PATENT ABSTRACT
Disclosed herein is a decoding apparatus for decoding an encoded signal on the basis of a plurality of state-transition trellises having state counts different from each other. The decoding apparatus including: a decoding section for decoding the encoded signal on the basis of a first state-transition trellis; and a mode selection section for selecting either a first operating mode based on the first state-transition trellis or a second operating mode based on a second state-transition trellis having a state count smaller than that of the first state-transition trellis. If the mode selection section selects the second operating mode, the decoding section decodes the encoded signal by carrying out switching of a state transition from a first state transition selected among state transitions of the first state-transition trellis as a first state transition not corresponding to the second state-transition trellis to a second state transition selected among state transitions of the second state-transition trellis as a second state transition not corresponding to the first state-transition trellis.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to decoding apparatus, decoding methods, program-recording mediums, programs and recording/reproduction apparatus. More particularly, the present invention relates to a decoding apparatus capable of keeping up with a plurality of operating modes without increasing a circuit size, a decoding method adopted by the decoding apparatus, a program implementing the decoding method, a program-recording medium for storing the program and a recording/reproduction apparatus employing the decoding apparatus.  
         [0002]     The capability exhibited by a contemporary recording/reproduction apparatus designed for optical disks as a capability of recording data into the optical disks mounted on the recording/reproduction apparatus has been becoming better remarkably. The conventional binary identification only is not sufficient means for guaranteeing a low reproduction-error rate. The number of cases has been increasing as cases in which a Viterbi decoding circuit capable of guaranteeing a low reproduction-error rate in spite of existence of inter-code interferences is used.  
         [0003]      FIG. 1  is a diagram showing a typical configuration of a recording/reproduction apparatus employing the Viterbi decoding circuit.  
         [0004]     In the typical configuration shown in  FIG. 1 , a modulation circuit  11  carries out a modulation process as part of processing to record data onto a recording medium  14 . The modulation circuit  11  converts an input sequence received from a preceding stage not shown in the figure as a sequence of pieces of information to be recorded into a modulated sequence X t  (t=0, 1, 2 and so on), and outputs the modulated sequence X t  to a precoder  12 .  
         [0005]     The precoder  12  carries out a preceding process for a partial response, which is abbreviated hereafter to a PR. That is to say, the precoder  12  encodes the modulated sequence X t  received from the modulation circuit  11  on the basis of a predetermined coding rule to generate an intermediate sequence y t . The precoder  12  then records the intermediate sequence y t  onto the recording medium  14  by way of a recording amplifier  13  including a recording head.  
         [0006]     The recording medium  14  is typically an optical disk such as a Blu-ray Disc, a CD-RW (Compact Disk ReWritable) or a DVD±RW (Digital Versatile Disk ReWritable). The recording amplifier  13  records data received by the modulation circuit  11  from the preceding stage not shown in the figure as a sequence of pieces of information to be recorded onto the recording medium  14 .  
         [0007]     A reproduction amplifier  15  including a reproduction head detects a signal reproduced from the recording medium  14  and outputs the result of the detection to an equalizer  16 . The equalizer  16  carries out an equalization process for a target transmission line model close to the frequency characteristic of a transmission line on the reproduced signal received from the reproduction amplifier  15  to generate a transmission-line output Z. An example of the equalization process is a PR equalization process. Then, the equalizer  16  outputs the transmission-line output Z to a PLL (Phase Locked Loop)  17  and a sampling circuit  18 .  
         [0008]     The PLL  17  extracts clock components from the transmission-line output Z on a transmission line including the recording medium  14  to generate a clock signal synchronized with the reproduced signal. The PLL  17  then outputs the generated clock signal to the sampling circuit  18 , a Viterbi decoding circuit  19  and a demodulation circuit  20 .  
         [0009]     The sampling circuit  18  samples the transmission-line output Z received from the equalizer  16  in synchronization with the clock signal received from the PLL  17  to convert the transmission-line output Z into data, which is shown in the figure as a sampled sequence Z t . The sampling circuit  18  then supplies the sampled sequence Z t  to the Viterbi decoding circuit  19 . The Viterbi decoding circuit  19  carries out a Viterbi decoding process on the sampled sequence Z t  received from the sampling circuit  18  to produce a most probable modulated sequence x t  corresponding to the output of the modulation circuit  11 .  
         [0010]     The demodulation circuit  20  is the counterpart of the modulation circuit  11 . The demodulation circuit  20  demodulates the most probable modulated sequence x t  received from the Viterbi decoding circuit  19  and outputs the result of the demodulation process to a succeeding stage not shown in the figure.  
         [0011]      FIG. 2  is a diagram showing a typical configuration of the Viterbi decoding circuit  19  employed in the recording/reproduction apparatus shown in  FIG. 1 .  
         [0012]     As shown in  FIG. 2 , the typical configuration of the Viterbi decoding circuit  19  comprises a BM (Branch Metric) computation circuit  41 , an ACS (Add, Compare and Select) circuit  42 , a path memory  43  and a most-probable determination circuit  44 .  
         [0013]     The BM computation circuit  41  uses the sampled sequence Z t  received as an input signal to compute branch-metric data for state transitions, which are each a transition from a state to another, and outputs the branch-metric data to the ACS circuit  42 .  
         [0014]     The ACS circuit  42  adds path-metric data of a state immediately preceding the present state to the branch-metric data received from the BM computation circuit  41  to produce a sum. If paths merge at the path memory  43  to be described later, the ACS circuit  42  adds path-metric data of a state immediately preceding the present state to the branch-metric data received from the BM computation circuit  41  to produce a sum for each of the merging paths, and compares the sums with each other to select the smallest one. The ACS circuit  42  then uses the selected sum resulting from of the addition, comparison and selection processes as updated path-metric data of the present state. Finally, the ACS circuit  42  outputs the result of the selection of the sums to the path memory  43  and the most-probable determination circuit  44 . In the following description, the path-metric data may be referred to simply as metric data in some cases.  
         [0015]     The path memory  43  is a plurality of shift registers provided at a plurality of stages each identified by a stage number as registers each composed of an array of flip-flops. Each of the flip-flops serves as a memory. A value stored in a memory is subjected to a select-shift operation before being shifted to a next memory. To put it in detail, a value to be stored in a memory provided at any specific stage is selected among values, which are received from memories provided at a stage preceding the specific stage through merging paths cited above, in dependence on the aforementioned selection result received from the ACS circuit  42 , and the selected value is then stored in the memory before being shifted to memories provided at a stage immediately following the specific stage. These operations to select, store and shift a value are carried out repeatedly. The operation to select a value to be stored in a memory provided at any specific stage among values from memories at a stage preceding the specific stage in dependence on the aforementioned selection result received from the ACS circuit  42  agrees with the operation carried out by the ACS circuit  42  to select a smallest sum among sums computed for merging paths as described above.  
         [0016]     The most-probable determination circuit  44  fetches an output signal from memories provided at the last stage and supplies the fetched output signal to the demodulation circuit  20  as the modulated sequence x t . For example, assume that the path memory  43  is composed of 16 stages. In this case, the most-probable determination circuit  44  fetches an output signal from memories composing a shift register provided at the 16 th  stage. In this way, the most probable signal reproduced at a time leading ahead of the present time by 16 clocks is firmly determined.  
         [0017]     It is to be noted that, if the above operation to select a value is not carried out in the path memory  43  as an operation agreeing with merging of paths, the most-probable determination circuit  44  carries out a most probable determination process to extract an output signal from path memories for storing minimum path-metric data for a state on the basis of a selection result received the ACS circuit  42 , and supplies the output signal extracted in the most probable determination process to the demodulation circuit  20  as the modulated sequence x t .  
         [0018]     By referring to  FIGS. 3 and 4 , the following description explains a PR transmission line for a case in which (1, 7) RLL (Run Length Limited) codes for d (minimum run length)=1 are used. It is to be noted that, in the typical configurations shown in  FIGS. 3 and 4 , a circle represents a state whereas a label attached to an arrow represents a branch or a transition. An RLL code is a code in which the number of 0s sandwiched between is in a modulated code is limited. A (d, k) RLL code is an RLL code with a minimum run length d set for a sequence of 0s sandwiched between 1s and a maximum run length k set for the sequence of 0s sandwiched between 1s. For example, a (1, 7) RLL code has a minimum run length of 1 and a maximum run length of 7 for the sequence of 0s sandwiched between 1s.  
         [0019]      FIG. 3  is a diagram showing state transitions of a PR (1, x, 1) transmission line with a constraint length of 3 for a case in which (1, 7) RLL codes are used. It is to be noted that the PR (1, x, 1) transmission line with a constraint length of 3 can be typically a PR (1, 1, 1) transmission line or a PR (1, 2, 1) transmission line. Since these typical transmission lines are different from each other only in that they have different theoretical values (or identification reference values) of transitions c, they are all explained below as the PR (1, x, 1) transmission line.  
         [0020]     In the case of the typical configuration shown in  FIG. 3 , notation c 000  denotes a transition from state S 00  to state S 00 . Notation c 001  denotes a transition from state S 00  to state S 01 . Notation c 011  denotes a transition from state S 01  to state S 11 . Notation c 111  denotes a transition from state S 11  to state S 11 . Notation c 110  denotes a transition from state S 11  to state S 10 . Notation c 100  denotes a transition from state S 10  to state S 00 .  
         [0021]     That is to say, in the diagram showing state transitions of a PR (1, x, 1) transmission line with a constraint length of 3, d (minimum run length)=1 shrinks the number of state transitions to 4 and the number of states to 4.  
         [0022]      FIG. 4  is a diagram showing transitions of states of a PR (1, x, x, 1) transmission line with a constraint length of 4 for a case in which (1, 7) RLL codes are used. It is to be noted that the PR (1, x, x, 1) transmission line with a constraint length of 4 can be typically a PR (1, 2, 2, 1) transmission line or a PR (1, 3, 3, 1) transmission line. Since these typical transmission lines are different from each other only in that they have different theoretical values (or identification reference values) of transitions c, they are all explained below as the PR (1, x, x, 1) transmission line.  
         [0023]     In the case of the typical configuration shown in  FIG. 4 , notation c 0000  denotes a transition from state S 000  to state S 000 . Notation c 0001  denotes a transition from state S 000  to state S 001 . Notation c 0011  denotes a transition from state S 001  to state S 011 . Notation c 0111  denotes a transition from state S 011  to state S 111 . Notation c 0110  denotes a transition from state S 011  to state S 110 . Notation c 1111  denotes a transition from state S 111  to state S 111 . Notation c 1110  denotes a transition from state S 111  to state S 110 . Notation c 0011  denotes a transition from state S 110  to state S 100 . Notation c 1001  denotes a transition from state S 100  to state S 001 . Notation c 1000  denotes a transition from state S 100  to state S 000 .  
         [0024]     That is to say, in the diagram showing transitions of states of a PR (1, x, x, 1) transmission line with a constraint length of 4, d (minimum run length)=1 shrinks the number of values to 7 and the number of states to 6.  
         [0025]     As described above, in a PR transmission line, a reproduced signal value is not confirmed by a state S itself. Instead, it is not until a transition c from a state S to a state S that a reproduced signal value is identified firmly.  
         [0026]     By referring to  FIGS. 5 and 6 , the following description explains details of the Viterbi decoding circuit  19  for the PR (1, x, 1) transmission line, the state transitions of which are shown in  FIG. 3 . It is to be noted that  FIG. 5  is a diagram showing typical configurations of the branch-metric computation circuit  41  and the ACS circuit  42  for the PR (1, x, 1) transmission line, the state transitions of which are shown in  FIG. 3 . On the other hand,  FIG. 6  is a diagram showing a typical configuration of the path memory  43  for the PR (1, x, 1) transmission line, the state transitions of which are shown in  FIG. 3 .  
         [0027]     In the typical configuration shown in  FIG. 5 , the branch-metric computation circuit  41  includes as many branch-metric computation sections  61 , which are used for computing BM (branch-metric data) for every transition from a state to another, as the state transitions. In the case of the typical configuration shown in  FIG. 5 , the number of state transitions is 6. Thus, the branch-metric computation circuit  41  includes 6 branch-metric computation sections  61 - 1  to  61 - 6 . Each of the branch-metric computation sections  61 - 1  to  61 - 6  calculates branch-metric data bm representing the likelihood of a state transition c and outputs the branch-metric data bm to the ACS circuit  42 . It is to be noted that symbol cABC (where suffixes A, B and C each represent the integer 0 or 1) assigned to a state transition c denotes the theoretical value (the identification reference value) of the state transition c. In addition, symbol nˆ2 used in the following description denotes the square of n.  
         [0028]     To put it concretely, let us assume that a reproduced signal (or a sampled sequence) completing a PR equalization at a time k is z k . In this case, the branch-metric computation section  61 - 1  computes branch-metric data bm 000   k  (=(z k −c 000 )ˆ2), which represents the likelihood of the state transition c 000 , and outputs the branch-metric data bm 000   k  to an ACS section  62 - 1 . By the same token, the branch-metric computation section  61 - 2  computes branch-metric data bm 100   k  (=(z k −c 100 )ˆ2), which represents the likelihood of the state transition c 100 , and outputs the branch-metric data bm 100   k  also to the ACS section  62 - 1 . In the same way, the branch-metric computation section  61 - 3  computes branch-metric data bm 001   k  (=(z k −c 001 )ˆ2), which represents the likelihood of the state transition c 001 , and outputs the branch-metric data bm 001   k  to an ACS section  62 - 2 . Likewise, the branch-metric computation section  61 - 4  computes branch-metric data bm 110   k  (=(z k −c 110 )ˆ2), which represents the likelihood of the state transition c 110 , and outputs the branch-metric data bm 110   k  to an ACS section  62 - 3 . Similarly, the branch-metric computation section  61 - 5  computes branch-metric data bm 001   k  (=(z k −c 011 ) ˆ2) which represents the likelihood of the state transition c 011 , and outputs the branch-metric data bm 001   k  to an ACS section  62 - 4 . By the same token, the branch-metric computation section  61 - 6  computes branch-metric data bm 111   k  (=(z k −c 111 )ˆ2), which represents the likelihood of the state transition c 111 , and outputs the branch-metric data bm 111 k also to the ACS section  62 - 4 .  
         [0029]     The ACS circuit  42  adds path-metric data stored internally as path-metric data of a state immediately preceding the present state to the branch-metric data received from the branch-metric computation circuit  41  to produce a sum. The ACS circuit  42  then uses the sum as updated path-metric data m of the present state. The path-metric data m is the likelihood of a history up to state S. The ACS circuit  42  includes as many ACS (add, compare and select) sections  62  as states. In the case of the typical configuration shown in  FIG. 5 , the number of states is 4. Thus, the ACS circuit  42  includes 4 ACS sections  62 - 1  to  62 - 4 . It is to be noted that the ACS sections  62 - 1  to  62 - 4  with path merging existing each compare the sum of path-metric data of a state immediately preceding the present state for one path and the branch-metric data with the sum of path-metric data of a state immediately preceding the present state for the other path and the branch-metric data, selecting the smaller one. The ACS circuit  42  then uses the selected smaller sum as updated path-metric data of the present state. Finally, the ACS circuit  42  outputs a selection result indicating which sum has been selected to the path memory  43 .  
         [0030]     To put it concretely, the ACS section  62 - 1  updates the path-metric data m 00   k , which is the likelihood of a history up to state S 00 . To be more specific, the ACS section  62 - 1  adds the path-metric data m 00   k-1  stored internally in the ACS section  62 - 1  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 000   k  received from the branch-metric computation section  61 - 1  to produce a first sum. The ACS section  62 - 1  also adds the path-metric data m 10   k-1  stored internally in the ACS section  62 - 3  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 100   k  received from the branch-metric computation section  61 - 2  to produce a second sum. Then, the ACS section  62 - 1  compares the first and second sums with each other in order to select the smaller one to be used as updated path-metric data m 00   k  of the present state. Finally, the ACS section  62 - 1  outputs a selection result sel 00  to a memory included in the path memory  43  as a memory used for storing the value of state S 00 . The computations and the comparison, which are carried out by the ACS section  62 - 1 , can be expressed by Eq. (1) given as follows:
 
 m 00 k =min { m 00 k-1   +bm 000 k   , m 10 k-1   +bm 100 k }  (1)
 
         [0031]     On the other hand, the ACS section  62 - 2  updates the path-metric data m 01   k , which is the likelihood of a history up to state S 01 . To put it concretely, the ACS section  62 - 2  adds the path-metric data m 00   k-1  stored internally in the ACS section  62 - 1  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 001   k  received from the branch-metric computation section  61 - 3  to produce a sum and uses the sum as updated path-metric data m 01   k  of the present state. The computation carried out by the ACS section  62 - 2  can be expressed by Eq. (2) given as follows:
 
 m 01 k   =m 00 k-1   +bm 001 k   (2)
 
         [0032]     By the same token, the ACS section  62 - 3  updates the path-metric data m 10   k , which is the likelihood of a history up to state S 10 . To put it concretely, the ACS section  62 - 3  adds the path-metric data m 11   k-1  stored internally in the ACS section  62 - 4  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 110   k  received from the branch-metric computation section  61 - 4  to produce a sum and uses the sum as updated path-metric data m 10   k  of the present state. The computation carried out by the ACS section  62 - 3  can be expressed by Eq. (3) given as follows:
 
 m 10 k   =m 11 k-1   +bm 110 k   (3)
 
         [0033]     In the same way as the ACS section  62 - 1 , the ACS section  62 - 4  updates the path-metric data m 11   k , which is the likelihood of a history up to state S 11 . To put it concretely, the ACS section  62 - 4  adds the path-metric data m 01   k-1  stored internally in the ACS section  62 - 2  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 011   k  received from the branch-metric computation section  61 - 5  to produce a first sum. The ACS section  62 - 4  also adds the path-metric data m 11   k-1  stored internally in the ACS section  62 - 4  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 111   k  received from the branch-metric computation section  61 - 6  to produce a second sum. Then, the ACS section  62 - 4  compares the first and second sums in order to select the smaller one to be used as updated path-metric data m 11   k  of the present state. Finally, the ACS section  62 - 4  outputs a selection result sel 11  to a memory included in the path memory  43  as a memory used for storing the value of state S 11 . The computations and the comparison, which are carried out by the ACS section  62 - 4 , can be expressed by Eq. (4) given as follows:
 
 m 11 k =min { m 11 k-1   +bm 111 k   , m 01 k-1   +bm 011 k }  (4)
 
