Digital signal reproducing apparatus with an amplitude limit sampling means

It is an object to provide a digital signal reproducing apparatus which can correctly reproduce a digital signal by suppressing a deterioration in decoding performance of a Viterbi decoding even when an asymmetry occurs in an information digital signal read from an optical disc. An amplitude-limited sample value is obtained by limiting an amplitude of a sample value obtained by sampling the read signal up to a predetermined amplitude limit value. The amplitude limit sample values are Viterbi decoded on the basis of a plurality of prediction samples including prediction sample whose values are respectively equal to the predetermined upper and lower amplitude limit values.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a reproducing apparatus of a digital 
signal recorded on a recording medium such as an optical disk or the like. 
2. Description of the Related Background Art 
A Viterbi decoding method (Viterbi Algorithm) has been known as a method of 
decoding a digital signal with a high reliability from a read signal read 
out from a recording medium on which the digital signal has been recorded 
at a high density. In the Viterbi algorithm, a train of sample values 
which are obtained by sampling the read signal is regarded as a time 
sequence and a digital signal sequence of "1" and/or "0" which seems to be 
most certain is obtained on the basis of the time sequence. 
FIG. 1 is a diagram showing a construction of a reproducing apparatus for 
reproducing a digital signal from an optical disk as an optical recording 
medium by applying the Viterbi algorithm. 
In FIG. 1, an optical pickup 1 irradiates a light beam onto an optical disk 
3 which is rotated by a spindle motor 2. A digital recording signal 
consisting of binary values each of which is "0" or "1" has been recorded 
on the optical disk 3. The optical pickup 1 photoelectrically converts a 
reflection light from the optical disk 3 and obtains a read signal p and 
supplies the signal to an A/D converter 10. The A/D converter 10 samples 
the read signal p at a predetermined sampling timing and obtains a sample 
value q and supplies it to a Viterbi decoder 20. 
Explanation will now be made with respect to the sample value q which is 
obtained in the case where a reproducing system shown in FIG. 1 is a 
partial response system of PR (1, 2, 2, 1) and a recording signal recorded 
on the optical disk 3 is a signal that was modulated under the rule of 
RLL(1, 7). 
In the reproducing system of PR (1, 2, 2, 1), a value to be obtained as a 
sample value q is decided every signal train of continuous four bits of 
the recording signal recorded on the optical disk 3. Since the recording 
signal has been modulated under the rule of RLL(1, 7), its minimum 
inversion interval is equal to 2. 
When the recording signal recorded on the optical disk 3 is considered on a 
unit basis of the 4-bit signal train, therefore, there are only ten kinds 
of patterns of the 4-bit train as follows. 
[0, 0, 0, 0] 
[0, 0, 0, 1] 
[0, 0, 1, 1] 
[0, 1, 1, 1] 
[1, 1, 1, 1] 
[1, 1, 1, 0] 
[1, 1, 0, 0] 
[1, 0, 0, 0] 
[0, 1, 1, 0] 
[1, 0, 0, 1] 
When the PR (1, 2, 2, 1) transmitting system is considered by making bit 
"1" and bit "0" in the 4-bit signal train to correspond to +1 and -1, 
respectively, values of the sample values q which are obtained each time 
the 4-bit signal train is read out from the optical disk 3 become as 
follows. 
