Abstract:
A multiplex data transmission system for transmitting N coded programs having differing bandwidth requirements over M separate channels. The N programs are separately encoded and allocated to the M channels in such a manner that optimum use is made of the available bandwidth in the channels. This is achieved by summing the bandwidth requirements for all the possible distinct combinations of the N programs taken M at a time, and selecting the combination which most closely matches the bandwidth capabilities of the available channels. A multiplexing device generates M separate coded data strings corresponding to the program allocation. At the receiver, a decoder is provided which selectively recovers a user-designated one of the N programs. The decoder includes an input device by which the user designates the desired program, a selecting device which selects the multiplexed code string including the designated program based on identifying information transmitted with the coded data strings, an inverse multiplexing device which extracts the designated coded program signal from the selected one of the M multiplexed code strings and a decoding circuit which decodes the encoded data signal extracted by the inverse multiplexing device to recreate the desired program.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a motion image encoder which inputs a plurality of motion image signals, codes the input motion image signals, multiplexes the coded motion image signals, and sends out the multiplexed motion image signals to a plurality of transponders, a motion image decoder which decodes multiplexed motion image signals transmitted via a plurality of transponders, and a readable storage medium storing an encoding or decoding program. 
     DESCRIPTION OF THE RELATED ART 
     Now, digital satellite broadcasting is put into practice pursuant to the IS-13818 (MPEG-2) standard of the international standard organization (ISO). An artificial satellite is provided with a plurality of transponders (transmission lines). A transmission ability per one transponder (bit rate) is approximately 30 Mbps. For example, assuming that 6 Mbps as the necessary bit rate required in coding motion image signals to meet image quality is required, 5 motion image signals can be transmitted per one transponder (multiplicity 5), and thus 20 transponders are required for broadcasting 100 motion image signals (100 channels of motion image signals) in digital satellite broadcasting. 
     Each channel is composed of a variety of programs, and the necessary bit rate changes depending on the contents of the programs. For instance, in a hard motion sports relay broadcast, a bit rate of approximately 10 Mbps is required. If a coding is carried out at approximately 6 Mbps in a similar manner to a usual motion sports relay broadcast, the image quality is degraded. In order to prevent such a problem, it is considered that the multiplicity of the transponders is reduced and the bit rate allocated to each motion image signal is increased. In such a case, when the number of the motion image signals is equal, the number of the transponders must be increased. Alternatively, when the number of the transponders is equal, the number of the motion image signals must be reduced. 
     In order to solve this problem, a statistical multiple coding has been developed, in which a bit rate allocated to each motion image signal is not fixed, but the sum of the bit rates is fixed. 
     In FIG. 1, there is shown a conventional motion image encoder using the statistical multiple coding. As shown in FIG. 1, the motion image encoder comprises two image motion encoder units  132 - 0  and  132 - 1 , in which 10 motion image signals V 0  to V 9  are coded and then multiplexed (5 signals in each image motion encoder unit  132 - 0  or  132 - 1 ) to output two multiplexed motion image signals to transponders (not shown) C 0  and C 1  from the image motion encoder units  132 - 0  and  132 - 1 , respectively. 
     More specifically, the motion image encoder unit  132 - 0  includes 5 coding means  130 - 0 ,  130 - 1 ,  130 - 2 ,  130 - 3  and  130 - 4  for coding the motion image signals V 0 , V 1 , V 2 , V 3  and V 4  to output code strings, and a multiplexer (MUX)  131 - 0  for multiplexing the code strings output from the coding means  130 - 0 ,  130 - 1 ,  130 - 2 ,  130 - 3  and  130 - 4  to send out a multiplexed code string to a transponder C 0 . Similarly, the motion image encoder unit  132 - 1  includes 5 coding means  130 - 5 ,  130 - 6 ,  130 - 7 ,  130 - 8  and  130 - 9  for coding the motion image signals V 5 , V 6 , V 7 , V 8  and V 9  to output code strings, and a multiplexer (MUX)  131 - 1  for multiplexing the code strings output from the coding means  130 - 5 ,  130 - 6 ,  130 - 7 ,  130 - 8  and  130 - 9  to send out a multiplexed code string to a transponder C 1 . 
     Now, for example, it is assumed that the transmission ability of the transponders C 0  and C 1  are each 30 Mbps, and that the necessary bit rates of the motion image signals V 0  and V 5  are each 10 Mbps and those of the motion image signals V 1  to V 4  and V 6  to V 9  are each 5 Mbps in a certain time period. The sum of the bit rates allocated to the 5 signals VO to V 4  or V 5  to V 9  becomes 30 Mbps which is equal to the transmission ability of the transponder C 0  or C 1 , inviting no image quality degradation. In this manner, in the conventional motion image encoder shown in FIG. 1, even when a large bit rate is required for parts of the motion image signals V 0  to V 9 , the sum of the bit rates for the motion image signals V 0  to V 4  is equal to or less than the transmission ability of the transponder C 0  and the sum of the bit rates for the motion image signals V 5  to V 9  is equal to or less than the transmission ability of the transponder C 1 , resulting in preventing the image quality degradation. 
     However, in this conventional motion image encoder, as described above, the motion image signals V 0  to V 4  are coded in the respective coding means and are then multiplexed in the MUX to output the multiplexed code string to the transponder C 0 , and similarly, the motion image signals V 5  to V 9  are coded in the respective coding means and are then multiplexed in the MUX to output the multiplexed code string to the transponder C 1 . That is, the groups of motion image signals correspond fixedly to the respective transponders. As a result, the transmission ability of a plurality of transponders cannot be sufficiently exploited, and the image quality may be deteriorated. 
