Abstract:
The image quality of video data is remarkably improved by a video data processing device and a video data processing method according to the invention. The input video data is adaptively filtered by adaptive filter  35  at the time coding as a function of the degree of coding difficulty of image and the decoded video data is adaptively compensated by adaptive image quality correcting circuit  50  as a function of the degree of coding difficulty of image to reduce the coding noise and appropriately correct the image quality.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a video data processing device and a video data processing method that can suitably be used for a recording/reproduction apparatus adapted to compression-coding input video data and recording them on an optical disc.  
           [0003]    2. Related Background Art  
           [0004]    Compression-coding conforming to the MPEG (Moving Picture Experts Group) Standard is known. With compression-coding conforming to the MPEG Standard, the amount of video data to be handled is reduced by utilizing spatial correlations of data in a same frame and temporal correlations of data in different frames.  
           [0005]    Therefore, the recording/reproduction apparatus using a coding system conforming to the MPEG Standard compression-codes the input video data to generate compression-coded video data and record them on an optical disc. For data reproduction, the recording/reproduction apparatus reproduces the compression-coded data from the optical disc and expansion-decodes the data to restore the original video data. In this way, the recording/reproduction apparatus can efficiently records a vast amount of video data on the optical disc by suing a coding system conforming to the MPEG Standard.  
           [0006]    Some of the recording/reproduction apparatus of the above described type are adapted to always output a predetermined volume of compression-coded data by adaptively filtering the video data to be compression-coded before the compression-coding operation. More specifically, of the frames of the video data to be compression-coded, those that show a weak temporal correlation are subjected to an operation of reducing the high frequency component of the frame before the compression-coding process whereas those that show a strong temporal correlation are compression-coded without being subjected to a reduction of the high frequency component.  
           [0007]    Thus, with such a recording/reproduction apparatus, if the volume of compression-coded video data is expected to increase because the temporal inter-frame correlation is low, the volume of information of the high frequency component is reduced before compression-coding the video data so as to always output a predetermined volume of compression-coded data because the image quality is not remarkably degraded to the sight of the viewer if the volume of the high frequency component of information is reduced.  
           [0008]    However, if the image compression rate is raised, the volume of the high frequency component can be reduced to such an extent that the degradation of the sharpness of the reproduced image is recognizable to the sight of the viewer. The operation of reducing the volume of the high frequency component is conducted in the coding preprocessing step or in the coding processing step. Then, an operation of compensating the reduced volume of the high frequency component is conducted after the decoding processing step.  
           [0009]    However, when the video data to be compression-coded is adaptively filtering before the compression-coding operation, the degree of filtering can vary depending on the image to be filtered. Therefore, if the operation of compensating the reduced volume of the high frequency component is conducted uniformly after the decoding processing step, some of the images to be compensated can be compensated inadequately.  
         BRIEF SUMMARY OF THE INVENTION  
         [0010]    In view of the above identified problem, it is therefore the object of the present invention to provide a video data processing device and a video data processing method that can improve the quality of output images if compared with conventional apparatus and methods.  
           [0011]    According to the invention, the above object is achieved by adaptively filtering video data at the time of coding as a function of the degree of difficulty of coding and adaptively compensating the video data after the decoding processing step as a function of the degree of difficulty of coding of the video data in order to reduce the coding noise and adequately correct the image quality.  
           [0012]    More specifically, in an aspect of the present invention, there is provided a video data processing device comprising:  
           [0013]    a degree of coding difficulty computing means for computing the degree of coding difficulty from the input video data;  
           [0014]    a filtering means for adaptively filtering said input video data on the basis of the degree of coding difficulty computed from said input data;  
           [0015]    a compression-coding means for compression-coding said input and filtered video data;  
           [0016]    a decoding means for decoding said compression-coded video data;  
           [0017]    a degree of coding difficulty computing means for computing the degree of coding difficulty from said decoded video data; and  
           [0018]    an image quality correcting means for adaptively correcting the image quality of said decoded video data on the basis of the degree of coding difficulty computed from said decoded video data.  
           [0019]    In another aspect of the invention, there is provided a video data processing method comprising steps of:  
           [0020]    computing the degree of coding difficulty from the input video data;  
           [0021]    adaptively filtering said input video data on the basis of the degree of coding difficulty computed from said input data;  
           [0022]    compression-coding said input and filtered video data;  
           [0023]    decoding said compression-coded video data;  
           [0024]    computing the degree of coding difficulty from said decoded video data; and  
           [0025]    adaptively correcting the image quality of said decoded video data on the basis of the degree of coding difficulty computed from said decoded video data.  
           [0026]    Thus, according to the invention, the coding noise can be reduced by adaptively filtering video data at the time of coding as a function of the degree of difficulty of coding and the image quality can be adequately corrected by adaptively compensating the video data after the decoding processing step as a function of the degree of difficulty of coding of the video data. Additionally, the amount of information can be effectively reduced by adaptively filtering the video data, using the degree of coding difficulty of the entire image and that of each of selected local areas of the image.  
