JP2806331B2 - Waveform equalization circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveform equalizing circuit, and more particularly, to a waveform equalizing circuit for equalizing a digital signal reproduced from a recording medium.

[0002]

2. Description of the Related Art Conventionally, in order to correct asymmetric waveform distortion of a digital signal reproduced from a recording medium, a reproduced digital signal is equalized in waveform by a waveform equalizing circuit. For example, a waveform equalization circuit in which a phase linear cos equalizer for compensating the frequency characteristics of a recording / reproducing optical system and an asymmetric transversal equalizer for correcting asymmetric waveform distortion of a reproduction signal generated by thermal recording are cascaded. It is conventionally known (JP-A-59-65936).

In this conventional waveform equalizer, the cos equalizer is composed of a delay line, a voltage attenuating element and a differential amplifier. If the delay time of the delay line is τ, it has a peak at a frequency ττ, It has a frequency characteristic having a predetermined amplitude at a frequency 0, the phase characteristic becomes linear, and the group delay time is a delay time τ
Therefore, the frequency characteristics of the optical recording / reproducing optical system can be compensated. The asymmetric transversal equalizer includes a delay line and a differential amplifier. The delay line is terminated with a plurality of intermediate taps having different delay times and a resistance equal to the characteristic impedance.

A plurality of signals having different delay times from the plurality of intermediate taps are respectively added and subtracted by a differential amplifier to remove nonlinear distortion.
The output signal of the differential amplifier is then returned to the original digital data by a data discriminating circuit using a threshold or a maximum likelihood decoding method.

[0005]

Generally, in order to obtain a good discrimination error rate in the reproduction of a digital signal,
It is desired that the S / N ratio of the reproduced signal be good. However,
In a certain kind of phase change medium, a sufficient S / N ratio cannot be obtained with a recording parameter value (for example, a recording current value of a recording laser in optical disk recording) that can provide a reproduced signal waveform with good symmetry. At the recording parameter value at which the best S / N ratio can be obtained, a phenomenon that the symmetry of the reproduced signal waveform is deteriorated is observed. Examples of these reproduced signal waveforms are shown in FIGS.

The reduction in the S / N ratio of the reproduced signal increases the error rate of the discriminating circuit when demodulating the reproduced signal into a digital signal. On the other hand, the deterioration of the symmetry of the waveform is caused by a difference between the upper and lower eye patterns used for the discrimination when multi-value discrimination, for example, ternary discrimination is used in the conversion into digital data. As a result, with respect to the eye with the smaller aperture, the noise margin with respect to the threshold value for determination decreases, and a problem occurs that the determination error rate increases as compared with the case where the waveform is symmetric. Therefore, in the conventional waveform equalizing circuit, when a reproduction signal from a specific recording medium is input, there is a problem that the S / N ratio and the vertical symmetry of the waveform cannot be compatible with each other, and the discrimination error rate decreases.

[0007] The present invention has been made in view of the above points,
It is an object of the present invention to provide a waveform equalization circuit that can achieve both an S / N ratio of a reproduced signal waveform of a phase change medium and vertical symmetry of the waveform and improve a discrimination error rate of the reproduced signal.

[0008]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a clamp circuit for clamping a digital signal reproduced from a recording medium and a nonlinear input / output characteristic for an output signal of the clamp circuit. And a non-linear equalizer circuit for outputting the same.

[0009] In addition, the non-linear input-output characteristics, Play Digi
Of Nonlinearity of Signal Amplitude Generated in Tall Signal Recording and Reproduction System
It has a characteristic nonlinearity .

According to the present invention, since the reproduced digital signal is given a non-linear amplitude characteristic after being clamped, even if the reproduced digital signal has a waveform asymmetry, the waveform is symmetric with respect to the center level. Amplitude correction can be performed.

[0011]

Next, an embodiment of the present invention will be described. FIG. 1 shows a block diagram of a reproducing apparatus provided with one embodiment of a waveform equalizing circuit according to the present invention. In FIG. 1, a waveform equalization circuit 4 is a waveform equalization circuit according to an embodiment of the present invention, and includes a clamp circuit 5 and a nonlinear circuit 6.
Although the waveform equalizing circuit 4 is provided between the reproduction preamplifier 3 and the transversal filter 7, it may be provided at a subsequent stage of the transversal filter 7.

Next, the operation of the reproducing apparatus will be described. The digital information signal recorded on the recording medium 1 is reproduced by a reproduction signal pickup 2 and supplied to a reproduction preamplifier 3, where it is preamplified and then supplied to a waveform equalization circuit 4, where the lower part of the waveform is After being clamped by the clamp circuit 5, a nonlinear characteristic is given by the nonlinear equalization circuit 6.

