Abstract:
An apparatus for use in a video signal decoding system for concealing errors in a video signal first detects a lost block in the video signal, the lost block having errors, and then provides pixel values of neighboring blocks for the lost block in the video signal based on information for the lost block. Subsequently, The apparatus computes edge gradients of neighboring pixels included in a predetermined range of pixels surrounding the lost block based on the pixel values of the neighboring blocks and determines a pixel interpolation direction based on the edge gradients of neighboring pixels. A spatially interpolated block is produced based on the pixel values of the neighboring blocks and the pixel interpolation direction and it is provided as a substitution block for compensating the lost block.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for use in a video signal decoding system having an error concealment; and, more particularly, to a method and apparatus for concealing errors present in a decoded video signal transmitted in a compressed form. 
     DESCRIPTION OF THE PRIOR ART 
     As is well known, transmission of digitized video signals can attain video images of a much higher quality than the transmission of analog signals. When a video signal is expressed in a digital form, a substantial amount of data is generated for transmission, especially in the case of a high definition television(HDTV) system. Since, however, the available frequency bandwidth of a conventional transmission channel is limited, in order to transmit the substantial amount of digital data therethrough, it is inevitable to compress or reduce the volume of the transmission data. 
     In conventional transmission systems, the video signal is compressed on a block-by-block basis by using, e.g., DCT(Discrete Cosine Transform) and motion compensation techniques; and the compressed video signal is transmitted to a receiving end through a noisy transmission channel, entailing data losses or damages in the transmitted video signal. Such transmission systems, however, do not provide sufficient overhead mechanism to restore all errors, but rather depend on error concealment at the receiving end in order to supply substitution data for approximating the unrestored data. 
     In such a conventional error concealment scheme, a lost block(which has not been restored) and its neighboring blocks are detected first in the transmitted video signal. That is, as shown in FIG. 4, a lost block, e.g., block 0, and neighboring blocks thereof, e.g., blocks 1 to 8, circumscribing the lost block, are determined. Subsequently, each pixel value within the lost block is compensated by a weighted mean value SB defined as follows: ##EQU1## wherein W 1  and W 2  denote predetermined weight factors, respectively; and B i  represents an average value of all pixel values of an ith neighboring block with i ranging from 1 to 8. 
     Since, however, the error concealment technique described above generates the lost block data by merely averaging all pixel values of the neighboring blocks without taking into account image particularities, e.g., edges, which may exist in the neighboring blocks, such substitution data may not be able to properly reflect the original image data of the lost block. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the invention to provide a method and apparatus capable of improving image quality by concealing errors in a transmitted video signal based on edge information included therein. 
     In accordance with one aspect of the present invention, there is provided a method, for use in a video signal decoding system, for concealing errors in a video signal transmitted in a compressed form, wherein the video signal is divided into a plurality of blocks each of which has N×M pixel values with N and M being positive integers, the method comprising the steps of: detecting a lost block containing errors; providing pixel values of blocks neighboring the lost block in the video signal; calculating an edge gradient at each of neighboring pixels included in a predetermined range of pixels surrounding the lost block based on the pixel values of the neighboring blocks and computing a magnitude and an angle of each of the edge gradients; generating a spatially interpolated block based on the edge gradients and the pixel values of the neighboring blocks; and providing the spatially interpolated block as a substitution block for compensating the lost block. 
     In accordance with another aspect of the present invention, there is provided an apparatus, for use in a video signal decoding system, for concealing errors in a video signal transmitted in a compressed form, wherein the video signal is divided into a plurality of blocks each of which has N×M pixel values with N and M being positive integers, which comprises: a lost block detector for detecting a lost block containing errors; a neighboring block determination circuit for providing pixel values of blocks neighboring the lost block in the video signal; a gradient calculator for computing an edge gradient at each of neighboring pixels included in a predetermined range of pixels surrounding the lost block based on the pixel values of the neighboring blocks and calculating a magnitude and an angle of each of the edge gradients; a direction determination circuit for determining a pixel interpolation direction based on the edge gradients of neighboring pixels; a block generator for producing a spatially interpolated block based on the pixel values of the neighboring blocks and the pixel interpolation direction; and a contribution circuit for providing the spatially interpolated block as a substitution block for compensating the lost block. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a schematic block diagram of an error concealment system; 
     FIGS. 2A and 2B represent an error concealment circuit 300 in FIG. 1 in accordance with the preferred embodiments of the present invention, respectively; 
     FIG. 3 denotes a detailed block diagram of a direction classification unit 210 shown in FIGS. 2A or 2B; and 
     FIG. 4 offers a schematic representation of a lost block and its neighboring blocks in a video signal. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a video signal is decoded at a decoding circuit 400 through the use of conventional decoding techniques such as a variable length decoding, an inverse quantization and an inverse DCT, the video signal being divided into a multiplicity of blocks, e.g., a block of N×M pixels with N and M being positive integers. And, the decoded video signal is stored at a frame memory 300. 
