JP3136727B2 - Adaptive filter control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to characteristic control of an adaptive filter for removing an interference component from a reproduced signal of an optical disk in which intersymbol interference has occurred.

[0002]

2. Description of the Related Art In an optical disc, in order to remove a waveform distortion generated in a signal passed through a recording / reproducing system, an equalizer is provided in a reproducing circuit to compensate for intersymbol interference in a recording sequence. When the transfer characteristic in the recording / reproducing system changes, or when the transfer characteristic cannot be specified, a method of adaptive equalization is adopted in which waveform distortion is estimated from the reproduced signal to determine the characteristics of the equalizer. .

FIG. 5 shows a configuration of a conventionally used adaptive filter. The signal before the equalization is input from the input terminal 1. The input signal has its waveform distortion removed by the transversal filter 2 including the delay element 3, the multiplier 4 and the adder 5, and is output from the output terminal 6. This output signal is fed back to extract an error signal used for updating the tap gain.

[0004] If the tap coefficient is not set to an optimum value, a signal with waveform distortion is output from the output terminal. The determiner 7 performs a binary determination from the signal R having the remaining waveform distortion, and outputs a binary signal D having a reference amplitude.
The tap gain coefficient is changed so as to minimize the power of the error signal E obtained by the difference between the output signal of the transversal filter 2 and the output signal of the decision unit 7. For this reason, after the tap coefficients converge to an appropriate value, the filter output signal takes the two values given as the discriminator output D.

The error signal is sent to a correlator 11 together with an appropriate time-delayed filter input signal, and the correlator outputs the correlation strength between the two. The output signal of the correlator is an integrator 12
And is given to the multiplier of each tap as a tap gain coefficient.

In the above example, data determination and error signal output are performed using only the reproduced signal having waveform distortion, so that when the distortion is large, an error occurs in the output of the decision unit itself, Correct output cannot be obtained. To avoid this, in many cases, a training signal having a predetermined pattern is added to the input signal following the synchronization signal. The tap gain coefficient is corrected so that the reproduction signal obtained following the synchronization signal matches the pattern of the training signal. FIG. 6 shows an example of a circuit for updating a coefficient using a training signal.

[0007] Transversal filter 2, correlator 11
The configuration of the integrator 12 is the same as that of the example of FIG. The synchronizing signal detection circuit 15 monitors the output R of the transversal filter 2 and, for a certain period of time from when the synchronizing signal appearing prior to the training signal is detected, generates the same training pattern as that included in the reproduced signal for a certain period of time. Prompt the training signal generation circuit 16. At the same time, by closing the switch 10, the error signal E 2 extracted as the difference between the output signal R from the transversal filter 2 and the training signal D is output to the correlator 11. On the other hand, while a signal other than the training signal is being input, the switch 10 is opened, and updating of the coefficient is prohibited by setting the error signal input E2 of the correlator to 0.

[0008] Originally, equalization by a transversal filter is intended to remove linear distortion. For this reason, the waveform distortion composed of only the linear distortion is effectively removed from the reproduced signal. However, for example, in the case of an optical disk on which thermal recording is performed using a light beam, if recording pits are formed continuously, pits overlap as shown by oblique lines in FIG. For this reason, the reproduced signal does not have a waveform like a broken line 21 obtained by superimposing the reproduced signal 19 with a single pit, but has a reproduction amplitude like a solid line 20. The nonlinear distortion component appearing here cannot be removed by the transversal filter and is left in the output signal.
This causes the error rate to deteriorate.

[0009] The nonlinear component included in the reproduced signal is shown in FIG.
In addition to the components due to the overlapping of the pits shown in (1), there is also known a component that appears because the size of the leading edge and the trailing edge of the recording pit changes due to the flow of heat on the recording medium. However, this nonlinear component can be removed by recording power control as disclosed in Japanese Patent Application No. 1-322340.

[0010]

In optical recording and magneto-optical recording, information is stored by changing the shape of the recording medium and the direction of magnetization. Appears.

On the other hand, a transversal filter circuit for equalizing a reproduced signal is constituted by only linear operations such as delay of the reproduced signal, multiplication of coefficients, and addition. Ingredients cannot be removed. Therefore, when a normal coefficient control method is used here, there is a disadvantage that the equalization ability is significantly reduced due to the influence of the non-linearity, and a decision error is increased.

An object of the present invention is to provide a coefficient control method which is hardly affected by nonlinear distortion in order to solve the above-mentioned problems, and to suppress a determination error of a filter output signal to a low level.

[0013]

According to the adaptive filter control method of the present invention, in an adaptive transversal filter for equalizing a binary reproduction signal of an optical disk, the sign of the output signal of the transversal filter is at least immediately before or Only when the code is different from any of the codes immediately after, or when the amplitude of the output signal is smaller than the reference level, a determination error is extracted from the output signal and the tap coefficient is updated.

[0014]

The non-linear distortion in the digital recording system of the optical disk is mainly caused by the overlap of the recording pits as shown in FIG. 7, and therefore appears characteristically when the same code continues. On the other hand, when judging a reproduced signal, it is more difficult to identify the inversion position when the code is inverted than when the same code is continuous, and the influence of bit shift and the like due to intersymbol interference is more difficult. easy.

In the present invention, tap coefficients are controlled using error signals before and after the code inversion position, which are relatively easily affected by intersymbol interference and less affected by nonlinear distortion. Thereby, good equalization characteristics are obtained before and after sign inversion, which is important in data determination. Further, when the same code continues, the coefficient is controlled such that the amplitude of the output signal is always larger than the reference level. Therefore, it is possible to obtain an output signal that is less affected by nonlinear distortion and that can reliably perform binary determination.

