Patent Publication Number: US-8542746-B2

Title: Decoding control apparatus which decodes the fewest pictures (although not displayed) prior to an edit point

Description:
This is a continuation of application Ser. No. 11/729,026, filed Mar. 28, 2007 now U.S. Pat. No. 8,098,738, which is a division of application Ser. No. 10/650,336, filed Aug. 28, 2003 now U.S. Pat. No. 7,263,129, which is entitled (under 35 USC 119) to the priority filing date of Japanese application(s) 2002-250137, filed on Aug. 29, 2002, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to decoding control for enhancing the freedom of a process to edit a moving picture. 
     An MPEG (Moving Picture Expert Group) technique is known as a typical method for compressing a moving picture, that is, for reducing the amount of data representing the moving picture, by carrying out a predictive encoding process on any specific one of frames, which compose the moving picture, with reference to frames other than the specific one. 
     In accordance with the MPEG technique, data is subjected to quantization processing in an encoding process and encoded data is subjected to inverse-quantization processing in a decoding process. Thus, if the encoding and decoding processes are carried out repeatedly, the quality of a picture obtained as a result of the last decoding process is poor. 
     In order to solve this problem, in an operation to edit a moving picture, which has been obtained as a result of an MPEG encoding process, the so-called edit points are set on an MPEG stream (or encoded data), which serves as an editing raw material representing the moving picture, without decoding the MPEG stream. The edit points are IN points and OUT points. Then, a list of edit points set on the MPEG stream is made. The list is referred to hereafter as an edit-point list. In an operation to reproduce a result of the editing operation, the MPEG stream is decoded in accordance with edit points shown on the edit-point list. 
     The aforementioned MPEG stream comprises GOPs each including a plurality of pictures. Symbol GOP (m) denotes an mth GOP as counted from the beginning of the MPEG stream. Assume for example that a start sequence of GOPs beginning with GOPi not shown in the figure and ending with GOP (m) on an MPEG stream  201  shown in  FIG. 1  has been reproduced. Then, GOP (n) located after GOP (m) is reproduced. Finally, another sequence of GOPs beginning with GOP (m+2) located before GOP (n) and ending with GOPj not shown in the figure is reproduced. The operations to reproduce the start sequence, GOP (n) and the other sequence is referred to as reproduction processing based on the so-called cut editing process. In this cut editing process, an edit-point list  202  shown in  FIG. 1  is made. It is to be noted that the edit-point list  202  is shown only in a conceptual manner focusing on edit points. Details of the edit-point list  202  will be explained later. 
     In the edit-point list  202  shown in  FIG. 1 , an edit point T i  represents the start position of GOPi, which is reproduced as the first GOP. The edit point T 1  represents the end position of GOP (m) or the start position of GOP (m+1). By the same token, an edit point T 2  represents the end position of GOP (m+1) or the start position of GOP (m+2). In the same way, an edit point T 3  represents the end position of GOP (n−1) or the start position of GOP (n). Similarly, an edit point T 4  represents the end position of GOP (n) or the start position of GOP (n+1). In addition, an edit point T j  represents the start position of GOPj, which is reproduced as the last GOP. It is to be noted that the edit points T 1  and T 4  are each an OUT point while the edit points T 2  and T 3  are each an IN point. 
     Notation ‘T i →T 1 ’ on the first line of the edit-point list  202  shown in  FIG. 1  indicates that the MPEG stream  201  is to be reproduced from the position (time code) T i  on the MPEG stream  201  to the time code T 1 . By the same token, notation ‘T 3 →T 4 ’ on the second line indicates that the MPEG stream  201  is to be reproduced the time code T 3  to the time code T 4 . In the same way, notation ‘T 2 →T j ’ on the third line indicates that the MPEG stream  201  is to be reproduced the time code T 2  to the time code T j . 
     Thus, in accordance with the edit-point list  202  shown in  FIG. 1 , the start sequence of GOPs beginning with GOPi and ending with GOP (m) on an MPEG stream  201  shown in  FIG. 1  is first reproduced. Then, GOP (n) located after GOP (m+2) and GOPj is reproduced. Finally, the other sequence of GOPs beginning with GOP (m+2) and ending with GOPj is reproduced. 
     As described above, the edit-point list  202  is first made in an editing operation and the MPEG stream  201  is then reproduced in a reproduction operation. Thus, by carrying out an editing operation without decoding the MPEG stream  201 , it is possible to implement a desired editing operation and, hence, obtain a desired result of editing. 
       FIG. 1  is a diagram showing an operation to edit only one MPEG stream as a raw editing material. It is to be noted, however, that a plurality of MPEG streams can also be used as a raw editing material to be edited in the same way as that shown in  FIG. 1 . 
     By the way, in an operation to reproduce an MPEG stream, which is obtained as a result to edit a raw MPEG stream by making an edit-point list as described above, it is necessary to decode the resulting MPEG stream, that is, to put the resulting MPEG stream in an MPEG decoding process. Assume that an MPEG decoder for decoding the MPEG stream has a decoding-process speed by one. In this case, by using only a unit of MPEG decoder having such a speed, it will be difficult to seamlessly reproduce an MPEG stream, which is obtained as a result of a process to edit a raw MPEG stream by making an edit-point list as described above. 
     In accordance with the MPEG technique, there are provided three picture types, namely, an Intra (I) picture, a Predictive (P) picture and a Bidirectionally predictive (B) picture. An I picture is a picture to be subjected to an intra encoding process without referencing other pictures. A P picture is a picture to be subjected to an intra encoding process or a predictive encoding process, which is an encoding process carried out by referencing an I picture or another P picture and using a predicted picture generated from the referenced picture. The referenced picture is a picture displayed at a time preceding the P picture being processed. A B picture is a picture to be subjected to an intra encoding process or a bidirectionally predictive encoding process, which is an encoding process carried out by referencing an I picture or another P picture and using a predicted picture generated from the referenced picture. The referenced picture in the bidirectionally predictive encoding process is a picture displayed at a time preceding or succeeding the P picture being processed. It is to be noted that, while an I or P picture can be used as a picture to be referenced in a predictive encoding process or a bidirectionally predictive encoding process, a B picture is never used as a picture to be referenced in any encoding process. 
     As an example, consider a long GOP with a structure consisting of 15 pictures (or 15 frames) wherein each of I and P pictures is placed as the last one of three consecutive pictures including two other B pictures. 
     Let alphabetical characters I, P and B denote respectively the I, P and B picture types of the pictures composing a GOP. In this case, an example of a GOP is represented by a list as follows: 
     B 1 , B 2 , I 3 , B 4 , B 5 , P 6 , B 7 , B 8 , P 9 , B 10 , B 11 , P 12 , B 13 , B 14 , P 15   
     where a numerical subscript appended to each of the alphabetical characters I, P and B denotes an order number of an order along the time axis. 
     As described above, a B picture is a picture encoded by referencing not only a picture displayed at a time preceding the B picture, but also a picture displayed at a time succeeding the B picture. Thus, a B picture cannot be decoded unless the picture used as a referenced picture to be displayed at a time succeeding the B picture is decoded before the B picture in a decoding operation. 
     In accordance with the MPEG technique, a picture used as a referenced picture in an operation to encode a B picture is encoded before the B picture. By the same token, a picture used as a referenced picture in an operation to decode B picture is decoded before the B picture. In the case of the aforementioned list representing the GOP consisting of pictures ranging from B 1  to P 15 , the pictures are encoded and decoded in the following order: 
     I 3 , B 1 , B 2 , P 6 , B 4 , B 5 , P 9 , B 7 , B 8 , P 12 , B 10 , B 11 , P 15 , B 13 , B 14 . 
     To put it in detail, pictures B 1  and B 2  are each encoded and decoded by referencing picture P 15  displayed at a preceding time and picture I 3  displayed at a succeeding time. Picture P 15  is a picture pertaining to the immediately preceding GOP while picture I 3  is a picture pertaining to the same GOP. 
     Picture I 3  is encoded and decoded without referencing another picture. 
     Pictures B 4  and B 5  are each encoded and decoded by referencing picture I 3  displayed at a preceding time and picture P 6  displayed at a succeeding time. Pictures I 3  and P 6  are each a picture pertaining to the same GOP. 
     Picture P 6  is encoded and decoded by referencing picture I 3  displayed at a preceding time. Picture I 3  is a picture pertaining to the same GOP. 
     Pictures B 7  and B 8  are each encoded and decoded by referencing picture P 6  displayed at a preceding time and picture P 9  displayed at a succeeding time. Pictures P 6  and P 9  are each a picture pertaining to the same GOP. 
     Picture P 9  is encoded and decoded by referencing picture P 6  displayed at a preceding time. Picture P 6  is a picture pertaining to the same GOP. 
     Pictures B 10  and B 11  are each encoded and decoded by referencing picture P 9  displayed at a preceding time and picture P 12  displayed at a succeeding time. Pictures P 9  and P 12  are each a picture pertaining to the same GOP. 
     Picture P 12  is encoded and decoded by referencing picture P 9  displayed at a preceding time. Picture P 9  is a picture pertaining to the same GOP. 
     Pictures B 13  and B 14  are each encoded and decoded by referencing picture P 12  displayed at a preceding time and picture P 15  displayed at a succeeding time. Pictures P 12  and P 15  are each a picture pertaining to the same GOP. 
     Picture P 15  is encoded and decoded by referencing picture P 12  displayed at a preceding time. Picture P 12  is a picture pertaining to the same GOP. 
     As described above, pictures B 1  and B 2  of any GOP (n) are encoded and decoded by referencing an I or P picture displayed at a succeeding time and an I or P picture displayed at a preceding time. In this case, the picture displayed at the succeeding time is picture I 3  pertaining to the same GOP (n) but the picture displayed at the preceding time is picture P 15  pertaining to GOP (n−1) immediately preceding GOP (n). 
     Assume that the current edit point is set at the head of GOP (n). In this case, in order to carry out a reproduction process starting from the head of GOP (n), GOP (n) cannot be decoded unless GOP (n−1) immediately preceding GOP (n) is decoded first as is obvious from a picture-decoding order  203  shown in  FIG. 2 . 
     To put it concretely, in order to decode pictures B 1  and B 2  pertaining to GOP (n), it is necessary to reference picture P 15  of GOP (n−1) immediately preceding GOP (n). In accordance with the conventional technology, an MPEG decoder is controlled in GOP units so that GOP (n) cannot be decoded unless GOP (n−1) immediately preceding GOP (n) is decoded first. 
     In accordance with the conventional technology, only the pictures composing GOP (n) are displayed without displaying the pictures composing GOP (n−1) as is obvious from a picture-decoding order  204  shown in  FIG. 2  in spite of the fact that both GOP (n) and GOP (n−1) are decoded. 
     As described above, the MPEG decoder needs to decode GOP (n−1), which will not be displayed, in addition to GOP (n). Thus, in order to obtain editing results obtained earlier by referring to  FIG. 1 , GOP (n) is decoded after GOP (m) separated away from GOP (n) is decoded but, in order to decode GOP (n), it is necessary to decode GOP (n−1) immediately preceding GOP (n). Accordingly, by using only a unit of MPEG decoder having a 1-time decoding-process speed, there is no time margin to decode GOP (n−1) after decoding GOP (m) but before decoding GOP (n). 
     As a result, by using only a unit of MPEG decoder having a 1-time decoding-process speed, on the basis of an edit-point list, it is impossible to reproduce results of editing an MPEG screen. 
     Japanese Patent Laid-open No. 2002-101379 discloses a method to reproduce results of editing an MPEG screen by using two MPEG decoders on the basis of an edit-point list. 
     In accordance with the method disclosed in Japanese Patent Laid-open No. 2002-101379, a duration required between a start edit point and an end edit point must be at least the length of a GOP. By using two MPEG decoders to alternately decode an MPEG stream, which has been edited under such a condition, it is possible to reproduce results of editing the MPEG screen. The start and end edit points can each be an IN or OUT point. If the start edit point is an IN point, the end edit point is an OUT point. If the start edit point is an OUT point, on the other hand, the end edit point is an IN point. In the following description, the minimum duration is appropriately referred to as a shortest cut duration or a margin. 
     An MPEG stream  211  shown in  FIG. 3  is the same as the MPEG stream  201  shown in  FIG. 1 . Assume that a start sequence of GOPs beginning with GOPi not shown in the figure and ending with GOP (m) on an MPEG stream  211  is to be reproduced. Then, GOP (n) located after GOP (m) is to be reproduced. Finally, another sequence of GOPs beginning with GOP (m+2) located before GOP (n) and ending with GOPj not shown in the figure is to be reproduced. In order to reproduce these GOPs, a cut editing process is carried out to set edit points T 1 , T 2 , T 3  and T 4 . 
     In this case, in an operation to reproduce these results of the editing, the start sequence of GOPs ending with picture P 15  at the tail of GOP (m) is first displayed to be followed by pictures B 1  to P 15  of GOP (n) before displaying the other sequence of GOPs beginning with picture B 1  at the head of GOP (m+2) as is obvious from a picture-displaying order  212  shown in  FIG. 3 . At the tail of GOP (m), the edit point T 1  has been set as an OUT point. At pictures B 1  and P 15  of GOP (n), the edit points T 3  and T 4  have been set as IN and OUT points respectively. At the head of GOP (m+2), the edit point T 2  has been set as an IN point. 
     In order to obtain a picture-displaying order  212  shown in  FIG. 3 , in accordance with the method disclosed in Japanese Patent Laid-open No. 2002-101379, a specific one of 2 MPEG decoders # 1  and # 2 , is used to decode all pictures ending with GOP (m) as shown in a picture-decoding order  213  of  FIG. 3 . For example, the specific MPEG decoder is MPEG decoder # 1 . It is to be noted that, in the process to decode GOP (m), the processing to decode picture B 14  to be decoded last among the pictures composing GOP (m) must be completed by a time t 2 , at which picture B 14  is to be displayed. Thus, the process to decode GOP (m) must be started before a time leading ahead of the time t 2  by a period of time it takes to display pictures composing a GOP. In the picture-decoding order  213  shown in  FIG. 3 , a time t 1  is determined to be the time to start the process to decode GOP (m). 
     The edit point T 1  is reached after decoder # 1  has decoded GOP (m) to be displayed. The process then is continued to processing to decode and display GOP (n). The processing to decode and display GOP (n) must start at the edit point T 3  and end at the edit point T 4 . 
     In order to decode GOP (n), it is necessary to decode GOP (n−1) immediately preceding GOP (n) as described earlier. In the process to decode GOP (n), the processing to decode picture B 14  to be decoded last among the pictures composing GOP (n) must be completed by a time t 3 , at which picture B 14  is to be displayed. Thus, the process to decode GOP (n) must be started before a time leading ahead of the time t 3  by a period of time it takes to display pictures composing a GOP. In a picture-decoding order  214  shown in  FIG. 3 , a time t 2  is determined to be the time to start the process to decode GOP (n). By the same token, the process to decode GOP (n−1) immediately preceding GOP (n) must be started before a time leading ahead of the time t 2  by a period of time it takes to display pictures composing a GOP in the picture-decoding order  214  shown in  FIG. 3 , a time t 1  is determined to be the time to start the process to decode GOP (n−1). 
     For the reason described above, MPEG decoder # 2  starts decoding GOP (n−1) at the time t 1  and continues the process by decoding GOP (n) as shown in the picture-decoding order  214  of  FIG. 3 . MPEG decoder # 2  is the other one of two decoders # 1  and # 2 . 
     In the mean time, MPEG decoder # 1  should naturally terminate the decoding process at the end of the processing to decode GOP (m). After GOP (n) is decoded by decoder # 2  and displayed, however, the edit point T 4  is reached. Then, MPEG decoder # 1  must decode the other sequence of GOPs beginning with GOP (m+2) at the edit point T 2  for a display. 
     In order to decode GOP (m+2), it is necessary to decode GOP (m+1) leading ahead of GOP (m+2) by a GOP as described earlier. In the process to decode GOP (m+2), the processing to decode picture B 14  to be decoded last among the pictures composing GOP (m+2) must be completed by a time t 4 , at which picture B 14  is to be displayed, as shown in the picture-decoding order  213  of  FIG. 3 . Thus, the process to decode GOP (m+2) must be started before a time leading ahead of the time t 4  by a period of time it takes to display pictures composing a GOP. In the picture-decoding order  213  shown in  FIG. 3 , the time t 3  is determined to be the time to start the process to decode GOP (m+2). By the same token, the process to decode GOP (m+1) leading ahead of GOP (m+2) by a GOP must be started before a time leading ahead of the time t 3  by a period of time it takes to display pictures composing a GOP. In the picture-decoding order  213  shown in  FIG. 3 , the time t 2  is determined to be the time to start the process to decode GOP (m+1). 
     The time t 2  is a time at which MPEG decoder # 1  ends the process to decode GOP (m). Thus, as soon as the process to decode GOP (m) is ended, the process to decode GOP (m+1) can be started immediately. 
     As described above, if the shortest cut duration is set at a period of time it takes to display a GOP, it is possible to reproduce results of editing an MPEG stream by using two MPEG recorders for carrying out the so-called alternate decoding process of GOPs between an edit point and the next edit point. 
     In accordance with the method disclosed in Japanese Patent Laid-open No. 2002-101379, however, the shortest cut duration, which is a minimum duration required between a start edit point and an end edit point, must be the period of time it takes to display a GOP, raising a problem of restricted freedom of editing. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to make restrictions on the shortest cut duration lenient in a process to seamlessly reproduce a result of editing the stream in order to enhance the freedom of editing in carrying out a process of editing the stream. 
     In order to achieve the object described above, in accordance with an aspect of the present invention, there is provided a decoding control apparatus, which is used for controlling a process to decode encoded data obtained as a result of a predictive encoding process and is provided with: a picture detector for determining one or fewest possible pictures, which must be decoded first before decoding a picture to be displayed after an edit point set in the encoded data but are not to be displayed; a start-point finder for finding a start point representing a timing to start a process to decode the one or fewest possible pictures determined by the picture detector even though the one or fewest possible pictures are not to be displayed; a decoding controller for controlling processes, which are carried out by a plurality of decoders for decoding the encoded data in order to decode the encoded data, in picture units on the basis of the start point; and a selector for selecting one of pictures, which are obtained as results of the processes carried out by the decoders, on the basis of the edit point and for outputting the selected picture. 
     In accordance with another aspect of the present invention, there is provided a decoding control method, which is used for controlling a process to decode encoded data obtained as a result of a predictive encoding process and is provided with the steps of: determining one or fewest possible pictures, which must be decoded first before decoding a picture to be displayed after an edit point set in the encoded data but are not to be displayed; finding a start point representing a timing to start a process to decode the one or fewest possible pictures even though the one or fewest possible pictures are not to be displayed; controlling processes, which are carried out by a plurality of decoders for decoding the encoded data in order to decode the encoded data, in picture units, on the basis of the start point; and selecting one of pictures, which are obtained as results of the processes carried out by the decoders, on the basis of the edit point and outputting the selected picture. 
     In accordance with a further aspect of the present invention, there is provided a program for driving a computer to carry out a decoding control process to control a process of decoding encoded data obtained as a result of a predictive encoding process wherein the decoding control process includes the steps of: determining one or fewest possible pictures, which must be decoded first before decoding a picture to be displayed after an edit point set in the encoded data but are not to be displayed; finding a start point representing a timing to start a process to decode the one or fewest possible pictures even though the one or fewest possible pictures are not to be displayed; controlling processes, which are carried out by a plurality of decoders for decoding the encoded data in order to decode the encoded data, in picture units; and selecting one of pictures, which are obtained as results of the processes carried out by the decoders, on the basis of the edit point and outputting the selected picture. 
     In accordance with a still further aspect of the present invention, there is provided a information-processing apparatus including: a picture detector for determining one or fewest possible pictures, which are not to be displayed but must be decoded first before decoding a picture to be displayed after an edit point described on an edit-point list, that is, a list of the edit point set in encoded data obtained as a result of a predictive encoding process; a start-point finder for finding a start point representing a timing to start a process to decode the one or fewest possible pictures determined by the picture detector even though the one or fewest possible pictures are not to be displayed; and a decoding control list maker for making a decoding control list describing at least the edit point and the start point as decoding control information for controlling processes carried out by a plurality of decoders for decoding the encoded data in order to decode the encoded data. 
     In accordance with a still further aspect of the present invention, there is provided a information-processing method including the steps of: determining one or fewest possible pictures, which are not to be displayed but must be decoded first before decoding a picture to be displayed after an edit point described on an edit-point list, that is, a list of the edit point set in encoded data obtained as a result of a predictive encoding process; finding a start point each representing a timing to start a process to decode the one or fewest possible pictures determined by the picture detector even though the one or fewest possible pictures are not to be displayed; and making a decoding control list describing at least the edit point and the start point as decoding control information for controlling processes carried out by a plurality of decoders for decoding the encoded data in order to decode the encoded data. 
     In accordance with a still further aspect of the present invention, there is provided a program, which is used for driving a computer to carry out a predetermined process and is provided with the steps of: determining one or fewest possible pictures, which are not to be displayed but must be decoded first before decoding a picture to be displayed after an edit point described on an edit-point list, that is, a list of the edit point set in encoded data obtained as a result of a predictive encoding process; finding a start point each representing a timing to start a process to decode the one or fewest possible pictures determined by the picture detector even though the one or fewest possible pictures are not to be displayed; and making a decoding control list describing at least the edit point and the start point as decoding control information for controlling processes carried out by a plurality of decoders for decoding the encoded data in order to decode the encoded data. 
     In accordance with a still further aspect of the present invention, there is provided a decoding control apparatus, which is used for controlling a process to decode encoded data obtained as a result of a predictive encoding process and is provided with: a decoding controller for controlling processes carried out by a plurality of decoders for decoding the encoded data in order to decode the encoded data in picture units on the basis of a decoding control list describing at least an edit point set in the encoded data and a start point representing a timing to start a process to decode fewest possible pictures, which are not to be displayed but must be decoded first before decoding a picture to be displayed after the edit points, as decoding control information for controlling the processes carried out by the decoders for decoding the encoded data in order to decode the encoded data; and a selector for selecting one of pictures, which are obtained as results of the processes carried out by the decoders, on the basis of the decoding control list and for outputting the selected picture. 
     In accordance with a still further aspect of the present invention, there is provided a decoding control method, which is used for controlling a process to decode encoded data obtained as a result of a predictive encoding process and is provided with the steps of: controlling processes carried out by a plurality of decoders for decoding the encoded data in order to decode the encoded data in picture units on the basis of a decoding control list describing at least an edit point set in the encoded data and a start point representing a timing to start a process to decode fewest possible pictures, which are not to be displayed but must be decoded first before decoding a picture to be displayed after the edit point as decoding control information for controlling the processes carried out by the decoders for decoding the encoded data in order to decode the encoded data; and selecting one of pictures, which are obtained as results of the processes carried out by the decoders, on the basis of the decoding control list and outputting the selected picture. 