     For instance, it is assumed that the transmission ability of the transponders C 0  and C 1  are each 30 Mbps, and that the necessary bit rates of the motion image signals V 0  and V 1  are each 3 Mbps, those of the motion image signals V 2  to V 4  are each 4 Mbps, those of the motion image signals V 5  and V 6  are each 6 Mbps, and V 7  to V 9  are each 10 Mbps in a certain time period. In the conventional motion image encoder, the motion image signals V 0  to V 4  and V 5  to V 9  are fixedly assigned to the respective transponders C 0  and C 1 , respectively, and the sum of the necessary bit rates for the motion image signals is 18 Mbps or 42 Mbps in the transponder C 0  or C 1 , respectively. The transmission ability of the transponders C 0  and C 1  is each 30 Mbps. In other words, the transponder C 0  has 12 Mbps surplus while the transponder C 1  is 12 Mbps short with the result of the image quality degradation. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a motion image encoder in view of the aforementioned problems of the prior art, which is capable of making an effective use of a plurality of transponders without surplus and shortage in their transmission ability, and preventing image quality degradation. 
     It is another object of the present invention to provide a motion image decoder for decoding motion image signals coded by the above motion image encoder, which is capable of preventing image quality degradation. 
     It is a further object of the present invention to provide a readable storage medium storing a program for realizing a motion image encoder and decoder using a computer, the program operating the computer as parts of the motion image encoder and decoder, which is capable of preventing image quality degradation. 
     In accordance with one aspect of the present invention, there is provided a motion image encoder, in which N-number of motion image signals are coded to produce N-number of code strings and the N-number of code strings are multiplexed to produce M-number of multiplexed code strings to be output to M-number of respective transponders, comprising N-number of coding means for coding N-number of respective motion image signals to produce N-number of code strings; and 
     multiplexing means for multiplexing the N-number of code strings per M-number of groups corresponding to M-number of transponders to produce M-number of multiplexed code strings, while grouping the N-number of code strings into M-number so that the sum of the necessary bit rates of each group of the motion image signals included in each of the M-number of multiplexed code strings becomes closest to bit rates of the corresponding transponder, and outputting the M-number of multiplexed code strings to the respective transponders. 
     In this construction, the N-number of coding means code the input motion image signals to produce the code strings. The multiplexing means then multiplexes the N-number of code strings per M-number of groups corresponding to M-number of transponders to produce M-number of multiplexed code strings, while grouping the N-number of code strings into M-number so that the sum of the necessary bit rates of each group of the motion image signals included in each of the M-number of multiplexed code strings becomes closest to bit rates of the corresponding transponder, and outputs the M-number of multiplexed code strings to the respective transponders. That is, in the above construction, the relationship between the motion image signals and the transponders is not fixed, and the motion image signals are properly allocated to the transponders so that the sum of the necessary bit rates of the motion image signals allocated to each transponder may be closest to the bit rates of the corresponding transponder. 
     For example, now, it is considered that 10 motion image signals V 0  to V 9  are coded and then multiplexed to output the multiplexed code strings to the transponders C 0  and C 1 . It is assumed that the transmission ability of both the transponders C 0  and C 1  is 30 Mbps in common, and the bit rate of the motion image signals V 0  and V 1  is 3 Mbps, that of the motion image signals V 2  to V 4  is 4 Mbps, that of the motion image signals V 5  and V 6  is 6 Mbps, and that of the motion image signals V 7  to V 9  is 10 Mbps in a certain time period. 
     In this motion image encoder, the corresponding relationship between the V 0  to V 9  and the transponders C 0  and C 1  is changed so that the sums of the necessary bit rates of the transponders may be closest to the transmission ability of the transponders. Thus, the motion image signals V 0  to V 6  are allocated to the transponder C 0  and the motion image signals V 7  to V 9  are allocated to the transponder C 1 . As a result, the sums of the necessary bit rates of the transponders C 0  and C 1  becomes 30 Mbps in common, and the image quality degradation can be presented. Alternatively, the motion image signals V 0  to V 2 , V 8  and V 9  are allocated to the transponder C 0  and the motion image signals V 3  to V 7  are allocated to the transponder C 1 . In this case, the sums of the necessary bit rates of the transponders C 0  and C 1  become 30 Mbps in common, and thus the image quality degradation can be avoided as well. 
     In order to achieve the above object of the present invention with a simple construction, the motion image encoder can further comprises a predetermined value table for storing predetermined values of the necessary bit rates of the N-number of motion image signals in each time period; and 
     allocation means for determining corresponding relationship between the transponders and the N-number of motion image signals allocated thereto in each time period so as to permit the sum of the necessary bit rates of the motion image signals included in each of the M-number of multiplexed code strings to be closest to the bit rates of the corresponding transponder on the basis of contents of the predetermined value table, and the multiplexing means multiplexes the N-number of code strings per the M-number of groups corresponding to the M-number of transponders to produce the M-number of multiplexed code strings according to the corresponding relationship determined by the allocation means, and outputs the M-number of multiplexed code strings to the respective transponders. 
     In this construction, the allocation means determines the corresponding relationship between the transponders and the N-number of motion image signals allocated thereto in each time period so as to permit the sum of the necessary bit rates of the motion image signals included in each of the M-number of multiplexed code strings to be closest to the bit rates of the corresponding transponder on the basis of contents of the predetermined value table, and the multiplexing means multiplexes the N-number of code strings per the M-number of groups corresponding to the M-number of transponders to produce the M-number of multiplexed code strings according to the corresponding relationship determined by the allocation means, and outputs the produced M-number of multiplexed code strings to the respective transponders. 