[0016]

Next, an embodiment of the present invention will be described with reference to FIGS.

FIG. 1 is a block diagram showing one embodiment.
The transversal filter 2 has the same configuration as the conventional example. The signal D, which has been binary-determined from the output signal by the determiner 7, is sent to a subtractor 8 to extract an error signal, and is also sent to an error signal selection circuit 9 for determining whether to use the error signal for coefficient control. Sent. The error signal selection circuit outputs a signal E1 obtained by delaying the output signal D of the determiner 7 and the output of the subtractor.
, The error selection signal S is activated according to the change in the sign of the signal D and the state of the sign of the signal E1. S
Is active only, switch 10 is closed and input E2 of correlator 11 equals E1. As a result, the tap coefficients are adaptively controlled by the correlation between the error signal and the input signal. On the other hand, when the switch 10 is open, the error signal E2 input to each correlator becomes 0, and the tap coefficient of the transversal filter 2 does not change.

FIG. 2 is a block diagram showing details of the error signal selection circuit 9. From the input D, two stages of delay elements 3 and an exclusive OR circuit 13 detect before and after sign inversion. The error selection signal S becomes active with a delay of one unit time T before and after the sign change of the input D. Further, the sign of the input E1 and the sign of the input D are compared, and even in the case of a different sign, the error selection signal S is activated with a delay of one unit time.

Since the error signal selection circuit 9 has a time delay of unit time T, the delay element 3 is required between the subtractor and the switch. Accordingly, the delay amount of the filter input signal input to the correlator 11 increases by the time T.

The error signal selection circuit 9 outputs only the error signal when the sign is inverted immediately before or after or when the amplitude of the output signal is smaller than the reference level to the correlator 13.
Send to For this reason, even when the output signal has an amplitude larger than the reference level in a series of the same code, the tap gain coefficient does not change.

FIG. 3 shows equalized waveforms when the conventional system is used and when the present system is used. The figure shows (1,7) NRZ
It is the equalization result which simulated the waveform of the recording density equivalent to bit length 0.5 micrometer using I code. According to the conventional equalization method, coefficient control is performed so that all determination points match the reference output level determined as the output of the determiner, while the output signal level from the filter approaches the reference level, The aperture ratio of the eye, which greatly affects the data determination error, is small. On the other hand, in the case of performing the coefficient control according to the present method, the continuous filter output signal of the same code passes further outside the reference level. The error rate is improved by about 3 dB in terms of the C / N ratio.

FIG. 4 is a block diagram showing a case where the equalization method of the present invention is applied to a recording signal to which a training signal is added. The synchronizing signal detection circuit 15 monitors the output signal from the transversal filter as in the prior art, and prompts the generation of a training signal during a training period that appears following the synchronizing signal. The output of the training signal generating circuit 16 is sent to the subtractor 8 and used to extract an error signal, and is also passed to the error signal selecting circuit 9.

The switch 10 provided on the feedback path of the error signal is closed only when the outputs of both the error signal selection circuit 9 and the synchronization signal detection circuit 15 are both active. Only by the error signal that satisfies the condition provided in the selection circuit,
Control of the coefficient is performed. This is effective when the waveform distortion of the input signal is large and the judgment error obtained from the reproduced signal is large.

In this circuit, control of the coefficient is performed only by the training signal. Therefore, when a recording code such as an RLL code is used, it is more preferable that the training signal also satisfies the RLL rule.
It is desirable that the training signal is performed simultaneously with the actual data recording in that the same recording condition is given to the training signal and the actual data.

[0025]

By using the control method of the present invention, it is possible to take out a good equalized output with little decision error even when the optical disc reproduction waveform inputted to the adaptive filter has a lot of nonlinear distortion.

[Brief description of the drawings]

FIG. 1 is a system diagram of a circuit for implementing the present invention.

FIG. 2 is a system diagram showing details of an error signal selection circuit.

FIG. 3 is a diagram for explaining an output result by the control method of the present invention.

FIG. 4 is a system diagram for explaining another embodiment of the present invention.

FIG. 5 is a system diagram for explaining a conventional technique.

FIG. 6 is a system diagram for explaining a conventional technique.

FIG. 7 is a diagram for explaining a cause of occurrence of nonlinear distortion.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Input terminal 2 Transversal filter 3 Delay element 4 Multiplier 5 Adder 6 Output terminal 7 Judgment device 8 Subtractor 9 Error signal selection circuit 11 Correlator 12 Integrator 15 Synchronization signal detection circuit 16 Training signal generation circuit 18 Recording pit 19 Single pit response waveform 20 Reproduction waveform

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-170613 (JP, A) JP-A-3-284014 (JP, A) JP-A-64-71209 (JP, A) JP-A-61- 30846 (JP, A) JP-A-58-50612 (JP, A) JP-B-3-18368 (JP, B2) JP-B 2-26406 (JP, B2) JP-B-58-34056 (JP, B2) (58) Field surveyed (Int. Cl. ⁷ , DB name) H03H 21/00 H03H 15/00 G11B 20/10

Claims (1)

(57) [Claims] 1. An adaptive transversal filter for equalizing a binary reproduction signal of an optical disk, wherein a sign of an output signal of the transversal filter is different from at least one of codes immediately before or immediately after, or of an output signal. Only if the amplitude is less than the reference level,
An adaptive filter control method characterized by extracting a decision error from an output signal and updating a tap coefficient.