GB2251353A

GB2251353A - Noise reduction

Info

Publication number: GB2251353A
Application number: GB9121382A
Authority: GB
Inventors: Gerhard Wischermann
Original assignee: BTS Broadcast Television Systems GmbH
Current assignee: Philips GmbH
Priority date: 1990-10-08
Filing date: 1991-10-08
Publication date: 1992-07-01
Anticipated expiration: 2011-10-08
Also published as: GB2251353B; DE4031785A1; GB9121382D0; DE4031785C2; FR2668325A1; FR2668325B1

Description

2251353 METHOD FOR REDUCING NOISE IN A VIDEO SIGNAL This invention relates

to a method for reducing noise in a video signal, and to a circuit arrangement for performing the method.

For reducing noise in a video signal it is known to use a recursive filter in which the input video signal is delayed by a picture period and the delayed signal is added to the input signal. The relative proportions of the delayed and input video signals are controllable in dependence upon movement in the picture. The greater the proportion of the delayed signal, that is the higher the feedback factor, the higher the degree of noise reduction. Thus, in the case of stationary masters or originals the noise can be integrated over a period of arbitrary long duration. When there is a high feedback factor and when there is movement, smearing effects occur, which are avoided by reducing the feedback factor when move- ment exists.

However, the detection of movement is often disturbed by the noise superimposed on the input video signal. The consequence is that either the desired noise reduction is not obtained, because the noise is misinterpreted as movement, or smearing occurs, because the movement is lost in the noise.

The object of the present invention is to provide a method for reducing noise in a video signal which permits a greater noise reduction to be obtained.

According to the present invention there is provided a method for reducing noise in a video signal in which the video signal is recursively filtered by adding the video signal.and the video signal delayed by one picture period with the proportions of the video signal and the delayed video signal which are added being controlled in dependence upon movement in the picture content, wherein the video signal also undergoes median filtering.

The invention has the advantage that much higher noise reduction factors can be obtained than with known methods.The invention is based on the finding that the interference caused by median filtering is so different compared with the aforementioned interference in the case of recursive filtering, that by the joint use of both filtering methods no accumulation of the e interference effects occurs.

Preferred circuit arrangements for performing the method according to the present invention advantageously use a common delay device for both median filtering and recursive filtering.

Embodiments of the invention will now be described more particularly hereinafter by way of example with reference to the accompanying drawings, wherein:

Figure 1 is a diagrammatic representation of a first embodiment of the inventive method, Figure 2 is a diagrammatic representation of a second embodiment of the inventive method, Figure 3 is an example of a median filtering 20 window usable in the inventive method, Figure 4 is a first embodiment of a first circui arrangement for performing the inventive method, and Figure 5 is a second embodiment of a circuit arrangement for performing the inventive method.

The same parts are given the same reference numerals throughout the drawings.

Referring to the drawings, inthe method shown in Figure 1 the input video signal is supplied at an input 1 to a recursive filter 2 and also to a median filter 3. The output signals of the filters 2 and 3 are added at 4. Noise-reduced video signals are available at the output 5.

In the method shown in Figure 2 there is first a recursive filtering 2 of th& input signal and then a median filtering 3, but the order can be reversed. This serial arrangement has the advantage compared with the parallel arrangement of Figure 1 of requiring reduced circuit expenditure for implementation. However, it is left to the expert to choose the more advantageous structure in any particular case.

Figure 3 shows an advantageous window for the median filter, in which the picture elements B, Cy Ey F immediately adjacent to a central picture element D in the horizontal and vertical directions, and the picture elements A and G having the same position in the pictures immediately preceding and following the element D as the element D has in its own picture, are supplied to a median selector, and in known manner the latter outputs the median amplitude value of the picture elements with respect to the input sequence.

In Figure 4,the video signal to be filtered is supplied to the input 1 and is multiplied by a factor (1 - k) in a multiplier 11. The output of the multiplier 11 is connected to a first input of an adder 12, whose other input is con- nected to the output of a further multiplier 13 which multiplies by the factor k. The factor k can assume values between 0 and 1. For k = 0 only the video signal supplied at 1 is present at the output of the adder 12, whereas for k = 1 only that supplied across the multiplier 13 is present. Thus, the multipliers 11, 13 and the adder 12 form a fade-over circuit, which is controllable with k.

