Abstract:
An apparatus has an input hold circuit for sequentially holding digital feedback signals sequentially transmitted at a sampling period, and for sequentially outputting the transmitted feedback signals or held feedback signals at a period which is shorter than the sampling period; a notch filter for applying a filtering processing to the feedback signals outputted from the input hold circuit; and an output hold circuit for sequentially outputting the feedback signals subjected to the filtering processing by said notch filter, while thinning the same, at a period which is same as the sampling period.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a feedback controller for performing a feedback control through detecting a driving state of a driving unit, a digital filter device which is preferably applicable to the feedback controller, and a storage device having a head for at least reproducing information stored in an information storage medium. 
     2. Description of the Related Art 
     Hitherto, there is widely used in various fields a feedback controller in which a feedback signal is generated through detecting a driving state of a driving unit, an arithmetic processing such as a phase compensation for a stabilization of a control system is performed, and a driving control of the driving unit is performed in accordance with the feedback signal after the arithmetic processing. 
     According to the feedback controller as mentioned above, in order to enhance the tracking efficiency, there is adopted such a scheme that a phase-lag compensation is established, a feedback gain is increased to establish a high frequency band. 
     It is known that when it is intended to increase a gain to a high frequency by the phase-lag compensation, a phase cross frequency is increased, and then the gain margin and the phase margin are decreased, so that a stability of a control system is damaged. 
     Therefore, there is a need to establish a high frequency band through increasing a gain cross frequency to some extent. However, in the even that in order to increase the gain cross frequency, the feedback gain is increased, there is a need to establish a phase-lead compensation over a high frequency so that a control system is stabilized in the high frequency. In this case, there is a possibility that the stability of the control system is damaged by a high-order resonance of a movable mechanism unit. 
     In order to solve this problem, there is adopted a scheme that a notch filter or a low pass filter is applied to a high-order resonance frequency. However, in case of a digital filter used in a digital control using a microprocessor unit (MPU), such as a digital signal processor (DSP) and the like which are used for the purpose of establishing low cost and high-performance, the digital filter does not sufficiently operate at a frequency close to the Nyquist frequency which is ½ of the sampling frequency. Therefore, in the event that a high-order resonance frequency of the movable mechanism unit exists at a frequency band close to the Nyquist frequency, it is difficult to ensure a sufficient stability of the control system. 
     The above-mentioned problem will be described referring to by way of example a feedback controller adopted in an optical storage device for accessing an optical disk. As the optical disk, for example, a phase change optical disk and a magneto optical disk exist. Here, typically, an optical storage device for accessing the magneto optical disk will be considered. 
     Hereinafter, first, a guide line of the optical storage device will be described, and then problems of the feedback control system will be described. 
     FIG. 1 is a perspective view of an optical storage device. 
     A spindle motor  101  for driving an optical disk  200  is fixed on a drive base  100 , for example, made of an aluminum. Further, on the drive base  100 , there are provided a movable mechanism unit  110  having an objective lens  111  and a magnetic coil  112 , and a magnetic circuit  121  having a permanent magnet disposed in such a manner that the movable mechanism unit  110  is sandwiched by the permanent magnet. The magnetic circuit  121  having the permanent magnet and the magnetic coil  112  constitutes a voice coil motor (VCM). As a current is supplied to the magnetic coil  112 , the movable mechanism unit  110  moves in a direction of an arrow A-A′ by an interaction of the current flowing through the magnetic coil  112  and the magnetic circuit. A laser beam is applied from a fixed optical unit  130  to the objective lens  111 . The laser beam is emitted from the objective lens  111  so that an optical spot is projected onto an optical disk  200  and reflected therefrom. The reflected light is returned through the objective lens  111  to the fixed optical unit  130 , so that information stored in the optical disk  200  is picked up. 
     FIG. 2 is a schematic construction view of an optical system of an optical storage device. 
     A laser beam emitted from a semiconductor laser  131  passes through a collimator lens  132  and a polarization beam splitter  133 , and reflects-on a reflecting mirror  113 , and further passes through the objective lens  111 , and finally be condensed on the optical disk  200 . 
     Here, only the reflecting mirror  113  other than the objective lens  111  is mounted on the movable mechanism unit  110 , and other all optical elements constitute the fixed optical unit  130 . 
