News Column

Patent Issued for Signal Processing Device, Signal Processing Method, and Optical Disc Apparatus

August 6, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventors Omaki, Masayuki (Tokyo, JP); Nakai, Kenya (Tokyo, JP); Takeshita, Nobuo (Tokyo, JP), filed on February 17, 2012, was published online on July 22, 2014.

The assignee for this patent, patent number 8787135, is Mitsubishi Electric Corporation (Tokyo, JP).

Reporters obtained the following quote from the background information supplied by the inventors: "Increase in capacity of various optical discs has been achieved by reducing size of record marks (including pits) which are binary data formed on tracks of the optical discs, and by reducing focus spot size on focal planes using an objective lens which makes a laser beam used for recording and reproduction have a shorter wavelength and has a large numerical aperture.

"For example, in CDs (compact discs), a disc substrate functioning as a light transmission layer has a thickness of approximately 1.2 (mm), a laser-beam wavelength of approximately 780 (nm) is employed, a numerical aperture of an objective lens is 0.45, and a recording capacity is 650 (MB). A resolution of pits for recording signals is restricted by a diffraction limit. The diffraction limit DL is given as

"DL=.lamda./(4.times.NA), in which a laser-beam wavelength .lamda. and a numerical aperture NA are used. The diffraction limit in CDs is calculated from this equation, which yields a value of approximately 430 (nm). In CDs, since a shortest data length (shortest pit length) is approximately 830 (nm), a size of the shortest data length is approximately 1.93 times of a focus spot size determined by the diffraction limit.

"Moreover, in DVDs (digital versatile discs), a light transmission layer has a thickness of approximately 0.6 (mm), a laser-beam wavelength of approximately 650 (nm) is employed, an NA is 0.6, and a recording capacity is 4.7 (GB). The diffraction limit in DVDs can be calculated from the same equation as that in the case of CDs, which yields a value of approximately 270 (nm). In DVDs, a shortest data length (shortest pit length) is approximately 400 (nm) and a size of the shortest data length is approximately 1.48 times of a focus spot size.

"Furthermore, in BDs (Blu-ray discs), a light transmission layer has a thickness of 0.1 (mm), a laser-beam wavelength of approximately 405 (nm) is employed, an NA is 0.85, and a recording capacity is 25 (GB) per one recording layer. The diffraction limit in BDs can be calculated from the same equation as that in the case of CDs, which yields a value of approximately 120 (nm). In BDs, a shortest data length (shortest pit length) is approximately 150 (nm) and a size of the shortest data length is approximately 1.25 times of a focus spot size.

"As described above, the increase in capacity of optical discs is achieved not only by reducing a focus spot in size but also by reducing a ratio of a size of the shortest data length (the shortest pit length) to a focus spot size (approximately 1.93 times in CDs and approximately 1.25 times in BDs). In order to reduce the ratio, it is necessary to reduce an SNR (Signal to Noise Ratio) required in reproduced signals which are read out. As a signal processing technique for this, a PRML scheme, in which a condition that reproduced waveforms from optical discs have known partial-response characteristics is combined with a maximum likelihood estimation method according to Viterbi decoding scheme, has been developed. This technique has contributed to improvement in error rates.

"For example, for BDs, a PRML scheme where (1, 2, 2, 1) is used as a partial response class is commonly used. The class (1, 2, 2, 1) is an expression of optical responses to recorded binary data (intersymbol interference) in seven gradation levels (amplitude levels), and it allows an expression approximately expressing actual reproduced waveforms. In the PRML scheme, ideal optical responses which approximately expresses reproduced waveforms are derived using the maximum likelihood estimation method (Viterbi decoding scheme), thereby estimating binary data recorded on BDs.

"Moreover, in HD DVDs (High-Definition Digital Versatile Disc), a shortest data length (shortest pit length) is approximately 200 (nm) and is less than a diffraction limit of approximately 270 (nm). For this reason, in a case of HD DVDs, the shortest data (shortest pit) can be read by using (1, 2, 2, 2, 1) as a partial response class and expressing an optical response (intersymbol interference) to recorded binary data in nine gradation levels (amplitude levels).