     In accordance with a still further aspect of the present invention, there is provided a program for driving a computer to carry out a decoding control process of decoding encoded data obtained as a result of a predictive encoding process wherein the decoding control process includes the steps of: controlling processes carried out by a plurality of decoders for decoding the encoded data in order to decode the encoded data in picture units on the basis of a decoding control list describing at least an edit point set in the encoded data and start point representing a timing to start a process to decode fewest possible pictures, which are not to be displayed but must be decoded first before decoding a picture to be displayed after the edit point as decoding control information for controlling the processes carried out by the decoders for decoding the encoded data in order to decode the encoded data; and selecting one of pictures, which are obtained as results of the processes carried out by the decoders, on the basis of the decoding control list and outputting the selected picture. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram to be referred to in describing a process to reproduce an MPEG stream on the basis of an edit-point list; 
         FIG. 2  is an explanatory diagram to be referred to in describing a process to decode and then display an MPEG stream; 
         FIG. 3  is an explanatory diagram to be referred to in describing a study case in which it is possible to seamlessly reproduce an MPEG stream; 
         FIG. 4  is an explanatory diagram to be referred to in describing another study case in which it is not possible to seamlessly reproduce an MPEG stream; 
         FIG. 5  is a block diagram showing a typical configuration of an embodiment implementing a disc system to which the present invention is applied; 
         FIG. 6  shows a flowchart to be referred to in explaining a reproduction process carried out by the disc system; 
         FIG. 7  is a diagram showing a sequence of pictures to be decoded sequentially by a decoding unit  4 ; 
         FIG. 8  is an explanatory diagram to be referred to in describing a further study case in which it is possible to seamlessly reproduce an MPEG stream; 
         FIG. 9  is an explanatory diagram to be referred to in describing a still further study case in which it is not possible to seamlessly reproduce an MPEG stream; 
         FIG. 10  is an explanatory diagram to be referred to in describing preparatorily decoded pictures and preparatory decoding times; 
         FIG. 11  shows a flowchart to be referred to in explaining a process to determine preparatorily decoded pictures; 
         FIG. 12  is a block diagram showing a typical configuration of another embodiment implementing a disc system to which the present invention is applied; 
         FIG. 13  is an explanatory diagram to be referred to in describing a reproduction process using a decoding unit  4  comprising decoders  4   1  to  4   3 ; 
         FIG. 14  is an explanatory diagram to be referred to in describing a reproduction process using a decoding unit  4  comprising decoders  4   1  to  4   4 ; 
         FIG. 15  is an explanatory diagram to be referred to in describing a reproduction process using a decoding unit  4  comprising decoders  4   1  to  4   7 ; 
         FIG. 16  is a diagram showing a relation between the number of decoders  4   1  to  4   k  (K) composing the decoding unit  4  and a shortest cut duration; 
         FIG. 17  shows a flowchart to be referred to in explaining a reproduction process carried out by the disc system; 
         FIG. 18  is a diagram showing an edit-point table; 
         FIG. 19  shows a flowchart to be referred to in explaining a process carried out by the disc system to make a decoding control list; 
         FIG. 20  is an explanatory diagram showing an edit-point list and a decoding control list made on the basis of the edit-point list; 
         FIG. 21  is a diagram showing a typical edit-point list; 
         FIG. 22  is a diagram showing a typical decoding control list; 
         FIG. 23  shows a flowchart to be referred to in explaining a reproduction process carried out by the disc system; 
         FIG. 24  is a diagram showing another typical decoding control list; 
         FIG. 25  shows a flowchart to be referred to in explaining a reproduction process carried out by the disc system; and 
         FIG. 26  is a diagram showing a typical configuration of an embodiment implementing a computer to which the present invention is applied. 
     
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     Before explaining preferred embodiments of the present invention, the shortest cut duration is explained. Consider a process to reproduce results of editing in the same way as that described above. In the results of editing, an edit point T 4  is not set at picture P 15  to be displayed last among the pictures composing GOP (n) as shown in a picture-displaying order  221  of  FIG. 4 , but set at picture B 14  to be displayed immediately before picture P 15  as shown in a picture-displaying order  222  of the same figure. In this process, the duration from an edit point T 3  set at picture B 1  at the beginning of GOP (n) to the next edit point, which is the edit point T 4  set at picture B 14  of same GOP (n), is smaller that the duration of a GOP, resulting in a state of an unsatisfied requirement of the shortest cut duration. 
     In this case, in an operation to reproduce these results of the editing, the start sequence of GOPs ending with picture P 15  at the tail of GOP (m) needs to be first displayed to be followed by pictures B 1  to B 14  of GOP (n) before displaying the other sequence of GOPs beginning with picture B 1  at the head of GOP (m+2) as is obvious from a picture-displaying order  222  shown in  FIG. 4 . At the tail of GOP (m), the edit point T 1  has been set. At pictures B 1  and B 14  of GOP (n), the edit points T 3  and T 4  have been set respectively. At the head of GOP (m+2), the edit point T 2  has been set. 
     In the operation to reproduce these results of editing, a specific one of two MPEG decoders # 1  and # 2  is used to decode all pictures ending with GOP (m) as shown in a picture-decoding order  223  of  FIG. 4 . For example, the specific MPEG decoder is MPEG decoder # 1 . It is to be noted that, in the process to decode GOP (m), the processing to decode picture B 14  to be decoded last among the pictures composing GOP (m) must be completed by a time t 2 , at which picture B 14  is to be displayed, as is the case with the afore described picture-decoding order  213  of  FIG. 3 . Thus, the process to decode GOP (m) must be started before a time leading ahead of the time t 2  by a period of time it takes to display pictures composing a GOP. In the picture-decoding order  213  shown in  FIG. 3 , a time t 1  is determined to be the time to start the process to decode GOP (m). 
     The edit point T 1  is reached after decoder # 1  has decoded GOP (m) to be displayed. The process is then continued to processing to decode and display pictures B 1  to B 14  of GOP (n). The processing to decode and display GOP (n) must start at the edit point T 3  and end at the edit point T 4 . 
     In order to decode GOP (n), it is necessary to first decode GOP (n−1) immediately preceding GOP (n) as described earlier. In the process to decode GOP (n), the processing to decode picture B 14  to be decoded last among the pictures composing GOP (n) must be completed by a time t 3 , at which picture B 14  is to be displayed. Thus, the process to decode GOP (n) must be started before a time leading ahead of the time t 3  by a period of time it takes to display pictures composing a GOP. In a picture-decoding order  225  shown in  FIG. 4 , a time t 2  is determined to be the time to start the process to decode GOP (n). By the same token, the process to decode GOP (n−1) immediately preceding GOP (n) must be started before a time leading ahead of the time t 2  by a period of time it takes to display pictures composing a GOP. In the picture-decoding order  225  shown in  FIG. 4 , a time t 1  is determined to be the time to start the process to decode GOP (n−1). 
     For the reason described above, MPEG decoder # 2  starts decoding GOP (n−1) at the time t 1  and continues the process by decoding GOP (n) as shown in the picture-decoding order  225  of  FIG. 4 . MPEG decoder # 2  is the other one of 2 decoders # 1  and # 2 . 
     In the mean time, MPEG decoder # 1  should naturally terminate the decoding process at the end of the processing to decode GOP (m). After GOP (n) is decoded by decoder # 2  and displayed, however, the edit point T 4  is reached. Then, MPEG decoder # 1  must decode the other sequence of GOPs beginning with GOP (m+2) at the edit point T 2  for a display. 
     In order to decode GOP (m+2), it is necessary to first decode GOP (m+1) leading ahead of GOP (m+2) by a GOP as described earlier. In the process to decode GOP (m+2), the processing to decode picture B 14  to be decoded last among the pictures composing GOP (m+2) must be completed by a time t 4 ′, at which picture B 14  is to be displayed, as shown in a picture-decoding order  224  of  FIG. 4 . 
     In this case, the edit point T 4  has been set at picture B 14  immediately preceding picture P 15 , which is the last picture in the display order of GOP (n), as shown the picture-displaying order  222  shown in  FIG. 4 . Thus, picture P 15  pertaining to GOP (n) is not displayed. That is to say, after picture B 14  pertaining to GOP (n) has been displayed, picture B 1  of GOP (m+2), at which the edit point T 2  has been set, is displayed without displaying picture P 15  pertaining to GOP (n). Picture B 1  pertaining to GOP (m+2) is the first picture in the display order of GOP (m+2). In the case of the case study shown in  FIG. 4 , the display time of GOP (m+2) is thus earlier than that of the conventional method shown in  FIG. 3  by a period of time it takes to decode one picture. 
     For the reason described above, in the process to decode GOP (m+2) in accordance with the case study shown in  FIG. 4 , the processing to decode picture B 14  to be decoded last among the pictures composing GOP (m+2) must be completed by the time t 4 ′ leading ahead of the time t 4  of the conventional method shown in  FIG. 3  by a period of time it takes to decode one picture. 
     As is the case with the conventional method shown in  FIG. 3 , the process to decode GOP (m+2) must be started before a time leading ahead of the time t 4 ′ by a period of time it takes to decode pictures composing a GOP. In the picture-decoding order  224  shown in  FIG. 4 , a time t 3 ′ is determined to be the time to start the process to decode GOP (m+2). By the same token, the process to decode GOP (m+1) leading ahead of GOP (m+2) by a GOP must be started before a time leading ahead of the time t 3 ′ by a period of time it takes to decode pictures composing a GOP. In the picture-decoding order  224  shown in  FIG. 4 , a time t 2 ′ is determined to be the time to start the process to decode GOP (m+1). 
     The time t 2 ′ leads ahead of the time t 2 , at which MPEG decoder # 1  completes the processing to decode GOP (m), by a period of time it takes to decode one picture. At the time t 2 , MPEG decoder # 1  starts an operation to decode picture B 14  of GOP (m) as shown in the picture-decoding order  223  of  FIG. 4 . 
     Thus, when the duration between the edit point T 3  and the next edit point T 4  does not satisfy the requirement of the shortest cut duration as shown in the picture-displaying order  222  of  FIG. 4 , MPEG decoder # 1  is not capable of starting an operation to decode GOP (m+1) at the time t 2 ′ so that it is impossible to seamlessly reproduce the results of editing in accordance with the edit-point list. 
     If the shortest cut duration is limited to a GOP, on the other hand, there is raised a problem of restricted freedom of editing. 
     In accordance with the present invention, however, it is possible to make the limitation imposed on the shortest cut duration more lenient while enhancing the freedom of editing. 
       FIG. 5  is a diagram showing a typical configuration of an embodiment implementing a disc system provided by the present invention. In this context, the technical term ‘system’ means a set of a plurality of apparatus logically combined with each other without regard to whether or not the apparatus composing the set is put in the same box. 
     An optical disc  1  typically contains a recorded MPEG stream obtained as a result of carrying out an MPEG encoding process on a moving picture. In addition, the optical disc  1  also includes an edit-point list made as another result of editing the recorded MPEG stream as a raw editing material. 
     A disc drive  2  has a configuration allowing the optical disc  1  to be mounted on and demounted from the disc drive  2 . The disc drive  2  is a component for reading out the MPEG stream as well as the edit-point list from the optical disc  1  and outputting the MPEG stream as well as the edit-point list to a bus  3 . 
     A decoding unit  4  includes a plurality of decoders. For example, the decoding unit  4  has two decoders, namely decoders  4   1  and  4   2 . The decoders  4   1  and  4   2  each issue a request for a portion of the MPEG stream to the disc drive  2  by way of the bus  3 . Portions of the MPEG stream, which are output by the disc drive  2  at such a request, are supplied to the decoders  4   1  and  4   2  by way of the bus  3 . In addition, the decoders  4   1  and  4   2  each carry out an MPEG decoding process on the received portions of the MPEG stream in accordance with control executed by a controller  7 , and output pictures obtained as a result of the MPEG decoding process to a switcher  5 . 
     It is to be noted that the decoders  4   1  and  4   2  composing the decoding unit  4  are assumed to operate at a 1-time decoding process speed even though the decoders  4   1  and  4   2  can each be a decoder operating at a speed other than the one-time decoding process speed. In addition, the disc drive  2  reads out an MPEG stream from the optical disc  1  at a read-out speed (or a transfer band) large enough for an operation to supply the MPEG stream to the decoders  4   1  and  4   2 . 
     The switcher  5  is a component for selecting either a picture decoded by the decoder  4   1  or a picture decoded by the decoder  4   2  in accordance with control executed by the controller  7 , and outputting a selected picture to a display unit  6 . The display unit  6  is a component for displaying a picture output by the switcher  5 . Thus, the display unit  6  displays whichever picture the switcher  5  selects among a picture decoded by the decoder  4   1  and a picture decoded by the decoder  4   2 . It is to be noted that the display unit  6  can be an external component connected to the disc system shown in  FIG. 5 . 
     The controller  7  is a component for controlling the disc drive  2 , the decoding unit  4  and the switcher  5  in accordance with an operation signal received from an operation unit  8 . To put it in detail, when receiving an operation signal requesting the disc system to reproduce a result of an operation to edit an MPEG stream in accordance with an edit-point list from the operation unit  8 , the controller  7  issues a request for the edit-point list to the disc drive  2 . The controller  7  then receives the edit-point list, which has been read out by the disc drive  2  from the optical disc  1  at the request, from the disc drive  2  by way of the bus  3 . Subsequently, the controller  7  detects edit points included on the edit-point list. In addition, for each of the edit points detected from the edit-point list, the controller  7  identifies fewest possible pictures, which are not to be displayed after the edit point but must be decoded first in order to decode pictures to be displayed after the edit point. Then, the controller  7  computes a start point representing a timing to start an operation to decode the one or fewest possible pictures not to be displayed after the edit point. Furthermore, the controller  7  controls operations, which are carried out by the decoders  4   1  and  4   2  to decode the MPEG stream in picture units, on the basis of the start point. Moreover, for each of the edit points detected from the edit-point list, the controller  7  controls the switcher  5 , driving the switcher  5  to select either a picture decoded by the decoder  4   1  or a picture decoded by the decoder  4   2  and output the selected picture. 
     The operation unit  8  is a component operated by the user to output an operation signal representing an operation carried out by the user on the operation unit  8  to the controller  7 . 
     The disc system with the configuration described above carries out a reproduction process to reproduce an MPEG stream from the optical disc  1  in accordance with an edit-point list, which has also been recorded on the same optical disc  1 . 
       FIG. 6  shows a flowchart representing the reproduction process carried out by the disc system shown in  FIG. 5 . 
     When the user operates the operation unit  8  to start a process to reproduce a result of an operation to edit an MPEG stream in accordance with an edit-point list, the operation unit  8  outputs an operation signal to start the reproduction process to the controller  7 . The operation signal to start the reproduction process is properly referred to hereafter as a reproduction start operation signal. 
     Receiving the reproduction start operation signal from the operation unit  8  at a step S 1 , the controller  7  issues a request for the edit-point list to the disc drive  2 . At this request, the disc drive  2  reads out the edit-point list from the optical disc  1  and then outputs the list to the bus  3 . The controller  7  then receives the edit-point list from the bus  3 . Then, at the next step S 2 , the controller  7  sets schedules of sequences of pictures to be decoded by the decoders  4   1  and  4   2  and a schedule of a select pattern for driving the switcher  5  to select the output of either the decoder  4   1  or the decoder  4   2 . 
     Subsequently, at the next step S 3 , the controller  7  controls the decoders  4   1  and  4   2  as well as the switcher  5  in accordance with the schedules set at the step S 2 . 
     In accordance with the control executed by the controller  7 , the decoders  4   1  and  4   2  issue a request for portions of an MPEG stream to the disc drive  2  by way of the bus  3 . Then, the decoders  4   1  and  4   2  decode the requested portions of the MPEG stream, which are supplied by the disc drive  2  by way of the bus  3 . The decoders  4   1  and  4   2  then output pictures obtained as a result of decoding to the switcher  5 . Subsequently, the switcher  5  selects a decoded picture output by either the decoder  1  or the decoder  2  and outputs the selected picture to the display unit  6 . 
     Then, at the next step S 4 , the controller  7  forms a judgment as to whether or not the user has operated the operation unit  8  to stop the reproduction process. If the outcome of the judgment formed at the step S 4  indicates that the user has operated the operation unit  8  to stop the reproduction process, that is, if the controller  7  has received an operation signal to stop the reproduction process from the operation unit  8 , the reproduction process is ended. The operation signal to stop the reproduction process is properly referred to hereafter as a reproduction stop operation signal. 
     If the outcome of the judgment formed at the step S 4  indicates that the user has not operated the operation unit  8  to stop the reproduction process, on the other hand, the flow of the reproduction process goes on to a step S 5  at which the controller  7  forms a judgment as to whether or not the process to reproduce the result of an operation to edit the MPEG stream in accordance with an edit-point list read out at the step S 1  has been completed. If the outcome of the judgment formed at the step S 5  indicates that the process to reproduce the result of an operation to edit the MPEG stream in accordance with an edit-point list has not been completed, the flow of the reproduction process goes back to the step S 3  to repeat the same processing. If the outcome of the judgment formed at the step S 5  indicates that the process to reproduce the result of an operation to edit the MPEG stream in accordance with an edit-point list has been completed, on the other hand, the execution of the reproduction process is ended. 
     The following description explains the processing carried out by the controller  7  at the step S 2  of the flowchart shown in  FIG. 6  to set schedules of sequences of pictures to be decoded by the decoders  4   1  and  4   2 . It is to be noted that, in this embodiment, a GOP typically includes pictures arranged in a sequence of B 1 , B 2 , I 3 , B 4 , B 5 , P 6 , B 7 , B 8 , P 9 , B 10 , B 11 , P 12 , B 13 , B 14 , P 15  and encoded/decoded in an order of I 3 , B 1 , B 2 , P 6 , B 4 , B 5 , P 9 , B 7 , B 8 , P 12 , B 10 , B 11 , P 15 , B 13 , B 14 . It is also worth noting, however, that the configuration of a GOP is not limited to the above sequence. 
     The controller  7  specifies pictures to be decoded by the decoder  4   1  or  4   2  in processing to decode the MPEG stream, and requests the decoders  4   1  and  4   2  to decode only fewest possible pictures, which are not to be displayed after an edit point but must be first decoded in order to decode pictures to be displayed after the edit point. In this way, the requirement of the shortest cut duration can be made more lenient. 
     That is to say, it may be necessary to decode GOP (n−1) immediately preceding GOP (n) in order to decode GOP (n) itself as described before. 
     To put it concretely, in order to decode pictures B 1  and B 2  pertaining to GOP (n), it is necessary to reference picture P 15  pertaining to GOP (n−1) immediately preceding GOP (n). Furthermore, in order to decode picture P 12  pertaining to GOP (n−1), it is necessary to reference picture P 9  pertaining to GOP (n−1). In the same way, for other pictures pertaining to GOP (n−1), in order to decode picture P 9 , it is necessary to reference picture P 6  and, in order to decode picture P 6 , it is necessary to reference picture I 3 . 
     In accordance with the conventional technology, the processing to decode pictures is controlled in GOP units. Thus, in order to decode pictures B 1  and B 2  pertaining to GOP (n), it is necessary to first decode all pictures pertaining to GOP (n−1) immediately preceding GOP (n). 
     Accordingly, if an edit point is set at picture B 1  or B 2  pertaining to GOP (n), in order to start a display operation beginning with picture B 1  or B 2 , it is necessary to first decode all pictures pertaining to GOP (n−1) even though these pictures are not to be displayed before pictures B 1  and B 2  in the case studies explained before. 
     In addition, for pictures pertaining to GOP (n), in order to decode a picture other than pictures B 1  and B 2 , it is necessary to reference a picture pertaining to the same GOP (n) even though it is not necessary to reference picture pertaining to GOP (n−1) immediately preceding GOP (n). In order to decode picture B 4  pertaining to GOP (n), for example, it is necessary to first reference pictures I 3  and P 6 , which also pertain to GOP (n). Thus, in order to decode picture B 4  pertaining to GOP (n), it is necessary to first decode all pictures I 3 , B 1 , B 2  and P 6 , which precede picture B 4  in the decoding order of GOP (n). Otherwise, picture B 4  cannot be decoded. 
     Thus, if an edit point is set at picture B 4  pertaining to GOP (n), it is necessary to first decode all pictures I 3 , B 1 , B 2  and P 6 , which also pertain to GOP (n) and not to be displayed, in order to start a display operation beginning with picture B 4 . 
     In order to implement the above scheme, the controller  7  controls the decoders  4   1  and  4   2  by specifying objects of decoding in picture units in place of GOP units. In this way, only pictures that have been displayed are referenced in case such pictures are required for decoding another picture to be displayed. On the other hand, pictures that have not been displayed are, if possible, not referenced in decoding a specific picture to be displayed with the exception of as fewest such pictures as possible. 
     That is to say, if objects of decoding are specified in GOP units, with an edit point set at picture B 1  or B 2  pertaining to GOP (n), in order to start a display operation beginning with picture B 1  or B 2 , as described above, it is necessary to first decode all pictures pertaining to GOP (n−1) even though these pictures are not to be displayed before pictures B 1  and B 2  in the case studies explained before. However, only picture P 15  pertaining to GOP (n−1) is actually referenced in decoding pictures B 1  and B 2  pertaining to GOP (n) and only picture P 12  pertaining to GOP (n−1) is actually referenced in decoding picture P 15  pertaining to GOP (n−1). By the same token, for other pictures pertaining to GOP (n−1), only picture P 9  is actually referenced in decoding picture P 12 , only picture P 6  is actually referenced in decoding picture P 9 , and only picture I 3  is actually referenced in decoding picture P 6 . 
     Thus, the fewest possible specific pictures of GOP (n−1), which need to be decoded first before decoding pictures B 1  and B 2  pertaining to GOP (n) even though these specific pictures are not to be displayed before pictures B 1  and B 2  in the case studies explained before, are only 5 pictures, namely, I 3 , P 6 , P 9 , P 12  and P 15 . Pictures of GOP (n−1), which do not need to be referenced in decoding pictures B 1  and B 2  pertaining to GOP (n), are B 1 , B 2 , B 4 , B 5 , B 7 , B 8 , B 10 , B 11 , B 13  and B 14 . It is thus unnecessary to decode the pictures of GOP (n−1), which are not referenced. A picture not to be displayed is not supplied to the display unit  6  shown in  FIG. 5 . 
