Abstract:
In a method and a circuit for detecting and concealing video signal errors caused by vertically extending film scratches during television film scanning, an error signal is derived at error locations generated by disturbed pixels and is used in the form of a control signal for concealing errors in the video signal. The error signal is generated only when an adjustable, locally bounded scratch width in the horizontal direction is not exceeded and when each scratch essentially extends vertically, and when the scratch length substantially corresponds to the film frame height. Finally, the disturbed locations in the video signal are replaced by the corresponding low-pass-filtered video signal with the aid of the error signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method of detecting and concealing video signal errors, caused by vertically extending film scratches, during television film scanning, in which an error signal is derived at error locations generated by disturbed pixels, and is used in the form of a control signal for concealing errors in the video signal. 
     Older, frequently used films often show two kinds of errors in the image area: dirt and dust on the surface of the film, and scratches in the running direction of the film, referred to as running scratches, which are caused by mechanical contact between the film-guiding elements and the image area. These errors, appearing as white spots or scratches, are particularly disturbing in electronic scanning and reversal of negative films. 
     2. Description of the Related Art 
     German Patent Application P 43 43 095.3 describes a method of concealing dust and scratches in connection with electronic film scanning. By means of temporally adaptive median filtering, film dust and coarse film dirt, as well as scratches accidentally spread across the film image, can be effectively filtered out in so far as they do not occur at the same location in consecutive film frames. 
     SUMMARY OF THE INVENTION 
     In contrast, it is an object of the present invention to provide a method of the type described in the opening paragraph in which stationary, vertically extending scratches which may also extend across a plurality of film frames can be effectively concealed. 
     This object is solved in that the error signal is generated only when an adjustable, locally bounded scratch width in the horizontal direction is not exceeded and when each scratch essentially extends vertically, and when the scratch length substantially corresponds to the film frame height, and in that the disturbed locations in the video signal are replaced by the corresponding low-pass-filtered video signal with the aid of the error signal. 
     The method according to the invention, has the advantage that both fine scratches on the film surface and deep scratches penetrating the color coating of the film can be concealed in such a way that no disturbances are visible at these locations in the image displayed. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a block diagram showing the circuits for performing the method according to the invention; 
     FIG. 2 shows the median filter used as a horizontal low-pass filter in FIG. 1; 
     FIG. 3 shows time diagrams to explain the scratch concealment; 
     FIG. 4 shows the horizontal high-pass filter of FIG. 1; 
     FIGS. 5a-5c show time diagrams of the signals occurring in FIG. 4; 
     FIG. 6 shows the vertical low-pass filter with line sorter of FIG. 1; 
     FIG. 7 shows time diagrams of the signals occurring in FIG. 6; 
     FIGS. 8a and 8b show a first graphic representation of the mode of operation of the vertical low-pass filter shown in FIG. 6; 
     FIGS. 9a and 9b show a second graphic representation of the mode of operation of the vertical low-pass filter shown in FIG. 6; 
     FIG. 10 shows the vertical integrator circuit occurring in FIG. 1; 
     FIG. 11 shows the comparator and signal extension circuits of FIG. 1; and 
     FIG. 12 shows time diagrams of the signals occurring in FIG. 11. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a circuit for performing the method according to the invention. The upper path represents the main signal path in which the preferably digital component signals Y, Cr, Cb supplied via input 1 are processed in an identical manner. These signals are first applied via a delay device 2 to a horizontal low-pass filter 3 and to a further delay device 4 used for delay compensation for the filter 3. The output signals of the filter 3 and of the delay device 4 are then each applied to an input of a multiplexer 5, while the video signals which are free from error locations can be taken off its output. 
     In the lower path, a control signal is derived from the luminance signal Y in a plurality of processing steps, which control signal marks the horizontal location of one or more film scratches. At these marked locations, the disturbed pixels are replaced in the main signal path by the output values of the low-pass filter 3 operating in the horizontal direction. The horizontal low-pass filter 3 may be implemented as a transversal filter or, according to FIG. 2, as a median filter. 
     The median filter 3, according to FIG. 1 and shown in FIG. 2, comprises eight pixel delay devices 21 having nine taps which are connected to the nine inputs of a median selector 22. The horizontal median-filtered video signal can then be taken off the output 23 of the median selector 22. The length of the filter or the required number of filter taps depends on the width of the scratches to be expected. With N=9 taps up to (N-1)/2=4 pixel-wide disturbances can be suppressed by means of the median filter. At a scanning frequency of 13.5 MHz, this corresponds to a duration of 300 ns which is quite sufficient in practice. It is sensible to adapt the filter length to the maximum scratch width so as to minimize the horizontal loss of resolution in the surroundings of the scratch. 
     FIG. 3 shows time diagrams, for example, for the luminance signal Y so as to elucidate the scratch concealment. This signal Y is disturbed, for example, in the center of the line, by a scratch SCR which is two clock pulses wide, which is detected by the control signal with a &#34;1-pulse&#34; generated in the scratch recognition circuit described below. By means of the multiplexer 5 in FIG. 1, the output values of the horizontal low-pass filter 3 are added at this location, which low-pass filter is implemented in this embodiment as a median filter having five taps. The output signal Y is free from disturbance at this location without details being lost in other areas. 
