In keeping up with the recent tendency towards using multimedia equipment, a need has been felt to process a larger quantity of the information, including the video information, and to increase the capacity of a storage device used for recording the information. In particular, in the field of storage of the high-quality video information, the tendency is towards using a BD (Blu-ray Disc) which is larger in storage capacity than the DVD (Digital Versatile Disc). To increase the recording capacity of an optical disc or a HDD device, it is necessary to increase the recording density. Concomitantly, reducing an error rate to provide for sufficient reliability has become a crucial task. The following description is centered mainly about increasing the density of the optical disc.
In an optical disc, a light beam collected by an optical element is illuminated on a disc medium, and the information is detected by lightness/darkness of reflected light or by light polarization. A beam spot collected is finite and, the smaller its diameter, the higher may be the density in recording/reproduction. In this situation, an optical approach towards reducing the beam spot is progressing. The spot diameter is inversely proportionate to the NA (Numerical Aperture) of an objective lens and directly proportionate to the wavelength λ of the laser beam. The spot diameter may thus be reduced by increasing the NA and by decreasing the wavelength λ. However, if NA is increased, the depth of focus becomes shallower, such that it becomes necessary to decrease the distance between the disc surface and the lens. Thus, limitations are imposed on increasing the NA. On the other hand, in a short wavelength laser, stability in high output oscillation and long useful life are requirements. Although the wavelength used is becoming shorter, as evidenced by use of an infrared laser (λ=780 nm) for a CD, a red laser (λ=650 nm) for DVD and a blue laser (λ=405 nm) for BD, limitations are again imposed on further reducing the wavelength.
Meanwhile, the MTF (Modulation Transfer Function), which is the frequency characteristic of a transmission route between an optical head and a disc medium, is in the form of an LPF (Low-Pass Filter) in which the gain in the high frequency range is decreased by reason of the finite beam spot. Hence, even if a rectangular wave is recorded, the readout waveform from a disc becomes dull. If the recording density is increased, a waveform to be read out at a specified time point interferes with a wavelength to be read out at another time point, a phenomenon known as inter-symbol interference. By reason of this inter-symbol interference, a recorded mark shorter than a preset length becomes difficult to reproduce. If conversely the recording mark is long, the frequency with which the phase information used for extracting the clock for synchronization is output is decreased, thus causing pulling-out of synchronization. It is therefore necessary to limit the mark length to less than a certain length. The recording data for the optical disc is encoded for recording from the above described perspective of signal processing. In particular, the RLL code (Run Length Limited Code) in which limitations are imposed on the length between transitions is preferentially used. For example, EFM (Eight to Fourteen Modulation), an 8/16 code or (1, 7) PP is used. The minimum run length of the PFM modulation code used for a CD and that of the 8/16 modulation code used for a DVD are 2 (d=2). The minimum run length of the (1, 7) PP, used in a BD of higher density, is 1. This code is 2/3 as is the (1, 7) RLL, and is featured by limitations imposed on the number of succession of shortest marks.
There is also known a technique termed waveform equalization. This technique uses an inverse filter that eliminates the inter-symbol interference. The technique emphasizes the high frequency components of the readout signal and hence suppresses the inter-symbol interference. However, the high frequency components of the noise are simultaneously emphasized, thus possibly deteriorating the SNR (Signal to Noise Ratio). In particular, if the recording density is increased, the deterioration of the SNR caused by waveform equalization is mainly responsible for errors in data detection. The PR (Partial Response) equalization is a system of waveform equalization that intentionally causes known inter-symbol interference to occur on purpose. In this technique, high frequency components are usually not emphasized, thus suppressing the SNR from deterioration.
Another effective detection system is the maximum likelihood detection system, according to which detection is carried out on a data sequence known to undergo certain state transitions. The detection performance may be improved by selecting such a time-series pattern, out of all possible time-series patterns, which has a smallest value of a mean square sum of errors. However, if the above processing is carried out on real circuits, difficulties are encountered in connection with the circuit size and the operational speed. Hence, an algorithm termed a Viterbi algorithm is usually employed to progressively select the path. In this case, the Viterbi algorithm is termed the Viterbi decoding or Viterbi detection.
A detection system, which is a combination of the above mentioned PR equalization and Viterbi detection, is known as PRML (Partial Response Maximum Likelihood) system, and detects data as it performs a sort of error correction. By PR equalization, correlation along the time direction is imparted to the readout signal. Hence, only specified state transitions are presented in the data sequence obtained on sampling the readout signal. The state transitions, thus specified, and a data sequence of the noise-corrupted actual readout signal, are compared to each other, and most probable state transitions are selected, whereby errors in data detection may be reduced. A PRML detection system, in which the modulation code with the minimum run length and a PR (1, 2, 2, 2, 1) channel are used, is described in Non-Patent Document 1. With this detection system, a broader detection margin may be obtained in high density recording/reproduction.