           [0027]    Therefore, according to the invention, there are provided a video data processing device and a video data processing method that can improve the quality of output images if compared with conventional apparatus and methods. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0028]    [0028]FIG. 1 is a schematic block diagram of a recording/reproduction apparatus realized by applying the present invention;  
         [0029]    [0029]FIG. 2 is a schematic block diagram of the noise reduction circuit arranged in the recording/reproduction apparatus of FIG. 2;  
         [0030]    [0030]FIG. 3 is a schematic block diagram of the motion compensated field recursive type noise reduction circuit arranged in the noise reduction circuit of FIG. 2;  
         [0031]    [0031]FIG. 4 is a schematic illustration of the operation of phase compensation of the V filter arranged in the motion compensated field recursive type noise reduction circuit of FIG. 3;  
         [0032]    [0032]FIG. 5 is a schematic block diagram of the degree of coding difficulty computing circuit arranged in said noise reduction circuit of FIG. 2;  
         [0033]    [0033]FIG. 6 is a schematic illustration of the horizontal and vertical filtering operation of said motion compensated field recursive type noise reduction circuit of FIG. 3;  
         [0034]    [0034]FIG. 7 is a schematic illustration of the multiplexing operation of the noise reduction circuit of FIG. 2;  
         [0035]    [0035]FIG. 8 is a schematic illustration of the timings of writing data in and those of reading data from the field memory of said noise reduction circuit of FIG. 2;  
         [0036]    [0036]FIG. 9 is a schematic illustration of the filtering operation using transfer function G of the adaptive type pre-filter arranged in said recording/reproduction apparatus of FIG. 1;  
         [0037]    [0037]FIG. 10 is a schematic illustration of the filtering operation using transfer function H of the adaptive type pre-filter arranged in said recording/reproduction apparatus of FIG. 1;  
         [0038]    [0038]FIG. 11 is a schematic illustration of the timing of writing a data in and that of reading a data from the frame memory of said noise reduction circuit of FIG. 2;  
         [0039]    [0039]FIG. 12 is a schematic illustration of the relationship between the filter coefficient and the degree of block coding difficulty that can be used for the purpose of the invention;  
         [0040]    [0040]FIG. 13 is a schematic illustration of the relationship between the table for selecting said filter coefficient and the degree of field coding difficulty;  
         [0041]    [0041]FIG. 14 is a flow chart of the processing operation of detecting a still image, detecting a scene change and a time base filtering operation conducted in the course of computing the degree of coding difficulty of each field;  
         [0042]    [0042]FIG. 15 is a timing chart illustrating the change in the attenuation of the degree of coding difficulty; and  
         [0043]    [0043]FIG. 16 is a timing chart illustrating the change in the degree of coding difficulty after an operation of processing a scene change. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0044]    Now, the present invention will be described by referring to the accompanying drawing that illustrate preferred embodiments of the invention.  
         [0045]    The present invention is typically applied to a recording/reproduction apparatus  1  having a configuration as shown in the block diagram of FIG. 1.  
         [0046]    Referring to FIG. 1, this recording/reproduction apparatus  1  is so designed as to receiving an NTSC(National Television System Committee) video signal S 1  from the outside through analog/digital (A/D) conversion circuit  2  and then the video data D 2  obtained by the A/D conversion of the A/D conversion circuit  2  is sent out from the A/D conversion circuit  2  to NTSC decoder  3 . The NTSC decoder  3  separates the luminance signal and the color signal of the NTSC video data D 2  from each other and transmits the video data D 3  including the luminance signal and the color signal that are separated from each other to noise reduction circuit  4  and scene change detection circuit  5 .  
         [0047]    Referring now to FIG. 2, the noise reduction circuit  4  comprises a motion compensated field recursive type noise reduction circuit  6 , a degree of coding difficulty computing circuit  7 , a multiplexer circuit  8  and a field memory  9 . The noise reduction circuit  4  is so designed that the video data D 3  from the NTSC detector  3  is supplied to the motion compensated field recursive type noise reduction circuit  6  whereas the scene change data D 26  indicating if there is a scene change in the video data D 3 , which may typically be those of a movie picture or television drama, or not is supplied to the degree of coding difficulty computing circuit  7 .  
         [0048]    Now, referring to FIG. 3, the video data D 3  is input to subtracter  10  and V filter  11  of the motion compensated field recursive type noise reduction circuit  6 . The field-delayed video data D 5  obtained by delaying the video data D 3  from the field memory  9  of the noise reduction circuit  4  (FIG. 2) by a field is input to V filter  13  of the motion compensated field recursive type noise reduction circuit  6 .  
         [0049]    The V filter  11  performs an operation of phase compensation on the video data of the interlaced scanning system relative to the field-delayed video D 5  in the vertical direction and sends out the obtained video data D 6  to motion compensation circuit  14 . Similarly, the V filter  13  performs an operation of phase compensation on the field-delayed video data D 5  of the interlaced scanning system relative to the video data D 3  in the vertical direction and sends out the obtained video data D 7  to motion compensation circuit  14 .  
         [0050]    More specifically, referring to FIG. 4, on the basis of the pixel values of two pixels of an even number field of the video data D 3  having a same phase in the horizontal direction and located on adjacent even number lines, the V filter  11  computes the pixel value of the pixel obtained by dividing the phase of each of the pixels to a predetermined ratio (e.g., 3:1) in the vertical direction. At the same time, on the basis of the pixel values of the two pixels of an odd number field of the field-delayed video data D 5  having a same phase in the vertical direction and located on adjacent odd number lines, the V filter  13  computes the pixel value of the pixel obtained by dividing the phase of each of the pixels to a predetermined ratio (e.g., 1:3) in the vertical direction. As a result of the phase compensation, the phase of the video D 3  and that of the field-delayed video data D 5  are made to agree with each other in the vertical direction.  
