Abstract:
A method of de-interlacing used to convert an interlaced video signal to a progressively scanned format utilizing vertical temporal filtering to generate the missing lines, utilizing appropriate filter coefficients to give a desired vertical frequency response, and filter utilizing coefficients such that the total combined contribution from all fields is unity while the total contribution from each individual field is chosen so as to boost higher temporal frequencies which has the perceived effect of increasing the sharpness of moving edges. Furthermore, in order to avoid certain unwanted artifacts, the lines of the current field are modified using a vertical temporal filter with similar temporal boosting properties to that which was used to generate the missing lines.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to digital video signal processing, and more particularly to a method and apparatus for converting an interlaced video signal to a progressively scanned format in which the perceived sharpness of moving edges is enhanced. 
     BACKGROUND OF THE INVENTION 
     Most television systems utilize signals which have been generated by scanning a source image in a 2:1 interlaced format. Many display devices are available which are capable of reproducing such images directly from the interlaced signal. For instance, in a CRT monitor, the interlaced signal may be used directly to modulate the intensity of a beam as it is swept across the screen in an interlaced raster format. Such displays, however, may suffer from artifacts such as visible line structure, flicker and twitter which are related to the interlaced nature of the scanning. In particular, these undesirable artifacts tend to become more noticeable for screens with larger diagonal sizes. It is often desirable to convert signals from an interlaced scan format to a progressive scan format in order to reduce the artifacts associated with interlaced scanning. Furthermore, some display devices are inherently progressive in nature and therefore require conversion to a progressive format before display is possible. 
     A number of solutions to the problem of conversion from interlaced to progressive scan format have been proposed in the prior art. One such method involves the simple merging of two interlaced video fields to produce a progressively scanned video frame in which the even lines come from the even field and the odd lines come from the odd field. This technique works well for sequences which contain little or no motion but results in objectionable artifacts when motion is present due to the simultaneous display of video data which represents the image at different points in time. 
     Various forms of spatial and/or temporal interpolation have also been proposed. One such method involves spatial interpolation within a single interlaced field in order to produce a progressive frame. This approach does not suffer from the motion artifacts described above but, among other problems, suffers from a loss of vertical detail since each field contains only half of the spatial picture data. Alternatively, it is also possible to generate the missing lines by means of purely temporal interpolation. This approach yields maximum vertical detail for static images but results in serious blur when motion is present. Various attempts have also been made to combine spatial and temporal interpolation in order to reap the benefits of both approaches. As described in U.S. Pat. No. 4,789,893 (Weston), it is possible to generate the missing lines as a weighted average of neighboring lines from both the current and adjacent fields. The weightings applied to each of the neighboring lines are chosen such that low vertical frequency components are contributed mainly by the current field and higher vertical frequency components are contributed partly by the current field and partly by the adjacent fields. This approach has the benefit that vertical resolution is enhanced at low temporal frequencies by the contribution from the adjacent fields, however, for higher temporal frequencies the contribution from the adjacent fields actually reduces the vertical resolution. Although this last method does not suffer from motion blur artifacts, it has been found that further enhancement of the image is possible. According to the present invention, a method is provided whereby the apparent sharpness of moving detail may be enhanced in both spatial dimensions. 
     The following patents are relevant as prior art relative to the present invention: 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 U.S. Pat. Documents 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 4,789,893 - Weston 
                 Dec 6/88 
                 Interpolating lines of video 
               
               
                   
                   
                 signals 
               
               
                 3,920,889 - Connor 
                 Nov 18/75 
                 Method and apparatus for 
               
               
                   
                   
                 crispening video signals by 
               
               
                   
                   
                 the use of temporal filters 
               
               
                 4,792,854 - Glenn 
                 Dec 20/88 
                 Apparatus for temporally 
               
               
                   
                   
                 processing a video signal 
               
               
                 5,227,883 - Dischert et al. 
                 Jul 13/93 
                 Method and apparatus for 
               
               
                   
                   
                 motion aperture correction 
               
               
                   
               
             
          
         
       
     
