Abstract:
The method and apparatus capable of adaptively coding intra-video signals by separately predicting intra DC and AC coefficients is provided. The method comprises the steps of finding a DC reference block among the previously coded blocks based on a correlation of the DC coefficients of the previously coded blocks; setting a DC coefficient of the DC reference block as a DC reference coefficient; determining one of the previously coded blocks as an AC reference block based on a correlation of selected AC coefficients of the previously coded blocks; generating AC reference coefficients based on AC coefficients of the AC reference block; and encoding coefficients of the current block based on the DC and the AC reference coefficients. The apparatus includes a transform block, a quantization block, a memory, a prediction block, and an entropy coding block.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for adaptively coding video signals; and, more particularly, to a method and apparatus capable of adaptively the coding video signals by way of predicting intra DC and AC coefficients, separately. 
     DESCRIPTION OF THE PRIOR ART 
     In digitally televised systems such as video-telephone, teleconference and high definition television systems, a large amount of digital data is needed to define each video frame signal since the video frame signal comprises a sequence of digital data referred to as pixel values. Since, however, an available frequency bandwidth of a conventional transmission channel is limited, in order to transmit the large amount of digital data therethrough, it is necessary to compress or reduce the volume of the data through the use of various data compression techniques, especially in the case of such low bit-rate video signal encoders as video-telephone and teleconference systems. 
     Among various video compression techniques, the so-called hybrid coding technique, which combines temporal and spatial compression techniques together with a statistical coding technique, is known to be most effective. Most hybrid coding techniques employ a motion compensated DPCM (Differential Pulse Coded Modulation), two-dimensional transform, e.g., DCT (Discrete Cosine Transform), quantization of transform coefficients, and an entropy coding, e.g., VLC (Variable Length Coding). 
     In the conventional MPEG-4 (Moving Picture Expert Group-4) coding scheme, an intra-mode input video signal is divided into blocks of M×N, e.g., 8×8, pixels and each block is transformed to a set of quantized coefficients, i.e., a DC and a plurality of AC quantized coefficients, through the use of two-dimensional transform, e.g., DCT and quantization thereof, M and N being positive integers. A set of quantized coefficients of a block is then DPCM coded based on a set of quantized coefficients of one of previously coded blocks in the frame. 
     Referring to FIG. 1, there is shown a portion of blocks of an intra-frame. In MPEG-4, a reference block RB of a current block is determined among a left and an upper blocks of the current block based on DC coefficients of the left and the upper and a left-upper blocks of the current block. If a current block is  108 , a reference block RB thereof is determined as: 
     
       
         If | D   3 − D   1 |&lt;|&lt; D   1 − D   2 |, then 
       
     
     RB is the upper block  104 , 
     else 
     
       
         RB is the left block  106   (Eq. 1) 
       
     
     wherein D 1 , D 2  and D 3  are DC quantized coefficients of the blocks  102 ,  104  and  106 , respectively. 
     In other words, if the horizontal correlation between the DC quantized coefficients of the previously coded blocks is greater than the vertical correlation, i.e., |D 1 −D 2 |&lt;|D 3 −D 1 |, the previously coded left block  106  is selected as the RB for the current block  108 ; and if otherwise, the previously coded upper block  104  is selected. Thereafter, a DPCM DC coefficient for the current block  108  is generated by calculating the difference between the DC quantized coefficient of the RB and that of the current block  108 . 
     According to the conventional MPEG-4 coding method, the RB determined based on the DC quantized coefficients of the previously coded blocks is also used in predicting the AC quantized coefficients of the current block without considering the correlations between the AC quantized coefficients of the previously coded blocks, which may result in degraded coding efficiency. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a method and apparatus capable of adaptively coding intra-video signals by separately predicting intra DC and AC coefficients. 
     In accordance with the present invention, there is provided a method for encoding a current block of a video signal based on P previously coded blocks, each of the current block and the previously coded blocks having M×N coefficients of a DC and a plurality of AC coefficients and P being an integer greater than 2, comprising the steps of: 
     (a) finding a DC reference block among the previously coded blocks based on a correlation of the DC coefficients of the previously coded blocks; 
     (b) setting a DC coefficient of the DC reference block as a DC reference coefficient; 
     (c) determining one of the previously coded blocks as an AC reference block based on a correlation of selected AC coefficients of the previously coded blocks; 
     (d) generating AC reference coefficients based on AC coefficients of the AC reference block; and 
     (e) encoding coefficients of the current block based on the DC and the AC reference coefficients. 
     In accordance with the present invention, there is provided an apparatus for encoding a video signal, the video signal is divided into a plurality of blocks of M×N pixels, comprising: a transform block for transforming the video signal on a block-by-block basis to thereby a set of M×N transform coefficients for each block, wherein the transform coefficients includes a DC and a multiplicity of AC transform coefficients; a quantization block for quantizing the set of transform coefficients for each block to provide a set of M×N quantized DC and AC coefficients; a memory for storing the set of quantized coefficients for each block; a prediction block for determining a DC reference block and an AC reference block of a current block based on quantized DC coefficients and quantized AC coefficients of previously coded blocks to thereby provide a set of DPCM (differential Pulse Coded Modulation) DC and AC coefficients of the current block based on quantized DC and AC coefficients of the DC and the AC reference blocks; and an entropy coding block for entropy coding the DPCM DC and the DPCM AC coefficients. 
    