EQU q[0,0,0,0]=(-1).times.1+(-1).times.2+(-1).times.2+(-1).times.1=-6 
EQU q[0,0,0,1]=(-1).times.1+(-1).times.2+(-1).times.2+(+1).times.1=-4 
EQU q[0,0,1,1]=(-1).times.1+(-1).times.2+(+1).times.2+(+1).times.1=0 
EQU q[0,1,1,1]=(-1).times.1+(+1).times.2+(+1).times.2+(+1).times.1=4 
EQU q[1,1,1,1]=(+1).times.1+(+1).times.2+(+1).times.2+(+1).times.1=6 
EQU q[1,1,1,0]=(+1).times.1+(+1).times.2+(+1).times.2+(-1).times.1=4 
EQU q[1,1,0,0]=(+1).times.1+(+1).times.2+(-1).times.2+(-1).times.1=0 
EQU q[1,0,0,0]=(+1).times.1+(-1).times.2+(-1).times.2+(-1).times.1=-4 
EQU q[0,1,1,0]=(-1).times.1+(+1).times.2+(+1).times.2+(-1).times.1=2 
EQU q[1,0,0,0]=(+1).times.1+(-1).times.2+(-1).times.2+(+1).times.1=-2 
Namely, in the case where the reproducing system shown in FIG. 1 is the PR 
(1, 2, 2, 1) transmitting system and the recording signal recorded on the 
optical disk 3 has been modulated under the format of RLL(1, 7), a value 
to be predicted as a sample value q is set to any one of 6, 4, 2, 0, -2, 
-4, and -6. 
A branch-metric calculation circuit 21 in the Viterbi decoder 20 obtains a 
square error, that is, 
{[sample value q]-[prediction sample K]}.sup.2 
between each of a plurality of prediction samples which can be predicted as 
a sample value q, namely, 
prediction sample K.sub.0 =6 
prediction sample K.sub.1 =4 
prediction sample K.sub.2 =2 
prediction sample K.sub.3 =0 
prediction sample K.sub.4 =-2 
prediction sample K.sub.5 =-4 
prediction sample K.sub.6 =-6 
and the actual sample value q, respectively, and supplies those square 
errors as branch-metric values to a path-metric calculation circuit 22. 
FIG. 2 is a diagram showing an example of an internal construction of the 
branch-metric calculation circuit 21 for arithmetically calculating the 
branch-metric values by using the prediction samples K.sub.0 to K.sub.6. 
In FIG. 2, the prediction samples K.sub.0 to K.sub.6 are fixedly supplied 
to subtracters 210 to 216, respectively. The subtracter 210 and a 
multiplier 217 calculate a square error between the sample value q 
supplied from the A/D converter 10 and the prediction sample K.sub.0 and 
supply the resulted square error as a branch-metric value e.sub.0 to the 
path-metric calculation circuit 22. The subtracter 211 and a multiplier 
218 obtain a square error between the sample value q and the prediction 
sample K.sub.1 and supply it as a branch-metric value e.sub.1 to the 
path-metric calculation circuit 22. The subtracter 212 and a multiplier 
219 obtain a square error between the sample value q and the prediction 
sample K.sub.2 and supply it as a branch-metric value e.sub.2 to the 
path-metric calculation circuit 22. The subtracter 213 and a multiplier 
220 obtain a square error between the sample value q and the prediction 
sample K.sub.3 and supply it as a branch-metric value e.sub.3 to the 
path-metric calculation circuit 22. The subtracter 214 and a multiplier 
221 obtain a square error between the sample value q and the prediction 
sample K.sub.4 and supply it as a branch-metric value e.sub.4 to the 
path-metric calculation circuit 22. The subtracter 215 and a multiplier 
222 obtain a square error between the sample value q and the prediction 
sample K.sub.5 and supply it as a branch-metric value e.sub.5 to the 
path-metric calculation circuit 22. The subtracter 216 and a multiplier 
223 obtain a square error between the sample value q and the prediction 
sample K.sub.6 and supply it as a branch-metric value e.sub.6 to the 
path-metric calculation circuit 22. 
FIG. 3 is a diagram showing an example of the so-called eye pattern of the 
read signal p which is ideally obtained in the case where the reproducing 
system shown in FIG. 1 is the PR (1, 2, 2, 1) transmitting system and the 
recording signal recorded on the optical disk 3 is the RLL (1, 7) 
modulated signal. 
The value of the sample value q which is obtained on the basis of the read 
signal p is equal to any one of the prediction samples K.sub.0 to K.sub.6. 
