GB2246265A

GB2246265A - Telecine video signal filtering circuit

Info

Publication number: GB2246265A
Application number: GB9110054A
Authority: GB
Inventors: Gerhard Wischermann
Original assignee: BTS Broadcast Television Systems GmbH
Current assignee: Philips GmbH
Priority date: 1990-05-10
Filing date: 1991-05-09
Publication date: 1992-01-22
Anticipated expiration: 2011-05-09
Also published as: JP3280997B2; JPH04230179A; GB2246265B; DE4014971C2; DE4014971A1; GB9110054D0

Description

A I,.

I- CIRCUIT ARRANGEMENT FOR FILTERING A VIDEO SIGNAL OBTAINED BY SCANNING A FILM.

This invention relates to a circuit arrangement for filtering a video signal obtained by scanning a film.

In the case of electronic film scanning interference occurs as a result of film dust and scratches in addition to the film grain. Although known noisereduction techniques based on time-recursive filtering are able effectively to reduce the noise caused by the film grain, they are unsuccessful in the case of interference caused by dirty or scratched films.

Patent Specification No. GB A2 202 706 describes a video signal processing circuit which is used for suppressing interference pulses and in which ' signals from three successive pictures are supplied to a median selector. Complicated movement detection measures are required so that the successive pictures can be compared in the case of movement.

The object of the present invention is to provide a circuit arrangement for filtering video signals, 1 which reduces interference of the type occurring with dirty and scratched films.

According to the present invention there is provided a circuit arrangement for filtering a video signal obtained by scanning a film, wherein the scanned values of a first picture element and of a plurality of further picture elements are supplied to respective inputs of a median selector,-each of the further picture elements being displaced both in the horizontal direction and in the vertical direction relative to the first picture element.

The invention is able significantly to reduce the interference caused by scratches or dust on films and also may be implemented at low cost.

Embodiments of circuit arrangements according to the present invention will now be particularly described by way of example with reference to-the accompanying drawings, wherein:

Figure 1 is a diagrammatic representation of those picture elements whose scanned values are supplied to a median selector according to a first embodiment of the invention during a scanning (digital sample) period, 1 1 i 1 1 1 1 i 1 i i i j - 1 i 1 1 1, if Figure 2 shows those picture elements whose scanned values are supplied to the median selector according to a further embodiment of the invention, Figure 3 shows those picture elements whose scanning values are supplied to the median selector according to a third embodiment of the invention, and Figures 4 to 7 are block circuit diagrams of different embodiments of the invention.

The same components are given the same references throughout the figures of the drawings.

Referring to the drawings, Figure 1 shows those picture elements of a television-type picture whose scanned (digital sample) values are simultaneously supplied to respective inputs of a median selector in a first embodiment of the invention. These picture elements are referred to as a 11window". In addition to a central picture element M, the window includes four further picture elements A to D, each of which is displaced both in the horizontal direction and in the vertical direction relative to the central picture element M. Thus in the horizontal direction (line) the pictureelement A is located to the left of the central picture element M and in the vertical 4 4 direction the picture element A is located above the central picture element M. Likewise, each of the picture elements B, C and D is also displaced both in the horizontal direction and in the 5 vertical direction from the element M.

In Figure 1 each of the elements A to D is spaced by one picture eleme nt spacing s from the central element M in both the horizontal and vertical directions. However, each element A to D may be spaced from the element M by a multiple of s in the horizontal and/or vertical directions. Preferably, however, the picture elements A to D form a cross with the central picture element M, that is to say, a notional line joining element A to element D and a notional line joining element B to element C will intersect at the central element M. Prferably, further, such cross is symmetrical about the vertical and/or horizontal axis.

It is to be understood that the central picture element M is not fixed but will change to the next consecutive picture element in the video signal on each sample clock or picture element period. However, the elements A to D will likewise change, so preserving the positional relationship shown in Figure 1.

1 i 1 1 i i 1 i 1 i i i t i 1 4 4 Scratches on the film normally extend in the direction in which the film runs, that is they form vertical lines. As in general scratches are very narrow, a scratch will at the most affect two of the five picture elements shown in Figure 1. Since a median filter passes to the output that input value which is in the centre with respect to the rank sequence, one scanning value not affected by the scratch is always passed on. From a spectral standpoint, a median selector with the window shown in Figure 1 is mainly transmissive of diagonal local frequencies, whilst purely horizontal, high frequency spectra are damped. These include vertically directed scratches.

The circuit arrangement according to the invention functions more advantageously than a conventional recursive filter with respect to the concealment of film dirt and dust particles. Since the spatial extent of the window of Figure 2 is relatively large, it is also possible to conceal larger interfering surfaces in a substantially complete manner. It is particularly advantageous that no special fault or error detection signal is required for this purpose.

In a filter with a window as shown in Figure 2, 1 i 1 two further picture elements E and F have been added compared with those of Figure 1. The picture element E is from the picture preceding the picture containing the central picture element M and the picture element F is from the picture following the picture containing the central picture element M. However, the additional picture elements E and F have the same relative position in their respective pictures as the central picture element M has in its picture. As scratches and other interfering influences on the film constantly alternate between the individual pictures, this measure more reliably ensures that the scanning value of a picture element not affected by interference is passed on to the median selector. If as a result of movement in the picture one or both of the picture elements E and F assume extreme values compared with the picture elements M and A to D, then they are eliminated by the median selector. A filter with the window shown in Figure 2 then acts in a similar manner to that shown in Figure 1.