     Further, in order to achieve the above object of the present invention with a simple construction and to reduce the danger of the image quality degradation effectively, the motion image encoder further comprises a predetermined value table for storing predetermined values and variation rates of the necessary bit rates of the N-number of motion image signals in each time period; and 
     allocation means for determining corresponding relationship between the transponders and the N-number of motion image signals allocated thereto in each time period so as to permit the statistical sum of the necessary bit rates of the motion image signals included in each of the M-number of multiplexed code strings to be closest to the bit rates of the corresponding transponder on the basis of contents of the predetermined value table, and the multiplexing means multiplexes the N-number of code strings per the M-number of groups corresponding to the M-number of transponders to produce the M-number of multiplexed code strings according to the corresponding relationship determined by the allocation means, and outputs the M-number of multiplexed code strings to the respective transponders. 
     In this construction, the allocation means determines the corresponding relationship between the transponders and the N-number of motion image signals allocated thereto in each time period so as to permit the statistical sum of the necessary bit rates of the motion image signals included in each of the M-number of multiplexed code strings to be closest to the bit rates of the corresponding transponder on the basis of contents of the predetermined value table, and the multiplexing means multiplexes the N-number of code strings per the M-number of groups corresponding to the M-number of transponders to produce the M-number of multiplexed code strings, and outputs the produced M-number of multiplexed code strings to the respective transponders. 
     In accordance with another aspect of the present invention, there is provided a motion image encoder, in which N-number of motion image signals are coded to produce N-number of code strings and the N-number of code strings are multiplexed to produce M-number of multiplexed code strings to be output to M-number of respective transponders, comprising M-number of multiplexing means corresponding to M-number of transponders, for multiplexing input code strings to produce M-number of multiplexed code strings to output the produced multiplexed code strings to the corresponding transponders; 
     M-number of coding units, each including a plurality of coding means, corresponding to the M-number of multiplexing means, for coding input motion image signals to produce code strings to output the produced code strings to the corresponding multiplexing means; and 
     switch means for allocating the N-number of motion image signals to the M-number of coding units so as to permit the sum of the necessary bit rates of the motion image signals allocated to each coding unit to be closest to bit rates of the corresponding transponder. 
     In this construction, the switch means allocates the input N-number of motion image signals to the M-number of coding units corresponding to the respective transponders. At this time, the switch means allocates the motion image signals so that the sum of the necessary bit rates of the motion image signals allocated to each coding unit may be closest to the bit rates of the corresponding transponder. Each coding unit codes the motion image signals allocated by the switch means to produce the code string, and each multiplexing means multiplexes the input code strings to produce the multiplexed code string and outputs the produced multiplexed code string to the corresponding transponder. 
     Moreover, in order to achieve the above object of the present invention with a simple construction, the motion image encoder can further comprises a predetermined value table for storing predetermined values of the necessary bit rates of the N-number of motion image signals in each time period; and 
     allocation means for determining corresponding relationship between the coding units and the N-number of motion image signals allocated thereto in each time period so as to permit the sum of the necessary bit rates of the motion image signals included in each of the M-number of multiplexed code strings to be closest to the bit rates of the corresponding transponder on the basis of contents of the predetermined value table, and the switch means allocates the N-number of motion image signals to the M-number of coding units according to the corresponding relationship determined by the allocation means. 
     In this construction, the allocation means determines the corresponding relationship between the coding units and the N-number of motion image signals allocated thereto in each time period so as to permit the sum of the necessary bit rates of the motion image signals included in each of the M-number of multiplexed code strings to be closest to the bit rates of the corresponding transponder on the basis of contents of the predetermined value table, and the switch means allocates the N-number of motion image signals to the M-number of coding units according to the corresponding relationship determined by the allocation means. 
     Further, in order to achieve the above object of the present invention with a simple construction and to reduce the danger of the image quality degradation effectively, the motion image encoder can further comprises a predetermined value table for storing predetermined values and variation rates of the necessary bit rates of the N-number of motion image signals in each time period; and 
     allocation means for determining corresponding relationship between the coding units and the N-number of motion image signals allocated thereto in each time period so as to permit the statistical sum of the necessary bit rates of the motion image signals included in each of the M-number of multiplexed code strings to be closest to the bit rates of the corresponding transponder on the basis of contents of the predetermined value table, and the switch means allocates the N-number of motion image signals to the M-number of coding units according to the corresponding relationship determined by the allocation means. 
     In this construction, the allocation means determines the corresponding relationship between the coding units and the N-number of motion image signals allocated thereto in each time period so as to permit the statistical sum of the necessary bit rates of the motion image signals included in each of the M-number of multiplexed code strings to be closest to the bit rates of the corresponding transponder on the basis of contents of the predetermined value table, and the switch means allocates the N-number of motion image signals to the M-number of coding units according to the corresponding relationship determined by the allocation means. 
     In order to decode the multiplexed code strings coded by the motion image encoder of the present invention to obtain the motion image signals, the motion image decoder for decoding multiplexed code strings input to output one motion image signal, comprises selecting means for selecting one multiplexed code string including a code string corresponding to a motion image signal designated by a user from M-number of multiplexed code strings transmitted from a transmitter side on the basis of corresponding information showing that what kinds of motion image signals are included in the M-number of multiplexed code strings; 
     inverse multiplexing means for picking out one code string corresponding to the motion image signal designated by the user from the multiplexed code string selected by the selecting means on the basis of the corresponding information; and 
     decoding means for decoding the code string output from the inverse multiplexing means to obtain a motion image signal. 
     In this construction, the selecting means selects one multiplexed code string including a code string corresponding to a motion image signal designated by a user from M-number of multiplexed code strings, and the inverse multiplexing means picks out one code string corresponding to the motion image signal designated by the user from the multiplexed code string selected by the selecting means. The decoding means decodes the code string output from the inverse multiplexing means to obtain a motion image signal. 