         [0051]    Referring back to FIG. 3, motion compensation circuit  14  operates as degree of correlation computing means firstly divides the video data D 6  into blocks of a predetermined size. Then, the motion compensation circuit  14  extracts one of the blocks as reference block and detects the similarity candidate block that is most similar to the reference block out of a number of candidate blocks found in a predetermined search area of the field-delayed video data D 7 . Thereafter, it determines the difference between the reference block and the detected similarity candidate block as noise. Subsequently, the motion compensation circuit  14  determines the difference between the similarity candidate block and each of all the remaining blocks in the video data D 6  and sends out the obtained data to Hadamard transform circuit  15  and degree of coding difficulty computing circuit  7  (FIG. 2) as motion compensation remaining difference data D 8 .  
         [0052]    The Hadamard transform circuit  15  divides the motion compensation remaining difference data D 8  transmitted from the motion compensation circuit  14  into, for instance, eight horizontal frequency components and sends out the motion compensation remaining difference data D 9  of each of the horizontal frequency components to non-linear circuit  16 . The non-linear circuit  16  comprises a non-linear limiter circuit. It generates noise data D 10  of each of the horizontal frequency components by adaptively changing each of the limiter values to be applied to the motion compensation remaining difference data D 9  of the horizontal frequency component and limiting the upper limit value of the motion compensation remaining difference data D 9  and sends out the obtained data to inverse Hadamard transform circuit  17 . Thus, a high limiter value is selected for the motion compensation remaining difference data D 9  if its horizontal frequency component is found in a low frequency band because the data can produce noticeable noises, whereas a low limiter value is selected for the motion compensation remaining difference data D 9  if its horizontal frequency component is found in a high frequency band because the data can produce only less noticeable noises.  
         [0053]    The inverse Hadamard transform circuit  17  transforms the noise data D 10  of each of the horizontal frequency components into noise data D 11  on the time base and transmits them to the subtracter  10 . The subtracter  10  generates noise-reduced video data D 12  involving no temporal correlations among different fields by subtracting the noise data D 11  from the video data D 3  fed from the NTSC decoder  3  (FIG. 1) and sends out the obtained data to multiplexer circuit  8  (FIG. 2).  
         [0054]    In this way the motion compensated field recursive type noise reduction circuit  6  reduces the noise in the video data D 3  in advance to consequently reduce block distortions that make the boundaries of the blocks discontinuous and linking mosquito noises that appear around the edges.  
         [0055]    As shown in FIG. 5, the degree of coding difficulty computing circuit  7  inputs the motion compensation remaining difference data D 8  supplied from the motion compensated field recursive type noise reduction circuit  6  (FIG. 2) to adding/averaging circuit  28 . The adding/averaging circuit  28  adds the motion compensation remaining difference data D 8  of a field to obtain the degree of field coding difficulty data D 29  for each field. The obtained degree of field coding difficulty data D 29  of each field is then input to temporal filter circuit  29  for a filtering operation to be conducted on the time base. The filtering operation is conducted on the time base by using the transfer function F of (1) to generate degree of field coding difficulty data  29 ; 
           F= (1− K )/(1− K×Z   −F )  (1) 
         [0056]    where K is a constant having a value of 0&lt;K&lt;1 and used to determine the time constant of the filter and Z −F  represents a field delay.  
         [0057]    The motion compensation remaining difference data D 8  are also supplied to H block filter circuit  21 . As shown in FIG. 6 (A), the H block filter circuit  21  performs an inter-block filter operation as expressed by formula (2) below on each of the block data dmc [m, n] of the motion compensation remaining difference data D 8  to produce motion compensation remaining difference data D 15  containing horizontally smoothed block data dmc_hfil [m, n] and sends them to V block filter circuit  22  and multiplexer circuit  23 ;  
               dmc                 _                   hfil              [     m   ,   n     ]       =         dmc        [       m   +   1     ,   n     ]       +     dmc              [     m   ,   n     ]     +     dmc        [       m   -   1     ,   n     ]         4             (   2   )                               
 
         [0058]    where m represents the horizontal address and n represents the vertical address of each block.  
         [0059]    As shown in FIG. 6(B), the V block filter circuit  22  performs a vertical inter-block filtering operation as expressed by formula (3) below on each of the block data dmc_hfil [m, n] of the motion compensation remaining difference data D 15  fed from the H block filter circuit  21 , using the block data dmc_hfil [m, n-1] contained in the motion compensation remaining difference multiplexed data D 16  fed from serial/parallel conversion circuit  24 , to generate motion compensation remaining difference data D 17  containing vertically smoothed block data dmc_hvfil [m, n] as expressed by formula (4) below and transmit them to the multiplexer circuit  23 .  
               dmc                 _                   hvfil              [     m   ,   n     ]       =         dmc                 _                   hfil              [     m   ,   n     ]       +     dmc                 _                   hfil              [     m   ,     n   -   1       ]         2             (   3   )                 dmc                 _                   hvfil              [     m   ,   n     ]       =               dmc              [       m   +   1     ,   n     ]     +     dmc        [     m   ,   n     ]       +     dmc              [       m   -   1     ,   n     ]     +                 dmc        [       m   +   1     ,     n   -   1       ]       +     dmc              [     m   ,     n   -   1       ]     +     dmc              [       m   -   1     ,     n   -   1       ]             8             (   4   )                               
 
         [0060]    Thus, as shown in FIG. 6(C), the motion compensation remaining difference data D 17  are obtained by spatially smoothing each of the block data dmc [m, n] of the motion compensation remaining difference data D 8 .  