In the case of the window shown in Figure 3, in addition to the picture elements shown in the window of Figure 2, two further picture elements G and H are also used, and these are taken respectively from the picture preceding the picture con- i 1 i 1 i i i i 1 1 1 i i 1 1 1 1 1 i i 1 i i i ii 0 taining the element E and from the picture following the picture containing the picture element F. These further picture elements G and H have the same relative position in their respective pictures as the picture elements M, E and F have in their pictures.

It is to be understood that picture elements such as E, F, G and H, that is, picture elements from pictures other than that containing the central element M but having the same relative position as the element M, do not need to be taken from pictures immediately adjacent the picture containing the element M. According to the required chracteristics these additional picture elements can be taken from pictures displaced by one or more pictures from that containing the element M.

Figure 4 shows a block circuit diagram of a first embodiment of a filter, having the window described in connection with Figure 1. The median selector 1 comprises in known manner a digital signal processor with five inputs m and a to d. The median rank value is passed to the output 2. This is possible in simple manner by, for example, sorting all the values as a function of their level and passing only the third highest value, that is the middle value, to the output 2.

0 The video signal to be filtered is fed to the input 3 and is so delayed in delay means 4, 5, 6 and 7 that the central picture element M is supplied to the input m of the median selector and the further picture elements A to D shwn in Figure 1 are fed respectively to the further inputs a to D of the median selector 1. Thus, the delay means 5 and 6 each provides a delay of one line period T L less a sample clock or picture element period TP, and the delay means 4 and 7 each provides a delay of two sample clock periods T P The delay means 4 to 7 can, for example, be formed by shift registers or FIFO memories. In circuits for the digital processing of video signals, in which digital memories, for example picture memories, are already present, it is possible to implement the delay means 4 to 7 in simple manner by reading the scanned values from appropriate addresses of the memory at appropriate times.

A block circuit diagram of a circuit arrangement having a window as shown in Figure 2 is shown in Figure 5. Compared with the median selector 11 shown in Figure 4, that in Figure 5 has two further inputs e and f. Between the input f and the input d there is a delay means 12 with a delay time of one picture period T F' less a line period T L and i i j 1 a less a clock period T P Together with the delay times of the delay means 4 and 5 a delay time of one picture period T F is obtained relative to the input m, so that the corresponding picture element F of the following picture is supplied to the input f. Similarly, the delay means 7 is followed by a delay means 13, which has the same delay time as the delay means 12 and supplies a signal corresponding to the picture element E to the input e 10 of the median selector 11.

The median selector of the embodiment according to Figure 6 has been extended by adding two further inputs e' and f' to which are supplied the same signals as are supplied to inputs e and f. This leads to the advantage that the planar resolution for still pictures is maintained. In addition, as a result of the more pronounced emphasis of the time axis of the filter there is a better concealment of interference only occurring in a single film picture.

A similar advantage is obtained if, as shown in Figure 7, further picture elements G and H preceding and following the picture elements E and F by one picture (Figure 3) are supplied to further median selector inputs & and h. Thus, in the circuit arrangement according to Figure 7 4 - 10 two further delay means 15 and 16 are provided each providing a delay of one picture period T...

1 1 i i z j 1 1 1 v - 11

Claims

CLAIMS_

1. A circuit arrangement for filtering a video signal obtained by scanning a film, wherein the scanned values of a first picture element and of a plurality of further picture elements are supplied to respective inputs of a median selector, each of the further picture elements being displaced both in the horizontal direction and in the vertical direction relative to the first picture element.

2. A circuit arrangement as claimed in claim 1, wherein the values of four further picture elements are provided, each further picture element being displaced relative to the first picture element by one picture element in each of the horizontal and vertical directions.

3. A circuit arrangemert- as claimed in claim 1, wherein the values of four further picture elements are provided, the further picture elements forming a cross with the first picture element.

4. A circuit arrangement as claimed in claim 1, wherein the scanned values of additional picture elements from at least one picture preceding the 1 1 1 111 a A picture containing the first picture element and from at least one picture following the picture containing the first picture element are also supplied to the median selector, the additional picture elements having the same relative position in their respective pictures as the first picture element has in its picture.

5. A circuit arrangement as claimed in claim 4, wherein the scanned value of at least one additional picture element from a picture following a picture containing the first picture element and the scanned value of at least one additional picure element from a picture preceding the picture containing the first picture element are each supplied in common to two inputs of the median selector.

6. A circuit arrangement for filtering a video signal, substantially as described herein with reference to Figure 4, 5, 6 or 7 of the accompanying drawings.

Published 1992 at The Patent Office. Concept House. Cardiff Road. Newport, Gwent NP9 I RH. Further copies may be obtained from, Sales Branch. Unit 6. Nine Mile Point. Culnifelinfach, Cross Keys. Newport. NP1 7HZ. Printed by Multiplex techniques lid. St Mary Cray. Kent 1 i 1 I i i I i 1 I 1 i