Abstract:
The present invention relates to converting a video signal comprising input image samples conforming to an interlace scanning lattice into a video signal comprising output image samples conforming to a progressive scanning lattice. First and second temporal filters receive and divide the input image samples into first and second temporal sub-bands. First and second low-pass vertical filters vertically filter each temporal sub-band such that the higher temporal sub-band is vertically filtered to a greater degree than the lower temporal sub-band. The filtered image samples from each sub-band are combined to form a filter output signal and a re-sampler re-samples the filter output signal to form the output image samples.

Description:
BACKGROUND OF THE INVENTION 
     1). Field of the Invention 
     The present invention relates to converting a video signal comprising input image samples conforming to an interlace scanning lattice into a video signal comprising output image samples conforming to a progressive scanning lattice. 
     2) Description of the Related Art 
     Image transmission systems require an initial generation of a multidimensional signal which is a function of three independent variables (x,y,t) in space and time. The initial signal is sampled and formatted so as to form a one-dimensional signal suitable for transmission. The sampling operation can be described by a sampling lattice which expresses the sample locations as a linear combination of sample indices which are the horizontal, temporal and vertical indices respectively. The two most common lattices used in transmitting video signals are the progressive scanning lattice and the interlace scanning lattice. 
     Considerable attention has been paid to providing higher quality pictures for television picture transmissions and the possibility for conversion between different sampling lattices. Such conversion requires attention to the conversion of video signals which employ an interlace scanning lattice into video signals which conform to a progressive scanning lattice. 
     A problem with such conversion arises because of the generation of unwanted image artefacts. 
     SUMMARY OF THE INVENTION 
     It is an aim of the present invention to provide improved filtering in the conversion of a video signal from an interlace scanning lattice to a progressive scanning lattice. 
     According to the present invention there is now provided a method of converting a video signal comprising input image samples conforming to an interlace scanning lattice into a video signal comprising output image samples conforming to a progressive scanning lattice, the method comprising the steps of: applying the input image samples to first and second temporal filters to divide the input image samples into first and second temporal sub-bands; vertically filtering each temporal sub-band by means of a low-pass vertical filter; the higher temporal sub-band being vertically filtered to a greater degree than the lower temporal sub-band; and, recombining the filtered image samples from each sub-band to form a filter output signal. 
     Further according to the present invention, there is provided apparatus for converting a video signal comprising input image samples conforming to an interlace scanning lattice into a video signal comprising output image samples conforming to a progressive scanning lattice, the apparatus comprising: first and second temporal filters to receive and divide the input image samples into first and second temporal sub-bands; first and second low-pass vertical filters to vertically filter each temporal sub-band such that the higher temporal sub-band is vertically filtered to a greater degree than the lower temporal sub-band; and, recombining means to recombine the filtered image samples from each sub-band to form a filter output signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
     FIG. 1 shows interlaced and progressive scanning lattices; 
     FIG. 2 shows spectra associated with the interlaced scanning lattice of FIG. 1; 
     FIG. 3 shows the effect of motion on the spectra of FIG. 2; 
     FIG. 4 shows a video signal transmission system embodying the present invention; 
     FIG. 5 shows a filter configuration used in the transmission system of FIG. 4; 
     FIGS. 6,  7  and  8  show filter elements used in the filter configuration of FIG. 5; 
     FIGS. 6A and 7A show alternatives to the filter elements of FIGS. 6 and 7 respectively; 
     FIGS. 9 and 10 show the frequency responses of the filter configuration of FIG. 5; 
     FIG. 11 shows an adaptive form of the filter configuration of FIG. 5; 
     FIG. 11A shows filter coefficients applicable to the filter configuration of FIG. 11; 
     FIGS. 12 and 13 show gain control circuits used in the adaptive filter configuration of FIG. 11; 
     FIG. 12A shows a modification of the gain control circuit of FIG. 12; 
     FIG. 14 shows the filter response of a vertical low pass filter included in the filter configuration of FIG. 11; and, 
     FIG. 15 shows the overall frequency response of the transmission system of FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the upper part of FIG. 1 is shown a scanning lattice for an interlaced video signal and in the lower part of FIG. 1 is shown a scanning lattice for the corresponding progressively scanned video signal. As is well known, each field consists of a raster scan of lines of image samples of the video signal. In FIG. 1, each line is represented by one filled dot and a field is represented by a vertical column of filled dots along the y axis and successive fields are spaced along the time axis, t. For the interlaced video signal seen at the top of FIG. 1, alternate fields have image samples which are displaced in the vertical direction relative to the image samples of the adjoining fields, the spacing interval being represented by unfilled dots in FIG.  1 . 
