Abstract:
There is provided a coding apparatus in which an input image signal is divided into blocks, the block data is compression coded by using an orthogonal transformation, and (n) sync blocks are produced from the (m) compression coded block data, wherein each sync block has a first area in which important data in the orthogonal transformation coded block data of a specific number of blocks is arranged and a second area in which non-important data in the orthogonal transformaton coded block data of an unspecific number of blocks is arranged. There is also provided a decoding apparatus in which an image signal is divided into blocks, the block formed block data is compression coded, and the compression coded block data is decoded on an (m) unit basis (m is a natural number), wherein the apparatus comprises a first decoding unit for decoding the coding data of the inputted (m) blocks in accordance with the inputting order on a block unit basis, a second decoding unit for decoding the coding data of the inputted (m) blocks in accordance with the order opposite to the inputting order on a block unit basis, and an output unit for integrating the data decoded by the first and second decoding units and for outputting decoding data of the (m) blocks.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a coding/decoding apparatus and, more particularly, to an apparatus for variable length coding image data after it was orthogonal transformed on a block unit basis comprising a predetermined number of pixels and to a decoding apparatus for decoding the coding data which was coded by the coding apparatus.  
           [0003]    2. Related Background Art  
           [0004]    Generally, since an information amount of an image signal is very large, in case of recording the image signal by a digital recording and reproducing apparatus such as a digital VTR or the like, the information is reduced by a high efficient compression coding and the reduced image signal is recorded.  
           [0005]    An orthogonal transformation coding using a DCT (Discrete Cosine Transform) is known as an example of the high efficient compression coding techniques.  
           [0006]    According to the orthogonal transformation coding, an input image signal is separated into blocks and each of the frequency components obtained by frequency dividing the image signal on such a block unit basis is coded. By using a method such that a data amount which is allocated is reduced or the like with respect to the high frequency component in which an influence by the deterioration on the eyesight characteristics is small, the information amount of the input image signal can be reduced.  
           [0007]    The case where the signal which was compression coded by the above method is recorded and reproduced by the digital VTR will now be described.  
           [0008]    When the coded data is recorded onto a tape, it is separated into sync blocks of a predetermined data amount unit as a minimum synchronous unit in the reproducing mode and the data is recorded.  
           [0009]    [0009]FIG. 1 shows a construction of a sync block.  
           [0010]    In FIG. 1, SYNC  1  denotes a synchronous pattern; and ID  2  indicates identification information comprising address information or the like to indicate the position on the screen of the data recorded; and DATA  3  denotes image data which was compression coded.  
           [0011]    [0011]FIG. 2 shows an arrangement on the tape of the sync blocks recorded by a conventional VTR.  
           [0012]    Reference numerals  2   a ,  2   b , and  2   c  in FIG. 2 denote tracks. The sync blocks are arranged in those tracks.  
           [0013]    Each track is a locus of a recording head in the recording mode. In the normal reproducing mode, a reproducing head scans on the same locus as the locus which was scanned by the recording head in the recording mode, so that the image data in each sync block can be reproduced.  
           [0014]    A method of reproducing the recording data in a special reproducing mode will now be described.  
           [0015]    In FIG. 2, reference numeral  10  denotes a locus of the reproducing head in a high speed searching mode as one of special reproducing modes.  
           [0016]    Since a tape travelling speed in the high speed searching mode is faster than that in the normal reproducing mode, the locus  10  of the reproducing head is a locus such that the reproducing head scans a plurality of tracks.  
           [0017]    Therefore, the data of only the sync blocks shown by hatched regions in FIG. 2 can be extracted from the tape by the reproducing head.  
           [0018]    In the above conventional recording/reproducing method of image data mentioned above, however, a sync block in which only a part can be scanned exists in the reproducing head locus  10 .  
           [0019]    For example, there is a sync block like a sync block  11  in FIG. 2 such that the whole portion cannot be scanned although only the front portion of the sync block can be scanned. Therefore, since the whole DATA arranged in the rear portion in the sync block  11  cannot be fetched, there is a problem such that the data which is necessary for the expanding process cannot be prepared and the whole image data existing in the sync block  11  cannot be reproduced.  