Any one of the branch-metric values e.sub.0 to e.sub.6 is, therefore, 
equal to 0. For example, when a sample value q.sub.0 is obtained at a 
sampling timing S2 shown in FIG. 3, the sample value q.sub.0 is equal to 
the prediction sample K.sub.0. In this instance, among the branch-metric 
values e.sub.0 to e.sub.6, the branch-metric value e.sub.0 is equal to 0. 
In the case where a sample value q.sub.6 is obtained at the sampling 
timing S.sub.2 shown in FIG. 3, the sample value q.sub.6 is equal to the 
prediction sample K.sub.6. In this instance, among the branch-metric 
values e.sub.0 to e.sub.6, the branch-metric value e.sub.6 is equal to 0. 
The path-metric calculation circuit 22 individually obtains an accumulated 
addition value of each of the branch-metric values e.sub.0 to e.sub.6 
every path and supplies a path selection signal indicative of a path in 
which the accumulated addition value is minimum to a path memory 23. While 
updating a serial digital signal sequence consisting of binary values of 
"0" and "1" in accordance with the path selection signal, the path memory 
23 sequentially generates the updated digital signal sequence as a 
reproduction digital signal corresponding to the recording signal. 
As mentioned above, the Viterbi decoder 20 obtains the square error values 
between the sample value q that is supplied from the A/D converter 10 and 
the prediction samples K.sub.0 to K.sub.6, respectively, and generates the 
data sequence corresponding to the path in which the accumulated addition 
value of the square error values is minimum as a reproduction digital 
signal corresponding to the recording signal. 
When an asymmetry occurs in the read signal p and its signal waveform 
becomes asymmetrical with respect to the center level, however, the sample 
value q is not equal to any one of the prediction samples K.sub.0 to 
K.sub.6, so that such a problem occurs that a decoding performance of the 
Viterbi decoder deteriorates. 
FIG. 4 is a diagram showing an example of the eye pattern in case an 
asymmetry occurs in the read signal p. 
In FIG. 4, when the sample value obtained at the sampling timing S.sub.2 is 
equal to q.sub.0, the sample value q.sub.0 is not equal to any one of the 
prediction samples K.sub.0 to K.sub.6. In this instance, an error .DELTA.q 
occurs even for the prediction sample K.sub.0 existing at the nearest 
position. Since the error .DELTA.q occurs, each of the branch-metric 
values e.sub.0 to e.sub.6 increases. In the Viterbi decoding for obtaining 
a most certain data signal sequence on the basis of the accumulated 
addition value of the branch-metric value, consequently, its decoding 
performance deteriorates. 
SUMMARY AND OBJECTS OF THE INVENTION 
The present invention has been conceived in order to solve the 
above-mentioned problem and therefore it is an object of the invention to 
provide a digital signal reproducing apparatus which can correctly 
reproduce a digital signal by suppressing a deterioration in decoding 
performance of a Viterbi decoding even when an asymmetry occurs in a read 
signal. 
According to the present invention, there is provided a digital signal 
reproducing apparatus for reproducing a recorded digital signal from a 
read signal read out from a recording medium on which the digital signal 
has been recorded, thereby obtaining a reproduction digital signal, 
comprising: amplitude limit sampling means for obtaining an amplitude 
limit sample value whose amplitude is limited by a predetermined amplitude 
limit value from the read signal; and a Viterbi decoder for performing a 
Viterbi decoding to the amplitude limit sample value on the basis of a 
plurality of prediction samples including a prediction sample whose value 
may be equal to the predetermined amplitude limit value, thereby obtaining 
the reproduction digital signal. 
In the digital signal reproducing apparatus according to the present 
invention, an amplitude of the sample value obtained by sampling the read 
signal is limited up to the predetermined amplitude limit value, thereby 
obtaining an amplitude limit sample value, and the amplitude limit sample 
value is Viterbi decoded on the basis of a plurality of prediction samples 
including a prediction sample whose value may be equal to the 
predetermined amplitude limit value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention will now be described hereinbelow. 