     SUMMARY OF THE INVENTION 
     According to the present invention, a method and apparatus are provided for converting an interlaced video image into a progressively scanned image by way of vertical temporal processing while enhancing the apparent sharpness of moving edges. According to the invention, missing lines are generated as a weighted average of neighboring lines from both the current and adjacent fields. The weightings are chosen such that the contribution from the current field is greater than unity, the combined contribution from all adjacent fields is negative and the total combined contribution from all fields is unity. In this way, the response to high temporal frequencies is boosted which has the effect of increasing the perceived sharpness of moving edges. Whereas if the contribution of the adjacent fields were to sum to zero as in some prior art approaches, then the interpolated lines could be merged with the unmodified lines of the current field without certain unwanted motion artifacts, in the present invention it becomes necessary to replace the lines of the current field with a weighted average of neighboring lines from both the current and adjacent fields such that the total contribution from each field is the same as the total contribution from each field used to generate the missing lines. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A description of the prior art and of the preferred embodiment of the present invention is provided hereinbelow with reference to the following drawings in which: 
     FIG. 1A is a schematic representation of how missing video lines may be derived by means of vertical interpolation, according to the prior art. 
     FIG. 1B is a schematic representation of how missing video lines may be derived by means of temporal interpolation, according to the prior art. 
     FIG. 1C is a schematic representation of how missing video lines may be derived by means of combined vertical and temporal interpolation in which the total contribution from the adjacent fields is substantially positive, according to the prior art. 
     FIG. 1D is a schematic representation of how missing video lines may be derived by means of combined vertical and temporal interpolation in which the total contribution from the adjacent fields is substantially zero, according to the prior art. 
     FIG. 2 is a plot of the temporal frequency responses of the various prior art methods shown in FIGS. 1A-1D. 
     FIG. 3A is a schematic representation of how missing video lines are derived using one of the preferred embodiments of the present invention. 
     FIG. 3B is a schematic representation of how the lines of the current field are modified using one of the preferred embodiments of the present invention. 
     FIG. 4A is a schematic representation of a current input video field in which exists a vertically oriented edge. 
     FIG. 4B is a schematic representation of the next input video field in which the vertically oriented edge has moved one pixel to the right. 
     FIG. 4C is a schematic representation of combining the missing video lines which are generated using the method of FIG. 3A with the unmodified lines of the current field. 
     FIG. 4D is a schematic representation of combining the missing video lines which are generated using the method of FIG. 3A with the lines of the current field which have been modified using the method of FIG.  3 B. 
     FIG. 5A is a schematic representation of how missing video lines are derived using a second embodiment of the present invention. 
     FIG. 5B is a schematic representation of how the lines of the current field are modified using a second embodiment of the present invention. 
     FIG. 6 is a plot of the temporal frequency responses of the methods shown in FIG.  3 A and FIG.  3 B and in FIG.  5 A and FIG.  5 B. 
     FIG. 7 is a block diagram of an apparatus for implementing the method according to the preferred embodiment of the present invention. 
     FIG. 8 is a block diagram of an apparatus according to an alternative but equally preferable embodiment for implementing the method of the present invention and for performing arbitrary vertical scaling. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, the specific case of line doubling is used to illustrate the methods of the prior art and of the present invention. However, the techniques described may be extended to other ratios of output lines to input lines including non-integer multiples. 
     For the case of line doubling, progressively scanned frames are typically produced by combining the lines of the current field with interpolated lines which fall spatially between the lines of the current field. The interpolated or ‘missing’ lines have, in the prior art, been generated using a variety of techniques. Turning to FIG. 1A, an example is shown of how missing video lines may be generated using vertical interpolation. In this example, the missing lines are generated as the average of the lines above and below each missing line. The vertical frequency response associated with this method rolls off faster than with some other methods described below, resulting in a loss of vertical detail. The temporal frequency response associated with this method is flat since there is no contribution from any field other than the current one. For completeness, the temporal response is illustrated in FIG. 2 by the flat line identified by reference A. 
     In FIG. 1B, an example is shown of how missing video lines may be generated using temporal interpolation. In this example, the missing lines are generated as the average of the lines in the two adjacent fields which are spatially coincident with each missing line. The vertical frequency response associated with this approach is flat when there is no motion between fields and thus all vertical detail is preserved. When motion is present, however, significant blur will occur since the output is produced by averaging samples which represent the image at different points in time. This motion blur corresponds with a roll off in the temporal response as illustrated by the line B in FIG.  2 . 
     FIG. 1C shows an example of how vertical and temporal interpolation may be combined. In this example, the missing lines are generated as the average of the lines in the two adjacent fields which are spatially coincident with each missing line and of the current lines above and below. This method represents a compromise between purely vertical and purely temporal interpolation. As a result, this method has some of the benefits of both approaches but also some of the disadvantages of both, such as motion blur. Since this method involves processing in both the spatial and temporal dimensions, the frequency response is also two dimensional. A plot of the temporal frequency response corresponding to zero vertical frequency is shown as line C in FIG.  2 . As with the previous method, the occurrence of motion blur is suggested by the roll off in the temporal response. 
