Abstract:
A decoding method and coding method of moving image signal, and a decoding apparatus and coding apparatus using the same, provides for coding or decoding at least two or more motion vectors, relating to the present processing pixel block, compensating the motion of the coded frame corresponding to each motion vector, generating two or more predicted images relating to the present processing image block, and selecting the predicted image for use in reconstruction of the present processing pixel block depending on presence or absence of decoding error contained in these two or more predicted images. Accordingly, if a decoding error occurs due to bit error in the bit stream, propagation of picture quality deterioration in time can be suppressed, so that excellent decoding of digital moving image signal is realized.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a decoding method and coding method of digital moving image signal, and a decoding apparatus and coding apparatus using the same methods, for use in television telephone, television conference, and recording of video signals and communication of various signals. More particularly, it relates to a decoding method and coding method of digital moving image signal, and a decoding apparatus and coding apparatus using the same, being designed so that effects of decoding errors may not propagate to frames and fields later in time. 
     BACKGROUND OF THE INVENTION 
     In television telephone or television conference, in order to transmit a moving image efficiently even in a narrow transmission route, the moving image is digitized, and this signal is further processed by digital image compression, and is transmitted, and at the receiving side, the compressed digital moving image signal is decoded, and the moving image is reproduced. 
     In a video disc or the like, in order to accumulate more moving images in one disc, the digital moving image signal is coded and the image is compressed. 
     The conventional coding method and decoding method of digital moving image signal include the ITU-T Recommendation H. 261 recommended in March 1993, and the MPEG by the ISO, and the ITU-T Recommendation is mainly described in this specification. For the sake of convenience, the coding method is first described, and then the decoding method, by referring to  FIG. 4  to  FIG. 6 . 
     The coding method of H. 261 is realized, as shown in  FIG. 4 , by a coding apparatus comprising motion vector detecting means  401 , motion compensation means  402 , prediction error calculating means  403 , intra-inter judging means  404 , DCT  405 , quantizing means  406 , variable length coding means  407 , inverse quantizing means  408 , inverse DCT  409 , reconstruction means  410 , frame memory  411 , and intra/inter switch  412 . 
     Each constituent element of H. 261 coding method is described below. First, the motion vector detecting means  401  detects the motion vector between a previous reconstruction frame stored in the frame memory  411 , and an entered present frame. The motion vector is detected, as shown in  FIG. 5 , in a unit of pixel block called macro block. The motion vector may be regarded as a spatial displacement amount of pixel block between the previous reconstruction frame and the present frame. Next, the motion compensation means  402  displaces the pixel block of the previous reconstruction frame stored in the frame memory  411  according to the motion vector, and generates a predicted image. This motion compensation is a process for reducing the prediction error in the prediction error calculation at a next step. 
     The prediction error calculating means  403  calculates the differential value between the macro block of the entered present frame and the predicted image in the pixel unit, and issues as a prediction error. Further, both the macro block of the present frame and the prediction error are entered into the intra/inter judging means  404 . Herein, for example, by comparison between the variance of pixels in the macro block of the present frame and the variance of the prediction error, it is judged whether coding is more efficient by directly coding the macro block of the present frame (intra-coding), or coding is more efficient by coding the prediction error (inter-coding), and according to the result, either the macro block of the present frame or the prediction error is issued. At the same time, the result of judgement is issued as an intra/inter control signal. 
     The macro block of the present frame or the prediction error issued from the intra/inter judging means  404  is transformed into a frequency region by the DCT  405  and is further transformed into a quantizing coefficient by the quantizing means  406  to be curtailed in redundancy, and further the quantizing coefficient is transformed into a variable length code on the basis of the statistic character by the variable length coding means  407 , and the quantity of information is curtailed. In the variable length coding means  407 , further, the intra/inter control signal and motion vector are entered, and coded into variable length codes. The variable length coding means  407  multiplexes all these variable length codes, and issues as a bit stream. 