FIG. 5 is a diagram showing an example of a construction of a digital 
signal reproducing apparatus according to the invention. 
In FIG. 5, the optical pickup 1 irradiates a light beam onto the optical 
disk 3 which is rotated by the spindle motor 2. A digital information 
signal consisting of binary values each being "0" or "1" has been recorded 
on the optical disk 3. The optical pickup 1 photoelectrically converts a 
light beam reflected from the optical disk 3 into a read signal p and 
supplies it to the A/D converter 10. The A/D converter 10 samples the read 
signal p at a predetermined sampling timing, thereby obtaining the sample 
value q. 
Explanation will now be made with respect to the sample value q in case a 
reproducing system shown in FIG. 5 is a partial response system of PR (1, 
2, 2, 1) and the original digital signal recorded on the optical disk 3 is 
the RLL (1, 7) modulated signal. 
In the reproducing system of PR (1, 2, 2, 1), a value which can be obtained 
as a sample value q is decided every signal train of continuous four bits 
of the digital signal recorded on the optical disk 3. Since the original 
digital signal has been RLL (1, 7) modulated, its minimum inversion 
interval is equal to 2. 
When the digital signal recorded on the optical disk 3 is considered on a 
unit basis of the 4-bit signal train, therefore, there are only ten kinds 
of patterns of the 4-bit train as follows. 
[0, 0, 0, 0] 
[0, 0, 0, 1] 
[0, 0, 1, 1] 
[0, 1, 1, 1] 
[1, 1, 1, 1] 
[1, 1, 1, 0] 
[1, 1, 0, 0] 
[1, 0, 0, 0] 
[0, 1, 1, 0] 
[1, 0, 0, 1] 
where, when the PR (1, 2, 2, 1) transmitting system is considered by making 
bit "1" and bit "0" among the 4-bit train correspond to +1 and -1, 
respectively, the sample value q obtained each time the 4-bit signal train 
is read out from the optical disk 3 is as follows. 
EQU q[0,0,0,0]=(-1).times.1+(-1).times.2+(-1).times.2+(-1).times.1=-6 
EQU q[0,0,0,1]=(-1).times.1+(-1).times.2+(-1).times.2+(+1).times.1=-4 
EQU q[0,0,1,1]=(-1).times.1+(-1).times.2+(+1).times.2+(+1).times.1=0 
EQU q[0,1,1,1]=(-1).times.1+(+1).times.2+(+1).times.2+(+1).times.1=4 
EQU q[1,1,1,1]=(+1).times.1+(+1).times.2+(+1).times.2+(+1).times.1=6 
EQU q[1,1,1,0]=(+1).times.1+(+1).times.2+(+1).times.2+(-1).times.1=4 
EQU q[1,1,0,0]=(+1).times.1+(+1).times.2+(-1).times.2+(-1).times.1=0 
EQU q[1,0,0,0]=(+1).times.1+(-1).times.2+(-1).times.2+(-1).times.1=-4 
EQU q[0,1,1,0]=(-1).times.1+(+1).times.2+(+1).times.2+(-1).times.1=2 
EQU q[1,0,0,0]=(+1).times.1+(-1).times.2+(-1).times.2+(+1).times.1=-2 
That is, in the case where the reproducing system shown in FIG. 5 is the PR 
(1, 2, 2, 1) transmitting system and the original digital signal recorded 
on the optical disk 3 has been RLL (1, 7) modulated, a value which is 
predicted as a sample value q is equal to any one of 6, 4, 2, 0, -2, -4, 
and -6. 