     A signal light reflected on the optical disk  200 , which carries information recorded on the optical disk  200 , passes through the objective lens  111 , reflects on the reflecting mirror  113 , enters the polarization beam splitter  133 , and goes to a beam splitter  134  side. An incident light to the beam splitter  134  is divided into two parts one of which enters Wollaston prism  135  whereby the light is separated in accordance with the polarization direction. And the light thus separated enters via a lens  136  a photo detector  137  for picking up information recorded on the optical disk  200 . 
     On the other hand, another of the two parts of light divided by the beam splitter  134  enters via a lens  138  a beam splitter  139  wherein the light is further divided into two parts one of which enters a photo detector  140  for a tracking error detection, and another enters a wedge prism  141  wherein a light beam is further divided into two parts and is projected onto a photo detector  142  for a focus error detection. 
     FIG. 3 is a block diagram of a feedback controller for driving the movable mechanism unit  110  of the optical storage device. 
     A position of the movable mechanism unit  110  is detected by a positional sensor  150 . The positional sensor  150  comprises the photo detector  140  for tracking an error detection, as shown in FIG. 2, and a signal processing circuit (not illustrated) for processing signals obtained by the photo detector  140 . A detection signal obtained by the positional sensor  150  is fed to a differential circuit  152  in which a difference between the detection signal and a target value signal representative of a position of the movable mechanism unit  110 , outputted from a target value generating circuit  151 , is calculated to generate an error signal. The error signal thus generated is attenuated in high frequency component by an anti-aliasing filter  153  for suppressing a frequency component exceeding Nyquist frequency which is the half of a sampling frequency of an A/D converter  154 . The A/D converter  154  converts the error signal thus obtained into a digital signal. A signal outputted from the A/D converter  154  is fed to a phase compensation filter  155  wherein the signal is subjected to a phase compensation processing so as to ensure a stability of a control system through a digital operation. A signal subjected to the phase compensation processing by the phase compensation filter  155  is fed to a driving circuit  156  in the form of a control signal to control the movable mechanism unit  110  to a target position. Incidentally, it is acceptable that the driving circuit  156  receives a digital control signal, or alternatively it is acceptable that the driving circuit  156  receives an analog control signal in such a manner that the output of the phase compensation filter  155  is converted into the analog control signal by a D/A conversion. 
     The driving circuit  156  supplies a driving signal (here a current signal) to an electromagnetic coil  112  (cf. FIG. 1) of the movable mechanism unit  110  in accordance with the entered control signal, and whereby the movable mechanism unit  110  is controlled to a target position. 
     FIGS.  4 (A) and  4 (B) are views showing an example of a frequency characteristic of a displacement of the movable mechanism unit  110  to a current supplied from the driving circuit  156 . FIG.  4 (A) shows a gain characteristic, and FIG.  4 (B) shows a phase characteristic. 
     Here, the sampling frequency 55 kHz is used. Therefore, the Nyquist frequency is 27.5 kHz which is ½ of the sampling frequency. 
     The anti-aliasing filter  153  can sufficiently attenuate a high-order of resonance and an electric noise component exceeding 30 kHz. In the event that a cut-off frequency of the anti-aliasing filter  153  is set up to a further low frequency side, it would have an effect on a frequency band related to a control stability in a phase delay. Consequently, it is impossible to set up the cut-off frequency to the lower frequency side than 30 kHz. 
     At the lower frequency side than 30 kHz, as shown in FIG.  4 (A), there exists three high-order of resonance of frequencies A, B and C, where A is about 16 kHz, B is about 22 kHz, and C is about 27.5 kHz which is substantially the same as the Nyquist frequency. 
     These high-order of resonance of frequencies cause the control system to be insecure. 
     FIGS.  5 (A) and  5 (B) are views showing frequency characteristics wherein two notch filters of 16 kHz and 22 kHz and a phase-lead compensation are disposed in a feedback system. 
     An effect of the differentiation appears up to the vicinity of the Nyquist frequency to ensure a phase margin at the gain close frequency (a frequency at the point where a gain curve crosses 0 dB), and the gain rises. This filter characteristic shows a driving sensitivity to the input error signal, and as seen from FIG.  5 (A), the driving sensitivity offers the highest value to an input of the Nyquist frequency (about 27.5 kHz). 
     FIGS.  6 (A) and  6 (B) are views showing frequency characteristics wherein two notch filters of 16 kHz and 22 kHz and a phase-lead compensation are disposed in a feedback system, in a similar fashion to that of FIGS.  5 (A) and  5 (B), and in addition a notch filter of 27.5 kHz is disposed in the feedback system. 
