JP4200113B2 - Equalizer and magnetic recording / reproducing apparatus - Google Patents

Description

  The present invention relates to a magnetic recording / reproducing apparatus and a magnetic recording / reproducing signal processing circuit using a high-sensitivity reproducing head using a perpendicular magnetic recording double-layer medium having a soft magnetic backing layer and a magnetoresistive effect element.

  In a perpendicular magnetic recording system that realizes high-density magnetic recording / reproduction, a recording system that combines a single-pole head and a double-layered medium composed of a soft magnetic underlayer / recording magnetic layer is often used. In this recording system, the recording magnetic field emitted from the head main magnetic pole is guided to the backing layer on the back side of the recording layer, and forms a magnetic path that returns from the auxiliary magnetic pole to the recording head again. By switching the direction of the recording magnetic field, the recording magnetic layer is magnetized in two directions in the medium thickness direction corresponding to the recording information code, and information is stored. In such recording by the recording head / medium structure, a strong and steep perpendicular recording magnetic field can be applied to the recording magnetic layer, so that information storage with high resolution can be realized. In addition, when reproducing magnetization information from a perpendicular magnetic recording medium magnetized in this way by a high-sensitivity reproducing head using a magnetoresistive (MR) effect element, a reproduction signal from the head is recorded on the medium. The recording magnetization distribution is directly sensed and a rectangular waveform corresponding to this is obtained.

  For the in-plane magnetic recording system, the SNML of the reproduction signal is improved by using the PRML system that combines the partial response (Partial-Response) equalization system and the maximum likelihood (Maximum-Likelihood) decoding system. Signal processing systems that perform reliable data reproduction are widely used. On the other hand, a signal processing method suitable for demodulating a perpendicular magnetic recording reproduction signal as described above has not been known so far. However, the reproduction signal contains a large amount of DC signal components and has a recording magnetization distribution and recording. Since it has the same rectangular wave shape as the current waveform, there are several techniques similar to partial response class 1 used in an optical recording / reproducing apparatus, etc., and this extended method (Japanese Patent Laid-Open No. 11-66755) and integrated signal detection. Proposed.

  In addition, for the purpose of easily processing the signal processing method similar to the in-plane recording method, the reproduction signal from the vertical recording medium is differentiated in advance to simulate the reproduction signal of the in-plane recording method. By creating a similar pulse signal waveform, a method of processing this by a signal processing method similar to the conventional one is proposed.

  An object of the present invention is to provide PRML (Partial-Response) which can realize the most reliable data demodulation with respect to a reproduction signal from a perpendicular magnetic recording head medium system using a double-layer recording medium and a single magnetic pole recording head / MR reproducing head. (Maximum-Likelihood) signal processing method. By providing waveform equalization conditions to better match the reproduced signal waveform and noise characteristics of the high density perpendicular magnetic recording method to the partial response (PR) waveform equalization method, And a means for reducing the error rate of data demodulation in the maximum likelihood (ML) decoding in the subsequent stage.

  As described above, the reproduction signal in the perpendicular magnetic recording system is a signal containing a large amount of rectangular wave DC signal components, but various noises from the recording medium and reproduction amplifiers are present near the DC component of the reproduction signal. Many disturbance factors such as distortion due to low-frequency loss in signal transmission systems such as circuits are localized. In order to eliminate this influence, a waveform equalization process having a frequency characteristic that blocks the DC signal component of the reproduction signal and suppresses the vicinity of the low-frequency signal is performed.

  However, the DC component cannot be removed in the partial response class 1 described in the above prior art, this extended method, or the signal processing method such as integral signal detection. In addition, in the signal processing method that differentiates the playback signal before equalization, the DC component is cut, so the effects of noise and distortion existing in the low-frequency region can be removed, but the high-frequency noise component due to differentiation is emphasized. There is a problem that the code error rate increases.