When the sample value q is larger than a predetermined upper limit value 
L.sub.MAX, a limiter 30 supplies the upper limit value L.sub.MAX as an 
amplitude limit sample value Q to the Viterbi decoder 20. When the sample 
value q is smaller than the upper limit value L.sub.MAX and is larger than 
a predetermined lower limit value L.sub.MIN, the limiter 30 supplies the 
supplied sample value q as it is as an amplitude limit sample value Q to 
the Viterbi decoder 20. In case the sample value q is smaller than the 
lower limit value L.sub.MIN, the limiter 30 supplies the lower limit value 
L.sub.MIN as an amplitude limit sample value Q to the Viterbi decoder 20. 
Namely, the limiter 30 supplies the value obtained by limiting the 
amplitude of the sample value q corresponding to the read signal p by the 
above-mentioned upper limit value L.sub.MAX and lower limit value 
L.sub.MIN as an amplitude limit sample value Q to the Viterbi decoder 20. 
In this instance, the upper limit value L.sub.MAX is set to a value which 
is smaller than the maximum value of the sample value q to be obtained 
when the level of the read signal p is fluctuated due to an influence by 
an asymmetry or the like and which is smaller than the maximum value of 
the prediction sample. The lower limit value L.sub.MIN is, further, set to 
a value which is larger than the minimum value of the sample value q to be 
obtained when the level of the read signal p is fluctuated due to the 
influence by the asymmetry or the like and which is larger than the 
minimum value of the prediction sample. 
FIG. 6 is a diagram showing a construction of a branch-metric calculation 
circuit 21' in the Viterbi decoder 20. 
FIG. 6 shows an example of an internal construction of the branch-metric 
calculation circuit 21' which is used in the case where the reproducing 
system shown in FIG. 5 is considered as a PR (1, 2, 2, 1) transmitting 
system and the digital signal recorded on the optical disk 3 is the RLL 
(1, 7) modulated signal. 
Values which are predicted as sample values q, namely, 6, 4, 2, 0, -2, -4, 
and -6 are made correspond as follows, respectively. 
prediction sample K.sub.0 =6 
prediction sample K.sub.1 =4 
prediction sample K.sub.2 =2 
prediction sample K.sub.3 =0 
prediction sample K.sub.4 =-2 
prediction sample K.sub.5 =-4 
prediction sample K.sub.6 =-6 
The branch-metric calculation circuit 21' shown in FIG. 6 uses the 
prediction samples K.sub.1 to K.sub.5 excluding the prediction sample 
K.sub.0 whose value is maximum and the prediction sample K.sub.6 whose 
value is minimum from among the prediction samples K.sub.0 to K.sub.6. 
In FIG. 6, the subtracter 210 and multiplier 217 obtain a square error 
between the amplitude limit sample value Q supplied from the limiter 30 
and the upper limit value L.sub.MAX as an amplitude limit value of the 
limiter 30 and set the square error to the branch-metric value e.sub.0. 
The subtracter 211 and multiplier 218 obtain a square error between the 
amplitude limit sample value Q and the prediction sample K.sub.1 and set 
it to the branch-metric value e.sub.1. The subtracter 212 and multiplier 
219 obtain a square error between the amplitude limit sample value Q and 
the prediction sample K.sub.2 and set it to the branch-metric value 
e.sub.2. The subtracter 213 and multiplier 220 obtain a square error 
between the amplitude limit sample value Q and the prediction sample 
K.sub.3 and set it to the branch-metric value e.sub.3. The subtracter 214 
and multiplier 221 obtain a square error between the amplitude limit 
sample value Q and the prediction sample K.sub.4 and set it to the 
branch-metric value e.sub.4. The subtracter 215 and multiplier 222 obtain 
a square error between the amplitude limit sample value Q and the 
prediction sample K.sub.5 and set it to the branch-metric value e.sub.5. 
The subtracter 216 and multiplier 223 obtain a square error between the 
amplitude limit sample value Q and the lower limit value L.sub.MIN as an 
amplitude limit value of the limiter 30 and set it to the branch-metric 
value e.sub.6. 
