Visualization of the tracks of pits on the metalized CD data surface by use of a scanning electron microscope. The horizontal line of the scan indicates a scale of 5 μ m. The track pitch, the distance between adjacent laps of the pit spiral, is 1.6 μ m so that there are about 600 tracks to a mm. Each pit has a width of about 0.5 μ m. In comparison, the cross-section of a human hair has a width of 75 μ m. The minimum pit length is 0.833 μ m to 0.972 μ m, the maximum pit length is 3.05 μ m to 3.56 μ m so that a track of pits might contain about 3 billion pits precisely arranged on a spiral. Unspiraled, the track would stretch about 3.5 miles. Because each outer track revolution contains more pits than each inner track revolution, the CD must be slowed down as it plays in order to maintain a constant rate of data. Based on the timing of the master clock, the CD player automatically regulates the disc rotational speed to maintain a constant bit rate of 4.3218MHz during holographic readout 

Contexts in source publication

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... methods of audio storage and detection have evolved since Thomas Alva Edison (1847–1931) made the first audio recording in 1877 on a cylinder covered with tin foil. Ironically, the invention of the analog phonograph was performed while he was experimenting with a device for storing digital data, a telegraphic code repeater. Early acoustical recordings were made on wax cylinder and shellac disc. Subsequently, numerous magnetic tape formats were developed. However, all of these audio systems recorded and reproduced analog signals by using a mechanical pickup. Because optical holography was far ahead, conceptionally as well as technically, time was not mature for the highly complex process of disc manufacturing. In IT the CD is certainly one of the most advanced storage media available. The CD-DA format stores its information digitally and uses a laser optoelectronic pickup. The length of its data represents the binary bits which represent the original audio signal. A laser beam of carrier frequency ν is focused to read the data stream. The data is physically contained in the disc’s pits which are impressed along its top surface and are covered with a 50 to 100nm metal layer. The data storage in pits on a flat surface is not directly visible to the naked eye. A scanning electron microscope is needed to get a sufficiently good look on the track of pits (Figure 1) arranged ...

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... continuous spiral running from the inner circumference to the outer one. Another 10 μ m to 30 μ m plastic layer protects the metalized pit surface. The laser beam is focused on the metalized data surface embedded inside the disc and passes through the transparent plastic substrate and back again. The fact that the laser beam passes the disc substrate provides one of the significant assets of the CD system. The plastic substrate has refractive index of 1.55 whereas air has normalized refractive index 1.0. The speed of light slows from c = 3 · 10 8 m/s to c = 1 . 9 · 10 8 m/s and changes the carrier frequency ν to the fraction νy . It is the instantaneous frequency νy which characterizes the frequency modulation of the carrier frequency ν induced by the substrate. Because of the refractive index, the thickness of the CD, and the numerical aperture of the objective lens, the size of the laser beam on the disc surface is approximately 2 μ m. Hence, the laser beam is focused to a point slightly larger than a pit width but does not overlap the tracks of pits (Figure 1). The reflective flat surface, called land (Figure 2), causes almost ninety percent of the laser light to be reflected into the optoelectronic pickup. When considered from the laser’s perspective, the pits are viewed as tracks of bumps. The height of each bump is between 0.11 μ m and 0.13 μ m. This height is slightly smaller than the semiconductor laser’s wavelength λ = 780nm in air. Inside the polycarbonate substrate with a refractive index of 1.55, the laser’s wavelength is about λ = 500 nm. The height of the bumps is therefore approximately 4 1 of the laser’s wavelength λ inside the disc ...