     The frequency spectra of the interlaced sequences for a stationary image are shown in FIG.  2 . The spectrum of each sequence is confined approximately to a plane. In the conversion from an interlaced to a progressive scanning format it is desirable to remove from the video signal the spectral component  10  introduced by the conversion. 
     The spectrum of a video signal representing a moving image differs from that of a stationary image and is a function of the motion in the scene being depicted. The effect of motion is seen in FIG.  3 . The spectral components which need to be removed from the video signal in the conversion from an interlaced to a progressive scanning format will therefore be a function of the motion in the scene being depicted. 
     In FIG. 4, an image transmission system includes a camera and image sampler to form an input video signal  20  which is described by horizontal and vertical indices of values X and Y and a temporal index of  50 . The values X and Y are intended to demonstrate that the system of FIG. 4 is not limited to any specific selection of horizontal or vertical index. Furthermore, the system of FIG. 4 is not limited to a temporal index of 50 but it is convenient to describe the operation of the system by reference to this specific number. 
     The signal  20  is comprised of a stream of image samples which conform to a progressive scanning lattice. A resampler  21  receives the input video signal  20  and subjects the signal  20  to a resampling operation to form a video signal in which the horizontal index is 720 and the vertical index is 576. In other words, there are 50 fields per second in the video signal from the resampler  21 , with 720 samples per line and 576 lines per field. The resampled signal from the resampler  21  is passed to a pre-filter  22  to filter the resampled signal before passing the resampled signal to an interlace module  23 . The interlace module  23  converts the progressive sequence from the pre-filter to an interlaced sequence which has half the data rate. The interlaced sequence is transmitted through an interlace transmission channel having a transmitter end  24  and a receiver end  25 . The transmission channel is configured to transmit signals which conform to the well-known MPEG standard. 
     The receiver  25  passes the transmitted video signal to a zero padding module  26  in which the received signal is padded with zeros. As indicated diagrammatically, each interval between adjacent lines of each field is padded with a zero so that in each field, alternate lattice positions have a zero value. The padded signal is applied to a post-filter  27  which filters the padded signal in a manner which will be explained. The filtering process changes the values of all the image samples so that the filtered signal includes image sample values in all the lattice positions of the same progressive sequence that was provided to the interlace module  23 . The filtered signal is re-sampled by a resampler  28  to recover the original signal having the spatial indices of X and Y and a temporal index of 50. 
     FIG. 5 shows a filter configuration used for the post-filter  27 . As will be explained later, this configuration can also be used for the pre-filter  22 . The filter  27  receives the stream of image samples from the zero padding module  26  at an input terminal  30 . The stream of image samples, including the padding zeros, is divided into a temporal low-pass band by means of a filter element  31  and a temporal high-pass band by means of a filter element  32 . The temporal low-pass band is filtered by means of a vertical low-pass filter element  33  and the temporal high-pass band is filtered by a vertical low-pass filter element  34 . The output from the channel including the filter elements  31  and  33  is combined with the output from the channel including the filter elements  32  and  34  to yield the progressive sequence to be applied to the resampler  28  of FIG.  4 . 
