DE4031785C2 - Method of reducing noise in video signals - Google Patents

Description

The invention is based on a method according to the type of the main claim and further relates to an advantageous circuit arrangement for performing the Procedure.

Recursive filters are known for reducing the noise in video signals (US Pat. No. 4,841,36 and Sandbank "Digital Television", John Wiley & Sons, New York, 1990, pages 261 to 264, in which the video signals are delayed by one frame period and the delayed signals are added to the supplied signals. The proportion of delayed or supplied video signals is present controllable by movement. The greater the proportion of delayed signals, i.e. the The feedback factor is, the higher the degree of noise reduction. At resting templates can thus reduce the noise for any length of time to get integrated. With a high feedback factor and when However, there are movement effects that are avoided by the fact that if there is movement, the feedback factor is reduced.

The detection of motion, however, is due to the video signals supplied superimposed noise disturbed. The result of this is that either the one you want Noise reduction is not achieved because the noise is constantly moving is misinterpreted, or that streaking effects occur because the movement in the Noise disappears.

Furthermore, an arrangement for adaptive is from DE 37 09 068 A1 Median filtering is known, with samples of a possibly containing noise Input signal can be filtered depending on a control signal.  

In Chiariglione "Signal Processing of HDTV, II, Proceeding of the Third Internatiol Workshop on HDTV, August 30 to September 1, 1989, pages 677 to 680" is for the conversion of interlaced images into progressively scanned (in HDTV sequences ) given median filtering an indication of "motion-compensated intermediate image algorithms".

The object of the present invention is to provide a method for reducing Noise in video signals indicate a greater reduction in noise allowed.

The inventive method with the characterizing features of The main claim has the advantage that significantly higher noise reduction factors than can be achieved in the known method. The invention is the Based on the knowledge that the interference caused by median filtering different from the disturbances mentioned in recursive filtering are that by using both filter methods together none Accumulation of the interference effects occurs.

The circuit arrangement for performing the method according to the invention advantageously uses a delay device both for Median filtering as well as for recursive filtering.

An embodiment of the invention is shown in the drawing and in the following description explained in more detail. It shows:

Fig. 1 is a schematic representation of an embodiment for the inventive method,

Fig. 2 shows an example of an applicable in the inventive method advantageously window for the median filtering,

Fig. 3 shows an embodiment of a circuit arrangement for performing the method according to the invention.

In the block diagram according to FIG. 1, the video signals are fed via an input 1 to a recursive filter 2 and then to a median filter 3 . Noise-reduced video signals are then available at output 5 .

FIG. 2 shows an advantageous window for the median filter, in which the image elements B, C, E, F that are adjacent to a central image element D and the respectively temporally adjacent image elements A and G are fed to a median selector, the median selector in a manner known per se outputs the mean amplitude value of the picture elements with respect to the sequence.

The input 1 of the circuit arrangement according to FIG. 3 is supplied with the video signals to be filtered and multiplied in a multiplier 11 by a factor (1-k). The output of the multiplier 11 is connected to a first input of an adder 12 , the other input of which is preceded by a further multiplier 13 . k can have values between 0 and 1. If k = 0, only the video signal fed in at 1 is transmitted to the output of the adder 12 , when k = 1 only the video signal fed in via the multiplier 13 . The multipliers 11 , 13 and the adder 12 thus form a cross-fade circuit which can be controlled with k.

Two delay devices 14 , 15 , each with a delay time T _{f of} an image period, are connected to the adder 12 . The delay devices 14 , 15 are preferably each formed by a digital image memory. The signal at output D of the first delay device is delayed by one frame period compared to the signal at input A - that is, by the European television standard, by 40 ms. Accordingly, the video signal at the output G of the delay device 15 is delayed by a total of two picture periods.

The delay devices 14 , 15 have further outputs B, C, E, F. Compared to the video signal at input A of delay device 14 , the video signals at these inputs have the following delays:
B one image period minus one line period,
C is one frame period minus one sampling period (duration of a picture element),
E one image period plus one line period and
F one frame period plus one sampling period.

Sampling values are thus simultaneously at the inputs of a median selector 16 , which belong to the picture elements with the same name in FIG. 2. Of the sample values present at the inputs A to G of the median selector 16 , the one which is in the middle in order is passed on to the output 17 .

The input of the multiplier 13 and an input of a motion detector 18 are also connected to the output 17 of the median selector 16 and are supplied with the video signals supplied at 1 via a further input. The motion detector 18 generates the signals k and (1-k) depending on the presence of motion. When the originals are stationary, k approaches 1, while when originals are moving, k approaches 0.

The delay devices 14 , 15 together with the crossfade circuit 11 , 12 , 13 and the motion detector 18 represent a motion-adaptive recursive filter. The delay devices 14 , 15 are additionally used for the median filter.

The median filter is therefore in the return path of the recursive filter. As a result, the median filter is run through several times, depending on the feedback factor k, which makes it work even more effectively. A further advantage of the circuit arrangement according to FIG. 3 lies in the better recognizability of movement, since signals with reduced noise are already supplied to the movement detector 18 via the input connected to the delay device 14 .

In addition, further outputs 19 , 20 are provided in the circuit arrangement according to FIG. 3, at which the signal, which is only recursively filtered, is present with the transit time T _f or 0.

Claims (2)

1. Method for reducing noise in video signals, both a recursive filtering and a median filtering of the video signals being carried out in succession,
characterized,
that both the input video signal and the already media-filtered video signal are evaluated depending on the movement,
that these signals obtained in this way are then added together,
and that the sum signal thus formed is finally filtered using a medium. 2. Circuit arrangement for performing the method according to claim 1,
characterized,
that an input ( 1 ) for the supplied video signals is connected to a first input of a cross-fade circuit ( 11 to 13 ) and a first input of a motion detector ( 18 ),
that the output of the fader circuit is connected to a delay device ( 14 , 15 ),
that the input (A) and the outputs (B to G) of the delay device ( 14 , 15 ) are connected to inputs of a median selector ( 16 ), the output of which forms the output ( 17 ) of the circuit arrangement,
and that this output ( 17 ) is further connected to a second input of the cross-fade circuit ( 11 to 13 ) and a second input of the motion detector ( 18 ).