Wave-shaping apparatus and reproduction signal processing apparatus including the same

A reproduction signal processing apparatus 200 in a first example according to the present invention will be described with reference to FIG. 1 . FIG. 1 is a block diagram of the reproduction signal processing apparatus 200 . As shown in FIG. 1, the reproduction signal processing apparatus 200 includes a wave-shaping apparatus 100 and a digitizing circuit 1300 .

A signal recorded on an optical disk 1 is reproduced by an optical pickup circuit 2 as a reproduction signal RS.

The wave-shaping apparatus 100 includes a first detection circuit 3 for detecting the reproduction signal RS from the optical pickup circuit 2 at a prescribed time constant and thus detecting an upper envelope RSU, a second detection circuit 4 for detecting the reproduction signal RS from the optical pickup circuit 2 at a prescribed time constant and thus detecting a lower envelope RSD, an averaging circuit 5 for finding an average value AVE of the upper envelope RSU and the lower envelope RSD, a low pass filter 6 for smoothing the AVE obtained by the averaging circuit 5 to output a smoothing signal SM 1 , and a subtraction circuit 7 for subtracting the smoothing signal SM 1 from the reproduction signal RS to output a wave-shaped signal SS. The digitizing circuit 1300 digitizes the wave-shaped signal SS to output a digitized signal.

With reference to FIGS. 2A through 2D, an exemplary operation of the wave-shaping apparatus 100 will be described.

As described above, a signal stored in the optical disk 1 (FIG. 1) is read by the optical pickup circuit 2 as a reproduction signal RS. When the optical disk 1 has a defect such as, for example, a scratch, the reproduction signal RS includes a disturbance D 1 as shown in FIG. 2 A.

When the reproduction signal RS is directly input to the digitizing circuit 1300 , the reproduction signal RS is digitized with a negative feedback digitized slicing level SL 1 , which follows an average value of the upper envelope RSU and the lower envelope RSD or the vicinity thereof. However, when the disturbance D 1 caused by a defect has an excessively short cycle, the negative feedback digitizing slicing level SL 1 cannot appropriately follow the disturbance D 1 , resulting in a slicing error.

In order to avoid such an inconvenience, the reproduction signal processing apparatus 200 includes the above-described elements and operates as follows.

Referring to FIG. 2A, the first detection circuit 3 (FIG. 1) detects the reproduction signal RS from the optical pickup circuit 2 and detects the upper envelope RSU. The second detection circuit 4 (FIG. 1) detects the reproduction signal RS from the optical pickup circuit 2 and detects the lower envelope RSD.

Referring to FIG. 2B, the averaging circuit 5 puts a weight represented by a weighting coefficient M:N to the upper envelope RSU and the lower envelope RSD to calculate the average value AVE.

Referring to FIG. 2C, the low pass filter 6 removes a noise component included in the average value AVE and extracts a hazardous DC fluctuation component included in the reproduction signal RS obtained when the optical disk (FIG. 1) has a defect. The signal containing substantially only the extracted component is output as a smoothing signal SM 1 .

Referring to FIG. 2D, the subtraction circuit 7 subtracts the smoothing signal SM 1 from the reproduction signal RS to output a wave-shaped signal SS which has a lines of symmetry between the upper half and the lower half.

When the wave-shaped signal SS obtained as a result of such pre-processing is input to the digitizing circuit 1300 , the wave-shaped signal SS is digitized with a negative feedback digitizing slicing level SL 2 . The negative feedback digitizing slicing level SL 2 does not substantially fluctuate even when the disturbance D 1 caused by the defect has an excessively short cycle and can appropriately follow the disturbance D 1 . Thus, slicing errors are significantly reduced. FIG. 3 shows an exemplary structure of the averaging circuit 5 . As shown in FIG. 3, the averaging circuit 5 can be realized by a simple structure.

With reference to FIGS. 4A and 4B, a frequency spectrum of the reproduction signal RS will be described.

FIG. 4A shows the frequency spectrum obtained when the reproduction signal RS is processed using a high pass filter 105 described with reference to FIG. 15 . As shown in FIG. 4A, a component represented by area A 1 is removed from the reproduction signal RS represented by area S by the high pass filter 105 . Generation of jitters caused by the lack of the component represented by area A 1 is increased.

FIG. 4B shows the frequency spectrum obtained when the reproduction signal RS is processed by the reproduction signal processing apparatus 200 . As shown in FIG. 4B, a component represented by an area as small as area B 1 is removed from the reproduction signal RS. Thus, the generation of jitters can be minimized. As a result, the slicing performance obtained when the light beam passes through the defect of the optical disk 1 (FIG. 1) is improved while the generation of jitters when the optical disk 1 has no defect is not deteriorated.