     On the other hand, the quantizing coefficient which is the output of the quantizing means  406  is inversely quantized by the inverse quantizing means  408 , and is inversely transformed from the frequency region by the inverse DCT  409 . When the macro block being presently coded is intra-coding, the intra/inter switch  412  is turned off, and the output from the inverse DCT  409  is directly stored in the frame memory  411 . To the contrary, when the macro block being presently coded is inter-coding, the intra/inter switch  412  is turned on, and the output of the inverse DCT  409  and the output of the motion compensation means  402  are added in the reconstruction means  410 , and stored in the frame memory  411 . Thus, the present reconstruction frame is stored in the frame memory  411 , and it is used in prediction of next frame. 
     Next, the decoding method of H. 261 is realized by an apparatus, as shown in  FIG. 6 , comprising variable length code decoding means  601 , inverse quantizing means  602 , inverse DCT  603 , frame memory  604 , motion compensation means  605 , intra/inter switch  606 , and reconstruction means  607 . 
     Each constituent element of the H. 261 decoding method is described below. First, the variable length code decoding means  601  separates and decodes the variable length code from the entered bit stream, and issues the quantizing coefficient, intra/inter control signal, and motion vector in the macro block unit. The quantizing coefficient is inversely quantized by the inverse quantizing means  602 , and is inversely transformed from the frequency region by the inverse DCT  603 . The motion compensation means  605  displaces the pixel block of the previous reconstruction frame stored in the frame memory  604 , on the basis of the motion vector, and generates a predicted image. 
     When the intra/inter control signal instructs intra-coding, the intra/inter switch  606  is turned off, and the output from the inverse DCT  603  is directly stored in the frame memory  604 . On the other hand, when the intra/inter control signal instructs inter-coding, the intra/inter switch  606  is turned on, and the output of the inverse DCT  603  and the output of the motion compensation means  605  are added in the reconstruction means  607 , and stored in the frame memory  604 . Thus, the present reconstruction frame is stored in the frame memory  604 , and it is used in prediction of next frame and is also issued as a decoded image. 
     In the coding method and decoding method of H. 261, the inverse quantizing means, inverse DCT, and motion compensation means process the same, and therefore the reconstruction frames stored in the frame memory of the coding method and decoding method always coincide with each other. 
     Generally, since the bit stream issued by the coding method of moving image signal is composed of variable length codes, in case an error occurs in transmission or accumulation, if it is an error of one bit, the decoding error occurs in a wide range, and the picture quality of the reconstruction frame deteriorates. In the conventional decoding method of moving image signal, the reconstruction frame lowed in picture quality due to decoding error is used in prediction of next frame, this picture quality deterioration can propagate to the succeeding frames later in time. 
     As one of the prior arts presenting a method of solving this problem, “Error Resilient Video Coding by Dynamic Replacing of Reference Pictures” (S. Fukunaka et al.) (1996 IEEE) is known, but when a decoding error occurs, the previously decoded video signal is directly used again, the improving effect of picture quality deterioration was trifling. 
     SUMMARY OF THE INVENTION 
     It is hence an object of the invention to present an excellent decoding method and coding method of digital moving image signal, and a decoding apparatus and coding apparatus using the same, capable of preventing propagation of picture quality deterioration in time even if a decoding error occurs due to bit error in the bit stream, and decoding with less picture quality deterioration. 
     Relating to the present processing pixel block, in the coding method and decoding method of digital moving image signal of the invention, at least two or more motion vectors are coded or decoded, and the motion of the decoded frame corresponding to each motion vector is compensated, and two or more predicted images relating to the present processing pixel block are generated. In the decoding method of moving image signal of the invention, depending on the presence or absence of decoding error contained in the two or more predicted images, the predicted image to be used reconstruction of the present processing pixel block is selected. 
     According to the invention thus constituted, if decoding error occurs due to bit error in the bit stream, an excellent coding method and decoding method of digital moving image signal capable of suppressing propagation of picture quality deterioration in time can be presented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing showing a constitution of a decoding apparatus for realizing a decoding method of moving image signal in first and second embodiments of the invention. 
         FIG. 2  is a diagram showing an estimating method of macro block having a decoding error in the decoding method of moving image signal in the first and second embodiments of the invention. 
         FIG. 3  is a diagram showing a constitution of a coding apparatus for realizing a coding method of moving image signal in third and fourth embodiments of the invention. 
         FIG. 4  is a diagram showing a constitution of a coding apparatus for realizing a coding method of moving image signal of prior art. 
         FIG. 5  is a diagram showing a detecting method of motion vector. 
         FIG. 6  is a diagram showing a constitution of a decoding apparatus for realizing a decoding method of moving image signal of prior art. 
       