     As will be understood from FIGS.  6 (A) and  6 (B), the notch filter almost has no effect at the frequency band (high-order of resonance C) near the Nyquist frequency. 
     FIG. 7 is a view showing a signal wave form of an error signal outputted from the differential circuit  152  when the noise of 27.5 kHz is added to the feedback control system, and a driving signal outputted from the driving circuit  156 . FIG. 8 is a view showing a signal wave form in which a part of the signals shown in FIG. 7 is enlarged on a time basis. 
     As shown in FIGS. 7 and 8, it will be understood that both the error signal and the driving signal vibrate at 27.5 kHz. FIGS. 7 and 8 show a result when the noise of 27.5 kHz is added intentionally to the feedback control system for the purpose of a confirmation of the stability of the control system. However, it is poor in the stability of the control system, and in the event that the movable mechanism unit  110  is considered in connection with an unevenness on production, there is a high possibility that a defective unit in which the movable mechanism unit  110  is excited unstably appears. 
     In the event that a high-order resonance frequency of the movable mechanism unit exists at the frequency band near the Nyquist frequency, as mentioned above, in order to ensure a sufficient stability of the control system, hitherto, there is a need to increase the sampling frequency, or alternatively to change a design of the movable mechanism unit so as to increase the high-order resonance frequency. 
     However, in order to increase the sampling frequency, there is a need that the A/D converter and other digital signal processing system are operated at high speed. In many cases, it is not permitted in view of the cost of manufacturing. On the other hand, there is a limit in a point that the high-order resonance frequency of the movable mechanism unit is increased, and also with respect to the change of the design, in many cases, it is difficult through a change of the design to expect that the high-order resonance frequency of the movable mechanism unit is increased to a sufficiently high frequency. Further, in some case, it is difficult to change the design per se. 
     FIGS.  9 (A)-(C) are explanatory views useful for understanding another problem involved in the vibration of the driving signal. 
     FIG.  9 (A) shows a signal waveform of a driving signal which is essentially necessary. FIG.  9 (B) shows a signal waveform of a signal in which a high frequency noise is superposed on the essentially necessary driving signal. FIG.  9 (C) shows a signal waveform of a signal in which a further large high frequency noise is superposed on the essentially necessary driving signal and thereby reaching a saturation level. 
     As the high frequency noise is superposed on the driving signal, even if the high-order resonance of the movable mechanism unit does not exist on the frequency, the saturation of the high frequency noise brings about disappearance or attenuation of the essentially necessary driving signal as shown in FIG.  9 (C). Thus, there is a possibility that a normal feedback control is not performed. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide a feedback controller capable of ensuring a sufficient stability, a digital filter which is preferably applicable to the feedback controller, and a storage device having a head for at least reproducing information stored in an information storage medium, and being capable of driving the head stably and with a sufficient tracking performance. 
     To achieve the above-mentioned objects, the present invention provides a feedback controller wherein a driving state of a driving unit is detected to generate feedback signals so that a driving control for the driving unit is performed, said feedback controller comprising: 
     a sampling unit for sampling on a digital basis the feedback signals at a predetermined first period; 
     a filter input unit for sequentially holding the feedback signals sequentially transmitted from said sampling unit at the first period, and for sequentially outputting the transmitted feedback signals or held feedback signals at a second period which is shorter than the first period; 
     a digital filter for applying a filtering processing to the feedback signals outputted from said filter input unit; and 
     a filter output unit for sequentially outputting the feedback signals subjected to the filtering processing by said digital filter, while thinning the same, at the first period. 
     In the above-mentioned feedback controller, it is acceptable that the feedback signals, which are generated upon detection of a driving state of the driving unit, are signals per se derived through a sensor for detecting the driving state, or alternatively it is acceptable that the feedback signals are signals processed involved in the signals derived from the sensor, for example, error signals representative of differences between the signals derived from the sensor and a target positional signal. 
     According to the present invention, the filter input unit generates the feedback signals at the second period which is shorter than the first period (a sampling period), and the digital filter applies a filtering processing to the feedback signals at the second period. This feature makes it possible to expect a sufficient filtering effect as to a frequency near for example a Nyquist frequency. Further, according to the present invention, the filter output unit restores the signals outputted from the digital filter to signals of the original first period. Thus, it is sufficient that only the portion of the digital filter is subjected to a high frequency arithmetic operation. Therefore, as compared with a case where a sampling frequency in the sampling unit is increased per se, there is no need to increase an operating speed for a digital arithmetic operation processing for the sampling unit and other than the portion of the digital filter. Thus it is possible to greatly suppress a manufacturing cost. 
