Resumen
Along with the optics and servomechanics, the decoding circuitry plays one of the main roles within a CD-ROM system. Also designated as channel decoder, the decoding electronics processes the read-out HF signal, regenerating the digital data embedded into the disc relief structure.
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A FIFO buffer is only needed in CD-ROM drives based on constant linear velocity (CLV) motor control. The subject will be further discussed throughout Section 4.8.
The intersymbol interference (ISI) is due to the finite bandwidth of any communication system. A signal consisting of abrupt pulses will be filtered improperly as it passes through the communication channel and the pulses, corresponding to transmitted symbols, may be smeared into adjacent time slots. The resulted waveform does not display a clear separation between neighboring symbols, impeding therefore the subsequent data detection.
It is assumed that all DC offsets introduced by the preprocessing electronics are removed by the coupling capacitor depicted in Fig. 4.2.
This maximum slope is obtained at a zero-crossing, between two adjacent longest land and pit (or vice versa).
The maximum variation in the detector current corresponds to the difference between 100 % and 0% reflection levels.
In general, one should talk about block synchronization. This operation, which separates the words, frames or data packets from each other, can be accomplished by embed-ding predefined patterns into the data stream [6].
The terms digitally- or numerically-controlled oscillator are also used.
The jitter cumulates statistically all small time variations of a repetitive signal, within a given frequency range, reflecting therefore the quality of the whole transmission channel. For example, the cumulated small variations exhibited by the period of a transmitted digital signal is usually designated as data-to-data jitter. Similarly, the jitter can be measured between the edges of digital data and recovered clock, respectively (clock-to-data jitter).
It is assumed that 0 and 1 are the logical levels on which the binary sequence relies.
The sync pattern consists of two consecutive maximum-length sequences (10000000000) followed by 10xxx, where ‘xxx’ denotes three merging bits.
See See further Section 6.1.
For an audio disc, the demodulated frame from Fig. 4.8 contains two streams of 12 symbols, for the left and right channel, respectively. Each audio sample contains 16 bits and hence, one frame carries 12 × 8/16 = 6 samples per audio channel. As the sampling rate of the audio signal equals 44.1 kHz (see Section 5.4), there are 44.1 • 103 /6 = 7350 demodulated frames needed to generate one second of music. In case of the CD-ROM system, the 2 x 12 symbols simply contain computer data instead of sampled audio, preserving therefore the number of 7350 frames/second.
A block code uses a sequence of n symbols (called codeword), each symbol comprising m bits. Only k symbols out of n carry user data, while the other n k symbols contain parity information. A linear code has the property that any two codewords of n symbols can be added and their (modulo-2) sum represents a valid codeword too.
Details on this subject are beyond the scope of this book. However, it is important to mention the notation GF(q) which is encountered in all CD standards and designates a Galois field of q elements [78]. The Galois fields are needed for the description of Reed-Solomon codes and their associated encoding/decoding techniques.
When a received symbol is unreliable (but not necessarily in error), its known position within the codeword is called erasure.
The distance d between any two codewords is given by the number of places in which these codewords differ from each other. The minimum distance dm of a code is the smallest value of the distance among all possible combinations of two codewords. For an (n, k) RS code, dm = n k + 1 and t errors can be corrected if 2t + 1 ≤ dm. However, erasure correction is more efficient, since d m — 1 erasure corrections can be performed.
A11-zero code words can be obtained, for instance, if the HF signal vanishes due to an interruption along its path. An all-zero codeword will be validated by the decoder as being without errors.
The symbols remained uncorrected at the output of the C2 decoder can cause undesired noise when an audio disc is played back. As this decoder marks any uncorrected codeword by attaching flags to all corresponding symbols, it will be possible to replace an unreliable 16-bit audio sample by a new one obtained according to a given algorithm. This process is called concealment and consists of either holding a previous reliable audio sample or interpolating linearly between two other reliable audio samples neighboring the one in error [12,45,77]. In order for the concealment to be successful, the scrambling and 2-symbol delay operations separate the adjacent audio samples during encoding.
As already discussed in Section 3.3.3, the total spiral eccentricity is the sum of track radial run-out and mechanical eccentricities.
The output bit rate should be an integer multiple of the audio sampling frequency F s = 44.1 kHz. Further details are given in Section 5.4.
A specific design case is represented by portable CD-ROM drives which may be affected by gyroscopic effects. These effects introduce additional torques in the motor equation and must be taken into account when designing the control loop of the turntable motor.
Some CD-ROM drives can buffer audio data that is read out at higher disc speeds and deliver it at 1X to the sound device of the host system (see further Section 6.3). In these cases, audio playback in CLV mode becomes obsolete.
As mentioned in Section 4.8.1, portable CD-ROM drives form a particular design case.
A calibration procedure is initiated after power-up by the basic engine microcontroller (see also Section 5.5) in order to determine some system parameters, like track pitch, speed va of recorded data, etc.
Usually, the speed measurement relies on Hall sensors which are symmetrically mounted inside the turntable motor. If incorrectly positioned, components of high frequency as well as a DC offset are introduced in the feedback signal. A Hall sensor is a device able to generate an electrical current when subjected to a magnetic field [31].
With the exception of portable CD-ROM drives.
The performance of CD-ROM drives will be further discussed in Section 7.4.
See Section 5.2.
See Section 5.3.
For instance, a new speed setpoint may be provided for each 2-minute increment of the read subcode timing. Details about the construction of the subcode timing are given in Section 6.1.
Due to the linear distribution of data along the disc spiral, the overspeed factor reaches a peak value at the end of the sledge-actuator displacement (see also Fig. 7.1) . For good access performance, these peaks must be situated within the decoding speed range of the channel and data path electronics.
See further Section 7.4.2 for details regarding the calculation of the average bit rate.
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© 1998 Springer Science+Business Media Dordrecht
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Stan, S.G. (1998). The decoding circuitry. In: The CD-ROM Drive. Springer, Boston, MA. https://doi.org/10.1007/978-1-4757-2843-9_4
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