         
           
                 
               
                 
                 
               
             
                 
                     
                 
                 
                   Reference Numerals 
                 
                 
                     
                 
               
               
                 
                     
                 
               
            
             
                 
                   101 
                   Variable length code decoding means 
                 
                 
                   102 
                   Inverse quantizing means 
                 
                 
                   103 
                   Inverse DCT 
                 
                 
                   104 
                   Frame memory A 
                 
                 
                   105 
                   Motion compensation means A 
                 
                 
                   106 
                   Intra/inter switch 
                 
                 
                   107 
                   Reconstruction means 
                 
                 
                   108 
                   Frame memory changeover switch 
                 
                 
                   109 
                   Frame memory B 
                 
                 
                   110 
                   Motion compensation means B 
                 
                 
                   111 
                   Predicted image selecting means 
                 
                 
                   112 
                   Decoding error map A 
                 
                 
                   113 
                   Decoding error map B 
                 
                 
                   114 
                   Decoding error map changeover switch 
                 
                 
                   301 
                   Motion vector detecting means A 
                 
                 
                   302 
                   Motion compensation means A 
                 
                 
                   303 
                   Prediction error calculating means 
                 
                 
                   304 
                   Intra/inter judging means 
                 
                 
                   305 
                   DCT 
                 
                 
                   306 
                   Quantizing means 
                 
                 
                   307 
                   Variable length coding means 
                 
                 
                   308 
                   Inverse quantizing means 
                 
                 
                   309 
                   Inverse DCT 
                 
                 
                   310 
                   Reconstruction means 
                 
                 
                   311 
                   Frame memory A 
                 
                 
                   312 
                   Intra/inter switch 
                 
                 
                   313 
                   Motion vector detecting means B 
                 
                 
                   314 
                   Motion compensation means B 
                 
                 
                   315 
                   Predicted image combining means 
                 
                 
                   316 
                   Frame memory B 
                 
                 
                   317 
                   Frame memory changeover switch 
                 
                 
                   401 
                   Motion vector detecting means 
                 
                 
                   402 
                   Motion compensation means 
                 
                 
                   403 
                   Prediction error calculating means 
                 
                 
                   404 
                   Intra/inter judging means 
                 
                 
                   405 
                   DCT 
                 
                 
                   406 
                   Quantizing means 
                 
                 
                   407 
                   Variable length coding means 
                 
                 
                   408 
                   Inverse quantizing means 
                 
                 
                   409 
                   Inverse DCT 
                 
                 
                   410 
                   Reconstruction means 
                 
                 
                   411 
                   Frame memory 
                 
                 
                   412 
                   Intra/inter switch 
                 
                 
                   601 
                   Variable length code decoding means 
                 
                 
                   602 
                   Inverse quantizing means 
                 
                 
                   603 
                   Inverse DCT 
                 
                 
                   604 
                   Frame memory 
                 
                 
                   605 
                   Motion compensation means 
                 
                 
                   606 
                   Intra/inter switch 
                 
                 
                   607 
                   Reconstruction means 
                 
                 
                     
                 
               
            
           
         
       