     Moreover, in order to decode the multiplexed code strings coded by the motion image encoder of the present invention to obtain the motion image signals, the motion image decoder for decoding multiplexed code strings input to output one motion image signal, comprises M-number of inverse multiplexing means for picking out one code string to be output from M-number of multiplexed code strings input to output the picked code string; 
     instructing means for instructing the inverse multiplexing means inputting the multiplexed code string including the code string corresponding to the motion image signal designated by a user to output the code string designated by the user on the basis of corresponding information showing that what kinds of motion image signals are included in the M-number of multiplexed code strings; 
     synthesizing means for synthesizing the code strings output from the M-number of inverse multiplexing means to output a synthesized code string; and 
     decoding means for decoding the synthesized code string output from the synthesizing means to output a motion image signal. 
     In this construction, the instructing means instructs the inverse multiplexing means inputting the multiplexed code string including the code string corresponding to the motion image signal designated by a user to output the code string designated by the user, and one inverse multiplexing means instructed by the instructing means among the M-number of inverse multiplexing means outputs the code string designated by the user. The synthesizing means synthesizes the code strings output from the inverse multiplexing means, and the decoding means decodes the synthesized code string output from the synthesizing means to obtain a motion image signal. 
     In order to achieve the above object of the present invention, a readable storage medium stores a program for realizing a motion image encoder using a computer, in which N-number of motion image signals are coded to produce N-number of code strings and the N-number of code strings are multiplexed to produce M-number of multiplexed code strings to be output to M-number of respective transponders, the program functioning the computer as N-number of coding means for coding N-number of respective motion image signals to produce N-number of code strings; and 
     multiplexing means for multiplexing the N-number of code strings per M-number of groups corresponding to M-number of transponders to produce M-number of multiplexed code strings, while grouping the N-number of code strings into M-number so that the sum of the necessary bit rates of each group of the motion image signals included in each of the M-number of multiplexed code strings becomes closest to bit rates of the corresponding transponder, and outputting the M-number of multiplexed code strings to the respective transponders. 
     Moreover, in order to achieve the above object of the present invention, a readable storage medium stores a program for realizing a motion image decoder for decoding multiplexed code strings input to output one motion image signal using a computer, the program functioning the computer as selecting means for selecting one multiplexed code string including a code string corresponding to a motion image signal designated by a user from M-number of multiplexed code strings transmitted from a transmitter side on the basis of corresponding information showing that what kinds of motion image signals are included in the M-number of multiplexed code strings; 
     inverse multiplexing means for picking out one code string corresponding to the motion image signal designated by the user from the multiplexed code string selected by the selecting means on the basis of the corresponding information; and 
     decoding means for decoding the code string output from the inverse multiplexing means to obtain a motion image signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects, features and advantages of the present invention will become more apparent from the consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a conventional motion image encoder; 
     FIG. 2 is a block diagram of a motion image encoder according to a first embodiment of the present invention; 
     FIG. 3 is a block diagram of a motion image encoder according to a second embodiment of the present invention; 
     FIG. 4 is a schematic diagram showing contents of a predetermined value table storing the predetermined values of necessary bit rates, shown in FIG. 3; 
     FIG. 5 is a flow chart showing an operation of an allocation means, shown in FIG. 3; 
     FIG. 6 is a block diagram of a motion image encoder according to a third embodiment of the present invention; 
     FIG. 7 is a schematic diagram showing contents of a predetermined value table storing the predetermined values and variation rates of necessary bit rates, shown in FIG. 6; 
     FIG. 8 is a block diagram of a motion image encoder according to a fourth embodiment of the present invention; 
     FIG. 9 is a block diagram of a motion image encoder according to a fifth embodiment of the present invention; 
     FIG. 10 is a block diagram of a motion image encoder according to a sixth embodiment of the present invention; 
     FIG. 11 is a block diagram of a motion image decoder according to a first embodiment of the present invention; 
     FIG. 12 is a block diagram of a motion image decoder according to a second embodiment of the present invention; and 
     FIG. 13 is a block diagram showing a hardware construction of a motion image encoder and a motion image decoder according to one embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, there is shown in FIG. 2 a motion image encoder according to a first embodiment of the present invention. 
     In FIG. 2, the motion image encoder, which codes 10 motion image signals V 0  to V 9  and multiplexes 10 code strings to output two multiplexed code strings to transponders (not shown) C 0  and C 1  as hereinafter described in detail, comprises a motion image encoder unit  12  and a program output controller  13 . 
     The motion image encoder unit  12  includes 10 coding means  10 - 0 ,  10 - 1 ,  10 - 2 ,  10 - 3 ,  10 - 4 ,  10 - 5 ,  10 - 6 ,  10 - 7 ,  10 - 8  and  10 - 9 , and a multiplexer (MUX)  11 . 
     The program output controller  13  outputs a control signal representing the corresponding relationship between the motion image signals V 0  to V 9  and the transponders C 0  and C 1  so as to allocate the necessary bit rates of the motion image signals V 0  to V 9  to the transponders C 0  and C 1  equally. This corresponding relationship represented by the control signal show which of the transponders C 0  and C 1  and which places of the transponder C 0  or C 1  the motion image signals are multiplexed to. Moreover, the corresponding relationship is changed with the elapse of time depending on the contents of the motion image signals V 0  to V 9 . 
     The 10 coding means  10 - 0 ,  10 - 1 ,  10 - 2 ,  10 - 3 ,  10 - 4 ,  10 - 5 ,  10 - 6 ,  10 - 7 ,  10 - 8  and  10 - 9  code the respective  10  input motion image signals V 0  to V 9  to output  10  motion image code strings to the multiplexer  11 . The multiplexer  11  multiplexes the  10  motion image code strings to output two multiplexed code strings to the transponders C 0  and C 1  according to the control signal sent from the program output controller  13 . The multiplexer  11  also multiplexes the corresponding relationship represented by the control signal. 