Where the weight coefficient M:N provided by the averaging circuit 5 to the upper and lower envelopes for finding the average value AVE is 1:1, an averaging circuit having a simple structure is obtained. Such an average circuit 5 A is shown in FIG. 5 .

FIG. 6 is a block diagram of a reproduction signal processing apparatus 200 A in a second example according to the present invention. Identical elements previously discussed with respect to FIG. 1 bear identical reference numerals and the descriptions thereof will be omitted. The reproduction signal processing apparatus 200 A includes a wave-shaping apparatus 100 A and the digitizing circuit 1300 .

The wave-shaping apparatus 100 A includes an asymmetry detection circuit 8 in addition to the elements shown in FIG. 1 . The weighting coefficient M:N to the upper and lower envelopes used by the averaging circuit 5 for calculating the average value AVE is determined to be substantially in proportion to an asymmetry quantity of the reproduction signal RS.

Asymmetry is usually generated under certain disc cutting conditions or recording conditions. When a recording pit tends to be written to be larger than a prescribed reference pit, the negative feedback digitizing slicing level to realize a post-digitization duty ratio of 50:50 is offset upward (or downward) from an intermediate level of the reproduction signal. When a recording pit tends to be written to be smaller than the prescribed reference pit, the negative feedback digitizing slicing level to realize a post-digitization duty ratio of 50:50 is offset downward (or upward) from the intermediate level. The offset amount of the negative feedback digitizing slicing level relative to the intermediate level of the reproduction signal RS is referred to as the “asymmetry quantity”.

With reference to FIGS. 7A through 7D, a frequency spectrum of the reproduction signal RS will be described.

FIG. 7A shows the frequency spectrum obtained when the asymmetry occurs. As shown in FIG. 7A, a negative feedback digitizing slicing level SL 3 is offset from an intermediate level ML by an asymmetry quantity ASQ. The ratio S:T, i.e., the ratio of (i) the distance between negative feedback digitizing slicing level SL 3 and the upper envelope RSU and (ii) the distance between negative feedback digitizing slicing level SL 3 and the lower envelope RSD is referred to as an “asymmetry coefficient ASC”.

In a period corresponding to the disturbance D 1 caused by a defect, the negative feedback digitizing slicing level SL 3 fluctuates while maintaining the asymmetry coefficient ASC=S:T. However, in a period where the disturbance D 1 has an excessively short cycle, the negative feedback digitizing slicing level SL 3 cannot appropriately follow the disturbance D 1 , resulting in a slicing error.

In order to avoid such an inconvenience, the reproduction signal processing apparatus 200 A includes the above-described elements and operates as follows.

Referring to FIG. 7B, the averaging circuit 5 sets a weighting coefficient M:N to the upper envelope RSU and the lower envelope RSD used for calculating an average value AVE 1 at the asymmetry coefficient ASC (S:T) which is output from the asymmetry detection circuit 8 , and puts a weight represented by the asymmetry coefficient ASC (S:T) to each of the upper envelope RSU and the lower envelope RSD to calculate the average value AVE 1 .

Referring to FIG. 7C, the low pass filter 6 removes a noise component included in the average value AVE 1 and outputs a smoothing signal SM 2 . Referring to FIG. 7D, the subtraction circuit 7 subtracts the smoothing signal SM 2 from the reproduction signal RS to output a wave-shaped signal SS 1 .

Where the weighting coefficient M:N is set at the asymmetry coefficient ASC (S:T), the fluctuation of the negative feedback digitizing slicing level SL 3 is minimized as shown in FIG. 7 D. Thus, the slicing performance when the light beam passes through the defect of the optical disk 1 (FIG. 1) is further improved.

The weighting coefficient M:N can also be 1:1 when the asymmetry occurs. With such setting, the fluctuation of the negative feedback digitizing slicing level SL 3 is within a range causing no practical problem, although the fluctuation is slightly larger as compared with when the weighting coefficient M:N is set at the asymmetry coefficient.

FIG. 8 shows an exemplary structure of the asymmetry detection circuit 8 . The asymmetry detection circuit 8 includes a peak detection circuit 9 for detecting a peak of the reproduction signal RS, a second low pass filter 10 for detecting a DC level of the reproduction signal RS, a bottom level detection circuit 11 for detecting a bottom level of the reproduction signal RS, second subtraction circuits 12 A and 12 B, and an asymmetry coefficient calculation circuit 12 C. The asymmetry coefficient calculation circuit 12 C calculates the asymmetry coefficient ASC (S:T) based on the difference between the output from the peak detection circuit 9 and the output from the second low pass filter 10 and the difference between the output from the second low pass filter 10 and the bottom level detection circuit 11 .

At least one of a first detection time constant used by the first detection circuit 3 and a second detection time constant used by the second detection circuit 4 is determined to be substantially in proportion to the diameter of a light spot on the optical disk 1 (FIG. 1 ).