     In the feedback controller according to the present invention as mentioned above, it is preferable that said filter input unit sequentially outputs the transmitted feedback signals or the held feedback signals at the second period which is 1/N (where N=integer) of the first period. 
     Adoption of the period which is 1/N (where N=integer) of the first period makes it possible to simplify an arrangement of the filter input unit and the filter output unit. 
     In the feedback controller according to the present invention as mentioned above, it is acceptable that said digital filter is a notch filter for eliminating or attenuating a specific frequency component, for example, a frequency component which is the same as a Nyquist frequency. 
     In the feedback controller according to the present invention as mentioned above, while the feedback controller according to the present invention is not restricted in a use, it is preferable that the feedback controller controls a detection head for picking up information stored in an information recording medium in such a manner that the detection head comes close to the information recording medium and moves, and more particularly the feedback controller controls a movement of the detection head. 
     Here, it is acceptable that the information recording medium is an optical disk or alternatively a magnetic disk. And thus, it is acceptable that the detection head is the optical head as show in FIGS. 1 and 2, or alternatively a magnetic head for electro-magnetically picking up information. 
     Further, according to the present invention, there is provided a digital filter device comprising: 
     a filter input unit for sequentially holding digital signals, which are sequentially transmitted at a predetermined first period, at the first period, and for sequentially outputting the transmitted digital signals or held digital signals at a second period which is shorter than the first period; 
     a digital filter for applying a filtering processing to the digital signals outputted from said filter input unit; and 
     a filter output unit for sequentially outputting the digital signals subjected to the filtering processing by said digital filter, while thinning the same, at the first period. 
     According to the digital filter of the present invention as mentioned above, it is possible to have an effective filtering function on the frequency near the Nyquist frequency. 
     Furthermore, according to the present invention, there is provided a storage device having a head for at least reproducing information stored in an information storage medium, said storage device comprising: 
     a driving unit for moving and controlling said head; 
     a feedback signal generating unit for detecting a position of said head and generating feedback signals to be fed to said driving unit on a feedback basis; 
     a sampling unit for sampling on a digital basis the feedback signals at a predetermined first period; 
     a filter input unit for sequentially holding the feedback signals sequentially transmitted from said sampling unit at the first period, and for sequentially outputting the transmitted feedback signals or held feedback signals at a second period which is shorter than the first period; 
     a digital filter for applying a filtering processing to the feedback signals outputted from said filter input unit; and 
     a filter output unit for sequentially outputting the feedback signals subjected to the filtering processing by said digital filter, while thinning the same, at the first period. 
     In the storage device according to the present invention as mentioned above, it is acceptable that said driving unit is a track driving unit for moving said head in a track direction, or alternatively it is acceptable that said driving unit is a focus driving unit for moving said head in a focus direction. 
     As seen in the storage device having a head for reproducing information stored in, for example, an optical disk, the driving in the track direction is to perform a double servo by both the carriage and the track actuator. In some cases, there is provided a single servo in which only a carriage has a driving unit. The storage device according to the present invention is applicable to both the single servo and the double servo. And further, in case of the double servo, the storage device according to the present invention is applicable to either one or both of the carriage and the track actuator. 
     According to the storage device of the present invention, it is possible to improve the follow-up performance of the head. That is, an application of the storage device to a track driving unit for moving a head in a track direction makes it possible to improve a track follow-up performance of the head, since the head moves promptly, and thus a control band is expanded. Further, an application of the storage device to a focus driving unit for moving the head in a focus direction makes it possible to improve the follow-up performance of the head in the focus direction, and whereby the head promptly responds to being out of focus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an optical storage device. 
     FIG. 2 is a schematic construction view of an optical system of an optical storage device. 
     FIG. 3 is a block diagram of a feedback controller for driving the movable mechanism unit of the optical storage device. 
     FIGS.  4 (A) and  4 (B) are views showing an example of a frequency characteristic of a displacement of the movable mechanism unit  110  to a current supplied from the driving circuit  156 . 
     FIGS.  5 (A) and  5 (B) are views showing frequency characteristics wherein two notch filters of 16 kHz and 22 kHz and a phase-lead compensation are disposed in a feedback system. 
     FIGS.  6 (A) and  6 (B) are views showing frequency characteristics wherein two notch filters of 16 kHz and 22 kHz and a phase-lead compensation are disposed in a feedback system, in a similar fashion to that of FIGS.  5 (A) and  4 (B), and in addition a notch filter of 27.5 kHz is disposed in the feedback system. 