           [0020]    In the case where a transmission error exceeding an error correction ability occurs on a transmission path, there is also a problem such that the data after the position at which the transmission error occurred cannot be reproduced and the data of a fixed length unit disappears in the worst case.  
         SUMMARY OF THE INVENTION  
         [0021]    Under such circumstances, it is an object of the invention to provide a coding/decoding apparatus which can reproduce a wider region on a screen than that of a conventional apparatus in case of the occurrence of a transmission error or in a special reproducing mode and can minimize the deterioration of a picture quality.  
           [0022]    According to one preferred embodiment, the above object is accomplished by a coding apparatus comprising: block forming means for dividing an input image signal into blocks; coding means for compression coding the block data obtained by dividing by the block forming means by using an orthogonal transformation; and generating means for generating (n) sync blocks from the coding data of the (m) blocks, wherein each sync block has a first area in which important data in the orthogonal transformation coded block data of the number as many as a special number of blocks is arranged and a second area in which non-important data in the orthogonal transformation coded block data of the number of an unspecific number of blocks is arranged.  
           [0023]    According to another preferred embodiment, there is provided a decoding apparatus in which an image signal is divided into blocks and the block-formed block data is compression coded and the compression coded block data is decoded on an (m) unit basis (m is a natural number), wherein the decoding apparatus comprises: first decoding means for decoding the coding data of the (m) inputted blocks on a block unit basis in accordance with the inputting order; second decoding means for decoding the coding data of the (m) inputted blocks on a block unit basis in accordance with the reversed inputting order; and output means for integrating the data decoded by the first and second decoding means and for outputting the decoding data of the (m) blocks.  
           [0024]    The above and other object and features of the invention will become apparent from the following detailed description taken in conjunction with the appended claims with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is an explanatory diagram showing a construction of sync blocks;  
         [0026]    [0026]FIG. 2 is an explanatory diagram showing an arrangement of sync blocks on a tape;  
         [0027]    [0027]FIG. 3 is a block diagram of a digital recording apparatus of an embodiment according to the invention;  
         [0028]    [0028]FIG. 4 is a diagram for explaining DCT transformed block data;  
         [0029]    [0029]FIG. 5 is a diagram showing a construction of sync blocks in the embodiment;  
         [0030]    [0030]FIG. 6 is a block diagram of a digital reproducing apparatus of the embodiment according to the invention; and  
         [0031]    [0031]FIG. 7 is a diagram showing schematically an internal structure of a sync block memory  204 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    A digital recording/reproducing apparatus according to an embodiment of the invention will now be described hereinbelow.  
         [0033]    [0033]FIG. 3 is a block diagram of a digital recording apparatus of the embodiment.  
         [0034]    An image signal inputted from an input terminal  101  is converted into a digital image signal by an A/D converting circuit  102 .  
         [0035]    The converted digital image signal is divided into blocks on a DCT processing unit basis by a block forming circuit  103 . In the embodiment, a 2-dimensional block (hereinbelow, referred to as a DCT block) of total 64 pixels (8 pixels in the lateral direction×8 pixels in the vertical direction) is used.  
         [0036]    The DCT block formed image signal is converted into the DCT component by a DCT circuit  104 . The block data which was converted into the DCT component is supplied to a delay circuit  105  and a code amount control circuit  106 .  
         [0037]    The DCT converted block data will now be described with reference to FIG. 4. In the embodiment, total 64 pixels (8 pixels in each of the vertical and lateral directions) are set to one block. FIG. 4 shows the correspondence relation of 64 DCT coefficients obtained by DCT arithmetic operating the block data. The left upper DC component indicates the mean value of 64 pixels in the block and is called a direct current (DC) coefficient. The remaining 63 DC components are called alternating current (AC) coefficients and indicate magnitudes of AC component electric powers in the block.  
         [0038]    Returning to the description of FIG. 3, the code amount control circuit  106  controls quantization steps of a quantization circuit  107  in a manner such that a data amount when a predetermined number of DCT blocks are collected and coded is set to an almost constant value. In the embodiment, 30 DCT blocks are set to a fixed length unit for simplicity of explanation.  
         [0039]    The delay circuit  105  delays 30 DCT block data by a time which is expended to execute the processes in the code amount control circuit  106 .  
