JP3507248B2 - Noise whitening equipment - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to noise whitening of a transmission system reproduction signal having colored noise, and more particularly to a digital system for reproducing data recorded by bringing a head into contact with a flexible magnetic recording medium such as a tape or a flexible disk. The present invention relates to signal processing of a magnetic recording / reproducing signal. The present invention will be described below by taking a digital VTR (Video Tape Recorder), which is a typical magnetic recording / reproducing apparatus, as an example.

[0002]

2. Description of the Related Art There are the following methods (I) and (II) as a way of thinking to reduce the code error rate in a digital VTR. (I) PR (Partial Response) method, Viterbi decoding, recording modulation, reproduction equalization, code detection, and decoding are devised at each stage. (II) Redundant bits are added to the recorded code and error correction is performed after code detection during reproduction. An actual digital VTR uses both of the above methods, and the parameters in each method are
The settings are made according to the characteristics of.

It should be noted here that the noise superimposed on the reproduced signal at the time of reproducing the VTR includes not only equipment noise such as thermal noise of the amplifier system but also medium noise of considerable magnitude. Among the medium noises, the modulation noises are particularly prominent in the VTR. Modulating noise refers to noise that has a correlation with a reproduction signal unlike white noise and that varies depending on the reproduction signal amplitude and the reproduction signal frequency.

However, in the reproduction signal processing of the conventional digital VTR, the modulating noise peculiar to the magnetic recording / reproducing system and the device noise having the property as the white noise are not distinguished from each other, and the white noise is totally generated. Have been treated as overlapping.

[0005]

In recent years, there is an increasing expectation for a small-sized and large-capacity digital VTR because of the desire to record high-quality information compactly and for a long time. In order to realize a small-sized and large-capacity digital VTR, it is necessary to perform high-density digital recording. However, when high-density recording is performed, the SN ratio during reproduction becomes small and the code error rate increases. Therefore, it is necessary to take measures to prevent the code error rate from increasing even if high-density recording is performed.

As a signal processing method that does not increase the code error rate when the SN ratio during reproduction is small, for example, PRML.
There is a method using maximum likelihood decoding such as (Partial Response Maximum Likelihood), but these maximum likelihood decoding methods assume that noise is white. However, the reproduction noise of the VTR is actually colored noise because it includes amplitude modulation noise that is correlated with the recording signal, and the desired improvement cannot be expected if the maximum likelihood decoding method is applied as it is.

Therefore, an object of the present invention is, in a reproducing system of a digital VTR for performing high density recording, on the premise that the reproducing system is combined with a maximum likelihood decoding method, the noise for whitening the reproducing noise by removing the amplitude modulation noise. It is intended to provide a whitening device.

[0008]

In order to achieve this object, the noise whitening device of the present invention is a noise whitening device for a reproduction signal of a transmission system having colored noise. In the noise whitening device, the reproduced signal does not contain colored noise. Signal component estimating means for estimating a typical signal component, linear filter means for convolving the estimated signal component and its transfer function, and a reproduced signal obtained by subtracting the estimated signal component from the reproduced signal. Correlation means for correlating the total noise with the output of the linear filter, and the output of the linear filter means and the output of the correlation means are multiplied to estimate an amplitude modulation noise component correlated with the signal component. It comprises a multiplication means and a subtraction means for removing the estimated amplitude-modulating noise component from the reproduction signal, and eliminates noise contained in the reproduction signal from white. It is characterized in that it has configured to reduction.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the accompanying drawings by way of examples, but before that, modulation noise contained in a reproduced signal of a digital VTR will be considered.

The main component of this modulation noise is amplitude modulation noise that varies the envelope of the reproduced signal waveform ⁽¹⁾ . Considering the noise component, the VTR reproduction signal is approximated by the following equation ⁽²⁾ .