     Now, when the corresponding relationship represented by the control signal shows that the motion image signals V 0  to V 6  are to be multiplexed to the first to seventh orders of the transponder C 0  and the motion image signals V 7  to V 9  are to be multiplexed to the first to third orders of the transponder C 1 , the multiplexer  11  multiplexes the code strings sent from the coding means  10 - 0  to  10 - 6  to the first to seventh orders along with multiplexing the corresponding relationship represented by the control signal to output a multiplexed code string to the transponder C 0 , and multiplexes the code strings fed from the coding means  10 - 7  to  10 - 9  to the first to third orders along with multiplexing the corresponding relationship to output another multiplexed code string to the transponder C 1 . 
     FIG. 3 shows a motion image encoder according to a second embodiment of the present invention. In this embodiment, the motion image encoder comprises a motion image encoder unit  12 , an allocation means  20  and a predetermined value table  21  of the necessary bit rates. The motion image encoder unit  12  has the same construction and functions as those of that of the first embodiment shown in FIG.  2 . 
     The predetermined value table  21  stores the predetermined values of the necessary bit rates of the  10  motion image signals V 0  to V 9  in each time period. The predetermined value table  21  is constructed on a memory and its contents can be freely changed from the outside. 
     FIG. 4 illustrates the contents of the predetermined value table  21 . It is readily understood from FIG. 4, that the predetermined values A of the necessary bit rates of the motion image signals V 0  to V 9  in a time period T 1 ≦T&lt;T 2  (T: present time) are 3, 3, 4, 4, 4, 6, 6, 10, 10 and 10 (Mbps), respectively, and those in another time period T 2 ≦T&lt;T 3  are 6, 6, 6, 6, 4, 4, 10, 6, 6 and 6 (Mbps), respectively. 
     The allocation means  20  calculates the sum of the necessary bit rates of each transponder C 0  or C 1  with respect to the effective combination according to the corresponding relationship between the two transponders C 0  and C 1  and the  10  motion image signals V 0  to V 9  allocated thereto on the basis of the contents of the predetermined value table  21 , adopts a combination of the motion image signals so as to minimize the maximum value of the sum of the necessary bit rates to determine the corresponding relationship in each time period, and outputs the determined corresponding relationship to the motion image encoder unit  12 . 
     Next, an operation of the motion image encoder described above will be described in connection with FIG. 5 which shows a processing of the allocation means  20 . 
     When the present time T becomes a set time (T 1 , T 2 , T 3 , . . . ) stored in the predetermined value table  21 , the allocation means  20  starts the processing shown in FIG.  5 . 
     An initial value of a combination is set to a variable P, and infinity is set to a variable MQ in step S 40 . The variable P is composed of  10  elements P 0  to P 9  (available value is “0” or “1”). An i-th element Pi (i=0 to 9) represents whether an i-th motion image signal Vi is allocated to the transponder C 0  or C 1 . That is, when the variable Pi is “0”, the motion image signal Vi is allocated to the transponder C 1 . The variable MQ holds the minimum value among evaluation values obtained during the processing, and a variable MP holds the value of the variable P at this time (see step S 44 ). 
     The allocation means  20  inspects whether the combination shown by the variable P is an effective or ineffective combination in step S 41 . When it is an effective combination, move to step S 42 , or when it is an ineffective combination, move to step S 45 . An ineffective combination, for example, means a combination of which all the motion image signals V 0  to V 9  are allocated to only one transponder, or the like. 
     The sum of the necessary bit rates for each transponder C 0  or C 1  in the combination shown by the variable P is calculated, and their maximum value is determined as an evaluation value Q of its combination in step S 42 . The sum Tc of the necessary bit rates of a c-th transponder (c=“0” or “1”) is obtained as follows.              Tc   =       ∑     i   =   0     9            δ     Pi   ,   c          Ai               (   1   )                                
     In formula (1), δx,y is a function which is “1” when x=y, otherwise “0”, and Ai is a predetermined value A of the necessary bit rate of the motion image signal Vi and is obtained -from the predetermined value table  21  of the necessary bit rates. For instance, when the present time T is T 1 ≦T&lt;T 2 , the predetermined values A={3, 3, 4, 4, 4, 6, 6, 10, 10, 10} are read out. Thereafter, an evaluation value Q is obtained as follows.              Q   =       max     C   =   0.1          Tc             (   2   )                                
     The evaluation value Q is compared with the variable MQ in step S 43 . When Q&lt;MQ, move to step S 44 , otherwise move to step S 45 . Substitute the evaluation value Q for the variable MQ and substitute the variable P for the variable MP in step S 44 . The variable P is changed to show the next combination in step S 45 . At this time, when the variable P shows the last combination, it is renewed to a code showing the completion. The variable P is inspected in step S 46 . When it does not show the completion, return to step S 41 . When it shows the completion, move to step S 47 . The variable MP is adopted as the last combination and outputs as the corresponding relationship between the motion image signals and the transponder in step S 47 . The operation is ended. 
     Now, for example, it is assumed that the contents of the predetermined value table  21  are shown in FIG. 4, and the allocation means  20  starts the processing shown in FIG. 5 at a time T 1 . 
     First, the allocation means  20  initialize a variable P as follows and sets infinity to a variable MQ in step S 40  in FIG.  5 . 
     P={0, 0, 0, 0, 0, 0, 0, 0, 0, 0,} 
     Thereafter, evaluation values Q are calculated in regard to effective combinations of the variable P as follows. 
