A data readout device generates decoded data based on a data bit determined by performing a hard-decision on likelihood data. The likelihood data is generated using an iterative decoding method corresponding to a turbo-coding process. The decoding process is performed on a sampled value obtained by
A data readout device generates decoded data based on a data bit determined by performing a hard-decision on likelihood data. The likelihood data is generated using an iterative decoding method corresponding to a turbo-coding process. The decoding process is performed on a sampled value obtained by sampling a signal read out from a magneto-optical disk at a predetermined sampling period. A reliability determination part detects whether each obtained data bit was obtained from likelihood data within a predetermined range defined with respect to a histogram of log likelihood ratios. When it is detected that a data bit was obtained by a hard-decision on likelihood data within the predetermined range, the data bit is regarded as being obtained by a hard-decision whose reliability is not sufficient. In this way, the error correcting decoder can perform ECC-based decoding operations including loss-correction processes depending on respective reliability-determination results that distinguish reliabilities among respective data bits.
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What is claimed is: 1. A data readout device comprising: a likelihood data generating part generating a likelihood data in accordance with an iterative decoding method corresponding to a turbo-coding process, wherein the iterative decoding process is performed on a sampled value obtained by samplin
What is claimed is: 1. A data readout device comprising: a likelihood data generating part generating a likelihood data in accordance with an iterative decoding method corresponding to a turbo-coding process, wherein the iterative decoding process is performed on a sampled value obtained by sampling a signal read out from a data recording medium at a predetermined sampling period; a hard-decision part performing a hard-decision on the likelihood data provided by said likelihood data generating part, and determining whether or not each data bit is one obtained from the likelihood data within a predetermined range in which the hard-decision is regarded as lacking sufficient reliability; and an error correcting decoder that performs an error correcting decoding operation using a predetermined rule and using a result of the determination made by said hard-decision part corresponding to said data bit so as to generate corrected decoded data, wherein said corrected decoded data provided by said error correcting decoder is then output from said data readout device without being provided to said likelihood data generating part, and further comprising a range-setting part that can set and adjust said predetermined range, wherein said range-setting part sets said predetermined range based on the number of repetitions of the decoding process in accordance with said iterative decoding method. 2. The data readout device as claimed in claim 1, wherein said reliability determination part outputs said result of reliability determination as flag data that is paired with said corresponding data bit. 3. The data readout device as claimed in claim 1, wherein said error correcting decoder handles said data bit as an erroneous data when said reliability determination part determines that said data bit is one obtained from said likelihood data within said predetermined range. 4. The data readout device as claimed in claim 3, wherein said error correcting decoder performs an ECC decoding operation, wherein said data bit determined as one obtained from said likelihood data within said predetermined range is handled as lost data to perform a process based on a loss correcting decoding method. 5. The data readout device as claimed in claim 1, wherein said reliability determination part employs two slice levels that define said predetermined range in order to determine whether each data bit is one obtained from said likelihood data falling within said predetermined range. 6. The data readout device as claimed in claim 5, wherein said likelihood data generated in accordance with said iterative decoding method is a value in a continuously expressed numerical range corresponding to probabilities from the maximum probability of said data bit being 1 through the maximum probability of said data bit being 0, wherein said two slice levels comprise a first slice level that is larger than the median of said numerical range and a second slice level that is smaller than the median of said numerical range; wherein the median of said numerical range is used as a slice level for making said hard-decision on said likelihood data. 7. The data readout device as claimed in claim 6, wherein said likelihood data generated in accordance with said iterative decoding method is a log likelihood ratio defined based on a ratio of the probability of said data bit being 1 and the probability of said data bit being 0. 8. The data readout device as claimed in claim 5, wherein said likelihood data generated in accordance with said iterative decoding method is a value in a continuously expressed numerical range corresponding to probabilities from the maximum probability of said data bit being 1 to the maximum probability of said data bit being 0, wherein said two slice levels comprise a first slice level that is larger than the median of said numerical range and a second slice level that is smaller than the median of said numerical range, wherein one hard-decision for deciding whether said data bit is 1 based on said first slice level and one hard-decision for deciding whether said data bit is 0 based on said second slice level are respectively performed, wherein said data bit as the hard-decision result corresponding to said likelihood data within said predetermined range defined by said first and second slice levels regarded as lacking sufficient reliability due to said hard-decision is set to a predetermined value of either 1 or 0. 9. The data readout device as claimed in claim 8, wherein said likelihood data generated in accordance with said iterative decoding method is a log likelihood ratio defined based on a ratio of the probability of said data bit being 1 and the probability of said data bit being 0. 