     By the same token, if an edit point has been set at picture B 4  pertaining to GOP (n), for example, fewest possible specific pictures of GOP (n), which need to be decoded first before decoding picture B 4  even though these specific pictures are not to be displayed due to the location of the edit point, are reduced to only picture I 3  pertaining to GOP (n). Since pictures B 1  and B 2  pertaining to GOP (n) are not referenced in decoding picture B 4  pertaining to GOP (n), it is not necessary to decode pictures B 1  and B 2 . 
     As described earlier, in accordance with the MPEG technique, while an I or P picture can be used as a picture to be referenced in decoding another picture, a B picture is never used as a picture to be referenced in decoding another picture. Thus, only pictures I and P can be categorized as fewest possible specific pictures, which need to be decoded first before decoding other pictures even though these specific pictures are not to be displayed. 
     As described earlier, at the step S 2  of the flowchart shown in  FIG. 6 , the controller  7  sets schedules of sequences of pictures, which are to be decoded by the decoders  4   1  and  4   2 , in accordance with the edit-point list. Typically, the schedule is set as shown in  FIG. 7 . 
     To put it in detail, the controller  7  detects first edit points from the edit-point list. For each detected edit point set as an IN point, the controller  7  then sets a schedule of a sequence of pictures to be decoded by the decoder  4   1  or  4   2  in accordance with a picture at which the detected edit point is set. 
     To put it concretely, If a detected edit point has been set as an IN point at picture B 1  pertaining to GOP (n) as indicated by the first line of the schedules shown in  FIG. 7 , for example, the fewest possible specific pictures of GOP (n−1), which need to be decoded first before decoding picture B 1  pertaining to GOP (n) even though these specific pictures are not to be displayed before picture B 1  in the case studies explained before, are only 5 pictures, namely, I 3 , P 6 , P 9 , P 12  and P 15  as described earlier. Thus, the controller  7  sets a schedule to first decode these pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n−1). Then, thereafter, subsequent pictures pertaining to GOP (n) need to be sequentially decoded. That is to say, the schedule includes decoding of subsequent pictures of GOP (n) in the normal decoding order of (I 3 , B 1 , B 2 , P 6 , B 4  and so on) after decoding pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n−1). 
     If a detected edit point has been set as an IN point at picture B 2  pertaining to GOP (n) as indicated by the second line of the schedules shown in  FIG. 7 , the fewest possible specific pictures of GOP (n−1), which need to be decoded first before decoding picture B 2  pertaining to GOP (n) even though these specific pictures are not to be displayed before picture B 2  in the case studies explained before, are only 5 pictures, namely, I 3 , P 6 , P 9 , P 12  and P 15  as described earlier. Thus, the controller  7  sets a schedule to first decode these pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n−1). Then, thereafter, pictures I 3 , B 1 , B 2 , P 6 , B 4  and so on pertaining to GOP (n) would naturally need to be sequentially decoded. However, picture B 1  supposed to be displayed before picture B 2 , at which a detected edit point has been set as an IN point, need to be neither referenced because picture B 1  is a B picture, nor displayed because the detected edit point has been set at picture B 2  following picture B 1  in the display order. Thus, it is not necessary to decode picture B 1 . That is to say, the schedule includes decoding of subsequent pictures of GOP (n) in a decoding order of (I 3 , B 2 , P 6 , B 4  and so on) after decoding pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n−1). That is to say, picture B 2  pertaining to GOP (n) is decoded after picture I 3  to be followed by decoding of pictures P 6 , B 4  and so on. 
     If a detected edit point has been set as an IN point at picture I 3  pertaining to GOP (n) as indicated by the third line of the schedules shown in  FIG. 7 , no other pictures need to be referenced in order to decode picture I 3 . Thus, pictures I 3 , B 1 , B 2 , P 6 , B 4  and so on pertaining to GOP (n) would naturally need to be sequentially decoded. Since a detected edit point is set as an IN point at picture I 3 , however, it is not necessary to display pictures B 1  and B 2 , which are supposed to be displayed before picture I 3  following pictures B 1  and B 2  in the display order, and it is also unnecessary to reference pictures B 1  and B 2  because pictures B 1  and B 2  are each a B picture. Thus, it is not necessary to decode pictures B 1  and B 2 . For this reason, the controller  7  sets a schedule to decode picture P 6  after picture I 3  without decoding pictures B 1  and B 2 . After decoding picture P 6 , the pictures remaining in the original decoding sequence are decoded sequentially in the normal order. 
     If a detected edit point has been set as an IN point at picture B 4  pertaining to GOP (n) as indicated by the fourth line of the schedules shown in  FIG. 7 , the fewest possible specific pictures, which need to be decoded first before decoding picture B 4  even though these specific pictures are not to be displayed before picture B 4  because of the position of the detected edit point, are reduced to only picture I 3  pertaining to GOP (n). Thus, the controller  7  sets a schedule to first decode picture I 3  pertaining to GOP (n). Then, thereafter, pictures B 1 , B 2 , P 6 , B 4 , B 5  and so on of GOP (n), which follow picture I 3  in accordance with the normal decoding order, would naturally need to be sequentially decoded. However, pictures B 1  and B 2  supposed to be displayed before picture B 4 , at which a detected edit point has been set as an IN point, need to be neither referenced because pictures B 1  and B 2  are each a B picture, nor displayed because the detected edit point has been set at picture B 4  following pictures B 1  and B 2  in the display order. Thus, it is not necessary to decode pictures B 1  and B 2 . For this reason, the controller  7  sets a schedule to decode picture P 6  pertaining to GOP (n) after picture I 3  without decoding pictures B 1  and B 2 . After decoding picture P 6 , the pictures remaining in the original decoding sequence are decoded sequentially in the normal order. 
     If a detected edit point has been set as an IN point at picture B 5  pertaining to GOP (n) as indicated by the fifth line of the schedules shown in  FIG. 7 , the fewest possible specific pictures, which need to be decoded first before decoding picture B 5  even though these specific pictures are not to be displayed before picture B 5  because of the position of the detected edit point, are reduced to only picture I 3  pertaining to GOP (n). Thus, the controller  7  sets a schedule to first decode picture I 3  pertaining to GOP (n). Then, thereafter, pictures B 1 , B 2 , P 6 , B 4 , B 5  and so on of GOP (n), which follow picture I 3  in accordance with the normal decoding order, would naturally need to be sequentially decoded. However, pictures B 1 , B 2  and B 4  supposed to be displayed before picture B 5 , at which a detected edit point has been set as an IN point, need to be neither referenced because pictures B 1 , B 2  and B 4  are each a B picture, nor displayed because the detected edit point has been set at picture B 5  following pictures B 1 , B 2  and B 4  in the display order. Thus, it is not necessary to decode pictures B 1 , B 2  and B 4 . For this reason, the controller  7  sets a schedule to decode picture P 6  pertaining to GOP (n) after picture I 3  without decoding pictures B 1  and B 2 . Then, after decoding picture P 6 , picture B 5  is decoded without decoding picture B 4 . Thereafter, the pictures remaining in the original decoding sequence are decoded sequentially in the normal order. 
     If a detected edit point has been set as an IN point at picture P 6  pertaining to GOP (n) as indicated by the sixth line of the schedules shown in  FIG. 7 , the fewest possible specific pictures, which need to be decoded first before decoding picture P 6  even though these specific pictures are not to be displayed before picture P 6  because of the position of the detected edit point, are reduced to only picture I 3  pertaining to GOP (n). Thus, the controller  7  sets a schedule to first decode picture I 3  pertaining to GOP (n). Then, thereafter, pictures B 1 , B 2 , P 6 , B 4 , B 5 , P 9  and so on of GOP (n), which follow picture I 3  in accordance with the normal decoding order, would naturally need to be sequentially decoded. However, pictures B 1 , B 2 , B 4  and B 5  supposed to be displayed before picture P 6 , at which a detected edit point has been set as an IN point, need to be neither referenced because pictures B 1 , B 2 , B 4  and B 5  are each a B picture, nor displayed because the detected edit point has been set at picture P 6  following pictures B 1 , B 2 , B 4  and B 5  in the display order. Thus, it is not necessary to decode pictures B 1 , B 2 , B 4  and B 5 . For this reason, the controller  7  sets a schedule to decode picture P 6  pertaining to GOP (n) after picture I 3  without decoding pictures B 1  and B 2 . Then, after decoding picture P 6 , picture P 9  is decoded without decoding pictures B 4  and B 5 . Thereafter, the pictures remaining in the original decoding sequence are decoded sequentially in the normal order. 
     If a detected edit point has been set as an IN point at picture B 7  pertaining to GOP (n) as indicated by the seventh line of the schedules shown in  FIG. 7 , the fewest possible specific pictures, which need to be decoded first before decoding picture B 7  even though these specific pictures are not to be displayed before picture B 7  because of the position of the detected edit point, are reduced to only pictures I 3  and P 6  pertaining to GOP (n). Thus, the controller  7  sets a schedule to first decode pictures I 3  and P 6  pertaining to GOP (n). Then, thereafter, pictures B 4 , B 5 , P 9 , B 7 , B 8  and so on of GOP (n), which follow picture P 6  in accordance with the normal decoding order, would naturally need to be sequentially decoded. However, pictures B 4  and B 5  supposed to be displayed before picture B 7 , at which a detected edit point has been set as an IN point, need to be neither referenced because pictures B 4  and B 5  are each a B picture, nor displayed because the detected edit point has been set at picture B 7  following pictures B 4  and B 5  in the display order. Thus, it is not necessary to decode pictures B 4  and B 5 . For this reason, the controller  7  sets a schedule to decode picture P 9  pertaining to GOP (n) after picture P 6  without decoding pictures B 4  and B 5 . Thereafter, the pictures remaining in the original decoding sequence are decoded sequentially in the normal order. 
     If a detected edit point has been set as an IN point at picture B 8  pertaining to GOP (n) as indicated by the eighth line of the schedules shown in  FIG. 7 , the fewest possible specific pictures, which need to be decoded first before decoding picture B 8  even though these specific pictures are not to be displayed before picture B 8  because of the position of the detected edit point, are reduced to only pictures I 3  and P 6  pertaining to GOP (n). Thus, the controller  7  sets a schedule to first decode pictures I 3  and P 6  pertaining to GOP (n). Then, thereafter, pictures B 4 , B 5 , P 9 , B 7 , B 8  and so on of GOP (n), which follow picture P 6  in accordance with the normal decoding order, would naturally need to be sequentially decoded. However, pictures B 4 , B 5  and B 7  supposed to be displayed before picture B 8 , at which a detected edit point has been set as an IN point, need to be neither referenced because pictures B 4 , B 5  and B 7  are each a B picture, nor displayed because the detected edit point has been set at picture B 8  following pictures B 4 , B 5  and B 7  in the display order. Thus, it is not necessary to decode pictures B 4 , B 5  and B 7 . For this reason, the controller  7  sets a schedule to decode picture P 9  pertaining to GOP (n) after picture P 6  without decoding pictures B 4  and B 5 . Then, after decoding picture P 9 , picture B 8  is decoded without decoding picture B 7 . Thereafter, the pictures remaining in the original decoding sequence are decoded sequentially in the normal order. 
     If a detected edit point has been set as an IN point at picture P 9  pertaining to GOP (n) as indicated by the ninth line of the schedules shown in  FIG. 7 , the fewest possible specific pictures, which need to be decoded first before decoding picture P 9  even though these specific pictures are not to be displayed before picture P 9  because of the position of the detected edit point, are reduced to only pictures I 3  and P 6  pertaining to GOP (n). Thus, the controller  7  sets a schedule to first decode pictures I 3  and P 6  pertaining to GOP (n). Then, thereafter, pictures B 4 , B 5 , P 9 , B 7 , B 8 , P 12 , B 10 , B 11  and so on of GOP (n), which follow picture P 6  in accordance with the normal decoding order, would naturally need to be sequentially decoded. However, pictures B 4 , B 5 , B 7  and B 8  supposed to be displayed before picture P 9 , at which a detected edit point has been set as an IN point, need to be neither referenced because pictures B 4 , B 5 , B 7  and B 8  are each a B picture, nor displayed because the detected edit point has been set at picture P 9  following pictures B 4 , B 5 , B 7  and B 8  in the display order. Thus, it is not necessary to decode pictures B 4 , B 5 , B 7  and B 8 . For this reason, the controller  7  sets a schedule to decode picture P 9  pertaining to GOP (n) after picture P 6  without decoding pictures B 4  and B 5 . Then, after decoding picture P 9 , picture P 12  is decoded without decoding pictures B 7  and B 8 . Thereafter, the pictures remaining in the original decoding sequence are decoded sequentially in the normal order. 
     If a detected edit point has been set as an IN point at picture B 10  pertaining to GOP (n) as indicated by the tenth line of the schedules shown in  FIG. 7 , the fewest possible specific pictures, which need to be decoded first before decoding picture B 10  even though these specific pictures are not to be displayed before picture B 10  because of the position of the detected edit point, are reduced to only pictures I 3 , P 6  and P 9  pertaining to GOP (n). Thus, the controller  7  sets a schedule to first decode pictures I 3 , P 6  and P 9  pertaining to GOP (n). Then, thereafter, pictures B 7 , B 8 , P 12 , B 10 , B 11  and so on of GOP (n), which follow picture P 9  in accordance with the normal decoding order, would naturally need to be sequentially decoded. However, pictures B 7  and B 8  supposed to be displayed before picture B 10 , at which a detected edit point has been set as an IN point, need to be neither referenced because pictures B 7  and B 8  are each a B picture, nor displayed because the detected edit point has been set at picture B 10  following pictures B 7  and B 8  in the display order. Thus, it is not necessary to decode pictures B 7  and B 8 . For this reason, the controller  7  sets a schedule to decode picture P 12  pertaining to GOP (n) after picture P 9  without decoding pictures B 7  and B 8 . Thereafter, the pictures remaining in the original decoding sequence are decoded sequentially in the normal order. 
     If a detected edit point has been set as an IN point at picture B 11  pertaining to GOP (n) as indicated by the eleventh line of the schedules shown in  FIG. 7 , the fewest possible specific pictures, which need to be decoded first before decoding picture B 11  even though these specific pictures are not to be displayed before picture B 11  because of the position of the detected edit point, are reduced to only pictures I 3 , P 6  and P 9  pertaining to GOP (n). Thus, the controller  7  sets a schedule to first decode pictures I 3 , P 6  and P 9  pertaining to GOP (n). Then, thereafter, pictures B 7 , B 8 , P 12 , B 10 , B 11  and so on of GOP (n), which follow picture P 9  in accordance with the normal decoding order, would naturally need to be sequentially decoded. However, pictures B 7 , B 8  and B 10  supposed to be displayed before picture B 11 , at which a detected edit point has been set as an IN point, need to be neither referenced because pictures B 7 , B 8  and B 10  are each a B picture, nor displayed because the detected edit point has been set at picture B 11  following pictures B 7 , B 8  and B 10  in the display order. Thus, it is not necessary to decode pictures B 7 , B 8  and B 10 . For this reason, the controller  7  sets a schedule to decode picture P 12  pertaining to GOP (n) after picture P 9  without decoding pictures B 7  and B 8 . Then, after decoding picture P 12 , picture B 11  is decoded without decoding picture B 10 . Thereafter, the pictures remaining in the original decoding sequence are decoded sequentially in the normal order. 
     If a detected edit point has been set as an IN point at picture P 12  pertaining to GOP (n) as indicated by the twelfth line of the schedules shown in  FIG. 7 , the fewest possible specific pictures, which need to be decoded first before decoding picture P 12  even though these specific pictures are not to be displayed before picture P 12  because of the position of the detected edit point, are reduced to only pictures I 3 , P 6  and P 9  pertaining to GOP (n). Thus, the controller  7  sets a schedule to first decode pictures I 3 , P 6  and P 9  pertaining to GOP (n). Then, thereafter, pictures B 7 , B 8 , P 12 , B 10 , B 11 , P 15 , B 13 , B 14  and so on of GOP (n), which follow picture P 9  in accordance with the normal decoding order, would naturally need to be sequentially decoded. However, pictures B 7 , B 8 , B 10  and B 11  supposed to be displayed before picture P 12 , at which a detected edit point has been set as an IN point, need to be neither referenced because pictures B 7 , B 8 , B 10  and B 11  are each a B picture, nor displayed because the detected edit point has been set at picture B 12  following pictures B 7 , B 8 , B 10  and B 11  in the display order. Thus, it is not necessary to decode pictures B 7 , B 8 , B 10  and B 11 . For this reason, the controller  7  sets a schedule to decode picture P 12  pertaining to GOP (n) after picture P 9  without decoding pictures B 7  and B 8 . Then, after decoding picture P 12 , picture P 15  is decoded without decoding pictures B 10  and B 11 . Thereafter, the pictures remaining in the original decoding sequence are decoded sequentially in the normal order. 
     If a detected edit point has been set as an IN point at picture B 13  pertaining to GOP (n) as indicated by the thirteenth line of the schedules shown in  FIG. 7 , the fewest possible specific pictures, which need to be decoded first before decoding picture B 13  even though these specific pictures are not to be displayed before picture B 13  because of the position of the detected edit point, are reduced to only pictures I 3 , P 6 , P 9  and P 12  pertaining to GOP (n). Thus, the controller  7  sets a schedule to first decode pictures I 3 , P 6 , P 9  and P 12  pertaining to GOP (n). Then, thereafter, pictures B 10 , B 11 , P 15 , B 13 , B 14  and so on of GOP (n), which follow picture P 12  in accordance with the normal decoding order, would naturally need to be sequentially decoded. However, pictures B 10  and B 11  supposed to be displayed before picture B 13 , at which a detected edit point has been set as an IN point, need to be neither referenced because pictures B 10  and B 11  are each a B picture, nor displayed because the detected edit point has been set at picture B 13  following pictures B 10  and B 11  in the display order. Thus, it is not necessary to decode pictures B 10  and B 11 . For this reason, the controller  7  sets a schedule to decode picture P 15  pertaining to GOP (n) after picture P 12  without decoding pictures B 10  and B 11 . Thereafter, the pictures remaining in the original decoding sequence are decoded sequentially in the normal order. 
     If a detected edit point has been set as an IN point at picture B 14  pertaining to GOP (n) as indicated by the fourteenth line of the schedules shown in  FIG. 7 , the fewest possible specific pictures, which need to be decoded first before decoding picture B 14  even though these specific pictures are not to be displayed before picture B 14  because of the position of the detected edit point, are reduced to only pictures I 3 , P 6 , P 9  and P 12  pertaining to GOP (n). Thus, the controller  7  sets a schedule to first decode pictures I 3 , P 6 , P 9  and P 12  pertaining to GOP (n). Then, thereafter, pictures B 10 , B 11 , P 15 , B 13 , B 14  and so on of GOP (n), which follow picture P 12  in accordance with the normal decoding order, would naturally need to be sequentially decoded. However, pictures B 10 , B 11  and B 13  supposed to be displayed before picture B 14 , at which a detected edit point has been set as an IN point, need to be neither referenced because pictures B 10 , B 11  and B 13  are each a B picture, nor displayed because the detected edit point has been set at picture B 14  following pictures B 10 , B 11  and B 13  in the display order. Thus, it is not necessary to decode pictures B 10 , B 11  and B 13 . For this reason, the controller  7  sets a schedule to decode picture P 15  pertaining to GOP (n) after picture P 12  without decoding pictures B 10  and B 11 . Then, after decoding picture P 15 , picture B 14  is decoded without decoding pictures B 13 . Thereafter, the pictures remaining in the original decoding sequence are decoded sequentially in the normal order. 
     If a detected edit point has been set as an IN point at picture P 15  pertaining to GOP (n) as indicated by the fifteenth line of the schedules shown in  FIG. 7 , the fewest possible specific pictures, which need to be decoded first before decoding picture P 15  even though these specific pictures are not to be displayed before picture P 15  because of the position of the detected edit point, are reduced to only pictures I 3 , P 6 , P 9  and P 12  pertaining to GOP (n). Thus, the controller  7  sets a schedule to first decode pictures I 3 , P 6 , P 9  and P 12  pertaining to GOP (n). Then, thereafter, pictures B 10 , B 11 , P 15 , B 13 , B 14  and so on of GOP (n), which follow picture P 12  in accordance with the normal decoding order, would naturally need to be sequentially decoded. However, pictures B 10 , B 11 , B 13  and B 14  supposed to be displayed before picture P 15 , at which a detected edit point has been set as an IN point, need to be neither referenced because pictures B 10 , B 11 , B 13  and B 14  are each a B picture, nor displayed because the detected edit point has been set at picture P 15  following pictures B 10 , B 11 , B 13  and B 14  in the display order. Thus, it is not necessary to decode pictures B 10 , B 11 , B 13  and B 14 . For this reason, the controller  7  sets a schedule to decode picture P 15  pertaining to GOP (n) after picture P 12  without decoding pictures B 10  and B 11 . Then, after decoding picture P 15 , picture I 3  pertaining to the next GOP is decoded without decoding pictures B 13  and B 14 . Thereafter, subsequent pictures are decoded sequentially in the normal decoding order. 
     By setting schedules as described above, the disc system shown in  FIG. 5  is capable of decoding an MPEG stream at a shortest cut duration of 6 frames (or 6 pictures) as shown in  FIG. 8 . 
     To put it in detail, in the case study shown in  FIG. 3 , the edit point T 1  is set at picture P 15  pertaining to GOP (m) as an OUT point, the edit point T 3  is set at picture B 1  pertaining to GOP (n) as an IN point, the edit point T 4  is set at picture P 15  pertaining to GOP (n) as an OUT point and the edit point T 2  is set at picture B 1  pertaining to GOP (m+2) as an IN point as indicated by a picture-displaying order  231  shown in  FIG. 8 . Now, the picture-displaying order  231  is changed to a picture-displaying order  232  also shown in  FIG. 8 . In accordance with the picture-displaying order  232 , the edit point T 1  is set at picture P 6  pertaining to GOP (m) instead of being set at picture P 15  as an OUT point and the edit point T 4  is set at picture P 6  pertaining to GOP (n) instead of being set at picture P 15  as an OUT point. 
     As a result of such a change, the duration between the edit point T 3  set at picture B 1  pertaining to GOP (n) as an IN point and the edit point T 4  set at picture P 6  pertaining to GOP (n) as an OUT point is a shortest cut duration, which is six frames in this case. 