To improve the detection performance by Viterbi detection, it is necessary that the frequency characteristic of the read channel is made to be coincident with a specified PR equalization characteristic. In this case, such PR equalization characteristic that is as close to that of the read channel as possible is to be selected. In general, the frequency characteristic is corrected using a waveform equalizer so that the PR characteristic will be as close to the preset PR characteristic as possible. Among the techniques of adaptively correcting signal deterioration with time to improve the detection performance, there are an automatic equalization system and an adaptive equalization system. Among the algorithms of adaptive equalization of the sequential type, there are a zero forcing method and a mean square method. The adaptive equalization technique is highly advantageous in that initial device adjustment is unneeded.
The operation of Viterbi detection will now be described. FIG. 12 shows signal transition states of a signal xn obtained by sampling a readout signal of a DVD in timed relation to the channel clock followed by PR (1, 3, 3, 1) equalization. FIG. 13 depicts a trellis diagram in which state transitions of FIG. 12 are plotted along the time axis. The numbers affixed to branches extending from six states of the state transition diagram represent ideal amplitude values rn of xa. For example, in an area composed of 4T spaces and 4T marks in succession in this order, the state sequence is S0→S1→S3→S7→S6→S4→S0→S0→S1→ . . . . xn at this time is ideally −4, −3, 0, 3, 4, 3, 0, −3, −4, −3, . . . . However, due to e.g., the noise, xa may become −3.9, −2.9, 0.1, 2.7, 3.8, 2.9, 0.2, −2.6, −3.9, −3.1, . . . . Now suppose that the Gaussian noise be superposed on xn and rn, assume any one of five reference levels (±14, ±3, 0), it is then the maximum likelihood detection to find rn that minimizes Σ(xn−rn)2. It is however difficult to compare all combinations in real time. In this consideration, the Viterbi algorithm sequentially performs the operation. As shown in FIG. 12, possible previous states of S0 are S0 and S4, and possible previous states of S7 are S3 and S7. Then referring to FIG. 14, the operation of selecting more plausible one of a plurality of paths entered simultaneously, such as states S0 and S7, is carried out at each time point. By tracing back the paths towards the past, the paths merge at a certain time point in one path (path merge). In short, the information prior to the merge time point may be identified.
For selecting one of the paths, an index for plausibility, termed a metric, is introduced. The plausibility Pan that a time point n is a state Sa is termed a path metric. A square of the difference between xn and the reference level r is termed a branch metric bn(r).bn(r)=(xn−r)2  (1)
Since the path metric is an integration of the branch metrics from the past, the smaller the path metric, the more plausible is the path metric. Since the path metric P1n is necessarily the state S0 one time point before, the path metric P1n is a path metric P0n-1, which is state S0 one time point before, plus the branch metric bn (−3) at the present time point, as indicated by the equation (2):P1n=P0n-1+bn(−3)  (2)
In similar manner, P3n, P4n and P6n are as indicated by the following equations (3) to (5):P3n=P1n-1+bn(0)  (3)P4n=P6n-1+bn(0)  (4)P6n=P7n-1+bn(3)  (5)
P0n is indicated by the equation (6). P0n is S0 or S4 one time point before, such that there are two paths. Out of the path metrics, a smaller one is selected. It should be noted that Min[a, b] denotes a or b, whichever is smaller.P0n=Min[P0n-1+bn(−4),P4n-1+bn(−3)]  (6)
In similar manner, P7n becomes as indicated by the following equation (7):P7n=Min[P7n-1+bn(4),P3n-1+bn(3)]  (7)
The path metrics are updated at every time point to select the path. The paths entered in all states merge in one path. Hence, by tracing back from a given time point towards the past, the paths merge to determine the information. Meanwhile, in the equations 6 and 7, it is sufficient that large-small comparison may be made between the path metrics. Since the terms xn2 in bn(r) are common to all path metrics, the following equation:bn(r)′=r2−2rxn may be used as a branch metrics, whereby the circuit may be simplified to advantage.Meanwhile, this processing is usually carried out in terms of a channel clock as a unit. Hence, a high speed processing is required. For example, at a speed eight times as fast as the DVD, the channel clock frequency exceeds 200 MHz. Hence, the processing is generally carried out by a dedicated circuit. FIG. 15 depicts a block diagram of a Viterbi detector. A path metric-branch metric addition circuit (Add), a path metric value comparator circuit (Compare) and a path selection circuit that selects the path based on the result of comparison (Select) are collectively termed an ACS circuit 52. Further, a circuit for calculating the branch metric 51 (BMG) and a memory (MEM) 53 that holds the path selection information, termed a path memory, are needed. It has been known that, if the signal level distribution after adaptive equalization for each reference level is proximate to the normal distribution, PRML detection by a Viterbi detector is able to detect even a low SNR signal that may not be detected by level detection. The low SNR signal not being detected by level detection may be exemplified by such a case where a signal has a good recording mark quality but suffers from significant jitter, a case where a signal suffers from significant inter-symbol interference, and a case where a signal has been reproduced in the defocused state.