10. The data readout device as claimed in claim 1, comprising a controller that can substantially disable the function of said range-setting part when the number of repetitions of the decoding process in accordance with said iterative decoding method does not exceed a predetermined number. 11. The data readout device comprising: a likelihood data generating part generating a likelihood data in accordance with an iterative decoding method corresponding to a turbo-coding process, wherein the iterative decoding process is performed on a sampled value obtained by sampling a signal read out from a data recording medium at a predetermined sampling period; a hard-decision part performing a hard-decision on the likelihood data provided by said likelihood data generating part, and determining whether or not each data bit is one obtained from the likelihood data within a predetermined range in which the hard-decision is regarded as lacking sufficient reliability; and an error correcting decoder that performs an error correcting decoding operation using a predetermined rule and using a result of the determination made by said hard-decision part corresponding to said data bit so as to generate corrected decoded data, wherein said corrected decoded data provided by said error correcting decoder is then output from said data readout device without being provided to said likelihood data generating part, and further comprising a range-setting part that can set and adjust said predetermined range, wherein said two slice levels that define said predetermined range can be set and adjusted using said range-setting part, wherein said likelihood data generated in accordance with said iterative decoding method is a value in a continuously expressed numerical range corresponding to probabilities from the maximum probability of said data bit being 1 to the maximum probability of said data bit being 0, wherein said two slice levels comprise a first slice level that is larger than the median of said numerical range and a second slice level that is smaller than the median of said numerical range, and wherein said range-setting part sets said first and second slice levels to respective values based on the number of repetitions of the decoding process in accordance with said iterative decoding method. 12. The data readout device as claimed in claim 11, wherein said range-setting part sets said predetermined range by making reference to likelihood data that corresponds to a peak position in a likelihood data histogram generated on random data in accordance with said iterative decoding method. 13. The data readout device as claimed in claim 11, wherein said likelihood data generated in accordance with said iterative decoding method is a value in a continuously expressed numerical range corresponding to probabilities from the maximum probability of said data bit being 1 to the maximum probability of said data bit being 0, wherein said two slice levels comprise a first slice level that is larger than the median of said numerical range and a second slice level that is smaller than the median of said numerical range, wherein said range-setting part sets said first and second slice levels to respective numerical values that are respectively closer to the median than numerical values of likelihood data that correspond to peak positions in a likelihood data histogram generated on random data in accordance with said iterative decoding method. 14. The data readout device as claimed in claim 11, wherein said range-setting part sets said predetermined range by using the standard deviation of a likelihood data histogram generated on random data in accordance with said iterative decoding method. 15. The data readout device as claimed in claim 11, wherein said likelihood data generated in accordance with said iterative decoding method is a value in a continuously expressed numerical range corresponding to probabilities from the maximum probability of said data bit being 1 to the maximum probability of said data bit being 0, wherein said two slice levels comprise a first slice level that is larger than the median of said numerical range and a second slice level that is smaller than the median of said numerical range, wherein said range-setting part sets said first and second slice levels to respective numerical values that are respectively larger and smaller than the median by the standard deviation of a likelihood data histogram generated on random data in accordance with said iterative decoding method. 16. The data readout device as claimed in claim 11, wherein said range-setting part sets said predetermined range based on a range of likelihood data whose frequencies in a likelihood data histogram generated on random data in accordance with said iterative decoding method do not exceed a predetermined number. 17. The data readout device as claimed in claim 11, wherein said likelihood data generated in accordance with said iterative decoding method is a value in a continuously expressed numerical range corresponding to probabilities from the maximum probability of said data bit being 1 to the maximum probability of said data bit being 0, wherein said two slice levels comprise a first slice level that is larger than the median of said numerical range and a second slice level that is smaller than the median of said numerical range, wherein said range-setting part sets said first and second slice levels to respective values that make the limits of a range that includes the median of said numerical range, wherein said range that includes the median of said numerical range is a range of likelihood data whose frequencies in a likelihood data histogram generated on random data in accordance with said iterative decoding method do not exceed a predetermined number. 18. The data readout device as claimed in claim 11, comprising a controller that can substantially disable the function of said range-setting part when the number of repetitions of the decoding process in accordance with said iterative decoding method does not exceed a predetermined number.
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