     In this case, in a process to reproduce a result of editing in accordance with the picture-displaying order  232  shown in  FIG. 8 , a start sequence of GOPs is first reproduced to be followed by a middle sequence of GOPs before finally displaying an end sequence of GOPs. The start sequence of GOPs ends at the edit point T 1  set at picture P 6  pertaining to GOP (m) as an OUT point. The middle sequence of GOPs begins at the edit point T 3  set at picture B 1  pertaining to GOP (n) as an IN point and ends at the edit point T 4  set at picture P 6  pertaining to GOP (n) as an OUT point. The end sequence of GOPs begins at the edit point T 2  set at picture B 1  pertaining to GOP (m+2) as an IN point. 
     In order to carry out the process to reproduce a result of editing in accordance with the picture-displaying order  232  as described above, the controller  7  controls one of the two decoders  4   1  and  4   2  such as the decoder  4   1  to decode pictures I 3 , B 1 , B 2 , P 6 , B 4  and B 5  to be displayed as pictures pertaining to GOP (m) as shown in a picture-decoding order  233  of  FIG. 8 . It is to be noted that, in the process to decode pictures I 3 , B 1 , B 2 , P 6 , B 4  and B 5  pertaining to GOP (m), the processing to decode picture B 5  to be decoded last among the pictures composing GOP (m) must be completed by a time t n−2 , at which picture B 5  is to be displayed. Thus, the process to decode GOP (m) must be started before a time leading ahead of the time t n−2  by a period of time it takes to display six pictures, namely, pictures B 1  to P 6 . In the picture-decoding order  233  shown in  FIG. 8 , a time t n−8  is determined to be the time to start the process to decode GOP (m). 
     In the case study shown in  FIG. 8 , reference symbol t n  denotes a time at which the display of the first picture B 1  pertaining to GOP (n) is started and the display of picture P 6  pertaining to GOP (m) is ended. Reference symbol t n−i  denotes a time leading ahead of the time t n  by a period of time it takes to display i frames. 
     After the decoder  4   1  decodes and displays pictures B 1  to P 6  pertaining to GOP (m), the edit point T 1  is reached as shown in the picture-displaying order  232  of  FIG. 8 . The process is then continued to processing to decode and display pictures B 1  to P 6  pertaining to GOP (n). The processing to decode and display GOP (n) must start at the edit point T 3  and end at the edit point T 4 . 
     Since picture B 1  pertaining to group GOP (n) is a picture at which the edit point T 3  has been set, as shown in a picture-decoding order  235  of  FIG. 8 , the controller  7  controls the other one of the decoders  4   1  and  4   2  (that is, the decoder  4   2  in this case) to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n−1) immediately preceding GOP (n) and then pictures I 3 , B 1 , B 2 , P 6 , B 4  and B 5  pertaining to GOP (n) in accordance with the schedule shown on the first line explained earlier by referring to  FIG. 7 . It is to be noted that, in the process to decode pictures I 3 , B 1 , B 2 , P 6 , B 4  and B 5  pertaining to GOP (n), the processing to decode picture B 1  at which the edit point T 3  has been set must be completed by a time t n , that is, a point of time picture B 1  is to be displayed. Thus, the process to decode picture I 3  pertaining to GOP (n−1) must be started before a time leading ahead of the time t n  by a period of time it takes to display seven pictures because the start of the processing to decode picture I 3  pertaining to GOP (n−1) leads ahead of the end of the processing to decode picture B 1  pertaining to GOP (n) by a period of time it takes to display seven pictures. In the picture-decoding order  235  shown in  FIG. 8 , a time t n−7  is determined to be the time to start the processing to decode I 3  pertaining to GOP (n−1). 
     In the mean time, the decoder  4   1  ends the decoding process after the processing to decode pictures I 3 , B 1 , B 2 , P 6 , B 4  and B 5  pertaining to GOP (m) is completed. When the decoder  4   1  ends the decoding process, the decoding process in the decoder  4   2  is going on till picture B 5  pertaining to GOP (n) is decoded. After picture B 5  pertaining to GOP (n) is decoded, the next picture pertaining to GOP (n), namely, picture P 6  decoded earlier than picture B 5 , is displayed. Then, the edit point T 4  is reached as shown in the picture-displaying order  232  of  FIG. 8 . Thereafter, GOP (m+2) starting at the edit point T 2  and subsequent GOPs must be decoded and displayed. 
     Since picture B 1  pertaining to group GOP (m+2) is a picture at which the edit point T 2  has been set, as shown in a picture-decoding order  234  of  FIG. 8 , the controller  7  controls the decoder  4   1  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (m+1) immediately preceding GOP (m+2) and then pictures I 3 , B 1 , B 2 , P 6 , B 4 , B 5  and so on pertaining to GOP (m+2) in accordance with the schedule shown on the first line explained earlier by referring to  FIG. 7 . 
     In the process to decode pictures I 3 , B 1 , B 2 , P 6 , B 4 , B 5  and so on pertaining to GOP (m+2), the processing to decode picture B 1  at which the edit point T 2  has been set must be completed by a time t n ′, that is, a point of time picture B 1  is to be displayed. Thus, the process to decode picture I 3  pertaining to GOP (m+1) must be started before a time leading ahead of the time t n ′ by a period of time it takes to display seven pictures because the start of the processing to decode picture I 3  pertaining to GOP (m+1) leads ahead of the end of the processing to decode picture B 1  pertaining to GOP (m+2) by a period of time it takes to display seven pictures. In the picture-decoding order  234  shown in  FIG. 8 , a time t n−7 ′ is determined to be the time to start the processing to decode I 3  pertaining to GOP (m+1). 
     In the case study shown in  FIG. 8 , reference symbol t n ′ denotes a time at which the display of the first picture B 1  pertaining to GOP (m+2) is started and the display of picture P 6  pertaining to GOP (n) is ended. Reference symbol t n−i ′ denotes a time leading ahead of the time t n ′ by a period of time it takes to display i frames. 
     In the case study shown in  FIG. 8 , the time t n−2  is a time at which the processing carried out by the decoder  4   1  to decode picture B 5  pertaining to GOP (m) is ended. On the other hand, the time t n−7 ′ is a time at which the processing carried out by the decoder  4   1  to decode GOP (m+1) is started. Thus, the decoding process carried out by the decoder  4   1  does not fail as long as the difference (t n−7 ′−t n−2 ) is at least 0. 
     Let the unity (or 1) represent a period of time it takes to display one frame (or one picture). In this case, the times t n−2  and t n−7 ′ can be represented by expressions (t n−2 ) and (t n ′−7) respectively. Thus, the difference (t n−7 ′−t n−2 ) can be represented by an expression {(t n ′−7)−(t n −2)}=(t n ′−t n −5). 
     On the other hand, in the case study shown in  FIG. 8 , the time t n  is a time to start an operation to display picture B 1  pertaining to GOP (n) as a picture at which the edit point T 3  has been set as an IN point whereas the time t n ′ is a time to end an operation to display picture P 6  pertaining to GOP (n) as a picture at which the edit point T 4  has been set as the OUT point following the IN point. The difference (t n ′−t n ) is the shortest cut duration, which is 6. 
     Accordingly, the difference (t n−7 ′−t n−2 ) is 1 so that the decoding process carried out by the decoder  4   1  does not fail. 
     Since the disc system shown in  FIG. 5  allows the shortest cut duration to be reduced to as small as a period of time it takes to display six frames as described above, the freedom of editing can be improved. 
     If the period between the edit point T 3  set as an IN point and the edit point T 4  set as the OUT point following the IN point is shorter than the shortest cut duration, the decoder  4   1  will fail. 
     Now, consider a reproduction process identical with the reproduction process explained above by referring to  FIG. 8  except that the picture-displaying order  232  shown in  FIG. 8  is changed to a picture-displaying order  242  shown in  FIG. 9 . In accordance with the picture-displaying order  242 , the edit point T 1  is set as an OUT point at picture B 5  pertaining to GOP (m) instead of being set at picture P 6  immediately following picture B 5  and the edit point T 4  is set as an OUT point at picture B 5  pertaining to GOP (n) instead of being set at picture P 6  immediately following picture B 5 . As a result of such a change, the duration between the edit point T 3  set at picture B 1  pertaining to GOP (n) as an IN point and the edit point T 4  set at picture B 5  pertaining to GOP (n) as an OUT point is equal to a period of only five frames, which is shorter than the shortest cut duration of six frames. 
     In this case, in a process to reproduce a result of editing in accordance with the picture-displaying order  242  shown in  FIG. 9 , a start sequence of GOPs is first reproduced to be followed by a middle sequence of GOPs before finally displaying an end sequence of GOPs. The start sequence of GOPs ends at the edit point T 1  set at picture B 5  pertaining to GOP (m) as an OUT point. The middle sequence of GOPs begins at the edit point T 3  set at picture B 1  pertaining to GOP (n) as an IN point and ends at the edit point T 4  set at picture B 5  pertaining to GOP (n) as an OUT point. The end sequence of GOPs begins at the edit point T 2  set at picture B 1  pertaining to GOP (m+2) as an IN point. 
     In order to carry out the process to reproduce a result of editing in accordance with the picture-displaying order  242  as described above, the controller  7  controls one of the two decoders  4   1  and  4   2  such as the decoder  4   1  to decode pictures I 3 , B 1 , B 2 , P 6 , B 4  and B 5  to be displayed as pictures pertaining to GOP (m) as shown in a picture-decoding order  243  of  FIG. 9 . In the case study shown in  FIG. 9 , picture P 6  pertaining to GOP (m) is not displayed. In decoding pictures B 4  and B 5  pertaining to GOP (m), however, picture P 6  is referenced. Thus, picture P 6  pertaining to GOP (m) must be decoded first before pictures B 4  and B 5 . 
     It is to be noted that, in the process to decode pictures I 3 , B 1 , B 2 , P 6 , B 4  and B 5  pertaining to GOP (m), the processing to decode picture B 5  to be decoded last among the pictures composing GOP (m) must be completed by a time t n−1 , at which picture B 5  is to be displayed. Thus, the process to decode GOP (m) must be started before a time leading ahead of the time t n−1  by a period of time it takes to display six pictures, namely, pictures B 1  to P 6 . In the picture-decoding order  243  shown in  FIG. 9 , a time t n−7  is determined to be the time to start the process to decode GOP (m). 
     In the case study shown in  FIG. 9 , reference symbol t n  denotes a time at which the display of the first picture B 1  pertaining to GOP (n) is started and the display of picture B 5  pertaining to GOP (m) is ended. 
     After the decoder  4   1  decodes pictures B 1  to P 6  pertaining to GOP (m) and displays only some of the pictures, namely, pictures B 1  to B 5 , the edit point T 1  is reached as shown in the picture-displaying order  242  of  FIG. 9 . The process then is continued to processing to decode pictures B 1  to P 6  pertaining to GOP (n) and display only some of the pictures, namely, pictures B 1  to B 5 . This is because picture P 6  pertaining to GOP (n) is not displayed. In decoding pictures B 4  and B 5  pertaining to GOP (n), however, picture P 6  pertaining to GOP (n) is referenced. Thus, picture P 6  pertaining to GOP (n) must be decoded first before pictures B 4  and B 5 . By the way, the processing to decode and display GOP (n) must start at the edit point T 3  and end at the edit point T 4 . 
     Since picture B 1  pertaining to group GOP (n) is a picture at which the edit point T 3  has been set, as shown in a picture-decoding order  245  of  FIG. 9 , the controller  7  controls the other one of the two decoders  4   1  and  4   2  (that is, the decoder  4   2  in this case) to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n−1) immediately preceding GOP (n) and then pictures I 3 , B 1 , B 2 , P 6 , B 4  and B 5  pertaining to GOP (n) in accordance with the schedule shown on the first line explained earlier by referring to  FIG. 7 . It is to be noted that, in the process to decode pictures I 3 , B 1 , B 2 , P 6 , B 4  and B 5  pertaining to GOP (n), the processing to decode picture B 1  at which the edit point T 3  has been set must be completed by a time t n , that is, a point of time picture B 1  is to be displayed. Thus, the process to decode picture I 3  pertaining to GOP (n−1) must be started before a time leading ahead of the time t n  by a period of time it takes to display seven pictures because the start of the processing to decode picture I 3  pertaining to GOP (n−1) leads ahead of the end of the processing to decode picture B 1  pertaining to GOP (n) by a period of time it takes to display seven pictures. In the picture-decoding order  245  shown in  FIG. 9 , a time t n−7  is determined to be the time to start the processing to decode I 3  pertaining to GOP (n−1). 
     In the mean time, the decoder  4   1  ends the decoding process after the processing to decode pictures I 3 , B 1 , B 2 , P 6 , B 4  and B 5  pertaining to GOP (m) is completed. When the decoder  4   1  ends the decoding process, the decoding process in the decoder  4   2  is going on till picture B 5  pertaining to GOP (n) is decoded. After picture B 5  pertaining to GOP (n) is displayed, the edit point T 4  is reached as shown in the picture-displaying order  242  shown in  FIG. 9 . Thereafter, GOP (m+2) starting at the edit point T 2  and subsequent GOPs must be decoded and displayed. 
     Since picture B 1  pertaining to group GOP (m+2) is a picture at which the edit point T 2  has been set, as shown in a picture-decoding order  244  of  FIG. 9 , the controller  7  controls the decoder  4   1  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (m+1) immediately preceding GOP (m+2) and then pictures I 3 , B 1 , B 2 , P 6 , B 4 , B 5  and so on pertaining to GOP (m+2) in accordance with the schedule shown on the first line explained earlier by referring to  FIG. 7 . 
     In the process to decode pictures I 3 , B 1 , B 2 , P 6 , B 4 , B 5  and so on pertaining to GOP (m+2), the processing to decode picture B 1  at which the edit point T 2  has been set must be completed by a time t n ′, that is, a point of time picture B 1  is to be displayed. Thus, the process to decode picture I 3  pertaining to GOP (m+1) must be started before a time leading ahead of the time t n ′ by a period of time it takes to display seven pictures because the start of the processing to decode picture I 3  pertaining to GOP (m+1) leads ahead of the end of the processing to decode picture B 1  pertaining to GOP (m+2) by a period of time it takes to display seven pictures. In the picture-decoding order  244  shown in  FIG. 9 , a time t n−7 ′ is determined to be the time to start the processing to decode I 3  pertaining to GOP (m+1). 
     In the case study shown in  FIG. 9 , reference symbol t n ′ denotes a time at which the display of the first picture B 1  pertaining to GOP (m+2) is started and the display of the last picture B 5  pertaining to GOP (n) is ended. 
     In the picture-decoding orders  243  and  244  shown in  FIG. 9 , the time t n−1  is a time at which the processing carried out by the decoder  4   1  to decode picture B 5  pertaining to GOP (m) is ended. On the other hand, the time t n−7 ′ is a time at which the processing carried out by the decoder  4   1  to decode GOP (m+1) is started. Thus, the decoding process carried out by the decoder  4   1  does not fail as long as the difference (t n−7 ′−t n−1 ) is at least 0. 
     Let the unity (or 1) represent a period of time it takes to display one frame (or one picture). In this case, the times t n−1  and t n−7 ′ can be represented by expressions (t n −1) and (t n ′−7) respectively. Thus, the difference (t n−7 ′−t n−1 ) can be represented by an expression {(t n ′−7)−(t n −1)}=(t n ′−t n −6). 
     On the other hand, in the case study shown in  FIG. 9 , the time t n  is a time to start an operation to display picture B 1  pertaining to GOP (n) as a picture at which the edit point T 3  has been set as an IN point whereas the time t n ′ is a time to end an operation to display picture B 5  pertaining to GOP (n) as a picture at which the edit point T 4  has been set as the OUT point following the IN point. The difference (t n ′−t n ) is the shortest cut duration, which is only 5 smaller than 6. 
     Accordingly, the difference (t n−7 ′−t n−1 ) is −1 so that the decoding process carried out by the decoder  4   1  does fail. 
     To put it in detail, in the decoding process carried out by the decoder  4   1 , a timing to decode picture B 5  pertaining to GOP (m) as shown in the picture-decoding order  243  of  FIG. 9  coincides with a timing to decode picture I 3  pertaining to GOP (m+1) as shown in the picture-decoding order  244  of the same figure. As a result, a seamless reproduction operation cannot be carried out. 
     Next, consider a case in which an edit point is set at a picture pertaining to GOP (m) as an OUT point and another edit point is set at a picture pertaining to GOP (n) as an IN point as shown in  FIG. 10 . In this case, after displaying frame A 7 , the operation to display frame C 4  shown in  FIG. 10  must be started as shown in a picture-displaying order  251  of  FIG. 10 . Frame A 7  pertaining to GOP (m) as shown in  FIG. 10  is the picture at which the aforementioned OUT point is set. On the other hand, frame C 4  pertaining to GOP (n) as shown in  FIG. 10  is the picture at which the aforementioned IN point is set. 
     Reference numeral  252  shown in  FIG. 10  denotes a sequence of pictures output by the decoder  4   1 , which is employed in the disc system shown in  FIG. 5 , as a result of decoding. On the other hand, reference numeral  253  shown in  FIG. 10  denotes a sequence of pictures output by the decoder  4   2  as a result of decoding. Thus, the switcher  5  selects the pictures output by the decoder  4   1 , ending with the picture at which the OUT point has been set. Thereafter, the switcher  5  selects the pictures output by the decoder  4   2 , starting with the picture at which the IN point has been set. 
     The decoder  4   2  must thus decode and output the pictures to be selected by the switcher  5 , starting with frame C 4  at which the IN point has been set. In order for the decoder  4   2  to decode the pictures to be selected by the switcher  5 , fewest possible specific pictures, which are referenced in decoding the pictures to be selected by the switcher  5  even though the specific pictures themselves are not to be displayed, must have been decoded before the processing to decode frame C 4  serving as an IN-point picture is decoded as described earlier. 
     Let the fewest possible specific pictures, which are referenced in decoding the picture to be selected by the switcher  5  even though the specific pictures themselves are not to be displayed, be referred to as preparatorily decoded pictures. Also, let a period of time it takes to decode the preparatorily decoded pictures be referred to as a preparatory decoding time. In the processing to set schedules at the step S 2  of the flowchart shown in  FIG. 6 , the controller  7  carries out preparatory-picture computation processing to determine the preparatorily decoded pictures and find the preparatory decoding time. Then, on the basis of the preparatorily decoded pictures and the preparatory decoding time, the controller  7  sets schedules of picture sequences to be decoded by the decoders  4   1  and  4   2 . 
     The preparatory-picture computation processing carried out by the controller  7  is explained by referring to a flowchart shown in  FIG. 11  as follows. 
     In the preparatory-picture computation processing, the controller  7  picks one of edit points that are each described on the edit-point list as an IN point as an STP. An STP is an IN point that has not been processed before in the preparatory-picture computation processing and must thus be chosen as an object of processing. At a step S 11 , the controller  7  forms a judgment as to whether or not the specific GOP (n) picture, at which the picked observed edit point STP has been set, is picture B 1  or B 2 . 
     If the outcome of the judgment formed at the step S 11  indicates that the specific picture, at which the picked observed edit point STP has been set, is picture B 1  or B 2 , the flow of the processing goes on to a step S 12  at which the controller  7  determines pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n−1) immediately preceding GOP (n) to be the preparatorily decoded pictures for the observed edit point STP. Then, the flow of the processing goes on to a step S 13 . 
     The controller  7  determines pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n−1) immediately preceding GOP (n) to be the preparatorily decoded pictures for the observed edit point STP as follows. 
     The controller  7  searches pictures along the sequence of displayed pictures starting with the picture, at which the observed edit point STP has been set, in an upstream direction for a first I picture. Then, the controller  7  makes a list of I and P pictures on the sequence in the downstream direction, between the first I picture and the picture, at which the observed edit point STP has been set. The list including the first I picture but excluding the picture, at which the observed edit point STP has been set, is a list of the preparatorily decoded pictures. 
     At the step S 13 , the controller  7  computes the picture count of the list of the preparatorily decoded pictures, namely, pictures I 3 , P 6 , P 9 , P 12  and P 15  which have been determined at the step S 12 . Since the picture count is five, which is the number of pictures on the list of preparatorily decoded pictures I 3 , P 6 , P 9 , P 12  ad P 15 , the controller  7  finds out that the preparatory decoding time is equal to a period of time it takes to decode five frames. Then, the flow of the processing goes back to the step S 11  to repeat the processing described above. To be more specific, at this step S 11 , the controller  7  picks one of edit points that are each described on the edit-point list as an IN point as a new edit point STP not processed yet in the preparatory-picture computation processing. 
     If the outcome of the judgment formed at the step S 11  indicates that the specific picture, at which the picked observed edit point STP has been set, is neither picture B 1  nor B 2 , on the other hand, the flow of the processing goes on to a step S 14 . At the step S 14 , the controller  7  forms a judgment as to whether or not the specific GOP (n) picture, at which the picked observed edit point STP has been set, is picture I 3 . 
     If the outcome of the judgment formed at the step S 14  indicates that the specific GOP (n) picture, at which the picked observed edit point STP has been set, is picture I 3 , the flow of the processing goes on to a step S 15  at which the controller  7  confirms that this specific picture pertaining to GOP (n) does not have pictures to be preparatorily decoded for this specific picture. In this case, the flow of the processing goes on to a step S 16 . At the step S 16 , the controller  7  computes the picture count of the list of the preparatorily decoded pictures. In this case, the picture count is zero. Since the picture count is zero, which is the number of pictures on the list of preparatorily decoded pictures, the controller  7  finds out that the preparatory decoding time is equal to a period of time it takes to decode 0 frames. That is to say, the preparatory decoding time is 0. Then, the flow of the processing goes back to the step S 11  to repeat the processing described above. To be more specific, at this step S 11 , the controller  7  picks one of edit points that are each described on the edit-point list as an IN point as a new edit point STP not processed yet in the preparatory-picture computation processing. 
     If the outcome of the judgment formed at the step S 14  indicates that the specific GOP (n) picture, at which the picked observed edit point STP has been set, is not picture I 3 , on the other hand, the flow of the processing goes on to a step S 17  at which the controller  7  forms a judgment as to whether or not the specific GOP (n) picture, at which the picked observed edit point STP has been set, is picture B 4 , B 5  or P 6 . 
     If the outcome of the judgment formed at the step S 17  indicates that the specific GOP (n) picture, at which the picked observed edit point STP has been set, is picture B 4 , B 5  or P 6 , the flow of the processing goes on to a step S 18  at which the controller  7  determines picture I 3  pertaining to GOP (n) to be the preparatorily decoded picture for the observed edit point STP. Then, the flow of the processing goes on to a step S 19 . At the step S 19 , the controller  7  computes the picture count of the list of the preparatorily decoded pictures. In this case, the picture count is one. Since the picture count is one, which is the number of pictures on the list of preparatorily decoded picture I 3 , the controller  7  finds out that the preparatory decoding time is equal to a period of time it takes to decode one frame. Then, the flow of the processing goes back to the step S 11  to repeat the processing described above. To be more specific, at this step S 11 , the controller  7  picks one of edit points that are each described on the edit-point list as an IN point as a new edit point STP not processed yet in the preparatory-picture computation processing. 