     In FIG. 6, the temporal low-pass filter element  31  is shown in further detail. The filter element includes an input connection  36  which applies the stream of input image samples to a delay element  37  which subjects the image samples to a delay equal to one field interval. The input connection  36  is also connected to pass 0.25 of the value of each input image sample directly to an output line  38 . The delay element  37  is connected to supply the delayed image sample values to a second delay element  39  which also subjects the image samples to a delay equal to one field interval. In addition the output from the delay element  37  is connected to pass 0.5 of the value of each image sample to the output line  38 . Finally, the delay element  39  is connected to pass 0.25 of the value of each image sample to the output line  38 . The coefficients (0.25, 0.5, 0.25) which determine the proportion of each image sample passed to the output line  38  are chosen to characterise the filter of FIG. 6 as a low-pass temporal filter. Low frequency temporal changes pass through the filter so that the original image samples, represented by the filled dots of FIG. 1, pass through substantially unchanged and the zero padded image samples are modified in value towards an average of the temporally adjacent samples. 
     FIG. 6A shows an alternative low-pass filter element in which the two delay elements  37  and  39  are replaced by a single delay element  37   a  which delays the image samples by a delay equal to one field interval. The input connection  36  is connected to pass 0.5 of the value of each input image sample directly to the output line  38  and the delay element is connected to supply 0.5 of the value of each image sample to output line  38 . 
     FIG. 7 shows the temporal high-pass filter element  32  which includes an input connection  40 , a delay element  41 , an output line  42 , and a delay element  43 . The delay elements  41  and  43  subject the image samples to a delay equal to one field interval. The configuration of the high-pass temporal filter in FIG. 7 is similar to the configuration of the low-pass filter element of FIG.  6 . It will be seen from FIG. 7 that the proportions of the image sample values passed to the output line  42  from the input connection  40 , the delay element  41  and the delay element  43  are determined by the coefficients −0.25, 0.5 and −0.25 respectively. These coefficients characterise the filter as a high-pass temporal filter. 
     FIG. 7A shows an alternative high pass filter in which the two delay elements  41  and  43  of FIG. 7 are replaced by a single delay element  41   a  which delays the image samples by a delay equal to one field interval. The filter of FIG. 7A has the coefficients of −0.5 and 0.5 as shown. 
     In FIG. 8, the configuration of the vertical low-pass filter elements  33  and  34  is shown in further detail. The filter element includes an input connection  45  connected to supply image sample values to a series of delay elements  46 . The number of the delay elements  46  may be varied to alter the frequency response. An output line  48  is connected to receive a proportion of the value of each input sample from the input connection  45  and each of the delay elements  46 . The proportions of the values of the image samples received by the output line  48  are determined by coefficients a N , a N−1 , . . . a 1  where N represents the number of the delay elements  46 . These coefficients are chosen to define the filter as a low-pass vertical filter. The coefficients selected for the filter element  33  in FIG. 5 differ from the coefficients selected for the filter element  34  in FIG. 5 so as to increase the degree of filtering in the channel including the filter  34  as compared to the degree of vertical filtering in the channel including the filter  33 . 
     FIG. 9 shows the frequency response of the post-filter  27  of FIG.  4 . The pass band of the filter is shown as a shaded area overlying the spectra already described with reference to FIG.  2 . The shaded area has a temporal high-pass band adjoining a temporal low-pass band. It will be seen that the high-pass temporal band is filtered to a greater degree in the vertical direction than the low-pass temporal band so that the pass band has a step-shape as depicted in FIG.  9 . The positioning of the stepped pass band in relation to the spectra  10  is effective to exclude the spectra  10 . The overall gain of the filter  27  is  2  within the passband and  0  within the spectral region to be excluded. 
     The pre-filter  22  has a pass-band which is similar to that of the post-filter  27  and is shown in FIG.  10 . In the case of the pre-filter  22 , however, the overall gain is approximately 1.0 for use with the progressive lattice input from the resampler  21 . 
     The pre-filter  22  and the post-filter  27  are of substantially the same configuration as has been described. In either case, the number of temporal sub-bands into which the input stream of image samples is divided is two. The reason is to provide the step in the frequency response as shown in FIGS. 9 and 10. The number of temporal sub-bands may be increased beyond two in either of the filters  22  and  27  by increasing the number of channels in the filter through which the image samples pass. If the temporal sub-bands number three for example, the frequency response would include a double step which again would be designed to exclude the unwanted spectral components  10  already referred to. 