In order to remove the influence of the noise / distortion localized in the low frequency region described above, the present invention can remove the DC component from the signal after waveform equalization while maintaining the effect of suppressing the increase of high frequency equalization noise. A partial response waveform equalization method and a waveform equalization circuit are provided.
The partial response waveform equalization circuit according to the present invention is configured by connecting delay circuit elements that delay an input signal by 1 bit in multiple stages, and for each input signal delayed by approximately 1 bit, a predetermined tap coefficient {h1, h2, h3,. ... A so-called transversal type filter composed of a plurality of multipliers for multiplying hL} and an adder for adding the input signals multiplied by the tap coefficients, but the tap coefficients are h1 + h2 + h3. It is characterized by a certain point. By determining the tap coefficient in this way, the DC component can be cut from the equalized signal waveform.

  Further, since the noise input to the maximum likelihood decoder is more optimally whitened, the error rate of the demodulated data is reduced. Furthermore, by performing signal processing using the waveform equalization method of the present invention, the reliability of data demodulation is higher than in the case of using the conventional disclosed technique, and a reproduced signal with lower SN quality can be allowed. It is possible to provide a magnetic recording / reproducing apparatus capable of realizing high-density information storage.

According to the present invention, the reliability of data demodulation in the maximum likelihood decoding is further improved with respect to the reproduction signal of the perpendicular magnetic recording system by the double-layer film medium and the high-sensitivity MR reproducing head, as compared with the case where the conventional disclosed technique is used. It is possible to provide a magnetic recording / reproducing apparatus that can allow low SN signal quality and realize higher-density information storage, and a magnetic recording / reproducing signal processing circuit using the same.

  In addition, in the present invention, the influence of noise from the recording medium can be reduced more effectively, and the reproduced waveform is subjected to low-frequency deterioration distortion due to the transfer characteristics of the preceding signal processing transmission system such as a reproduction system amplifier circuit. As a precondition, it is possible to perform waveform equalization processing that suppresses this, so there is no need for special compensation circuits, etc., and this effect is reduced, recording and playback signals that allow deterioration of the characteristics of the preceding signal processing transmission system A processing circuit can be provided.

By eliminating the DC reproduction detection from the reproduction signal, the influence of the low frequency disturbance is eliminated, and the adaptive learning of the equalization process becomes high accuracy and high speed. In addition, the contact between the recording medium and the MR reproduction element (thermal Asperity) and DC offset and fluctuation of the reproduced waveform caused by fluctuations in head characteristics, the influence on the maximum likelihood decoded data demodulation can be eliminated.

(Example 1)
FIG. 1 shows a basic implementation configuration of a magnetic recording / reproducing apparatus provided by the present invention. In this embodiment, the information code data 1 {ak} (k is an integer indicating a bit time) input to the recording signal processing circuit 9a is predetermined in the encoder 2 because of run-length restrictions and error correction code addition. Are converted into recording code data {bk}. The recording code data {bk} is converted into an analog recording current signal {ck} through a recording current conversion processing circuit 3a and a recording amplifier 3b, and then supplied to the recording perpendicular magnetic recording head medium system 4. Information is stored.

  In the perpendicular magnetic recording head medium system 4, a double-layer perpendicular magnetic recording medium having a recording magnetic layer 6a and a soft magnetic backing layer 6b on a substrate 6c is used as the recording medium 6. The recording head 5 is a single pole head. A coil 5b is wound around the main magnetic pole 5a of the recording head, and a recording magnetic field is induced by a recording current passing through the coil 5b. This recording magnetic field forms a magnetic path that returns to the auxiliary magnetic pole 5c through the backing layer 6b and then returns to the main magnetic pole 5a, thereby applying a steep vertical magnetic field to the recording magnetic layer 6a. The recording medium is magnetized in the medium thickness direction. When reproducing the magnetization information from the perpendicular magnetic recording medium recorded in this way by using the reproducing head 7 having the magnetoresistive (MR) effect element 7a placed between the shield film 7b and the head, As shown in FIG. 2, the reproduction signal 8 is a rectangular wave signal corresponding to the recording magnetization distribution on the medium. In other words, a binary recording current signal {ck} in the forward and reverse directions corresponding to the two code values {0, 1} of the recording code data {bk} is sent to the recording head 5 and the recording medium 6 is moved, Corresponding to this, when recording magnetization in the downward direction is formed (NRZ recording), this recording magnetization pattern 17 is directly sensed from the reproducing head 7 running on the recording medium 6, and in the recording magnetization direction. A reproduction signal sequence {dk} having a rectangular waveform with a slow rise in which the voltage changes stepwise at the transition position is reproduced and output in an analog manner. The width of the waveform rise is determined by the structure and characteristics of the perpendicular magnetic recording head medium system 4 and recording / reproducing conditions, and limits the increase in recording density for both the output voltage of the signal. Further, noise due to various factors may be superimposed on the waveform, and waveform distortion depending on the frequency transfer characteristics of the head medium system and other electronic components may be superimposed.