     If the outcome of the judgment formed at the step S 17  indicates that the specific GOP (n) picture, at which the picked observed edit point STP has been set, is not picture B 4 , B 5  or P 6 , on the other hand, the flow of the processing goes on to a step S 20  at which the controller  7  forms a judgment as to whether or not the specific GOP (n) picture, at which the picked observed edit point STP has been set, is picture B 7 , B 8  or P 9 . 
     If the outcome of the judgment formed at the step S 20  indicates that the specific GOP (n) picture, at which the picked observed edit point STP has been set, is picture B 7 , B 8  or P 9 , the flow of the processing goes on to a step S 21  at which the controller  7  determines pictures I 3  and P 6  pertaining to GOP (n) to be the preparatorily decoded pictures for the observed edit point STP. Then, the flow of the processing goes on to a step S 22 . At the step S 22 , the controller  7  computes the picture count of the list of the preparatorily decoded pictures. In this case, the picture count is two. Since the picture count is two, which is the number of pictures on the list of preparatorily decoded pictures I 3  and P 6 , the controller  7  finds out that the preparatory decoding time is equal to a period of time it takes to decode two frames. Then, the flow of the processing goes back to the step S 11  to repeat the processing described above. To be more specific, at this step S 11 , the controller  7  picks one of edit points that are each described on the edit-point list as an IN point as a new edit point STP not processed yet in the preparatory-picture computation processing. 
     If the outcome of the judgment formed at the step S 20  indicates that the specific GOP (n) picture, at which the picked observed edit point STP has been set, is not picture B 7 , B 8  or P 9 , on the other hand, the flow of the processing goes on to a step S 23  at which the controller  7  forms a judgment as to whether or not the specific GOP (n) picture, at which the picked observed edit point STP has been set, is picture B 10 , B 11  or P 12 . 
     If the outcome of the judgment formed at the step S 23  indicates that the specific GOP (n) picture, at which the picked observed edit point STP has been set, is picture B 10 , B 11  or P 12 , the flow of the processing goes on to a step S 24  at which the controller  7  determines pictures I 3 , P 6  and P 9  pertaining to GOP (n) to be the preparatorily decoded pictures for the observed edit point STP. Then, the flow of the processing goes on to a step S 25 . At the step S 25 , the controller  7  computes the picture count of the list of the preparatorily decoded pictures. In this case, the picture count is three. Since the picture count is three, which is the number of pictures on the list of preparatorily decoded pictures I 3 , P 6  and P 9 , the controller  7  finds out that the preparatory decoding time is equal to a period of time it takes to decode three frames. Then, the flow of the processing goes back to the step S 11  to repeat the processing described above. To be more specific, at this step S 11 , the controller  7  picks one of edit points that are each described on the edit-point list as an IN point as a new edit point STP not processed yet in the preparatory-picture computation processing. 
     If the outcome of the judgment formed at the step S 23  indicates that the specific GOP (n) picture, at which the picked observed edit point STP has been set, is not picture B 10 , B 11  or P 12 , on the other hand, the flow of the processing goes on to a step S 26  at which the controller  7  determines pictures I 3 , P 6 , P 9  and P 12  pertaining to GOP (n) to be the preparatorily decoded pictures for the observed edit point STP. Then, the flow of the processing goes on to a step S 27 . At the step S 27 , the controller  7  computes the picture count of the list of the preparatorily decoded pictures. In this case, the picture count is four. Since the picture count is four, which is the number of pictures on the list of preparatorily decoded pictures I 3 , P 6 , P 9  and P 12 , the controller  7  finds out that the preparatory decoding time is equal to a period of time it takes to decode four frames. Then, the flow of the processing goes back to the step S 11  to repeat the processing described above. To be more specific, at this step S 11 , the controller  7  picks one of edit points that are each described on the edit-point list as an IN point as a new edit point STP not processed yet in the preparatory-picture computation processing. 
     It is to be noted that the execution of the preparatory-picture computation processing represented by the flowchart shown in  FIG. 11  is ended when the controller  7  no longer finds one of edit points that are each described on the edit-point list as an IN point as a new edit point STP not processed yet in the preparatory-picture computation processing. 
       FIG. 12  is a diagram showing a typical configuration of another embodiment implementing the disc system, to which the present invention is applied. Components of the other embodiment, which are identical with their respective counterparts employed in the embodiment shown in  FIG. 5 , are denoted by the same reference numerals as the counterparts. Explanations of the components identical with their counterparts are omitted if unnecessary. In the case of the disc system shown in  FIG. 2 , the number of decoders employed in the decoding unit  4  is not limited to two, namely, the decoders  4   1  and  4   2 . Instead, the decoding unit  4  includes k decoders  4   1 ,  4   2 , - - - and  4   k . The rest is the same as the embodiment shown in  FIG. 5 . It is to be noted that, typically, the decoders  4   k  (where k=1, 2, - - - and K) shown in  FIG. 12  each have a one-time decoding process speed as is the case with the decoders  4   1  and  4   2  shown in  FIG. 5 . 
     The disc system shown in  FIG. 5  is identical with the disc system shown in  FIG. 12  for K=2. As described above, the shortest cut duration of the embodiment shown in  FIG. 5  is equal to the period of time it takes to display six frames. In the case of the embodiment shown in  FIG. 12 , by increasing the number of decoders  4   1  to  4   k  composing the decoding unit  4 , that is, by setting K at a number greater than 2, the shortest cut duration can be made shorter than the period of time it takes to display six frames. 
     To put it concretely, for example, K is set at 3, that is, the decoding unit  4  includes three decoders  4   1 ,  4   2  and  4   3 . In this case, the shortest cut duration can be reduced to a period of time it takes to display three frames. 
     Now, assume that an editing process has been carried out to display pictures B 1 , B 2  and I 3  pertaining to GOP (n) beginning at a time t a , then pictures B 1 , B 2  and I 3  pertaining to GOP (n+1) beginning at a time t b , then pictures B 1 , B 2  and I 3  pertaining to GOP (n+2) beginning at a time t c  and, finally, pictures pertaining to GOP (n+3) and subsequent GOPs beginning at a time t d  as shown in a picture-displaying order  261  of  FIG. 13 . In this case, as IN points, edit points are set at picture B 1  pertaining to GOP (n) to be displayed at the time t a , picture B 1  pertaining to GOP (n+1) to be displayed at the time t b , picture B 1  pertaining to GOP (n+2) to be displayed at the time t c  and picture B 1  pertaining to GOP (n+3) to be displayed at the time t d . 
     It is to be noted that, in this case, the time t b  lags behind the time t a  by a period of time it takes to display three frames, the time t c  lags behind the time t b  by a period of time it takes to display three frames and the time t d  lags behind the time t c  by a period of time it takes to display three frames. Thus, in the picture-displaying order  261  shown in  FIG. 13 , the shortest cut duration is a period of time it takes to display the three frames between the times t a  and t b  (or the three frames between the times t b  and t c  or the three frames between the times t c  and t d ). 
     In this case, first of all, pictures B 1 , B 2  and I 3  pertaining to GOP (n) must be displayed beginning at the time t a  with picture B 1  at which an edit point has been set as an IN point. Thus, as described earlier, the controller  7  drives a specific one of the three decoders  4   1  to  4   3  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n−1) immediately preceding GOP (n) and, then, pictures I 3 , B 1  and B 2  pertaining to GOP (n) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  262  of  FIG. 13 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. For example, the decoder  4   1  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3 , B 1  and B 2  pertaining to GOP (n), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t a  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n−1) and leading ahead of picture B 1  pertaining to GOP (n) by seven pictures must be started before a time leading ahead of the time t a  by a period of time it takes to display seven pictures. In the picture-decoding order  262  shown in  FIG. 13 , a time t a−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n−1). Thus, the process to decode picture I 3  pertaining to GOP (n−1) is started at the time t a−7  whereas the process to decode picture B 2  pertaining to GOP (n) is completed by a time t a+1  lagging behind the time t a  by a period of time it takes to display one picture. Picture B 2  pertaining to GOP (n) is a picture to be decoded last among pictures I 3 , B 1  and B 2  pertaining to GOP (n). 
     After the decoder  4   1  displays decoded pictures B 1 , B 2  and I 3  pertaining to GOP (n), the time t b  is reached as shown in the picture-displaying order  261  of  FIG. 13 . Thus, pictures B 1 , B 2  and I 3  pertaining to GOP (n+1), which are to be displayed starting at the time t b , must be decoded and displayed since an edit point has been set at picture B 1  pertaining to GOP (n+1) as an IN point. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the three decoders  4   1  to  4   3  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n) immediately preceding GOP (n+1) and, then, pictures I 3 , B 1  and B 2  pertaining to GOP (n) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  264  of  FIG. 13 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. For example, the decoder  4   2  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3 , B 1  and B 2  pertaining to GOP (n+1), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t b  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n) and leading ahead of picture B 1  pertaining to GOP (n+1) by seven pictures must be started before a time leading ahead of the time t b  by a period of time it takes to display seven pictures. In the picture-decoding order  264  shown in  FIG. 13 , a time t b−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n). Thus, the process to decode picture I 3  pertaining to GOP (n) is started at the time t b−7  whereas the process to decode picture B 2  pertaining to GOP (n+1) is completed by a time t b+1  lagging behind the time t b  by a period of time it takes to display one picture. Picture B 2  pertaining to GOP (n+1) is a picture to be decoded last among pictures I 3 , B 1  and B 2  pertaining to GOP (n+1). 
     After the decoder  4   2  displays decoded pictures B 1 , B 2  and I 3  pertaining to GOP (n+1), the time t c  is reached as shown in the picture-displaying order  261  of  FIG. 13 . Thus, pictures B 1 , B 2  and I 3  pertaining to GOP (n+2), which are to be displayed starting at the time t c , must be decoded and displayed since an edit point has been set at picture B 1  pertaining to GOP (n+2) as an IN point. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the three decoders  4   1  to  4   3  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+1) immediately preceding GOP (n+2) and, then, pictures I 3 , B 1  and B 2  pertaining to GOP (n+2) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  265  of  FIG. 13 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. In this case, the only remaining decoder  4   3  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3 , B 1  and B 2  pertaining to GOP (n+2), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t c  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n+1) and leading ahead of picture B 1  pertaining to GOP (n+2) by seven pictures must be started before a time leading ahead of the time t c  by a period of time it takes to display seven pictures. In the picture-decoding order  265  shown in  FIG. 13 , a time t c−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n+1). Thus, the process to decode picture I 3  pertaining to GOP (n+1) is started at the time t c−7  whereas the process to decode picture B 2  pertaining to GOP (n+2) is completed by a time t c+1  lagging behind the time t c  by a period of time it takes to display one picture. Picture B 2  pertaining to GOP (n+2) is a picture to be decoded last among pictures I 3 , B 1  and B 2  pertaining to GOP (n+2). 
     After the decoder  4   3  displays decoded pictures B 1 , B 2  and I 3  pertaining to GOP (n+2), the time t d  is reached as shown in the picture-displaying order  261  of  FIG. 13 . Thus, pictures B 1 , B 2  and I 3  pertaining to GOP (n+3), which are to be displayed starting at the time td, and the subsequent pictures, must be decoded and displayed since an edit point has been set at picture B 1  pertaining to GOP (n+3) as an IN point. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the 3 decoders  4   1  to  4   3  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+2) immediately preceding GOP (n+3) in order to decode picture B 1  pertaining to GOP (n+3) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  263  of  FIG. 13 . 
     It is to be noted that, in the process to decode pictures pertaining to GOP (n+3), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t d  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n+2) and leading ahead of picture B 1  pertaining to GOP (n+3) by seven pictures must be started before a time leading ahead of the time t d  by a period of time it takes to display seven pictures. In the picture-decoding order  263  shown in  FIG. 13 , a time t d−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n+2). 
     By the way, with the timing of the time t d−7 , while the decoders  4   2  and  4   3  are each still carrying out a decoding process as is obvious from respectively the picture-decoding orders  264  and  265  shown in  FIG. 13 , the decoder  4   1  has completed its decoding process at the time t a+1  leading ahead of the time t d−7  by a period of time it takes to display one picture as shown in the picture-decoding orders  262  and  263  of  FIG. 13 . Thus, at the time t d−7 , the decoder  4   1  is available for a decoding process. For this reason, the controller  7  drives the decoder  4   1  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+2) and, then, pictures I 3 , B 1  and B 2  pertaining to GOP (n+3). 
     As is obvious from the above description, if the shortest cut duration is equal to a period of time it takes to display three frames, there is no case in which the decoders  4   1 ,  4   2  and  4   3  must decode a plurality of pictures at the same time. 
     Thus, by designing the decoding unit  4  to include the three decoders  4   1 ,  4   2  and  4   3 , the shortest cut duration can be reduced to a period of time it takes to display three frames. That is to say, the embodiment employing three decoders allows the shortest cut duration to be reduced to a period shorter than the embodiment wherein the decoding unit  4  includes only the two decoders  4   1  and  4   2  as shown in  FIG. 5 . As a result, the freedom of editing can be further improved. 
     The following description explains an implementation of the embodiment realizing the disc system as shown in  FIG. 12  wherein the shortest cut duration can be further reduced to a period of time it takes to display two frames by designing the decoding unit  4  to include four decoders  4   1 ,  4   2 ,  4   3  and  4   4 , that is, by setting the value of K at 4. 
     To put it in detail, now, assume that an editing process has been carried out to display pictures B 1  and B 2  pertaining to GOP (n) beginning at a time t a , then pictures B 1  and B 2  pertaining to GOP (n+1) beginning at a time t b , then pictures B 1  and B 2  pertaining to GOP (n+2) beginning at a time t c , then pictures B 1  and B 2  pertaining to GOP (n+3) beginning at a time t d  and, finally, pictures pertaining to GOP (n+4) and subsequent GOPs beginning at a time t e  as shown in a picture-displaying order  271  of  FIG. 14 . In this case, as IN points, edit points are set at picture B 1  pertaining to GOP (n) to be displayed at the time t a , picture B 1  pertaining to GOP (n+1) to be displayed at the time t b , picture B 1  pertaining to GOP (n+2) to be displayed at the time t c , picture B 1  pertaining to GOP (n+3) to be displayed at the time t d  and picture B 1  pertaining to GOP (n+4) to be displayed at the time t e . 
     It is to be noted that, in this case, the time t b  lags behind the time t a  by a period of time it takes to display two frames, the time t c  lags behind the time t b  by a period of time it takes to display two frames, the time t d  lags behind the time t c  by a period of time it takes to display two frames and the time t e  lags behind the time t d  by a period of time it takes to display two frames. Thus, in the picture-displaying order  271  shown in  FIG. 14 , the shortest cut duration is a period of time it takes to display the two frames between the times t a  and t b  (or the two frames between the times t b  and t c , the two frames between the times t c  and t d  or the two frames between the times t d  and t e ). 
     In this case, first of all, pictures B 1  and B 2  pertaining to GOP (n) must be displayed beginning at the time t a  with picture B 1  at which an edit point has been set as an IN point. Thus, as described earlier, the controller  7  drives a specific one of the four decoders  4   1  to  4   4  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n−1) immediately preceding GOP (n) and, then, pictures I 3 , B 1  and B 2  pertaining to GOP (n) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  272  of  FIG. 14 . Even though picture I 3  pertaining to GOP (n) is not to be displayed, picture I 3  is referenced in decoding pictures B 1  and B 2  pertaining to the same group. Thus, picture I 3  pertaining to GOP (n) must also be decoded. The specific decoder is a decoder that is not carrying out a decoding process at the present time. For example, the decoder  4   1  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3 , B 1  and B 2  pertaining to GOP (n), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t a  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n−1) and leading ahead of picture B 1  pertaining to GOP (n) by seven pictures must be started before a time leading ahead of the time t a  by a period of time it takes to display seven pictures. In the picture-decoding order  272  shown in  FIG. 14 , a time t a−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n−1). Thus, the process to decode picture I 3  pertaining to GOP (n−1) is started at the time t a−7  whereas the process to decode picture B 2  pertaining to GOP (n) is completed by a time t a+1  lagging behind the time t a  by a period of time it takes to display one picture. Picture B 2  pertaining to GOP (n) is a picture to be decoded last among pictures I 3 , B 1  and B 2  pertaining to GOP (n). 
     After the decoder  4   1  decodes pictures I 3 , B 1 , and B 2  pertaining to GOP (n) and displays only pictures B 1  and B 2  pertaining to the same group, the time t b  is reached as shown in the picture-displaying order  271  of  FIG. 14 . Thus, pictures B 1 , B 2  and I 3  pertaining to GOP (n+1) must be decoded in order to display pictures B 1  and B 2  pertaining to the same group, starting at the display time t b  of picture B 1  pertaining to the same group since an edit point has been set at picture B 1  as an IN point. Even though picture I 3  pertaining to GOP (n+1) is not to be displayed, picture I 3  is referenced in decoding pictures B 1  and B 2  pertaining to the same group. Thus, picture I 3  pertaining to GOP (n+1) must also be decoded. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the four decoders  4   1  to  4   4  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n) immediately preceding GOP (n+1) and, then, pictures I 3 , B 1  and B 2  pertaining to GOP (n+1) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  273  of  FIG. 14 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. For example, the decoder  4   2  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3 , B 1  and B 2  pertaining to GOP (n+1), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t b  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n) and leading ahead of picture B 1  pertaining to GOP (n+1) by seven pictures must be started before a time leading ahead of the time t b  by a period of time it takes to display seven pictures. In the picture-decoding order  274  shown in  FIG. 14 , a time t b−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n). Thus, the process to decode picture I 3  pertaining to GOP (n) is started at the time t b−7  whereas the process to decode picture B 2  pertaining to GOP (n+1) is completed by a time t b+1  lagging behind the time t b  by a period of time it takes to display one picture. Picture B 2  pertaining to GOP (n+1) is a picture to be decoded last among pictures I 3 , B 1  and B 2  pertaining to GOP (n+1). 
     After the decoder  4   2  decodes pictures I 3 , B 1  and B 2  pertaining to GOP (n) and displays only pictures B 1  and B 2  pertaining to the same group, the time t c  is reached as shown in the picture-displaying order  271  of  FIG. 14 . Thus, pictures B 1 , B 2  and I 3  pertaining to GOP (n+2) must be decoded in order to display pictures B 1  and B 2  pertaining to the same group, starting at the display time t c  of picture B 1  pertaining to the same group since an edit point has been set at picture B 1  pertaining to GOP (n+2) as an IN point. Even though picture I 3  pertaining to GOP (n+2) is not to be displayed, picture I 3  is referenced in decoding pictures B 1  and B 2  pertaining to the same group. Thus, picture I 3  pertaining to GOP (n+2) must also be decoded. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the four decoders  4   1  to  4   4  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+1) immediately preceding GOP (n+2) and, then, pictures I 3 , B 1  and B 2  pertaining to GOP (n+2) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  274  of  FIG. 14 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. For example, the decoder  4   3  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3 , B 1  and B 2  pertaining to GOP (n+2), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t c  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n+1) and leading ahead of picture B 1  pertaining to GOP (n+2) by seven pictures must be started before a time leading ahead of the time t c  by a period of time it takes to display seven pictures. In the picture-decoding order  274  shown in  FIG. 14 , a time t c−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n+1). Thus, the process to decode picture I 3  pertaining to GOP (n+1) is started at the time t c−7  whereas the process to decode picture B 2  pertaining to GOP (n+2) is completed by a time t c+1  lagging behind the time t c  by a period of time it takes to display one picture. Picture B 2  pertaining to GOP (n+2) is a picture to be decoded last among pictures I 3 , B 1  and B 2  pertaining to GOP (n+2). 
     After the decoder  4   3  decodes pictures I 3 , B 1  and B 2  pertaining to GOP (n+2) and displays only pictures B 1  and B 2  pertaining to the same group, the time t d  is reached as shown in the picture-displaying order  271  of  FIG. 14 . Thus, pictures B 1 , B 2  and I 3  pertaining to GOP (n+3) must be decoded in order to display pictures B 1  and B 2  pertaining to the same group, starting at the display time t d  of picture B 1  pertaining to the same group since an edit point has been set at picture B 1  as an IN point. Even though picture I 3  pertaining to GOP (n+3) is not to be displayed, picture I 3  is referenced in decoding pictures B 1  and B 2  pertaining to the same group. Thus, picture I 3  pertaining to GOP (n+3) must also be decoded. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the four decoders  4   1  to  4   4  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+2) immediately preceding GOP (n+3) and, then, pictures I 3 , B 1  and B 2  pertaining to GOP (n+3) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  275  of  FIG. 14 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. In this case, the only remaining decoder  4   4  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3 , B 1  and B 2  pertaining to GOP (n+3), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t d  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n+2) and leading ahead of picture B 1  pertaining to GOP (n+3) by seven pictures must be started before a time leading ahead of the time t d  by a period of time it takes to display seven pictures. In the picture-decoding order  275  shown in  FIG. 14 , a time t d−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n+2). Thus, the process to decode picture I 3  pertaining to GOP (n+2) is started at the time t d−7  whereas the process to decode picture B 2  pertaining to GOP (n+3) is completed by a time t d+1  lagging behind the time t d  by a period of time it takes to display one picture. Picture B 2  pertaining to GOP (n+3) is a picture to be decoded last among pictures I 3 , B 1  and B 2  pertaining to GOP (n+3). 
     After the decoder  4   4  decodes pictures I 3 , B 1  and B 2  pertaining to GOP (n+3) and displays only pictures B 1  and B 2  pertaining to the same group, the time t e  is reached as shown in the picture-displaying order  271  of  FIG. 14 . Thus, pictures B 1 , B 2  and I 3  pertaining to GOP (n+4) and subsequent pictures must be decoded, starting at the display time t e  of picture B 1  pertaining to the same group since an edit point has been set at picture B 1  pertaining to GOP (n+4) as an IN point. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the four decoders  4   1  to  4   4  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+3) immediately preceding GOP (n+4) in order to decode picture B 1  pertaining to GOP (n+4) in accordance with the schedule shown on the first line of  FIG. 7  as shown in the picture-decoding order  272  of  FIG. 14 . 