Here, the symmetry of the waveform of the reproduced signal input to the waveform equalizing circuit 4 is poor, and the eye patterns thereof are, as shown in FIG.
In the case where a difference occurs in the magnitude of the waveform, the instantaneous waveform of the reproduced signal waveform which becomes such an eye is distorted on one side (lower portion of the waveform) as shown in FIG. I have. When the reproduced signal having the waveform shown in FIG. 3 is input to the waveform equalizing circuit 4, the lower part of the waveform is clamped by the clamp circuit 5, and the direct current at the lower end of the waveform is fixed. The purpose of fixing the DC level is that the characteristics of the next-stage nonlinear equalizing circuit 6 generate an output corresponding to the DC level.

The reproduced signal waveform is subsequently input to a non-linear equalizer 6 having an amplitude response characteristic (input / output characteristic) indicated by I in FIG. The input / output characteristic I is obtained and set from the recording / reproducing response characteristic of the medium. When the input signal is at a low level below a predetermined level, the input signal is output at almost the same level, and when the input signal is at a high level above the predetermined level, the signal level is reduced. This is a nonlinear amplitude response characteristic that is output after compression. Therefore, when a reproduced signal having a waveform whose lower part is relatively contracted with respect to the upper part of the waveform as shown by II in FIG.
As shown by III, the output signal has a substantially symmetric waveform by compressing the noise level at the upper part of the waveform. As described above, the asymmetry of the waveform of the input reproduction signal is suppressed by the non-linear equalization circuit 6, that is, the symmetry is improved and the noise level is also compressed and output.

Examples of the non-linear equalizing circuit 6 use a voltage-current characteristic of a diode, or read-only memory (RO) data obtained by A / D converting a reproduced signal.
M), there is a method in which the data after the A / D conversion is used as an address, a response value is written to the data at the address, and decoding is performed.

The reproduced signal whose waveform has been equalized by the nonlinear equalizer 6 is corrected for the amplitude characteristic degraded in the recording / reproducing process by the transversal filter 7, and the amplitude is made constant by the automatic gain control (AGC) circuit 8. After being controlled so as to be, the signal is supplied to the PLL circuit 9 and the data determination circuit 10.

The PLL circuit 9 binarizes the input reproduction signal by a comparator, and synchronizes the output pulse of the voltage controlled oscillator with the rise and fall of the output pulse of the comparator. The extracted clock signal is supplied to a data discriminating circuit 10 for discriminating a reproduced signal based on a threshold value or performing maximum likelihood decoding, and converting the signal into digital data. Data discriminating circuit 1
The output digital data of 0 is transmitted to a subsequent signal processing circuit (not shown).

In this embodiment, since the symmetry of the reproduced signal waveform is improved, the jitter of the output clock signal of the PLL circuit 9 due to the poor symmetry of the reproduced signal waveform is reduced. The eye pattern opening of the signal can be sampled almost accurately, and the erroneous detection rate of data discrimination can be reduced.

According to the present invention, after a reproduced signal is converted into a digital signal through an A / D converter, the digital signal may be subjected to clamp processing and nonlinear equalization by digital processing. The nonlinear input / output characteristics of the nonlinear equalizer 6 are not limited to the characteristics shown in FIG. 5, but may be any inverse characteristics of the nonlinearity of the reproduced digital signal .

[0020]

As described above, according to the present invention,
By applying a nonlinear amplitude characteristic after the reproduced digital signal is clamped, even if the reproduced digital signal has a waveform asymmetry, the amplitude is corrected so that the waveform is symmetric with respect to the center level. When the recording is performed so that the S / N ratio of the reproduced signal is the best, the S / N ratio and the S / N ratio can be reduced even when using a recording medium that deteriorates the waveform symmetry.
A reproduced signal having both waveform symmetry can be obtained. Therefore, even in the case of multi-valued discrimination, the noise margins of the respective eye pattern openings with respect to the threshold values are substantially equal, and the discrimination of the originally possessed reproduced signal is performed. An error rate can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram of a playback device having one embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of an eye pattern of a reproduced signal having poor waveform symmetry.

FIG. 3 is an instantaneous waveform of the eye pattern of FIG. 2;

FIG. 4 is an input / output characteristic diagram of the nonlinear equalization circuit in FIG. 1;

FIG. 5 is a diagram showing a reproduction signal waveform.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 recording medium 2 reproduction signal pickup 3 reproduction preamplifier 4 waveform equalization circuit 5 clamp circuit 6 nonlinear equalization circuit 7 transversal filter 8 AGC (automatic gain control) circuit 9 PLL circuit 10 data discrimination circuit

Claims (1)

(57) [Claims] 1. A clamp circuit for clamping a digital signal reproduced from the recording medium, the output signal of the clamp circuit, the reproduced digital
A waveform equalization circuit comprising: a nonlinear equalization circuit that applies a reverse characteristic of a nonlinearity generated in a recording / reproduction system to a signal waveform amplitude and outputs the signal.