Y (t) = f (t) + n _t (t) {h ₁ (t) * f (t)} + n _a (t) (1) where f (t): in the reproduced signal An ideal signal component that does not include noise (hereinafter simply referred to as a signal component) n _t (t): Proportional coefficient h ₁ (t) based on tape surface variation: Fourier transform of H ₁ (ω) = ω , Transfer function h ₁ (t) * f (t): convolution of h ₁ (t) and f (t) n _a (t): superposed noise

In the equation (1), the second term on the right side represents the amplitude modulation noise, and the sum of the second term and the third term represents the total noise. The amplitude modulation noise has a correlation with the signal component and also has a proportional relationship with the tape surface fluctuation. Since amplitude-modulating noise correlates with signal components, total noise is colored noise that correlates with signal components ⁽²⁾ .

FIG. 1 shows a TSS (Tilted Sendust Sputter).
With head, relative speed between tape and head is 7.1m
VTR of / sec, using a MP (Metal Particle) tape, a repetitive pulse with a duty of 50% is recorded as a recording signal, and when this is reproduced, a reproduced signal waveform and an instantaneous waveform of a recording signal repetitive frequency neighboring component of noise Shows an example of the relationship with. From the figure, it can be seen that the noise waveform exhibits amplitude modulation. Further, in the figure, the noise waveform changes in phase with the reproduction signal, but the relationship between the noise waveform and the reproduction signal may have an opposite phase depending on the reproduction condition.

FIG. 2 shows a clock rate of 12.5 under the same conditions as described above with respect to the type of head, relative speed, and tape used.
The reproduction output and the spectral density of noise when a random pulse of MHz is recorded as a recording signal are shown.
It can be seen from FIG. 2 that the noise spectrum and the signal component spectrum change with the same tendency in the signal band of 12.5 MHz or less, and there is a correlation between the signal component and the noise component.

FIG. 3 shows the instantaneous waveforms of the reproduced output and noise for the components within the signal band. In FIG.
Although the case where the noise waveform changes in phase with the reproduction signal is shown, the relationship between the noise waveform and the reproduction signal may be in the opposite phase depending on the reproduction condition. It can be seen from FIG. 3 that the noise exhibits amplitude modulation, as in the case of the repetitive pulse.

As described above, the noise contained in the reproduced signal is colored noise having a correlation with the signal component.

Since the reproduced signal of the VTR can be approximated by the equation (1), if the signal component in the reproduced signal and the surface variation of the tape can be estimated by some means, the amplitude modulation noise can be estimated from the second term of the equation (1). The value can be calculated. It is considered that the total noise in the reproduced signal can be whitened by subtracting the estimated value of the amplitude modulation noise from the reproduced signal.

Therefore, in the present invention, the whitening of the noise is performed by estimating the signal component in the reproduced signal and the surface fluctuation of the tape by the signal estimating circuit and the linear filter a.

Since the ultimate object of the present invention is to reduce the code error after the reproduction signal processing at the time of high density recording, if the signal estimation circuit can correctly estimate the signal, a complicated reproduction signal is dared. No action required. That is, since the above signal estimation circuit cannot perform perfect signal estimation, this circuit may be composed of extremely simple hardware such as a comparator because of the circuit scale. In addition, when it is desired to suppress the code error rate to an extremely low level, a demodulation circuit with high error correction capability should be selected from the demodulation circuits proposed in the past, even if it is a slightly complicated circuit configuration. It may be adopted as a signal estimation circuit.

Finally, the present invention can be combined with any existing scheme aimed at reducing the bit error rate,
The new code error rate reducing means that can further reduce the code error rate can be realized by whitening noise from the estimated value of the signal component and tape surface fluctuation.

An embodiment of the present invention will be described with reference to the circuit configuration block diagram of FIG. The device of the present invention uses the reproduction signal of the digital magnetic recording / reproducing device as an input signal. The input reproduction signal is divided into three, and is input to the delay circuit a (1), the delay circuit b (2), and the signal component estimation circuit (3), respectively.

In the signal component estimation circuit (3), the reproduced signal y
The ideal signal component f (t) that does not contain noise in (t) is estimated. In FIG. 4, the estimation result is expressed as f ^* (t).
The signal component estimation circuit (3) is composed of, for example, the circuit shown in FIG. The operation of the signal component estimation circuit shown in FIG. 5 will be described. The reproduction signal y (t) is input to the code determination circuit (11), the code “1” or “0” is detected, and the NRZL corresponding to that code is detected. (NonReturn to Zero Level) Outputs a waveform.