  In the reproduction signal processing circuit 9b, the reproduction signal 8 is amplified by the reproduction amplifier 10, and after removing unnecessary high frequency noise and signal components by the low-pass filter 11, the analog / digital (A / D) converter 12 is used. Then, at a bit timing of the recording code data {bk}, it is converted into a discrete reproduction signal sequence {ek} sampled into a digital value. In the present invention, the partial response waveform equalization suitable for the reproduction signal 8 is performed by the equalizer 13 at the subsequent stage in order to perform the most efficient and reliable data code demodulation from the discrete reproduction signal sequence {ek}. At the same time, the output signal from the equalizer 13 is converted by the maximum likelihood decoder 14 into a demodulated data code string {gk} that seems to have the lowest error rate. In particular, the equalizer 14 allows a known waveform interference value over a finite bit length on the output signal waveform, thereby reducing an increase in high-frequency noise due to enhancement of a high-frequency signal component in waveform processing as much as possible. The waveform processing is performed to avoid the influence of signal distortion and noise in the low frequency range including the direct current (DC) component of the reproduction signal 8. (This will be described in detail later with reference to FIG. 3.) In addition, the maximum likelihood decoder 14 uses a known waveform interference amount added by the equalizer 13 using the Viterbi algorithm, Data demodulation processing with higher reliability against noise is realized. The demodulated data code string {gk} is subjected to an inverse transform process through the decoder 15, and reproduced code data 16 {ak '} corresponding to the original information code data 1 {ak} is reproduced and output.

FIG. 3 illustrates the details of the partial response waveform processing in the equalizer 13 in the embodiment of FIG. In FIG. 3A, a reproduced waveform 19 is obtained from the reproducing head 7 when the dibit recording magnetization pattern 18 (two adjacent recording magnetization reversal pairs 18a recorded at the shortest bit interval) on the recording medium 6 is reproduced. It is an output waveform. As described above, at the timing of two magnetization transitions, the step response signals 19a having a dull rise depending on the frequency characteristics of the head medium system overlap with each other, so that an isolated pulse waveform is output. In a general perpendicular magnetic recording / reproducing system having the above head medium system, each step response signal 19a is known to be approximated by a tanh type function. By parameter K that determines the rise width,
H (t) = V * tanh ((3.415t) / (π · K · Tb)) − V * tanh (3.4514t) / (π · K · Tb)) (1)
Is approximated by This can be regarded as an impulse response output waveform (response to the isolated bit “1” on the recording code data {bk}) for the head medium system, and the power spectrum 20 in the frequency domain of the dibit reproduction waveform 19 is: As shown in FIG. 3 (b), the energy for bit detection is concentrated in a lower frequency area centering on the DC component. However, on the other hand, the reproduced signal 8 from the reproducing head 7 is reproduced through the electronic parts / signal transmission path characteristics such as the reproducing amplifier 10 and is affected by the waveform distortion due to the deterioration of the frequency characteristics. That is inevitable. In particular, the reproduction amplifier 10 must allow a low-frequency cutoff characteristic including a DC component in order to realize a wide band, and as a result, waveform distortion due to the loss of the low-frequency signal component is significant on the output signal. Become. Compensation of this low-frequency waveform distortion in waveform processing requires an excessive compensation circuit and causes excessive emphasis on the superimposed noise component, thereby causing a harmful effect.