     It is to be noted that, in the process to decode pictures pertaining to GOP (n+4), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t e  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n+3) and leading ahead of picture B 1  pertaining to GOP (n+4) by seven pictures must be started before a time leading ahead of the time t e  by a period of time it takes to display seven pictures. In the picture-decoding order  272  shown in  FIG. 14 , a time t e−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n+3). 
     By the way, with the timing of the time t e−7 , while the decoders  4   2 ,  4   3  and  4   4  are each still carrying out a decoding process as is obvious from respectively the picture-decoding orders  273 ,  274  and  275  shown in  FIG. 14 , the decoder  4   1  has completed its decoding process at the time t a+1  coinciding with the time t e−7  as shown in the picture-decoding order  272  of  FIG. 14 . Thus, at the time t e−7 , the decoder  4   1  is available for a decoding process. 
     For this reason, the controller  7  drives the decoder  4   1  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+3) and, then, pictures I 3 , B 1  and B 2  pertaining to GOP (n+4) as shown in the picture-decoding order  272  of  FIG. 14 . 
     As is obvious from the above description, even the shortest cut duration is equal to a period of time it takes to display two frames, there is no case in which the decoders  4   1 ,  4   2 ,  4   3  and  4   4  must decode a plurality of pictures at the same time. Thus, by designing the decoding unit  4  to include the four decoders  4   1 ,  4   2 ,  4   3  and  4   4 , the shortest cut duration can be reduced to a period of time it takes to display two frames. That is to say, the embodiment employing four decoders allows the shortest cut duration to be reduced to a period shorter than the embodiment wherein the decoding unit  4  includes only the three decoders  4   1 ,  4   2  and  4   3  as described earlier by referring to  FIG. 13 . As a result, the freedom of editing can be further improved. 
     The following description explains an implementation of the embodiment realizing the disc system as shown in  FIG. 12  wherein the shortest cut duration can be further reduced to a period of time it takes to display one frame by designing the decoding unit  4  to comprise seven decoders  4   1 ,  4   2 ,  4   3 ,  4   4 ,  4   5 ,  4   6  and  4   7 , that is, by setting the value of K at 7. 
     To put it in detail, now, assume that an editing process has been carried out to display picture B 1  pertaining to GOP (n) beginning at a time t a , then picture B 1  pertaining to GOP (n+1) beginning at a time t b , then picture B 1  pertaining to GOP (n+2) beginning at a time t c , then pictures B 1  and B 2  pertaining to GOP (n+3) beginning at a time t d , then pictures B 1  and B 2  pertaining to GOP (n+4) beginning at a time t e , then pictures B 1  and B 2  pertaining to GOP (n+5) beginning at a time t f , then pictures B 1  and B 2  pertaining to GOP (n+6) beginning at a time t g  and, finally, pictures pertaining to GOP (n+7) and subsequent GOPs beginning at a time t h  as shown in a picture-displaying order  281  of  FIG. 15 . In this case, as IN points, edit points are set at picture B 1  pertaining to GOP (n) to be displayed at the time t a , picture B 1  pertaining to GOP (n+1) to be displayed at the time t b , picture B 1  pertaining to GOP (n+2) to be displayed at the time t c , picture B 1  pertaining to GOP (n+3) to be displayed at the time t d , B 1  pertaining to GOP (n+4) to be displayed at the time t e , B 1  pertaining to GOP (n+5) to be displayed at the time t f , B 1  pertaining to GOP (n+6) to be displayed at the time t g  and picture B 1  pertaining to GOP (n+7) to be displayed at the time t h . 
     It is to be noted that, in this case, the time t b  lags behind the time t e  by a period of time it takes to display one frame, the time t c  lags behind the time t b  by a period of time it takes to display one frame, the time t d  lags behind the time t c  by a period of time it takes to display one frame, the time t e  lags behind the time t d  by a period of time it takes to display one frame, the time t f  lags behind the time t e  by a period of time it takes to display one frame, the time t g  lags behind the time t f  by a period of time it takes to display one frame and the time t h  lags behind the time t g  by a period of time it takes to display one frame. Thus, in the picture-displaying order  281  shown in  FIG. 15 , the shortest cut duration is a period of time it takes to display the one frame between the times t a  and t b  (or the one frame between the times t b  and t c , the one frame between the times t c  and td, the one frame between the times td and t e , the one frame between the times t e  and t f , the one frame between the times t f  and t g  or the one frame between the times t g  and t h ). 
     In this case, first of all, pictures B 1  and I 3  pertaining to GOP (n) must be displayed beginning at the time t a  with picture B 1  at which an edit point has been set as an IN point. Thus, as described earlier, the controller  7  drives a specific one of the seven decoders  4   1  to  4   7  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n−1) immediately preceding GOP (n) and, then, pictures I 3  and B 1  pertaining to GOP (n) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  282  of  FIG. 15 . Even though picture I 3  pertaining to GOP (n) is not to be displayed, picture I 3  is referenced in decoding pictures B 1  and B 2  pertaining to the same group. Thus, picture I 3  pertaining to GOP (n) must also be decoded. The specific decoder is a decoder that is not carrying out a decoding process at the present time. For example, the decoder  4   1  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3  and B 1  pertaining to GOP (n), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t a  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n−1) and leading ahead of picture B 1  pertaining to GOP (n) by seven pictures must be started before a time leading ahead of the time t a  by a period of time it takes to display seven pictures. In the picture-decoding order  282  shown in  FIG. 15 , a time t a−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n−1). Thus, the process to decode picture I 3  pertaining to GOP (n−1) is started at the time t a−7  whereas the process to decode picture B 1  pertaining to GOP (n) is completed by the time t a . Picture B 1  pertaining to GOP (n) is a picture to be decoded last among pictures I 3  and B 1  pertaining to GOP (n). 
     After the decoder  4   1  decodes pictures I 3  and B 1  pertaining to GOP (n) and displays only picture B 1  pertaining to the same group, the time t b  is reached as shown in the picture-displaying order  281  of  FIG. 15 . Thus, pictures B 1  and I 3  pertaining to GOP (n+1) must be decoded in order to display picture B 1  pertaining to the same group, starting at the display time t b  of picture B 1  pertaining to the same group since an edit point has been set at picture B 1  as an IN point. Even though picture I 3  pertaining to GOP (n+1) is not to be displayed, picture I 3  is referenced in decoding picture B 1  pertaining to the same group. Thus, picture I 3  pertaining to GOP (n+1) must also be decoded. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the seven decoders  4   1  to  4   7  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n) immediately preceding GOP (n+1) and, then, pictures I 3  and B 1  pertaining to GOP (n+1) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  283  of  FIG. 15 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. For example, the decoder  4   2  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3  and B 1  pertaining to GOP (n+1), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t b  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n) and leading ahead of picture B 1  pertaining to GOP (n+1) by seven pictures must be started before a time leading ahead of the time t b  by a period of time it takes to display seven pictures. In the picture-decoding order  284  shown in  FIG. 15 , a time t b−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n). Thus, the process to decode picture I 3  pertaining to GOP (n) is started at the time t b−7  whereas the process to decode picture B 1  pertaining to GOP (n+1) is completed by the time t b . Picture B 1  pertaining to GOP (n+1) is a picture to be decoded last among pictures I 3  and B 1  pertaining to GOP (n+1). 
     After the decoder  4   2  decodes pictures I 3  and B 1  pertaining to GOP (n+1) and displays only picture B 1  pertaining to the same group, the time t c  is reached as shown in the picture-displaying order  281  of  FIG. 15 . Thus, pictures B 1  and I 3  pertaining to GOP (n+2) must be decoded in order to display picture B 1  pertaining to the same group, starting at the display time t c  of picture B 1  pertaining to the group since an edit point has been set at picture B 1  as an IN point. Even though picture I 3  pertaining to GOP (n+2) is not to be displayed, picture I 3  is referenced in decoding picture B 1  pertaining to the same group. Thus, picture I 3  pertaining to GOP (n+2) must also be decoded. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the seven decoders  4   1  to  4   7  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+1) immediately preceding GOP (n+2) and, then, pictures I 3  and B 1  pertaining to GOP (n+2) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  284  of  FIG. 15 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. For example, the decoder  4   3  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3  and B 1  pertaining to GOP (n+2), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t c  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n+1) and leading ahead of picture B 1  pertaining to GOP (n+2) by seven pictures must be started before a time leading ahead of the time t c  by a period of time it takes to display seven pictures. In the picture-decoding order  284  shown in  FIG. 15 , a time t c−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n+1). Thus, the process to decode picture I 3  pertaining to GOP (n+1) is started at the time t c−7  whereas the process to decode picture B 1  pertaining to GOP (n+2) is completed by the time t c . Picture B 1  pertaining to GOP (n+2) is a picture to be decoded last among pictures I 3  and B 1  pertaining to GOP (n+2). 
     After the decoder  4   3  decodes pictures I 3  and B 1  pertaining to GOP (n+2) and displays only picture B 1  pertaining to the same group, the time t d  is reached as shown in the picture-displaying order  281  of  FIG. 15 . Thus, pictures B 1  and I 3  pertaining to GOP (n+3) must be decoded in order to display picture B 1  pertaining to the same group, starting at the display time t d  of picture B 1  pertaining to the group since an edit point has been set at picture B 1  pertaining to GOP (n+3) as an IN point. Even though picture I 3  pertaining to GOP (n+3) is not to be displayed, picture I 3  is referenced in decoding picture B 1  pertaining to the same group. Thus, picture I 3  pertaining to GOP (n+3) must also be decoded. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the seven decoders  4   1  to  4   7  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+2) immediately preceding GOP (n+3) and, then, pictures I 3  and B 1  pertaining to GOP (n+3) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  285  of  FIG. 15 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. For example, the decoder  4   4  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3  and B 1  pertaining to GOP (n+3), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t d  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n+2) and leading ahead of picture B 1  pertaining to GOP (n+3) by seven pictures must be started before a time leading ahead of the time t d  by a period of time it takes to display seven pictures. In the picture-decoding order  285  shown in  FIG. 15 , a time t d−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n+2). Thus, the process to decode picture I 3  pertaining to GOP (n+2) is started at the time t d−7  whereas the process to decode picture B 1  pertaining to GOP (n+3) is completed by the time t d . Picture B 1  pertaining to GOP (n+3) is a picture to be decoded last among pictures I 3  and B 1  pertaining to GOP (n+3). 
     After the decoder  4   4  decodes pictures I 3  and B 1  pertaining to GOP (n+3) and displays only picture B 1  pertaining to the same group, the time t e  is reached as shown in the picture-displaying order  281  of  FIG. 15 . Thus, pictures B 1  and I 3  pertaining to GOP (n+4) must be decoded in order to display picture B 1  pertaining to the same group, starting at the display time t e  of picture B 1  pertaining to the group since an edit point has been set at picture B 1  pertaining to GOP (n+4) as an IN point. Even though picture I 3  pertaining to GOP (n+4) is not to be displayed, picture I 3  is referenced in decoding picture B 1  pertaining to the same group. Thus, picture I 3  pertaining to GOP (n+4) must also be decoded. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the seven decoders  4   1  to  4   7  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+3) immediately preceding GOP (n+4) and, then, pictures I 3  and B 1  pertaining to GOP (n+4) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  286  of  FIG. 15 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. For example, the decoder  4   5  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3  and B 1  pertaining to GOP (n+4), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t e  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n+3) and leading ahead of picture B 1  pertaining to GOP (n+4) by seven pictures must be started before a time leading ahead of the time t e  by a period of time it takes to display seven pictures. In the picture-decoding order  286  shown in  FIG. 15 , a time t e−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n+3). Thus, the process to decode picture I 3  pertaining to GOP (n+3) is started at the time t e−7  whereas the process to decode picture B 1  pertaining to GOP (n+4) is completed by the time t e . Picture B 1  pertaining to GOP (n+4) is a picture to be decoded last among pictures I 3  and B 1  pertaining to GOP (n+3). 
     After the decoder  4   5  decodes pictures I 3  and B 1  pertaining to GOP (n+4) and displays only picture B 1  pertaining to the same group, the time t f  is reached as shown in the picture-displaying order  281  of  FIG. 15 . Thus, pictures B 1  and I 3  pertaining to GOP (n+5) must be decoded in order to display picture B 1  pertaining to the same group, starting at the display time t f  of picture B 1  pertaining to the group since an edit point has been set at picture B 1  pertaining to GOP (n+5) as an IN point. Even though picture I 3  pertaining to GOP (n+5) is not to be displayed, picture I 3  is referenced in decoding picture B 1  pertaining to the same group. Thus, picture I 3  pertaining to GOP (n+5) must also be decoded. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the seven decoders  4   1  to  4   7  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+4) immediately preceding GOP (n+5) and, then, pictures I 3  and B 1  pertaining to GOP (n+5) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  287  of  FIG. 15 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. For example, the decoder  4   6  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3  and B 1  pertaining to GOP (n+5), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t f  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n+4) and leading ahead of picture B 1  pertaining to GOP (n+5) by seven pictures must be started before a time leading ahead of the time t f  by a period of time it takes to display seven pictures. In the picture-decoding order  287  shown in  FIG. 15 , a time t f−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n+4). Thus, the process to decode picture I 3  pertaining to GOP (n+4) is started at the time t f−7  whereas the process to decode picture B 1  pertaining to GOP (n+5) is completed by the time t f . Picture B 1  pertaining to GOP (n+5) is a picture to be decoded last among pictures I 3  and B 1  pertaining to GOP (n+5). 
     After the decoder  4   6  decodes pictures I 3  and B 1  pertaining to GOP (n+5) and displays only picture B 1  pertaining to the same group, the time t g  is reached as shown in the picture-displaying order  281  of  FIG. 15 . Thus, pictures B 1  and I 3  pertaining to GOP (n+6) must be decoded in order to display picture B 1  pertaining to the same group, starting at the display time t g  of picture B 1  pertaining to the group since an edit point has been set at picture B 1  pertaining to GOP (n+6) as an IN point. Even though picture I 3  pertaining to GOP (n+6) is not to be displayed, picture I 3  is referenced in decoding picture B 1  pertaining to the same group. Thus, picture I 3  pertaining to GOP (n+6) must also be decoded. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the seven decoders  4   1  to  4   7  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+5) immediately preceding GOP (n+6) and, then, pictures I 3  and B 1  pertaining to GOP (n+6) in accordance with the schedule shown on the first line of  FIG. 7  as shown in a picture-decoding order  288  of  FIG. 15 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. In this case, the only remaining decoder  4   7  is selected as the specific decoder. 
     It is to be noted that, in the process to decode pictures I 3  and B 1  pertaining to GOP (n+6), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t g  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n+5) and leading ahead of picture B 1  pertaining to GOP (n+6) by seven pictures must be started before a time leading ahead of the time t g  by a period of time it takes to display seven pictures. In the picture-decoding order  288  shown in  FIG. 15 , a time t g−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n+5). Thus, the process to decode picture I 3  pertaining to GOP (n+5) is started at the time t g−7  whereas the process to decode picture B 1  pertaining to GOP (n+6) is completed by the time t g . Picture B 1  pertaining to GOP (n+6) is a picture to be decoded last among pictures I 3  and B 1  pertaining to GOP (n+6). 
     After the decoder  4   7  decodes pictures I 3  and B 1  pertaining to GOP (n+6) and displays only picture B 1  pertaining to the same group, the time t h  is reached as shown in the picture-displaying order  281  of  FIG. 15 . Thus, pictures B 1 , B 2  and I 3  pertaining to GOP (n+7) and subsequent pictures must be decoded, starting at the display time t h  of picture B 1  pertaining to the group since an edit point has been set at picture B 1  pertaining to GOP (n+7) as an IN point. 
     For the reason described above, as described earlier, the controller  7  drives a specific one of the seven decoders  4   1  to  4   7  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+6) immediately preceding GOP (n+7) in order to decode picture B 1  pertaining to GOP (n+7) in accordance with the schedule shown on the first line of  FIG. 7  as shown in the picture-decoding order  282  of  FIG. 15 . The specific decoder is a decoder that is not carrying out a decoding process at the present time. In this case, the decoder  4   1  is again selected as the specific decoder for a reason described later. 
     It is to be noted that, in the process to decode pictures pertaining to GOP (n+7), the processing to decode picture B 1 , at which an edit point has been set as an IN point, must be completed by the display time t h  of picture B 1 . Thus, the process to decode picture I 3  pertaining to GOP (n+6) and leading ahead of picture B 1  pertaining to GOP (n+7) by seven pictures must be started before a time leading ahead of the time t h  by a period of time it takes to display seven pictures. In the picture-decoding order  282  shown in  FIG. 15 , a time t h−7  is determined to be the time to start the process to decode picture I 3  pertaining to GOP (n+6). 
     By the way, with the timing of the time t h−7 , while the decoders  4   2  to  4   7  are each still carrying out a decoding process as is obvious from respectively the picture-decoding orders  283  to  288  shown in  FIG. 15 , the decoder  4   1  has completed its decoding process at the time t a  coinciding with the time t h−7  as shown in the picture-decoding order  282  of  FIG. 15 . 
     Thus, at the time t h−7 , the decoder  4   1  is available for a decoding process. For this reason, the controller  7  drives the decoder  4   1  to decode pictures I 3 , P 6 , P 9 , P 12  and P 15  pertaining to GOP (n+6) and, then, pictures I 3  and B 1  pertaining to GOP (n+7) as shown in the picture-decoding order  282  shown in  FIG. 15 . 
     As is obvious from the above description, with the decoding unit  4  designed to include seven decoders  4   1  to  4   7 , there is no case in which the decoders  4   1  to  4   7  must decode a plurality of pictures at the same time even if the shortest cut duration is reduced to a period of time it takes to display one frame. Thus, by designing the decoding unit  4  to include the seven decoders  4   1  to  4   7 , the shortest cut duration can be reduced to a period of time it takes to display one frame. As a result, an editing process can be carried out without restrictions caused by the shortest cut duration. 
       FIG. 16  is a diagram showing a relation between the number of decoders  4   1  to  4   k  (K) composing the decoding unit  4  and the shortest cut duration. As shown in  FIG. 16 , if the number of decoders  4   1  to  4   k  (K) is set at 2, the shortest cut duration is equal to a period of time it takes to display six frames. If the number of decoders  4   1  to  4   k  (K) is set at 3, the shortest cut duration is equal to a period of time it takes to display three frames. If the number of decoders  4   1  to  4   k  (K) is set at 3, 4 or 5, the shortest cut duration is equal to a period of time it takes to display two frames. If the number of decoders  4   1  to  4   k  (K) is set at 7 or a greater number, the shortest cut duration is equal to a period of time it takes to display one frame. 
     It is to be noted that the relation shown in  FIG. 16  is established between the number of decoders  4   1  to  4   k  (K) composing the decoding unit  4  and the shortest cut duration for the decoders  4   1  to  4   k  (K) of the decoding unit  4  each having a one-time decoding process speed. If the decoders  4   1  to  4   k  composing the decoding unit  4  each have an L-time decoding process speed where L is a positive number), the shortest cut duration can be reduced to a period of time it takes to display one frame by composing the decoding unit  4  of at least 7/L decoders  4   1 ,  4   2  and so on. In this case, an editing process can be carried out without restrictions caused by the shortest cut duration. 
     Next, the reproduction process carried out by the disc system shown in  FIG. 12  is further explained by referring to a flowchart shown in  FIG. 17 . 
     When the user operates the operation unit  8  to start a process to reproduce a result of edit processing of an MPEG stream in accordance with an edit-point list, the operation unit  8  outputs a reproduction-start operation signal, which is an operation signal making a request for a start of a reproduction process, to the controller  7 . 
     When receiving the reproduction-start operation signal from the operation unit  8  at a step S 31 , the controller  7  issues a request for an edit-point list to the disc drive  2 . At this request, the disc drive  2  reads out the edit-point list from the optical disc  1  and then outputs the list to the bus  3 . Then, the controller  7  receives the edit-point list from the bus  3 . Subsequently, at the next step S 32 , the controller detects all IN and OUT points included on the edit-point list and makes a table of edit points. The table of edit points is properly referred to hereafter as an edit-point table. 
     It is to be noted that, in the edit-point table, edit points are described in a display-time order. 
     Then, at the next step S 33 , the controller  7  carries out a process to find preparatorily decoded pictures for each of the edit points, which is described in the edit-point table as an IN point. The process to find preparatorily decoded pictures has been explained before by referring to the flowchart shown in  FIG. 11 . In addition, at the step S 33 , the controller  7  reflects the preparatorily decoded pictures and a preparatory decoding time, which are found in the process to find preparatorily decoded pictures, in the edit-point table for each IN point. Subsequently, the flow of the reproduction process goes on to a step S 34 . At the step S 34 , the controller  7  finds a decoding start time for each of the edit points, which is described in the edit-point table as an IN point, from the preparatory decoding time for the IN point, and reflects the decoding start time in the edit-point table. The decoding start time of an IN point is a time at which a process to decode the preparatorily decoded pictures for the IN time must be started. 
     It is to be noted that, basically, a decoding start time of an edit point serving as an IN point is set at a time leading ahead of the IN point by at least the preparatory decoding time for the IN point. If picture B 1  or B 2  has been set at the edit point, however, a decoding start time of the edit point is set at a time leading ahead of the edit point by at least the sum of the preparatory decoding time for the edit point and the decoding time of one frame. 
     The reason why the decoding time of one frame is added to the preparatory decoding time as described above is explained as follows. Assume that picture B 1  or B 2  pertaining to GOP (n) is to be decoded. In this case, it is necessary to reference picture P 15  pertaining to GOP (n−1) immediately preceding GOP (n) in the display order and picture I 3  pertaining to GOP (n). Picture I 3  pertaining to GOP (n) is to be displayed after picture B 1  and B 2 . Picture P 15  pertaining to GOP (n−1) and picture I 3  pertaining to GOP (n) are each called a referenced picture for picture B 1  or B 2  pertaining to GOP (n), but only picture P 15  pertaining to GOP (n−1) immediately preceding GOP (n) in the display order is regarded as one of the preparatorily decoded pictures because picture P 15  is not to be displayed after picture B 1  or B 2  pertaining to GOP (n). Since picture I 3  pertaining to GOP (n) is to be displayed after picture B 1  or B 2  pertaining to the same group, however, picture I 3  is not regarded as one of the preparatorily decoded pictures. 
     Because picture I 3  pertaining to GOP (n) is referenced in decoding picture B 1  or B 2  pertaining to GOP (n), nevertheless, it is necessary to decode picture I 3  before decoding picture B 1  or B 2 . 