     The video signal is also fed to a block detector 100 and the block detector 100 first checks a lost block in the video signal to thereby provide lost block information including position data representing the position of the lost block in the video signal onto a line L20, the lost block having lost or damaged pixel values; and provides neighboring block information retrieved through a line L30 from the decoded video signal stored at the frame memory 300 based on the lost block information to an error concealment circuit 200 via a line L10, the neighboring block information including pixel values for each of neighboring blocks for the lost block. 
     The error concealment circuit 200 generates a substitution block for compensating the lost block within the video signal, the substitution block being produced based on the neighboring block information on the line L10 from the block detector 100. 
     In FIGS. 2A and 2B, there are represented detailed block diagrams of the error concealment circuit 200 shown in FIG. 1 in accordance with the preferred embodiments of the present invention. And, a direction classification unit 210 commonly included in FIGS. 2A and 2B is described in FIG. 3 in more detail. The structure of the error concealment circuit 200 and its operation will be understood with reference to FIGS. 2A, 2B and 3. 
     First of all, the direction classification unit 210 and a comparison unit 220 which are commonly used in the preferred embodiments given in conjunction with FIGS. 2A and 2B will be introduced with reference to the drawings. 
     The neighboring block information on the line L10 from the block detector 100 is coupled to a direction classification unit 210 having a gradient calculation sector 212, a direction quantization sector 214 and a maximum gradient determination sector 216 as shown in FIG. 3. Subsequently, the gradient calculation sector 212 computes gradients at neighboring pixels included in a predetermined range of pixels, e.g., three or four rows and columns of pixels, surrounding the lost block, by using gradient measures in the spatial domain based on the neighboring block information. The local edge gradients Gx and Gy at a pixel location x(i,j) of the neighboring pixels are first determined as follows: 
     
         Gx=x.sub.i-1j+1 -x.sub.i-1j-1 +2x.sub.ij+1 -2x.sub.ij-1 +x.sub.i+1j+1 -x.sub.i+1j-1                                             EQ. 2 
    
     
         Gy=x.sub.i-1j-1 -x.sub.i+1j-1 +2x.sub.i-1j -2x.sub.i+1j +x.sub.i-1j+1 -x.sub.i+1j+1                                             EQ. 3 
    
     wherein x ij  represents a pixel value at the pixel location x(i,j), i and j being positive integers representing the pixel position in the block along the horizontal and the vertical direction, respectively. This is equivalent to applying the following 3×3 Sobel operators to the neighboring pixels. ##EQU2## Based on the local edge gradients Gx and Gy computed according to the EQ. 2 and EQ. 3, the magnitude, Gmag, and angle, Gpha, of the gradient at the pixel location x(i,j) are given as follows: ##EQU3## The above gradient measures are calculated for each of the neighboring pixels surrounding the lost block and the results, i.e., the magnitude and angle of the gradient for each of the neighboring pixels are provided to the direction quantization sector 214. 
     At the direction quantization sector 214, the angles coupled thereto are rounded to, e.g., the nearest multiple of 45 degrees and, thereafter, each of them corresponds to one of four directional categories D1 to D4 which represent, e.g., horizontal, vertical, diagonally ascending and diagonally descending directions, respectively. Each gradient along one of the four directional categories D1 to D4 is fed to the maximum gradient determination sector 216. 
     At maximum gradient determination sector 216, if a line drawn from the pixel location x(i,j) along the direction determined by the angle of the gradient thereat passes through the lost block, then that particular pixel location x(i,j) is classified as a voting pixel. The directional categories D1 to D4 at the voting pixel locations are mutually exclusively accumulated by summing the magnitudes of the associated directional gradients. That is, the magnitudes of the gradients at the voting pixel locations classified to a directional category D1 are accumulated to thereby produce a first sum, the magnitudes of the gradients of the voting pixel locations contained in a directional category D2 are accumulated to thereby provide a second sum and so on. By comparing the sums for the directional categories D1 to D4 with one another, the sector 216 selects a directional category exhibiting a largest accumulated sum as a pixel interpolation direction. The selected directional category and the associated sum are supplied to a second block compensation block 240A or 240B via a line L220 and the comparison unit 220 through a line L210, respectively. 
     Referring back to FIGS. 2A or 2B, the sum corresponding to the selected directional category is compared with a predetermined threshold value at the comparison unit 220 in order to provide selection signals to a selection unit 250A or 250B. 