Next, this NRZL waveform is treated as an analog waveform and input to the linear filter b (12) having a transfer characteristic equal to the recording / reproducing characteristic of the tape head system of the VTR. An example of the linear filter b (12) is the filter shown in FIG. Here, the filter gain is adjusted so that the signal level of the filter output matches the reproduction signal level. Note that τ ₁ is a delay line with a delay time τ ₁ .

The delay circuit b (2) is composed of a delay line, a phase shift circuit or the like, and delays the input reproduction signal by a time equal to the time required for the processing in the signal component estimation circuit (3). The difference between the output signal y (t) of the delay circuit b (2) and the output signal f ^* (t) of the signal component estimation circuit (3) is calculated (4) to determine the time required for the processing of the linear filter a (6). The estimated total noise is used as one input signal of the correlator (7) via the delay circuit c (5) having the same delay time.

The correlator (7) is constructed by connecting a multiplication circuit and a low-pass filter in cascade as shown in FIG. 8, for example, and outputs the low-frequency component of the multiplication waveform of two input signals. Therefore, in this embodiment, this correlator operates so as to calculate a coefficient indicating how the component having a correlation with the signal component in the estimated total noise is proportional to the tape surface variation, and (1) and it outputs the second term of equation _{{h 1 (t) * f} (t)} estimates n ^* _t coefficients _{n t (t) (t)} . This correlator can estimate n ^* _t (t) because the total noise (y
The superposed noise ((3) in the equation (1)) in (t) − f ^* (t)) does not correlate with the signal component, and the surface fluctuation of the tape is gentler than that of the signal, and most of the component Is in the low range.

Output signal f of signal component estimation circuit (3)
^* (t) is also input to the linear filter a (6). The linear filter a (6) is a high-frequency emphasis filter used for the purpose of reflecting the influence on the tape surface fluctuation in the magnetic recording / reproducing system as the high-frequency component is greater. Therefore, when the colored noise does not depend on the parameter having the frequency characteristic like the influence of the tape surface variation, the linear filter a (6) and the delay circuit c (5) can be omitted. The linear filter a (6) is, for example, a transversal filter as shown in FIG. 9, and convolves with the transfer function h ₁ (t). Here, τ ₂ is a delay line with delay time τ ₂ . The output signal of the linear filter a (6) becomes an input signal to the correlator and is correlated with the above estimated total noise (y (t) -f ^* (t)), and the noise having a correlation with the signal component is the tape surface. The proportional relationship with the fluctuation is output as a time function.

Multiplication of the output signal of the correlator (7) and the output signal of the linear filter a (6) via the delay circuit d (8) having a delay time equal to the time required for processing in the correlator (7). And the estimated amplitude modulation noise (n ^* _t (t) {h ₁ (t) *
f ^* (t)}) is calculated.

The delay circuit a (1) is composed of a delay line, a phase shift circuit, or the like, and the delay time of each of the signal component estimation circuit (3), the linear filter a (6) and the correlator (7) is Delay the playback signal by a time equal to the sum. The output signal of the delay circuit a (1) and the estimated amplitude modulation noise _{^{(n * t (t) {}} h 1 (t) *
f ^* (t)}) is taken and the reproduced signal from which amplitude modulation noise is removed is output.

Here, the result of a simulation experiment for confirming the operation of the reproduction signal processing circuit of the present invention will be shown. Maximum repetition frequency of recorded signal is 6.25MHz
The M-series for 255 cycles, the recording and reproducing conditions are MP tape for the tape, 20 μm track width for the head and TSS structure, and the relative speed is 7.1 m / s.

Since the purpose of this simulation experiment was to confirm the whitening of noise, by using an M-sequence signal with periodicity, the ensemble average of the reproduced signals was calculated at the signal period to estimate the signal component. An NRZL waveform with almost no error was used as the output signal of the code determination circuit.
⁽³⁾ .

