speed does not need to be controlled. However, with constant linear velocity, the disc rotation speed must vary according to the position of the pick-up head so that the speed remains at exactly 1.2 m/s. With constant angular velocity, the disc capacity is smaller than under constant linear velocity (74 minutes and 30 seconds of high-fidelity sound at 44.1 kHz). 
A CD-R consists of a plastic disc covered with a number of manufactured layers. On one side, (the side where the dye, gold, etc., is applied) the polycarbonate disc has an extremely fine design which is basically a spiral. It has the following dimensions :
It is formed at the same time the polycarbonate disc is molded. The part of the mold which creates the spiral design is called the stamper. The curve is not regular, but instead wobbles around an average curve :  Oscillations frequency is 22.05 kHz. Oscillation causes the pick-up head to follow the curve and adjust CD rotation speed. The laser beam follows the spiral in the same way the sapphire followed the groove in a 33 or 45 vinyl disc. It crosses the polycarbonate, the layer of dye and reflects off the gold (or silver) layer. The returning signal is analyzed and broken down into two main sets of data: refraction (Ref) and the error signal from tracking the groove (Tracking error). Refraction allows the information on the disc to be read: the dye will be more dark where the laser has burnt it to write binary data (two possibilities: 0 or 1 similar to an on-off switch) and refraction will differ depending on the condition of the dye (burnt, and therefore dark, or unburnt and light). On one hand, the tracking error signal means the spiral can be tracked, and on the other that the disc rotation speed can be controlled. As mentioned above, the spiral does not follow a regular curve, but instead wobbles. The reflected laser beam is broken down into two parts (right and left) and analysis of the variations in reflected strength of the two parts gives the rotation velocity (signal frequency).
Since the frequency is 22050 Hertz, the period must be constant to guarantee a smooth flow of data when reading the written area or when recording. Since the pick-up head has to move in order to follow the variation in the spiral, the variation in amplitude of the signal is also used to correct its position. One signal will be stronger than another as the laser beam becomes more distant from the average line of the spiral's wobble. The result of the analysis of this difference will control the movement of the pick-up head.
The areas burned by the laser beam (mark) are variable in size but standardized: 3T to 11T, where T equals 231.4 nanoseconds. The minimum size of a piece of data is thus 3x231.4 ns which gives 694.2 ns or 0.833 µ (11T = 3.054 µ).This size is needed because the laser beam is broad (1.7 µ). The signal returned by the laser is cleaned for translation into binary code (0 or 1) as the following diagram shows:  If the reflected signal changes, the binary value is 1. If the reflection is constant, it will be 0. A pit (synonymous with mark) or a land of 4T will return three 0 values (the occurrence of 0 values is nT-1). Bits of data are not simply written onto the disc one after another. To avoid the risk of data loss due to dust for example, the data is arranged into a particular order. One word is cut up and placed in different places so that, if there is a defect in the disc, the loss of data concerning a single word or block is limited. Thus the error tracking system can reconstitute the lost data more easily.
The coding system detects errors and corrects them according to how serious they are. Since there is always the risk of error, and data loss cannot be allowed, correction systems have been invented. The most simple would be to repeat the data two or three times, but this would result in a loss of space. Another option is as follows: a table of data (matrix) is used to add up the numbers on each line and each column. In this way, if data is lost, it can be correctly calculated from the total values in the table. Several error measurements are used to give the Block Error Rate (BLER): 7,350 blocks are read in one second and a 3% error rate is tolerated. In other words, there should be no more than 220 unusable blocks every second (Orange Book). If the BLER is high, the reason is most likely that the dye has been contaminated or dust was present during molding. MAM-A's specifications are for an average BLER of less than 50 every 10 seconds and a maximum BLER of less than 100 in any one second. Uncorrectable errors (E32): These are very significant defects which means the error can not be corrected nor the information recovered, and are not acceptable. The disc undergoes an optical test (CCD), and the Basler equipment we operate tells us the size of the defect and its depth within the disc. When necessary, the disc is rejected (for a single defect size, the further it is from the layer of dye, the less serious it becomes).
During the recording phase, the CD-Recorder identifies the CD-R manufacturer and while writing, continuously adjusts the strength of the laser according to the CD-R quality. As the laser records the CD-R, it reads at the same time. It uses the signal to calculate a parameter called which is equal to divided by the maximum value of the signal :  The wrong strength increases signal deviation. If deviation is measured by varying the strength, the following diagram is obtained :  |
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Operating Principles
