Decoding method of moving image signal, and decoding apparatus of moving image signal using the same

Abstract: The invention relates to a decoding method and coding method of moving image signal, and a decoding apparatus and coding apparatus using the same, 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 of digital moving image signal, and a decoding apparatus using the same method, 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 of digital moving image signal, and a decoding 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 of digital moving image signal, and a decoding 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. Thus, the present invention concerns a decoding method and apparatus as defined in the appended claims. Relating to the present processing pixel block, in the decoding method of digital moving image signal of the invention, at least two or more motion vectors are 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. 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 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 code 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. 2. (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.

Claims: A decoding method of moving image signal, being a decoding method of moving image signal for decoding two or more motion vectors relating to the present processing pixel block, compensating the motion of the decoded frame corresponding to each one of said two or more motion vectors, and generating two or more predicted images relating to the present processing pixel block, wherein the predicted image used in reconstruction of the present processing pixel block is selected depending on presence or absence of decoding error contained in said two or more predicted images. A decoding method of moving image signal of claim 1, wherein if there are plural predicted images free from decoding error in said two or more predicted images, the predicted image produced from the latest decoded frame in time out of said plural predicted images free from decoding error is used in reconstruction of the present processing pixel block. A decoding apparatus of moving image signal comprising: variable length code decoding means for decoding two or more motion vectors relating to the present processing pixel block, motion compensation means for compensating the motion of the decoded frame corresponding to each one of said two or more motion vectors, and generating two or more predicted images relating to the present processing pixel block, bit error detecting means for detecting a bit error from the output of said variable length code decoding means, memory means for storing the bit error detecting result of said bit error detecting means, and predicted image selecting means for recognizing presence or absence of decoding error contained in said two or more predicted images, and in dependence thereon selecting the predicted image to be used in reconstruction of the present processing pixel block. A decoding apparatus of moving image signal of claim 3, wherein the bit error detecting means detects bit error in the pixel block when the variable length code of the pixel block decoded by the variable length code decoding means is contradictory to a specified standard. A decoding apparatus of moving image signal of claim 3, wherein the memory means stores bit errors in plural frames by plotting the pixel blocks in which bit error is detected in each frame in a map form. A decoding apparatus of moving image signal of claim 5, wherein the memory means comprises plural decoding error map memories storing each frame consecutive in time, and also has changeover means, and therefore said plural decoding error map memories are changed over by said changeover means, and issued.

Inventors: MACHIDA YUTAKA, MACHIDA, YUTAKA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1998
Main classification: H04N7
IPC codes: H04N7, H03M7