         [0034]     The path memory  43  shown in  FIG. 6  depicts a trellis expressing the state-transition diagram shown in  FIG. 3  in terms of sequences along the time axis. A circle represents a state S shown in  FIG. 3  whereas an arrow represents a state transition c. Each shift register employed in the path memory  43  shown in  FIG. 6  has 4 memories having the same form as the 4-state trellis expressing the state-transition diagram shown in  FIG. 3  in terms of sequences along the time axis. That is to say, the Viterbi decoding circuit  19  for a PR (1, x, 1) transmission line carries out a decoding process on the basis of the trellis expressing the state-transition diagram shown in  FIG. 3  in terms of sequences along the time axis.  
         [0035]     Thus, a circle in the path memory  43  also represents a memory such as a flip-flop. In the typical configuration shown in  FIG. 6 , the number of stages of memories composing the path memory  43  is 3. It is to be noted, however, that the number of stages can be actually 16 or 32 for example.  
         [0036]     In the path memory  43 , an operation to select a value among values stored in memories at a preceding stage is carried out in dependence on a selection result, which is the result of selection carried out by the ACS circuit  42 , and the selected value is shifted to a memory at the stage immediately following the preceding stage repeatedly. To put it concretely, in the path memory  43 , a value to be stored in a memory for state S 00  at any specific stage is a value selected among a value stored in a memory for state S 00  at a stage immediately preceding the specific stage and a value stored in a memory for state S 10  at the stage immediately preceding the specific stage in accordance with a selection result sel 00  received from the ACS section  62 - 1 . The selected value stored in the memory for state S 00  at the specific stage is then shifted (output) to a memory for state S 00  at a stage immediately following the specific stage and a memory for state S 00  at the stage immediately following the specific stage. By the same token, in the path memory  43 , a value to be stored in a memory for state S 11  at the specific stage is a value selected among a value stored in a memory for state S 11  at the stage immediately preceding the specific stage and a value stored in a memory for state S 01  at the stage immediately preceding the specific stage in accordance with a selection result sel 11  received from the ACS section  62 - 4 . The selected value stored in the memory for state S 11  at the specific stage is then shifted (output) to a memory for state S 11  at the stage immediately following the specific stage and a memory for state S 10  at the stage immediately following the specific stage.  
         [0037]     It is to be noted that, by way of a memory for state S 01  at each specific stage, a value is shifted from a memory at a stage immediately preceding the specific stage to a memory existing at a stage immediately following the specific stage as a memory according to a transition c. Thus, for any specific stage in the path memory  43 , a value stored in a memory for state S 00  at a stage immediately preceding the specific stage is shifted to a memory for state S 11  at a stage immediately following the specific stage by way of a memory for state S 01  at the specific stage. By the same token, by way of a memory for state S 10  at each specific stage, a value is shifted from a memory at a stage immediately preceding the specific stage to a memory existing at a stage immediately following the specific stage as a memory according to a transition c. Thus, for any specific stage in the path memory  43 , a value stored in a memory for state S 11  at a stage immediately preceding the specific stage is shifted to a memory for state S 00  at a stage immediately following the specific stage by way of a memory for state S 10  at the specific stage.  
         [0038]     As described above, a value to be stored in any path memory provided at any specific stage identified by a certain stage number as a path memory at which paths merge is selected among values stored in path memories provided at stages immediately following the specific stage in accordance with a selection result produced by a process carried out by the ACS circuit to select a sum of metric data as a result of comparison of sums. As a result, the most-probable determination circuit  44  fetches data originated from the most probable paths from memories provided at the last stage, and supplies the fetched data to the demodulation circuit  20  as the modulated sequence x t .  
         [0039]     By referring to  FIGS. 7 and 8 , the following description explains details of the Viterbi decoding circuit  19  for the PR (1, x, x, 1) transmission line, the state transitions of which are shown in  FIG. 4 . It is to be noted that  FIG. 7  is a diagram showing typical configurations of the branch-metric computation circuit  41  and the ACS circuit  42  for the PR (1, x, x, 1) transmission line, the state transitions of which are shown in  FIG. 4 . On the other hand,  FIG. 8  is a diagram showing a typical configuration of the path memory  43  for the PR (1, x, x, 1) transmission line, the state transitions of which are shown in  FIG. 4 . The typical configurations shown in  FIGS. 7 and 8  are basically the same as their counterparts included in the Viterbi decoding circuit  19  as shown in  FIGS. 5 and 6  respectively except that, in the typical configurations shown in  FIGS. 7 and 8 , the number of states is 6 and the number of state transitions is 10. Thus, descriptions of their details are not repeated to avoid duplications.  
         [0040]     In the typical configuration shown in  FIG. 7 , the branch-metric computation circuit  41  includes as many branch-metric computation sections  71  as the state transitions. In the case of the typical configuration shown in  FIG. 7 , the number of state transitions is 10. Thus, the branch-metric computation circuit  41  includes 10 computation sections  71 - 1  to  71 - 10 .  
         [0041]     The branch-metric computation section  71 - 1  computes branch-metric data bm 0000   k  (=(z k −c 0000 )ˆ2), which represents the likelihood of the state transition c 0000 , and outputs the branch-metric data bm 0000 k to an ACS section  72 - 1 . By the same token, the branch-metric computation section  71 - 2  computes branch-metric data bm 1000   k  (=(z k −c 1000 )ˆ2), which represents the likelihood of the state transition c 1000 , and outputs the branch-metric data bm 1000   k  also to the ACS section  72 - 1 . Likewise, the branch-metric computation section  71 - 3  computes branch-metric data bm 0001   k  (=(z k −c 0001 )ˆ2), which represents the likelihood of the state transition c 0001 , and outputs the branch-metric data bm 0001   k  to an ACS section  72 - 2 . By the same token, the branch-metric computation section  71 - 4  computes branch-metric data bm 1001   k  (=(z k −c 1001 )ˆ2), which represents the likelihood of the state transition c 1001 , and outputs the branch-metric data bm 1001   k  also to the ACS section  72 - 2 . In the same way, the branch-metric computation section  71 - 5  computes branch-metric data bm 0011   k  (=(z k −c 0011 )ˆ2), which represents the likelihood of the state transition c 0011 , and outputs the branch-metric data bm 0011   k  to an ACS section  72 - 3 .  
         [0042]     Likewise, the branch-metric computation section  71 - 6  computes branch-metric data bm 1100   k  (=(z k −c 1100 )ˆ2), which represents the likelihood of the state transition c 1100 , and outputs the branch-metric data bm 1100   k  to an ACS section  72 - 4 . Similarly, the branch-metric computation section  71 - 7  computes branch-metric data bm 0110   k  (=(z k −c 0110 )ˆ2), which represents the likelihood of the state transition c 0110 , and outputs the branch-metric data bm 0110   k  to an ACS section  72 - 5 . By the same token, the branch-metric computation section  71 - 8  computes branch-metric data bm 1110   k  (=(z k −c 1110 )ˆ2), which represents the likelihood of the state transition c 1110 , and outputs the branch-metric data bm 1110   k  also to the ACS section  72 - 5 . Similarly, the branch-metric computation section  71 - 9  computes branch-metric data bm 0111   k  (=(z k −c 0111 )ˆ2), which represents the likelihood of the state transition c 0111 , and outputs the branch-metric data bm 0111   k  to an ACS section  72 - 6 . By the same token, the branch-metric computation section  71 - 10  computes branch-metric data bm 1111   k  (=(z k −c 1111 )ˆ2), which represents the likelihood of the state transition c 1111 , and outputs the branch-metric data bm 1111   k  also to the ACS section  72 - 6 .  
         [0043]     The ACS circuit  42  includes as many ACS (add, compare and select) sections  72  as states. In the case of the typical configuration shown in  FIG. 7 , the number of states is 6. Thus, the ACS circuit  42  includes 6 ACS sections  72 - 1  to  72 - 6 .  
         [0044]     The ACS section  72 - 1  updates path-metric data m 000   k , which is the likelihood of a history up to state S 000 . To be more specific, the ACS section  72 - 1  adds the path-metric data m 000   k-1  stored internally in the ACS section  72 - 1  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 0000   k  received from the branch-metric computation section  71 - 1  to produce a first sum. The ACS section  72 - 1  also adds the path-metric data m 100   k-1  stored internally in the ACS section  72 - 4  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 1000   k  received from the branch-metric computation section  71 - 2  to produce a second sum. Then, the ACS section  72 - 1  compares the first and second sums in order to select the smaller one to be used as updated path-metric data m 000   k  of the present state. Finally, the ACS section  72 - 1  outputs a selection result sel 000  to a memory included in the path memory  43  as a memory used for storing the value of state S 000 . The computation and the comparison, which are carried out by the ACS section  72 - 1 , can be expressed by Eq. (5) given as follows:
 
 m 000 k =min { m 000 k-1   +bm 0000 k   , m 100 k-1   +bm 1000 k}   (5)
 
         [0045]     By the same token, the ACS section  72 - 2  updates path-metric data m 001   k , which is the likelihood of a history up to state S 001 . To be more specific, the ACS section  72 - 2  adds the path-metric data m 000   k-1  stored internally in the ACS section  72 - 1  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 0001   k  received from the branch-metric computation section  71 - 3  to produce a first sum. The ACS section  72 - 2  also adds the path-metric data m 100   k-1  stored internally in the ACS section  72 - 4  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 1001   k  received from the branch-metric computation section  71 - 4  to produce a second sum. Then, the ACS section  72 - 2  compares the first and second sums in order to select the smaller one to be used as updated path-metric data m 001   k  of the present state. Finally, the ACS section  72 - 2  outputs a selection result sel 001  to a memory included in the path memory  43  as a memory used for storing the value of state S 001 . The computation and the comparison, which are carried out by the ACS section  72 - 2 , can be expressed by Eq. (6) given as follows:
 
 m 001 k =min { m 000 k-1   +bm 0001 k   , m 100 k-1   +bm 1001 k }  (6)
 
         [0046]     On the other hand, the ACS section  72 - 3  updates path-metric data m 011   k , which is the likelihood of a history up to state S 011 . To be more specific, the ACS section  72 - 3  adds the path-metric data m 001   k-1  stored internally in the ACS section  72 - 2  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 0011   k  received from the branch-metric computation section  71 - 5  to produce a sum. Then, the ACS section  72 - 3  uses the sum as updated path-metric data m 011   k  of the present state. The computation carried out by the ACS section  72 - 3  can be expressed by Eq. (7) given as follows:
 
 m 011 k   =m 001 k-1   +bm 0011 k   (7)
 
         [0047]     By the same token, the ACS section  72 - 4  updates path-metric data m 100   k , which is the likelihood of a history up to state S 100 . To be more specific, the ACS section  72 - 4  adds the path-metric data m 110   k-1  stored internally in the ACS section  72 - 5  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 1100   k  received from the branch-metric computation section  71 - 6  to produce a sum. Then, the ACS section  72 - 4  uses the sum as updated path-metric data m 100   k  of the present state. The computation carried out by the ACS section  72 - 4  can be expressed by Eq. (8) given as follows:
 
 m 100 k   =m 110 k-1   +bm 1100 k   (8)
 
         [0048]     The ACS section  72 - 5  updates path-metric data m 110   k , which is the likelihood of a history up to state S 110 . To be more specific, the ACS section  72 - 5  adds the path-metric data m 111   k-1  stored internally in the ACS section  72 - 6  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 1110   k  received from the branch-metric computation section  71 - 8  to produce a first sum. The ACS section  72 - 5  also adds the path-metric data m 011   k-1  stored internally in the ACS section  72 - 3  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 0110   k  received from the branch-metric computation section  71 - 7  to produce a second sum. Then, the ACS section  72 - 5  compares the first and second sums in order to select the smaller one to be used as updated path-metric data m 110   k  of the present state. Finally, the ACS section  72 - 5  outputs a selection result sel 110  to a memory included in the path memory  43  as a memory used for storing the value of state S 110 . The computation and the comparison, which are carried out by the ACS section  72 - 5 , can be expressed by Eq. (9) given as follows:
 
 m 110 k =min { m 111 k-1   +bm 1110 k   , m 011 k-1   +bm 0110 k }  (9)
 
         [0049]     By the same token, the ACS section  72 - 6  updates path-metric data m 111   k , which is the likelihood of a history up to state S 111 . To be more specific, the ACS section  72 - 6  adds the path-metric data m 111   k-1  stored internally in the ACS section  72 - 6  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 1111   k  received from the branch-metric computation section  71 - 10  to produce a first sum. The ACS section  72 - 6  also adds the path-metric data m 011   k-1  stored internally in the ACS section  72 - 3  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 0111   k  received from the branch-metric computation section  71 - 9  to produce a second sum. Then, the ACS section  72 - 6  compares the first and second sums in order to select the smaller one to be used as updated path-metric data m 111   k  of the present state. Finally, the ACS section  72 - 6  outputs a selection result sel 111  to a memory included in the path memory  43  as a memory used for storing the value of state S 111 . The computation and the comparison, which are carried out by the ACS section  72 - 6 , can be expressed by Eq. (10) given as follows:
 
 m 111 k =min { m 111 k-1   +bm 1111 k , ( m 011 k-1   +bm 0111 k }  (10)
 
         [0050]     The path memory  43  shown in  FIG. 8  depicts a trellis expressing the state-transition diagram shown in  FIG. 4  in terms of sequences along the time axis. A circle represents a state S shown in  FIG. 4  whereas an arrow represents a state transition c. Each shift register employed in the path memory  43  shown in  FIG. 8  has 6 memories having the same form as the 6-state trellis expressing the state-transition diagram shown in  FIG. 4  in terms of sequences along the time axis. That is to say, the Viterbi decoding circuit  19  for a PR (1, x, x, 1) transmission line carries out a decoding process on the basis of the trellis expressing the state-transition diagram shown in  FIG. 4  in terms of sequences along the time axis.  
         [0051]     In the path memory  43  shown in  FIG. 8 , an operation to select a value among values stored in memories at a preceding stage is carried out in dependence on the result of the selection carried out by the ACS circuit  42 , and the selected value is shifted to a memory at the stage immediately following the preceding stage repeatedly. That is to say, in the path memory  43 , a value to be stored in a memory for state S 000  at any specific stage is a value selected among a value stored in a memory for state S 000  at a stage immediately preceding the specific stage and a value stored in a memory for state S 100  at the stage immediately preceding the specific stage in accordance with a selection result sel 000  received from the ACS section  72 - 1 . The selected value stored in the memory for state S 000  at the specific stage is then shifted (output) to a memory for state S 000  at a stage immediately following the specific stage and a memory for state S 000  at the stage immediately following the specific stage. By the same token, in the path memory  43 , a value to be stored in a memory for state S 001  at the specific stage is a value selected among a value stored in a memory for state S 000  at the stage immediately preceding the specific stage and a value stored in a memory for state S 100  at the stage immediately preceding the specific stage in accordance with a selection result sel 001  received from the ACS section  72 - 2 . The selected value stored in the memory for state S 001  at the specific stage is then shifted (output) to a memory for state S 011  at the stage immediately following the specific stage.  
         [0052]     In addition, in the path memory  43 , a value to be stored in a memory for state S 110  at any specific stage is a value selected among a value stored in a memory for state S 011  at a stage immediately preceding the specific stage and a value stored in a memory for state S 111  at the stage immediately preceding the specific stage in accordance with a selection result sel 001  received from the ACS section  72 - 5 . The selected value stored in the memory for state S 110  at the specific stage is then shifted (output) to a memory for state S 100  at a stage immediately following the specific stage. By the same token, in the path memory  43 , a value to be stored in a memory for state S 111  at the specific stage is a value selected among a value stored in a memory for state S 011  at the stage immediately preceding the specific stage and a value stored in a memory for state S 111  at the stage immediately preceding the specific stage in accordance with a selection result sel 111  received from the ACS section  72 - 6 . The selected value stored in the memory for state S 111  at the specific stage is then shifted (output) to a memory for state S 110  at the stage immediately following the specific stage and a memory for state S 111  at the stage immediately following the specific stage.  
         [0053]     It is to be noted that, by way of a memory for state S 011  at each specific stage, a value is shifted from a memory at a stage immediately preceding the specific stage repeatedly to a memory existing at a stage immediately following the specific stage as a memory according to a transition c. Thus, for any specific stage in the path memory  43 , a value stored in a memory for state S 001  at a stage immediately preceding the specific stage is shifted to a memory for state S 110  at a stage immediately following the specific stage and a memory for state S 111  at the same following stage by way of a memory for state S 011  at the specific stage. By the same token, by way of a memory for state S 100  at each specific stage, a value is shifted from a memory at a stage immediately preceding the specific stage repeatedly to a memory existing at a stage immediately following the specific stage as a memory according to a transition c. Thus, for any specific stage in the path memory  43 , a value stored in a memory for state S 110  at a stage immediately preceding the specific stage is shifted to a memory for state S 000  at a stage immediately following the specific stage and a memory for state S 001  at the same following stage by way of a memory for state S 100  at the specific stage.  
         [0054]     As is obvious from the above descriptions, the branch-metric computation circuit  41 , the ACS circuit  42  and the path memory  43  each have different configurations for different constraint lengths, that is, different state counts. In general, the smaller the d (minimum run length) and the larger the constraint length, the larger the size of the circuit.  
         [0055]     However, a change in constraint length or a change in inter-code interference length is attributed to a change of the frequency characteristic of the PR transmission line and, normally, if the structure of the recording medium as well as the structure of the recording head are determined, an optimum PRML (Partial Response Maximum Likelihood) method is generally determined to be a method of a certain type.  
         [0056]     On the other hand, a demand for downward compatibility with specifications of the optical disk, which has been becoming popular so far, is also high. In addition, even for the same generation, a plurality of specification sets exists so that a recording/reproduction apparatus for recent optical disks is required absolutely to keep up with different specifications of frequency characteristics. The specification sets include specifications for disks of addition-recording and renewal-recording types, single-layer and multiple-layer disks as well as low-density and high-density disks.  
         [0057]     For the reasons described above, a single recording/reproduction apparatus is required to have a plurality of operating modes for different constraint lengths such as PR (1, 2, 1) and PR (1, 3, 3, 1). Thus, it is necessary to provide such a single recording/reproduction apparatus with a plurality of Viterbi decoding circuits of different types having different constraint lengths and use the apparatus by switching the Viterbi decoding circuit from one having a certain constraint length to another having a different constraint length. In addition, it is necessary to also provide a branch-metric computation circuit  41 , an ACS circuit  42  and a path memory  43 , which have been explained by referring to FIGS.  5  to  8 , for each type having a constraint length. If a single recording/reproduction apparatus needs to be provided with a plurality of operating modes for different constraint lengths as described above, however, the recording/reproduction apparatus will have a problem of a large size of the circuit or a problem of limitation on the type of the Viterbi decoding circuit.  
         [0058]     In addition, high speed operations are required also in a Viterbi decoding circuit employed in the recording/reproduction apparatus in order to meet a higher demand raised in recent years as a demand for a high recording/reproduction rate. A portion determining the operating speed of a Viterbi decoding circuit is the ACS circuit for carrying out addition, subtraction (or comparison) and comparison operations in a short possible period such as 1 clock cycle. In addition, if the subtraction (or comparison) processing carried out by the ACS circuit as processing having an amount corresponding to a plurality of clock cycles can all be completed in a time slot, the operating speed can be further increased due to such early completion. The portion determining the operating speed of a Viterbi decoding circuit is referred to as a critical path.  
         [0059]     Japanese Patent Laid-open No. Hei 8-84082 discloses a proposed technique to increase the operating speed by at least two times by handling state transitions occurring during at least two time slots as one state transition occurring in one time slot. In this case, however, the size of the circuit must be increased by up to two times in exchange for the increase in operating speed.  
         [0060]     By referring to FIGS.  9  to  12 , the following description explains a Viterbi decoding circuit  19  having its operating speed increased by carrying out processing of the amount corresponding to two time slots in just one time slot.  
         [0061]      FIGS. 9 and 10  are diagrams showing a Viterbi decoding circuit  19  provided for the PR (1, x, 1) transmission line shown in  FIG. 3  as a Viterbi decoding circuit  19  capable of carrying out processing of the amount corresponding to two time slots in just one time slot. That is to say, the Viterbi decoding circuit  19  shown in  FIGS. 9 and 10  is capable of carrying out processing, which is performed by the Viterbi decoding circuit  19  shown in  FIGS. 5 and 6  in two time slots, in just one time slot.  
         [0062]     In the typical configuration shown in  FIG. 9 , the branch-metric computation circuit  41  has branch-metric computation sections  81 - 1  to  81 - 10  each used for calculating branch-metric data for state transitions occurring over two time slots in the configuration shown in  FIG. 5 . Each of the branch-metric computation sections  81 - 1  to  81 - 10  computes branch-metric data bm for state transitions occurring over two time slots and outputs the branch-metric data bm to an ACS circuit  42 . That is to say, the branch-metric computation circuit  41  shown in  FIG. 9  computes branch-metric data bmABCD k  (=bmABC k-1 +bmBCD k ) where bmABC k-1  is equal to the square (z k-1 −cABC)ˆ2, bmBCD k  is equal to the square (z k −cBCD)ˆ2 whereas suffixes A, B, C and D are each the integer 0 or 1.  
         [0063]     To put it concretely, the branch-metric computation section  81 - 1  computes branch-metric data bm 0000   k  (=bm 000   k-1 +bm 000   k ) where branch-metric data bm 000   k-1  is equal to the square (z k-1 −c 000 )ˆ2 and branch-metric data bm 000   k  is equal to the square (z k −c 000 )ˆ2, outputting the branch-metric data bm 0000   k  to the ACS section  82 - 1 . In this case, notation bm 0000   k  denotes branch-metric data corresponding to a state transition occurring over two time slots, notation bm 000   k  denotes branch-metric data corresponding to a state transition from a state immediately preceding the present state and notation bm 0000   k-1  denotes branch-metric data corresponding to a state transition from a state immediately leading ahead of the state immediately preceding the present state.  
         [0064]     In the following description, bmABC k-1  is equal to the square (z k-1 −cABC)ˆ2, bmBCD k  is equal to the square (z k −cBCD)ˆ2 whereas suffixes A, B, C and D are each the integer 0 or 1 as described above. Thus, by the same token, the branch-metric computation section  81 - 2  computes branch-metric data bm 1000   k  (=bm 100   k-1 +bm 000   k ) and outputs the branch-metric data bm 1000   k  also to the ACS section  82 - 1  where notation bm 1000   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the same way, the branch-metric computation section  81 - 3  computes branch-metric data bm 1100   k  (=bm 110   k-1 +bm 100   k ) and outputs the branch-metric data bm 1100   k  also to the ACS section  82 - 1  where notation bm 1100   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0065]     Similarly, the branch-metric computation section  81 - 4  computes branch-metric data bm 0001   k (=bm 000   k-1 +bm 001   k ) and outputs the branch-metric data bm 0001   k  to the ACS section  82 - 2  where notation bm 0001   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. Likewise, the branch-metric computation section  81 - 5  computes branch-metric data bm 1001   k  (=bm 100   k-1 +bm 001   k ) and outputs the branch-metric data bm 1001   k  also to the ACS section  82 - 2  where notation bm 1001   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. By the same token, the branch-metric computation section  81 - 6  computes branch-metric data bm 0110   k  (=bm 011   k-1 +bm 110   k ) and outputs the branch-metric data bm 0110   k  to the ACS section  82 - 3  where notation bm 0110   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the same way, the branch-metric computation section  81 - 7  computes branch-metric data bm 1110   k  (=bm 111   k-1 +bm 110   k ) and outputs the branch-metric data bm 1110   k  also to the ACS section  82 - 3  where notation bm 1110   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0066]     Similarly, the branch-metric computation section  81 - 8  computes branch-metric data bm 0011   k  (=bm 001   k-1 +bm 011   k ) and outputs the branch-metric data bm 0011   k  to the ACS section  82 - 4  where notation bm 0011   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. Likewise, the branch-metric computation section  81 - 9  computes branch-metric data bm 0111   k  (=bm 011   k-1 +bm 111   k ) and outputs the branch-metric data bm 0111   k  also to the ACS section  82 - 4  where notation bm 0111   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. By the same token, the branch-metric computation section  81 - 10  computes branch-metric data bm 1111   k  (=bm 111   k-1 +bm 111   k ) and outputs the branch-metric data bm 111   k  also to the ACS section  82 - 4  where notation bm 1111   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0067]     The ACS circuit  42  adds path-metric data stored internally as path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data received from the branch-metric computation circuit  41  to produce a sum. If paths merge at the path memory  43 , the ACS circuit  42  adds path-metric data of a state immediately leading ahead of a state immediately preceding the present state to branch-metric data received from the branch-metric computation circuit  41  to produce a sum for each of the merging paths, and compares the sums with each other to select the smallest one. The ACS circuit  42  then uses the sum or the smallest sum as updated path-metric data m of the present state. The path-metric data m is the likelihood of a history up to state S. The ACS circuit  42  includes as many ACS sections  82  as states. In the case of the typical configuration shown in  FIG. 9 , the number of states is  4 . Thus, the ACS circuit  42  includes  4  ACS sections  82 - 1  to  82 - 4 .  
         [0068]     The ACS section  82 - 1  updates the path-metric data m 00   k , which is the likelihood of a history up to state S 00 . To be more specific, the ACS section  82 - 1  adds the path-metric data m 00   k-2  stored internally in the ACS section  82 - 1  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 0000   k  received from the branch-metric computation section  81 - 1  to produce a first sum. The ACS section  82 - 1  also adds the path-metric data m 10   k-2  stored internally in the ACS section  82 - 3  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 1000   k  received from the branch-metric computation section  81 - 2  to produce a second sum. In addition, the ACS section  82 - 1  also adds the path-metric data m 11   k-2  stored internally in the ACS section  82 - 4  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 1100   k  received from the branch-metric computation section  81 - 3  to produce a third sum. Then, the ACS section  82 - 1  compares the first, second and third sums with each other in order to select the smallest one to be used as updated path-metric data m 00   k  of the present state. Finally, the ACS section  82 - 1  outputs a selection result sel 00  to a memory included in the path memory  43  as a memory used for storing the value of state S 00 . The computations and the comparison, which are carried out by the ACS section  82 - 1 , can be expressed by Eq. (11) given as follows:
 
 m 00 k =min { m 00 k-2   +bm 0000 k   , m 10 k-2   +bm 1000 k   , m 11 k-2   +bm 1100 k }  (11)
 