     FIG. 7 is a view showing a signal wave form of an error signal outputted from the differential circuit when the noise of 27.5 kHz is added to the feedback control system, and a driving signal outputted from the driving circuit. 
     FIG. 8 is a view showing a signal wave form in which a part of the signals shown in FIG. 7 is enlarged on a time basis. 
     FIGS.  9 (A)- 9 (C) are explanatory views useful for understanding another problem involved in the vibration of the driving signal. 
     FIG. 10 is a block diagram of an optical storage device to which the present invention is applied. 
     FIG. 11 is a block diagram of a feedback controller for driving a movable mechanism unit of an optical storage device according to the present invention. 
     FIG. 12 is a time chart useful for understanding a control at the time of detection of information of an optical storage device. 
     FIGS.  13 (A) and  13 (B) are explanatory views useful for understanding an operation of an input hold circuit. 
     FIG. 14 is a circuit block diagram of a notch filter wherein signals of a notch operation ( 1 ) are added. 
     FIG. 15 is a circuit block diagram of a notch filter wherein signals of a notch operation ( 2 ) are added. 
     FIGS.  16 (A) and  16 (B) are views showing frequency characteristics wherein two notch filters of 16 kHz and 22 kHz and a phase-lead compensation are disposed in a feedback system of a track servo, in a similar fashion to that of FIGS.  5 (A) and  5 (B) and FIGS.  6 (A) and  6 (B), and in addition a notch filter of 27.5 kHz having an structure shown in FIG. 11 is disposed in the feedback system. 
     FIG. 17 is a view showing a signal wave form of an error signal when the noise of 27.5 kHz is added to the feedback control system having the frequency characteristics shown in FIGS.  16 (A) and  16 (B), and a driving signal. 
     FIG. 18 is a view showing a signal wave form in which a part of the signals shown in FIG. 17 is enlarged on a time basis. 
     FIG. 19 is a block diagram of a feedback controller according to an alternative embodiment of the present invention. 
     FIG. 20 is a block diagram of a feedback controller according to a further alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, there will be described embodiments of the present invention. Here, there will be described an optical storage device by way of example. 
     FIG. 10 is a is a block diagram of an optical storage device to which the present invention is applied. In FIG. 10, an optical disk  310  is a medium which is optically readable and writable and is accommodated in a cartridge (not illustrated). As the cartridge of the optical disk  310  is loaded onto the device, a spindle motor  312  rotates the optical disk  310  at a constant speed. 
     Against the optical disk  310 , a carriage  314  is disposed in such a manner that it is movable in a radius direction of the optical disk  310 . The carriage  314  is loaded with an optical head movable section  318 . The carriage  314  is moved by a carriage driving coil in the radius direction of the optical disk  310 . Specifically, the voice coil motor  316 , which is referred to in FIG. 1, is used to move the carriage  314 . 
     The optical head movable section  318 , which is loaded onto the carriage  314 , is provided with an objective lens  322 . The objective lens  322  condenses laser beams emitted from an optical head fixed section  320  on a medium surface of the optical disk  310  so as to form a beam spot. Further, the objective lens  322  is moved by a focus actuator driving coil  326  in an optical axis direction, so that a focal point control is performed to form a prescribed beam spot on the medium surface of the optical disk  310 . 
     On the optical head fixed section  320 , there is provided a laser photo detector  330  for detecting returned light of the beam spot imaged on the optical disk  310  by the objective lens  322 . A received optical signal of the laser photo detector  330  is fed to an AGC amplifier  354 . The AGC amplifier  354  outputs a focus error signal E 5  and a track error signal E 6 . Of course, the optical head fixed section  320  is provided with a laser light source for emitting a laser beam to the optical head movable section  318 . The detail of the optical head fixed section  320  is the same as FIG.  2 . 
     A DSP  340  is provided to control a servo mechanism of the carriage  314  and the focus actuator driving coil  326 . 
     The DSP  340  includes AD converters (ADC) and DA converters (DAC). As the DSP  340 , for example, Fujitu product MB86312 can be used. The DSP  340  further comprises a focus servo unit  358  which serves as a focus servo arithmetic unit, a track servo unit  364  which serves as a track servo arithmetic unit, and a seek control unit  372  which serves as a seek arithmetic unit. 