     Thus, before decoding picture B 1  or B 2  pertaining to GOP (n), it is necessary to decode picture I 3  pertaining to GOP (n) in addition to the pictures referred to as the preparatorily decoded pictures. For this reason, if picture B 1  or B 2  has been set at an edit point, a decoding start time of the edit point is set at a time leading ahead of the edit point by at least the sum of the preparatory decoding time it takes to decode the preparatorily decoded pictures for the edit point and the decoding time of one frame, which is picture I 3  pertaining to the same group as picture B 1  or B 2 . 
     As is obvious from the above description, for each edit point included on the edit-point list, a created edit-point table shows preparatorily decoded pictures, a preparatory decoding time and a decoding start time, which are associated with the edit point like for example one shown in  FIG. 18 . 
     Thus, the pieces of processing carried out at the steps S 32  to S 34  described above correspond to the scheduling performed at the step S 2  of the flowchart shown in  FIG. 6 . 
     Then, at the next step S 35 , the controller  7  forms a judgment as to whether or not the decoders  4   1  to  4   k  have completed preparations for decoding operations. The judgment is formed repeatedly at the step S 35  till the preparations are completed. As the outcome of the judgment formed at the step S 35  indicates that the decoders  4   1  to  4   k  have completed the preparations for decoding operations, the flow of the reproduction process goes on to a step S 36  at which the controller  7  sets a variable n at an initial value of, for example, 1 and a variable m at an initial value of, for example, 1. The variable n is a variable identifying a decoder  4   n , which is supposed to start a decoding operation. On the other hand, the variable m is a variable identifying a decoder  4   m , which is supposed to be selected by the switcher  5 . Subsequently, the flow of the reproduction process goes on to a step S 37 . 
     At the step S 37 , the controller  7  drives the decoder  4   n  to start a decoding operation starting with a picture at which the first IN point described in the edit-point table has been set. In addition, the controller  7  controls the switcher  5  to select a decoding result output by the decoder  4   m  and output the selected decoding result to the display unit  6 . Thus, a picture obtained as a result of a decoding operation carried out by the decoder  4   m  is displayed on the display unit  6 . 
     Then, at the next step S 38 , the controller  7  forms a judgment as to whether or not the current timing is a timing for one of decoding start points described in the edit-point table. 
     If the outcome of the judgment formed at the step S 38  indicates that the current timing is not a timing for one of decoding start points described in the edit-point table, the flow of the reproduction process goes on to a step S 43 , skipping steps S 39  to S 42 . 
     If the outcome of the judgment formed at the step S 38  indicates that the current timing is a timing for one of decoding start points described in the edit-point table, on the other hand, the flow of the reproduction process goes on to the step S 39  at which the controller  7  increments the variable n by typically 1. Then, the flow of the reproduction process goes on to a step S 40 . 
     A decoding start point judged at the step S 38  to coincide with the current timing is appropriately referred to hereafter as an observed decoding start point. 
     At the step S 40 , the controller  7  forms a judgment as to whether or not the variable n is greater than the decoder count K of the counters  4   1  to  4   k . If the outcome of the judgment formed at the step S 40  indicates that the variable n is greater than the decoder count K, the flow of the reproduction process goes on to a step S 41  at which the controller  7  initializes the variable n to a value of 1. Then, the flow of the reproduction process goes on to a step S 42 . 
     If the outcome of the judgment formed at the step S 40  indicates that the variable n is not greater than the decoder count K, on the other hand, the flow of the reproduction process goes on directly to the step S 42 . At this step, the controller  7  drives the decoder  4   n  to start an operation to decode preparatorily decoded pictures associated with the observed decoding start point described in the edit-point table. Then, the flow of the reproduction process goes on to the step S 43 . 
     At the step S 43 , the controller  7  forms a judgment as to whether or not a picture to be decoded by a currently operating decoder is an IN-point picture. A currently operating decoder is one of the decoders  4   1  to  4   k , which is currently carrying out a decoding operation. An IN-point picture is a picture at which an edit point has been set as an IN point. 
     If the outcome of the judgment formed at the step S 43  indicates that the picture to be decoded by a currently operating decoder is not an IN-point picture, the flow of the reproduction process goes on to a step S 49 , skipping steps S 44  to S 48 . 
     If the outcome of the judgment formed at the step S 43  indicates that the picture to be decoded by a currently operating decoder is an IN-point picture, on the other hand, the flow of the reproduction process goes on to the step S 44  at which the controller  7  copies the value of the variable m to a variable m′ and increments the variable m by typically 1. The variable m is a variable identifying a currently operating decoder  4   m , the decoding output of which is selected by the switcher  5 . Then, the flow of the reproduction process goes on to a step S 45 . 
     At the step  45 , the controller  7  forms a judgment as to whether or not the variable m is greater than the decoder count K of the counters  4   1  to  4   k . If the outcome of the judgment formed at the step S 45  indicates that the variable m is greater than the decoder count K, the flow of the reproduction process goes on to a step S 46  at which the controller  7  initializes the variable m to a value of 1. Then, the flow of the reproduction process goes on to a step S 4   7 . 
     If the outcome of the judgment formed at the step S 45  indicates that the variable m is not greater than the decoder count K, on the other hand, the flow of the reproduction process goes on directly to the step S 4   7 . At this step, the controller  7  controls the switcher  5  to select a decoding result output by the decoder  4   m  identified by the variable m, which was incremented by 1 at the step S 44  or initialized to 1 at the step S 46 . 
     As a result, the screen appearing on the display unit  6  is switched from a previous decoded picture to a new decoded picture. The previous decoded picture is a picture obtained as a result of a decoding operation carried out by the decoder  4   m ′ identified by the variable m′, which is equal to the variable m before being incremented by 1 at the step S 44  and, of course, before being initialized to 1 at the step S 46 . On the other hand, the new decoded picture is a picture obtained as a result of a decoding operation carried out by the decoder  4   m  identified by the variable m, which has been incremented by 1 at the step S 44  or, has been incremented by 1 at the step S 44  and then initialized to 1 at the step S 46 . 
     Subsequently, at the next step S 48 , the controller  7  stops the decoding operation carried out by the decoder  4   m ′ identified by the variable m′, which is equal to the variable m before being incremented by 1 at the step S 44  and, of course, before being initialized to 1 at the step S 46  as described above. Then, the flow of the reproduction process goes on to the step S 49 . 
     The pieces of processing carried out at the steps S 35  to S 48  described above correspond to the control of the decoding unit  4  and the switcher  5 . This control is executed at the step S 3  of the flowchart shown in  FIG. 6 . 
     At the step S 49 , the controller  7  forms a judgment as to whether or not the user has operated the operation unit  8  to stop the reproduction process much like the step S 4  of the flowchart shown in  FIG. 6 . If the outcome of the judgment formed at the step S 49  indicates that the user has operated the operation unit  8  to stop the reproduction process, that is, if the controller  7  has received an operation signal to stop the reproduction process from the operation unit  8 , the execution of the reproduction process is ended. The operation signal to stop the reproduction process is properly referred to hereafter as a reproduction stop operation signal. 
     If the outcome of the judgment formed at the step S 49  indicates that the user has not operated the operation unit  8  to stop the reproduction process, on the other hand, the flow of the reproduction process goes on to a step S 50  at which the controller  7  forms a judgment as to whether or not the process to reproduce an editing result of the MPEG stream in accordance with the edit-point list read out at the step S 31  has been completed much like the step S 5  of the flowchart shown in  FIG. 6 . If the outcome of the judgment formed at the step S 50  indicates that the process to reproduce an editing result of the MPEG stream in accordance with the edit-point list has not been completed, the flow of the reproduction process goes back to the step S 38  to repeat the same processing. If the outcome of the judgment formed at the step S 50  indicates that the process to reproduce an editing result of the MPEG stream in accordance with the edit-point list has been completed, on the other hand, the execution of the reproduction process is ended. 
     In accordance with the reproduction process represented by the flowchart shown in  FIG. 17 , decoding operations are carried out in the decoders  4   1  to  4   k  in a decoding order of decoders  4   1 ,  4   2 , - - - ,  4   k ,  4   1 ,  4   2  and so on, and the switcher  5  is controlled to select a picture obtained as a result of a decoding operation. It is to be noted, however, that the order of decoders to carry out decoding operations and the order of decoders selected by the switcher  5  are not limited to those indicated by the flowchart. 
     As described above by referring to the flowchart shown in  FIG. 17 , in the disc system shown in  FIG. 12 , the controller  7  reads out the edit-point list from the optical disc  1  and makes an edit-point table from the edit-point list. Since the controller  7  controls the decoders  4   1  to  4   k  to carry out decoding operations of the MPEG stream on the basis of information included in the edit-point table, the information included in the edit-point table can be said to be decoding control information for controlling the decoding operations. 
     The disc system shown in  FIG. 12  is capable of making a decoding control list described in accordance with a predetermined format from the decoding control information. In addition, if such a decoding control list already exists, the disc system shown in  FIG. 12  is capable of controlling the decoders  4   1  to  4   k  to carry out decoding operations of the MPEG stream on the basis of the decoding control list. 
     Thus, the disc system shown in  FIG. 12  has the function of a reproduction apparatus for reproducing information from the optical disc  1  in accordance with an edit-point list. In addition, the disc system shown in  FIG. 12  also has the function of a list-making apparatus for making a decoding control list from the edit-point list and the function of a reproduction apparatus for reproducing information from the optical disc  1  in accordance with the decoding control list. It is to be noted that, for an operation to reproduce information from the optical disc  1  in accordance with the decoding control list, the disc system shown in  FIG. 12  is capable of reproducing information from the optical disc  1  in accordance with the decoding control list, which is typically stored in advance in the controller  7 . As an alternative, the decoding control list is recorded onto the optical disc  1  and, in an operation to reproduce information from the optical disc  1  in accordance with the decoding control list, the decoding control list is first retrieved from the optical disc  1 . 
     The following description explains functions of the disc system shown in  FIG. 12  with the disc system working as a list-making apparatus for making a decoding control list as well as a reproduction apparatus for reproducing information from the optical disc  1  in accordance with the decoding control list. The list-making apparatus is virtually an information-processing apparatus for making a decoding control list. It is to be noted that some of the blocks of the disc system shown in  FIG. 12  carry out the same operations as those described above. Thus, the explanation of such blocks does not need to be repeated. 
     The description begins with an explanation of a function executed by the disc system of  FIG. 12  as a list-making apparatus for making a decoding control list. It is to be noted that the optical disc  1  shown in  FIG. 12  is assumed to be a disc for recording an MPEG stream obtained typically as a result of carrying out an MPEG encoding process on a moving picture. In addition, the optical disc  1  is also used for storing an edit-point list made as a result of carrying out an editing process with an MPEG stream in the optical disc  1  used as a raw editing material. 
     The controller  7  makes an edit-point list as requested for example by an operation signal generated by the operation unit  8 . To put it in detail, when the controller  7  receives an operation signal requesting creation of an edit-point list from the operation unit  8 , for example, the controller  7  issues a request for the edit-point list to the disc drive  2 . At this request, the disc drive  2  reads out the edit-point list from the optical disc  1  and then outputs the list to the bus  3 . Subsequently, the controller  7  receives the edit-point list and then detects edit points from the list. In addition, for each of the edit points detected from the edit-point list, fewest possible pictures are detected. The one or fewest possible pictures are pictures that must be decoded first in order to decode pictures to be displayed at and after the edit point, even though the one or fewest possible pictures themselves are not to be displayed. The controller  7  then finds a decoding start point representing a timing with which an operation to decode the one or fewest possible pictures is started. 
     The controller  7  further makes a decoding control list describing decoding control information. The decoding control information includes each of edit points detected from the edit-point list and additional data associated with each of the edit points. The additional data includes the number of pictures, which are not to be displayed but must be decoded first in order to decode pictures to be displayed at and after the edit point, and a decoding start point representing a timing to start an operation to decode the pictures, which are not to be displayed but must be decoded. The controller  7  then supplies the decoding control list to the disc drive  2 , onto which the list is to be stored, by way of the bus  3 . It is to be noted that details of the decoding control list will be explained later by referring to  FIG. 22 . 
     By referring to a flowchart shown in  FIG. 19 , the following description explains a decoding control list creation process carried out by the disc system shown in  FIG. 12  to make a decoding control list. 
     When the user operates the operation unit  8  to make a decoding control list from an edit-point list, the operation unit  8  supplies a decoding control list operation signal to the controller  7 . The decoding control list operation signal is an operation signal requesting the controller  7  to make a decoding control list. 
     When receiving the decoding control list operation signal from the operation unit  8  at a step S 61 , the controller  7  issues a request for an edit-point list to the disc drive  2 . At this request, the disc drive  2  reads out the edit-point list from the optical disc  1  and then outputs the list to the bus  3 . Subsequently, the controller  7  receives the edit-point list from the bus  3 . Then, at the next step S 62 , the controller detects all edit points described on the edit-point list as IN and OUT points. Subsequently, for each of the detected edit points, the controller  7  finds fewest possible pictures and a decoding start point representing a timing with which an operation to decode the one or fewest possible pictures is started. As described earlier, the one or fewest possible pictures are pictures that must be decoded first in order to decode pictures to be displayed at and after the edit point even though the one or fewest possible pictures themselves are not to be displayed. The controller  7  further makes a decoding control list based on information such as the detected edit points, the number of fewest possible pictures, which are not to be displayed but must be decoded for each of the edit points, and the computed decoding start point for each of the edit points. That is to say, the controller  7  makes a decoding control list showing decoding control information including the detected edit points, the number of fewest possible pictures, which are not to be displayed but to be decoded for each of the edit points, and the computed decoding start point for each of the edit points. 
     Then, the flow of the list creation process goes on from the step S 62  to a step S 63  at which the controller  7  outputs the decoding control list made at the step S 62  to the bus  3 . The disc drive  2  receives the decoding control list from the bus  3  and then stores the list onto the optical disc  1  to end the execution of the list creation process. 
     As described above, in the decoding control list creation process represented by the flowchart shown in  FIG. 19 , the disc system shown in  FIG. 12  is capable of making a decoding control list based on an edit-point list recorded on the optical disc  1  and storing the decoding control list onto the optical disc  1 . By carrying out a reproduction process to be described later with reference to a flowchart shown in  FIG. 23 , from the optical disc  1 , the disc system shown in  FIG. 12  or another apparatus including a plurality of decoders is capable of reproducing an MPEG stream including a decoding control list also recorded on the disc  1 . 
     By referring to  FIGS. 20 to 22 , the following description explains the decoding control list made in the decoding control list creation process represented by the flowchart shown in  FIG. 19 . 
     Now, assume that an editing operation like one shown in  FIG. 20  has been carried out. 
     An MPEG-stream portion between an IN point set on an MPEG stream used as a raw editing material and an OUT point for the IN point is referred to as a clip. If a cut editing operation is carried out, the result of the cut editing operation is a sequence of a plurality of clips. 
     The upper portion of  FIG. 20  shows a result of a cut editing operation carried out on raw material data including 2 MPEG streams  301  and  302 . 
     In  FIG. 20 , the file names of the MPEG streams  301  and  302  are ../0001/video01.mpg and ../0002/video02.mpg respectively. It is to be noted that, strictly speaking, ../0001/of the file name ../0001/video01.mpg is the name of a directory or a folder including a file containing the MPEG stream  301 , and video01.mpg is the name of the file. ‘../’ represents a directory at a level immediately above the current directory. This file naming applies to ../0002/video02.mpg as well. 
     As shown in  FIG. 20 , a pair of IN and OUT points is set on the MPEG stream  301  to result in clip # 1  between the IN and OUT points. By the same token, a pair of IN and OUT points is set on the MPEG stream  302  to result in clip # 2  between the IN and OUT points. Thus, clip # 1  is a portion of the file ../0001/video01.mpg and clip # 2  is a portion of the file ../0002/video02.mpg. 
     The so-called time code TC is attached to each of frames composing the MPEG streams  301  and  302 . Used for representing the display timing of a frame to which the time code is attached, the time code TC has a format of ‘Hours:Minutes:Seconds:Frame number’. Each edit point set as an IN or OUT point is represented by a time code. 
     In  FIG. 20 , the IN point of clip # 1  is represented by a TC of 00:00:00:00 while the OUT point of clip # 1  is represented by a TC of 00:05:00:12. On the other hand, the IN point of clip # 2  is represented by a TC of 00:10:00:03 while the OUT point of clip # 2  is represented by a TC of 00:15:00:00. 
     Assume that an editing process was carried out to reproduce clip # 1  to be followed by clip # 2 . As shown in displayed frames  303  of  FIG. 20 , an edit point has been set as an OUT point at picture A 7  pertaining to GOP (m) of clip # 1  on the MPEG stream  301  and as an IN point at picture C 4  pertaining to GOP (m) of clip # 2  on the MPEG stream  302 . In this case, in an operation to reproduce the post-editing MPEG streams, as shown in output frames  304  of  FIG. 20 , the decoder  4   1  is driven to decode and output pictures ending with picture A 7  at which an edit point has been set as the OUT point of clip # 1  and, as shown in output frames  305  of  FIG. 20 , the decoder  4   2  is driven to decode and output pictures starting with picture C 4  at which an edit point has been set as the IN point of clip # 2 . 
     However, in order for the decoder  4   2  to decode frames starting with frame C 4  at which an edit point has been set as an IN point, specific pictures to be referenced in decoding the frames even though not to be displayed must be decoded first prior to an operation to decode the frame C 4  at which an edit point has been set as an IN point. As described earlier, such specific pictures are referred to as preparatorily decoded pictures. As shown in the displayed frames  305  of  FIG. 20 , four pictures C 0 , C 1 , C 2  and C 3  are the preparatorily decoded pictures of frame C 4  and must thus be decoded prior to an operation to decode the frame C 4 . Since the IN point of clip # 2  is represented by a ‘TC=00:10:00:03’, the decoding start point of clip # 2  is a picture represented by a ‘TC=00:10:59:29’, which is leading ahead of ‘TC=00:10:00:03’ by a period of time it takes to decode four pictures. 
       FIG. 21  is a diagram showing a typical edit-point list made as a result of an editing operation explained by referring to  FIG. 20 . The edit-point list shown in  FIG. 21  can be prescribed in a definition using a language such as the XML (eXtensible, Markup Language). 
     To be more specific,  FIG. 21  shows typical codes in a range specified by a &lt;body&gt; tag placed on the first line of the edit-point list to serve as a start tag and a &lt;/body&gt; tag placed on the eleventh line to serve as an end tag. It is to be noted that, in the definition of the edit-point list shown in  FIG. 21 , a number placed at the beginning or each line and a colon ‘:’ following the number are not codes but merely added properly to make the description easy to understand. Lines and colons shown in  FIGS. 22 and 24  are added for the same purpose as those shown in  FIG. 21 . 
     The edit-point list shown in  FIG. 21  is a list of time codes each representing an edit point set in a raw-material data in order to identify a clip and a list of time codes each representing a timing to reproduce a clip represented by data following an edit point. Pieces of information included on the list begin at a &lt;par&gt; tag placed on the second line and end at a &lt;/par&gt; tag placed on the tenth line. 
     ‘&lt;!--Clip 1 --&gt;’ on the third line indicates that codes described on the fourth and fifth lines are codes of clip # 1 . It is to be noted that a comment can be described between ‘&lt;!--’ and ‘--&gt;’. 
     On the fourth and fifth lines, codes ‘&lt;ref src=“../0001/video01.mpg” begin=“smpte=00:00:00:00” clipBegin=“smpte=00:00:00:00” clipEnd=“00:05:00:12”/&gt;’ are described. The code ‘&lt;ref src=“../0001/video01.mpg”’ thereof indicates that the ‘../0001/video01.mpg’ is the name of a file containing the MPEG stream of clip # 1 . The code ‘begin=“smpte=00:00:00:00”’ thereof indicates that an operation to reproduce clip # 1  begins at a time code TC of 00:00:00:00 of data following the edit point. The code ‘begin=“smpte=00:00:00:00”’ can be omitted. The code ‘clipBegin=“smpte=00:00:00:00”’ thereof indicates that the IN point of clip # 1  is set at the time code TC of 00:00:00:00 of file ../0001/video01.mpg. On the other hand, the code ‘clipEnd=“smpte=00:05:00:12”’ thereof indicates that the OUT point of clip # 1  is set at the time code TC of 00:05:00:12 of file ../0001/video01.mpg. It is to be noted that the code ‘smpte=’ indicates that information following this code is a time code conforming to standards set by the SMPTE (Society of Motion Picture and Television Engineers). It is also worth noting that the afore described format of the time code is no more than a typical format. In actuality, any other format can be adopted as long as the other format indicates time information of an order in which pictures are displayed. 
     ‘&lt;!--Clip 2 --&gt;’ on the seventh line indicates that codes described on the eighth and ninth lines are codes of clip # 2 . 
     On the eighth and ninth lines, codes ‘&lt;ref src=“../0002/video02.mpg” begin=“smpte=00:05:00:12” clipBegin=“smpte=00:10:00:03” clipEnd=“00:15:00:00”/&gt;’ are described. The code ‘&lt;ref src=“../0002/video02.mpg”’ thereof indicates that the ‘../0002/video02.mpg’ is the name of a file containing the MPEG stream of clip # 2 . The code ‘begin=“smpte=00:05:00:13”’ thereof indicates that an operation to reproduce clip # 2  begins at a time code TC=00:05:00:13 of data following the edit point. This is because the MPEG stream of clip # 2  is reproduced after the MPEG stream of clip # 1  as explained earlier by referring to  FIG. 20 . Since the reproduction period of time for the MPEG stream of clip # 1  is ‘00:05:00:12’ between the time code TC=00:00:00:00 of file ../0001/video01.mpg and the time code TC=00:05:00:12 of file ../0001/video01.mpg as described on the fifth line, the operation to reproduce clip # 2  begins at a time code TC=00:05:00:13. The code ‘clipBegin=“smpte=00:10:00:03”’ thereof indicates that the IN point of clip # 2  is set at the time code TC=00:10:00:03 of file ../0002/video02.mpg. On the other hand, the code ‘clipEnd=“smpte=00:15:00:00”’ thereof indicates that the OUT point of clip # 2  is set at the time code TC=00:15:00:00 of file ../0002/video02.mpg. 
       FIG. 22  is a diagram showing a typical decoding control list made by carrying out the process to make a decoding control list in accordance with the flowchart shown in  FIG. 19  from the edit-point list shown in  FIG. 21 . As described before, the edit-point list shown in  FIG. 21  is made as a result of an editing operation explained earlier by referring to  FIG. 20 . It is to be noted that, much like the edit-point list shown in  FIG. 21 , the decoding control list shown in  FIG. 22  can be prescribed in a definition using a language such as the XML. 
