METHOD AND CHICOT FOR DETECTING
AND CONCEALING VIDEO SIGNAL
ERRORS
BACKGROUND OF THE INVENTION 5
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 10 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 15 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 20 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.
BRIEF DESCRIPTION OF THE DRAWINGS
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 35 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 40 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. 45
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.
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 60 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; 55 FIGS. 5a-Sc 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-l)/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 "1-pulse" 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.