     The method shown in FIG. 1D is a somewhat more sophisticated approach to combining the benefits of vertical and temporal interpolation as described in U.S. Pat. No. 4,789,893 (Weston). In this approach, the missing lines are generated as a weighted average of neighboring pixels in the current and adjacent fields. The weightings are chosen such that low vertical frequencies are contributed mainly from the current field and high vertical frequencies are contributed partly from the current field and partly from the adjacent field. In order for this to be satisfied and in order to minimize motion blur, the weights of the adjacent fields sum to zero while the weights of the current field sum to unity. At low temporal frequencies, the vertical response owing to the contribution from the current field is boosted by the contribution from the adjacent fields, hence vertical detail is enhanced. At high temporal frequencies corresponding to vertical motion, the vertical response is reduced by the contribution from the adjacent fields. Hence, vertical detail is reduced. A plot of the temporal frequency response corresponding to zero vertical frequency is shown by the line D in FIG.  2 . The temporal response in this case is flat (i.e. equivalent to the flat line response A in FIG. 2) since the total contribution from the adjacent fields is zero and such that motion blur does not occur. 
     In all of the prior art methods described above, the temporal response is either flat and therefore relatively free from motion artifacts, or rolls off at higher temporal frequencies resulting in motion blur artifacts. Subjective testing has shown that increased sharpness of moving image detail is often found to be more pleasing to viewers than images without such enhanced sharpness. According to the present invention, the apparent sharpness of moving edges is enhanced by boosting the response at higher temporal frequencies. FIG. 3A shows how the missing video lines are generated according to the preferred embodiment of the present invention. The weightings are chosen such that appropriate contributions are derived from each field in order to give the desired vertical frequency response. The temporal response is achieved by selecting the weightings such that the contribution from the current field is greater than unity, the contribution from the adjacent field is negative and the combined contribution from both fields is unity. The fact that the combined contribution from both fields sums to unity ensures that the average luminance of the image is preserved. 
     As a consequence of the fact that the total contribution from the current field does not sum to unity, it is not desirable to simply merge the interpolated missing lines with the lines from the current field as is possible with the system shown in FIG.  1 D. FIG. 4 illustrates the problem which arises if such line merging were to be performed. FIG. 4A shows a field of interlaced video within which exists a vertically oriented edge. FIG. 4B shows the subsequent field in which the vertically oriented edge has moved one pixel to the right. FIG. 4C shows the result of combining the interpolated missing lines which are derived by the method depicted in FIG. 3A with the lines of the current field. It will be noted that the pixel values shown in FIG. 4C will result in the appearance of a serrated edge. When viewed as a moving sequence, this artifact detracts substantially from the perceived image quality. FIG. 4D shows the result of combining the interpolated missing lines which are derived using the method depicted in FIG. 3A, with the lines of the current field which have been modified according to the method shown in FIG.  3 B. It can be seen that the serration effect does not occur when this technique is employed. The weightings used to modify the lines of the current field are chosen such that the total contribution from each field is substantially equal to the corresponding contribution from each field which was used to generate the interpolated missing lines. In this way, an equal amount of temporal boosting is applied to both the missing and current lines. 
     The methods shown in FIG.  3 A and FIG. 3B use contributions from the current input video field and the next occurring adjacent field to produce an output frame. Although this method may produce acceptable results, at times it may be desirable to use contributions from the current field and the adjacent earlier field. It may also be desirable to use more than two input video fields to produce an output frame in order to achieve the desired temporal response. FIG. 5A shows how the missing video lines are generated in a second embodiment of the present invention in which contributions are taken from three input video fields to produce an output frame. As in the first embodiment, the weightings are chosen such that appropriate contributions are derived from each field in order to give the desired vertical frequency response. The temporal response is achieved by selecting the weightings such that the contribution from the current field is greater than unity, the combined contribution from the two adjacent fields is negative and the total contribution from all fields is unity. FIG. 5B shows how the lines of the current field are modified in the second embodiment of the present invention in order to avoid an effect similar to that illustrated in FIG. 4C in which vertically oriented edges which move horizontally may appear serrated. As in the first embodiment, the weightings used to modify the lines of the current field are chosen such that the total contribution from each field is substantially equal to the corresponding contribution from each field which was used to generate the interpolated missing lines. 
     The line A in FIG. 6 shows the temporal frequency response corresponding to zero vertical frequency for the combined methodologies in FIG.  3 A and FIG.  3 B. The response increases for higher temporal frequencies and thus the apparent sharpness of moving edges is enhanced regardless of whether the motion is in the horizontal direction or vertical direction or a combination of both. The line B in FIG. 6 shows the temporal frequency response corresponding to zero vertical frequency for the combined methodologies in FIG.  5 A and FIG.  5 B. The responses illustrated by line A and line B are only slightly different and may appear indistinguishable. Line A is the higher of the two. 