"As described above, since it is difficult to physically improve a resolution which is restricted by the diffraction limit, signal processing plays a more major role in achieving the increase in capacity of optical discs. In particular, it is not expected that a laser-beam wavelength shorter than a wavelength of 405 (nm) for BDs is put to practical use, from viewpoints of inviting deteriorations in optical elements and expecting harmful effects on the human body. For this reason, it is intended to realize the increase in capacity by a method that uses nearfield light, multilayering of recording layers, use of holography or other methods. If asymmetry of a reproduced waveform is deteriorated or signal intensity near the shortest data length decreases, quality of a reproduced signal is further deteriorated and therefore a further improvement in signal processing techniques is required. Moreover, deterioration of a reproduced signal in quality harmfully influences extraction of a clock signal.

"For example, Non-Patent Documents 1 and 2 disclose optical super resolution techniques called Super-RENS (Super REsolution Nearfield Structure). According to the techniques, by causing a refractive-index change at a local part where light intensity is large or a temperature is high in a focus spot on an optical disc, it is possible to reproduce record marks that are smaller than the diffraction limit .lamda./(4.times.NA) determined by a numerical aperture NA of a condenser lens which is an optical element of an optical disc apparatus and a wavelength .lamda. of light. The local part where the refractive-index change is caused is now simply referred to as an aperture. Since this aperture is excited by energy and is derived by the refractive-index change accompanied by a crystal-structure change, there is a temporal delay in response to light. If this delay is not negligible to a rotation speed of the optical disc, a signal read out by near-field light is undesirably partially delayed, thereby producing harmful influence on decoding of a signal and extraction of a clock signal.

"In optical disc apparatuses, data recorded in an optical disc itself is recorded with a stable clock signal. However, at a time of reproducing from the optical disc, it is impossible to regenerate a spindle rotation which is completely the same as that at a time of recording, and therefore it is necessary to reproduce the clock signal each time. In optical disc apparatuses, it is common to adopt a method of extracting a clock signal from a reproduced signal itself using a PLL (Phase-Locked Loop) circuit. In general, a PLL circuit is formed by a phase comparator, a loop filter and a voltage-controlled oscillator. The phase comparator compares a phase which is calculated from a reproduced signal sampled by a clock signal and a phase of the clock signal itself, thereby outputting a phase error signal corresponding to a phase difference between them. The loop filter supplies a control voltage which is obtained by filtering the phase error signal from the phase comparator, to the voltage-controlled oscillator. The voltage-controlled oscillator outputs a clock signal of a frequency proportional to the control voltage. A reproduced signal is sequentially sampled by the output clock signal from the voltage-controlled oscillator, the output clock signal from the voltage-controlled oscillator influences a calculation of a phase of a reproduced signal, and thus the PLL circuit forms a loop feedback circuit. By the loop feedback circuit, a frequency and a phase difference of the output clock signal vary in accordance with a frequency of an input signal. When a phase error between a sampling point of the reproduced signal and the clock signal is calculated, a point (a crossing point) where the reproduced signal intersects a certain slice level is defined as a clock point. In other words, a difference between the crossing point and the sampling point is a phase error between the reproduced signal (reproduced waveform) and the clock signal, and the loop feedback circuit works so as to make them equal. As a slice level, a center level (average level) of a reproduced waveform is usually used and it is a level where there are largest number of the crossing points in a reproduced waveform of an optical disc.

"Moreover, there is a method of calculating a phase error between a reproduced waveform and a clock signal, in combination with a PRML scheme (see Patent Document 1, for example). This is a method of calculating a phase error at each sampling point, without setting a slice level. In this method, an ideal waveform is predicted according to the PRML scheme and a difference between the ideal waveform and the reproduced waveform is calculated as a phase error."

In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventors' summary information for this patent: "Problem to be Solved by the Invention

"However, methods described in the Non-Patent Documents 1 and 2 are not effective for a reproduced signal (reproduced waveform) having a greatly deteriorated waveform. For example, in optical discs, as a data length (record mark length) of recorded binary data (record marks) is shorter, an SNR tends to decrease and an amplitude level of a reproduced waveform of binary data having a short data length tends to concentrate at a center level. For this reason, in a case of the method in which a phase error between a crossing point (clock point) where a reproduced signal intersects a slice level and a sampling point is calculated, there is a problem that a variation in a phase error increases and a clock signal is unstabilized and quality of binarized data as a reproduced signal is deteriorated.