FIGS. 9A and 9C respectively show light spots SP 1 and SP 2 having diameters d 1 and d 2 passing through a defect DF on the optical disk 1 . In FIGS. 9A and 9C, the size of the defect DF is the same. FIGS. 9B and 9D respectively show the fluctuation of the reproduction signal RS in the states shown in FIGS. 9A and 9C.

The diameter d 1 of the light spot SP 1 is n times the diameter d 2 of the light spot SP 2 (n>1). When the diameter d 1 is n times the diameter d 2 , the decreasing rate of the level of the reproduction signal RS (represented as RS 1 ) in FIG. 9B is 1/n times the decreasing rate of the level of the reproduction signal RS (represented as RS 2 ) in FIG. 9 D. Thus, the level of the reproduction signal RS 1 decreases more slowly than the reproduction signal RS 2 .

The first detection circuit 3 detects the reproduction signals RS 1 and RS 2 using detection signals DS 1 and DS 2 , respectively. A detection time constant T 1 of the detection signal DS 1 is determined to be n times a detection time constant T 2 of the detection signal DS 2 . As can be appreciated from this, the detection time constant used by the first detection circuit 3 is determined to be substantially in proportion to the diameter of the light spot. The detection time constant used by the second detection circuit 4 is also determined to be substantially in proportion to the diameter of the light spot.

The detection time constants T 1 and T 2 are determined so as to detect a decrease in the level of the reproduction signals RS 1 and RS 2 . Therefore, the detection time constants T 1 and T 2 can be determined to be short so long as the slopes of the detection signals DS 1 and DS 2 shown in FIGS. 9B and 9D are obtained.

It should be noted that when the detection time constant is determined to be excessively short so that a rapidly decreasing reproduction signal level is detected, unnecessary noise is picked up. Accordingly, the detection time constant is determined to be appropriate in consideration of the dimensions of the light spot.

It is preferable to determine the detection time constant to be substantially in proportion to the diameter of the light spot. In this case, the slicing performance obtained when the light beam passes through the defect of the optical disk 1 (FIG. 1) is improved while the generation of jitters when the optical disk 1 has no defect is not deteriorated without picking up the unnecessary noise.

The detection time constant used by the first detection circuit 3 or the second detection circuit 4 can be determined so as to be substantially in inverse proportion to the reproduction linear velocity. FIGS. 10A and 10B show fluctuations of the reproduction signal RS when the light beam passes through the defect of the optical disk 1 (FIG. 1 ), with different linear velocities of the same defect. FIG. 10A shows the fluctuation when the optical disk 1 is reproduced at the normal speed, and FIG. 10B shows the fluctuation when the optical disk 1 is reproduced at the speed of n times the normal speed.

As can be appreciated from the comparison between FIG. 10 A and FIG. 10B, when the linear velocity is n times higher, the decreasing rate of the level of the reproduction signal RS is also n times higher, and accordingly the optimum detection time constant is 1/n. In this manner, the detection time constant is determined to be substantially in inverse proportion to the reproduction linear velocity.

The cutoff frequency of the low pass filter 6 can be determined to be substantially in inverse proportion to the diameter of the light spot on the optical disk 1 (FIGS. 9 A and 9 C). As shown in FIGS. 9A and 9B, when the diameter of the light spot is n times larger, the decreasing rate of the level of the reproduction signal RS is 1/n. When the diameter of the light spot is 1/n, the decreasing rate of the level of the reproduction signal RS is n times higher.

When the decreasing rate of the level of the reproduction signal RS is n times higher, the frequency of the hazardous DC fluctuation component (FIG. 7C) included in the smoothing signal SM 2 becomes n times higher. In order to allow the fluctuation component having the frequency of n times higher to pass through the low pass filter 6 , the cutoff frequency of the low pass filter 6 needs to be set at n times higher.

Accordingly, when the diameter of the light spot is 1/n, the cutoff frequency of the low pass filter 6 is set at n times higher. In this manner, the cutoff frequency of the low pass filter 6 is determined to be substantially in inverse proportion to the diameter of the light spot.

In this case, the slicing performance obtained when the light beam passes through the defect of the optical disk 1 (FIG. 1) is improved while the generation of jitters when the optical disk 1 has no defect is not deteriorated.

The cutoff frequency of the low pass filter 6 can be determined to be substantially in proportion to the reproduction linear velocity. As shown in FIG. 10B, when the reproduction linear velocity is n times higher, the decreasing rate of the level of the reproduction signal RS obtained when the light beam passes through the defect of the optical disk 1 is also n times higher.

When the decreasing rate of the level of the reproduction signal RS is n times higher, the frequency of the hazardous DC fluctuation component (FIG. 7C) included in the smoothing signal SM 2 becomes n times higher. In order to allow the fluctuation component having the frequency of n times higher to pass through the low pass filter 6 , the cutoff frequency of the low pass filter 6 needs to be set at n times higher.