FIG. 6 shows the power components of the signal components in the reproduced signal and the noise contained in the reproduced signal before and after applying the whitening method of the present invention. It is considered that the noise after applying the whitening method of the present invention is almost flat, the amplitude modulation noise is almost removed, and the noise is whitened.

Further, the removal of the amplitude modulation noise also improves the SN ratio of the reproduced signal. In the simulation experiment of the present invention, S in the signal band of 12.5 MHz or less
The N ratio improvement amount was 1.2 dB.

Reference (1) Kamijo, Matsui: "Analysis of near-carrier noise in digital VTR" IEICE Technical Report, MR92-82, 19
93 (2) Matsui, Kamijo: "Extraction of Modulating Noise and Examination of Mathematical Model in Digital Magnetic Recording" TV Technical Report, VIR
94-78, 1994 (3) Matsui, Kamijo: "A Study on Modulating Noise in Digital Magnetic Recording," TV Annual University, 20-6, 199
Four

[0034]

By using the present invention, the amplitude modulation noise included in the reproduction signal of the magnetic recording / reproducing apparatus can be removed and the noise can be easily whitened. Also,
The removal of the amplitude modulation noise also improved the SN ratio of the reproduced signal. By using the present invention in the preceding stage of the reproduction signal processing circuit utilizing the maximum likelihood decoding such as PRML, the error rate of the high density digital magnetic recording / reproducing apparatus could be reduced. Although the description has been made so far for the digital magnetic recording / reproducing apparatus, the present invention is not limited to the digital magnetic recording / reproducing apparatus, but can be applied to any transmission system having colored noise. It goes without saying that the same effect can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a relationship between a reproduction signal waveform and an instantaneous waveform of a noise signal near a recording signal repetition frequency when a repeating pulse is recorded as a recording signal.

FIG. 2 is a diagram showing a spectral density of a reproduced signal and noise when a random pulse is recorded as a recording signal.

FIG. 3 is a diagram showing instantaneous waveforms of a reproduced signal and noise for components within a signal band regarding recording / reproduction in FIG.

FIG. 4 is a circuit block diagram of an embodiment of a noise whitening device by removing amplitude modulation noise according to the present invention.

FIG. 5 is a diagram showing a configuration example of a signal estimation circuit.

FIG. 6 is a diagram showing an effect of applying the whitening method of the present invention.

FIG. 7 is a configuration diagram of a linear filter b.

FIG. 8 is a block diagram of a correlator.

FIG. 9 is a configuration diagram of a linear filter a.

[Explanation of symbols]

1 delay circuit a 2 delay circuit b 3 Signal component estimation circuit 4,10 Subtractor 5 Delay circuit c 6 Linear filter a 7 Correlator 8 Delay circuit d 9 multiplier 11 Code determination circuit 12 Linear filter b

Continuation of the front page (56) References JP-A-6-275015 (JP, A) JP-A-9-106504 (JP, A) Matsui et al. A study on the influence of modulation noise on the error rate in digital magnetic recording. , IEICE Technical Report, Japan, The Institute of Electronics, Information and Communication Engineers, October 13, 1995, MR95-38, pp. 1-6. (58) Fields surveyed (Int.Cl. ⁷ , DB name) G11B 5/00-5/024 G11B 5/09 G11B 20/10

Claims (2)

(57) [Claims] 1. A noise whitening device for a reproduction signal of a transmission system having colored noise, wherein a signal component estimating means for estimating an ideal signal component which does not contain colored noise in the reproduced signal, and the estimated signal component. Linear filter means for performing convolution with the transfer function; correlating means for subtracting the estimated signal component from the reproduced signal, for correlating total noise in the reproduced signal with the output of the linear filter; Multiplier means for multiplying the output of the filter means and the output of the correlation means to estimate an amplitude modulation noise component correlated with the signal component, and the estimated amplitude modulation noise component is removed from the reproduced signal. A noise whitening device, which is configured to whiten noise included in the reproduction signal. 2. The noise whitening device according to claim 1, wherein the transmission system reproduction signal having the colored noise is a reproduction signal of a digital magnetic recording / reproducing device.