On the other hand, the medium noise spectrum 20a sensed by the reproducing head 7 from the recording medium 6 is also localized in the low frequency region centered on the DC component, so that this and the low frequency component of the reproduced signal are separated and detected. It is also difficult. In order to avoid these problems, in the present invention, the equalizer 13 performs waveform processing of the above-described dibit reproduction waveform 19 into an equalization waveform 22 having the shape of the equalization waveform power spectrum 21 in FIG. To do. Like conventional partial response equalization, by allowing a gentle high-frequency cutoff characteristic that matches the power spectrum 20 of the reproduced waveform 19 to be suppressed, high-frequency noise enhancement in equalization processing is suppressed, and the DC component is set to zero. By suppressing the low-frequency signal component and processing the waveform to allow the low-frequency cutoff characteristic, waveform distortion and medium noise localized in the low-frequency region can be suppressed on the reproduction signal. From the equalized signal having such frequency characteristics, it is possible to demodulate data in a situation with less noise and distortion. Further, since noise enhancement in waveform processing in the equalizer 13 is suppressed and noise frequency characteristics close to white are maintained, a more suitable noise environment is provided to the maximum likelihood decoder 14 in the subsequent stage, and the demodulated data The code error rate of the code string {gk} is reduced. The equalized waveform 22 having such a frequency characteristic is a known non-zero waveform interference amount (s1, s2,...) Over a sufficient bit length n as shown in FIG. can be obtained by selecting a waveform shape having s3,.
In the conventional partial response equalization process, it is possible to set an equalized waveform having a gentle high-frequency cutoff characteristic by appropriately selecting this value for the reproduced waveform 19. In the present invention, in order to suppress the DC signal component to zero on the reproduced waveform 19, the following expression is applied to the non-zero waveform interference amount (s1, s2, s3,... Sn) added at the time of partial response equalization. A constraint condition such as (2) is added.
s1 + s2 + s3 +... sn = 0 (2)
The constraint condition of Expression (2) is that the frequency expression H (f) of the equalized waveform 22 (f is the frequency)
H (f) = s1 * exp (−2πjfTb) + s2 * exp (−2πjf2Tb) + s3 * exp (−2πjf2Tb) +... Sn * exp (−2πjfnTb) (3)
Thus, it is easily derived from the condition that H (0) = 0 at frequency = 0. A method for determining a noise whitening filter characteristic that minimizes the output noise energy from the equalizer 13 with respect to the reproduced waveform 19 under the constraint condition (2) is a number of classical filter theories. And disclosed in cited references “Design of finite impulse response for the Viterbi algorithm and decision-feedback equalizer, D. G. Messerschit., Proceeding. By using many disclosed algorithms such as a linear prediction filter learning algorithm, it can be easily performed on an actual reproduced waveform. As a result, an appropriate waveform interference amount (s1, s2, s3,... Sn) in the partial response equalization process given by the equalizer 13 is uniquely determined for a certain reproduced waveform 19, Equalizer characteristics for realizing can also be uniquely determined.
FIG. 4 shows the amplitude ratios of the optimum waveform interference amounts (s1, s2, s3,... Sn) to be set for the reproduced waveform 19 of the equation (1) having various parameters K. It is. If the bit length n of the waveform interference is 5 or more, an equalizer characteristic close to the optimum can be obtained. However, excessively increasing this increases the realization scale of the maximum likelihood decoder 14 in the subsequent stage, and therefore, as much as possible. Should be suppressed. By selecting an integer ratio that can approximate the ratio of these optimum interference amounts, the decoder is simplified.