         [0061]    Now, referring to FIG. 7, the multiplexer circuit  23  multiplexes the horizontally smoothed motion compensation remaining difference data D 15  and the horizontally and vertically smoothed motion compensation remaining difference data D 17  for each block and transmits the motion compensation remaining difference data D 18  obtained as a result of the multiplexing to parallel/serial conversion circuit  25 . The parallel/serial conversion circuit  25  performs an operation of parallel/serial conversion on the 4-bit multiplexed motion compensation remaining difference data and sends out the obtained 1-bit multiplexed motion compensation remaining difference data D 19  to selector circuit  26 .  
         [0062]    The selector circuit  26  switches the multiplexed motion compensation remaining difference data D 19  fed from the parallel/serial conversion circuit  25  and the multiplexed motion compensation remaining difference data D 20  read out from memory  27  and sends out either of the switched data to the memory  27  as multiplexed motion compensation remaining difference data D 21  so that the memory  27  stores the transmitted multiplexed motion compensation remaining difference data D 21 . Then, the memory  27  reads out the stored multiplexed motion compensation remaining difference data D 21  at a predetermined timing and transmits them to the selector circuit  26  and the serial/parallel conversion circuit  24  as multiplexed motion compensation remaining difference data D 20 . The serial/parallel conversion circuit  24  performs an operation of serial/parallel conversion on the 1-bit multiplexed motion compensation remaining difference data D 21  and transmits the obtained 4-bit multiplexed motion compensation remaining difference data D 16  to the V block filter circuit  22  and temporal filter circuit  30 .  
         [0063]    The temporal filter circuit  30  operates as correlation smoothing means to turn the horizontally and vertically smoothed motion compensation remaining difference data of the motion compensation remaining difference data D 16  fed from the serial/parallel conversion circuit  24  into dfc_in and also the field-delayed degree of coding difficulty data D 25  comprising the motion compensation remaining difference data obtained by delaying the motion compensation remaining difference data dfc_in fed from the field memory  9  (FIG. 2) by a frame into motion compensation remaining difference data dfc_fd and also smooths the motion compensation remaining difference data D 8  input to the degree of coding difficulty computing circuit  7  by both horizontally and vertically filtering them on the time base. The obtained data are defined as degree of block coding difficulty data for each block.  
         [0064]    More specifically, the temporal filter circuit  30  generates degree of block coding difficulty data tfil by performing a filtering operation on the time base as expressed by formula (5) on the basis of the horizontally and vertically smoothed motion compensation remaining difference data dfc_in and the motion compensation remaining difference data dfc_fd obtained by delaying the motion compensation remaining difference data dfc_in by a field;  
                                                                 dif_fd = dfc_in-fdc_fd   (5)       adif_fd = abs (dif_fd)       case (s_dif_fd)        0;                if (adif_fd) &gt; c_tmp_fil_h×2)            tfil = dfc_fd + c_tmp_fil_h           else                               tfil   =       dfc_in   +   dfc_fd     2                                                     1:                if (adif_fd) ’2 c_tmp_fil_h×2)            tfil = dfc_fd-c_tmp_fil_1           else                               tfil   =       dfc_in   +   dfc_fd     2                                                  
 
         [0065]    where dif_fd represents the difference data between the motion compensation remaining difference data dfc_in and the motion compensation remaining difference data dfc_fd, adif_fd represents the absolute value of the difference data dif_fd, s_dif_fd represents the code bit of the difference data dif_fd, indicating that the difference data dif_fd is positive when s_dif_fd is 0 and the difference data dif_fd is negative when s_dif_fd is 1, and c_tmp_fil_h and c_tmp_fil — 1 represent respective constants.  
         [0066]    Thus, the temporal filter circuit  30  selects the average of the motion compensation remaining difference data dfc_in and the motion compensation remaining difference data dfc_fd for the degree of block coding difficulty data tfil when the difference data dif_fd of the horizontally and vertically smoothed motion compensation remaining difference data fdc_in and the motion compensation remaining difference data dfc_fd obtained by delaying the motion compensation remaining difference data dfc_in by a field is found within a predetermined range, whereas it selects the outcome of adding a predetermined constant to or subtracting it from the motion compensation remaining difference data dfc_fd as degree of block coding difficulty data tfil when the difference data dif_fd is found outside the predetermined range.  
         [0067]    The degree of coding difficulty computing circuit  7  (FIG. 2) is fed with a scene change data D 26  indicating if there is a scene change in the video data D 3 , which may typically be those of a movie picture or television drama, or not by the scene change detection circuit  5  (FIG. 1) and inputs the supplied scene change data D 26  to the temporal filter circuit  30 . If the scene change detection circuit  5  outputs 1 as scene change data D 26  when it detects a scene change it but outputs 0 in any other occasion.  