     P={1, 0, 0, 0, 0, 0, 0, 0, 0, 0}, Q=57 
     P={0, 1, 0, 0, 0, 0, 0, 0, 0, 0}, Q=57 
     P={1, 1, 0, 0, 0, 0, 0, 0, 0, 0}, Q=54 
     P={0, 0, 1, 0, 0, 0, 0, 0, 0, 0}, Q=56 
     P={1, 0, 1, 0, 0, 0, 0, 0, 0, 0}, Q=53 
     P={0, 1, 1, 0, 0, 0, 0, 0, 0, 0}, Q=53 
     P={1, 1, 1, 0, 0, 0, 0, 0, 0, 0}, Q=50 
     : 
     P={1, 1, 1, 1, 1, 1, 1, 0, 0, 0}, Q=30 
     : 
     P={1, 1, 1, 0, 1, 1, 1, 1, 1, 1}, Q=56 
     P={0, 0, 0, 1, 1, 1, 1, 1, 1, 1}, Q=50 
     P={1, 0, 0, 1, 1, 1, 1, 1, 1, 1}, Q=53 
     P={0, 1, 0, 1, 1, 1, 1, 1, 1, 1}, Q=53 
     P={1, 1, 0, 1, 1, 1, 1, 1, 1, 1}, Q=56 
     P={0, 0, 1, 1, 1, 1, 1, 1, 1, 1}, Q=54 
     P={1, 0, 1, 1, 1, 1, 1, 1, 1, 1}, Q=57 
     P={0, 1, 1, 1, 1, 1, 1, 1, 1, 1}, Q=57 
     In this case, the predetermined values A of the necessary bit rates to be required for calculating the evaluation values Q are read out of the predetermined value table  21 , and at the time T 1 , the allocation means  20  obtains the predetermined values A={3, 3, 4, 4, 4, 6, 6, 10, 10, 10}. 
     From the calculation results of the above evaluation values Q, the allocation means  20  adopts the minimum evaluation value Q as the optimum combination as follows, 
     MP={1, 1, 1, 1, 1, 1, 1, 0, 0, 0}, MQ=30 
     and outputs the corresponding relationship to the motion image encoder unit  12 . In other words, the allocation means  20  outputs the corresponding relationship instructing the motion image encoder unit  12  to multiplex the motion image signals V 0  to V 6  to the first to seventh orders of the transponder C 1  and to multiplex the motion image signals V 7  to V 9  to the first to third orders of the transponder C 0 . The motion image encoder unit  12  thus conducts the aforementioned operation according to the corresponding relationship sent from the allocation means  20 . 
     FIG. 6 shows a motion image encoder according to a third embodiment of the present invention. In this embodiment, the motion image encoder comprises a motion image encoder unit  12 , an allocation means  50  and a predetermined value table  51 . 
     The predetermined value table  51  stores the predetermined values A and variation rates B of the necessary bit rates of the  10  motion image signals V 0  to V 9  in a certain time period. The predetermined value table  51  is constructed on a memory and its contents can be freely changed from the outside. 
     FIG. 7 illustrates contents of a predetermined value table  51 . In the example shown in FIG. 4, it is apparent that the predetermined values A of the necessary bit rates of the motion image signals V 0  to V 9  in a time period T 1 ≦T&lt;T 2  (T: present time) are 3, 3, 4, 4, 4, 6, 6, 10, 10 and 10 (Mbps), respectively, and the variation rates B of the necessary bit rates are 1, 1, 1, 1, 2, 1, 3, 2, 5 and 3 (Mbps), respectively. 
     The allocation means  50  calculates the statistical sum of the necessary bit rates of each transponder C 0  or C 1  with respect to the effective combination according to the corresponding relationship between the two transponders CO and C 1  and the  10  motion image signals V 0  to V 9  allocated thereto on the basis of the contents of the predetermined value table  51 , adopts a combination of the motion image signals so as to minimize the maximum value of the sum of the necessary bit rates to determine the corresponding relationship in each time period, and outputs the determined corresponding relationship to the motion image encoder unit  12 . 
     The allocation means  50  implements the same operation as one shown in FIG. 5 by the allocation means  20  shown in FIG. 3 except the following processing in place of that in step S 42  shown in FIG.  5 . 
     That is, the statistical sum of the necessary bit rates of each transponder C 0  or C 1  in the combination shown by the variable P is calculated, and the maximum value of their sums is determined as an evaluation value Q of its combination. The statistical sum Tc is calculated as follows.              Tc   =         ∑     i   =   0     9            δ     Pi   ,   c            (     Ai   -     α                 Bi       )         +       α        [         ∑     i   =   0     9          δ   Pi       ,     Bi   2       ]         1   /   2                 (   3   )                                
     In formula (3), Bi represents a variation rate of the necessary bit rates of the motion image signals V 0  to V 9  to be multiplexed and is read out of the predetermined value table  51 . For instance, when the present time T is T 1 ≦T&lt;T 2 , the variation rates B={1, 1, 1, 1. 2. 1, 3, 2, 5, 3 25} are read out. Further, α is a parameter for determining quality and is determined to be “2” in this case. 
     Now, for example, it is assumed that the contents of the predetermined value table  51  are shown in FIG. 7, and the allocation means  50  starts the processing at a time T 1 . 
     First, the allocation means  50  initialize a variable P as follows and sets infinity to a variable MQ. 
     P={0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0} 
     Thereafter, evaluation values Q are calculated as effective combinations of the variable P are changed in order as follows. 