         [0069]     Likewise, the ACS section  82 - 2  updates the path-metric data m 01   k , which is the likelihood of a history up to state S 01 . To be more specific, the ACS section  82 - 2  adds the path-metric data m 00   k-2  stored internally in the ACS section  82 - 1  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 0001   k  received from the branch-metric computation section  81 - 4  to produce a first sum. The ACS section  82 - 2  also adds the path-metric data m 10   k-2  stored internally in the ACS section  82 - 3  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 1001   k  received from the branch-metric computation section  81 - 5  to produce a second sum. Then, the ACS section  82 - 2  compares the first and second sums with each other in order to select the smaller one to be used as updated path-metric data m 01   k  of the present state. Finally, the ACS section  82 - 2  outputs a selection result sel 01  to a memory included in the path memory  43  as a memory used for storing the value of state S 01 . The computations and the comparison, which are carried out by the ACS section  82 - 2 , can be expressed by Eq. (12) given as follows:
 
 m 01 k =min { m 00 k-2   +bm 0001 k   , m 10 k-2   +bm 1001 k }  (12)
 
         [0070]     By the same token, the ACS section  82 - 3  updates the path-metric data m 10   k , which is the likelihood of a history up to state S 10 . To be more specific, the ACS section  82 - 3  adds the path-metric data m 11   k-2  stored internally in the ACS section  82 - 4  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 1110   k  received from the branch-metric computation section  81 - 7  to produce a first sum. The ACS section  82 - 3  also adds the path-metric data m 01   k-2  stored internally in the ACS section  82 - 2  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 0110   k  received from the branch-metric computation section  81 - 6  to produce a second sum. Then, the ACS section  82 - 3  compares the first and second sums with each other in order to select the smaller one to be used as updated path-metric data m 10   k  of the present state. Finally, the ACS section  82 - 3  outputs a selection result sel 10  to a memory included in the path memory  43  as a memory used for storing the value of state S 10 . The computations and the comparison, which are carried out by the ACS section  82 - 3 , can be expressed by Eq. (13) given as follows:
 
 m 10 k =min { m 11 k-2   +bm 1110     k   , m 01 k-2   +bm 0110 k }  (13)
 
         [0071]     The ACS section  82 - 4  updates the path-metric data m 11   k , which is the likelihood of a history up to state S 11 . To be more specific, the ACS section  82 - 4  adds the path-metric data m 11   k-2  stored internally in the ACS section  82 - 4  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 1111   k  received from the branch-metric computation section  81 - 10  to produce a first sum. The ACS section  82 - 4  also adds the path-metric data m 01   k-2  stored internally in the ACS section  82 - 2  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 0111   k  received from the branch-metric computation section  81 - 9  to produce a second sum. In addition, the ACS section  82 - 4  also adds the path-metric data m 00   k-2  stored internally in the ACS section  82 - 1  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 0011   k  received from the branch-metric computation section  81 - 8  to produce a third sum. Then, the ACS section  82 - 4  compares the first, second and third sums with each other in order to select the smallest one to be used as updated path-metric data m 11   k  of the present state. Finally, the ACS section  82 - 4  outputs a selection result sel 11  to a memory included in the path memory  43  as a memory used for storing the value of state S 11 . The computations and the comparison, which are carried out by the ACS section  82 - 1 , can be expressed by Eq. (14) given as follows:
 
 m 11 k =min { m 11 k-2   +bm 1111 k   , m 01 k-2   +bm 0111 k   , m 00 k-2   +bm 0011 k }  (14)
 
         [0072]     A trellis depicted in the path memory  43  of the typical configuration shown in  FIG. 10  has only one stage. The number of states is the same as that for the trellis depicted in the path memory  43  shown in  FIG. 6 . However, the select and shift operations are carried out for every 2 bits (that is, for every two time slots).  
         [0073]     The trellis depicted in the path memory  43  shown in  FIG. 10  indicates that it is quite within the bounds of possibility that a transition occurs from state S 00  to state S 00 , S 01  or S 11  at a time immediately following the next time, a transition occurs from state S 01  to state S 10  or S 11  at a time immediately following the next time, a transition occurs from state S 10  to state S 00  or S 01  at a time immediately following the next time and a transition occurs from state S 11  to state S 00 , S 10  or S 11  at a time immediately following the next time. That is to say, the Viterbi decoding circuit  19  of this case carries out a decoding process on the basis of the trellis expressing a state-transition diagram shown in  FIG. 3  as a state-transition diagram of the PR (1, x, 1) transmission line in terms of sequences along the time axis based on units of two time slots.  
         [0074]     Thus, in the path memory  43  shown in  FIG. 10 , a value to be stored instantly in a memory for state S 00  at any specific stage is a value selected among a value stored in a memory for state S 00  at a stage immediately preceding the specific stage, a value stored in a memory for state S 10  at a stage immediately preceding the specific stage and a value stored in a memory for state S 11  at the stage immediately preceding the specific stage in accordance with a selection result sel 00  received from the ACS section  82 - 1 . The selected value stored instantly in the memory for state S 00  at the specific stage is shifted in the same time slot to a memory for state S 00  at a stage immediately following the specific stage, a memory for state S 01  at a stage immediately following the specific stage and a memory for state S 11  at the stage immediately following the specific stage. Similarly, in the path memory  43 , a value to be stored instantly in a memory for state S 00  at any specific stage is a value selected among a value stored in a memory for state S 00  at a stage immediately preceding the specific stage and a value stored in a memory for state S 10  at the stage immediately preceding the specific stage in accordance with a selection result sel 01  received from the ACS section  82 - 2 . The selected value stored instantly in the memory for state S 01  at the specific stage is shifted in the same time slot to a memory for state S 10  at a stage immediately following the specific stage and a memory for state S 11  at the stage immediately following the specific stage.  
         [0075]     By the same token, in the path memory  43 , a value to be stored instantly in a memory for state S 10  at any specific stage is a value selected among a value stored in a memory for state S 01  at a stage immediately preceding the specific stage and a value stored in a memory for state S 11  at the stage immediately preceding the specific stage in accordance with a selection result sel 10  received from the ACS section  82 - 3 . The selected value stored instantly in the memory for state S 10  at the specific stage is shifted in the same time slot to a memory for state S 00  at a stage immediately following the specific stage and a memory for state S 01  at the stage immediately following the specific stage. In the same way as stage S 00 , in the path memory  43 , a value to be stored instantly in a memory for state S 11  at any specific stage is a value selected among a value stored in a memory for state S 00  at a stage immediately preceding the specific stage, a value stored in a memory for state S 01  at a stage immediately preceding the specific stage and a value stored in a memory for state S 11  at the stage immediately preceding the specific stage in accordance with a selection result sel 11  received from the ACS section  82 - 4 . The selected value stored instantly in the memory for state S 11  at the specific stage is shifted in the same time slot to a memory for state S 00  at a stage immediately following the specific stage, a memory for state S 10  at a stage immediately following the specific stage and a memory for state S 11  at the stage immediately following the specific stage. As a result, data of two time slots is output to the demodulation circuit  20  as the modulated sequence x t .  
         [0076]     As is obvious from the above descriptions, the number of branch-metric computation sections increases from 6 in the branch-metric computation circuit  41  shown in  FIG. 5  to  10  in the branch-metric computation circuit  41  shown in  FIG. 9 . However, the number of states employed the ACS circuit  42  remains at 4 as it is. Thus, the number of ACS sections employed in the ACS circuit  42  shown in  FIG. 9  also remains the same as that for the ACS circuit  42  shown in  FIG. 5 . Nevertheless, the number of states preceding the present state by two time slots for the ACS circuit  42  shown in  FIG. 9  increases to 3 from 2 states preceding the present state by one time slot for the ACS circuit  42  shown in  FIG. 5 . Thus, in the ACS circuit  42  shown in  FIG. 9 , the smallest one among 3 different results of addition is selected as path-metric data.  
         [0077]     That is to say, the Viterbi decoding circuit  19  shown in  FIGS. 9 and 10  as a Viterbi decoding circuit  19  for carrying out processing of an amount corresponding to two time slots of the configurations shown in  FIGS. 5 and 6  in just one time slot has a circuit size much larger than that of the Viterbi decoding circuit  19  shown in  FIGS. 5 and 6  as a Viterbi decoding circuit  19  for carrying out processing of one time slot.  
         [0078]      FIGS. 11 and 12  are diagrams showing a Viterbi decoding circuit  19  provided for the PR (1, x, x, 1) transmission line shown in  FIG. 4  as a Viterbi decoding circuit  19  capable of carrying out processing of the amount corresponding to two time slots of the configurations shown in  FIGS. 7 and 8  in just one time slot. That is to say, the Viterbi decoding circuit  19  shown in  FIGS. 11 and 12  is capable of carrying out processing, which is performed by the Viterbi decoding circuit  19  shown in  FIGS. 7 and 8  in two time slots, in just one time slot.  
         [0079]     In addition, the Viterbi decoding circuit  19  shown in  FIGS. 11 and 12  is different from the Viterbi decoding circuit  19  shown in  FIGS. 9 and 10  only in that the number of states increases to 6 from 4 for the Viterbi decoding circuit  19  shown in  FIGS. 9 and 10  whereas the number of state transitions in the Viterbi decoding circuit  19  shown in  FIGS. 11 and 12  increases to 16 from 10 for the Viterbi decoding circuit  19  shown in  FIGS. 9 and 10 . Thus, the Viterbi decoding circuit  19  shown in  FIGS. 11 and 12  basically has a configuration identical with that of the Viterbi decoding circuit  19  shown in  FIGS. 9 and 10 , making it unnecessary to explain the Viterbi decoding circuit  19  shown in  FIGS. 11 and 12  in detail.  
         [0080]     That is to say, the branch-metric computation circuit  41  shown in  FIG. 11  has branch-metric computation sections  91 - 1  through  91 - 16  each used for computing branch-metric data corresponding to a state transition occurring over two time slots from a state immediately leading ahead of a state immediately preceding the present state.  
         [0081]     The branch-metric computation section  91 - 1  computes branch-metric data bm 00000   k = 0000   k-1 +bm 0000   k  and outputs the branch-metric data bm 00000   k  to the ACS section  92 - 1  where notation bm 00000   k  denotes branch-metric data corresponding to a state transition occurring over two time slots in the configuration shown in  FIG. 7 . By the same token, the branch-metric computation section  91 - 2  computes branch-metric data bm 10000   k = 1000   k-1 +bm 0000   k  and outputs the branch-metric data bm 10000   k  also to the ACS section  92 - 1  where notation bm 10000   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the same way, the branch-metric computation section  91 - 3  computes branch-metric data bm 11000   k = 1100   k-1 +bm 1000   k  and outputs the branch-metric data bm 11000   k  also to the ACS section  92 - 1  where notation bm 11000   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0082]     Likewise, the branch-metric computation section  91 - 4  computes branch-metric data bm 00001   k = 0000   k-1 +bm 0001   k  and outputs the branch-metric data bm 00001   k  to the ACS section  92 - 2  where notation bm 00001   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. By the same token, the branch-metric computation section  91 - 5  computes branch-metric data bm 10001   k = 1000   k-1 +bm 0001   k  and outputs the branch-metric data bm 10001   k  also to the ACS section  92 - 2  where notation bm 10001   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the same way, the branch-metric computation section  91 - 6  computes branch-metric data bm 11001   k = 1100   k-1 +bm 1001   k  and outputs the branch-metric data bm 11001   k  also to the ACS section  92 - 2  where notation bm 11001   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0083]     Likewise, the branch-metric computation section  91 - 7  computes branch-metric data bm 00011   k = 0001   k-1 +bm 0011   k  and outputs the branch-metric data bm 00011   k  to the ACS section  92 - 3  where notation bm 00011   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. By the same token, the branch-metric computation section  91 - 8  computes branch-metric data bm 10011   k = 1001   k-1 +bm 0001   k  and outputs the branch-metric data bm 10011   k  also to the ACS section  92 - 3  where notation bm 10011   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the same way, the branch-metric computation section  91 - 9  computes branch-metric data bm 01100   k = 0110   k-1 +bm 1100   k  and outputs the branch-metric data bm 01100   k  to the ACS section  92 - 4  where notation bm 01100   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. Similarly, the branch-metric computation section  91 - 10  computes branch-metric data bm 11100   k = 1110   k-1 +bm 1100   k  and outputs the branch-metric data bm 11100   k  also to the ACS section  92 - 4  where notation bm 11100   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0084]     Likewise, the branch-metric computation section  91 - 11  computes branch-metric data bm 00110   k = 0011   k-1 +bm 0110   k  and outputs the branch-metric data bm 00110   k  to the ACS section  92 - 5  where notation bm 00110   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. By the same token, the branch-metric computation section  91 - 12  computes branch-metric data bm 01110   k = 0111   k-1 +bm 1110   k  and outputs the branch-metric data bm 01110   k  also to the ACS section  92 - 5  where notation bm 01110   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the same way, the branch-metric computation section  91 - 13  computes branch-metric data bm 11110   k = 1111   k-1 +bm 1110   k  and outputs the branch-metric data bm 11110   k  also to the ACS section  92 - 5  where notation bm 11110   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0085]     Likewise, the branch-metric computation section  91 - 14  computes branch-metric data bm 00111   k = 0011   k-1 +bm 0111   k  and outputs the branch-metric data bm 00111   k  to the ACS section  92 - 6  where notation bm 00111   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. By the same token, the branch-metric computation section  91 - 15  computes branch-metric data bm 01111   k = 0111   k-1 +bm 1111   k  and outputs the branch-metric data bm 01111   k  also to the ACS section  92 - 6  where notation bm 01111   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the same way, the branch-metric computation section  91 - 16  computes branch-metric data bm 11111   k = 1111   k-1 +bm 1111   k  and outputs the branch-metric data bm 11111   k  also to the ACS section  92 - 6  where notation bm 11111   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0086]     The ACS circuit  42  adds path-metric data stored internally as path-metric data of a state immediately leading ahead of the state immediately preceding the present state to branch-metric data received from the branch-metric computation circuit  41  to produce a sum. If paths merge in the path memory  43 , the ACS circuit  42  adds path-metric data of a state immediately leading ahead of a state immediately preceding the present state to branch-metric data received from the branch-metric computation circuit  41  to produce a sum for each of the merging paths, and compares the sums with each other to select the smallest one. The ACS circuit  42  then uses the sum or the selected smallest sum as updated path-metric data m of the present state. The path-metric data m is the likelihood of a history up to state S. The ACS circuit  42  includes as many ACS (add, compare and select) sections  62  as states. In the case of the typical configuration shown in  FIG. 11 , the number of states is 6. Thus, the ACS circuit  42  includes 6 ACS sections  92 - 1  to  92 - 6 .  
         [0087]     To put it concretely, the ACS section  92 - 1  updates the path-metric data m 000   k , which is the likelihood of a history up to state S 000 . To be more specific, the ACS section  92 - 1  adds the path-metric data m 000   k-2  stored internally in the ACS section  92 - 1  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 00000   k  received from the branch-metric computation section  91 - 1  to produce a first sum. The ACS section  92 - 1  also adds the path-metric data m 100   k-2  stored internally in the ACS section  92 - 4  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 10000   k  received from the branch-metric computation section  91 - 2  to produce a second sum. In addition, the ACS section  92 - 1  also adds the path-metric data m 110   k-2  stored internally in the ACS section  92 - 5  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 11000   k  received from the branch-metric computation section  91 - 3  to produce a third sum. Then, the ACS section  92 - 1  compares the first, second and third sums with each other in order to select the smallest one to be used as updated path-metric data m 000   k  of the present state. Finally, the ACS section  92 - 1  outputs a selection result sel 000  to a memory included in the path memory  43  as a memory used for storing the value of state S 000 . The computations and the comparison, which are carried out by the ACS section  92 - 1 , can be expressed by Eq. (15) given as follows:
 
 m 000 k =min { m 000 k-2   +bm 00000 k   , m 100 k-2   +bm 10000 k   , m 110 k-2   +bm 11000 k }  (15)
 
         [0088]     By the same token, the ACS section  92 - 2  updates the path-metric data m 001   k , which is the likelihood of a history up to state S 001 . To be more specific, the ACS section  92 - 2  adds the path-metric data m 000   k-2  stored internally in the ACS section  92 - 1  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 00001   k  received from the branch-metric computation section  91 - 4  to produce a first sum. The ACS section  92 - 2  also adds the path-metric data m 100   k-2  stored internally in the ACS section  92 - 4  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 10001   k  received from the branch-metric computation section  91 - 5  to produce a second sum. In addition, the ACS section  92 - 2  also adds the path-metric data m 110   k-2  stored internally in the ACS section  92 - 5  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 11001   k  received from the branch-metric computation section  91 - 6  to produce a third sum. Then, the ACS section  92 - 2  compares the first, second and third sums with each other in order to select the smallest one to be used as updated path-metric data m 001   k  of the present state. Finally, the ACS section  92 - 2  outputs a selection result sel 001  to a memory included in the path memory  43  as a memory used for storing the value of state S 001 . The computations and the comparison, which are carried out by the ACS section  92 - 2 , can be expressed by Eq. (16) given as follows:
 
 m 001 k =min { m 000 k-2   +bm 00001 k   , m 100 k-2   +bm 10001 k   , m 110 k-2   +bm 11001 k }  (16)
 
         [0089]     Likewise, the ACS section  92 - 3  updates the path-metric data m 011   k , which is the likelihood of a history up to state S 011 . To be more specific, the ACS section  92 - 3  adds the path-metric data m 000   k-2  stored internally in the ACS section  92 - 1  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 00011   k  received from the branch-metric computation section  91 - 7  to produce a first sum. The ACS section  92 - 3  also adds the path-metric data m 100   k-2  stored internally in the ACS section  92 - 4  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 10011   k  received from the branch-metric computation section  91 - 8  to produce a second sum. Then, the ACS section  92 - 3  compares the first and second sums with each other in order to select the smaller one to be used as updated path-metric data m 011   k  of the present state. Finally, the ACS section  92 - 3  outputs a selection result sel 011  to a memory included in the path memory  43  as a memory used for storing the value of state S 011 . The computations and the comparison, which are carried out by the ACS section  92 - 3 , can be expressed by Eq. (17) given as follows:
 
 m 011 k =min { m 000 k-2   +bm 00011 k   , m 100 k-2   +bm 10011 k }  (17)
 
         [0090]     By the same token, the ACS section  92 - 4  updates the path-metric data m 100   k , which is the likelihood of a history up to state S 100 . To be more specific, the ACS section  92 - 4  adds the path-metric data m 111   k-2  stored internally in the ACS section  92 - 6  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 11100   k  received from the branch-metric computation section  91 - 10  to produce a first sum. The ACS section  92 - 4  also adds the path-metric data m 011   k-2  stored internally in the ACS section  92 - 3  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 01100   k  received from the branch-metric computation section  91 - 9  to produce a second sum. Then, the ACS section  92 - 4  compares the first and second sums with each other in order to select the smaller one to be used as updated path-metric data m 100   k  of the present state. Finally, the ACS section  92 - 4  outputs a selection result sel 100  to a memory included in the path memory  43  as a memory used for storing the value of state S 100 . The computations and the comparison, which are carried out by the ACS section  92 - 4 , can be expressed by Eq. (18) given as follows:
 
 m 100 k =min { m 111 k-2   +bm 11100 k   , m 011 k-2   +bm 01100 k }  (18)
 