     The decoding control list shown in  FIG. 22  is made by adding preDecBegin and preDecDur to the edit-point list shown in  FIG. 21 . preDecBegin is attributive information representing a decoding start point, which is a point of time to start an operation to decode pictures not to be displayed. On the other hand, preDecDur is attributive information representing the number of I and P pictures, which must be decoded first before decoding a picture at which an edit point has been set as an IN point. 
     To put it concretely, a code of ‘preDecBegin=“smpte=00:09:59:29”’ on the tenth line is information specifying that an operation to decode clip # 2  specified by codes described on the eighth and ninth lines is started at a time code TC=00:09:59:29 set for file ../0002/video02.mpg. A code of ‘preDecDur=“4”’ is information indicating that the number of I and P pictures, which must be decoded first before decoding a picture to be displayed at a TC of 00:10:00:03 is 4. In the decoding control list shown in  FIG. 22 , the picture to be displayed at a TC of 00:10:00:03 is picture C 4  shown in  FIG. 20 . On the other hand, the I and P pictures, which must be decoded first before decoding picture C 4 , are pictures C 0  to C 3  shown in  FIG. 20 . 
     By starting the operation to decode the four pictures, namely, C 0  to C 3  specified by preDecDur at the time code TC=00:09:59:29 specified by preDecBegin, the disc system shown in  FIG. 12  is capable of seamlessly reproducing C 4 , which is the first picture of clip # 2  to be displayed when a TC of 00:10:00:03 is reached. 
     It is to be noted that the operation to decode the four pictures, which starts at the time code TC=00:09:59:29 specified by preDecBegin, is carried out by a decoder different from a decoder used so far for carrying out an operation to decode clip # 1  specified by codes described on the fourth and fifth lines. 
     In the decoding control list shown in  FIG. 20 , preDecBegin is described to specify a time code for file ./0002/video02.mpg containing clip # 2 . However, preDecBegin can also be described for specifying another time code such as a time code of post-editing data. In this case, preDecBegin is described as ‘TC=00:05:00:08’, which leads ahead of ‘TC=00:05:00:12’ by a period of time it takes to decode four frames. 
     As described above, the disc system shown in  FIG. 12  makes a decoding control list and then stores the list onto the optical disc  1  also shown in  FIG. 12 . 
     The following description explains the disc system of  FIG. 12 , which functions as a reproduction apparatus reproducing an MPEG stream from the optical disc  1  in accordance with a decoding control list. 
     It is to be noted that the optical disc  1  shown in  FIG. 12  is assumed to contain an MPEG stream obtained typically as a result of carrying out an MPEG encoding process on a moving picture and contain a decoding control list made from an edit-point list. 
     The controller  7  controls the disc drive  2 , the decoding unit  4  and the switcher  5  typically in accordance with operation signals received from the operation unit  8 . When receiving an operation signal, which requests an operation to reproduce an MPEG stream from the optical disc  1 , from the operation unit  8 , for example, the controller  7  issues a request for a decoding control list to the disc drive  2 . At this request, the disc drive  2  reads out a decoding control list from the optical disc  1  and then outputs the list to the bus  3 . Then, the controller  7  receives the decoding control list from the bus  3 . 
     Subsequently, the controller  7  controls the decoders  4   1  to  4   k  in picture units to carry out operations to decode encoded data on the basis of the decoding control list. To put it in detail, the controller  7  controls the decoders  4   1  to  4   k  in picture units to carry out operations to decode an MPEG stream on the basis of edit points described on the decoding control list as decoding control information, the number of fewest possible pictures for each of the edit points and a start point representing a timing to start an operation to decode the one or fewest possible pictures. As described before, the one or fewest possible pictures are pictures to be decoded first before decoding the picture displayed at and after the edit point. However, the one or fewest possible pictures themselves are not to be displayed. In addition, the controller  7  also controls the switcher  5  on the basis of the decoding control list so that the switcher  5  correctly selects a decoded picture output by one of the decoders  4   1  to  4   k . 
     By referring to a flowchart shown in  FIG. 23 , the following description explains a reproduction process carried out by the disc system shown in  FIG. 12  on the basis of a decoding control list such as the one shown in  FIG. 22 . 
     When the user operates the operation unit  8  to start a process to reproduce a result of edit processing of an MPEG stream in accordance with an edit-point list, the operation unit  8  outputs a reproduction-start operation signal, which is an operation signal making a request for a start of a reproduction process, to the controller  7 . 
     When receiving an operation signal to start a reproduction process from the operation unit  8  at a step S 71 , the controller  7  issues a request for a decoding control list to the disc drive  2 . At this request, the disc drive  2  reads out the decoding control list from the optical disc  1  and then outputs the list to the bus  3 . The controller  7  then receives the decoding control list from the bus  3 . Subsequently, the flow of the reproduction process goes on to a step S 72 . 
     At the step S 72 , the controller  7  forms a judgment as to whether or not the decoders  4   1  to  4   k  have completed preparations for decoding operations. If the preparations for decoding operations have not been completed, the formation of the judgment at the step S 72  is repeated. The judgment is formed at this step repeatedly till the outcome of the judgment indicates that the decoders  4   1  to  4   k  have completed the preparations for decoding operations. As the outcome of the judgment indicates that the decoders  4   1  to  4   k  have completed the preparations for decoding operations, the flow of the reproduction process goes on to a step S 73  at which the controller  7  sets a variable n at an initial value of, for example, 1 and a variable m at an initial value of, for example, 1. The variable n is a variable identifying a decoder  4   n , which is supposed to start a decoding operation. On the other hand, the variable m is a variable identifying a decoder  4   m , which is supposed to be selected by the switcher  5 . Subsequently, the flow of the reproduction process goes on to a step S 74 . 
     At the step S 74 , the controller  7  drives the decoder  4   n  to start a decoding operation starting with a picture at which an IN point described on the decoding control list has been set. The IN point is described for the clip to be reproduced first. In addition, the controller  7  controls the switcher  5  to select a decoding result output by the decoder  4   m  and output the selected decoding result to the display unit  6 . Thus, a picture obtained as a result of a decoding operation carried out by the decoder  4   m  is displayed on the display unit  6 . 
     Then, at the next step S 75 , the controller  7  forms a judgment as to whether or not the current timing is a timing for one of decoding start points described on the decoding control list, that is, whether or not the current timing is a timing for a time code specified by preDecBegin. 
     If the outcome of the judgment formed at the step S 75  indicates that the current timing is not a timing for one of decoding start points described on the decoding control list, that is, the outcome indicates the current timing is not a timing for a time code specified by preDecBegin, the flow of the reproduction process goes on to a step S 80 , skipping steps S 76  to S 79 . 
     If the outcome of the judgment formed at the step S 75  indicates that the current timing is a timing for one of decoding start points described on the decoding control list, that is, the outcome indicates the current timing is a timing for a time code specified by preDecBegin, on the other hand, the flow of the reproduction process goes on to the step S 76  at which the controller  7  increments the variable n by typically 1. Then, the flow of the reproduction process goes on to a step S 77 . 
     preDecBegin specifying a time code, whose timing is judged at the step S 75  to coincide with the current timing, is appropriately referred to hereafter as observed preDecBegin. 
     At the step S 77 , the controller  7  forms a judgment as to whether or not the variable n is greater than the decoder count K of the counters  4   1  to  4   k . If the outcome of the judgment formed at the step S 77  indicates that the variable n is greater than the decoder count K, the flow of the reproduction process goes on to a step S 78  at which the controller  7  initializes the variable n to a value of 1. Then, the flow of the reproduction process goes on to a step S 79 . 
     If the outcome of the judgment formed at the step S 77  indicates that the variable n is not greater than the decoder count K, on the other hand, the flow of the reproduction process goes on directly to the step S 79 . At this step, the controller  7  drives the decoder  4   n  to start an operation to decode preparatorily decoded pictures, the number of which is specified by preDecDur described on the decoding control list, being associated with observed preDecBegin. Then, the flow of the reproduction process goes on to a step S 80 . 
     At the step S 80 , the controller  7  forms a judgment as to whether or not a picture to be decoded by a currently operating decoder is an IN-point picture. A currently operating decoder is one of the decoders  4   1  to  4   k , which is currently carrying out a decoding operation. An IN-point picture is a picture at which an edit point has been set as an IN point, that is, a picture with the time code thereof specified by any one of clipBegin codes on the decoding control list. 
     If the outcome of the judgment formed at the step S 80  indicates that the picture to be decoded by a currently operating decoder is not a picture with the time code thereof specified by clipBegin, the flow of the reproduction process goes on to a step S 86 , skipping steps S 81  to S 85 . 
     If the outcome of the judgment formed at the step S 80  indicates that the picture to be decoded by a currently operating decoder is a picture with the time code thereof specified by clipBegin, on the other hand, the flow of the reproduction process goes on to the step S 81  at which the controller  7  copies the value of the variable m to a variable m′ and increments the variable m by typically 1. The variable m is a variable identifying a currently operating decoder  4   m , the decoding output of which is selected by the switcher  5 . Then, the flow of the reproduction process goes on to a step S 82 . 
     At the step S 82 , the controller  7  forms a judgment as to whether or not the variable m is greater than the decoder count K of the counters  4   1  to  4   k . If the outcome of the judgment formed at the step S 82  indicates that the variable m is greater than the decoder count K, the flow of the reproduction process goes on to a step S 83  at which the controller  7  initializes the variable m to a value of 1. Then, the flow of the reproduction process goes on to a step S 84 . 
     If the outcome of the judgment formed at the step S 82  indicates that the variable m is not greater than the decoder count K, on the other hand, the flow of the reproduction process goes on directly to the step S 84 . At this step, the controller  7  controls the switcher  5  to select a decoding result output by the decoder  4   m  identified by the variable m, which was incremented by 1 at the step S 81  or initialized to 1 at the step S 83 . 
     As a result, the screen appearing on the display unit  6  is switched from a previous decoded picture to a new decoded picture. The previous decoded picture is a picture obtained as a result of a decoding operation carried out by the decoder  4   m ′ identified by the variable m′, which is equal to the variable m before being incremented by 1 at the step S 81  and, of course, before being initialized to 1 at the step S 83 . On the other hand, the new decoded picture is a picture obtained as a result of a decoding operation carried out by the decoder  4   m  identified by the variable m, which has been incremented by 1 at the step S 81  or, has been incremented by 1 at the step S 81  and then initialized to 1 at the step S 83 . 
     Subsequently, at the next step S 85 , the controller  7  stops the decoding operation carried out by the decoder  4   m ′ identified by the variable m′, which is equal to the variable m before being incremented by 1 at the step S 81  and, of course, before being initialized to 1 at the step S 83  as described above. Then, the flow of the reproduction process goes on to the step S 86 . 
     At the step S 86 , the controller  7  forms a judgment as to whether or not the user has operated the operation unit  8  to stop the reproduction. If the outcome of the judgment formed at the step S 86  indicates that the user has operated the operation unit  8  to stop the reproduction process, that is, if the controller  7  has received an operation signal to stop the reproduction process from the operation unit  8 , the execution of the reproduction process is ended. The operation signal to stop the reproduction process is properly referred to hereafter as a reproduction stop operation signal. 
     If the outcome of the judgment formed at the step S 86  indicates that the user has not operated the operation unit  8  to stop the reproduction process, on the other hand, the flow of the reproduction process goes on to a step S 87  at which the controller  7  forms a judgment as to whether or not the process to reproduce an editing result of the MPEG stream in accordance with the decoding control list read out at the step S 71  has been completed. If the outcome of the judgment formed at the step S 87  indicates that the process to reproduce an editing result of the MPEG stream in accordance with the decoding control list has not been completed, the flow of the reproduction process goes back to the step S 75  to repeat the same processing. If the outcome of the judgment formed at the step S 87  indicates that the process to reproduce an editing result of the MPEG stream in accordance with the decoding control list has been completed, on the other hand, the execution of the reproduction process is ended. 
     As described above, in accordance with the reproduction process represented by the flowchart shown in  FIG. 23 , the disc system shown in  FIG. 12  is capable of reading out a decoding control list recorded on the optical disc  1  and then reproducing an MPEG stream from the optical disc  1  in accordance with the decoding control list. 
     Thus, by carrying out the reproduction process represented by the flowchart shown in  FIG. 23 , the disc system shown in  FIG. 12  is capable of reproducing an MPEG stream from the optical disc  1 , on which a decoding control list has been recorded in a process to make the decoding control list in accordance with the flowchart shown in  FIG. 19 . In addition, the disc system shown in  FIG. 12  is capable of reproducing an MPEG stream from the optical disc  1 , on which a decoding control list made by another apparatus has been recorded by the other apparatus. 
     In accordance with the reproduction process represented by the flowchart shown in  FIG. 23 , the disc system shown in  FIG. 12  is capable reproducing an MPEG stream from the optical disc  1  in accordance with a decoding control list provided that the decoding control list has been stored in the optical disc  1  even if an edit-point list has not been recorded on the optical disc  1 . 
       FIG. 24  is a diagram showing another typical decoding control list. 
     The decoding control list shown in  FIG. 24  is identical with that shown in  FIG. 22  except that, in the case of the decoding control list shown in  FIG. 24 , descriptions each specifying a decoder are added to the list on the sixth and eleventh lines. 
     It is to be noted that, in the decoding control list shown in  FIG. 24 , a description of ‘decoder=“0”’ specifying a decoder indicates that a decoder  4   p  is a decoder for decoding clip # 1  to which the description is added. The decoder  4   p  is one of the K decoders  4   1  to  4   k  employed in the disc system shown in  FIG. 12 . By the same token, a description of ‘decoder=“1”’ specifying a decoder indicates that a decoder  4   q  is a decoder for decoding clip # 2  to which the description is added. The decoder  4   q  is also one of the K decoders  4   1  to  4   k . In this case, p≠q. In addition, p and q are each an integer in the range 1 to K. 
     To be more specific, the description of ‘decoder=“0”’ on the sixth line of the decoding control list shown in  FIG. 24  specifies that the decoder  4   p  is a decoder for decoding clip # 1  specified by codes described on the fourth and fifth lines as a clip of file ../0001/video01.mpg from a TC of 00:00:00:00 to a TC of 00:05:00:12. 
     On the other hand, the description of ‘decoder=“1”’ on the eleventh line of the decoding control list shown in  FIG. 24  specifies that the decoder  4   q  is a decoder for decoding I and P pictures, which must be decoded first during a decoding period of time starting at a TC of 00:09:59:29 in order to decode a first picture of clip # 2  specified by codes described on the ninth and tenth lines as a clip of file ../0002/video02.mpg from a TC of 00:10:00:03 to a TC of 00:15:00:00 so as to display the first picture at the TC of 00:10:00:03. 
     As described above, the decoding control list can be made as a list including informative descriptions each specifying a decoder. In this case, the controller  7  employed in the disc system shown in  FIG. 12  selects a decoder specified on the decoding control list and drives the selected decoder to decode an MPEG stream. 
     That is to say, in the reproduction process carried out in accordance with the flowchart shown in  FIG. 23  on the basis of a decoding control list specifying no decoders, the variable n identifying a decoder  4   n  among the K decoders  4   1  to  4   k  as a decoder to carry out a decoding operation and the variable m identifying a decoder  4   m  among the K decoders  4   1  to  4   k  as a decoder to be selected by the switcher  5  are set initially at 1 and then incremented sequentially thereafter. In a reproduction process carried out on the basis of a decoding control list specifying decoders as shown in  FIG. 24 , on the other hand, the disc system shown in  FIG. 12  recognizes a decoder specified on the decoding control list and then drives the recognized decoder to start a decoding operation with a timing of a time code specified by preDecBegin on the same list. Then, as a picture at a time code specified by clipBegin becomes an object of decoding, a decoding-result output by the recognized decoder is selected. 
     By referring to a flowchart shown in  FIG. 25 , the following description explains a reproduction process carried out by the disc system shown in  FIG. 12  to make a decoding control list like for example the one shown in  FIG. 22  from an edit-point list stored in the optical disc  1  and reproduce an MPEG stream on the basis of the decoding control list. 
     Steps S 121  to S 123  are executed for the same purpose as respectively steps S 61  to S 63  of the flowchart shown in  FIG. 19 . As described earlier, at the steps S 61  to S 63 , the disc system shown in  FIG. 12  makes a decoding control list based on an edit-point list stored in the optical disc  1  and stores the decoding control list onto the optical disc  1 . 
     Steps S 124  to S 140  are executed for the same purpose as respectively steps S 71  to S 87  of the flowchart shown in  FIG. 23 . As described earlier, at the steps S 71  to S 87 , the disc system shown in  FIG. 12  reads out the decoding control list from the optical disc  1 , and reproduces an MPEG stream from the optical disc  1  on the basis of the decoding control list. 
     It is to be noted that, by saving the decoding control list made at the step S 122  in the controller  7 , for example, the pieces of processing carried out at the steps S 123  and S 124  can be eliminated. 
     As described above, the series of the operations is carried out by the controller  7 , which can be implemented by hardware or software. If a controller  7  implemented by software is used for carrying out the series of operations described above, a program composing the software is installed into typically a general-purpose computer implementing the controller  7 . 
       FIG. 26  is a diagram showing a typical configuration of an embodiment implementing a computer, into which a program composing the software to be executed to carry out the series of operations described above is installed. 
     The program is installed in a hard disc  105  or stored in advance in a ROM  103 . The hard disc  105  and the ROM  103  each serve as a recording medium embedded in the computer. 
     In order to install the program into the hard disc  105 , the program is first temporarily or permanently stored in advance in a removable recording medium  111  such as a flexible disc, a CD-ROM (Compact Disc Read Only Memory), an MO (Magneto Optical) disc, a DVD (Digital Versatile Disc), a magnetic disc or a semiconductor memory. The program stored in the removable recording medium  111  can then be presented to the user as package software. Then, the program is installed into the hard disc  105  employed in the computer from the removable recording medium  111  described above. 
     It is to be noted that, as an alternative to the installation of the program from the removable recording medium  111 , the program is downloaded to the computer from a download source through a radio communication using an artificial satellite for digital satellite broadcasting or by way of a wire communication means such as a LAN (Local Area Network) or the Internet. A communication unit  108  employed in the computer receives the program downloaded as such and then finally installs the program into the embedded hard disc  105 . 
     The computer also has an embedded CPU (Central Processing Unit)  102 . The CPU  102  is connected to an input/output interface  110  by using a bus  101 . The user enters a command to the computer by operating one of a keyboard, a mouse, a mike and another input means, which compose an input unit  107 . The input unit  107  then passes on the command to the CPU  102  by way of the input/output interface  110 . The program stored in advance in the ROM (Read Only Memory)  103  is executed in accordance with the command. Instead of being stored in the ROM  103 , the program to be executed is installed in the hard disc  105  as described above. In this case, the program is loaded into a RAM  104  from the hard disc  105 . As explained before, the program has been installed in the hard disc  105  from a removable recording medium  111  mounted on a drive  109  or from a download source by way of a satellite or a network to be received by the communication unit  108 . By execution of the program, the CPU  102  carries out the processes according to the flowcharts described above in any of the configurations shown in the block diagrams explained earlier. Then, if necessary, the CPU  102  typically outputs results of the processes to an output unit  106  including an LCD (Liquid Crystal Display), a speaker and other components by way of the input/output interface  110 , transmits the results to the satellite or the network by way of the communication unit  108  or records the results onto the hard disc  105 . 
     In this specification, processing steps prescribing the program to be executed by the computer to implement a variety of operations do not necessarily have to be carried out sequentially along the time axis in accordance with a predetermined order represented by a flowchart. Instead, the steps may include operations to be carried out concurrently or individually. 
     In addition, the program can be executed by using a computer or a plurality of computers in a distributed processing system. Furthermore, the program can also be transmitted to a remote computer for execution. 
     As described above, the embodiments reproduce an edited MPEG stream. It is to be noted, however, that the present invention can also be applied to reproduction of encoded data other than an MPEG stream. An example of such encoded data is data obtained as a result of a predictive encoding process carried out by referring to other pictures. 
     In addition, in the embodiments, an edit-point list is recorded on the optical disc  1  in advance and the controller  7  acquires the edit-point list from the optical disc  1 . However, the edit-point list can also for example be recorded in a recording medium other than the optical disc  1  or acquired from transmission media such as the Internet. The decoding control list can be acquired in the same way as the edit-point list. 
     Furthermore, in the embodiments, a GOP is a sequence of pictures B 1 , B 2 , I 3 , B 4 , B 5 , P 6 , B 7 , B 8 , P 9 , B 10 , B 11 , P 12 , B 13 , B 14 , P 15 . However, the present invention can also be applied to a GOP of a different sequence of pictures. For example, the present invention can be applied to a GOP including no B and/or P pictures. Moreover, the present invention can also be applied to a closed or open GOP. 
     It is to be noted that, if the GOP is a GOP of a different sequence of pictures, preparatorily decoded pictures will be different from those found by carrying out the process represented by the flowchart shown in  FIG. 11 . 
     In addition, in the embodiments, the optical disc  1  is a recording medium for storing an MPEG stream to be reproduced. However, the present invention can also be applied to reproduction of an MPEG stream from a recording medium other than the optical disc  1 . Examples of the other recording medium are a magnetic disc, a magnetic tape and a semiconductor memory. The present invention can also be applied to reproduction of an MPEG stream downloaded from transmission media such as the Internet or a satellite. 
     As described above,  FIGS. 8 and 13  to  15  each show an example with picture B 1  always taken as a picture at which an edit point is set as an IN point. As explained earlier by referring to  FIG. 11 , however, a case in which an edit point is set at picture B 1  or B 2  is a case having a largest number of preparatorily decoded pictures. Thus, if a seamless reproduction process can be carried out with an edit point set at picture B 1  or B 2 , naturally, a seamless reproduction process can also be carried out with an edit point set at a picture other than picture B 1  or B 2 . 
     Furthermore, in the embodiments, the controller  7  detects edit points set on an MPEG stream from an edit-point list. However, the user is also allowed to supply an edit point set on an MPEG stream to the controller  7  by, for example, operating the operation unit  8 . As another alternative, an external means is used for detecting an edit point set on an MPEG stream and supplying the edit point to the controller  7 . That is to say, there is specially no restriction on the method to supply an edit point set on an MPEG stream to the controller  7 . 
     Moreover, in the embodiments, the same apparatus functions as an apparatus for making a decoding control list from an edit-point list and an apparatus for executing decoding control based on the decoding control list. However, the present invention is not limited to this scheme. For example, the apparatus for making a decoding control list from an edit-point list can be an apparatus different from the apparatus for executing decoding control based on the decoding control list. 
     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.