    
     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 portion of blocks of a frame; 
     FIG. 2 represents a schematic block diagram of an apparatus capable of adaptively coding video signals in accordance with the present invention; 
     FIG. 3 illustrates a schematic block diagram of a prediction block shown in FIG. 2; 
     FIG. 4 depicts a detailed block diagram of a DC coefficients prediction circuit shown in FIG. 3; 
     FIG. 5 presents a detailed block diagram of an AC coefficients prediction circuit shown in FIG. 3; 
     FIG. 6 exemplifies a portion of a frame in the transform domain; and 
     FIGS. 7A and 7B are a flow chart of the procedure for adaptively coding the video signals in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2, there is shown a schematic block diagram of an apparatus capable of adaptively coding intra-video signals in accordance with the present invention. 
     The apparatus of the present invention comprises a transform block  202 , a quantization block  204 , a memory block  206 , a prediction block  208  and an entropy coding block  210 . A intra-frame signal divided into blocks of M×N, e.g., 8×8, pixels, M and N being positive integers, is inputted to the transform block  202  on a block basis. The transform block  202  transforms each input block into a set of transform coefficients including one DC and, e.g., 63, AC coefficients by using, e.g., DCT method. The set of DC and AC transform coefficients constitutes a coefficient block of M×N, e.g., 8×8, coefficients, wherein the first coefficient positioned at the upper-left corner of the coefficient block corresponds to the DC transform coefficient and the others are AC transform coefficients whose horizontal and vertical frequencies increase along the zigzag scanning order. Thereafter, the transform block  202  provides the quantization block  204  with the generated set of transform coefficients. 
     The quantization block  204  quantizes the set of transform coefficients to create a set of quantized coefficients for each block. The set of quantized coefficients are transmitted to the prediction block  208  and the memory block  206  via a line L 21 . The memory  206  stores the set of quantized coefficients fed thereto. 
     The prediction block  208  performs a predictive coding on a current block based on the sets of quantized coefficients of previously coded blocks and generates a set of DPCM coefficients for the current block. Thereafter, the entropy coding block  210  codes the set of DPCM coefficients through the use of, e.g., VLC technique, and provides the coded DPCM coefficients to a transmitter (not shown) for the transmission thereof. 
     Now, the detailed operation of the prediction block  208  will be described with reference to FIGS. 3 to  6 . Referring to FIG. 3, there is shown a schematic block diagram of the prediction block  208  shown in FIG.  2 . As shown in FIG. 3, the prediction block  208  includes a position detection circuit  302 , a coefficient detection circuit  304 , a switch  306  having terminals TA, TB and TC, a DC coefficient prediction circuit  308 , an AC coefficient prediction circuit  310  and a selector  312 . 
     The position detection circuit  302  identifies the position of a current block being processed in response to position data of the current block fed thereto along with the set of quantized coefficients from the quantization block  204  via the line L 21 . The position data may be provided from a system controller (not shown), separately from the set of quantized coefficients. The position detection circuit  302  provides the detected position of the current block to the DC and the AC coefficient prediction circuits  308  and  310  through a line L 31 . The coefficient detection circuit  304  detects whether each coefficient in the set fed thereto via the line L 21  is a DC or an AC quantized coefficient and generates a corresponding signal for controlling the switch  306  and the selector  312  via a line L 34 . 
     If the detected coefficient is a DC coefficient, the coefficient detection circuit  304  generates a first selection signal that controls the switch  306  to connect the terminal TA to the terminal TB and the selector  312  to select an output on a line L 35 ; and if otherwise, it generates a second selection signal that controls the switch  306  to connect the terminal TA to the terminal TC and the selector  312  to select an output on a line L 36 . 