         [0040]    Each of the DCT block data outputted from the delay circuit  105  is quantized by the quantization circuit  107 .  
         [0041]    The quantized DCT block data is zigzag scanned as shown in FIG. 4 and is converted into a one-dimensional data train from a low frequency component of the DCT transformation coefficients to a high frequency component and is coded every DCT block by the variable length coding circuit  108 .  
         [0042]    The variable length coded data is supplied to a sync block generation circuit  109 , by which sync blocks are generated.  
         [0043]    A construction of the sync block in the embodiment will now be described.  
         [0044]    [0044]FIG. 5 is a diagram showing a construction of the sync block in the embodiment.  
         [0045]    In FIG. 5, each sync block has synchronous information (SYNC)  50 , identification information (ID)  51 , and coding data  52  and  53  from the head.  
         [0046]    The coding data is divided into two regions  52  and  53  and inserted into the sync block.  
         [0047]    The region  52  is used for the coding data of the low frequency component of the DCT transformation coefficient. The region  53  is used for the coding data of the high frequency component of the DCT transformation coefficient.  
         [0048]    In the embodiment, the data of a predetermined code amount is sequentially written into the region  52  of each sync block every block in accordance with the order from the coding data of the low frequency components of the DCT transformation coefficients of six blocks. The coding data of the high frequency components of the DCT transformation coefficients which could not be written into the region  52  is written into the regions  53  of five sync blocks after an ID code (EOB) of each DCT block unit was added to such coding data. Further specifically speaking, L n  and H n  shown in FIG. 5 are multiplied and the coding data of one DCT block is enclosed (n is an integer of 1 to 30). Each L n  has a predetermined code amount and a code amount of each H n  differs.  
         [0049]    The EOB is written in a hatched region  53   a  of shown in FIG. 5. A hatched region  53   b  shown in the region  53  of the sync block #5 in FIG. 5 denotes a space region which is not yet used.  
         [0050]    In the embodiment, 30 blocks are set to one unit and quantizing characteristics (for instance, quantization steps) are controlled so that the coding data of 30 DCT blocks is enclosed in five such blocks.  
         [0051]    Returning to the explanation of FIG. 3, an error detection correction code is added to each sync block data with the above construction by an error detection correction coding circuit (ECC)  110 .  
         [0052]    The data which is outputted from the ECC circuit  110  is subjected to a predetermined modulating process by a modulation circuit  111  and is recorded onto a magnetic tape  113  by a recording head  112 .  
         [0053]    [0053]FIG. 6 is a block diagram of a digital reproducing apparatus of the embodiment. In FIG. 6, the portions corresponding to those shown in FIG. 3 are designated by the same reference numerals and their descriptions are omitted here.  
         [0054]    In FIG. 6, the data recorded on the magnetic tape  113  is read out by the reproducing head  112 . The read-out data is subjected to a predetermined demodulating process by a demodulation circuit  202  and is supplied to an error correction circuit  203 .  
         [0055]    The error correction circuit  203  corrects erroneous data by the error detection correction code which was added in the recording mode. With respect to the sync block in which uncorrectable data occurs, an error flag is outputted to a sync block memory  204 .  
         [0056]    The sync block memory  204  stores the error corrected coding data on a sync block unit basis.  
         [0057]    The error correction circuit  203  outputs an error flag to the sync block in which the error uncorrectable data exists.  
         [0058]    That is, the error correction circuit  203  generates “1” in the case where the sync block has error uncorrectable data and generates “0” when no error uncorrectable data exists on a sync block unit basis. The value “1” or “0” is written into a predetermined region in the sync block memory  204 .  
         [0059]    [0059]FIG. 7 schematically shows an internal structure of the sync block memory  204 .  
         [0060]    The error flag outputted from the error correction circuit  203  is written into a region  71 .  
         [0061]    The coding data of the low frequency component of the DCT transformation coefficient in the region  52  (refer to FIG. 5) of the sync block is written into a region  72 . The coding data of the high frequency component of the DCT transformation coefficient in the region  53  (refer to FIG. 5) of the sync block is written into a region  73 .  