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the invention are described below while referring to  FIG. 1  to  FIG. 3 . 
     First Embodiment 
     A first embodiment of the invention relates to a decoding method of moving image signal designed to select a predicted image to be used in reconstruction of present processing pixel block, depending on presence or absence of decoding error contained in two or more predicted images. 
     The decoding method of moving image signal in the first embodiment of the invention is realized, as shown in  FIG. 1 , by a decoding apparatus comprising variable length code decoding means  101 , inverse quantizing means  102 , inverse DCT  103 , frame memory A  104 , motion compensation means A  105 , intra/inter switch  106 , reconstruction means  107 , frame memory changeover switch  108 , frame memory B  109 , motion compensation means B  110 , predicted image selecting means  111 , decoding error map A  112 , decoding error map B  113 , and decoding error map changeover switch  114 . 
     The operation of the decoding method of moving image signal in the first embodiment of the invention is described below. First, the decoding error map changeover switch  114  is set at the decoding error map B  113  side, and the frame memory changeover switch  108  is set at the frame memory B  109  side. 
     The variable length code decoding means  101  separates and decodes a variable length code from the entered bit stream, and issues quantizing coefficient, intra/inter control signal, motion vector A, and motion vector B in macro block unit, and also writes presence or absence of decoding error in the decoding error map B  113 . The decoding error map is a memory for storing the decoded state about all macro blocks in one frame, by expressing, for example, a correctly decoded macro block as 1 and a macro block having decoding error as 0. In the decoding error map A  112 , the decoded state one frame before in time from the present processing macro block is stored, and in the decoding error map B  113 , the decoded state two frames before in time from the present processing macro block is stored. 
     In the variable length code decoding means  101 , various means for detecting decoding error of macro block may be considered, and, for example, it may be realized as follows. When decoding a moving image signal, if there is a bit error in the entered bit stream, all macro blocks from the bit having the error until a next synchronizing code (start code) appears cannot be decoded correctly. Generally, unless bit error detecting means is provided, macro blocks that cannot be decoded correctly due to bit error cannot be identified. However, while decoding the bit stream from the wrong bit until appearance of next synchronizing code, (1) a variable length code not specified preliminarily may appear, (2) a value exceeding an allowable range may be decoded, and other contradictions may occur at high probability. 
     By making use of the character of (1) or (2), it may be possible to estimate that there is a decoding error in N macro blocks tracing back from the macro block having such decoding contradiction, and all macro blocks after the macro block generating decoding contradiction until appearance of synchronizing code. An example of N=4 is shown in  FIG. 2 . In the example in  FIG. 2 , since decoding contradiction took place when decoding the (n+5)-th macro block, it is estimated that there is a decoding error in all macro blocks from the (n+1)-th macro block, four macro blocks before, till the (n+7)-th macro block just before the synchronizing signal. 
     Consequently, the quantizing coefficient issued from the variable length code decoding means  101  is inversely quantized by the inverse quantizing means  102 , and is inversely transformed from the frequency region by the inverse DCT  103 . 
     On the other hand, in the frame memory A  104 , the reconstruction image of one frame before in time from the present processing macro block is stored, and in the frame memory B  109 , the reconstruction image of two frames before in time from the present processing macro block is stored. The motion vector A is a motion vector corresponding the frame memory A  104 , and the motion vector B is a motion vector corresponding to the frame memory B  109 . That is, the motion compensation means A  105  generates a predicted image A by displacing the pixel block of the reconstruction frame of one frame before stored in the frame memory A  104 , according to the motion vector A. Similarly, the motion compensation means B  110  generates a predicted image B by displacing the pixel block of the reconstruction frame of two frames before stored in the frame memory B  109 , according to the motion vector B. 
     The predicted image selecting means  111  first identifies the position of the pixel block to be compensated of motion, by using the value of the entered motion vector A. Comparing the position of this pixel block and the position of the error information stored in the decoding error map A  112 , presence or absence of decoding error contained in the predicted image A is judged. Similarly, predicted image selecting means  111  judges presence or absence of decoding error contained in the predicted image B, from the contents of the value of the entered motion vector B and decoding error map B  113 . When judging that a decoding error is contained in the predicted image A, the predicted image selecting means  111  issues only the predicted image B, or, to the contrary, when judging that a decoding error is contained in the predicted image B, it issues only the predicted image A. If it is judged that error is not contained in either predicted image A or predicted image B, the average of the predicted image A and predicted image B is calculated and issued in the pixel unit. 
     If it is judged that error is contained in both predicted image A and predicted image B, the average of the predicted image A and predicted image B may be calculated and issued in the pixel unit, the predicted image A may be issued, or the predicted image B may be issued, as determined preliminarily. 
     When the intra/inter control signal issued from the variable length code decoding means  101  instructs intra-coding, the intra/inter switch  106  is turned off, and the output from the inverse DCT  103  is directly stored in the frame memory B  109 . On the other hand, when the intra/inter control signal instructs inter-coding, the intra/inter switch  106  is turned on, and the output of the inverse DCT  103  and the output of the predicted image selecting means  111  are added in the reconstruction means  107 , and stored in the frame memory B  109 . Thus, the present reconstruction frame is stored in the frame memory B  109 , and it is issued as decoded image. 