         [0091]     In the same way as the ACS section  92 - 1 , the ACS section  92 - 5  updates the path-metric data m 110   k , which is the likelihood of a history up to state S 110 . To be more specific, the ACS section  92 - 5  adds the path-metric data m 111   k-2  stored internally in the ACS section  92 - 6  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 11110   k  received from the branch-metric computation section  91 - 13  to produce a first sum. The ACS section  92 - 5  also adds the path-metric data m 011   k-2  stored internally in the ACS section  92 - 3  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 01110   k  received from the branch-metric computation section  91 - 12  to produce a second sum. In addition, the ACS section  92 - 5  also adds the path-metric data m 001   k-2  stored internally in the ACS section  92 - 2  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 00110   k  received from the branch-metric computation section  91 - 11  to produce a third sum. Then, the ACS section  92 - 5  compares the first, second and third sums with each other in order to select the smallest one to be used as updated path-metric data m 110   k  of the present state. Finally, the ACS section  92 - 5  outputs a selection result sel 110  to a memory included in the path memory  43  as a memory used for storing the value of state S 110 . The computations and the comparison, which are carried out by the ACS section  92 - 5 , can be expressed by Eq. (19) given as follows:
 
 m 110 k =min { m 111 k-2   +bm 11110 k   , m 011 k-2   +bm 01110 k   , m 001 k-2   +bm 00110 k }  (19)
 
         [0092]     By the same token, the ACS section  92 - 6  updates the path-metric data m 111   k , which is the likelihood of a history up to state S 111 . To be more specific, the ACS section  92 - 6  adds the path-metric data m 111   k-2  stored internally in the ACS section  92 - 6  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 11111   k  received from the branch-metric computation section  91 - 16  to produce a first sum. The ACS section  92 - 6  also adds the path-metric data m 011   k-2  stored internally in the ACS section  92 - 3  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 01111   k  received from the branch-metric computation section  91 - 15  to produce a second sum. In addition, the ACS section  92 - 6  also adds the path-metric data m 001   k-2  stored internally in the ACS section  92 - 2  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 00111   k  received from the branch-metric computation section  91 - 14  to produce a third sum. Then, the ACS section  92 - 6  compares the first, second and third sums with each other in order to select the smallest one to be used as updated path-metric data m 111   k  of the present state. Finally, the ACS section  92 - 6  outputs a selection result sel 111  to a memory included in the path memory  43  as a memory used for storing the value of state S 111 . The computations and the comparison, which are carried out by the ACS section  92 - 6 , can be expressed by Eq. (20) given as follows:
 
 m 111 k =min { m 111 k-2   +bm 11111 k   , m 011 k-2   +bm 01111 k   , m 001 k-2   +bm 00111 k }  (20)
 