To the adder 12 there are connected two delay devices 14, 15 each having a delay time T f of one picture period. The delay devices 14, 15 are each preferably formed by a digital picture memory. The signal at the output D of the first delay device 14 is delayed by one picture period compared with the signal at the input A, that is for the European television standard by 40ms. Similarly the video signal at the output G of the delay device 15 is delayed by in all two picture periods compared to the signal at A.

The delay devices 14, 15 have four further outputs B, C, E and F. Compared with the video signal at the input A of the delay device 14, the video signal at these outputs have the following delays:

B one picture period minus a line period, C one picture period minus a scan period (dura tion of a picture element), E one picture period plus a line period, and F one picture period plus a scan period.

The picture elements at A to G are provided simultaneously to the inputs of a median selector 16, and constitute the same set of pictureelements as are represented in Figure 3. Of the picture element values at the inputs A to G of the median selector 16 at any given moment, that which is in succession in the centre is passed to the output 17.

To the output D of the delay device 14 there is also connected the input of the multiplier 13 nd one input of a movement detector 18, which is also supplied with the input video signal 1 at 1 via a second input. The movement detector 18 produces signals k and (1-k) as a function of the amount and degree of movement in the picture content. In the case of stationary originals k approaches 1, whereas with moving originals k moves towards zero.

Together with the fade-over circuit 11, 12, 13 and the movement detector 18, the delay device 14 10 constitutes a movement-adaptive recursive filter. The delay device 14 is additionally used as part of the median filter, which also comprises the delay device 15 and the median selector 16.

The circuit arrangement shown in Figure 5 comprises the same components as the circuit arrangement of Figure 4, so that there is no need to describe the individual components again. However, in the circuit arrangement of Figure 5, unlike that of Figure 4, it is not the video signal delayed by one picture period which is used for feedback within the recursive filter, but the already median-filtered signal. Therefore the median filter is in the return or feedback path of the recursive filter. Thus, depending on the feedback factor k, the signal will pass through the median filter several times and consequently the filter will function even more effectively. Another 8 - advantage of the circuit arrangement of Figure 5 is the improved detectability of movement, because noise-reduced signals are supplied to the movement detector 18 from the median selector 5 16. In addition, in the circuit arrangement of Figure 5, further outputs 19, 20 are provided, at which there is the merely recursively filtered signal with the delay T f or 0.

k

Claims

CLAIMS.

1. A method for reducing noise in a video signal in which the video signal is recursively filtered by adding the video signal and the video signal delayed by one picture period with the proportion of the video signal and the delayed video signal which are added being controlled in dependence upon movement in the picture content, wherein the video signal also undergoes median filtering.

2. A method as claimed in claim 1, wherein the video'signal successively undergoes recursive filtering and median filtering.

3. A method as claimed in claim 1, wherein the video signal undergoes both recursive filtering and median filtering and the recursively filtered video signal and the median filtered video signal are added.

4. A method as claimed in claim 1, wherein use is made for median filtering of picture elements which are adjacent in position and in time to a median picture element.

S

5. A method for reducing noise in a video signal, wherein the sum of the video signal and the output - 10 signal of a median filter are supplied to the input of the median filter, and the proportions of the video signal and the output signal of the median filter which are added are controlled in dependence upon movement in the picture content.

6. A circuit arrangement for performing the method claimed in claim 1, comprising a delay device having outputs providing respective delays of two picture periods, one picture period and further delay periods which together with the input of the delay device correspond to a window for median filtering, wherein the input and the outputs of the delay device are connected to the inputs of a median selector whose output forms the output of the circuit arrangement, and wherein at least part of the delay device is used for delaying the video signal in a recursive filter.

7. A circuit arrangement as claimed in claim 6, wherein the video signal is connected to a first input of a fade-over circuit and to a first input of a movement detector, wherein the output of the fade-over circuit is connected to the delay device, and wherein an output of the delay device providing a delay of one picture period is connected to a second input of the fadeover circuit and to a second input of the - 11 movement detector.

8. A circuit arrangement as claimed in claim 6, wherein the video signal is connected to a first input of a fade-over circuit and to a first input of a movement detector, wherein the output of the fade-over circuit is connected to the delay device, and wherein the output of the median selector is connected to a second input of the fade-over circuit and to a second input of the movement detector.

9. A circuit arrangement as claimed in claim 6, 7 or 8, wherein the said further delay periods of the delay device are one picture period minus one line period, one picture period minus one scan period, one picture period plus one line period and one picture period plus one scan period.

10. A method as claimed in claim 1, substantially as described with reference to the accompanying drawings.