"Moreover, as shown in Patent Document 1, in a case where a predicted waveform (ideal waveform) which is predicted according to the PRML scheme is used as a target waveform, the target waveform itself includes an error and a phase error is undesirably calculated using the target waveform including the error as a reference. For this reason, the art shown in Patent Document 1 has a problem that a variation in a phase error increases and a clock signal is unstabilized and quality of binarized data as a reproduced signal is deteriorated.

"Therefore, the present invention is made to solve the above-described problems in the conventional arts and an object of the present invention is to provide a signal processing device, a signal processing method and an optical disc apparatus that can improve quality of binarized data as a reproduced signal by generating a stable clock signal.

"Means for Solving the Problem

"A signal processing device according to an aspect of the present invention includes: an adaptive filter for filtering adaptively a reproduced waveform of a reproduced signal from a recording medium so as to make the reproduced waveform be closer to a target waveform; a PRML circuit for sequentially generating binarized data from the filtered reproduced waveform using a PRML scheme by sampling at sampling points in a period based on a clock signal and sequentially generating a partial response waveform which is to be the target waveform from the binarized data; a calculating unit for sequentially calculating first phase errors with regard to the sampling points from a difference between the target waveform and the filtered reproduced waveform; a limiting unit for outputting second phase errors by excluding a specific phase error from the first phase errors; and a clock generating unit for generating the clock signal of a frequency corresponding to the second phase errors; wherein the specific phase error includes a phase error at a time when the partial response waveform reaches a specific level which excludes at least a level not less than a predetermined amplitude level.

"A signal processing method according to an aspect of the present invention includes the steps of: filtering adaptively a reproduced waveform of a reproduced signal from a recording medium so as to make the reproduced waveform be closer to a target waveform; sequentially generating binarized data from the filtered reproduced waveform using a PRML scheme by sampling at sampling points in a period based on a clock signal and sequentially generating a partial response waveform which is to be the target waveform from the binarized data; sequentially calculating first phase errors with regard to the sampling points from a difference between the target waveform and the filtered reproduced waveform; outputting second phase errors by excluding a specific phase error from the first phase errors; and generating the clock signal of a frequency corresponding to the second phase errors; wherein the specific phase error includes a phase error at a time when the partial response waveform reaches a specific level which excludes at least a level not less than a predetermined amplitude level.

"A signal processing method according to another aspect of the present invention includes: a step of filtering adaptively a reproduced waveform of a reproduced signal from a recording medium so as to make the reproduced waveform be closer to a target waveform; a step of sequentially generating binarized data from the filtered reproduced waveform using a PRML scheme by sampling at sampling points in a period based on a clock signal, while sequentially generating a partial response waveform from the binarized data; a step of generating the target waveform by equalizing the partial response waveform; a step of sequentially calculating a phase error with regard to the sampling point from a difference between the partial response waveform and the filtered reproduced waveform; and a step of generating the clock signal of a frequency corresponding to the phase error.

"A disc apparatus according to an aspect of the present invention includes: an optical head device for optically reading binary data recorded in a recording medium; a reproduced signal processing unit for generating a reproduced signal from a signal output from the optical head device; and the signal processing device for generating the binarized data from the reproduced signal.

"Effect of the Invention

"The signal processing device, the signal processing method and the optical disc apparatus according to the present invention have an effect that a stable clock signal can be generated and therefore quality of the binarized data as the reproduced signal can be improved."

For more information, see this patent: Omaki, Masayuki; Nakai, Kenya; Takeshita, Nobuo. Signal Processing Device, Signal Processing Method, and Optical Disc Apparatus. U.S. Patent Number 8787135, filed February 17, 2012, and published online on July 22, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8787135.PN.&OS=PN/8787135RS=PN/8787135

Keywords for this news article include: Electronics, Signal Processing, Mitsubishi Electric Corporation.

Our reports deliver fact-based news of research and discoveries from around the world. Copyright 2014, NewsRx LLC


For more stories covering the world of technology, please see HispanicBusiness' Tech Channel



Source: Electronics Newsweekly


Story Tools






HispanicBusiness.com Facebook Linkedin Twitter RSS Feed Email Alerts & Newsletters