JP2551235B2 - Adaptive filter control method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to characteristic control of an adaptive filter for removing an interference component from a reproduction signal of an optical disc in which intersymbol interference has occurred.

(Prior Art) In an optical disc, intersymbol interference in a recording sequence is compensated by providing an equalizer in a reproducing circuit in order to remove a waveform distortion generated in a signal passing through a recording / reproducing system. If the transfer characteristic in the recording / reproducing system changes or if 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 characteristic of the equalizer. .

FIG. 4 shows the structure of a conventionally used adaptive filter. The signal before equalization is input from the input terminal 1. The input signal is subjected to waveform distortion removal 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
It is fed back to extract the error signal used for updating the tap gain.

When the tap gain is not set to the optimum value, a signal with waveform distortion remains is output from the output terminal. Judge 7
Performs binary determination from the signal with the waveform distortion remaining, and outputs a binary signal having a reference amplitude. The tap gain coefficient is determined by the output signal of the transversal filter 2 and the decision unit 7
The power of the error signal obtained by the difference with the output signal of is changed so as to minimize. Therefore, after the tap coefficient converges to an appropriate value, the filter output signal takes the two values given as the output of the determiner.

The error signal is sent to the correlator 13 together with the input signal of the filter which is delayed by an appropriate time, and the correlator 13 outputs the correlation strength of both. The output signal of the correlator 13 is integrated by the integrator 14 and given to the multiplier of each tap as a tap gain coefficient.

In the above example, since only the reproduced signal having the waveform distortion is used to determine the data and output the error signal,
If the distortion is large, an error occurs in the output of the determiner 7, and a correct output cannot be obtained. In order to avoid this, in many cases, the training signal having a predetermined pattern is added to the input signal following the synchronization signal. The tap gain coefficient is modified so that the reproduction signal obtained subsequent to the synchronization signal matches the pattern of the training signal. FIG. 5 shows an example of a circuit for updating the coefficient using the training signal.

Transversal filter 2, correlator 13 and integrator
The structure of 14 is the same as that of the example of FIG. The sync signal detection circuit 15 monitors the output of the transversal filter 2,
The training signal generation circuit generates the same training pattern as that included in the reproduction signal for a certain period of time from the time when the sync signal that precedes the training signal is detected.
Prompt to 16. Also, by closing the switch 12 at the same time, the error signal extracted as the difference between the output signal from the transversal filter 2 and the training signal is output to the correlator 13. On the other hand, the switch is opened while the signals other than the training signal are input, and the error signal input to the correlator 13 is set to 0, whereby the coefficient update is prohibited.

Originally, the equalization by the transversal filter aims at removing linear distortion. Therefore, the waveform distortion composed of only linear distortion is effectively removed from the reproduced signal. However, for example, in the case of an optical disc that performs thermal recording using a light beam, if recording pits are continuously formed,
Overlap between the pits occurs as shown by the diagonal lines in FIG.
Therefore, the reproduced signal does not have a waveform as shown by the broken line 21 obtained by superposing the reproduced signals 19 by a single pit, but has a reproduced amplitude as shown by the solid line 20. The non-linear distortion component appearing here cannot be removed by the transversal filter, and is therefore left in the output signal, which causes deterioration of the error rate.

The non-linear component contained in the reproduced signal 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, in addition to the component due to the pit overlap shown in FIG. The ingredients are also known. However, this non-linear component can be removed by the recording power control as disclosed in Japanese Patent Application No. 1-322340.

(Problems to be Solved by the Invention) In optical recording and optical timing recording, since information is stored by changing the shape and magnetization direction of the recording medium, the nonlinearity of the input / output characteristics increases as the recording density increases. Appears.

On the other hand, the transversal filter circuit that equalizes the reproduced signal is configured only by linear operations such as delay of the reproduced signal, multiplication of coefficients, and addition. Therefore, if the signal has nonlinear distortion, the component is removed. I can't. Therefore, when the normal coefficient control method is used here, there is a drawback in that the equalization ability is significantly reduced due to the influence of the non-linearity, and the judgment error is increased.

SUMMARY OF THE INVENTION An object of the present invention is to provide a coefficient control method which is less likely to be affected by non-linear distortion with respect to the above problems, and to suppress the determination error of the filter output signal to a low level.

(Means for Solving the Problems) The adaptive filter control method according to the present invention is applied to an optical disc
In a method of controlling tap coefficients in an adaptive transversal filter for equalizing a value reproduction signal, the output is output only when a sign of an output signal of the transversal filter is different from at least one of a code immediately before and a code immediately after. The determination error is extracted from the signal, and the tap coefficient is updated.