     FIG.  3 A and FIG. 3B show how the method of the present invention may be applied to a single video channel. It is common, however, for a colour video signal to be split into several components and represented using multiple video channels. For instance, video systems often utilize a single channel to carry luminance information and two other channels to carry chrominance information. In such a case, it may be desirable to apply different levels of temporal boosting in the luminance and chrominance channels in order to avoid colour aberrations. 
     The method of the present invention which is shown in FIG.  3 A and FIG. 3B can be implemented using an arrangement of storage elements, multiplexers, and arithmetic elements as shown in FIG.  7 . As each field of an interlaced video signal  1  arrives at the input to a memory controller  2 , it is written into a memory  3 . Concurrently with the operation of writing data into memory, data from the previous field is retrieved from memory and applied to the input of line store element  6  and multiplier  19  while data from the most recently received field is routed through the memory controller and applied to the input of line store element  4  and multiplier  16 . In an alternative mode of operation, data from the most recently received field may be retrieved from memory and applied to the input of line store element  4  and multiplier  16  rather than being routed directly through the memory controller. As new data is stored in line store  4 , the previously stored data is output and applied to the input of line store  5  where it replaces the data which was previously there. In a similar fashion, the data in line store  6  replaces the data in line store  7  which replaces the data in line store  8 . Data which is placed into any line store will remain there for one input line period which is equal to two output line periods before it is replaced by new data. During the first output line period, a selector signal S causes multiplexers  9 - 15  to select a first set of coefficients which are connected to the A side of each multiplexer and which are used to generate the interpolated missing lines. During the second output line period, the selector signal causes an alternate set of coefficients which are connected to the B side of each multiplexer to be selected, which are used to modify the existing lines. The selector signal S may be generated by a controller (not shown) in a well known manner. Regardless of which set of coefficients is selected, the coefficients which appear at the outputs of multiplexers  9 - 15  are applied to one of the inputs of each of multipliers  16 - 22 , respectively. The other input of each multiplier  17 ,  18  and  20 - 22  is driven by the outputs of line stores  4 - 8 , respectively. When the outputs from multipliers  16 - 22  are summed together using adder  23 , the output  24  is the desired progressively scanned video signal. 
     The foregoing description of a preferred embodiment of the system of the present invention is not restricted to the specific best mode set forth herein. Indeed, the flexible nature of software programming is such that the broad concepts of the invention may be implemented using software rather than hardware as set forth herein. Also, as indicated above, the principles of the invention are not limited to the specific case of line doubling. An obvious extension of this method would be to combine the method of the present invention with additional operations such as scaling in order to produce a greater or lesser number of lines. Furthermore, such scaling may include scaling by a non-integer multiple and could either be implemented using a separate processing stage or could be combined with the method of the present invention and implemented using a single composite structure. In the latter case, the coefficients may be split into a number of phases corresponding to an equal number of spatial regions which fall between the input video lines. When an output line must be generated which falls within a given spatial region, the coefficient set which corresponds to that phase is selected. As taught by the present invention, each set of coefficients corresponding to a unique phase should satisfy the property that the contribution from the current field is greater than unity, the combined contribution from all adjacent fields is negative and the total contribution from all fields is unity. FIG. 8 shows an apparatus for implementing the above method in which coefficients are split into phases to enable interpolation at arbitrary positions. The apparatus is similar to that shown in FIG. 7 except that multiplexers  9 - 15  have been replaced by coefficient storage banks  25 - 31 . All of the phases for each filter tap are stored within an associated local coefficient storage bank. Depending on the desired spatial position of the output video line, the phase selector signal PHASE which is generated by a controller (not shown) in a well known manner, is used to address the appropriate set of coefficients. 
     The weightings shown in FIG.  3 A and FIG. 3B comprise only one of the possible embodiments of the present invention and many other variations are possible. For instance, the weightings could be chosen to result in a greater or lesser amount of temporal boosting. Similarly, the weightings could be adjusted to modify the frequency response in the vertical dimension to achieve a desired characteristic. Furthermore, additional operations on the data which might otherwise be performed separately, could be performed by adjusting the coefficients so as to combine the additional operation with the method of the present invention. The adjustment of contrast is an example of an additional operation which could easily be implemented with only minor modification to the method of the present invention. Such an operation would be equivalent to scaling of the coefficients by a real number. In addition, contributions from a different number of lines in each field other than the number shown in the illustrative examples herein above could also be used. Among other variations, it would also be possible for contributions to be taken from a greater number of fields than shown in the examples. Any of the above variations are believed to be within the scope of this invention as defined by the claims appended hereto.