Setting the circuit parameters of the equalizer 13 from the relationship with the input reproduced waveform 19 with respect to the optimum waveform interference amount (s1, s2, s3,... Sn) determined as described above is as follows. It is very easy by the known filter design theory. In many cases, as shown in FIG. 5, the equalizer 13 includes a shift register in which storage delay elements 23 for storing a 1-bit signal are connected in series, and predetermined tap coefficients (h1, h2, h3,. .., HL; L is a transversal filter composed of a multiplier 24, an adder 25, and the like for multiplying each stored content by a tap length) and performing a product-sum operation. Under the constraint condition (2), the tap coefficient hK (1 ≦ K ≦ L) for the reproduced waveform 19 having a DC frequency component is
h1 + h2 + h3 +..., hK,... + hL = 0 (4)
It has the following characteristics. Under the constraint condition (4) of the tap coefficient, this is calculated from the input reproduced waveform 19 and the known waveform interference amounts (s1, s2, s3,. The method of determining the filter tap coefficient hL for realizing is a technique generally described in detail in a cited document “Introduction to Adaptive Filter (1984), by Simon Haykin”. As disclosed in the cited document, the signal output from the equalizer 13 is observed online, and an error from the target signal determined by the waveform interference amount (s1, s2, s3,... Sn) is evaluated. However, the adaptive learning circuit 26 uses the adaptive algorithm such as the MSE (Mean-Square Error) method or the LSM (Least Mean Square) method to sequentially update each tap coefficient hK while satisfying the constraint condition (4). It is also possible to obtain the optimum tap coefficient. In particular, giving the constraint conditions (2) to (4) eliminates the need for learning estimation of signal components in the vicinity of the DC component in this adaptive tap learning operation. This makes it possible to perform an adaptive learning operation that is more stable and accurate for an actual recording / reproducing system signal having medium noise.

  The signal output from the equalizer 13 as described above is demodulated by the maximum likelihood decoder 14 using a Viterbi algorithm or the like. FIG. 6 shows an example (n = 5) of the state transition trellis diagram of this Viterbi decoding, and each arrow indicates recording code data 3 {bk} assumed at bit time k and corresponding to this. The value of the output signal value {fk} from the generator 13 is indicated. The output signal value {fk} is given by the code value {bk} processed at the current time k when the above-described waveform interference amount (s1, s2, s3,... Sn) is given, and (n−1). From the state of the code values {bk−n + 1, bk−n + 1,..., Bk−1} up to the previous bit, it is uniquely determined by a linear convolution operation. Therefore, the number of states in the trellis diagram is 2 ^ (n−1). From the viewpoint of reducing the number of states, it is necessary to sufficiently and sufficiently suppress the bit length n of waveform interference in the equalizer 13. In all the state transitions indicated by the temporal transition of the trellis diagram, the code sequence indicated by the most probable transition is selected as decoded data.

Since the equalizer 13 in the present embodiment has a low-frequency cutoff characteristic including a DC component, it is possible to largely eliminate the influence of DC component offset and low-frequency fluctuation / distortion that often occur on the actual reproduction signal. In particular, in a high-density recording / reproducing system, due to fluctuations in characteristics of the reproducing head 7 and a decrease in the distance between the reproducing head 7 and the recording medium 6, the magnetoresistive (MR) effect element 7 a and the recording medium 6 are in contact. A TA (Thermal Asperity) phenomenon occurs in which the magnetoresistive characteristics greatly change with a rise in temperature, and at this time, a large DC offset fluctuation occurs in the reproduction signal. The equalizer 13 of the present invention can eliminate this influence on the subsequent stage. Further, since the maximum likelihood decoding to the partial response equalization waveform having the constraint condition (2) does not detect the DC frequency component, data demodulation can be performed without being affected by such a phenomenon.
(Example 2)
FIG. 7 shows a second embodiment of the present invention. In this embodiment, the equalizer 13 includes a (1, 0, −1) equalizer 28a and (s1, s2, s3,... Sn).
The (1,0, -1) equalizer 28a is composed of a divert reproduction waveform 19 and a partial response class 4 waveform interference (1,0, -1). After shaping into a short bit waveform, the desired partial response interference amount is added by the (s1, s2, s3,..., Sn) partial response interference adding circuit 28b. At this time, in the equalizer 13 as a whole, (s1, s2, s3-s1,..., Sk-sk-2,..., Sn-sn-2, − with respect to the input of the dibit reproduction waveform 19. (sn−1, −sn) is obtained as a waveform response with an interference amount. While observing the output of the (1, 0, −1) equalizer 28a, the gain control signal 29c for the reproduction signal 8 is adaptively generated using the automatic gain control circuit 29a, and the automatic gain amplifier 27 is used. The fluctuation of the reproduction signal amplitude can be suppressed.