     Thus, the focus error signal E 5  and the track error signal E 6  outputted from the AGC amplifier  354  are fed via AD converters  356  and  362  to the focus servo unit  358  and the track servo unit  364 , respectively. Current indication data E 11  and E 12  outputted from the focus servo unit  358  and the track servo unit  364  are fed on a feedback basis via DA converters  360  and  366  to driving circuits  410  and  412 , respectively, so that a focal point control for a beam spot and an on-track control are performed. 
     The seek control unit  372  performs a control for a position determination of the carriage  314  to a target track position in accordance with a seek command from a microprocessor unit (MPU)  420 , for example, Fujitsu product MB86312. The track error signal E 6  outputted from the AGC amplifier  354  is fed to a comparator  368  to detect a TES zero cross signal E 7 . 
     The TES zero cross signal E 7  is counted by a counter  371  during a predetermined period of time measured by a timer  370 , and is compared with a target velocity by the seek control unit  372 . The track servo unit  364  outputs a current indication data E 12  to the DA converter  366  so that the deviation becomes zero, and performs a feedback control for the driving circuit  412 . 
     To perform a seek control, the seek control unit  372  generates a track jump output instruction and simultaneously inhibits a generation of a track servo on signal E 8 , so that the on-track control by the track servo unit  364  is released. Thus, an inhibition of a generation of a track servo on signal E 8  releases the on-track control and performs a seek operation in accordance with the track jump output instruction. 
     In this manner, the optical storage device is provided with three feedback controls in the focus servo, the track servo and the seek control. 
     FIG. 11 is a block diagram of a feedback controller for driving a movable mechanism unit of an optical storage device according to the present invention. In FIG. 11, the same parts are denoted by the same reference numbers as those of FIG. 3, and a redundant description will be omitted. 
     In the feedback controller shown in FIG. 11, between the phase compensation filter  155  and the driving circuit  156 , which are also shown in FIG. 3, there are disposed an input hold circuit  157 , a notch filter  158  and an output hold circuit  159 . The input hold circuit  157 , the notch filter  158  and the output hold circuit  159  correspond to examples of the filter input unit, the digital filters and the filter output unit, respectively, referred to in the present invention. These three elements constitute a digital filter according to the embodiment of the present invention. Operation of these three elements will be described later. 
     The present invention is to perform a feedback control and is applicable to ones involved in the above-mentioned problems. Consequently, the present invention is applicable to three feedback controls in the focus servo, the track servo and the seek control in the optical storage device as shown in FIG.  10 . 
     Particularly, in the event that the present invention is applied to the focus servo and the track servo, the movable mechanism unit  110  serves as the carriage and the focus actuator. The positional sensor  150 , the target value generating circuit  151 , and the differential circuit  152  correspond to an error signal generating circuit (omitted in FIG. 10) for generating the track error signal and the focus error signal. 
     The track error signal and the focus error signal amplified by the AGC amplifier  354  are applied to the anti-aliasing filter and then applied to the AD converters  362  and  356  of the DSP 340 , respectively. 
     In the focus servo unit  358  and the track servo unit  364 , the associated signals pass through the phase compensation filter  155 , the input hold circuit  157 , the notch filter  158  and the output hold circuit  159 , as shown in FIG. 11, and then be applied through a D/A converter (not illustrated) to the driving circuit  156  on a feedback basis. 
     FIG. 12 is a time chart useful for understanding a control at the time of detection of information of an optical storage device. 
     During a sampling time T which is a time interval between the adjacent sampling clocks (part (A) of FIG.  12 ), a focus servo arithmetic unit (part (B) of FIG. 12) is operate, and then a track servo arithmetic unit (part (C) of FIG. 12) is operated. 
     The focus servo arithmetic unit performs a focusing in such a manner that the objective lens  111  (cf. 
     FIG. 1) is translated in a direction wherein the objective lens  111  approaches the optical disk  200  or goes away from the optical disk  200 , on the basis of a detection signal of the photo detector  142  (cf. FIG. 2) for a focus error detection. This is not related to the present invention on a direct basis, and thus the detailed description will be omitted. 
     The track servo arithmetic unit corresponds to the portion of the A/D converter  154  to the output hold circuit  159  of the feedback controller shown in FIG.  11 . 