     P={1, 0, 0, 0, 0, 0, 0, 0, 0, 0}, Q=33.8324 
     P={0, 1, 0, 0, 0, 0, 0, 0, 0, 0}, Q=33.8324 
     P={1, 1, 0, 0, 0, 0, 0, 0, 0, 0}, Q=32.6969 
     P={0, 0, 1, 0, 0, 0, 0, 0, 0, 0}, Q=32.8324 
     P={1, 0, 1, 0, 0, 0, 0, 0, 0, 0}, Q=31.6969 
     P={0, 1, 1, 0, 0, 0, 0, 0, 0, 0}, Q=31.6969 
     P={1, 1, 1, 0, 0, 0, 0, 0, 0, 0}, Q=30.5602 
     : 
     P={1, 1, 1, 0, 0, 1, 0, 1, 0, 0}, Q=19.8564 
     : 
     P={1, 1, 1, 0, 1, 1, 1, 1, 1, 1}, Q=32.8324 
     P={0, 0, 0, 1, 1, 1, 1, 1, 1, 1}, Q=30.5602 
     P={1, 0, 0, 1, 1, 1, 1, 1, 1, 1}, Q=31.6969 
     P={0, 1, 0, 1, 1, 1, 1, 1, 1, 1}, Q=31.6969 
     P={1, 1, 0, 1, 1, 1, 1, 1, 1, 1}, Q=32.8324 
     P={0, 0, 1, 1, 1, 1, 1, 1, 1, 1}, Q=32.6969 
     P={1, 0, 1, 1, 1, 1, 1, 1, 1, 1}, Q=33.8324 
     P={0, 1, 1, 1, 1, 1, 1, 1, 1, 1}, Q=33.8324 
     In this case, the predetermined values A and the variation rates B of the necessary bit rates to be required for calculating the evaluation values Q are read out of the predetermined value table  51 , and at the time T 1 , the allocation means  50  obtains the predetermined values A={3, 3, 4, 4, 4, 6, 6, 10, 10, 10} and the variation rates B={1, 1, 1, 1, 2, 1, 3, 2, 5, 3}. 
     From the calculation results of the above evaluation values Q, the allocation means  50  adopts the minimum evaluation value Q as the optimum combination as follows, 
     MP={1, 1, 1, 0, 0, 1, 0, 1, 0, 0}, MQ=19.8564 
     and outputs the corresponding relationship to the motion image encoder unit  12 . That is, the allocation means  50  outputs the corresponding relationship instructing the motion image encoder unit  12  to multiplex the motion image signals V 0  to V 2 , V 5  and V 7  to the first to fifth orders of the transponder C 1  and to multiplex the motion image signals V 3 , V 4 , V 6 , V 8  and V 9  to the first to fifth orders of the transponder C 0 . The motion image encoder unit  12  thus conducts the aforementioned operation according to the corresponding relationship output from the allocation means  50 . 
     FIG. 8 shows a motion image encoder according to a fourth embodiment of the present invention. In this embodiment, the motion image encoder comprises a motion image encoder unit  72  and a program output controller  73 . 
     The motion image encoder unit  72  includes a switch means  70 ,  14  coding means  10 - 0  to  10 - 13 , and two multiplexers  71 - 0  and  71 - 1  which correspond to  7  coding means  10 - 0  to  10 - 6  and  7  coding means  10 - 7  to  10 - 13 , respectively. 
     The program output controller  73  outputs a control signal representing the corresponding relationship between the motion image signals V 0  to V 9  and the transponders C 0  and C 1  so as to allocate the necessary bit rates of the motion image signals V 0  to V 9  to the transponders C 0  and C 1  equally. This corresponding relationship represented by the control signal shows which of the transponders C 0  and C 1  and which places of the transponder C 0  or C 1  the motion image signals are multiplexed to. Moreover, the corresponding relationship is changed with the elapse of time depending on the contents of the motion image signals V 0  to V 9 . 
     The switch means  70  outputs the input  10  motion image signals V 0  to V 9  to  10  of the  14  coding means  10 - 0  to  10 - 13  according to the control signal sent from the program output controller  73  and outputs nothing to the other  4  coding means. 
     For instance, when the corresponding relationship represented by the control signal instructs that the motion image signals V 0  to V 4  are to be multiplexed to the first to fifth orders of the transponder C 0  and the motion image signals V 5  to V 9  are to be multiplexed to the first to fifth orders of the transponder C 1 , the switch means  70  sends the motion image signals V 0  to V 4  to  5  of the  7  coding means  10 - 0  to  10 - 6  corresponding to the transponder C 0  and the motion image signals V 5  to V 9  to  5  of the  7  coding means  10 - 7  to  10 - 13  corresponding to the transponder C 1 . 
     When inputting the motion image signal, each of the coding means  10 - 0  to  10 - 13  codes the motion image signal to output a code string, or, when inputting no motion image signal, each coding means does not output anything. 
     Each multiplexer  71 - 0  or  71 - 1  multiplexes at most the  7  code strings output from the  7  coding means  10 - 0  to  10 - 6  or  10 - 7  to  10 - 13  according to the corresponding relationship indicated by the control signal and outputs a multiplexed code string to the respective transponder C 0  or C 1 . Moreover, the multiplexers  71 - 0  and  71 - 1  also multiplex the corresponding relationship represented by the control signal. 
     FIG. 9 shows a motion image encoder according to a fifth embodiment of the present invention. In this embodiment, the motion image encoder comprises a motion image encoder unit  72  having the same construction and functions as the fourth embodiment shown in FIG. 8, the allocation means  80  and a predetermined value table  21  having the same construction and functions as the second embodiment shown in FIG.  3 . 
     The allocation means  80 , similar to the allocation means  20  shown in FIG. 3, calculates the sum of the necessary bit rates of each transponder C 0  or C 1  with respect to the combination of at most 7 motion image signals allocated to one transponder according to the corresponding relationship of the  10  motion image signals V 0  to V 9  allocated to the two transponders C 0  and C 1  on the basis of the contents of the predetermined value table  21 , adopts a combination of the motion image signals so as to permit the sum of the necessary bit rates to be closest to the transmission ability of each transponder to determine the corresponding relationship in each time period, and outputs the determined corresponding relationship to the motion image encoder unit  72 . The motion image encoder unit  72  executes the operation in the same manner as described above according to the corresponding relationship received from the allocation means  80 . 