         [0068]    When transmitting the degree of block coding difficulty data tfil to shift register  31  as degree of block coding difficulty data D 27 , the temporal filter circuit  30  sends the scene change data D 26  on a time division basis in the vertical blanking interval of the degree of block coding difficulty data D 27 . Then, the shift register  31  temporarily holds the scene change data D 26  and the degree of block coding difficulty data D 27  and outputs them at a predetermined timing to the multiplexer circuit  8  of the noise reduction circuit  4 .  
         [0069]    When outputting the noise-reduced video data D 12  fed from the motion compensated field recursive type noise reduction circuit  6 , the multiplexer circuit  8  multiplexes the noise-reduced video data D 12 , the scene change data D 26 , the degree of block coding difficulty data D 27  and the degree of field coding difficulty data D 28  by outputting the scene change data D 26 , the degree of block coding difficulty data D 27  and the degree of field coding difficulty data D 28  in the horizontal blanking interval of the noise-reduced video data D 12 . Then, it transmits the noise-reduced video data D 12 , the scene change data D 26 , the degree of block coding difficulty data D 27  and the degree of field coding difficulty data D 28  that are multiplexed to and stores them in the field memory  9 .  
         [0070]    The field memory  9  delays the video data D 3  fed from the NTSC decoder  3  by a field and transmits the field-delayed video data D 5  to the motion compensated field recursive type noise reduction circuit  6  and the scene change detection circuit  5 . It also delays the degree of block coding difficulty data D 27  by a field and transmits the field-delayed degree of block coding difficulty data D 25  to the degree of coding difficulty computing circuit  7 . Additionally, it transmits the field-delayed degree of block coding difficulty data D 25 , the field-delayed degree of coding difficulty data D 28  and the scene change data D 26  to adaptive pre-filter  35  (FIG. 1) and causes the noise-reduced video data D 12  to be delayed by a field. Finally, it transmits the field-delayed noise-reduced video data D 30  to the adaptive pre-filter  35 .  
         [0071]    In FIG. 8, (A) through (H) show the timings of writing the above data in and reading them from the field memory  9 .  
         [0072]    Referring to FIG. 8, (A) shows the timing of writing the noise-reduced video data D 12  in the field memory  9  and (B) shows the timing of writing the degree of block coding difficulty data D 27  in the field memory  9  while (C) shows the timing of writing the degree of field coding difficulty data D 28  in the field memory  9  and (D) shows the timing of writing the scene change data D 26  in the field memory  9 . Similarly, in FIG. 8, (E) shows the timing of reading the field-delayed noise-reduced video data D 30  from the field memory  9  and (F) shows the timing of reading the field-delayed degree of coding difficulty data D 25  from the field memory  9 , while (G) shows the timing of reading the degree of field coding difficulty data D 28  from the field memory  9  and (H) shows the timing of reading the scene change data D 26  from the field memory  9 .  
         [0073]    The adaptive pre-filter  35  performs an adaptive pre-filtering operation on the field-delayed noise-reduced video data D 30  by using the transfer function H as expressed by formula (6) below on the basis of the field-delayed degree of block coding difficulty data D 25 , the degree of field coding difficulty data D 28  and the scene change data D 26  to reduce the volume of information of the field-delayed noise-reduced video data D 30  as a function of the temporal correlations of the fields of the field-delayed noise-reduced video data D 30 . Then, it transmits the noise-reduced video data D 31  obtained as a result of the above operation to MPEG encoder  36 . 
           H= 1−(1 −G )×α 
           G=a× ( a×Z   −1   +b+a×Z   +1 )× Z   −H   +b× ( a×Z   −1   +b+a×Z   +1 )+ a× ( a×Z   −1   +b+a×Z   −1 )× Z   +H   (6) 
         [0074]    As shown in FIG. 9, the transfer function G is adapted to perform a weighted operation of multiplying the pixel values of the pixels surrounding a pixel drawing attention by a predetermined coefficient and adding the obtained product to the pixel value of the pixel drawing attention.  
         [0075]    In formula (6), the filter coefficient α is a value within the range of 0 to 1 as determined on the basis of the values of the field-delayed degree of block coding difficulty data D 25 , the degree of field coding difficulty data D 28  and the scene change data D 26 . More specifically, a value close to 1 is selected for the filter coefficient α if the field-delayed degree of block coding difficulty data D 25  and the degree of field coding difficulty data D 28  show respective values not lower than a predetermined level and the scene change data D 26  shows a value equal to 0, whereas a value of 0 is selected for the filter coefficient α if the field-delayed degree of block coding difficulty data D 25  and the degree of field coding difficulty data D 28  show respective values not higher than a predetermined level and the scene change data D 26  shows a value equal to 0. However, a value of 0 is compulsively selected for the filter coefficient α regardless of the value of the field-delayed degree of block coding difficulty data D 25  and that of the degree of field coding difficulty data D 28  if the the value of the scene change data D 26  is equal to 1.  