     After completion of decoding process of the present frame, the decoding error map changeover switch  114  is changed over to the decoding error map A  112  side, and the frame memory changeover switch  108  is changed over to the frame memory A  104  side. By this changeover, in decoding of next frame, the decoded state of two frames before in time is stored in the decoding error map A  112 , and the decoded state of one frame before in time is stored in the decoding error map B  113 . Likewise, the reconstruction image of two frames before in time is stored in the frame memory A  104 , and the reconstruction image of one frame before in time is stored in the frame memory B  109 . 
     In the first embodiment of the invention, since the decoding method of moving image signal is thus constituted, by using only the predicted image not containing decoding error in reconstruction of the present processing pixel block, the picture quality deterioration due to decoding error is prevented from propagating into the frames subsequent in time. 
     In the decoding method of moving image signal of the invention, the decoding error map and frame memory are provided by two pieces each, but by using three pieces or more, the decoded state and reconstruction image of three frames or more before in time may be stored. Incidentally, the means for estimating the macro block having decoding error by decoding contradiction, and the means for judging decoding error contained in the predicted image by using the decoding error map are only example, and they may be composed of other means. 
     Second Embodiment 
     A second embodiment of the invention relates to a decoding method of moving image signal, in which if decoding error is not contained in plural predicted images out of two or more predicted images, the predicted image produced from the latest decoded frame in time out of the predicted images free from decoding error is used in reconstruction of the present processing pixel block. 
     The decoding method of moving image signal of the second embodiment of the invention is realized by the same decoding apparatus as in the first embodiment shown in  FIG. 1 . The operation of the decoding method of the moving image signal of the second embodiment of the invention is described below. The operation other than that of the predicted image selecting means  111  is not particularly different from that in the first embodiment. 
     In the predicted image selecting means  111 , if it is judged that there is no error contained in either predicted image A or predicted image B, it is constituted to issue the newer one of the predicted image A or predicted image B. That is, if it is judged that error is not contained in either predicted image A or predicted image B, it is constituted to issue always only the predicted image produced from the reconstruction image of one frame before. 
     In the decoding method of the moving image signal realized in the decoding apparatus shown in  FIG. 1 , the decoding error map and frame memory are provided by two pieces each, but by using three pieces or more, the decoded state and reconstruction image of three frames or more before in time may be stored. If there are three predicted images, supposing, for example, there is an error in the latest predicted image A, out of predicted image A, predicted image B, and predicted image C, and no error in the predicted image B and predicted image C, it is constituted to issue the latest predicted image B out of the predicted image B and predicted image C. It is the same if there are four or more images. If all predicted images are wrong, one predetermined predicted image may be issued. 
     In the second embodiment of the invention, since the decoding method of the moving image signal is thus constituted, the correlation of the present processing pixel block to be reconstructed and the predicted image is heightened due to similarity of moving images in time, so that the code quantity necessary for reconstruction of present processing block is decreased. 
     Third Embodiment 
     A third embodiment of the invention is a coding method of moving image signal for inter-coding the present processing pixel block when the correlation is high between two or more predicted images compensated of motion by two or more motion vectors, and intra-coding the present processing pixel block when the correlation is low between two or more predicted images. 
     The coding method of the moving image signal of third embodiment of the invention is realized, as shown in  FIG. 3 , by the coding apparatus comprising motion vector detecting means A  301 , motion compensation means A  302 , prediction error calculating means  303 , intra/inter judging means  304 , DCT  305 , quantizing means  306 , variable length coding means  307 , inverse quantizing means  308 , inverse DCT  309 , reconstruction means  310 , frame memory A  311 , intra/inter switch  312 , motion vector detecting means B  313 , motion compensation means B  314 , predicted image combining means  315 , frame memory B  316 , and frame memory changeover switch  317 . 
     The operation of the coding method of moving image signal of the third embodiment of the invention is described below. First, the frame memory changeover switch  317  is supposed to be set at the frame memory B  316  side. 
     To begin with, the motion vector detecting means A  301  detects the motion vector A by the reconstruction frame stored in the frame memory A  311  and the entered present frame. Similarly, the motion vector detecting means B  313  detects the motion vector B by the reconstruction frame stored in the frame memory B  316  and the entered present frame. Herein, in the frame memory A  311 , the reconstruction image of one frame before in time from the present processing macro block is stored, and in the frame memory B  316 , the reconstruction image of two frames before in time from the present processing macro block is stored. 