         [0093]     A trellis depicted in the path memory  43  of the typical configuration shown in  FIG. 12  has only 1 stage. The number of states is the same as that for the trellis depicted in the path memory  43  shown in  FIG. 8 . However, the select and shift operations are carried out for every 2 bits (that is, for every two time slots).  
         [0094]     The trellis depicted in the path memory  43  shown in  FIG. 12  indicates that it is quite within the bounds of possibility that a transition occurs from state S 000  to state S 000 , S 001  or S 011  at a time following the next time, a transition occurs from state S 001  to state S 110  or Sill at a time following the next time, a transition occurs from state S 011  to state S 100 , S 110  or S 111  at a time following the next time, a transition occurs from state S 100  to state S 000 , S 001  or S 011  at a time following the next time, a transition occurs from state S 110  to state S 000  or S 001  at a time following the next time and a transition occurs from state S 111  to state S 100 , S 110  or Sill at a time following the next time. That is to say, the Viterbi decoding circuit  19  of this case carries out a decoding process on the basis of the trellis expressing a state-transition diagram shown in  FIG. 4  as a state-transition diagram of the PR (1, x, x, 1) transmission line in terms of sequences along the time axis based on two time slots.  
         [0095]     Thus, in the path memory  43 , a value to be stored instantly in a memory for state S 000  at any specific stage is a value selected among a value stored in a memory for state S 000  at a stage immediately preceding the specific stage, a value stored in a memory for state S 100  at a stage immediately preceding the specific stage and a value stored in a memory for state S 110  at the stage immediately preceding the specific stage in accordance with a selection result sel 000  received from the ACS section  92 - 1 . The selected value stored instantly in the memory for state S 000  at the specific stage is shifted in the same time slot to a memory for state S 000  at a stage immediately following the specific stage, a memory for state S 000  at a stage immediately following the specific stage and a memory for state S 011  at the stage immediately following the specific stage. Similarly, in the path memory  43 , a value to be stored instantly in a memory for state S 000  at any specific stage is a value selected among a value stored in a memory for state S 000  at a stage immediately preceding the specific stage, a value stored in a memory for state S 100  at a stage immediately preceding the specific stage and a value stored in a memory for state S 110  at the stage immediately preceding the specific stage in accordance with a selection result sel 001  received from the ACS section  92 - 2 . The selected value stored instantly in the memory for state S 001  at the specific stage is shifted in the same time slot to a memory for state S 110  at a stage immediately following the specific stage and a memory for state S 111  at the stage immediately following the specific stage.  
         [0096]     By the same token, in the path memory  43 , a value to be stored instantly in a memory for state S 011  at any specific stage is a value selected among a value stored in a memory for state S 000  at a stage immediately preceding the specific stage and a value stored in a memory for state S 100  at the stage immediately preceding the specific stage in accordance with a selection result sel 011  received from the ACS section  92 - 3 . The selected value stored instantly in the memory for state S 011  at the specific stage is shifted in the same time slot to a memory for state S 100  at a stage immediately following the specific stage, a memory for state S 110  at a stage immediately following the specific stage and a memory for state S 111  at the stage immediately following the specific stage. In the same way, in the path memory  43 , a value to be stored instantly in a memory for state S 100  at any specific stage is a value selected among a value stored in a memory for state S 011  at a stage immediately preceding the specific stage and a value stored in a memory for state S 111  at the stage immediately preceding the specific stage in accordance with a selection result sel 100  received from the ACS section  92 - 4 . The selected value stored instantly in the memory for state S 100  at the specific stage is shifted in the same time slot to a memory for state S 000  at a stage immediately following the specific stage, a memory for state S 001  at a stage immediately following the specific stage and a memory for state S 011  at the stage immediately following the specific stage.  
         [0097]     By the same token, in the path memory  43 , a value to be stored instantly in a memory for state S 110  at any specific stage is a value selected among a value stored in a memory for state S 001  at a stage immediately preceding the specific stage, a memory for state S 011  at a stage immediately preceding the specific stage and a value stored in a memory for state S 111  at the stage immediately preceding the specific stage in accordance with a selection result sel 110  received from the ACS section  92 - 5 . The selected value stored instantly in the memory for state S 110  at the specific stage is shifted in the same time slot to a memory for state S 000  at a stage immediately following the specific stage and a memory for state S 001  at the stage immediately following the specific stage. In the same way, in the path memory  43 , a value to be stored instantly in a memory for state S 111  at any specific stage is a value selected among a value stored in a memory for state S 001  at a stage immediately preceding the specific stage, a memory for state S 011  at a stage immediately preceding the specific stage and a value stored in a memory for state S 111  at the stage immediately preceding the specific stage in accordance with a selection result sel 111  received from the ACS section  92 - 6 . The selected value stored instantly in the memory for state S 111  at the specific stage is shifted in the same time slot to a memory for state S 100  at a stage immediately following the specific stage, a memory for state S 110  at a stage immediately following the specific stage and a memory for state S 111  at the stage immediately following the specific stage. As a result, data of two time slots is output to the demodulation circuit  20  as the modulated sequence x t .  
         [0098]     As is obvious from the above descriptions, the number of branch-metric computation sections increases from 10 in the branch-metric computation circuit  41  shown in  FIG. 7  to  16  in the branch-metric computation circuit  41  shown in  FIG. 11 . However, the number of states for the transmission line remains at 6 as it is. Thus, the number of ACS sections employed in the ACS circuit  42  shown in  FIG. 11  also remains the same as that for the ACS circuit  42  shown in  FIG. 7 . However, the number of states preceding the present state by two time slots for the ACS circuit  42  shown in  FIG. 11  increases to three from two states preceding the present state by one time slot for the ACS circuit  42  shown in  FIG. 7 . Thus, in the ACS circuit  42  shown in  FIG. 11 , the smallest one among 3 different results of addition is selected as path-metric data.  
         [0099]     That is to say, the Viterbi decoding circuit  19  shown in  FIGS. 11 and 12  for carrying out processing having an amount corresponding to two time slots in the Viterbi decoding circuit  19  shown in  FIGS. 7 and 8  in just one time slot has a circuit size much larger than that of the Viterbi decoding circuit  19  shown in  FIGS. 7 and 8  for carrying out processing of one time slot.  
         [0100]     It is to be noted that descriptions with reference to FIGS.  1  to  12  are applicable and properly referred to in the following explanation of the present invention.  
       SUMMARY OF THE INVENTION  
       [0101]     If it is necessary to provide a single recording/reproduction apparatus with a plurality of Viterbi decoding circuits of different types having different constraint lengths and use the apparatus by switching the Viterbi decoding circuit from one having a certain constraint length to another having a different constraint length as described above, the apparatus has a problem that, for each type of constraint length, it is necessary to provide a branch-metric computation circuit, an ACS (add, compare and select) circuit and a path memory so that the size of the circuit increases, or a problem of limitation on the type of the Viterbi decoding circuit.  
         [0102]     In addition, if a state transition occurring over two time slots is processed as a single state transition so as to carry out operations at a high speed, the recording/reproduction apparatus also has a problem of an increased circuit size.  
         [0103]     On top of that, as the size of the circuit increases, the recording/reproduction apparatus raises another problem that the computation becomes complicated and the design cost also rises as well.  
         [0104]     In order to solve the problems described above, the inventors of the present invention have devised a decoding apparatus capable of keeping up with a plurality of operating modes without increasing the size of its circuit.  
         [0105]     In accordance with an embodiment of the present invention, there is provided a decoding apparatus characterized in that the decoding apparatus includes: a decoding section for decoding an encoded signal on the basis of a first state-transition trellis; and a mode selection section for selecting either a first operating mode based on the first state-transition trellis or a second operating mode based on a second state-transition trellis having a state count smaller than that of the first state-transition trellis. If the mode selection section selects the second operating mode, the decoding section decodes an encoded signal by carrying out switching of a state transition from a first state transition selected among state transitions of the first state-transition trellis as a first state transition not corresponding to the second state-transition trellis to a second state transition selected among state transitions of the second state-transition trellis as a second state transition not corresponding to the first state-transition trellis.  
         [0106]     As the decoding section described above, it is possible to employ a decoding section including: a branch-metric computation section for calculating branch-metric data; a path-metric selection section for selecting most probable path-metric data on the basis of branch-metric data calculated by the branch-metric computation section; and a path memory for obtaining a decoded signal by shifting information, which is stored in memories employed in the path memory, in accordance with a selection result produced by the path-metric selection section.  
         [0107]     In accordance with another embodiment of the present invention, there is provided a decoding method characterized in that the decoding method includes: a decoding step of decoding an encoded signal on the basis of a first state-transition trellis; and a mode selection step of selecting either a first operating mode based on the first state-transition trellis or a second operating mode based on a second state-transition trellis having a state count smaller than that of the first state-transition trellis. If the second operating mode is selected at the mode selection step, at the decoding step, an encoded signal is decoded by carrying out switching of a state transition from a first state transition selected among state transitions of the first state-transition trellis as a first state transition not corresponding to the second state-transition trellis to a second state transition selected among state transitions of the second state-transition trellis as a second state transition not corresponding to the first state-transition trellis.  
         [0108]     In accordance with a further embodiment of the present invention, there is provided a program-recording medium for recording a program characterized in that the program includes: a decoding step of decoding an encoded signal on the basis of a first state-transition trellis; and a mode selection step of selecting either a first operating mode based on the first state-transition trellis or a second operating mode based on a second state-transition trellis having a state count smaller than that of the first state-transition trellis. If the second operating mode is selected at the mode selection step, at the decoding step, an encoded signal is decoded by carrying out switching of a state transition from a first state transition selected among state transitions of the first state-transition trellis as a first state transition not corresponding to the second state-transition trellis to a second state transition selected among state transitions of the second state-transition trellis as a second state transition not corresponding to the first state-transition trellis.  
         [0109]     In accordance with a still further embodiment of the present invention, there is provided a program characterized in that the program includes: a decoding step of decoding an encoded signal on the basis of a first state-transition trellis; and a mode selection step of selecting either a first operating mode based on the first state-transition trellis or a second operating mode based on a second state-transition trellis having a state count smaller than that of the first state-transition trellis. If the second operating mode is selected at the mode selection step, at the decoding step, an encoded signal is decoded by carrying out switching of a state transition from a first state transition selected among state transitions of the first state-transition trellis as a first state transition not corresponding to the second state-transition trellis to a second state transition selected among state transitions of the second state-transition trellis as a second state transition not corresponding to the first state-transition trellis.  
         [0110]     In accordance with a still further embodiment of the present invention, there is provided a recording/reproduction apparatus characterized in that the recording/reproduction apparatus includes: a reproduction section for reproducing a signal, which has been recorded by a recording section on a recording medium, to result in a reproduced signal by carrying out an equalization process on the signal to convert the signal into a signal having a PR (Partial Response) characteristic; a decoding section for decoding the reproduced signal, which has been reproduced by the reproduction section, on the basis of a first state-transition trellis; and a mode selection section for selecting either a first operating mode based on the first state-transition trellis or a second operating mode based on a second state-transition trellis having a state count smaller than that of the first state-transition trellis. If the mode selection section selects the second operating mode, the decoding section decodes the reproduced signal by carrying out switching of a state transition from a first state transition selected among state transitions of the first state-transition trellis as a first state transition not corresponding to the second state-transition trellis to a second state transition selected among state transitions of the second state-transition trellis as a second state transition not corresponding to the first state-transition trellis.  
         [0111]     In accordance with a first embodiment of the present invention, an encoded signal is decoded on the basis of a first state-transition trellis. Either a first operating mode based on the first state-transition trellis or a second operating mode based on a second state-transition trellis having a state count smaller than that of the first state-transition trellis is selected. If the second operating mode is selected, the encoded signal is decoded by carrying out switching of a state transition from a first state transition selected among state transitions of the first state-transition trellis as a first state transition not corresponding to the second state-transition trellis to a second state transition selected among state transitions of the second state-transition trellis as a second state transition not corresponding to the first state-transition trellis.  
         [0112]     In accordance with a second embodiment of the present invention, a signal recorded on a recording medium is reproduced by carrying out an equalization process on the signal to convert the signal into a signal having a PR (Partial Response) characteristic. The reproduced signal is then decoded on the basis of a first state-transition trellis. Either a first operating mode based on the first state-transition trellis or a second operating mode based on a second state-transition trellis having a state count smaller than that of the first state-transition trellis is selected. If the second operating mode is selected, the encoded signal is decoded by carrying out switching of a state transition from a first state transition selected among state transitions of the first state-transition trellis as a first state transition not corresponding to the second state-transition trellis to a second state transition selected among state transitions of the second state-transition trellis as a second state transition not corresponding to the first state-transition trellis.  
         [0113]     The decoding apparatus can be an independent apparatus or a block for carrying out a decoding process of a recording/reproduction apparatus. As an alternative, the decoding apparatus can also be a block for carrying out a decoding process of a communication apparatus.  
         [0114]     In accordance with the present invention, an increase in circuit size can be suppressed. In addition, in accordance with the present invention, an optimum operating mode is selected so that decoding performance can be enhanced.  
         [0115]     Before preferred embodiments of the present invention are explained, relations between disclosed inventions and the embodiments are explained in the following comparative description. It is to be noted that, even if there is an embodiment described in this specification but not included in the following comparative description as an embodiment corresponding to an invention, such an embodiment is not to be interpreted as an embodiment not corresponding to an invention. Conversely, an embodiment included in the following comparative description as an embodiment corresponding to a specific invention is not to be interpreted as an embodiment not corresponding to an invention other than the specific invention.  
         [0116]     In addition, the following comparative description is not to be interpreted as a comprehensive description covering all inventions disclosed in this specification. In other words, the following comparative description by no means denies existence of inventions disclosed in this specification but not included in claims as inventions for which a patent application is filed. That is to say, the following comparative description by no means denies existence of inventions to be included in a separate application for a patent, included in an amendment to this specification or added in the future.  
         [0117]     In accordance with an embodiment of the present invention, there is provided a decoding apparatus characterized in that the decoding apparatus includes: a decoding section (such as a Viterbi decoding circuit  112  shown in  FIG. 13 ) for decoding an encoded signal on the basis of a first state-transition trellis (such as a trellis indicated by solid and dotted lines in a path memory  123  shown in  FIG. 16 ); and a mode selection section (such as a system control section  111  shown in  FIG. 13 ) for selecting either a first operating mode (such as a PR (1, x, x, 1) mode) based on the first state-transition trellis or a second operating mode (such as a PR (1, x, 1) mode) based on a second state-transition trellis having a state count smaller than that of the first state-transition trellis, wherein, if the mode selection section selects the second operating mode, the decoding section decodes the encoded signal by carrying out switching of a state transition (such as switching in a trellis indicated by solid line and dotted thick lines in the path memory  123  shown in  FIG. 16 ) from a first state transition selected among state transitions of the first state-transition trellis as a first state transition (such as a state transition shown in  FIG. 16  as a transition from state S 001  to state S 011 ) not corresponding to the second state-transition trellis to a second state transition selected among state transitions of the second state-transition trellis as a second state transition (such as a state transition shown in  FIG. 16  as a transition from state S 000  to state S 011 ) not corresponding to the first state-transition trellis.  
         [0118]     As the decoding section described above, it is possible to employ a decoding section characterized by including: a branch-metric computation section (such as a branch-metric computation circuit  121  shown in  FIG. 15 ) for calculating branch-metric data; a path-metric selection section (such as an ACS (addition, comparison and selection) circuit  122  shown in  FIG. 15 ) for selecting most probable path-metric data on the basis of branch-metric data calculated by the branch-metric computation section; and a path memory (such as the path memory  123  shown in  FIG. 16 ) for obtaining a decoded signal by shifting information, which is stored in memories employed in the path memory, in accordance with a selection result produced by the path-metric selection section.  
         [0119]     In accordance with another embodiment of the present invention, there is provided a decoding method characterized in that the decoding method includes: a decoding step (such as a step S 14  of a flowchart shown in  FIG. 20 ) of decoding an encoded signal on the basis of a first state-transition trellis; and a mode selection step (such as a step S 11  of the flowchart shown in  FIG. 20 ) of selecting either a first operating mode based on the first state-transition trellis or a second operating mode based on a second state-transition trellis having a state count smaller than that of the first state-transition trellis, whereby, if the second operating mode is selected at the mode selection step, at the decoding step, an encoded signal is decoded by carrying out switching of a state transition from a first state transition selected among state transitions of the first state-transition trellis as a first state transition not corresponding to the second state-transition trellis to a second state transition selected among state transitions of the second state-transition trellis as a second state transition not corresponding to the first state-transition trellis (such as a process carried out at a step S 13  of the flowchart shown in  FIG. 20 ).  
         [0120]     It is to be noted that, in accordance with further embodiments of the present invention, there are provided a program-recording medium and a program. Since the program-recording medium and the program each basically have the same configuration as the encoding method described above, however, their descriptions are omitted to avoid duplications.  
         [0121]     In accordance with a still further embodiment of the present invention, there is provided a recording/reproduction apparatus characterized in that the recording/reproduction apparatus includes: a reproduction section (such as an equalizer  16  shown in  FIG. 13 ) for reproducing a signal, which has been recorded by a recording section (such as a recording amplifier  13  shown in  FIG. 13 ) on a recording medium (such as a recording medium  14  shown in  FIG. 13 ), to result in a reproduced signal by carrying out an equalization process on the signal to convert the signal into a signal having a PR (Partial Response) characteristic; a decoding section (such as the Viterbi decoding circuit  112  shown in  FIG. 13 ) for decoding the reproduced signal, which has been reproduced by the reproduction section, on the basis of a first state-transition trellis (such as the trellis indicated by solid and dotted lines in the path memory path memory  123  shown in  FIG. 16 ); and a mode selection section (such as the system control section  111  shown in  FIG. 13 ) for selecting either a first operating mode (such as the PR (1, x, x, 1) mode) based on the first state-transition trellis or a second operating mode (such as the PR (1, x, 1) mode) based on a second state-transition trellis having a state count smaller than that of the first state-transition trellis, wherein, if the mode selection section selects the second operating mode, the decoding section decodes the reproduced signal by carrying out switching of a state transition (such as switching in the trellis indicated by solid line and dotted thick lines in the path memory  123  shown in  FIG. 16 ) from a first state transition selected among state transitions of the first state-transition trellis as a first state transition (such as a state transition shown in  FIG. 16  as a transition from state S 001  to state S 011 ) not corresponding to the second state-transition trellis to a second state transition selected among state transitions of the second state-transition trellis as a second state transition (such as a state transition shown in  FIG. 16  as a transition from state S 000  to state S 011 ) not corresponding to the first state-transition trellis. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0122]     These and other objects and features of the present invention will become clear from the following description of the preferred embodiments given with reference to the accompanying diagrams, in which:  
         [0123]      FIG. 1  is a block diagram showing a typical configuration of a recording/reproduction apparatus in related art;  
         [0124]      FIG. 2  is a block diagram showing a typical configuration of a Viterbi decoding circuit employed in the recording/reproduction apparatus in related art shown in  FIG. 1 ;  
         [0125]      FIG. 3  is a diagram showing a typical configuration of state transitions for a PR (1, x, 1) transmission line having a minimum run length (d) of 1;  
         [0126]      FIG. 4  is a diagram showing a typical configuration of state transitions of for PR (1, x, x, 1) transmission line having a minimum run length (d) of 1;  
         [0127]      FIG. 5  is a diagram showing a typical configuration of a branch-metric computation circuit and a typical configuration of an ACS (Add, Compare and Select) circuit for the state transitions shown in  FIG. 3 ;  
         [0128]      FIG. 6  is a block diagram showing a typical configuration of a path memory for the state transitions shown in  FIG. 3 ;  
         [0129]      FIG. 7  is a diagram showing a typical configuration of a branch-metric computation circuit and a typical configuration of an ACS circuit for the state transitions shown in  FIG. 4 ;  
         [0130]      FIG. 8  is a block diagram showing a typical configuration of a path memory for the state transitions shown in  FIG. 4 ;  
         [0131]      FIG. 9  is a diagram showing a typical configuration of a branch-metric computation circuit and a typical configuration of an ACS circuit provided for the state transitions shown in  FIG. 3  as circuits for carrying out processing in units of two time slots;  
         [0132]      FIG. 10  is a block diagram showing a typical configuration of a path memory provided for the state transitions shown in  FIG. 3  as a circuit for carrying out processing in units of two time slots;  
         [0133]      FIG. 11  is a diagram showing a typical configuration of a branch-metric computation circuit and a typical configuration of an ACS circuit provided for the state transitions shown in  FIG. 4  as circuits for carrying out processing in units of two time slots;  
         [0134]      FIG. 12  is a block diagram showing a typical configuration of a path memory provided for the state transitions shown in  FIG. 4  as a circuit for carrying out processing in units of two time slots;  
         [0135]      FIG. 13  is a block diagram showing a typical configuration of a recording/reproduction apparatus according to an embodiment of the present invention;  
         [0136]      FIG. 14  is a block diagram showing a typical configuration of a Viterbi decoding circuit employed in the recording/reproduction apparatus shown in  FIG. 13 ;  
         [0137]      FIG. 15  is a diagram showing typical configurations of a branch-metric computation circuit and an ACS circuit, which are employed in the Viterbi decoding circuit shown in  FIG. 14 ;  
         [0138]      FIG. 16  is a block diagram showing a typical configuration of a path memory employed in the Viterbi decoding circuit shown in  FIG. 14 ;  
         [0139]      FIG. 17  is a block diagram showing a typical configuration of a branch-metric computation section employed in the branch-metric computation circuit shown in  FIG. 15 ;  
         [0140]      FIG. 18  is a block diagram showing a typical configuration of an ACS section employed in the ACS circuit shown in  FIG. 15 ;  
         [0141]      FIG. 19  is a circuit diagram showing the typical configuration of the path memory shown in  FIG. 16 ;  
         [0142]      FIG. 20  shows a flowchart referred to in explanation of processing carried out by the recording/reproduction apparatus shown in  FIG. 13 ;  
         [0143]      FIG. 21  is a diagram showing typical configurations of a branch-metric computation circuit and an ACS circuit, which are employed in the Viterbi decoding circuit shown in  FIG. 14  as a decoding circuit for carrying out processing in units of two time slots;  
         [0144]      FIG. 22  is a block diagram showing a typical configuration of a path memory employed in the Viterbi decoding circuit shown in  FIG. 14  as a decoding circuit for carrying out processing in units of two time slots; and  
         [0145]      FIG. 23  is a block diagram showing another typical configuration of the recording/reproduction apparatus according to the embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0146]     Embodiments of the present invention are explained by referring to diagrams as follows.  
         [0147]      FIG. 13  is a diagram showing a typical configuration of a recording/reproduction apparatus according to the present invention. It is to be noted that the typical configuration is basically the same as the configuration of the recording/reproduction apparatus shown in  FIG. 1  except that, in the typical configuration shown in  FIG. 13 , a system control section  111  is added and a Viterbi decoding circuit  112  serves as a substitute for the Viterbi decoding circuit  19  employed in the recording/reproduction apparatus shown in  FIG. 1 . For these reasons, descriptions of the typical configuration are properly omitted to avoid duplications.  
         [0148]     In the typical configuration shown in  FIG. 13 , the system control section  111  selects an operating mode among a plurality of operating modes provided for the Viterbi decoding circuit  112 , and supplies a mode select signal denoted by the word ‘mode’ in the figure to the Viterbi decoding circuit  112  as a selection result revealing which operating mode has been selected. An operating mode is a mode in which the Viterbi decoding circuit  112  decodes data reproduced from the recording medium  14 . In the case of the embodiment shown in  FIG. 13 , the Viterbi decoding circuit  112  has operating modes of 2 different types.  