         [0076]    [0076]FIG. 12 is a schematic illustration of the relationship between the filter coefficient α and the degree of block coding difficulty data D 25  that can be used for the purpose of the invention. In other words, the filter coefficient α is controlled in a manner as shown in FIG. 12 according to a function of the degree of block coding difficulty data obtained on a block by block basis. In FIG. 12, a table number (Tab. No.) of an optimally selected table is selected as a function of the value of the degree of field coding difficulty data D 28  shown in FIG. 13 for each image transmission rate. For example, if the image transmission rate is 4 Mbps and the value of the degree of field coding difficulty data D 28  obtained for each field is  14 , a table number (Tab. No.) of  15  is selected. Then, according to the selection of table number (Tab. No.)  15 , the filter coefficient α is controlled by the degree of block coding difficulty data D 25  obtained for each block of the scene.  
         [0077]    Thus, as shown in FIG. 10, the adaptive pre-filter  35  adaptively filters the field-delayed noise-reduced video data D 30  by selecting a value close to 1 for the filter coefficient for a field and a domain showing a low temporal correlation to reduce the volume of information for the high frequency component of the field and a value close to 0 for the filter coefficient for a field and a domain showing a high temporal correlation so as not to reduce the volume of information for the high frequency component of the field. With this arrangement, the volume of information of the compressed video data D 36  received by the downstream MPEG encoder  36  is held to a constant level.  
         [0078]    Thus, if the volume of information of the compressed video data D 36  is expected to increase because the inter-field temporal correlation is low, the adaptive pre-filter  35  reduces the volume of information of the high frequency component that does not noticeably degrade the image quality to the sight of the viewer if reduced before compression-coding the field-delayed noise-reduced video data D 30  in order to hold the volume of information of the compressed video data D 36  to a constant level.  
         [0079]    At this time, the adaptive pre-filter  35  forcibly select 0 for the filter coefficient α for a field that does not have any temporal correlation with the field showing an image change or the immediately preceding field so that no information of the field may omitted. As a result, the recording/reproduction apparatus  1  is left free from degrading any video data that comes immediately after a scene change when compression-coding and expansion decoding the noise-reduced video data D 31  produced by the adaptive pre-filter  35 .  
         [0080]    The MPEG encoder  36  performs a compression-coding operation on the noise-reduced video data D 31  according to the DCT (Discrete Cosine Transform) coding system to transmit compression-coded data D 32  including coded information such as that of quantization scale added to the video data to ECC (Error Correcting Circuit) encoder  37 .  
         [0081]    The ECC encoder  37  adds an error correction code to the compressed video data D 32  and transmits the compressed video data D 33  including the error correction code to  8 - 14  modulation circuit  38 . The  8 - 14  modulation circuit  38  modulates the compressed video data D 33  according to a predetermined  8 - 14  modulation system and transmits the obtained compressed video data D 34  to RF amplifier  39 . The RF amplifier  39  amplifies the compressed video data D 34  to a predetermined level. Then, the obtained compressed video data D 35  are recorded on an optical disc  41  by way of an optical pickup  40 .  
         [0082]    For data reproducing operation, the recording/reproduction apparatus  1  reproduces the compressed video data D 40  from the optical disc  41  by way of the optical pickup  40  and transmits them to the RF amplifier  45 . The RF amplifier  45  amplifies the compressed video data D 40  to predetermined level and transmits the obtained compressed video data D 41  to  8 - 14  demodulation circuit  46 . The  8 - 14  demodulation circuit  46  demodulates the compressed video data D 41  according to a predetermined  8 - 14  demodulation system and transmits the obtained compressed video data D 41  to ECC decoder  47 .  
         [0083]    The ECC decoder  47  performs an error correcting operation, using the error correction code added by the ECC encoder  37  and the obtained compressed video data D 43  to MPEG decoder  48 . The MPEG decoder  48  performs an expansion-decoding operation on the compressed video data D 43  to restore the original video data D 44  and transmits the restored video data D 44  to noise reduction circuit  49 . At this time, the MPEG decoder  48  extracts coding data D 45  such as quantization scale and transmits the extracted coding data D 45  also to the noise reduction circuit  49 .  
         [0084]    The noise reduction circuit  49  reduces the block distortions and the mosquito noises produced in the video data D 44  by conducting a filtering operation on the video data D 44 , using the coding information D 45  and transmits the obtained video data D 46  to image quality correction circuit  50 . Like the noise reduction circuit  4  of the recording system, the noise reduction circuit  49  of the reproduction system computationally determines the degree of field coding difficulty data D 50  for each block and also the degree of field coding difficulty data D 5   1  for each field and transmits them to adaptive image quality correction circuit  50 .  
         [0085]    The adaptive image quality correction circuit  50  adaptively performs an image quality correcting operation on the video data D 46 , using transfer function H as expressed by formula (7), on the basis of the degree of field coding difficulty data D 50  and the degree of field coding difficulty data D 51 . 