     Here, first, an A/D conversion is performed by the A/D converter  154  (part (C 1 ) of FIG.  12 ), then a phase compensation arithmetic operation is performed by the phase compensation filter  155  (part (C 2 ) of FIG.  12 ). Further, an input hold by the input hold circuit  157  (part (C 3 ) of FIG.  12 ), a notch arithmetic operation by the notch filter  158  (part (C 4 ) of FIG. 12) and an output hold by the output hold circuit  159  (part (C 5 ) of FIG. 12) are performed in the named order. The notch arithmetic operation by the notch filter  158  is divided into a notch arithmetic operation ( 1 ) and a notch arithmetic operation ( 2 ). 
     Hereinafter, the input hold, the notch arithmetic operation ( 1 ), the notch arithmetic operation ( 2 ), and the output hold are explained in the named order. 
     FIGS.  13 (A) and  13 (B) are explanatory views useful for understanding an operation of the input hold circuit  157 . Here, for the purpose of simplification, the phase compensation filter  155  is out of the consideration. 
     It is assumed that digital signals denoted by the black dots shown in FIG.  13 (B) are obtained in such a manner that an analog signal shown in FIG.  13 (A), which is a signal before the conversion by the A/D converter  154 , is subjected to a sampling with a sampling period T by the A/D converter  154  and then converted into a digital signal. The phase compensation filter  155  permits the digital signals denoted by the black dots to pass through as they are and to be fed to the input hold circuit  157 . Upon receipt of the digital signals denoted by the black dots, the input hold circuit  157  holds sequentially the digital signals at a period T of receipt of the digital signals, and outputs sequentially the received digital signals or the held digital signals at a period T/2. That is, the input hold circuit  157  sequentially outputs the digital signals represented by the black dots and white dots shown in FIG.  13 (B). Further, in other words, the input hold circuit  157  outputs the digital signals which are substantially equivalent to a matter that a sampling period T is changed to T/2. 
     FIG. 14 is a circuit block diagram of a notch filter wherein signals of the notch operation ( 1 ) are added. 
     Here, x(t) is a new input, and T is a sampling period. Thus, x(t−T) implies a sampling signal before one period with respect to x(t). z −1  denotes a delay unit of T/2. a 0 , a 1 , a 2 , b 1 , b 2  denote notch filter constants of sampling time T/2 and notch frequency  1 /(2T). That is, the notch filter shown in FIG. 14 is arranged to cut a frequency which is the same as the Nyquist frequency. 
     In timing represented by the signals added in FIG. 14, an output y (t−T/2) is expressed by the following equation. 
     
       
           y ( t−T/ 2) =a   0   x ( t ) +a   1   x ( t−T ) +a   2   x ( t−T ) −b   1   y ( t−T ) +b   2   y ( t −3 T /2) 
       
     
     It is noted that the output of the notch filter in this timing is not fed to the output hold circuit  159  and thus is not transmitted to the driving circuit  156 . 
     FIG. 15 is a circuit block diagram of a notch filter wherein signals of a notch operation ( 2 ) are added. 
     As time elapses by T/2 from the state of the signal shown in FIG. 14, the state is changed to a state of the signal shown in FIG.  15 . That is, in this timing, an output y (t) of the notch filter is expressed by the following equation. 
     
       
           y ( t ) =a   0   x ( t ) +a   1   x ( t ) +a   2   x ( t−T )− b   1   y ( t−T /2) +b   2   y ( t−T ) 
       
     
     As time further elapses by T/2, the state is changed to a state shown in FIG. 14, and as time furthermore elapses by T/2, the state is changed to a state shown in FIG.  15 . These are alternatively repeated. 
     The output hold circuit  159  does not receive the output y (t−T/2) in the state shown in FIG. 14, but receives and holds the output y (t) in the state shown in FIG.  15 . Thus, the output hold circuit  159  sequentially outputs the digital signals, which are subjected to a filtering processing by the notch filter  158 , at a period T which is the same as the sampling time T. 
     FIGS.  16 (A) and  16 (B) are views showing frequency characteristics wherein two notch filters of 16 kHz and 22 kHz and a phase-lead compensation are disposed in a feedback system of a track servo, in a similar fashion to that of FIGS.  5 (A) and  5 (B) and FIGS.  6 (A) and  6 (B), and in addition a notch filter of 27.5 kHz having an structure shown in FIG. 11 is disposed in the feedback system. Incidentally, while FIG. 11 fails to show notch filters of 16 kHz and 22 kHz, those filters are disposed as a need arises. 
     In comparison of the point C in FIG.  16 (A) with the point C in FIGS.  6 (A), it would be understood that the arrangement according to the present embodiment causes the frequency identical with the Nyquist frequency to greatly attenuate. 