         [0149]     In general, however, the Viterbi decoding circuit  112  has operating modes of a plurality of different types. For example, the Viterbi decoding circuit  112  has a PR (Partial Response) (1, x, 1) mode having a constraint length of 3 and a PR (1, x, x, 1) mode having a constraint length of 4. Both the modes are based on (1, 7) RLL codes having a minimum run length (d) of 1.  
         [0150]     The Viterbi decoding circuit  112  is basically configured as a Viterbi decoding circuit working in an operating mode having the largest constraint length, that is, an operating mode having the largest number of states. An example of such a Viterbi decoding circuit is the Viterbi decoding circuit  19  shown in  FIGS. 7 and 8  as a Viterbi decoding circuit operating in the PR (1, x, x, 1) mode. Sections composing the Viterbi decoding circuit  112  are switched to work in an operating mode indicated by a mode select signal received from the system control section  111 .  
         [0151]     The Viterbi decoding circuit  112  carries out a Viterbi decoding process on a sampled sequence z t  received from the sampling circuit  18  in an operating mode, which is either the PR (1, x, 1) mode or the PR (1, x, x, 1) mode, in order to reproduce a most probable modulated sequence x t  corresponding to the output of the modulation circuit  11 . The operating mode is selected on the basis of the mode select signal received from the system control section  111 .  
         [0152]      FIG. 14  is a diagram showing a typical configuration of the Viterbi decoding circuit  112  employed in the recording/reproduction apparatus shown in  FIG. 13 . It is to be noted that, in the typical configuration shown in  FIG. 14 , sections identical with their respective counterparts employed in the Viterbi decoding circuit  19  shown in  FIG. 2  are denoted by the same reference numerals as the counterparts and their explanations are not repeated in order to avoid duplications.  
         [0153]     The configuration of a BM (branch metric) computation circuit  121  is based on a branch-metric computation circuit working in an operating mode having the largest constraint length. An example of such a branch-metric computation circuit is the branch-metric computation circuit  41  shown in  FIG. 7  as a BM circuit operating in the PR (1, x, x, 1) mode. When the branch-metric computation circuit  121  receives a mode select signal from the system control section  111 , the branch-metric computation circuit  121  switches the operating mode to the PR (1, x, 1) mode or the PR (1, x, x, 1) mode in accordance with the received mode select signal and computes branch-metric data for each state transition on the basis of an input signal z t  received from the sampling circuit  18 . The branch-metric computation circuit  121  then outputs the computed branch-metric data to an ACS (add, compare and select) circuit  122 .  
         [0154]     The configuration of the ACS circuit  122  is based on an ACS circuit working in an operating mode having the largest constraint length. An example of such an ACS circuit is the ACS circuit  42  shown in  FIG. 7  as an ACS circuit operating in the PR (1, x, x, 1) mode. When the ACS circuit  122  receives a mode select signal from the system control section  111 , the ACS circuit  122  switches the operating mode to the PR (1, x, 1) mode or the PR (1, x, x, 1) mode in accordance with the received mode select signal. Then, the ACS circuit  122  adds path-metric data of a state immediately preceding the present state to branch-metric data received from the branch-metric computation circuit  121  to produce a sum. If paths merge in the path memory  123  to be described later, the ACS circuit  122  adds path-metric data of the state immediately preceding the present state to branch-metric data received from the branch-metric computation circuit  121  to produce a sum for each of the merging paths, and compares the sums to select the smallest one to be used as updated path-metric data of the present state. Finally, the ACS circuit  122  outputs a selection result, which is the result of the selection of the sums to the path memory  43  and a most-probable determination circuit  44 .  
         [0155]     The configuration of the path memory  123  is based on a path memory working in an operating mode having the largest constraint length. An example of such a path memory is the path memory  43  shown in  FIG. 8  as a path memory operating in the PR (1, x, x, 1) mode. When the path memory  123  receives a mode select signal from the system control section  111 , the path memory  123  switches the operating mode to the PR (1, x, 1) mode or the PR (1, x, x, 1) mode in accordance with the received mode select signal. Then, the path memory  123  carries out a select-shift operation described before on a value stored in each memory of the path memory  123  repeatedly in accordance with the selection result received from the ACS circuit  122 .  
         [0156]     The Viterbi decoding circuit  112  shown in  FIG. 14  is explained concretely by referring to  FIGS. 15 and 16  as follows.  
         [0157]      FIGS. 15 and 16  show a Viterbi decoding circuit  112  for a case in which both the PR (1, x, 1) and PR (1, x, x, 1) modes can be used. In the case of the typical configurations shown in  FIGS. 15 and 16 , the configuration of the Viterbi decoding circuit  112  is based on a Viterbi decoding circuit working in the operating mode having the largest constraint length. An example of such a Viterbi decoding circuit is the Viterbi decoding circuit  19  shown in  FIGS. 7 and 8  as a Viterbi decoding circuit operating in the PR (1, x, x, 1) mode. Sections composing the Viterbi decoding circuit  112  are switched to work in an operating mode indicated by a mode select signal received from the system control section  111 .  
         [0158]      FIG. 15  is a diagram showing a typical configuration of the branch-metric computation circuit  121  and a typical configuration of the ACS circuit  122 . It is to be noted that some blocks shown in  FIG. 15  include an equation given on an upper row and an equation given on a lower row. An equation given on an upper row represents an operation carried out in the PR (1, x, x, 1) mode whereas an equation given on a lower row represents an operation carried out in the PR (1, x, 1) mode. In addition, a line and a block, which are each shown by a dotted line, represent respectively a data transfer and an operation, which are not carried out in the PR (1, x, 1) mode. A hatched item represents a substitute for data, which is to be used in the PR (1, x, 1) mode, as a substitute used in conformity with an operation carried out in the PR (1, x, x, 1) mode.  
         [0159]     The branch-metric computation circuit  121  includes as many branch-metric computation sections as state transitions as the branch-metric computation circuit  41  shown in branch-metric computation section  131  in  FIG. 7 . In the case of the typical configuration shown in  FIG. 15 , the number of state transitions is 10. Thus, the branch-metric computation circuit  121  includes 10 branch-metric computation sections  131 - 1  to  131 - 10 . In the following description, the branch-metric computation sections  131 - 1  to  131 - 10  are each referred to simply as a branch-metric computation section  131  in case there is no need to distinguish them one another.  
         [0160]     When the branch-metric computation circuit  121  switches the operating mode to the PR (1, x, 1) mode, the branch-metric computation circuit  121  designates the branch-metric computation section  131  which computes branch-metric data bmABCD as a branch-metric computation circuit for computations of branch-metric data bmABC data where suffixes A, B, C and D each denote the integer 1 or 0. At that time, the branch-metric computation section  131  switches the theoretical value of a state transition c from cABCD of the PR (1, x, x, 1) mode to cABC of the PR (1, x, 1) mode for each state transition c and computes the square (z k −cABC)ˆ2 instead of computing the square (z k −cABCD)ˆ2. It is to be noted that symbol cABCD assigned to a state transition c denote the theoretical value (the identification reference value) of the state transition c in the PR (1, x, x, 1) mode and symbol cABCD assigned to a state transition c denote the theoretical value (the identification reference value) of the state transition c in the PR (1, x, 1) mode. In addition, symbol nˆ2 used in descriptions denotes the square of n where notation n denotes an expression.  
         [0161]     To put it concretely, in the PR (1, x, x, 1) mode, the branch-metric computation section  131 - 1  computes branch-metric data bm 0000   k =(z k −c 0000 )ˆ2, which represents the likelihood of the state transition c 0000 , and outputs the branch-metric data bm 0000   k  to an ACS section  132 - 1 . In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  131 - 1  computes branch-metric data bm 000   k =(z k −c 000 )ˆ2, which represents the likelihood of the state transition c 000 , and outputs the branch-metric data bm 000   k  to the ACS section  132 - 1 . By the same token, in the PR (1, x, x, 1) mode, the branch-metric computation section  131 - 2  computes branch-metric data bm 1000   k =(z k −c 1000 )ˆ2, which represents the likelihood of the state transition c 1000 , and outputs the branch-metric data bm 1000   k  also to the ACS section  132 - 1 . In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  131 - 2  computes branch-metric data bm 100   k =(z k −c 100 )ˆ2, which represents the likelihood of the state transition c 100 , and outputs the branch-metric data bm 100   k  also to the ACS section  132 - 1 .  
         [0162]     In the same way, in the PR (1, x, x, 1) mode, the branch-metric computation section  131 - 3  computes branch-metric data bm 0001   k =(z k −c 0001 )ˆ2, which represents the likelihood of the state transition c 0001 , and outputs the branch-metric data bm 0001 k to an ACS section  132 - 2 . In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  131 - 3  does not operate. Likewise, in the PR (1, x, x, 1) mode, the branch-metric computation section  131 - 4  computes branch-metric data bm 1001   k =(z k −c 1001 )ˆ2, which represents the likelihood of the state transition c 1001 , and outputs the branch-metric data bm 1001   k  also to the ACS section  132 - 2 . In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  131 - 4  does not operate.  
         [0163]     Similarly, in the PR (1, x, x, 1) mode, the branch-metric computation section  131 - 5  computes branch-metric data bm 0011   k =(z k −c 0011 )ˆ2, which represents the likelihood of the state transition c 0011 , and outputs the branch-metric data bm 0011   k  to an ACS section  132 - 3 . In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  131 - 5  computes branch-metric data bm 001   k =(z k −c 001 )ˆ2, which represents the likelihood of the state transition c 011 , and outputs the branch-metric data bm 001   k  to the ACS section  132 - 3 . By the same token, in the PR (1, x, x, 1) mode, the branch-metric computation section  131 - 6  computes branch-metric data bm 1100   k =(z k −c 1100 )ˆ2, which represents the likelihood of the state transition c 1100 , and outputs the branch-metric data bm 1100   k  to an ACS section  132 - 4 . In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  131 - 6  computes branch-metric data bm 110   k =(z k −c 110 )ˆ2, which represents the likelihood of the state transition c 110 , and outputs the branch-metric data bm 110   k  to the ACS section  132 - 4 .  
         [0164]     Likewise, in the PR (1, x, x, 1) mode, the branch-metric computation section  131 - 7  computes branch-metric data bm 0110   k =(z k −c 0110 )ˆ2, which represents the likelihood of the state transition c 0110 , and outputs the branch-metric data bm 0110   k  to an ACS section  132 - 5 . In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  131 - 7  does not operate. By the same token, in the PR (1, x, x, 1) mode, the branch-metric computation section  131 - 8  computes branch-metric data bm 1110   k =(z k −c 1110 )ˆ2, which represents the likelihood of the state transition c 1110 , and outputs the branch-metric data bm 1110   k  also to the ACS section  132 - 5 . In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  131 - 8  does not operate.  
         [0165]     Similarly, in the PR (1, x, x, 1) mode, the branch-metric computation section  131 - 9  computes branch-metric data bm 0111   k =(z k −c 0111 )ˆ2, which represents the likelihood of the state transition c 0111 , and outputs the branch-metric data bm 0111   k  to an ACS section  132 - 6 . In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  131 - 9  computes branch-metric data bm 011   k =(z k −c 011 )ˆ2, which represents the likelihood of the state transition coil, and outputs the branch-metric data bm 011   k  to the ACS section  132 - 6 . By the same token, the branch-metric computation section  131 - 10  computes branch-metric data bm 1111   k =(z 1 −c 1111 )ˆ2, which represents the likelihood of the state transition c 1111 , and outputs the branch-metric data bm 1111   k  also to the ACS section  132 - 6 . In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  131 - 10  computes branch-metric data bm 111   k =(z k −c 111 )ˆ2, which represents the likelihood of the state transition c 111 , and outputs the branch-metric data bm 111   k  also to the ACS section  132 - 6 .  
         [0166]     The ACS circuit  122  includes as many ACS sections and the number of states as the ACS circuit  42  shown in  FIG. 7  does. In the case of the typical configuration shown in  FIG. 15 , the number of states is 6. Thus, the ACS circuit  122  includes 6 ACS sections  132 - 1  to  132 - 6 . In the following description, the ACS sections  132 - 1  to  132 - 6  are each referred to simply as an ACS section  132  in case there is no need to distinguish them one another.  
         [0167]     Much like the branch-metric computation circuit  121 , when the ACS circuit  122  switches the operating mode to the PR (1, x, 1) mode, the ACS circuit  122  designates the ACS section  132  for computing path-metric data mABC as an ACS circuit for computations of path-metric data mAB.  
         [0168]     To put it concretely, in the PR (1, x, x, 1) mode, the ACS section  132 - 1  updates path-metric data m 000   k , which is the likelihood of a history up to state S 000 . To be more specific, the ACS section  132 - 1  adds the path-metric data m 000   k-1  stored internally in the ACS section  132 - 1  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 0000   k  received from the branch-metric computation section  131 - 1  to produce a first sum. The ACS section  132 - 1  also adds the path-metric data m 100   k-1  stored internally in the ACS section  132 - 4  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 1000   k  received from the branch-metric computation section  131 - 2  to produce a second sum. Then, the ACS section  132 - 1  compares the first and second sums with each other in order to select the smaller one to be used as updated path-metric data m 000   k  of the present state. The computation and the comparison are carried out by the ACS section  132 - 1  in accordance with Eq. (5) given before. Finally, the ACS section  132 - 1  outputs a selection result sel 000  to a memory included in the path memory  123  as a memory used for storing the value of state S 000 .  
         [0169]     In the PR (1, x, 1) mode, on the other hand, the ACS section  132 - 1  updates the path-metric data m 00   k , which is the likelihood of a history up to state S 00 . To be more specific, the ACS section  132 - 1  adds the path-metric data m 00   k-1  stored internally in the ACS section  132 - 1  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 000   k  received from the branch-metric computation section  131 - 1  to produce a first sum. The ACS section  132 - 1  also adds the path-metric data m 10   k-1  stored internally in the ACS section  132 - 4  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 100   k  received from the branch-metric computation section  131 - 2  to produce a second sum. Then, the ACS section  132 - 1  compares the first and second sums with each other in order to select the smaller one to be used as updated path-metric data m 00   k  of the present state. The computation and the comparison are carried out by the ACS section  132 - 1  in accordance with Eq. (1) given before. Finally, the ACS section  132 - 1  outputs a selection result sel 000  to a memory included in the path memory  123  as a memory used for storing the value of state S 00 .  
         [0170]     By the same token, in the PR (1, x, x, 1) mode, the ACS section  132 - 2  updates path-metric data m 001   k , which is the likelihood of a history up to state S 001 . To be more specific, the ACS section  132 - 2  adds the path-metric data m 000   k-1  stored internally in the ACS section  132 - 1  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 0001   k  received from the branch-metric computation section  131 - 3  to produce a first sum. The ACS section  132 - 2  also adds the path-metric data m 100   k-1  stored internally in the ACS section  132 - 4  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 1001   k  received from the branch-metric computation section  131 - 4  to produce a second sum. Then, the ACS section  132 - 2  compares the first and second sums in order to select the smaller one to be used as updated path-metric data m 001   k  of the present state. The computation and the comparison are carried out by the ACS section  132 - 2  in accordance with Eq. (6) given before. Finally, the ACS section  132 - 2  outputs a selection result sel 001  to a memory included in the path memory  123  as a memory used for storing the value of state S 001 .  
         [0171]     In the PR (1, x, 1) mode, on the other hand, the ACS section  132 - 2  does not operate.  
         [0172]     In the PR (1, x, x, 1) mode, the ACS section  132 - 3  updates path-metric data m 01   k , which is the likelihood of a history up to state S 011 . To be more specific, the ACS section  132 - 3  adds the path-metric data m 001   k-1  stored internally in the ACS section  132 - 2  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 0011   k  received from the branch-metric computation section  131 - 5  to produce a sum in accordance with Eq. (7) given before. Then, the ACS section  132 - 3  uses the sum as updated path-metric data m 011   k  of the present state.  
         [0173]     In the PR (1, x, 1) mode, on the other hand, the ACS section  132 - 3  updates the path-metric data m 01   k , which is the likelihood of a history up to state S 01 . To put it concretely, the ACS section  132 - 3  adds the updated path-metric data m 00   k-1  stored internally in the ACS section  132 - 1  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 001   k  received from the branch-metric computation section  131 - 5  to produce a sum in accordance with Eq. (2) given before and uses the sum as updated path-metric data m 01   k  of the present state.  
         [0174]     It is to be noted that, as described above, in the PR (1, x, 1) mode, the ACS section  132 - 3  updates the path-metric data m 01   k  on the basis of path-metric data m 00   k-1  stored internally in the ACS section  132 - 1  (used also for updating path-metric data m 000   k  in the PR (1, x, x, 1) mode) in place of the path-metric data m 001   k-1  that should be used for updating path-metric data m 01   k . This is because the ACS section  132 - 2  for finding path-metric data m 001   k-1  does not operate in the PR (1, x, 1) mode as described above. The path-metric data m 001   k-1  is path metric data of a state immediately preceding the present state having path-metric data m 011   k  corresponding to the path-metric data m 01   k  updated by the ACS section  132 - 3 .  
         [0175]     By the same token, in the PR (1, x, x, 1) mode, the ACS section  132 - 4  updates path-metric data m 100   k , which is the likelihood of a history up to state S 100 . To be more specific, the ACS section  132 - 4  adds the path-metric data m 110   k-1  stored internally in the ACS section  132 - 5  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 1100   k  received from the branch-metric computation section  131 - 6  to produce a sum in accordance with Eq. (8) given before. Then, the ACS section  132 - 4  uses the sum as updated path-metric data m 100   k  of the present state.  
         [0176]     In the PR (1, x, 1) mode, on the other hand, the ACS section  132 - 4  updates the path-metric data m 10   k , which is the likelihood of a history up to state S 10 . To put it concretely, the ACS section  132 - 4  adds the updated path-metric data m 11   k-1  stored internally in the ACS section  132 - 6  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 110   k  received from the branch-metric computation section  131 - 6  to produce a sum in accordance with Eq. (3) given before and uses the sum as updated path-metric data m 10   k  of the present state.  
         [0177]     It is to be noted that, as described above, in the PR (1, x, 1) mode, the ACS section  132 - 4  updates the path-metric data m 10   k  on the basis of path-metric data m 11   k-1  stored internally in the ACS section  132 - 6  (also used for finding path-metric data m 111   k  in the PR (1, x, x, 1) mode) in place of path-metric data m 110   k-1 , which should be used for updating path-metric data m 10   k , in the same way as the ACS section  132 - 3  updates the path-metric data m 01   k  as described above. This is because the ACS section  132 - 5  for finding path-metric data m 110   k-1  does not operate in the PR (1, x, 1) mode as will be described below. The path-metric data m 110   k-1  is path metric data of a state immediately preceding the present state having path-metric data m 100   k  corresponding to the path-metric data m 10   k  updated by the ACS section  132 - 4 .  
         [0178]     By using the path-metric data m 00   k-1  and the path-metric data m 11   k-1  as substitutes as described above, a trellis spread throughout the path memory  123  shown in  FIG. 16  as the trellis of state transitions occurring in the PR (1, x, x, 1) mode becomes compatible with the trellis spread throughout the path memory  43  shown in  FIG. 6  as the trellis of state transitions occurring in the PR (1, x, 1) mode so that the ACS circuit  122  is capable of operating in both the PR (1, x, x, 1) mode and the PR (1, x, 1) mode.  
         [0179]     In the same way as the ACS section  132 - 2 , in the PR (1, x, x, 1) mode, the ACS section  132 - 5  updates path-metric data m 110   k , which is the likelihood of a history up to state S 110 . To be more specific, the ACS section  132 - 5  adds the path-metric data m 111   k-1  stored internally in the ACS section  132 - 6  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 1110   k  received from the branch-metric computation section  131 - 8  to produce a first sum. The ACS section  132 - 5  also adds the path-metric data m 011   k-1  stored internally in the ACS section  132 - 3  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 0110   k  received from the branch-metric computation section  131 - 7  to produce a second sum. Then, the ACS section  132 - 5  compares the first and second sums in order to select the smaller one to be used as updated path-metric data m 110   k  of the present state. The computation and the comparison are carried out by the ACS section  132 - 5  in accordance with Eq. (9) given before. Finally, the ACS section  132 - 5  outputs a selection result sel 110  to a memory included in the path memory  123  as a memory used for storing the value of state S 110 .  
         [0180]     In the PR (1, x, 1) mode, on the other hand, the ACS section  132 - 5  does not operate.  
         [0181]     In the same way as the ACS section  132 - 1 , in the PR (1, x, x, 1) mode, the ACS section  132 - 6  updates path-metric data m 111   k , which is the likelihood of a history up to state S 111 . To be more specific, the ACS section  132 - 6  adds the path-metric data m 111   k-1  stored internally in the ACS section  132 - 6  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 1111   k  received from the branch-metric computation section  131 - 10  to produce a first sum. The ACS section  132 - 6  also adds the path-metric data m 011   k-1  stored internally in the ACS section  132 - 3  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 0111   k  received from the branch-metric computation section  131 - 9  to produce a second sum. Then, the ACS section  132 - 6  compares the first and second sums in order to select the smaller one to be used as updated path-metric data m 111   k  of the present state. Finally, the ACS section  132 - 6  outputs a selection result sel 111  to a memory included in the path memory  123  as a memory used for storing the value of state S 111 . The computation and the comparison are carried out by the ACS section  132 - 6  in accordance with Eq. (10) given before.  
         [0182]     In the PR (1, x, 1) mode, on the other hand, the ACS section  132 - 6  updates the path-metric data m 11   k , which is the likelihood of a history up to state S 11 . To put it concretely, the ACS section  132 - 6  adds the path-metric data m 01   k-1  stored internally in the ACS section  132 - 3  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 011   k  received from the branch-metric computation section  131 - 9  to produce a first sum. The ACS section  132 - 6  also adds the path-metric data m 11   k-1  stored internally in the ACS section  132 - 4  as the path-metric data of the state immediately preceding the present state to the branch-metric data bm 111   k  received from the branch-metric computation section  131 - 10  to produce a second sum. Then, the ACS section  132 - 6  compares the first and second sums in order to select the smaller one to be used as updated path-metric data m 11   k  of the present state. Finally, the ACS section  132 - 6  outputs a selection result sel 111  to a memory included in the path memory  123  as a memory used for storing the value of state S 11 . The computations and the comparison are carried out by the ACS section  132 - 6  in accordance with Eq. (4) given before.  
         [0183]      FIG. 16  is a diagram showing a typical configuration of the path memory  123 .  
         [0184]     Much like the path memory  43  shown in  FIG. 8 , the path memory  123  has a configuration for six states as a configuration expressing a state-transition diagram shown in  FIG. 4  in terms of sequences along the time axis. It is to be noted that, in the path memory  123  shown in  FIG. 16 , each solid-line circle represents a state existing in both the PR (1, x, x, 1) mode and the PR (1, x, 1) mode. On the other hand, each dotted-line circle represents a state existing only in the PR (1, x, x, 1) mode. In addition, each solid-line arrow represents a state transition possibly occurring in both the PR (1, x, x, 1) mode and the PR (1, x, 1) mode. On the other hand, each dotted-line arrow represents a state transition possibly occurring only in the PR (1, x, x, 1) mode. Furthermore, each arrow expressed by a thick dotted line represents a state transition modified for use as a state transition probably occurring only in the PR (1, x, 1) mode and thus never occurring in the PR (1, x, x, 1) mode.  
         [0185]     In the typical configuration shown in  FIG. 16  as the configuration of the path memory  123 , much like the ACS circuit  122 , when the operating mode is switched to the PR (1, x, 1) mode, a state transition from state S 001  to state S 011  is changed (or switched) to a state transition from state S 000  (or S 00  in the PR (1, x, 1) mode) to state S 011  (or S 01  in the PR (1, x, 1) mode) whereas a state transition from state S 110  to state S 100  is changed (or switched) to a state transition from state S 111  (or S 11  in the PR (1, x, 1) mode) to state S 110  (or S 10  in the PR (1, x, 1) mode) before carrying out operations.  
         [0186]     By replacing the state transitions of the two types cited above, a trellis spread throughout the path memory  123  shown in  FIG. 16  as the trellis of state transitions occurring in the PR (1, x, x, 1) mode becomes compatible with the trellis spread throughout the path memory  43  shown in  FIG. 6  as a typical trellis of state transitions occurring in the PR (1, x, 1) mode so that the path memory  123  is capable of operating in both the PR (1, x, x, 1) mode and the PR (1, x, 1) mode.  
         [0187]     That is to say, in the PR (1, x, x, 1) mode, the path memory  123  operates in the same way as the path memory  43  shown in  FIG. 8 . To put it in detail, in the path memory  123 , a value to be stored in a memory for state S 000  at any specific stage is a value selected among a value stored in a memory for state S 000  at a stage immediately preceding the specific stage and a value stored in a memory for state S 100  at the stage immediately preceding the specific stage in accordance with a selection result sel 000  received from the ACS section  132 - 1 . The selected value stored in the memory for state S 000  at the specific stage is then shifted (output) to a memory for state S 000  at a stage immediately following the specific stage and a memory for state S 001  at the stage immediately following the specific stage. In the path memory  123 , a value to be stored in a memory for state S 000  at the specific stage is a value selected among a value stored in a memory for state S 000  at the stage immediately preceding the specific stage and a value stored in a memory for state S 100  at the stage immediately preceding the specific stage in accordance with a selection result sel 001  received from the ACS section  132 - 2 . The selected value stored in the memory for state S 001  at the specific stage is then shifted (output) to a memory for state S 011  at the stage immediately following the specific stage.  
         [0188]     In addition, in the path memory  123 , a value to be stored in a memory for state S 110  at any specific stage is a value selected among a value stored in a memory for state S 011  at a stage immediately preceding the specific stage and a value stored in a memory for state S 111  at the stage immediately preceding the specific stage in accordance with a selection result sel 001  received from the ACS section  132 - 5 . The selected value stored in the memory for state S 110  at the specific stage is then shifted (output) to a memory for state S 110  at a stage immediately following the specific stage. In the path memory  123 , a value to be stored in a memory for state S 111  at the specific stage is a value selected among a value stored in a memory for state S 011  at the stage immediately preceding the specific stage and a value stored in a memory for state S 111  at the stage immediately preceding the specific stage in accordance with a selection result sel 111  received from the ACS section  132 - 6 . The selected value stored in the memory for state S 111  at the specific stage is then shifted (output) to a memory for state S 110  at the stage immediately following the specific stage and a memory for state S 111  at the stage immediately following the specific stage.  