         [0062]    L 1  to L 30  in the region  72  indicate coding data of the low frequency components of the DCT blocks #1 to #30. H 1  to H 30  denote coding data of the high frequency components of the DCT blocks #1 to #30. A hatched portion indicates a delimiter code EOB.  
         [0063]    In case of reading out the coding data every block from the memory  204 , since the cod g data of the low frequency component of the DCT transformation coefficient of each DCT block has been stored at a predetermined memory location in the region  72  in the memory  204 , it can be sequentially read out. However, although the memory location of the coding data of the high frequency component of each DCT block has been stored in the region  73  in the memory  204 , since the position in the region  73  is unspecified, the coding data of the high frequency component is sequentially read out on the basis of the EOB.  
         [0064]    A forward direction low frequency data readout circuit  205  sequentially reads out the coding data L 1  to L 30  of the low frequency components in accordance with the order from the head address, in the region  72  in the sync block memory  204  (forward direction decoding mode).  
         [0065]    A forward direction high frequency data readout circuit  206  sequentially reads out the coding data H 1  to H 30  of the high frequency components in accordance with the order from the head address in the region  73  in the sync block memory  204  on the basis of the EOB.  
         [0066]    The coding data L 1  to L 30  of low frequencies and coding data H 1  to H 30  of high frequencies which were read out by the forward direction low frequency read-out circuit  205  and the forward direction high frequency read-out circuit  206  are supplied to contacts (a) and (b) of a switch  207 .  
         [0067]    The switch  207  is controlled by a switch control circuit  208  so that the coding data of low frequency continues after the coding data of the high frequencies of each DCT block.  
         [0068]    Since a decoding circuit  209  restores the image data by executing the processes opposite to the processes upon coding, their detailed descriptions are omitted here. The decoding circuit  209  restores the image data by executing a variable length decoding, a reverse quantization, and reverse DCT transformation to the coding data of each DCT block which is supplied from the switch  207 . The restored image data is inputted to a frame memory  210 .  
         [0069]    On the other hand, a reverse direction low frequency data read-out circuit  211  sequentially reads out the coding data L 1  to L 30  in the region  72  in the sync block memory  204  in accordance with the order opposite to that in ease of the low frequency read-out circuit  205 , namely, in accordance with the order from L 30  to L 1 .  
         [0070]    A reverse direction high frequency data readout circuit  212  reads out the coding data H 1  to H 30  of the high frequencies in the region  73  in the sync block memory  204  in accordance with the order opposite to that in ease of the forward direction high frequency read-out circuit  206 , namely, in accordance with the order from H 30  to H 1  (reverse direction decoding mode).  
         [0071]    In case of reading out on a bit unit basis, it is convenient that the EOB is set to the same bit train even in case of reading in the direction from any of the LSB or MSB.  
         [0072]    The coding data L 30  to L 1  of the low frequencies and the coding data H 30  to H 1  of the high frequencies which were read out by the reverse direction low frequency read-out circuit  211  and reverse direction high frequency read-out circuit  212  are supplied to contacts (a) and (b) of a switch  213 .  
         [0073]    The switch  213  is controlled by the switch control circuit  208  in a manner such that the coding data of the high frequency continues after the coding data of the low frequencies of each DCT block.  
         [0074]    A decoding circuit  214  restores the image data by executing the variable length decoding, reverse quantization, and reverse DCT transformation to the coding data of each DCT block which is supplied from the switch  213  in a manner similar to the decoding circuit  209 . The restored image data is inputted to the frame memory  210 .  
         [0075]    As shown in FIG. 7, it is now assumed that an error occurs in the sync block #3 and the error flag has been set to “1” in the sync block memory  204 .  
         [0076]    The switch control circuit  208  refers to the error flag region  71  in the sync block memory  204  and controls the switches  207  and  213  as mentioned above and restores the image data so long as the error flag is not set to “1”.  
         [0077]    That is, since the image data is restored in both of the forward direction decoding mode and the reverse direction decoding mode, the block is restored in two decoding modes. However, it is sufficient that the image data restored by either one of the decoding modes is read out from the frame memory  210 . For example, in the embodiment, the image data restored in the forward direction decoding mode is outputted to the post stage.  