(Function) Non-linear distortion in the digital recording type of the optical disc is
It mainly appears due to the duplication of recording pits as shown in FIG.
On the other hand, in the determination of the reproduction signal, it is more difficult to identify the position when the code is reversed than when the same code is continuous, and is easily affected by bit shift due to intersymbol interference. .

According to the present invention, the tap coefficient is controlled only by using the error signal before and after the code inversion position, which is relatively easily affected by the intersymbol interference and less affected by the nonlinear distortion. This makes it possible to obtain an output signal that has good equalization characteristics before and after the sign inversion, which is important in data determination, and that is less affected by nonlinear distortion.

(Embodiment) Next, an embodiment of the present invention will be described with reference to FIGS. 1 to 3.

FIG. 1 is a block diagram showing an embodiment. The transversal filter 2 has the same configuration as the conventional example. The signal, which has been subjected to binary decision by the decision unit 7 from the output signal, is sent to the subtracter 8 for taking out the error signal and also to the sign inversion detection circuit 9 for detecting the inversion position of the code. The sign reversal detection circuit 9 detects the sign reversal position from the output signal of the determiner 7, and works to close the switch 12 for the error signal corresponding to the sign before and after the reversal. Since the sign inversion detection circuit 9 has a unit time T delay,
The delay element 3 is required between the subtractor 8 and the switch 12. Along with this, the delay amount of the filter input signal input to the correlator 13 also increases by the time T.

The sign inversion detection circuit 9 sends only the error signal of the sign having the sign inversion immediately before or immediately after to the correlator 13. Therefore, the tap gain coefficient converges so as to minimize only the error before and after the sign inversion, regardless of the reproduction amplitude when the same code continues.

FIG. 2 shows equalized waveforms when the conventional method is used and when the present method is used. The figure shows the equalization result of simulating the recording waveform of NRZ recording with a bit length of 0.5 μm. Non-linear distortion is included in the D / U ratio of about 10 dB. 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, so that the output signal level from the filter approaches the reference level, The aperture ratio of the eye, which greatly affects the data judgment error, is small. On the other hand, when the coefficient control according to this method is performed, the output signal amplitude increases as the continuous filter output signal of the same code passes outside the reference level, but the eye uses the conventional method. Compared with the case, the judgment margin is increased by about 20%.

Also, the output waveforms shown here are for the case where a code of which the minimum run length of the same code is not limited is used, but when an RLL code in which at least two identical codes are consecutive is used, the code inversion is performed. The effect does not appear continuously, so a higher effect can be obtained.

FIG. 3 shows a block diagram when the equalization method of the present system is applied to a recording signal to which a training signal is added. The sync signal detection circuit 15 monitors the output signal from the transversal filter 2 as in the conventional example, and prompts the generation of the training signal during the training period that appears after the sync signal. The output of the training signal generation circuit 16 is sent to the subtractor 8 and used for extracting the error signal, and at the same time, it is passed to the sign inversion detection circuit 9 for detecting the inversion position of the code.

Switch provided in the error signal feedback path
12 is closed only when both outputs of the sign inversion detection circuit 9 and the synchronization signal detection circuit 15 are active, and the coefficient control is performed only by the error signals before and after the sign inversion position during the training signal period. Done. 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, coefficient control is performed only by the training signal. Therefore, when a recording code such as an RLL code is used, the same RLL is used for the training signal.
It is more preferable that the rule is satisfied. Further, it is preferable to perform the training signal at the same time when the actual data is recorded, because the same recording condition is given to the training signal and the actual data.

(Effects of the Invention) By using the control method of the present invention, it is possible to extract a good equalized output with few determination errors even when the optical disk reproduction waveform input to the adaptive filter has a large amount of nonlinear distortion.

[Brief description of drawings]

FIG. 1 is a system diagram showing an embodiment of the present invention, FIG. 2 is a diagram for explaining an output result by this control method, FIG. 3 is a system diagram showing another embodiment of the present invention, and FIG. FIG. 5 and FIG. 5 are system diagrams for explaining the conventional technique, and FIG. 6 is a diagram for explaining the cause of non-linear distortion. In the figure, 1 ... input terminal, 2 ... transversal filter, 6 ... output terminal, 7 ... decider, 8 ... subtractor, 9 ... sign inversion detector, 13 ... correlator, 14 ... … Integrator, 15 …… sync signal detection circuit, 16 …… training signal generation circuit, 18 …… recording pit, 19 …… single pit response waveform, 20 …… playback waveform.

Claims (1)

(57) [Claims] 1. A method of controlling a tap coefficient in an adaptive transversal filter for equalizing a binary reproduction signal of an optical disc, wherein the sign of an output signal of the transversal filter is at least immediately before or after the sign. An adaptive filter control method, characterized in that a determination error is extracted from the output signal and the tap coefficient is updated only when the difference is different.