         [0189]     It is to be noted that, by way of a memory for state S 011  at each specific stage, a value is shifted from a memory at a stage immediately preceding the specific stage repeatedly to a memory existing at a stage immediately following the specific stage as a memory according to a transition c. Thus, for any specific stage in the path memory  123 , a value stored in a memory for state S 001  at a stage immediately preceding the specific stage is shifted to a memory for state S 110  at a stage immediately following the specific stage and a memory for state S 111  at the same following stage by way of a memory for state S 011  at the specific stage. By way of a memory for state S 100  at each specific stage, a value is shifted from a memory at a stage immediately preceding the specific stage repeatedly to a memory existing at a stage immediately following the specific stage as a memory according to a transition c. Thus, for any specific stage in the path memory  123 , a value stored in a memory for state S 110  at a stage immediately preceding the specific stage is shifted to a memory for state S 000  at a stage immediately following the specific stage and a memory for state S 001  at the same following stage by way of a memory for state S 100  at the specific stage.  
         [0190]     In the PR (1, x, 1) mode, on the other hand, the path memory  123  operates in the same way as the path memory  43  shown in  FIG. 4 . That is to say, in the path memory  43 , a value to be stored in a memory for state S 00  at any specific stage is a value selected among a value stored in a memory for state S 00  at a stage immediately preceding the specific stage and a value stored in a memory for state S 10  at the stage immediately preceding the specific stage in accordance with a selection result sel 000  received from the ACS section  132 - 1 . The selected value stored in the memory for state S 00  at the specific stage is then shifted (output) to a memory for state S 00  at a stage immediately following the specific stage and a memory for state S 01  at the stage immediately following the specific stage. In the path memory  123 , a value to be stored in a memory for state S 11  at the specific stage is a value selected among a value stored in a memory for state S 11  at the stage immediately preceding the specific stage and a value stored in a memory for state S 01  at the stage immediately preceding the specific stage in accordance with a selection result sel 111  received from the ACS section  132 - 6 . The selected value stored in the memory for state S 11  at the specific stage is then shifted (output) to a memory for state S 11  at the stage immediately following the specific stage and a memory for state S 10  at the stage immediately following the specific stage.  
         [0191]     It is to be noted that, by way of a memory for state S 01  and state S 10  at each specific stage, a value is shifted from a memory at a stage immediately preceding the specific stage repeatedly to a memory existing at a stage immediately following the specific stage as a memory according to a transition c. Thus, for any specific stage in the path memory  123 , a value stored in a memory for state S 00  at a stage immediately preceding the specific stage is shifted to a memory for state S 11  at a stage immediately following the specific stage by way of a memory for state S 01  at the specific stage. By the same token, by way of a memory for state S 10  at each specific stage, a value is shifted from a memory at a stage immediately preceding the specific stage repeatedly to a memory existing at a stage immediately following the specific stage as a memory according to a transition c. Thus, for any specific stage in the path memory  123 , a value stored in a memory for state S 11  at a stage immediately preceding the specific stage is shifted to a memory for state S 00  at a stage immediately following the specific stage by way of a memory for state S 10  at the specific stage.  
         [0192]     In addition, in the PR (1, x, 1) mode, the memories for states S 001  and S 110  of the PR (1, x, x, 1) mode do not operate.  
         [0193]     As described above, in the PR (1, x, x, 1) mode, the Viterbi decoding circuit  112  carries out a decoding process on the basis of a trellis expressing a state-transition diagram shown in  FIG. 4  as a state-transition diagram of a PR (1, x, x, 1) transmission line in terms of sequences along the time axis. The trellis is the trellis composed of state transitions represented by solid-line and dotted-line arrows in  FIG. 16 . In the PR (1, x, 1) mode, on the other hand, by replacing the state transitions of the two types cited above, the Viterbi decoding circuit  112  is capable of carrying out a decoding process on the basis of a trellis expressing a state-transition diagram shown in  FIG. 3  as a state-transition diagram of a PR (1, x, 1) transmission line in terms of sequences along the time axis. The trellis in this case is the trellis composed of state transitions represented by solid-line arrows and arrows each indicated by a dotted thick-line arrow in  FIG. 16 .  
         [0194]     Next, by referring to  FIG. 17 , computation processing carried out by the branch-metric computation section  131  is explained concretely. It is to be noted that the typical configuration shown in  FIG. 17  is the configuration of the branch-metric computation section  131 - 1 .  
         [0195]     The branch-metric computation section  131  shown in  FIG. 17  includes a selector  151 , which is typically a multiplexer, a subtractor  152  and a processing section  153 .  
         [0196]     The selector  151  selects the theoretical value c 0000  or c 000  and outputs the selected one to the subtractor  152 . As described earlier, c 0000  and c 000  are each a theoretical value (or an identification reference value) of a transition c. That is to say, when the selector  151  receives a mode select signal denoted by the word ‘mode’ from the system control section  111 , the selector  151  selects the theoretical value c 0000  or c 000  in dependence on the received mode select signal and outputs the selected theoretical value to the subtractor  152 .  
         [0197]     The subtractor  152  subtracts the theoretical value c 0000  or c 000  supplied by the selector  151  from an input signal z k  received from the sampling circuit  18  to produce a difference (z k −c 0000 ) or (z k −c 000 ) and outputs the difference (z k −c 0000 ) or (z k −c 000 ) to the processing section  153 . The processing section  153  computes the square of the difference (z k −c 0000 ) or (z k −c 000 ) supplied thereto to compute bm 0000   k  (=(z k −c 0000 )ˆ2) or bm 000   k  (=(z k −c 000 )ˆ2) and outputs the branch-metric data bm 0000   k  or bm 000   k  to the ACS section  132 .  
         [0198]     That is to say, in the PR (1, x, x, 1) mode, the selector  151  selects the theoretical value c 0000  and outputs the theoretical value c 0000  to the subtractor  152 . The subtractor  152  subtracts the theoretical value c 0000  supplied by the selector  151  from an input signal z k  received from the sampling circuit  18  to produce a difference (z k −c 0000 ) and outputs the difference (z k −c 0000 ) to the processing section  153 . The processing section  153  computes the square of the difference (z k −c 0000 ) supplied thereto to compute branch-metric data bm 0000   k  (=(z k −c 0000 )ˆ2) and outputs the branch-metric data bm 0000   k  to the ACS section  132 .  
         [0199]     In the PR (1, x, 1) mode, on the other hand, the selector  151  selects the theoretical value c 000  and outputs the theoretical value c 000  to the subtractor  152 . The subtractor  152  subtracts the theoretical value c 000  supplied by the selector  151  from an input signal z k  received from the sampling circuit  18  to produce a difference (z k −c 000 ) and outputs the difference (z k −c 000 ) to the processing section  153 . The processing section  153  computes the square of the difference (z k −c 000 ) supplied thereto to compute branch-metric data bm 000   k  (=(z k −c 000 )ˆ2) and outputs the branch-metric data bm 000   k  to the ACS section  132 .  
         [0200]     Next, by referring to  FIG. 18 , computation processing carried out by the ACS section  132  is explained concretely. It is to be noted that the typical configuration shown in  FIG. 18  is the configuration of the ACS section  132 - 3  for carrying out a computation process on data partially modified due to the computing mode. In addition, the ACS section  132 - 3  does not carry out a comparison process.  
         [0201]     The ACS section  132  shown in  FIG. 18  includes a selector  161 , which is a multiplexer, and a adder  162 .  
         [0202]     When the selector  161  receives a mode select signal denoted by the word ‘mode’ from the system control section  111 , the selector  161  selects patch-metric data m 001   k-1  stored in the ACS section  132 - 2  as the patch-metric data of a state immediately preceding the present state or patch-metric data m 000   k-1  (patch-metric data m 00   k-1 ) stored in the ACS section  132 - 1  as the patch-metric data of a state immediately preceding the present state in accordance with the mode select signal, and outputs the selected patch-metric data to the adder  162 .  
         [0203]     The adder  162  adds branch-metric data bm 0011   k  (or bm 001   k ) received from the branch-metric computation section  131 - 5  to the patch-metric data received from the selector  161  to produce a sum and uses the sum as updated path-metric data m 011   k-1  (or m 01   k-1 ) of the present state.  
         [0204]     That is to say, in the PR (1, x, x, 1) mode, the selector  161  selects patch-metric data m 001   k-1  stored in the ACS section  132 - 2  as the patch-metric data of a state immediately preceding the present state, and outputs the selected patch-metric data m 001   k-1  to the adder  162 . The adder  162  adds branch-metric data bm 0011   k  received from the branch-metric computation section  131 - 5  to the patch-metric data m 001   k-1  received from the selector  161  to produce a sum and uses the sum as updated path-metric data m 011   k-1  of the present state.  
         [0205]     In the PR (1, x, 1) mode, on the other hand, the selector  161  selects patch-metric data m 000   k-1  (patch-metric data m 00   k-1 ) stored in the ACS section  132 - 1  as the patch-metric data of a state immediately preceding the present state, and outputs the selected patch-metric data to the adder  162 . The adder  162  adds branch-metric data bm 0011   k  received from the branch-metric computation section  131 - 5  to the patch-metric data m 000   k-1  (patch-metric data m 00   k-1 ) received from the selector  161  to produce a sum and uses the sum as updated path-metric data m 011   k-1  (m 01   k-1 ) of the present state.  
         [0206]      FIG. 19  is a diagram showing a typical hardware configuration of the path memory  123  shown in  FIG. 16 . The typical hardware configuration shown in  FIG. 19  includes flip-flop arrays arranged at three stages. In actuality, however, the flip-flops of the path memory  123  are arranged typically at 16 or 32 stages. The flip-flops each serve as a memory for storing a state in a trellis of the path memory  123  shown in  FIG. 16 .  
         [0207]     To put it in detail, the path memory  123  shown in  FIG. 19  includes memories  181 - 1  to  181 - 3  arranged at three stages as memories for state S 000  (or S 00 ) in the trellis of the path memory  123  shown in  FIG. 16 , memories  182 - 1  to  182 - 3  arranged at three stages as memories for state S 001  in the trellis of the path memory  123 , memories  183 - 1  to  183 - 3  arranged at three stages as memories for state S 011  (or S 01 ) in the trellis of the path memory  123 , memories  184 - 1  to  184 - 3  arranged at three stages as memories for state S 100  (or S 10 ) in the trellis of the path memory  123 , memories  185 - 1  to  185 - 3  arranged at three stages as memories for state S 110  in the trellis of the path memory  123  and memories  186 - 1  to  186 - 3  arranged at three stages as memories for state S 111  (or S 1 ) in the trellis of the path memory  123 .  
         [0208]     The input terminals of the memories  181 - 1  to  183 - 1  are each connected to the ground. On the other hand, the input terminals of the memories  184 - 1  to  186 - 1  are each connected to a power-supply line VDD.  
         [0209]     In addition, a multiplexer serving as a selector  191 - 1  is provided at a stage in front of the memory  181 - 2 . By the same token, a multiplexer serving as a selector  191 - 2  is provided at a stage in front of the memory  181 - 3 . In the same way, a multiplexer serving as a selector  192 - 1  is provided at a stage in front of the memory  182 - 2 . Likewise, a multiplexer serving as a selector  192 - 2  is provided at a stage in front of the memory  182 - 3 . Similarly, a multiplexer serving as a selector  193 - 1  is provided at a stage in front of the memory  183 - 2 . By the same token, a multiplexer serving as a selector  193 - 2  is provided at a stage in front of the memory  183 - 3 .  
         [0210]     In the same way, a multiplexer serving as a selector  194 - 1  is provided at a stage in front of the memory  184 - 2 . By the same token, a multiplexer serving as a selector  194 - 2  is provided at a stage in front of the memory  184 - 3 . In the same way, a multiplexer serving as a selector  195 - 1  is provided at a stage in front of the memory  185 - 2 . Likewise, a multiplexer serving as a selector  195 - 2  is provided at a stage in front of the memory  185 - 3 . Similarly, a multiplexer serving as a selector  196 - 1  is provided at a stage in front of the memory  186 - 2 . By the same token, a multiplexer serving as a selector  196 - 2  is provided at a stage in front of the memory  186 - 3 .  
         [0211]     In the following description, the memories  181 - 1  to  181 - 3 ,  182 - 1  to  182 - 3 ,  183 - 1  to  183 - 3 ,  184 - 1  to  184 - 3 ,  185 - 1  to  185 - 3 , and  186 - 1  to  186 - 3  are each referred to simply as a memory  181 , a memory  182 , a memory  183 , a memory  184 , a memory  185 , and a memory  186  respectively.  
         [0212]     Similarly, the selectors  191 - 1  and  191 - 2 , the selectors  192 - 1  and  192 - 2 , the selectors  193 - 1  and  193 - 2 , the selectors  194 - 1  and  194 - 2 , the selectors  195 - 1  and  195 - 2 , and the selectors  196 - 1  and  196 - 2  are each referred to simply as a selector  191 , a selector  192 , a selector  193 , a selector  194 , selector  195 , and a selector  196  in case there is no need to distinguish them from each other.  
         [0213]     A value stored in a memory  181  provided at a specific stage is shifted to a memory  181  provided at a stage immediately following the specific stage by way of a selector  191  provided between the memories  181 , shifted to a memory  182  provided at a stage immediately following the specific stage by way of a selector  192  provided between the memories  181  and  182  and shifted to a memory  183  provided at a stage immediately following the specific stage by way of a selector  193  provided between the memories  181  and  183 . A value stored in a memory  182  provided at a specific stage is shifted to a memory  183  provided at a stage immediately following the specific stage by way of a selector  193  provided between the memories  182  and  183 .  
         [0214]     Likewise, a value stored in a memory  183  provided at a specific stage is shifted to a memory  185  provided at a stage immediately following the specific stage by way of a selector  195  provided between the memories  183  and  185  and shifted to a memory  186  provided at a stage immediately following the specific stage by way of a selector  196  provided between the memories  183  and  186 . A value stored in a memory  184  provided at a specific stage is shifted to a memory  181  provided at a stage immediately following the specific stage by way of a selector  191  provided between the memories  184  and  181  and shifted to a memory  182  provided at a stage immediately following the specific stage by way of a selector  192  provided between the memories  184  and  182 .  
         [0215]     In the same way, a value stored in a memory  185  provided at a specific stage is shifted to a memory  184  provided at a stage immediately following the specific stage by way of a selector  194  provided between the memories  185  and  184 . A value stored in a memory  186  provided at a specific stage is shifted to a memory  184  provided at a stage immediately following the specific stage by way of a selector  194  provided between the memories  186  and  184 , shifted to a memory  185  provided at a stage immediately following the specific stage by way of a selector  195  provided between the memories  185  and  186  and shifted to a memory  186  provided at a stage immediately following the specific stage by way of a selector  196  provided between the memories  186 .  
         [0216]     The selector  191  selects one of values shifted from the memories  181  and  184  provided at a stage immediately preceding the present stage in accordance with a selection result sel 000  received from the ACS section  132 - 1 , outputting the selected value to the memory  181  provided at the present stage. The selector  192  selects one of values shifted from the memories  181  and  184  provided at a stage immediately preceding the present stage in accordance with a selection result sel 001  received from the ACS section  132 - 2 , outputting the selected value to the memory  182  provided at the present stage.  
         [0217]     Likewise, the selector  193  selects one of values shifted from the memories  181  and  182  provided at a stage immediately preceding the present stage in accordance with the mode select signal ‘mode’ received from the system control section  111 , outputting the selected value to the memory  183  provided at the present stage. The selector  194  selects one of values shifted from the memories  185  and  186  provided at a stage immediately preceding the present stage in accordance with the mode select signal ‘mode’ received from the system control section  111 , outputting the selected value to the memory  184  provided at the present stage.  
         [0218]     Similarly, the selector  195  selects one of values shifted from the memories  183  and  186  provided at a stage immediately preceding the present stage in accordance with a selection result sel 1110  received from the ACS section  132 - 5 , outputting the selected value to the memory  185  provided at the present stage. The selector  196  selects one of values shifted from the memories  183  and  186  provided at a stage immediately preceding the present stage in accordance with a selection result sel 111  received from the ACS section  132 - 6 , outputting the selected value to the memory  186  provided at the present stage.  
         [0219]     That is to say, in the PR (1, x, x, 1) mode, the selector  191  selects one of values shifted from the memories  181  and  184  provided at a stage immediately preceding the present stage in accordance with a selection result sel 000  received from the ACS section  132 - 1 , outputting the selected value to the memory  181  provided at the present stage. The selector  192  selects one of values shifted from the memories  181  and  184  provided at a stage immediately preceding the present stage in accordance with a selection result sel 001  received from the ACS section  132 - 2 , outputting the selected value to the memory  182  provided at the present stage.  
         [0220]     Likewise, in the PR (1, x, x, 1) mode, the selector  193  selects a value shifted from the memory  182  among values shifted from the memories  181  and  182  provided at a stage immediately preceding the present stage, outputting the selected value to the memory  183  provided at the present stage. The selector  194  selects a value shifted from the memory  185  among values shifted from the memories  185  and  186  provided at a stage immediately preceding the present stage, outputting the selected value to the memory  184  provided at the present stage.  
         [0221]     Similarly, in the PR (1, x, x, 1) mode, the selector  195  selects one of values shifted from the memories  183  and  186  provided at a stage immediately preceding the present stage in accordance with a selection result sel 1110  received from the ACS section  132 - 5 , outputting the selected value to the memory  185  provided at the present stage. The selector  196  selects one of values shifted from the memories  183  and  186  provided at a stage immediately preceding the present stage in accordance with a selection result sel 111  received from the ACS section  132 - 6 , outputting the selected value to the memory  186  provided at the present stage.  
         [0222]     In the PR (1, x, 1) mode, on the other hand, the selector  191  selects one of values shifted from the memories  181  and  184  provided at a stage immediately preceding the present stage in accordance with a selection result sel 000  received from the ACS section  132 - 1 , outputting the selected value to the memory  181  provided at the present stage. In the PR (1, x, 1) mode, however, the selector  192  does not receive a selection result sel 001  from the ACS section  132 - 2  so that the selector  192  does not operate.  
         [0223]     In the PR (1, x, 1) mode, the selector  193  outputs a value shifted from the memory  181  provided at a stage immediately preceding the present stage to the memory  183  provided at the present stage. It is to be noted that, in the PR (1, x, 1) mode, the selector  193  does not receive a value from the memory  182  provided at a stage immediately preceding the present stage. The selector  194  outputs a value shifted from the memory  186  provided at a stage immediately preceding the present stage to the memory  184  provided at the present stage. It is to be noted that, in the PR (1, x, 1) mode, the selector  194  does not receive a value from the memory  185 .  
         [0224]     Much like the selector  192 , in the PR (1, x, 1) mode, however, the selector  195  does not receive a selection result sel 110  from the ACS section  132 - 5  so that the selector  195  does not operate. In the PR (1, x, 1) mode, however, the selector  196  selects one of values shifted from the memories  183  and  186  provided at a stage immediately preceding the present stage in accordance with a selection result sel 111  received from the ACS section  132 - 6 , outputting the selected value to the memory  186  provided at the present stage.  
         [0225]     As described above, the selectors  193  and  194  in the path memory  123  each select a value for the PR (1, x, 1) mode or the PR (1, x, x, 1) mode.  
         [0226]     It is to be noted that the path memory  123  shown in  FIG. 19  is obtained by merely adding a connection from the memory  181  to the memory  183  and a connection from the memory  186  to the memory  184  as well as adding of the selectors  193  and  194  to the configuration of the PR (1, x, x, 1) path memory  43  shown in  FIG. 8 . That is to say, by merely changing the configuration of the PR (1, x, x,  1 ) path memory  43  a little bit, it is possible to select the PR (1, x, 1) mode or the PR (1, x, x, 1) mode.  
         [0227]     By referring a flowchart shown in  FIG. 20 , the following description explains processing to carry out a decoding process executed by selecting an operating mode of the recording/reproduction apparatus shown in  FIG. 13 .  
         [0228]     Let us assume for example that a decoding process is to be carried out in the PR (1, x, x, 1) mode. In this case, the user enters a command to switch/change the operating mode via an operation input section or the like. As an alternative, a command is issued in the recording/reproduction apparatus shown in  FIG. 13  to change the operating mode in accordance with data recorded on the recording medium  14 .  
         [0229]     In response to these commands, at a step S 11 , the system control section  111  selects the PR (1, x, 1) mode or the PR (1, x, x, 1) mode as the operating mode. The system control section  111  then outputs a mode select signal ‘mode’ to the branch-metric computation circuit  121 , the ACS circuit  122  and the path memory  123  as a result of the selection. Subsequently, the flow of the processing goes on to a step S 12 .  
         [0230]     Receiving the mode select signal ‘mode’ from the system control section  111  at the step S 12 , the selectors employed in the branch-metric computation circuit  121 , the ACS circuit  122  and the path memory  123  produce a result of determination as to whether or not the PR (1, x, 1) mode has been selected. If the result of determination indicates that the PR (1, x, 1) mode has been selected, the flow of the processing goes on to a step S 13  at which the operating mode is switched to the PR (1, x, 1) mode prior to a decoding process carried out thereafter. After the decoding process has been carried out in the PR (1, x, 1) mode, the processing represented by the flowchart shown in  FIG. 20  is ended.  
         [0231]     In the decoding process carried out at the step S 13  in the PR (1, x, 1) mode, to put it concretely, the selector  151  employed in the branch-metric computation section  131  selects the theoretical value c 000  and outputs the theoretical value c 000  to the subtractor  152 . The subtractor  152  subtracts the theoretical value c 000  supplied by the selector  151  from an input signal z k  supplied by the sampling circuit  18  to find a difference (z k −c 000 ) and supplies the difference (z k −c 000 ) to the processing section  153 . The processing section  153  computes branch-metric data bm 000   k  (=(z k −c 000 )ˆ2), where symbol (z k −c 000 )ˆ2 denotes the square of the difference (z k −c 000 ), and supplies the branch-metric data bm 000   k  to the ACS section  132 .  
         [0232]     In the PR (1, x, 1) mode, the selector  161  employed in the ACS section  132  selects path-metric data m 000   k-1  or m 00   k-1  and outputs the selected path-metric data to the adder  162 . The adder  162  adds branch-metric data bm 0011   k  received from the branch-metric computation section  131 - 5  to the path-metric data m 000   k-1  (or m 00   k-1 ) received from the selector  161  to produce a sum, and uses the sum as updated path-metric data m 011   k-1  (or m 01   k-1 ) of the present state.  
         [0233]     As described above, in the PR (1, x, 1) mode, the selector  191  employed in the path memory  123  selects one of values shifted from the memories  181  and  184  provided at a stage immediately preceding the present stage in accordance with a selection result sel 000  received from the ACS section  132 - 1 , outputting the selected value to the memory  181  provided at the present stage. In the PR (1, x, 1) mode, however, the selector  192  employed in the path memory  123  does not receive a selection result sel 001  from the ACS section  132 - 2  so that the selector  192  does not operate.  
         [0234]     As described above, in the PR (1, x, 1) mode, the selector  193  employed in the path memory  123  outputs a value shifted from the memory  181  provided at a stage immediately preceding the present stage to the memory  183  provided at the present stage. It is to be noted that, in the PR (1, x, 1) mode, the selector  193  employed in the path memory  123  does not receive a value from the memory  182  provided at a stage immediately preceding the present stage. By the same token, in the PR (1, x, 1) mode, the selector  194  employed in the path memory  123  outputs a value shifted from the memory  186  provided at a stage immediately preceding the present stage to the memory  184  provided at the present stage. It is to be noted that, in the PR (1, x, 1) mode, the selector  194  employed in the path memory  123  does not receive a value from the memory  185 .  
         [0235]     Much like the selector  192 , in the PR (1, x, 1) mode, however, the selector  195  does not receive a selection result sel 110  from the ACS section  132 - 5  so that the selector  195  does not operate. In the PR (1, x, 1) mode, however, the selector  196  selects one of values shifted from the memories  183  and  186  provided at a stage immediately preceding the present stage in accordance with a selection result sel 111  received from the ACS section  132 - 6 , outputting the selected value to the memory  186  provided at the present stage.  
         [0236]     If the selectors employed in the branch-metric computation circuit  121 , the ACS circuit  122  and the path memory  123  produce a determination result indicating that the PR (1, x, 1) mode has not been selected, that is, a determination result indicating that the PR (1, x, x, 1) mode has been selected, on the other hand, the flow of the processing goes on from the step S 12  to a step S 14  at which the operating mode is switched to the PR (1, x, x, 1) mode prior to a decoding process carried out thereafter. After the decoding process has been carried out in the PR (1, x, x, 1) mode, the processing represented by the flowchart shown in  FIG. 20  is ended.  
         [0237]     In the decoding process carried out at the step S 14  in the PR (1, x, x, 1) mode, to put it concretely, the selector  151  employed in the branch-metric computation circuit  121  selects the theoretical value c 0000  and outputs the theoretical value c 0000  to the subtractor  152 . The subtractor  152  subtracts the theoretical value c 0000  supplied by the selector  151  from an input signal z k  supplied by the sampling circuit  18  to find a difference (z k −c 0000 ) and supplies the difference (z k −c 0000 ) to the processing section  153 . The processing section  153  computes branch-metric data bm 0000   k  (=(z k −c 0000 )ˆ2), where symbol (z k −c 0000 )ˆ2 denotes the square of the difference (z k −c 0000 ), and supplies the branch-metric data bm 0000   k  to the ACS section  132 .  
         [0238]     In the PR (1, x, x, 1) mode, the selector  161  employed in the ACS section  122  selects path-metric data m 001   k-1 , and outputs the selected path-metric data m 001   k-1  to the adder  162 . The adder  162  adds branch-metric data bm 0011   k  received from the branch-metric computation section  131 - 5  to the path-metric data m 001   k-1  received from the selector  161  to produce a sum, and uses the sum as updated path-metric data m 011   k-1  of the present state.  
         [0239]     That is to say, in the PR (1, x, x, 1) mode, the selector  191  employed in the path memory  123  selects one of values shifted from the memories  181  and  184  provided at a stage immediately preceding the present stage in accordance with a selection result sel 000  received from the ACS section  132 - 1 , outputting the selected value to the memory  181  provided at the present stage. By the same token, in the PR (1, x, x, 1) mode, the selector  192  employed in the path memory  123  selects one of values shifted from the memories  181  and  184  provided at a stage immediately preceding the present stage in accordance with a selection result sel 001  received from the ACS section  132 - 2 , outputting the selected value to the memory  182  provided at the present stage.  
         [0240]     Likewise, in the PR (1, x, x, 1) mode, the selector  193  employed in the path memory  123  selects a value shifted from the memory  182  among values shifted from the memories  181  and  182  provided at a stage immediately preceding the present stage, outputting the selected value to the memory  183  provided at the present stage. In the same way, in the PR (1, x, x, 1) mode, the selector  194  employed in the path memory  123  selects a value shifted from the memory  185  among values shifted from the memories  185  and  186  provided at a stage immediately preceding the present stage, outputting the selected value to the memory  184  provided at the present stage.  
