Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Patent
1997-05-19
1999-01-26
Swann, Tod R.
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
369 59, 369 48, G11B 700, G11B 2010
Patent
active
058645314
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a playback circuit, and more in particular to a playback circuit suitably used with a playback system for reproducing the signals read from a disk-shaped recording medium (such as the read-only optical disk, the write-once optical disk or the rewritable optical disk).
BACKGROUND ART
The digital data (original information) normally handled by us have a signal format with 0s and 1s aligned in an arbitrary sequence. When considering the characteristics of the recording and reproduction systems, however, this format in its direct form cannot be efficiently used as recording signals. In common practice, therefore, the original information is converted (modulated) into a form suitable for the above-mentioned recording media and recorded, The signal reproduced from the recording medium is restored (demodulated) to the original information. In recent years, various recording codes have been suggested for digital data recording schemes. The characteristics requirements for modulation-demodulation for data in an optical disk, for example, are as follows: the reproduced signal. level in digitization at the time of reproduction or in order to prevent fluctuations of the servo error signal at the time of recording or reproduction, the frequency spectrum of the modulated signal is DC free (contains no DC components). the successively arriving bit information
Familiar modulation schemes for recording data on an optical disk include NRZ, NRZI, FM, EFM and (2, 7 )-RLL schemes. The number of the same successive bits in the data sequence is called the Run.
The NRZ (Non Return to Zero) scheme is often used for magnetic recording of a hard disk. In this scheme, signal levels for logical values 0 and 1 of digital data exactly correspond to low and high signal levels, respectively. This scheme, therefore, is most easy to understand for conversion of an electrical signal for digital data.
The NRZI (Non Return to Zero Inverse) scheme, on the other hand, is one in which the signal level is inverted only when the logical value of the digital data is 1.
In the NRZ and NRZI schemes, the maximum inversion width Tmax of the data string is infinitely large, and it is understood that self-synchronization is difficult in these modulation schemes. Also, since they are not DC-free, these schemes are not used for recording data in the currently-available optical disks, for which the RLL and EFM modulation schemes are the current choices.
RLL (Run Length Limited) is such an encoding scheme that the maximum inversion width Tmax is finite. Especially, an encoding scheme in which the minimum run is d, the maximum run is k and m bits of data are mapped to n modulated bits is called the (d, k, m, n)-RLL encoding scheme. Depending on the parameters selected, there are several different RLL encoding schemes, which can be determined by the performance evaluation parameters indicating the characteristics of the respective schemes.
In the case where the minimum inversion width Tmin providing one of the evaluation parameters is excessively small, a reproduced waveform interference is caused by the OTF (Optical Transfer Function) or the diffraction limit of the optical pickup. The value Tmin, therefore, is advantageously larger.
Conversely, the maximum inversion width Tmax which is related to the ease of self-synchronization is advantageously smaller. More specifically, in order to enable the extracted sync signal to follow the temporal fluctuations (jitter), if any, of the reproduced signal, the signal is required to be inverted frequently. Also, a long inversion interval poses the problem of fluctuations of the DC component.
The detection window width Tw represents the tolerance of the temporal fluctuation (jitter) of the reproduced signal. The playback jitter in the optical disk is considered to derive from various factors. It is not desirable, however, that data displacement, etc. is caused by the temporal fluctuations of the reproduced signal. Therefore, the detection window width Tw is desirably l
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Chu Kim-Kwok
Sony Corporation
Swann Tod R.
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