     The following description will deal with the way in which the control signal is derived, in which first the luminance signal Y in FIG. 1 is applied to a horizontal high-pass filter 6 so as to extract all perpendicular structures having a small pulse width from the video signal. The vertical extension of the scratch is determined by means of the line sorter 7 and the vertical low-pass filter 8. Subsequently, the mean value is formed for all lines of a frame by means of a vertical integrator 9 so as to extract the scratch information. In this way, residual image information components and superimposed noise can be averaged out. Positive or negative threshold values for the occurrence of white or black scratches can be defined in the subsequent comparator circuit 10. The output signal of the comparator circuit is finally applied to a signal extension circuit 11, while the control signal for controlling the multiplexer 5 can be taken off its output. 
     Since high-pass filtering is known to be possible by subtraction of a complementary low-pass-filtered input signal from an unfiltered input signal (having the same delay), a median filter having a high-pass character and realized in this way is shown in FIG. 4. In contrast to linear transversal or recursive filtering, which can be mathematically described with a transfer function, a high-pass filter having a median character can only be realized in this way. The luminance signal Y is thus applied via an input 24 to a delay device 25 and to a median filter 26 whose output signals are applied to a subtraction circuit 27. A median-filtered luminance signal can thus be taken off output 28. 
     FIGS. 5a-5c show the signals a, b, c processed in accordance with FIG. 4. As can easily be seen, the advantage of median high-pass filtering is apparent in this case because the difference signal c represents an exact image of the scratch without any pulse distortion. The first criterion for a scratch, namely the pulse width, is checked with this high-pass filtering. If the median filter window covers five pixels, pulses having a maximum width of two pixels are passed unchanged. 
     The second feature of a scratch is its essentially perpendicular extension. In accordance with FIG. 1, this criterion is checked by means of the subsequent line sorter 7 in connection with the vertical low-pass filter 8. In FIG. 6, a median filter 31 having three taps of two line delay devices 32, 33 is used in connection with a median selector 34 which is preceded by three memory banks 35, 36, 37 for sorting the lines. The upper third part of a frame is written into the first memory bank 35, the middle third is written into the second memory bank 36 and the lower third of the frame is written into the third memory bank 37. During reading, the memory bank is cyclically changed after each line. The median filter 31 thus receives the lines in the sequence 1, 97, 193, 2, 98, 194 etc. The delays between the three taps of the median filter 31 thus each cover a third part of the active frame. 
     The time diagrams in FIG. 7 show that a real scratch (that is, a perpendicular scratch) can pass the median filter 31 unhindered, whereas a scratch-like image detail BD is already completely suppressed because of its slightly oblique position so that only the error signal SCR can be taken off the output 38 of the median filter 31. 
     The third feature of a potential scratch, namely the longitudinal extension is also checked by means of the vertical median filter 31. Since a median filtering operation represents the result of a majority decision, to a certain extent, narrow, perpendicular structures are shortened if their length is smaller than half the frame height, whereas they are lengthened if their length exceeds half the frame height. 
     FIGS. 8a and 8b graphically shows this behavior. Structures which are shorter than one-third of the frame height are completely suppressed, see the first four perpendicular stripes in FIG. 8a, whereas structures which are longer than two-thirds of the frame height are expanded throughout the length, see the last four perpendicular stripes. 
     In a median filtering operation with five taps, the selectivity between full expansion or full compression can be improved, cf. FIGS. 9a and 9b. In FIG. 6, the three memories 35, 36, 37 for sorting the lines should then be replaced accordingly by five memories, each time with the capacitance of one-fifth of a frame. Another possibility for improving the selectivity is a repeated use of the filter structure shown in FIG. 6. 
     After the horizontal high-pass filtering and the vertical low-pass filtering operations for extracting the scratch information, the mean value is now formed for all lines of a frame by means of the vertical integrator 9 shown in FIG. 10. Residual image information and superimposed noise can thereby be averaged out. An accumulator, comprising a line delay device 41 which is fed back to an input of a summing circuit 42 and whose other input conveys the output signal of the vertical low-pass filter 8 shown in FIG. 1, is set to zero by means of the pulse V-Clear (having the duration of one line) at the start of a frame via a multiplexer 43. The intermediate result of the integration is continually stored in the line memory 41 and transferred to a second line memory 44 at the end of a frame. A second multiplexer 45 causes the result of the accumulation to recirculate in the memory 44 for the duration of a frame, while the accumulator (41, 42, 43) is performing the next integration. 
     The output signal of the vertical integrator 9 is applied to a comparator circuit shown in FIG. 11. This comparator circuit comprises two comparators 46, 47, comparator 46 of which receives a positive threshold value WT for white scratches and comparator 47 receives a negative threshold value BT for black scratches. The two comparator signals are combined via an OR circuit 48 and applied to a subsequent signal extension circuit 49. This circuit comprises two pixel delay devices 51, 52 whose three taps are combined via a further OR circuit 53 so that the control signal derived from the scratch information for controlling the multiplexer 5 of FIG. 1 can be taken off output 54. 
     FIG. 12 shows the output signal d of the vertical integrator 9 with respect to time (one line). The signal mean value, which is around zero, has two peaks w and b which are caused by a white and a black scratch, respectively. These two peaks w and b are compared with the threshold values WT and BT in the comparators 46 and 47 and combined via the OR circuit 48 so that corresponding pulses e are produced at the output of the comparator. 
     This comparator output signal with the two pulses w and b is also applied to the signal extension circuit 49, while the pulses w&#39; and b&#39; thus extended can be taken off the output of this circuit as a control signal. Here it is readily evident that the bases of the scratch pulses are certainly detected by the control signal in this way. Without this extension circuit, the threshold values would have to be so low that there would be a risk of erroneous triggering due to residual image parts or noise at a small contrast of the scratches.