           H= 1+(1− G )×α 
           G=a× ( a×Z   −1   +b+a×Z   +1 )× Z   −H   b× ( a×Z   −1   +b+a×Z   +1 )+ a× ( a×Z   −1   +b+a×Z   +1 )× Z   +H   (7) 
         [0086]    The relationship between the filter coefficient α and the degree of block coding difficulty data for the recording system illustrated in FIGS. 12 and 13 also applies to the reproduction system. Thus, the filter coefficient α is controlled by the value of the degree of block coding difficulty data obtained for each block in a manner as shown in FIG. 12. In FIG. 12, a table number (Tab. No.) of an optimally selected table is selected as a function of the value of the degree of field coding difficulty data D 28  shown in FIG. 13 for each image transmission rate. For example, if the image transmission rate is 4 Mbps and the value of the degree of field coding difficulty data D 51  obtained for each field is  14 , a table number (Tab. No.) of  15  is selected. Then, according to the selection of table number (Tab. No.)  15 , the filter coefficient α is controlled by the degree of block coding difficulty data D 50  obtained for each block of the scene. Note that the table used in the reproduction system may well show characteristics different from those of the table used for the recording system as shown in FIGS. 12 and 13 without any problem. Additionally, the parameter to be used for controlling the degree of coding difficulty in the image quality operation is not limited to the filter coefficient α and the frequency characteristics of the filter and/or the core ring level may also be used as additional parameters.  
         [0087]    The adaptive image quality correcting circuit  50  sends out the video data D 47  obtained as a result of the adaptive image quality correcting operation to NTSC encoder  51 . The NTSC encoder  51  converts the video data D 47  into NTSC video data D 48  and transmits the obtained NTSC video data D 48  to digital/analog (D/A) conversion circuit  52 . The D/A conversion circuit  52  performs an operation of digital/analog conversion on the NTSC video data D 48  and transmits the obtained video signal S 49  to the outside.  
         [0088]    Referring to FIG. 1, operation input section  55  comprises switches including a switch for selecting if an operation of reducing the block distortions is to be carried out or not and one for controlling the operation of image correction and adapted to generate input data D 55  in response to the input operation on the part of the user and transmit the generated input data D 55  to control circuit  56 . The control circuit generates control data D 56  on the basis of the input data D 55  fed from the operation input section  55  and sends out the generated control data D 56  to the noise reduction circuit  49  and the image quality correction circuit  50  to control the operation of the noise reduction circuit  49  and that of the image quality correction circuit  50 .  
         [0089]    With the above arrangement, the scene change detection circuit  5  detects the scene change, if any, contained in the video data D 3  fed from the NTSC decoder  3  by analysing the correlation between the video data D 3  and the field-delayed video data D 5  obtained by delaying the video data D 3  by a field and transmits scene change data D 26  indicating if there is a scene change or not to the adaptive pre-filter  35 .  
         [0090]    The noise reduction circuit  4  analyses the correlation between the video data D 3  and the field-delayed video data D 5  and generates motion compensation remaining difference data D 8 . Subsequently, it smooths the generated motion compensation remaining difference data D 8  both horizontally and vertically on the time base to generate degree of field coding difficulty data D 25  and transmits the generated data to the adaptive pre-filter  35 .  
         [0091]    The adaptive pre-filter  35  reduces the amount of information of each field image constituting the field-delayed noise-reduced video data D 30  by raising the degree of band limiting for removing unnecessary frequency components from each field image showing a low correlation with and lowering the degree of band limiting for removing unnecessary frequency components from each field image showing a high correlation with the field image of the immediately preceding field but forcibly reducing the degree of band limiting for each field image showing no correlation with the field image of the immediately preceding field because of a scene change out of the field images of the field-delayed noise-reduced video data D 30  fed sequentially from the noise reduction circuit  4  on the basis of the field-delayed degree of field coding difficulty data D 25  and the scene change data D 26  fed to it.  
         [0092]    While the timing of reading out the data to be fed to each of the scene change detection circuit  5 , the motion compensated field recursive type noise reduction circuit  6 , the degree of coding difficulty computing circuit  7  and the adaptive pre-filter  35  by using the field memory  9  is described for the above embodiment, the present invention is by no means limited thereto and the timing of reading the data to be fed to each of the above circuits may alternatively be controlled by means of a frame memory.  
         [0093]    In FIG. 1, (A) through (H) show the timings of writing the above data in and reading them from the frame memory. Referring to FIG. 11, (A) shows the timing of writing the noise-reduced video data D 12  in the frame memory and (B) shows the timing of writing the degree of block coding difficulty data D 27  in the frame memory while (C) shows the timing of writing the degree of frame coding difficulty data D 28  in the frame memory and (D) shows the timing of writing the scene change data in the frame memory. Similarly, in FIG. 11, (E) shows the timing of reading the frame-delayed noise-reduced video data D 12  from the frame memory and (F) shows the timing of reading the frame-delayed degree of block coding difficulty data from the frame memory, while (G) shows the timing of reading the frame-delayed degree of coding difficulty data from the frame memory and (H) shows the timing of reading the scene change data D 26  from the frame memory.  
         [0094]    While the degree of coding difficulty data is computed from the motion compensation remaining difference data in the above embodiment, the degree of coding difficulty data may alternatively be computed by determining the difference value of the vectors of adjacent blocks and synthesizing the difference of vectors and the motion compensation remaining difference.  
         [0095]    Additionally, while a noise reduction circuit, an adaptive pre-filter circuit and an adaptive image quality correction circuit are provided independently in the recording system and in the reproduction system of the above embodiment, the same circuits may be used for the both systems if a recording (coding) operation and a reproduction (decoding) operation are not conducted simultaneously.  
         [0096]    Still additionally, while the above embodiment of the present invention is applied to a recording/reproduction apparatus  1  in the above description, the present invention is not limited thereto and can be equally applied a video data processing apparatus that is designed to adaptively filter the video data to be compression-coded and performs an adapted image quality correcting operation on the image data after expansion-decoding the video data.  