         [0241]     Similarly, in the PR (1, x, x, 1) mode, the selector  195  employed in the path memory  123  selects one of values shifted from the memories  183  and  186  provided at a stage immediately preceding the present stage in accordance with a selection result sel 1110  received from the ACS section  132 - 5 , outputting the selected value to the memory  185  provided at the present stage. By the same token, in the PR (1, x, x, 1) mode, the selector  196  employed in the path memory  123  selects one of values shifted from the memories  183  and  186  provided at a stage immediately preceding the present stage in accordance with a selection result sel 111  received from the ACS section  132 - 6 , outputting the selected value to the memory  186  provided at the present stage.  
         [0242]     As described above, the operating mode for the configurations of the branch-metric computation circuit  121 , the ACS circuit  122  and the path memory  123 , which are employed in the recording/reproduction apparatus shown in  FIG. 13 , can be switched from one to another with ease. Thus, it is not necessary to provide a circuit for each type of constraint length. As a result, the size of the circuit can be prevented from increasing.  
         [0243]     By referring to  FIGS. 21 and 22 , the following description explains a Viterbi decoding circuit capable of operating at a higher speed by carrying out processing of an amount corresponding to two clock cycles in just one clock cycle.  
         [0244]      FIGS. 21 and 22  are diagrams showing a Viterbi decoding circuit  112  capable of carrying out processing of an amount corresponding to two clock cycles in just one clock cycle. The configuration of the Viterbi decoding circuit  112  shown in  FIGS. 21 and 22  is basically the same as the configuration of the Viterbi decoding circuit  112  shown in  FIGS. 15 and 16  except that, in the case of the configuration of the Viterbi decoding circuit  112  shown in  FIGS. 21 and 22 , processing of an amount corresponding to two clock cycles is carried out in just one clock cycle while, in the case of the configuration of the Viterbi decoding circuit  112  shown in  FIGS. 15 and 16 , processing of an amount corresponding to two clock cycles is divided into two portions in two respective clock cycles, being carried out in the two clock cycles.  
         [0245]     That is to say, in the embodiment shown in  FIG. 21 , the configuration of the Viterbi decoding circuit  112  is based on a Viterbi encoding circuit working in an operating mode having the largest constraint length. An example of such a Viterbi encoding circuit is the Viterbi decoding circuit  19  shown in  FIGS. 11 and 12  as a Viterbi encoding circuit operating in the PR (1, x, x, 1) mode. When the Viterbi decoding circuit  112  receives a mode select signal from the system control section  111 , sections employed in the Viterbi decoding circuit  112  switch the operating mode to the PR (1, x, 1) mode or the PR (1, x, x, 1) mode in accordance with the received mode select signal and operates in the selected mode.  
         [0246]     To put it in detail,  FIGS. 21 and 22  are diagrams showing a branch-metric computation circuit  121  and an ACS circuit  122 , which are capable of carrying out processing of an amount corresponding to two clock cycles in just one clock cycle.  
         [0247]     The branch-metric computation circuit  121  includes as many branch-metric computation sections as state transitions as the branch-metric computation circuit  41  shown in  FIG. 11  does. In the case of the typical configuration shown in  FIG. 21 , the number of state transitions is 16. Thus, the branch-metric computation circuit  121  includes 16 branch-metric computation sections  231 - 1  to  231 - 16  each used for computing a branch-metric data bm corresponding to a state transition occurring over two time slots from a state immediately leading ahead of the state immediately preceding the present state. In the following description, the branch-metric computation sections  231 - 1  to  231 - 16  are each referred to simply as a branch-metric computation section  231  in case there is no need to distinguish them from each other. Thus, the branch-metric computation section  231  computes a branch-metric data bm corresponding to a state transition occurring over two time slots from a state immediately leading ahead of the state immediately preceding the present state. Then, the branch-metric computation section  231  outputs the computed branch-metric data bm to the ACS circuit  122 .  
         [0248]     When the branch-metric computation circuit  121  switches the operating mode to the PR (1, x, 1) mode, the branch-metric computation circuit  121  designates the branch-metric computation section for computing branch-metric data bmABCDE as a branch-metric circuit for computations of branch-metric data bmABCD where suffixes A, B, C, D and E each denotes the integer 1 or 0. At that time, the branch-metric computation section switches cABCD of the PR (1, x, x, 1) mode to cABC of the PR (1, x, 1) mode and cBCDE of the PR (1, x, x, 1) mode to cBCD of the PR (1, x, 1) for each state transition c and computes branch-metric data bmABCD (=(z k −cABC)ˆ2+(z k −cBCD)ˆ2) and instead of computing bmABCDE (=(z k −cABCD)ˆ2+(z k −cBCDE)ˆ2). It is to be noted that symbols cABCD and cBCDE assigned to a state transition c denote the theoretical value (the identification reference value) of the state transition c in the PR (1, x, x, 1) mode whereas symbols cABC and cBCD assigned to a state transition c denote the theoretical value (the identification reference value) of the state transition c in the PR (1, x, 1) mode.  
         [0249]     To put it concretely, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 1  computes branch-metric data bm 00000   k (=bm 0000   k-1 +bm 0000   k ) and outputs the branch-metric data bm 00000   k  to the ACS section  232 - 1  where branch-metric data bm 0000   k-1  is equal to the square (z k-1 −c 0000 )ˆ2 and branch-metric data bm 0000   k  is equal to the square (z k −c 0000 )ˆ2. In this case, notation bm 00000   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 1  computes branch-metric data bm 0000   k  (=bm 000   k-1 +bm 000   k ) where branch-metric data bm 000   k-1  is equal to the square (z k-1 −c 000 )ˆ2 and branch-metric data bm 000   k  is equal to the square (z k −c 000 )ˆ2 and outputs the branch-metric data bm 0000   k  also to the ACS section  232 - 1 . In this case, notation bm 0000   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0250]     In the following description, branch-metric data bmABCD k-1  is equal to the square (z k-1 −cABCD)ˆ2, branch-metric data bmBCDE k  is equal to the square (z k −cBCDE)ˆ2, branch-metric data bmABC k-1  is equal to the square (z k-1 −cABC)ˆ2, branch-metric data bmBCD k  is equal to the square (z k −cBCD)ˆ2 whereas suffixes A, B, C, D and E are each the integer 0 or 1 as described above. Thus, by the same token, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 2  computes branch-metric data bm 10000   k (=bm 1000   k-1 +bm 0000   k ) and outputs the branch-metric data bm 10000   k  also to the ACS section  232 - 1  where notation bm 10000   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 2  computes branch-metric data bm 1000   k  (=bm 100   k-1 +bm 000   k ) and outputs the branch-metric data bm 1000   k  also to the ACS section  232 - 1  where notation bm 1000   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0251]     In the same way, in the PR (1, x, x, 1) mode, branch-metric computation section  231 - 3  computes branch-metric data bm 11000   k  (=bm 1100   k-1 +bm 1000   k ) and outputs the branch-metric data bm 11000   k  also to the ACS section  232 - 1  where notation bm 11000   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 3  computes branch-metric data bm 1100   k  (=bm 110   k-1 +bm 100   k ) and outputs the branch-metric data bm 1100   k  also to the ACS section  232 - 1  where notation bm 1100   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0252]     By the same token, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 4  computes branch-metric data bm 00001   k  (=bm 0000   k-1 +bm 0001   k ) and outputs the branch-metric data bm 00001   k  to the ACS section  232 - 2  where notation bm 00001   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 4  does not operate. In the same way, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 5  computes branch-metric data bm 10001   k  (=bm 1000   k-1 +bm 0001   k ) and outputs the branch-metric data bm 10001   k  also to the ACS section  232 - 2  where notation bm 10001   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 5  does not operate. By the same token, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 6  computes branch-metric data bm 11001   k  (=bm 1100   k-1 +bm 1001   k ) and outputs the branch-metric data bm 11001   k  also to the ACS section  232 - 2  where notation bm 11001   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 6  does not operate.  
         [0253]     In the same say, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 7  computes branch-metric data bm 00011   k  (=bm 0001   k-1 +bm 0011   k ) and outputs the branch-metric data bm 00011   k  to the ACS section  232 - 3  where notation bm 00011   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 7  computes branch-metric data bm 0001   k  (=bm 000   k-1 +bm 001   k ) and outputs the branch-metric data bm 0001   k  also to the ACS section  232 - 3  where notation bm 0001   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0254]     By the same token, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 8  computes branch-metric data bm 10011   k  (=bm 1001   k-1 +bm 0001   k ) and outputs the branch-metric data bm 10011   k  also to the ACS section  232 - 3  where notation bm 10011   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 8  computes branch-metric data bm 1001   k  (=bm 100   k-1 +bm 001   k ) and outputs the branch-metric data bm 1001   k  also to the ACS section  232 - 3  where notation bm 1001   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0255]     In the same way, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 9  computes branch-metric data bm 01100   k  (=bm 0110   k-1 +bm 1100   k ) and outputs the branch-metric data bm 01100   k  to the ACS section  232 - 4  where notation bm 01100   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 9  computes branch-metric data bm 0110   k  (=bm 011   k-1 +bm 110   k ) and outputs the branch-metric data bm 0110   k  also to the ACS section  232 - 4  where notation bm 0110   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0256]     By the same token, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 10  computes branch-metric data bm 11100   k  (=bm 1110   k-1 +bm 1100   k ) and outputs the branch-metric data bm 11100   k  also to the ACS section  232 - 4  where notation bm 11100   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 10  computes branch-metric data bm 1110   k  (=bm 111   k-1 +bm 100   k ) and outputs the branch-metric data bm 1110   k  also to the ACS section  232 - 4  where notation bm 1110   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0257]     In the same way, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 11  computes branch-metric data bm 00110   k  (=bm 0011   k-1 +bm 0110   k ) and outputs the branch-metric data bm 00110   k  to the ACS section  232 - 5  where notation bm 00110   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 11  does not operate.  
         [0258]     In the same way, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 12  computes branch-metric data bm 01110   k  (=bm 0111   k-1 +bm 1110   k ) and outputs the branch-metric data bm 01110   k  also to the ACS section  232 - 5  where notation bm 01110   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 12  does not operate. By the same token, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 13  computes branch-metric data bm 11110   k  (=bm 1111   k-1 +bm 1110   k ) and outputs the branch-metric data bm 11110   k  also to the ACS section  232 - 5  where notation bm 11110   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 13  does not operate.  
         [0259]     By the same token, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 14  computes branch-metric data bm 00111   k  (=bm 0011   k-1 +bm 0111   k ) and outputs the branch-metric data bm 00111   k  to the ACS section  232 - 6  where notation bm 00111   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 8  computes branch-metric data bm 0011   k  (=bm 001   k-1 +bm 011   k ) and outputs the branch-metric data bm 0011   k  also to the ACS section  232 - 6  where notation bm 0011   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0260]     In the same way, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 15  computes branch-metric data bm 01111   k  (=bm 0111   k-1 +bm 1111   k ) and outputs the branch-metric data bm 01111   k  also to the ACS section  232 - 6  where notation bm 01111   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 15  computes branch-metric data bm 0111   k  (=bm 011   k-1 +bm 111   k ) and outputs the branch-metric data bm 0111   k  also to the ACS section  232 - 6  where notation bm 0111   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0261]     By the same token, in the PR (1, x, x, 1) mode, the branch-metric computation section  231 - 16  computes branch-metric data bm 11111   k  (=bm 1111   k-1 +bm 1111   k ) and outputs the branch-metric data bm 11111   k  also to the ACS section  232 - 6  where notation bm 11111   k  denotes branch-metric data corresponding to a state transition occurring over two time slots. In the PR (1, x, 1) mode, on the other hand, the branch-metric computation section  231 - 16  computes branch-metric data bm 1111   k  (=bm 111   k-1 +bm 111   k ) and outputs the branch-metric data bm 111   k  also to the ACS section  232 - 6  where notation bm 1111   k  denotes branch-metric data corresponding to a state transition occurring over two time slots.  
         [0262]     Much like the ACS circuit  42  shown in  FIG. 11 , the ACS circuit  122  adds branch-metric data received from the branch-metric computation circuit  121  to path-metric data of a state immediately leading ahead of the state immediately preceding the present state to produce a sum and uses the sum as updated path-metric data of the present state. As described earlier, path-metric data m of state S represents the likelihood of a history up to state S. The ACS circuit  122  includes as many ACS sections as states. In the case of the typical configuration shown in  FIG. 21 , the number of states is 6. Thus, the ACS circuit  122  includes ACS sections  232 - 1  to  232 - 6 . In the following description, the ACS sections  232 - 1  to  232 - 6  are each referred to simply as an ACS section  232  in case there is no need to distinguish them from each other.  
         [0263]     Much like the branch-metric computation circuit  121  shown in  FIG. 21 , when the operating mode is switched to the PR (1, x, 1) mode, the ACS circuit  122  also designates the ACS section  232  for computing path-metric data mABC as an ACS section for computations of path-metric data mAB.  
         [0264]     To put it concretely, in the PR (1, x, x, 1) mode, the ACS section  232 - 1  updates the path-metric data m 000   k , which is the likelihood of a history up to state S 000 . To be more specific, the ACS section  232 - 1  adds the path-metric data m 000   k-2  stored internally in the ACS section  232 - 1  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 00000   k  received from the branch-metric computation section  231 - 1  to produce a first sum. The ACS section  232 - 1  also adds the path-metric data m 100   k-2  stored internally in the ACS section  232 - 4  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 10000   k  received from the branch-metric computation section  231 - 2  to produce a second sum. In addition, the ACS section  232 - 1  also adds the path-metric data m 110   k-2  stored internally in the ACS section  232 - 5  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 11000   k  received from the branch-metric computation section  231 - 3  to produce a third sum. Then, the ACS section  232 - 1  compares the first, second and third sums with each other in order to select the smallest one to be used as updated path-metric data m 000   k  of the present state. Finally, the ACS section  232 - 1  outputs a selection result sel 000  to a memory included in the path memory  123  as a memory used for storing the value of state S 000 . The computations and the comparison are carried out by the ACS section  232 - 1  in accordance with Eq. (15) given before.  
         [0265]     In the PR (1, x, 1) mode, on the other hand, the ACS section  232 - 1  updates the path-metric data m 00   k , which is the likelihood of a history up to state S 00 . To be more specific, the ACS section  232 - 1  adds the path-metric data m 00   k-2  stored internally in the ACS section  232 - 1  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 0000   k  received from the branch-metric computation section  231 - 1  to produce a first sum. The ACS section  232 - 1  also adds the path-metric data m 10   k-2  stored internally in the ACS section  232 - 4  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 1000   k  received from the branch-metric computation section  231 - 2  to produce a second sum. In addition, the ACS section  232 - 1  also adds the updated path-metric data m 11   k-2  stored internally in the ACS section  232 - 6  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 1100   k  received from the branch-metric computation section  231 - 3  to produce a third sum. Then, the ACS section  232 - 1  compares the first, second and third sums with each other in order to select the smallest one to be used as updated path-metric data m 00   k  of the present state. Finally, the ACS section  232 - 1  outputs a selection result sel 000  to a memory used for storing the value of state S 00 . The computations and the comparison are carried out by the ACS section  232 - 1  in accordance with Eq. (11) given before.  
         [0266]     It is to be noted that, as described above, in the PR (1, x, 1) mode, the ACS section  232 - 1  updates the path-metric data m 00   k  on the basis of path-metric data m 11   k-2  stored internally in the ACS section  232 - 6  (also for updating path-metric data m 111   k  in the PR (1, x, x, 1) mode) in place of the path-metric data m 110   k-2  that should be used for updating path-metric data m 00   k . This is because the ACS section  232 - 2  for finding path-metric data m 110   k-2  does not operate in the PR (1, x, 1) mode as will be described below. The path-metric data m 110   k-2  is path metric data of a state immediately leading ahead of the state immediately preceding the present state having path-metric data m 000   k  corresponding to the path-metric data m 00   k  updated by the ACS section  232 - 1 .  
         [0267]     By the same token, in the PR (1, x, x, 1) mode, the ACS section  232 - 2  updates the path-metric data m 001   k , which is the likelihood of a history up to state S 001 . To be more specific, the ACS section  232 - 2  adds the path-metric data m 000   k-2  stored internally in the ACS section  232 - 1  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 00001   k  received from the branch-metric computation section  231 - 4  to produce a first sum. The ACS section  232 - 2  also adds the path-metric data m 100   k-2  stored internally in the ACS section  232 - 4  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 10001   k  received from the branch-metric computation section  231 - 5  to produce a second sum. In addition, the ACS section  232 - 2  also adds the path-metric data m 110   k-2  stored internally in the ACS section  232 - 5  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 11001   k  received from the branch-metric computation section  231 - 6  to produce a third sum. Then, the ACS section  232 - 2  compares the first, second and third sums with each other in order to select the smallest one to be used as updated path-metric data m 001   k  of the present state. Finally, the ACS section  232 - 2  outputs a selection result sel 001  to a memory used for storing the value of state S 001 . The computations and the comparison are carried out by the ACS section  232 - 2  in accordance with Eq. (16) given before.  
         [0268]     In the PR (1, x, 1) mode, on the other hand, the ACS section  232 - 2  does not operate.  
         [0269]     In the PR (1, x, x, 1) mode, the ACS section  232 - 3  updates the path-metric data m 011   k , which is the likelihood of a history up to state S 011 . To be more specific, the ACS section  232 - 3  adds the path-metric data m 000   k-2  stored internally in the ACS section  232 - 1  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 00011   k  received from the branch-metric computation section  231 - 7  to produce a first sum. The ACS section  232 - 3  also adds the path-metric data m 100   k-2  stored internally in the ACS section  232 - 4  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 10011   k  received from the branch-metric computation section  231 - 8  to produce a second sum. Then, the ACS section  232 - 3  compares the first and second sums with each other in order to select the smaller one to be used as updated path-metric data m 011   k  of the present state. Finally, the ACS section  232 - 3  outputs a selection result sel 011  to a memory used for storing the value of state S 011 . The computations and the comparison are carried out by the ACS section  232 - 3  in accordance with Eq. (17) given before.  
         [0270]     In the PR (1, x, 1) mode, on the other hand, the ACS section  232 - 3  updates the path-metric data m 01   k , which is the likelihood of a history up to state S 01 . To be more specific, the ACS section  232 - 3  adds the path-metric data m 00   k-2  stored internally in the ACS section  232 - 1  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 0001   k  received from the branch-metric computation section  231 - 7  to produce a first sum. The ACS section  232 - 3  also adds the path-metric data m 10   k-2  stored internally in the ACS section  232 - 4  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 1001   k  received from the branch-metric computation section  231 - 8  to produce a second sum. Then, the ACS section  232 - 3  compares the first and second sums with each other in order to select the smaller one to be used as updated path-metric data m 01   k  of the present state. Finally, the ACS section  232 - 3  outputs a selection result sel 011  to a memory used for storing the value of state S 01 . The computations and the comparison are carried out by the ACS section  232 - 3  in accordance with Eq. (12) given before.  
         [0271]     By the same token, in the PR (1, x, x, 1) mode, the ACS section  232 - 4  updates the path-metric data m 100   k , which is the likelihood of a history up to state S 100 . To be more specific, the ACS section  232 - 4  adds the path-metric data m 111   k-2  stored internally in the ACS section  232 - 6  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 11100   k  received from the branch-metric computation section  231 - 10  to produce a first sum. The ACS section  232 - 4  also adds the path-metric data m 011   k-2  stored internally in the ACS section  232 - 3  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 01100   k  received from the branch-metric computation section  231 - 9  to produce a second sum. Then, the ACS section  232 - 4  compares the first and second sums with each other in order to select the smaller one to be used as updated path-metric data m 100   k  of the present state. Finally, the ACS section  232 - 4  outputs a selection result sel 100  to a memory for storing the value of state S 100 . The computations and the comparison are carried out by the ACS section  232 - 4  in accordance with by Eq. (18) given before.  
         [0272]     In the PR (1, x, 1) mode, on the other hand, the ACS section  232 - 4  updates the path-metric data m 10   k , which is the likelihood of a history up to state S 10 . To be more specific, the ACS section  232 - 4  adds the path-metric data m 11   k-2  stored internally in the ACS section  232 - 6  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 1110   k  received from the branch-metric computation section  231 - 10  to produce a first sum. The ACS section  232 - 4  also adds the path-metric data m 01   k-2  stored internally in the ACS section  232 - 3  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 0110   k  received from the branch-metric computation section  231 - 9  to produce a second sum. Then, the ACS section  232 - 4  compares the first and second sums with each other in order to select the smaller one to be used as updated path-metric data m 10   k  of the present state. Finally, the ACS section  232 - 4  outputs a selection result sel 100  to a memory for storing the value of state S 10 . The computations and the comparison are carried out by the ACS section  232 - 4  in accordance with Eq. (13) given before.  
         [0273]     By the same token, in the PR (1, x, x, 1) mode, the ACS section  232 - 5  updates the path-metric data m 110   k , which is the likelihood of a history up to state S 110 . To be more specific, the ACS section  232 - 5  adds the path-metric data m 111   k-2  stored internally in the ACS section  232 - 6  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 11110   k  received from the branch-metric computation section  231 - 13  to produce a first sum. The ACS section  232 - 5  also adds the path-metric data m 011   k-2  stored internally in the ACS section  232 - 3  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 01110   k  received from the branch-metric computation section  231 - 12  to produce a second sum. In addition, the ACS section  232 - 5  also adds the path-metric data m 001   k-2  stored internally in the ACS section  232 - 3  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 00110   k  received from the branch-metric computation section  231 - 11  to produce a third sum. Then, the ACS section  232 - 5  compares the first, second and third sums with each other in order to select the smallest one to be used as updated path-metric data m 110   k  of the present state. Finally, the ACS section  232 - 5  outputs a selection result sel 110  to a memory used for storing the value of state S 110 . The computations and the comparison are carried out by the ACS section  232 - 5  in accordance with Eq. (19) given before.  
         [0274]     In the PR (1, x, 1) mode, on the other hand, the ACS section  232 - 5  does not operate.  
         [0275]     In the PR (1, x, x, 1) mode, the ACS section  232 - 6  updates the path-metric data m 111   k , which is the likelihood of a history up to state S 111 . To be more specific, the ACS section  232 - 6  adds the path-metric data m 111   k-2  stored internally in the ACS section  232 - 6  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 11111   k  received from the branch-metric computation section  231 - 16  to produce a first sum. The ACS section  232 - 6  also adds the path-metric data m 011   k-2  stored internally in the ACS section  232 - 3  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 01111   k  received from the branch-metric computation section  231 - 15  to produce a second sum. In addition, the ACS section  232 - 6  also adds the path-metric data m 001   k-2  stored internally in the ACS section  232 - 3  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 00111   k  received from the branch-metric computation section  231 - 14  to produce a third sum. Then, the ACS section  232 - 6  compares the first, second and third sums with each other in order to select the smallest one to be used as updated path-metric data m 111   k  of the present state. Finally, the ACS section  232 - 6  outputs a selection result sel 111  to a memory used for storing the value of state S 111 . The computations and the comparison are carried out by the ACS section  232 - 6  in accordance with Eq. (20) given before.  
         [0276]     In the PR (1, x, 1) mode, on the other hand, the ACS section  232 - 6  updates the path-metric data m 11   k , which is the likelihood of a history up to state S 11 . To be more specific, the ACS section  232 - 6  adds the path-metric data m 11   k-2  stored internally in the ACS section  232 - 6  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 1111   k  received from the branch-metric computation section  231 - 16  to produce a first sum. The ACS section  232 - 6  also adds the path-metric data m 01   k-2  stored internally in the ACS section  232 - 3  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 0111   k  received from the branch-metric computation section  231 - 15  to produce a second sum. In addition, the ACS section  232 - 6  also adds the updated path-metric data m 00   k-2  stored internally in the ACS section  232 - 1  as the path-metric data of a state immediately leading ahead of the state immediately preceding the present state to the branch-metric data bm 0011   k  received from the branch-metric computation section  231 - 14  to produce a third sum. Then, the ACS section  232 - 6  compares the first, second and third sums with each other in order to select the smallest one to be used as updated path-metric data m 11   k  of the present state. Finally, the ACS section  232 - 6  outputs a selection result sel 111  to a memory used for storing the value of state S 11 . The computations and the comparison are carried out by the ACS section  232 - 6  in accordance with Eq. (14) given before.  
         [0277]     It is to be noted that, as described above, in the PR (1, x, 1) mode, the ACS section  232 - 6  updates the path-metric data m 11   k  on the basis of path-metric data m 00   k-2  stored internally in the ACS section  232 - 1  (also used for finding path-metric data m 000   k  in the PR (1, x, x, 1) mode) in place of path-metric data m 001   k-2 , which should be used for updating path-metric data m 11   k , in the same way as the ACS section  232 - 1  updates the path-metric data m 00   k  as described above. This is because the ACS section  232 - 5  for finding path-metric data m 001   k-2  does not operate in the PR (1, x, 1) mode as described above. The path-metric data m 001   k-2  is path metric data of a state immediately leading ahead of the state immediately preceding the present state having path-metric data m 111   k  corresponding to the path-metric data m 11   k  updated by the ACS section  232 - 6 .  
         [0278]     By using the path-metric data m 11   k-2  and the path-metric data m 00   k-2  as substitutes as described above, a trellis spread throughout the path memory  123  shown in  FIG. 22  as the trellis of state transitions occurring in the PR (1, x, x, 1) mode becomes compatible with the trellis spread throughout the path memory  43  shown in  FIG. 10  as the trellis of state transitions occurring in the PR (1, x, 1) mode so that the ACS circuit  122  is capable of operating in both the PR (1, x, x, 1) mode and the PR (1, x, 1) mode.  
         [0279]      FIG. 22  is a diagram showing a typical configuration of the path memory  123  for carrying out processing of an amount corresponding to two time slots in just one time slot.