  Similarly, while observing the output of the (1, 0, −1) equalizer 28 a, the sample timing signal 29 d for the analog reproduction signal 8 is detected using the timing extraction circuit 29 b and sent to the analog / digital converter 12. The sampling phase can be supported. By observing the signal in the previous stage of the waveform equalization processing circuit 28b as in the present invention, the circuit configuration for automatic gain and timing extraction can be as simple as the conventional partial response class 4. Moreover, since the fluctuation of the DC signal component can be eliminated by 28a, this control can be avoided and a good control operation can be performed. Also in the configuration of the equalizer 28 shown in FIG. 5, the input signal of the adaptive learning circuit 26 is taken from the output of 28a, and only the (1, 0, -1) equalizer 28a is adaptively learned, so that it is simpler. And efficient equalization waveform processing is possible. At this time, the waveform equalization processing circuit 28b can be composed of a fixed tap transversal product-sum operation circuit.

  By using the magnetic recording / reproducing apparatus as described above, and a signal processing circuit or a semiconductor integrated circuit equipped with the magnetic recording / reproducing apparatus, a double-layer film medium having a soft magnetic backing layer and a magnetoresistive head having a shield film can be obtained. It is possible to record and reproduce information data with optimum reliability with respect to the perpendicular magnetic recording head / medium system used for the reproducing head. Further, since it is possible to allow a reduction in the resolution and SN quality of the reproduction signal, it is possible to realize recording and reproduction of higher density information.

The figure which shows the fundamental Example of this invention. The figure which shows the recording / reproducing process of a perpendicular magnetic recording system. (A) The figure explaining the partial response waveform equalization processing of the present invention (time waveform) (b) The figure explaining the partial response waveform equalization processing of the present invention (frequency spectrum). The table | surface which shows the relationship between the parameter K and the partial response waveform interference amount. The figure which shows the structure of an equalizer. FIG. 11 is a diagram (n = 5) illustrating an example of a state transition trellis diagram of Viterbi decoding. The figure which shows the 2nd Example of this invention.

Explanation of symbols

1: information code data, 2: encoder, 3a: recording current conversion processing circuit, 3b: recording amplifier, 4: perpendicular magnetic recording head medium system, 5: recording head, 5a: main pole, 5b: coil, 5c: auxiliary Magnetic pole, 6: recording medium, 6a: recording magnetic layer, 6b: soft magnetic backing layer, 6c: substrate, 7: reproducing head, 7a: magnetoresistive (MR) effect element, 7b: shield film, 8: reproducing signal, 9a : Recording signal processing circuit, 9b: reproduction signal processing circuit, 10: reproduction amplifier, 11: low-pass filter, 12: analog / digital (A / D) converter, 13: equalizer, 14: maximum likelihood decoder, 15: Decoder, 16: Reproduction code data, 17: Recording magnetization pattern, 18: Dibit recording magnetization pattern, 18a: Adjacent recording magnetization inversion pair, 19: Dibit reproduction waveform, 19a: Step response signal, 20: Dibit re-transmission Spectrum of raw waveform, 20a: Spectrum of medium noise / low frequency distortion, 21: Spectrum of equalized waveform, 22: Equalized waveform, 23: Memory delay element, 24: Multiplier, 25: Adder, 26: Adaptive learning Circuit 27; automatic gain amplifier 28a: (1, 0, -1) equalizer 28b: (s1, s2, s3,... Sn) waveform equalization processing circuit 29a: automatic gain control circuit 29b : Timing extraction circuit, 29c: gain control signal, 29d: sample timing signal.

