Abstract
We have only discussed until now the optics, servo-mechanics and the channel decoder as three essential parts of a CD-ROM basic engine. However, data delivery would not be possible without generating a laser beam, converting the optical power into electrical signals and without the supervision of a central microcontroller. In addition, any CD-ROM drive should also play back audio discs, which calls for an adequate digital-to-analog (D/A) interface.
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The coherence (temporal and spatial) represents a measure of the phase uniformity across the optical wavefront. In the process of stimulated emission, each photon added to the stimulated radiation has a phase, polarization, energy and direction identical to that of the amplified light wave in the laser cavity.
This operation mode is usually called continuous wave (CW), as opposed to the operation under a frequency-modulated current superposed on the DC bias.
The gain g is a measure of the incremental optical energy flux per unit length.
The quantum efficiency of a photonic emitting device is given by the number of photons generated per electron-hole pair.
The confinement factor is the ratio of the light intensity within the active layer to the sum of light intensities both within and outside the active layer.
The depletion region (also called the space charge region) is formed around the junction between two semiconductors differently doped and is characterized by zero-densities of the mobile carriers (electrons and holes) .
The external quantum efficiency of a photodetector is given by the number of carriers (electron-hole pairs) collected to produce the photocurrent, divided by the number of incident photons. The internal quantum efficiency ηint is the number of created electronhole pairs divided by the numbers of absorbed photons and is usually very high, if not unity, in defect-free materials [8].
Typical values [8] for a semiconductor material are Dp = 13 cm2/s and τ p = 10 – 7 s.
An intrinsic semiconductor (i.e., not doped) contains, per unit volume, the same number of electrons in the conduction band as number of holes in the valence band.
The 3-dB bandwidth is defined as the frequency at which the photocurrent amplitude drops to half of its value at low frequencies.
The Johnson noise results from the random motion of carriers that contributes to the dark current of the device. Other disturbance sources are the generation-recombination process which gives rise to shot noise, the surface and interface defects in the bulk semiconductor (determining the flicker or 1/f noise), and the optical signal itself, which generates the quantum noise.
We refer here to the signal-to-noise ratio of the photocurrents with respect to all disturbances, like digital clocks, not originating from the photodetector itself.
It is generally considered that frequencies between 5 and 20,000 Hz are sufficient for high-fidelity reproduction of audible sounds. When frequencies above 20 kHz are completely cut off, they are in proportion of 99 % inaudible [70].
The Philips format, for example, is called Inter-IC Sound (I2S) and assigns only one clock (BCLK) period for each audio bit. The serial pattern is coded in 2’s complement and begins always with the most significant bit (MSB). It is also allowed to reverse the polarity of the word clock (WCLK), which means that a zero logic of WCLK does not necessarily mark a left-channel sample.
Because the high-frequency contents of music is relatively small, a first-order preemphasis (+20 dB/decade) in this frequency range may be applied during recording. The reverse operation (de-emphasis) needs being performed during playback [50,791.
The noise shaper, also called single-bit DAC, bit-stream DAC or Sigma Delta (∑∆) modulator, is a 1-bit quantization device whose output data is sampled at very high frequencies when compared to the input sampling frequency. The difference between the output and input signals is fed back via a specific transfer function which creates a pulse density modulation (PDM) output stream. A benefit of the device at low (audio) frequencies is its excellent noise suppression. In fact, the total noise is shaped in frequency and pushed outside the audio band. Further details about ∑∆ modulation can be found in [19,54,71,72].
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© 1998 Springer Science+Business Media Dordrecht
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Stan, S.G. (1998). Other basic-engine building blocks. In: The CD-ROM Drive. Springer, Boston, MA. https://doi.org/10.1007/978-1-4757-2843-9_5
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DOI: https://doi.org/10.1007/978-1-4757-2843-9_5
Publisher Name: Springer, Boston, MA
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