The path-metric calculation circuit 22 obtains an accumulated addition 
value of each of the branch-metric values e.sub.0 to e.sub.6 every path 
and supplies the path selection signal indicative of a path in which the 
accumulated addition value is minimum to the path memory 23. While 
updating a serial digital signal sequence consisting of binary values of 
"0" and "1" in accordance with the path selection signal, the path memory 
23 sequentially generates the signal sequence as a reproduction digital 
signal corresponding to the recording signal. 
As mentioned above, the digital signal reproducing apparatus performs the 
Viterbi decoding by using the amplitude limit sample value Q in which an 
amplitude of the sample value q obtained in correspondence to the read 
signal p is limited by the limiter 30. Among a plurality of prediction 
samples which are used for Viterbi decoding, further, the value of each of 
the maximum and minimum prediction samples is equalized to the amplitude 
limit value of the limiter 30. 
According to the construction, even if an asymmetry occurs in the read 
signal p and the sample value q exceeds a range of the prediction samples 
K.sub.0 to K.sub.6, in this instance, the branch-metric value e.sub.0 or 
e.sub.6 can be forcedly set to 0. 
In FIG. 7, for instance, it is assumed that the sample value obtained at 
the sampling timing S.sub.2 is set to q.sub.0. In this instance, the 
sample value q.sub.0 is larger than the upper limit value L.sub.MAX as an 
amplitude limit value of the limiter 30 as shown in FIG. 7. The limiter 
30, therefore, sets the upper limit value L.sub.MAX itself to an amplitude 
limit sample value Q.sub.0 and supplies it to the branch-metric 
calculation circuit 21' of the Viterbi decoder 20. The amplitude limit 
sample value Q.sub.0 is supplied to each of the subtracters 210 to 216 of 
the branch-metric calculation circuit 21' shown in FIG. 6. Among the 
subtracters 210 to 216, the subtracter 210 performs a subtraction between 
the amplitude limit sample value Q.sub.0 as an upper limit value L.sub.MAX 
and the upper limit value L.sub.MAX. The subtraction result is, therefore, 
equal to 0 and the branch-metric value e.sub.0 is also equal to 0. 
According to the invention, therefore, even if the value of the obtained 
sample value q exceeds the range of the prediction samples K.sub.0 to 
K.sub.6 and is not equal to any one of the prediction samples K.sub.0 to 
K.sub.6 due to an influence by the asymmetry, an error which occurs by a 
deviation between the sample value q and each of the prediction samples 
K.sub.0 to K.sub.6 is not reflected to the branch-metric value, so that a 
deterioration in performance of the Viterbi decoding can be suppressed. 
In the embodiment shown in FIG. 5, although the amplitude limitation using 
the limiter 30 has been executed to the sample value q obtained by A/D 
converting the read signal p. However, the amplitude limitation may be 
applied to the read signal p. Even in that case, a desirable reproduction 
can be expected. 
FIG. 8 is a diagram showing a construction of a digital signal reproducing 
apparatus which is another embodiment of the invention made in 
consideration of the above points. 
In FIG. 8, functional blocks having the same functions as those of the 
functional blocks shown in FIG. 5 are designated by the same reference 
numerals. 
In the digital signal reproducing apparatus shown in FIG. 8, the read 
signal p read out from the optical disk 3 is supplied to a limiter 30'. As 
shown in FIG. 9, the limiter 30' limits an amplitude value of the read 
signal p at a signal level corresponding to the upper limit value 
L.sub.MAX and the lower limit value L.sub.MIN and supplies the resultant 
signal to the A/D converter 10. Since the A/D converter 10 A/D converts 
the amplitude-limited read signal, the sample value to be obtained is 
equal to the amplitude limit sample value Q. 
In brief, so long as the limiter is constructed in such a manner that the 
sample value which is obtained by the A/D converter 10 is equal to the 
amplitude limit sample value Q whose amplitude is limited by the 
predetermined amplitude limit value, the limiter 30 can be provided before 
or after the A/D converter 10. 