     FIG. 10 shows a motion image encoder according to the sixth embodiment of the present invention. In this embodiment, the motion image encoder comprises a motion image encoder unit  72 , an allocation means  90  and a predetermined value table  51  having the same construction and functions as the fourth embodiment shown in FIG.  6 . 
     The allocation means  90 , similar to the allocation means  50  shown in FIG. 6, calculates the statistical sum of the necessary bit rates of each transponder C 0  or C 1  with respect to the combination of at most 7 motion image signals allocated to one transponder according to the corresponding relationship of the  10  motion image signals V 0  to V 9  allocated to the two transponders C 0  and C 1  on the basis of the contents of the predetermined value table  51 , adopts a combination of the motion image signals so as to minimize the maximum value of the sum of the necessary bit rates to determine the corresponding relationship in each time period, and outputs the determined corresponding relationship to the motion image encoder unit  72 . The statistical sum of the necessary bit rates is calculated using formula (3) in the same manner described above. 
     In FIG. 11, there is shown a motion image decoder according to a first embodiment of the present invention. The motion image decoder comprises a selector  100 , an inverse multiplexer  101 , a decoder  102  and an indicator  103 . 
     A user inputs information showing a motion image signal to be decoded to the indicator  103 . The indicator  103  picks out information of corresponding relationship from the input multiplexed code string, transfers information showing a transponder for transmitting the multiplexed code string including the motion image signal designated by the user, to the selector  100  and also transfers information showing which place of the transponder the motion image signal designated by the user is multiplexed to, to the inverse multiplexer  101 . At this time, the information of the corresponding relationship is included in both multiplexed code strings sent from the transponders C 0  and C 1 , and the indicator  103  can pick out the information of the corresponding relationship irrespective of which the selector  100  selects either transponder C 0  or C 1 . 
     The selector  100  selects one of the multiplexed code strings sent via the transponders C 0  and C 1  according to the information transferred from the indicator  103  to output the selected multiplexed code string. The inverse multiplexer  101  picks out one code string from the multiplexed code string output from the selector  100  according to the information transferred from the indicator  103  to output the selected code string. The decoder  102  decodes the code string output from the inverse multiplexer  101  to output a motion image signal. 
     FIG. 12 shows a motion image decoder according to a second embodiment of the present invention. The motion image decoder comprises two inverse multiplexers  110 - 0  and  110 - 1 , a synthesizer  111 , a decoder  112  and an instructor  113 . 
     A user inputs information showing a motion image signal to be decoded to the instructor  113 . The instructor  113  picks out information of corresponding relationship from the input multiplexed code string, and also transfers information showing which place of the transponder the motion image signal designated by the user is multiplexed to, to one inverse multiplexer  110 - 0  or  110 - 1  including the motion image signal designated by the user. However, the instructor  113  does not send anything to the other inverse multiplexer  110 - 1  or  110 - 0 . 
     The inverse multiplexer  110 - 0  or  110 - 1  having been transferred the information from the instructor  113  selects one code string shown by the received information and outputs the selected code string to the synthesizer  111 . The synthesizer  111  synthesizes the code strings output from the inverse multiplexers  110 - 0  and  110 - 1  to output a synthesized code string to the encoder  112 . The decoder  112  decodes the synthesized code string to output a motion image signal. 
     FIG. 13 illustrates a hardware construction of the motion image encoder shown in FIG.  2  and the motion image decoder shown in FIG.  11 . This hardware construction comprises a computer  120  and a storage medium  121 . A semiconductor memory, a magnetic disk, and other suitable storage media can be used for the storage medium  121 . 
     When the motion image encoder shown in FIG. 2 is realized, a motion image encoding program is stored in the storage medium  121 . This motion image encoding program stored in the storage medium  121  is read into the computer  120 , and the operation of the computer  120  is controlled to realize the coding means  10 - 0  to  10 - 9 , the multiplexer  11  and the program output controller  13  shown in FIG. 2 on the computer  120 . 
     Further, when the motion image decoder shown in FIG. 11 is realized, a motion image decoding program is stored in the storage medium  121 . This motion image decoding program stored in the storage medium  121  is read into the computer  120 , and the operation of the computer  120  is controlled to realize the selector  100 , the inverse multiplexer  101 , the decoder  102  and the indicator  103  shown in FIG. 11 on the computer  120 . 
     As described above, in the motion image encoder according to the present invention, the relationship between the motion image signals and the transponders are not fixed, and the motion image signals are allocated to the transponders so that the bit rates of the multiplexed code strings output to each transponder may be closest to its transmission ability. Hence, a plurality of transponders can be utilized without overs and shorts, and as a result, image quality degradation can be prevented. 
     Further, in the motion image encoder of the present invention, including a predetermined value table for storing predetermined values of the necessary bit rates of N motion image signals in a certain time period, the corresponding relationship between transponders and the motion image signals allocated thereto in each time period is determined so that the sum of the necessary bit rates of the motion image signals allocated to each transponder may be closest to its transmission ability on the basis of the contents of the predetermined value table, resulting in preventing image quality degradation with a simple construction. 
     Further, in the motion image encoder of the present invention, including a predetermined value table for storing predetermined values and variation rates of the necessary bit rates of N motion image signals in a certain time period, the corresponding relationship between transponders and the motion image signals allocated thereto in each time period is determined so that the statistical sum of the necessary bit rates of the motion image signals allocated to each transponder may be closest to its transmission ability on the basis of the contents of the predetermined value table, resulting in preventing image quality degradation more effectively with a simple construction. 
     While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by those embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.