     FIG. 17 is a view showing a signal wave form of an error signal (an output signal of the differential circuit  152 ) when the noise of 27.5 kHz is added to the feedback control system having the frequency characteristics shown in FIGS.  16 (A) and  16 (B), and a driving signal (an output signal of the driving circuit  156 ). FIG. 18 is a view showing a signal wave form in which a part of the signals shown in FIG. 17 is enlarged on a time basis. 
     In comparison of FIGS. 17 and 18 with FIGS. 7 and 8, it would be understood that the control system is sufficiently stabilized as to the high frequency noises of noise of 27.5 kHz. 
     According to the present embodiment, the input hold circuit generates digital signals of a period T/2 to a sampling period T. However, it is not restricted to the period T/2. It is acceptable that the input hold circuit generates digital signals of 1/N (where N=integer) of the sampling period T (for example, T/3, T/4 etc.), or alternatively it is acceptable that the input hold circuit generates digital signals of an independent period, which is shorter than the sampling period T, but not 1/N (where N=integer) of the sampling period T. Even if the input hold circuit generates digital signals of any period, the output hold circuit restores the digital signals to digital signals of a period which is the same as the original sampling period T. 
     FIG. 19 is a block diagram of a feedback controller according to an alternative embodiment of the present invention. In FIG. 19, the same parts are denoted by the same reference numbers as those of FIG.  11 . And redundant description will be omitted. 
     The output of the positional sensor  150  is fed through an anti-aliasing filter  153 ′ to an A/D converter  154 ′ in which the output is converted into a digital signal. The digital signal thus obtained is fed to a differential circuit  152 ′ in which a difference between the digital signal and a target value signal outputted from a target value generating circuit  151  is calculated. 
     The target value generating circuit  151 ′ generates a digital signal representative of a target value signal. The differential circuit  152 ′ performs an arithmetic operation of a difference between digital signals. 
     As described above, it is acceptable that the A/D converter performs the A/D conversion on the detection signal of the positional sensor  150 , or alternatively it is acceptable that the A/D converter performs the A/D conversion on the error signal after processing of the detection signal, as shown in the embodiment of FIG.  11 . 
     FIG. 20 is a block diagram of a feedback controller according to a further alternative embodiment of the present invention. 
     The feedback controller according to the present embodiment is applied to a magnetic storage device for picking up information stored in a magnetic disk  500 . 
     The information stored in the magnetic disk  500  is read by a magnetic head  510 . A position detection signal of the magnetic head  510  is fed to a DSP  520 . An output of the DSP  520  is applied to a driving circuit  530 . The magnetic head  510  is driven by an output of the driving circuit  530 . 
     The DSP  520  comprises an A/D converter  521 , an input hold circuit  522 , a notch filter  523  and an output hold circuit  524 . The input hold circuit  522 , the notch filter  523  and the output hold circuit  524  are substantially the same as the input hold circuit  157 , the notch filter  158  and the output hold circuit  159  shown in FIGS. 11 and 19 in their functions, respectively, while they have their inherent portions as to the control system for driving the magnetic head. It should be noticed that the those elements are incorporated in the DSP  520 , and their functions are implemented in a combination of the hardware with the software. 
     Incidentally, according to the present embodiment, the notch filter is used as an example of the digital filter referred to in the present invention. However, the digital filter referred to in the present invention is not restricted to the notch filter, and a various types of digital filter can be adopted. It is acceptable to adopt any type of digital filter which makes it possible to deal with a frequency near the Nyquist frequency. 
     As mentioned above, according to a digital filter of the present invention, it is possible to have an effective filtering effect on a frequency band near the Nyquist frequency without increasing the sampling frequency, and thereby ensuring a sufficient stability in the even that the digital filter of the present invention is used for the feedback control. 
     In other words, the feedback controller according to the present invention is applicable to various types of driving units. And it is possible to improve a responsibility to the deviation from a target. For example, according to a storage device of the present invention to which a feedback controller of the present invention is applied, an application of the storage device to a track driving unit for moving a head in a track direction makes it possible to improve a track follow-up performance of the head. This feature may solve such a problem that when the head does not follow the track into an off-track, the head does not move promptly, and thug a control band is expanded so that a movement of the head is prompt. Further, an application of the storage device to a focus driving unit for moving the head in a focus direction makes it possible to improve the follow-up performance of the head in the focus direction, and whereby the head promptly responds to being out of focus. 
     While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by those embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and sprit of the present invention.