In the branch-metric calculation circuit 21' shown in FIG. 6, among the 
prediction samples K.sub.0 to K.sub.6, each of the prediction sample 
K.sub.0 whose value is maximum and the prediction sample K.sub.6 whose 
value is minimum is equalized to the amplitude limit value (the upper 
limit value L.sub.MAX and the lower limit value L.sub.MIN) in the limiter 
30. A plurality of prediction samples near the maximum and minimum values 
can be also equalized to the amplitude limit value. 
FIG. 10 is a diagram showing an internal construction of the branch-metric 
calculation circuit 21' according to another embodiment of the invention 
made in consideration of the above points. 
In FIG. 10, functional blocks having the same functions as those of the 
functional blocks shown in FIG. 6 are designated by the same reference 
numerals. 
In FIG. 10, the subtracter 210 and multiplier 217 obtain a square error 
between the amplitude limit sample value Q supplied from the limiter 30 
and the upper limit value L.sub.MAX as an amplitude limit value of the 
limiter 30 and supply the square error as branch-metric values e.sub.0 and 
e.sub.1 to the path-metric calculation circuit 22. The subtracter 212 and 
multiplier 219 obtain a square error between the amplitude limit sample 
value Q and the prediction sample K.sub.2 and supply the square error as a 
branch-metric value e.sub.2 to the path-metric calculation circuit 22. The 
subtracter 213 and multiplier 220 obtain a square error between the 
amplitude limit sample value Q and the prediction sample K.sub.3 and 
supply it as a branch-metric value e.sub.3 to the path-metric calculation 
circuit 22. The subtracter 214 and multiplier 221 obtain a square error 
between the amplitude limit sample value Q and the prediction sample 
K.sub.4 and supply it as a branch-metric value e.sub.4 to the path-metric 
calculation circuit 22. The subtracter 216 and multiplier 223 obtain a 
square error between the amplitude limit sample value Q and the lower 
limit value L.sub.MIN as an amplitude limit value of the limiter 30 and 
supply it as branch-metric values e.sub.5 and e.sub.6 to the path-metric 
calculation circuit 22. 
Namely, the branch-metric calculation circuit 21' shown in FIG. 10 
equalizes not only the prediction sample K.sub.0 but also the prediction 
sample K.sub.1 to the upper limit value L.sub.MAX of the limiter 30. The 
circuit 21' further equalizes not only the prediction sample K.sub.6 but 
also the prediction sample K.sub.5 to the lower limit value L.sub.MIN of 
the limiter 30. 
In the branch-metric calculation circuit 21' shown in FIG. 10, therefore, 
the construction comprising the subtracter 211 and multiplier 218 and the 
construction comprising the subtracter 215 and multiplier 222 which are 
used in the branch-metric calculation circuit 21' shown in FIG. 6 are 
substantially unnecessary. 
When the branch-metric calculation circuit 21' shown in FIG. 10 is used, 
therefore, the circuit scale can be reduced as compared with the 
branch-metric calculation circuit 21' of FIG. 6. 
According to the present invention, in the Viterbi algorithm as mentioned 
above, by paying attention to a point that values near the maximum and 
minimum levels of the read signal are not important in the decoding, the 
amplitude limit sample value is obtained in a manner such that the 
amplitude of the sample value obtained by sampling the read signal is 
limited by the predetermined amplitude limit value and, the amplitude 
limit sample value is Viterbi decoded on the basis of the plurality of 
prediction samples including the prediction sample whose value is equal to 
the predetermined amplitude limit value. 
According to the construction, the value of the branch-metric can be 
forcedly set to 0 under the situation even if the amplitude of the sample 
value obtained when the read signal is sampled is larger or smaller than 
the ideal value due to an influence by the asymmetry. 
The sample value obtained by sampling the read signal, therefore, is not 
remarkably deviated from the prediction sample value, so that the 
deterioration in decoding performance of the Viterbi decoding can be 
prevented. By equalizing the values of two or more prediction samples to 
the predetermined amplitude limit value, the circuit scale can be reduced.