         [0078]    On the other hand, in the case where the error flag has been set to “1”, with respect to the sync block with such an error flag, the switches  207  and  213  are held in a state in which they are connected to the contact (a).  
         [0079]    That is, in the example of FIG. 7, a part of H 13  and parts of H 14  to H 17  and H 18  are ignored. With respect to those DCT blocks #13 to #18, they are decoded by only the low frequency components.  
         [0080]    A circuit comprising the circuits  205  to  209  sequentially reads out the coding data from the head address and restores the image data of each DCT block by the coding data of the low and high frequencies. However, as for the sync block having the error flag, the image data of the DCT block is restored by only the coding data of low frequency. The restored image data is stored into the frame memory  210 .  
         [0081]    On the other hand, the circuit comprising the circuits  208  and  211  to  214  reads out the coding data in accordance with the order opposite to that mentioned above and restores the image data of each DCT block by the coding data of the low and high frequencies and writes the restored image data into the frame memory  210 . However, as for the sync blocks after the sync block having the error flag, the reading operation from the memory  204  is stopped and the writing operation into the frame memory  210  is also stopped.  
         [0082]    As mentioned above, in the sync blocks, the circuits  205  to  207  and  209  properly restores the image data by using the coding data of low and high frequencies from the first sync block to the sync block just before the sync block having errors in the forward direction. The circuits  211  to  214  properly restores the image data by using the coding data of low and high frequencies from the last sync block to the sync block just before the sync block having errors in the reverse direction.  
         [0083]    With respect to the DCT blocks included in the sync block with errors, the circuits  205  to  207  and  209  reproduces the image data by only the low frequency component.  
         [0084]    As mentioned above, when the image data is stored into the frame memory  210 , it is read out and supplied to a D/A converting circuit  215  in accordance with the raster scanning order and is converted into the analog signal. The analog signal is outputted to the outside from an output terminal  216 .  
         [0085]    In the embodiment, with regard to the DCT blocks included in the sync block having errors, the circuits  205  to  207  and  209  reproduces the image data by only the low frequency component. However, the circuits  211  to  214  can also reproduce the image data by only the low frequency component.  
         [0086]    For the errors of the sync block unit, a deterioration of the picture quality can be minimized by the construction of sync block and decoding method mentioned above. Even in a special reproducing mode, the reproduced image of a good picture quality can be obtained.  
         [0087]    The reason why such a good image is obtained will now be described with reference to FIG. 2. In a high speed reproducing mode, as for the sync block  11  in which only a part could be scanned by the reproducing head, the whole DATA arranged in the rear portion in the sync block  11  cannot be taken out in the conventional apparatus, so that there are problems such that the data necessary for the expanding process cannot be fully prepared and the whole image data existing in the sync block  11  cannot be reproduced.  
         [0088]    However, by constructing the sync block as in the embodiment, since the data of the low frequency portion of the data which was DCT transformed and coded is arranged in the front portion of the sync block, the image signal can be restored even from the sync block in which only a part of the image data can be taken out. Therefore, many newer picture plane regions can be reproduced on the screen.  
         [0089]    The present invention can be embodied by many other forms without departing from the spirit and main features of the invention.  
         [0090]    For example, in the description of the above embodiment, two reading and decoding circuit systems such as circuits  205 ,  206 ,  207 , and  209  and circuits  211 ,  212 ,  213 , and  214  have been provided. However, as already described with respect to one of those circuits, it is also possible to construct in a manner such that the reading and decoding operations are first executed from the head address and the decoding is executed by only the low frequency component for the sync block having the error flag and, after that, the reading and decoding operations are executed in accordance with the opposite order from the last address.  
         [0091]    In the embodiment, only an example has been shown and described with respect to the number of sync blocks corresponding to the DCT blocks or their relations and the number of sync blocks and their relations can be arbitrarily set. The fixed length unit of the code amount and the size of DCT block can be also similarly arbitrarily set.  
         [0092]    In other words, the foregoing description of embodiment has been given for illustrative purposes only and not to be construed as imposing any limitation in every respect.  
         [0093]    The scope of the invention is, therefore, to be determined solely by the following claims and not limited by the text of the specifications and alterations made within a scope equivalent to the scope of the claims fall within the true spirit and scope of the invention.