Claims (4)

A digital filter equalizer for equalizing a discretized signal from an analog / digital conversion circuit, wherein a plurality of delay elements that delay an input signal by 1 bit are connected in series, and the plurality of connected 1 bits A multiplier connected to each of the delay elements and multiplying the output signal of the 1-bit delay element by a predetermined tap coefficient, an adder for adding the output from the multiplier, and equalizing the output from the adder and a adaptive learning circuit for calculating said predetermined tap coefficients from the error between the target, _ the tap coefficient is the sum under the constraint becomes zero, by the tap coefficient is variable, a DC blocking it has a frequency characteristic, and by the tap coefficients with the response waveform characteristics of the asymmetric shape, the signal equalizer and having a frequency characteristic of the noise whitening filter suppresses high-frequency noise enhancement Two-layer vertical medium having a backing layer, a recording single magnetic pole head, a reproducing head, a recording signal processing circuit for supplying a recording signal to the single magnetic pole head, and a reproducing signal for processing the reproducing signal from the reproducing head In a perpendicular magnetic recording / reproducing apparatus having a processing circuit,
Reproduced signal processing circuit, a sample circuit (or an analog / digital converter) discretizing the reproduction signal, and a circuit connecting a plurality of delay elements to the discrete signal 1 bit delay in series, are the multi-connection connected to each of the 1-bit delay element, a multiplier for multiplying the predetermined tap coefficients to the output signal of the 1-bit delay element, an adder for adding outputs from the multiplier, an output from said adder And an adaptive learning circuit for calculating the predetermined tap coefficient from an error between the equalization target and
Under the restriction that the sum of the tap coefficients is zero, the tap coefficient is variable, so that it has a DC cutoff frequency characteristic and the tap coefficient with an asymmetrical response waveform characteristic enhances high-frequency noise. A perpendicular magnetic recording / reproducing apparatus comprising a digital signal equalizer having a frequency characteristic of a noise whitening filter for suppressing noise and a maximum likelihood decoder.   The reproduction signal processing circuit includes a reproduction amplifier that amplifies the reproduction signal from the reproduction head, an automatic gain amplifier that uses the output from the reproduction amplifier as an input signal, and a low-pass signal that uses the output from the automatic gain amplifier as an input. A filter, an analog / digital converter that receives the output of the low-pass filter, and a partial response class 4 waveform interference (1, 0, − 1) an equalizing circuit for shaping into a waveform, a partial response interference adding circuit for adding a predetermined partial response interference amount to the output from the equalizer, and an output from the partial response interference adding circuit as inputs. While observing the output from the maximum likelihood decoder and the equalization circuit, sample timing signal timing extraction for the reproduced signal is performed, and the analog signal is extracted. A timing extraction circuit for outputting to the digital / digital converter, and an automatic gain control circuit for outputting a signal for controlling the gain of the automatic gain amplifier to the substantially automatic gain amplifier while observing the output of the equalization circuit. The perpendicular magnetic recording / reproducing apparatus according to claim 2, wherein: A semiconductor integrated circuit comprising a digital filter equalizer for equalizing a discretized signal from an analog / digital conversion circuit, wherein a circuit in which a plurality of delay elements that delay an input signal by 1 bit are connected in series, A multiplier connected to each of the 1-bit delay elements and multiplying the output signal of the 1-bit delay element by a predetermined tap coefficient, an adder for adding the output from the multiplier, an output from the adder, etc. An adaptive learning circuit that calculates the predetermined tap coefficient from an error from the conversion target,
Under the constraint that the predetermined tap coefficient has a total sum of zero_, the tap coefficient is variable, so that it has a DC cutoff frequency characteristic and a higher tap coefficient with an asymmetrical response waveform characteristic. A semiconductor integrated circuit comprising a digital signal equalizer having a frequency characteristic of a noise whitening filter for suppressing band noise enhancement, and a reproduction signal processing circuit having a maximum likelihood decoder.

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