     The DC coefficient prediction circuit  308  generates a DPCM DC coefficient of the current block on the line L 35  and the AC coefficient prediction circuit  310  generates DPCM AC coefficients of the current block on the line L 35 . 
     Thereafter, the selector  312  selects the output of the DC coefficient prediction circuit  308  or that of the AC coefficient prediction circuit  310  according to the selection signals fed from the coefficient detection circuit  304  and provides the selected output to the entropy coding block  210  shown in FIG. 2 via the line L 24 . 
     Referring to FIG. 4, there is shown a detailed block diagram of the DC coefficient prediction circuit  308  shown in FIG.  3 . As shown in FIG. 4, the DC coefficient prediction circuit  308  has a reference DC determination circuit  402  and a DPCM DC coefficient determination circuit  404 . The reference DC determination circuit  402  receives the position information fed from the position detection circuit  302  shown in FIG. 3 via the line L 31  and determines to which one of the blocks  602 - 608  shown in FIG. 6 the current block belongs, wherein the position information represents the position of the current block in the frame. 
     If the current block is of the kind  602 , the first block in the frame having no previously coded blocks, the reference DC determination circuit  402  transmits zero as a reference DC value to the DPCM DC coefficient determination circuit  404  via a line L 41 . If the position information represents that the current block of the type, the block  604  or  606 , which is only one adjacent previously coded block, i.e.,  602 , the reference DC determination circuit  402  determines the block  602  as the reference block RB thereof to thereby provide, as the reference DC value, the quantized DC coefficient D 1  of the block  602  from the memory  206  to the DPCM DC coefficient determination circuit  404 . In case that the current block is one of the blocks, each having previously coded upper and left blocks, e.g., the block  608 , the reference DC determination circuit  402  retrieves the quantized DC coefficients D 1 , D 2  and D 3  of the upper-left, the upper and the left blocks  602 ,  604  and  606  of the current block  608 ; determines one of the upper and the left blocks  604  and  606  as a RB of the current block  608  by using Eq. 1 described above; and transmits the quantized DC coefficient of the RB as the reference DC value to the DPCM DC coefficient determination circuit  404  via the line L 41 . 
     Thereafter, the DPCM DC coefficient determination circuit  404  calculates the difference value between the reference DC value and the quantized DC coefficient of the current block fed from the switch  306  via the line L 32 ; and provides the selector  312  with same as a DPCM DC coefficient of the current block. 
     Referring to FIG. 5, there is shown a detailed block diagram of the AC coefficient prediction circuit  310  shown in FIG.  3 . As shown in FIG. 5, the AC coefficient prediction circuit  310  has a reference block determination circuit  502  and a DPCM AC coefficient determination circuit  504 . The reference block determination circuit  502  receives the position information of the current block fed from the position detection circuit  302  shown in FIG. 3 via the line L 31 . Thereafter, the reference block determination circuit  502  examines to which one of the block types  602 ,  604 ,  606  or  608  the current block belongs. 
     If the current block is of the block type  602 , since there is no previously coded blocks, the reference block determination circuit  502  set zeros as reference AC coefficients for the current block and transmits same to the DPCM AC coefficient determination circuits  504  via a line L 51 . If the current block belongs to one of the block types  604  or  606  which has only one previously coded adjacent block, i.e.,  602 , the reference block determination circuit  502  determines the unique previously coded adjacent block  602  as the RB for the current block. 
     In case that the current block is of the block type  608  having the upper and the left previously coded adjacent blocks  604  and  606 , the reference block determination circuit  502  retrieves from the memory  206  via the line L 22 , Ah 1  and Ah 2 , the first horizontal AC quantized coefficients of the upper-left and the upper blocks  602  and  604 , and Av 1  and Av 3 , the first vertical AC quantized coefficients of the upper-left and the left blocks  602  and  606 ; and determines a RB for the current block  608  by computing a horizontal and a vertical correlations as: 
      ( HC ) −1   =|Ah   1 − Ah   2 | 
     