     The motion compensation means A  302  generates a predicted image A by displacing the pixel block of the reconstruction frame of one frame before stored in the frame memory A  311 , according to the motion vector A. Similarly, the motion compensation means B  314  generates a predicted image B by displacing the pixel block of the reconstruction frame of two frames before stored in the frame memory B  316 , according to the motion vector B. The predicted image combining means  315  calculates and issues the average of the entered predicted image A and predicted image B in the pixel unit. The prediction error calculating means  303  calculates the difference of the macro block of the entered present frame and the output of the predicted image combining means  315  in the pixel unit, and issues as prediction error. 
     Further, the macro block of the present frame, this prediction error, and the predicted image A and predicted image B are entered into the intra/inter judging means  304 . Herein, as the processing at first stage, for example, by comparing the variance of pixels in the macro block of the present frame and the variance of the prediction error, it is judged whether the macro block of the present frame is processed by intra-coding or inter-coding. If the inter-coding is judged as a result of processing at the first stage, the correlation between the predicted image A and predicted image B is evaluated as the processing at second stage, and it is judged whether the macro block of the present frame is processed by intra-coding or inter-coding. 
     Evaluation of correlation of predicted image A and predicted image B and intra/inter judgement at the second stage are executed, for example, as follows. First, the absolute differential sum of predicted image A and predicted image B is calculated in the pixel unit. If the absolute differential sum is larger than a predetermined threshold, the correlation is small, and the macro block of the present frame is judged to be processed by intra-coding. To the contrary, if the absolute differential sum is larger than the predetermined threshold, the correlation is large, and the macro block of the present frame is judged to be processed by inter-coding. 
     According to the result of judgement by the intra/inter judging means  304  consisting of the processing in the first stage and second stage, either the macro block of the present frame or the pertinent prediction error is issued. At the same time, the result of judgement is also issued as intra/inter control signal. The macro block of the present frame or the pertinent prediction error issued from the intra/inter judging means  304  is put into the variable length coding means  307  through the DCT  305  and quantizing means  306 . In the variable length coding means  307 , the intra/inter control signal, and motion vector A and motion vector B are also entered, and are coded into variable length codes. The variable length coding means  307  multiplexes all these variable length codes, and issues as a bit stream. 
     On the other hand, the quantizing coefficient which is the output of the quantizing means  306  is put into the reconstruction means  310  through the inverse quantizing means  308  and inverse DCT  309 . When the macro block being presently coded is intra-coding, the intra/inter switch  312  is turned off, and the output from the inverse DCT  309  is directly stored in the frame memory B  316 . To the contrary, when the macro block being presently coded is inter-coding, the intra/inter switch  312  is turned on, and the output of the inverse DCT  309  and the output of the predicted image combining means  315  are added in the reconstruction means  310 , and stored in the frame memory B  316 . Thus, the present reconstruction frame is stored in the frame memory B  316 . 
     After completion of coding process of the present frame, the frame memory changeover switch  317  is changed over to the frame memory A  311  side. By this changeover, in coding of next frame, the reconstruction image of two frames before in time is stored in the frame memory A  311 , and the reconstruction image of one frame before in time is stored in the frame memory B  316 . 
     In the third embodiment of the invention, since the coding method of moving image signal is thus constituted, if a predicted image different from the coding side is selected in the decoding method of moving image signal, inter-coding is effected only when the correlation between predicted images is high, so that mis-matching of the predicted images may be kept to a minimum limit. 
     In the coding method of moving image signal of the third embodiment, two frame memories are provided, but by using three or more, the reconstruction image of three frames or more before in time may be stored. Incidentally, the means for evaluating the correlation of the predicted image A and predicted image B is only an example, and it may be composed of other means. 
     Fourth Embodiment 
     A fourth embodiment of the invention relates to a coding method of moving image signal for using the predicted image produced from the latest decoded frame in time out of two or more predicted images compensated of motion by two or more motion vectors in coding of the present processing pixel block. 
     The coding method of moving image signal of the fourth embodiment of the invention is realized by the same coding apparatus as in the third embodiment shown in  FIG. 3 . The operation of the coding method of the moving image signal of the fourth embodiment of the invention is described below. The operation other than that of the predicted image combining means  315  is not particularly different from that in the third embodiment. 
     The predicted image combining means  315  is constituted so as to issue only the newer one of the predicted image A or predicted image B. That is, it is constituted to issue always only the predicted image produced from the reconstruction image of one frame before. If three or more frame memories are used and the reconstruction image of three frames or more before in time is stored, only the latest predicted image may be issued. 
     In the fourth embodiment of the invention, since the coding method of the moving image signal is thus constituted, the prediction error about the present processing pixel block is smaller owing to the similarity of moving images in times, and the generated code quantity is smaller. 
     Thus, according to the invention, since the present processing pixel block is reconstructed by selecting the predicted image free from decoding error, the picture quality deterioration due to decoding error does not propagate in time, so that deterioration of reproduced picture quality due to bit error occurring in transmission or accumulation of bit stream may be kept to a minimum limit.