         [0097]    Now, an operation of detecting a still image and a scene change in the process of computing the degree of coding difficulty Kp_Fi for each field before a filtering operation on the time base for the purpose of the invention will be described by referring to FIGS. 14, 15 and  16 .  
         [0098]    Firstly in Step S 1 , it is determined if the input image is a still image or not. The input image is determined to be a still image when the degree of coding difficulty Kp_Fi keeps a same or similar value for a plurality of fields.  
         [0099]    If it is determined in Step S 1  that the input image is a still image, the operation proceeds to Step S 2 , where a constant, e.g., 2, is added to the attenuated value d of degree of coding difficulty. If, on the other hand, it is determined in Step S 1  that the input image is not a still image, the operation proceeds to Step S 3 , where the constant of 2 is subtracted from the attenuated value d of degree of coding difficulty.  
         [0100]    [0100]FIG. 15 shows a timing chart of this operation. Referring to FIG. 15, the input image is determined to be a still image when the degree of coding difficulty Kp_Fi for each field remains same for consecutive sixteen fields. If the still image continues from field number [n+ 1 ] to field number [n+23], constant 2 is added to the attenuated value d of degree of coding difficulty from the field number [n+16]. If it is assumed that the degree of coding difficulty Kp_Fi [0] is constantly equal to 10 for the purpose of simplicity, the attenuated value d of degree of coding difficulty is 10 from field number [n+20] to field number [n+23]. If it is determined that the input image is no longer a still image from field number [n+24], the attenuated value d of degree of coding difficulty will be counted down therefrom.  
         [0101]    In Step S 4 , the attenuated value d of degree of coding difficulty is subtracted from the degree of coding difficulty Kp_Fi for each field.  
         [0102]    Then, in Steps S 5  and S 6 , the outcome of the subtraction Kp_Fi stl in Step S 4  is restricted so as not to become negative. More specifically, it is determined in Step S 5  if the outcome of the subtraction Kp_Fi_stl [0] is smaller than 0 or not and, if the outcome of the subtraction Kp_Fi_stil [0] is smaller than 0, Kp_Fi_stl [0]=0 and d=Kp_Fi [0] are made to hold true in Step S 6 . It is also determined in Step S 5  if the outcome of the subtraction Kp_Fi_stl [0] is greater than the degree of coding difficulty KP_Fi [0] or not and, if the outcome of the subtraction Kp_Fi_stl [0] is greater than the degree of coding difficulty Kp_Fi [0], Kp_Fi_stl [0]=Kp —Fi [ 0] and d= 0  are made to hold true in Step S 6 .  
         [0103]    Then, in Step S 7 , it is determined if a scene change point is present or not. The flag xsc [0] for indicating the outcome of the judgment on a scene change point falls down to level Low for a field after a scene change point. Then, the flag xsc [0] is expanded on the time base by a logical multiplication operation of the flag xsc [F] of the immediately preceding field and the flag xsc [2F] of the field preceding the current field by two and a scene change signal xsc [0] &amp; xsc [F] &amp; xsc [2F] is produced to indicate that the three fields immediately succeeding the scene change point are held to level Low.  
         [0104]    If the scene change signal xsc [0] &amp; xsc [F] &amp; xsc [2F] is at level Low and hence the outcome of the judgement in Step S 7  is YES, a processing operation for a scene change is performed and the degree of scene change coding difficulty Kp_Fi_sc [F] of the immediately preceding frame is maintained in Step S 8 . If, on the other hand, the scene change signal xsc [0] &amp; xsc [F] &amp; xsc [2F] is at level High and hence the outcome of the judgement in Step S 7  is NO, the operation proceeds to Step S 9 , where the degree of coding difficulty Kp_Fi_stl [0] after the still image processing operation is made equal to the degree of coding difficulty Kp_Fi_sc [0] after the scene change processing operation.  
         [0105]    Then, in Step S 10 , a filtering operation is conducted on the time base by using the above degree of scene change coding difficulty Kp_Fi_sc [0], Kp_Fi_sc [F] and the transfer function F of formula (1).  
         [0106]    While FIGS. 12 and 13 show an instance of real time transmission, where the adaptive processing operation that is conducted as a function of image transmission rate, it may be needless to say that the adaptive processing operation may be conducted adaptively as a function of the image compression rate because the image transmission rate and the image compression rate show an invariable relationship.  
         [0107]    In the case of non-real time transmission, an adaptive control operation will be conducted as a function of the image compression rate instead of the image transmission rate.  
         [0108]    For example, the recording/reproduction apparatus  1  of FIG. 1 uses an optical disc drive for an optical disc. An example of non-real time transmission may be an occasion where a hard disc drive is used and the MPEG compression ratio of the MPEG encoder and the MPEG decoder is different from the recording/reproduction rate of the hard disc. Then, the difference between the compression ratio and the recording/reproduction rate is absorbed by the MPEG encoder, the modulation circuit, the demodulation circuit and the buffer memory in the MPEG decoder. If such is the case, the arrangement may be modified in such a way that the hard disc unit and the MPEG encoder and the MPEG decoder are connected by way of a digital transmission system or the hard disc unit is replaced by a semiconductor memory.