There are however cases where the performance of a PRML detection system is inferior to that of the conventional level detector. These cases may be met when a mark non-optimum in shape has been recorded on a disc mainly due to failure (unmatching) of power control in recording. FIG. 6 shows a typical readout waveform of a 5T space+11T mark, in which the mark is a long mark suffering from distortion. Such mark has been formed by a recording strategy in which the recording power is to be reduced at the leading end of the long mark. In the level detection, an edge (transition) position of 5T space to −11T mark is closer to the 5T side, and hence may be detected without errors by bi-level slice detection. On the other hand, suppose now that the edge position be detected by PRML. In this case, one out of an ideal path (5T+11T, correct solution) and an ideal path 2 (6T+10T, error) is selected. Thus, path metrics of the two paths are now to be found. In Table 1, square values of the differences between the ideal values and x, at each time point, viz., branch metric values, are shown for the ideal paths 1, 2 in the PR (1, 3, 3, 1) channel.
TABLE 1ideal path 1ideal path 2time nxn(rn)(xn − rn)2(r2n)(xn − r2n)23−4.1−40.01−40.014−3.5−30.25−40.255−1.101.21−33.6161.532.2502.2572.0443182.243.2443.24Σ——10.96—10.36
The sum of the branch metrics gives a path metric. In a Viterbi detector, one of the two paths having a smaller path metric value is determined to be more plausible. The path metric for the ideal path 1 is 10.96, whereas that for the ideal path 2 is 10.36. Hence, the path 2 with the smaller path metric is selected, which results in an error detection.
FIG. 8 shows a typical readout waveform of a 6T space+10T mark whose long mark suffers from distortion. Such mark has been formed by a recording strategy in which the recording power is to be increased at the leading end portion of the long mark. In the level detection, an edge position of 6T space to 10T mark is closer to the 10T mark side, and hence may be detected without errors by bi-level slice detection. On the other hand, suppose now that this be detected by PRML. In this case, an ideal path (5T+11T, error) or an ideal path 2 (6T+10T, correct solution) is to be selected. Path metrics of the two paths are now to be found. In Table 2, square values of the differences between the ideal values and x, at each time point, viz., branch metric values, are shown for the ideal paths 1, 2 in the PR (1, 3, 3, 1) channel.
TABLE 2ideal path 1ideal path 2time nxn(rn)(xn − rn)2(r2n)(xn − r2n)23−4.1−40.01−40.014−3.5−30.25−40.255−1.201.44−33.2461.034.001.074.540.2532.2584.140.0140.01Σ——5.96—6.76
The sum of the branch metrics gives a path metric. In the Viterbi detector, one of two paths which has a smaller path metric value is determined to be more plausible. The path metric for the ideal path 1 is 5.96, while that for the ideal path 2 is 6.76. Hence, the path 1 with the smaller path metric is selected, which results in an error detection.
The level detection is a conventional technique used in many optical disc drives. Even in an optical disc drive, provided with a Viterbi detector, which has come to be used these days, reproduction compatibility with respect to a disc recorded with a conventional drive is indispensable. It is true that, in detection by PRML, the reproduction performance may be expected to be improved. However, in detecting a readout signal with strong non-linearity, such as a case with a non-optimum recording mark, the detection performance may become inferior to that with threshold detection, which is counted as a drawback.
Patent Document 1 discloses a measure to be taken in avoiding the deterioration in the performance in the detection by PRML of a signal suffering from waveform distortion. FIG. 16 shows an arrangement of the Viterbi detector. An ACS circuit 102 takes charge of addition of path metrics and branch metrics generated by a branch metric generation circuit 101, comparison of path metric values, and path selection which is based on the results of comparison. A path memory 103 processes the path selection information. A maximum likelihood decision unit 104 selects and outputs data which represents the smallest path metric value. However, upon path selection, the information obtained on comparing an input signal entered to the Viterbi detector at a time point a preset number of channel clocks before, and a threshold value α, to each other by a comparator 106 is used to make the latest path decision, in addition to the path metric values. In particular, if the input signal at the time point a preset number of channel clocks before has an amplitude corresponding to that of a mark, such a path where the marks are consecutively arrayed is selected. If otherwise, path selection is made on the basis of the path metric values. By such path selection, such a path where marks are consecutively arrayed may be selected even in case there is mark distortion, thus prohibiting an error in data detection.
Patent Document 2 discloses an alternative method for detecting the waveform distortion by PRML. FIG. 17 depicts a block diagram showing its formulation. In this method, the information recorded on a magneto-optical recording medium of domain wall displacement detection type 204 is read out by a magnetic head 206 and an optical head 205. The waveform distortion produced at this time is removed at the outset by a limiter circuit 201. After equalization by a PR equalizing circuit 202 to a PR (1, −1) channel, the waveform distortion is detected by a Viterbi detection circuit 203.    [Patent Document 1] JP Patent Kokai Publication No. JP-P2008-287763A, which corresponds to US Patent Application Publication No. US2010/0135142A1.    [Patent Document 2] JP Patent Kokai Publication No. JP-P2005-011385A, which corresponds to US Patent Application Publication No. US2004/0252589A1.    [Non-Patent Document 1] Ogawa, Homma et al., ‘Development of Technology of FID DVD Device Implementation (Recording Technique), Technical Report for Society of Information Media, ITE Technical Report, Vol. 28, No. 43, pp. 17 to 20, MMS2004-38, CE2004-39 (July 2004)