Television – Image signal processing circuitry specific to television – Noise or undesired signal reduction
Reexamination Certificate
1999-04-21
2001-07-10
Kostak, Victor R. (Department: 2611)
Television
Image signal processing circuitry specific to television
Noise or undesired signal reduction
C348S618000, C348S701000
Reexamination Certificate
active
06259489
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to video noise reducers and method's of noise reduction.
A well known method for the reduction of noise In a video signal employs a recursive Filter, The method operates by averaging similar areas in successive images and relies on the fact that the true image information in a video signal is highly correlated, Noise, however, is generally random and is attenuated in the averaging process, A recursion constant k determines the relative weightings of the current and preceding frames in the averaging process and thus the number of frames over which effective averaging takes place. Significant attenuation of noise can be achieved with averaging over a relatively small number of frames.
It is well understood that the picture correlation upon which recursive noise reduction relies, breaks down where there is movement in the input images. It is important to disable the recursion when movement is detected; failure to do this will result in image smear. Motion adaptive noise reduction has the annoying artifact, that picture regions at the edges of moving objects are substantially noisier than the surrounding picture. An edge of a moving object is associated with disabling of recursive noise reduction so that each edge of the moving object will be followed by a region of signal which is not noise reduced. The problem is exacerbated by the need—usually—for the motion detector signal to be filtered spatially; this results in an even larger area which lacks noise reduction. It is also the case that the more effective is the noise reduction process over the image as a whole, the more noticeable are the noisy regions around moving objects.
Whilst the need to avoid smearing has been described in relation to the detection of moving objects, the disabling of recursion will apply also to global motion such as panning and also to shot changes and other edits.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an improved video noise reducer and method of noise reduction which removes or reduces the above problem.
Accordingly, the present invention consists in a method of noise reduction in a video signal, wherein pixel information is recursively filtered, with recursion being disabled at a disjuncture, wherein recursively filtered pixel Information prior to a disjuncture continues to be output until pixel information following the disjuncture has been recursively filtered to an approximate degree of noise reduction.
Advantageously, the output of a recursive filter is presented to a picture interval delay chain and following detection of a disjuncture and disabling of recursion, a selection is made between picture interval delayed filter outputs.
The present invention recognises that, taking the example of a shot change, the visually disturbing effect of replacing a quiet, heavily noise-reduced shot by the first noisy field of a now shot can be avoided by delaying showing the new shot until sufficient fields have been recursively filtered for the noise level on the new shot to have approached or equaled that of the current shot. The same approach applies of course pixel-by-pixel in the face of motion.
In this aspect, the present invention further consists in a video noise reduction filter comprising a temporal recursive filter; means for disabling recursion in the filter on detection of motion; a delay path affording multiple filter outputs mutually delayed by picture intervals; and means for selecting between said filter outputs on detection of motion to provide an output having maximum noise.
It is an object of a preferred form of the present invention to provide an improved video noise reduction filter and method of noise reduction utilising a temporal recursive filter, in which after a disjuncture—such as motion—leading to disabling of recursion, noise levels are more rapidly reduced.
Accordingly, it is advantageously provided that the recursion is controlled after a disjuncture such that available pixels contribute equally to the filter output.
It has been found, for examples that where pixels a and b are available, noise is minimized by taking the output (a+b)/2. Similarly, if pixels a, b and c are available, the preferred output is (a+b+c)/3.
Preferably, the recursion constant is varied in dependence upon the count of picture intervals since a disjuncture reading to disabling of recursion.
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Flannaghan Barry
Weston Martin
Kinberg Robert
Kostak Victor R.
Snell & Wilcox Limited
Venable
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