     Meantime, in accordance with the preferred embodiment of the present invention, the neighboring block information from the block detector 100 is also fed to a first and the second block compensation units 230A and 240A as shown in FIG. 2A. 
     At the first compensation unit 230A, a first compensated block is produced by using the before-mentioned conventional error concealment technique replacing each pixel value of the lost block with the weighted mean value of pixel values of the neighboring blocks for the lost block. 
     The second block compensation unit 240A makes a second compensated block through the use of a directional interpolation based on pixel values of the neighboring blocks along the direction corresponding to the selected directional category, the direction interpolation generating pixel values by performing one dimensional interpolation along a particular direction. For example, if the directional category D1 representing the horizontal direction is chosen as the pixel interpolation direction, the second block compensation unit 240A first calculates an average value of all pixel values of each of the blocks 4 and 5; computes a mean value of the average values for the blocks 4 and 5; and constructs the second compensated block, each pixel value of the second compensated block being the mean value. Likewise, the respective neighboring blocks 2 and 7, 3 and 6, and 1 and 8 along the directions corresponding to the rest directional categories D2, D3 and D4, respectively can be also used for producing the second compensated block in the same manner as this process. The first and the second compensated blocks produced as above are supplied to the selection unit 250A. 
     The selection unit 250A chooses either the first compensated block from the first block compensation unit 230A or the second compensated block from the second block compensation unit 240A in response to the selection signals outputted from the comparison unit 220. That is, if the sum corresponding to the selected directional category is smaller than the predetermined threshold value, the first compensated block is provided to the frame memory 300 via a line L40 as the substitution block, and, if otherwise, the second compensated block is supplied to the memory 300. 
     On the other hand, in accordance to another preferred embodiment of the present invention with reference to FIG. 2B, the neighboring block information coupled to the error concealment circuit 200 through the line L10 is converted through the use of the discrete cosine transform at a DCT unit 260. The transformed neighboring block information is fed to a first and the second block compensation units 230B and 240B. 
     At the first block compensation unit 230B, a first compensated DCT block is produced based on the transformed neighboring block information. Each DCT coefficient SC ij  of the first compensated DCT block is calculated by: ##EQU4## wherein W k  represents a predetermined weight factor for a kth transformed neighboring block for the lost block; Bk ij  denotes an ijth DCT coefficient of the kth transformed neighboring block; k varies from 1 to 8; i ranges from 1 to N; and j ranges from 1 to M. 
     In the preferred embodiment of the present invention, when the DCT coefficient SC ij  is computed according to the EQ. 6, all the weight factors for the vertically and horizontally neighboring blocks are identical and, likewise, all the weight factors for the diagonally neighboring blocks are also identical. That is, referring to FIG. 4, the weight factors for the blocks 2, 4, 5 and 7 are same and those for the blocks 1, 3, 6 and 8 are identical. 
     In the meantime, the second block compensation unit 240B generates a second compensated DCT block based on the transformed neighboring block information from the DCT unit 260 and the selected directional category from the maximum gradient determination sector 216 in the direction classification unit 210. That is to say, each DCT coefficient of the second compensated DCT block is determined by averaging two DCT coefficients selected from the two neighboring blocks along the direction corresponding to the selected directional category, the two DCT coefficients being of a same frequency of the two neighboring blocks. The neighboring blocks along the directions corresponding to the directional categories D1, D2, D3 and D4 are also determined as before-mentioned. Therefore, if the directional category D1 is chosen as the pixel interpolation direction, the second block compensation unit 240B also produces the second compensated DCT block by using DCT coefficients of the blocks 4 and 5 in FIG. 4. For the respective blocks related to the rest directional categories, this process can be applied in the same manner. The first and the second compensated DCT blocks from the first and the second block compensation units 230B and 240B, respectively, are supplied to the selection unit 250B. 
     At the selection unit 250B, likewise in the former preferred embodiment of the present invention, one of the first and the second compensated DCT blocks is selected in response to the selection signals outputted from the comparison unit 220 and it is fed to an IDCT unit 270. That is to say, as the result of the comparison process at the comparison unit 220, if the sum corresponding to the selected directional category is smaller than the predetermined threshold value, the first compensated DCT block is provided to the IDCT unit 270, and, if otherwise, the second compensated DCT block is supplied to the IDCT unit 270. 
     The IDCT unit 270 inversely transforms the selected block from the selection unit 250B by using an inverse discrete cosine transform and provides it to the frame memory 300 via the line L40 as the substitution block. 
     The lost block in the video signal is compensated by the substitution block on the line L40 supplied from the selection unit 250A or the IDCT unit 270 at the frame memory 300 based on the lost block information from the block detector 100. 
     While the present invention has been shown and described in connection with the preferred embodiments only, it will be readily apparent to those of ordinary skill in the art that many changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.