       
         ( VC ) −1   =Av   1 − Av   3 |  (Eq. 2) 
       
     
     wherein the HC and the VC represent the horizontal and the vertical correlations for the AC quantized coefficients of the previously coded blocks, respectively. 
     If the horizontal correlation HC is greater than the vertical correlation VC, i.e., |Ah 1 −Ah 2 |&lt;|Av 1 −Av 3 |, the reference block determination circuit  502  determines the previously coded block  606  as the RB for the current block  608 ; and if otherwise, it determines the previously coded block  604  as the RB. Thereafter, the reference block determination circuit  502  provides via the line L 51  the DPCM AC coefficients determination circuit  504  with a set of reference AC coefficients. In accordance with the present invention, the AC coefficients in the first row of the set of reference AC coefficients are identical to those of the RB and the remaining reference AC coefficients are all set to zeros if the RB is the upper block of the current block. Similarly, the AC coefficients in the first column of the set of reference AC coefficients are identical to those of the RB and the remaining reference AC coefficients are set to zeros in case the left block is determined as the RB. 
     The DPCM AC coefficients determination circuit  504  calculates DPCM AC coefficients by deriving differences between AC quantized coefficients of the current block on the line L 33  and their corresponding reference AC coefficients on the line L 51 . The DPCM AC coefficients for the current block are provided to the selector  312  via the line L 36 . 
     Referring to FIGS. 7A and 7B, there is shown a procedure describing the adaptive video signal coding scheme in accordance with the present invention. At step  702  shown in FIG. 7A, position information and a set of quantized coefficients having a DC and a plurality of AC coefficients of a current block are inputted. Thereafter, the process proceeds to step  704  to check whether each coefficients in the set is a DC or an AC quantized coefficient. If the check result is positive at step  704 , the process proceeds to step  720  shown in FIG.  7 B. 
     At step  720 , it is checked whether the current block belongs to which one of the blocks  602 - 608  shown in FIG. 6 based on the received position information thereof. If the check result is positive at step  720 , the process proceeds to step  730 ; and if otherwise, it proceeds to step  722 . At step  722 , it is again checked whether the current block belongs to the block  608 . If the check result is negative at step  722 , the process proceeds to step  728 ; and if otherwise, it proceeds to step  724  to retrieve the quantized DC coefficients D 1 , D 2  and D 3  of the upper-left, the upper and the left blocks  602 ,  604  and  606  of the current block  608 . Thereafter, at step  726 , the process calculates a correlation between the quantized DC coefficients D 1 , D 2  and D 3  of the blocks  602 ,  604  and  606  and proceeds to step  728 . 
     At step  728 , the process determines one of the upper and the left blocks  604  and  606  as a reference block RB of the current block by using Eq. 1 described above to thereby select the quantized DC coefficient of the RB as the reference DC value of the current block. Thereafter, at step  730 , the process calculates the difference value between the reference DC value and the quantized DC coefficients of the current block to thereby generate a DPCM DC coefficient of the current block. At step  732 , the DPCM DC coefficient is transmitted and the process is returns to step  704  shown in FIG.  7 A. 
     In case that the check result is negative at step  704 , the process proceeds to step  706 . At step  706 , it is checked whether the current block belongs to which one of the blocks  602 - 608  shown in FIG. 6 based on the received position information thereof. If the check result is positive at step  706 , the process proceeds to step  716 ; and if otherwise, it proceeds to step  708 . At step  708 , it is again checked whether the current block belongs to the block  608  having the upper and the left previously coded adjacent blocks  604  and  606 . If the check result is negative at step  708 , the process proceeds to step  714 ; and if otherwise, it proceeds to step  710  to retrieve the first horizontal AC quantized coefficients of the upper-left and the upper blocks  602  and  604 , and the first vertical AC quantized coefficients of the upper-left and the  602  and  606 . Thereafter, at step  712 , the process calculates correlations between the first horizontal AC quantized coefficients and the first vertical AC quantized coefficients by using Eq. 2 described above and proceeds to step  714 . 
     At step  714 , the process determines one of the upper and the left blocks  604  and  606  as a reference block RB of the current block based on the correlation result. If the RB is the left block of the current block, the AC coefficients in the first row of the set of reference AC coefficients are identical to those of the RB and the remaining reference AC coefficients are all set to zeros. If otherwise, i.e., the upper block is determined as the RB, the AC coefficients in the first column of the set of reference AC coefficients are identical to those of the RB and the remaining reference AC coefficients are set to zeros. 
     Thereafter, at step  716 , the process derives the differences between AC quantized coefficients of the current block and their corresponding reference AC coefficients of the RB to thereby generate DPCM AC coefficients for the current block. At step  718 , the DPCM AC coefficients are transmitted and the process is terminated. 
     While the present invention has been described with respect to certain preferred embodiments only, other modifications and variations may be made without departing from the spirit and scope of the present invention as set forth in the following claims.