     The pre-filter  22  and the post-filter  27  have each been shown and described as non-adaptive filters. Either of these filters may however be constructed as an adaptive filter as will now be described with reference to FIGS. 11 to  14 . In FIG. 11, the input terminal  30  and the filter elements  31  to  34  are shown in the same configuration as in FIG.  5 . The input terminal  30  is additionally connected to a vertical low-pass filter element  50  and a gain control  51 . The output from the filter elements  33 ,  34  and  50  are combined in a ratio of k/k−1 where k is a gain control quantity which is generated by means of the gain control  51 . The filter response of the low-pass vertical filter  50  is shown in FIG. 14 in relation to an input video signal in which the image samples conform to an interlace scanning lattice. 
     The filter coefficients applicable to the filter configuration of FIG. 11 are shown in FIG.  11 A. The low pass coefficients in the top line of FIG. 11A are applicable to the low pass filter  31 , the high pass coefficients in the middle line of FIG. 11A are applicable to the high pass filter  32  and the all pass coefficients in the bottom line of FIG. 11A are applicable to the vertical filter  50 . 
     Referring to FIG. 12, the gain control element  51  includes a serial connection of a horizontal low-pass filter element  52 , a vertical low-pass filter element  53 , and a temporal band-pass filter element  54  to produce a filtered signal. The filtered signal is passed through an absolute determination module  55  which provides a signal representing the absolute magnitude of the signal. A field delay element  56  receives the absolute value of the signal and applies a delay equal to one field interval. The delayed signal from the delay element  56  is applied to one input of a comparator  57 . Another input to the comparator  57  is provided by the output from the module  55 . The comparator  57  is effective to determine the maximum of the two supplied inputs. The maximum value is applied to a mapping circuit  58  which maps the values from the comparator  57  onto the required values k from which the gain control circuit. 
     The temporal band-pass filter element  54  is shown in FIG.  13  and includes two serially connected delay elements  59  and  60  between an input terminal  61  and an output line  62 . The coefficients of the values of the image samples passed between the input terminal  61  and the output line  62  are seen to be −1.0, 0 and 1.0. 
     In operation, those spectral components associated with motion pass through the filter elements  52 ,  53  and  54  of the gain control circuit. The absolute values associated with each field of the incoming signal are compared with the next preceding values so as to derive a measure of the magnitude of the motion and thereby to derive the quantity k. In the complete absence of motion in the scene being depicted by the incoming video signal samples, the value of k derived by the mapping circuit is increased so as to provide for an increased proportion of the filter throughput to come by way of the filter elements  31  to  34 . As motion increases, the orientation of the spectrum associated with each line shifts as already described and as seen in FIG.  14 . The pass band appropriate to a scene including high motion is that of the vertical low-pass filter element  50 . The decrease in the value k controls the gain through the filter element  50  to adapt to the spectral components in the input video signal which are associated with motion in the scene being depicted. 
     The gain control circuit of FIG. 12 may be modified as shown in FIG.  12 A. The elements  52  to  55  of FIG. 12 are included in the arrangement shown in FIG.  12 A. However, the field delay element  56  is supplemented by one or more optional additional branches, each of which includes field delay elements. A first such branch includes field delay elements  56   a  and  56   b . The second branch, if included, would have three field delay elements. Each successive option branch has one more delay element than the preceding branch and allows the motion to be detected over a successively larger number of fields. As before, the comparator  57  is effective to determine to maximum of all the inputs supplied to it. 
     The simplicity of the filter structures described make them eminently practical for hardware implementation in contrast to the prior art filters of the non-separable two-dimensional type which are difficult to implement. The algorithms described result in a filter frequency response which does not tile in the frequency domain as would be implied by conventional interpolation algorithms. 
     The pre-filter  22  and the post-filter  27  have frequency responses which are matched to provide an overall frequency response as shown in FIG. 15 in which a guard interval separates the unwanted spectral frequencies from the pass band of the matched filters thereby significantly reducing the level of artefacts.