Accordingly, when the reproduction linear velocity is n times higher, the cutoff frequency of the low pass filter 6 is set at n times higher. In this manner, the cutoff frequency of the low pass filter 6 is determined to be substantially in proportion to the reproduction linear velocity.

In this case, the slicing performance obtained when the light beam passes through the defect of the optical disk 1 (FIG. 1) is improved while the generation of jitters when the optical disk 1 has no defect is not deteriorated.

FIG. 11 is a block diagram of a reproduction signal processing apparatus 200 B in a third example according to the present invention. Identical elements previously discussed with respect to FIG. 1 bear identical reference numerals and the descriptions thereof will be omitted. The reproduction signal processing apparatus 200 B includes a wave-shaping apparatus 100 B and the digitizing circuit 1300 .

The wave-shaping apparatus 100 B includes a dropout detection circuit 13 for detecting a dropout indicating a decrease in the amplitude of the reproduction signal RS in addition to the elements shown in FIG. 1 .

When the dropout detection circuit 13 detects a dropout, at least one of the first detection time constant used by the first detection circuit 3 and the second detection time constant used by the second detection circuit 4 is determined to be shorter than the time constant when no dropout is detected. Accordingly, when a dropout is detected, a negative feedback digitizing slicing level can follow a disturbance more accurately in a period corresponding to the disturbance caused by a defect. When no dropout is detected (normal reproduction), the noise or the like generated in the first or second detection circuit 3 or 4 is suppressed. Accordingly, the slicing performance obtained when the light beam passes through the defect of the optical disk 1 (FIG. 1) is improved while the generation of jitters when the optical disk 1 has no defect is not deteriorated.

FIG. 12 is a block diagram of another reproduction signal processing apparatus 200 C in the third example according to the present invention. Identical elements previously discussed with respect to FIG. 11 bear identical reference numerals and the descriptions thereof will be omitted. The reproduction signal processing apparatus 200 C includes a wave-shaping apparatus 100 C and the digitizing circuit 1300 .

When the dropout detection circuit 13 detects a dropout, the cutoff frequency of the low pass filter 6 is set at higher than the time constant when no dropout is detected. Accordingly, when a dropout is detected, a hazardous DC fluctuation component generated in a period corresponding to a disturbance caused by a defect is allowed to pass through the low pass filter 6 . When no dropout is detected (normal reproduction), the noise or the like generated in the first or second detection circuit 3 or 4 is suppressed. Accordingly, the slicing performance obtained when the light beam passes through the defect of the optical disk 1 (FIG. 1) is improved while the generation of jitters when the optical disk 1 has no defect is not deteriorated.

In the first through third examples, the reproduction signal processing apparatuses respectively including the wave-shaping apparatuses 100 , 100 A and 100 B which are used for pre-processing the signal before the signal is input to the digitizing circuit 1300 . The present invention is not limited to these apparatuses and is applicable to any apparatus for pre-processing a signal before the signal is input to an A/D converter.

FIG. 13 is a block diagram illustrating a structure of still another reproduction signal processing apparatus 200 D in the third example according to the present invention. Identical elements previously discussed with respect to FIG. 1 bear identical reference numerals and the descriptions thereof will be omitted.

The reproduction signal processing apparatus 200 D includes the wave-shaping apparatus 100 , an A/D converter 115 for performing A/D conversion of a wave-shaped signal SS output by the wave-shaping apparatus 100 , and a digital signal processing circuit 116 for performing digital signal processing of the signal output by the A/D converter 115 and outputting reproduction data and a reproduction clock.

As described in the first through third examples, the wave-shaping apparatus outputs a wave-shaped signal SS which is accurately digitized reproduction signal even when the level of the reproduction signal significantly fluctuated by a defect on an optical disk. Such a function of the wave-shaping apparatus stabilizes the performance of the digital signal processing circuit 116 .

In this manner, the wave-shaping apparatus 100 is usable for pre-processing a signal before the signal is input to the A/D converter 115 . The wave-shaping apparatuses 100 A and 100 B are also usable for pre-processing a signal before the signal is input to an A/D converter.

As described above, the present invention provides a wave-shaping apparatus and a reproduction signal processing apparatus, which accurately digitizes a reproduction signal even when the level of the reproduction signal obtained from an optical disk significantly fluctuates by a defect on the optical disk as well as when the optical disk has no defect. The wave-shaping apparatus according to the present invention further makes maximum use of the dynamic range of an A/D converter, and improves the stability of the digital signal processing circuit. Thus, the wave-shaping apparatus and the reproduction signal processing apparatus according to the present invention alleviate the load of a digitizing circuit and significantly decrease the generation of jitters.

Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.