1. Field of the Invention
The present invention relates to a technology for accessing a storage medium and, more particularly, to a baseline correction apparatus for correcting a baseline of input signal.
2. Description of the Related Art
In the area of disk drives, attention has recently been focused on disk storage apparatuses of vertical magnetic recording system which are capable of increasing the recording density. With the conventional disk drives of longitudinal magnetic recording method, a magnetization corresponding to binary recorded data is formed in the longitudinal direction of the disk medium. In contrast to this, with the disk drives of vertical magnetic recording method, the same magnetization is formed in the depth direction of the disk medium.
Generally, with disk drives, data are recorded on the disk medium by a NRZ (non-return-to-zero) record encoding method. When the recorded data are read by the head from the surface of the disk medium, the reproduced signals (read signals) are a dipulse signal sequence for a longitudinal magnetic recording system. For a vertical magnetic recording system, on the other hand, the same reproduced signals are a pulse signal stream including a direct current (DC) low-frequency component.
Generally, the read channel system (reproduced signal processing system including a read amplifier) of a disk drive is such that the analog front-end circuit of a read amplifier, AC coupling, and the like has a low-frequency cutoff characteristic. This is intended in part to improve the SNR (signal-to-noise ratio) of reproduced signals by removing unnecessary low-frequency noise component from reproduced signals.
In a vertical magnetic recording system, the reproduced signals contain low-frequency components, and thus a phenomenon in which the baselines of reproduced signals are varied is observed when the low-frequency noise components are cut off by the analog front-end circuit having a low-frequency cutoff characteristic. If such variation in the baseline of reproduced signals occurs, a problem of higher decoding error rate will arise when the recorded data are decoded from the reproduced signals.
To resolve this problem, one possible solution may be to lower the low-frequency cutoff frequency of the read channel system. However, simply widening the passband will lead to an SNR deterioration of reproduced signals because it cannot cut off the low-frequency noise components. Moreover, the read amplifier, in particular, is normally sensitive to the low-frequency noise, such as 1/f noise, so that it is even more subject to the SNR deterioration. Hence, with a vertical magnetic recording system, simply lowering the low-frequency cutoff frequency of the read channel will rather result in raising the error rate.
As a conventional method for countering the baseline variation, there has been a proposed technique in which an ideal value of baseline and the difference of it from the actual value of baseline are determined and correction is made by feeding back the value of difference to a process before the input side of the A-D converter (See Reference (1) in the following Related Art List, for instance). Also, another proposed technique achieves a baseline without variation by first obtaining reverse characteristics of varying components of the baseline and then finding the differences from the varying baseline (See Reference (2), for instance). Also, there is a proposed method for correcting the baseline variation by the use of the total value of detected direct current components of analog signals (See Reference (3), for instance).
Related Art List
(1) Japanese Patent Application Laid-Open No. 2004-127409.
(2) Japanese Patent Application Laid-Open No. Hei11-185209.
(3) Japanese Patent Application Laid-Open No. Hei11-266185.
Under these circumstances, the inventors have come to realize the following problem. Conventionally, baseline correction has been done by calculating the necessary amount of correction and feeding it back to a preceding stage, which results in a delay in the timing of correction as much as the time taken to calculate the amount of correction. And this delay can be fatal and unacceptable to storage apparatuses of recent years which must make access for read and write at a speed exceeding 1 Gbps. In other words, a baseline correction, when done by the conventional method, can be inaccurate because the amount of correction to be used in the correction is based on past data. The problem therefore is that where access at higher speed is required, this baseline variation can adversely affect the subsequent circuits for error correction and the like.