Television – Image signal processing circuitry specific to television – Noise or undesired signal reduction
Reexamination Certificate
2000-06-15
2003-12-09
Lee, Michael H. (Department: 2614)
Television
Image signal processing circuitry specific to television
Noise or undesired signal reduction
C348S625000, C348S606000, C348S026000, C345S063000, C382S266000
Reexamination Certificate
active
06661469
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a false contour correcting apparatus and method used when an image signal is subjected to digital signal processing.
BACKGROUND ART
In recent years, as a television receiver or the like has increased in image quality and made multi-functional, an image signal has been frequently subjected to digital signal processing. For example, gray level correction has been made by the digital signal processing in order to increase the contrast of an image. When a quantized digital signal is subjected to such digital signal processing, an unnatural contour referred to as a false contour may, in some cases, be produced in an image displayed on a screen. To address this, a technique for removing a false contour produced by digital signal processing is disclosed in JP-A-6-62280, for example.
Referring now to the drawings, description is made of a conventional false contour correction circuit which is disclosed in JP-A-6-62280.
FIG. 9
is a block diagram showing the configuration of the conventional false contour correction circuit. The false contour correction circuit comprises a random number generator
5
, a judgment circuit
6
, and an addition circuit
7
, and receives an n-bit digital image signal A. A signal F composed of predetermined lower bits out of the n bits composing the inputted digital image signal A is fed to the judgment circuit
6
. The random number generator
5
outputs a digital random number H having the same bit width as the bit width of the signal F. The judgment circuit
6
compares a value represented by the signal F composed of the predetermined lower bits of the digital image signal A with the digital random number H outputted from the random number generator
5
, and outputs a signal representing “1” or “0” as a correction signal I depending on the results of the comparison. The addition circuit
7
is an adder having the same bit width as that of upper bits G of the digital image signal A, and adds the upper bits G of the digital image signal A and the correction signal I outputted from the judgment circuit
6
, to generate a corrected output signal J.
According to the above-mentioned false contour correction circuit, the correction signal I having no regularity is added to the upper bits G of the digital image signal A. Therefore, the signal F composed of the lower bits which are inputted to the judgment circuit
6
out of the n bits composing the digital image signal A and the correction signal I outputted from the judgment circuit
6
are not correlated with each other within the precision of the random number generator
5
. In a case where an image which hardly changes in brightness or hue is quantized by thus making a correction having no correlation with the image, the position on a screen where the quantization level changes is dispersed backward, forward, right, and left, so that an unnatural false contour is reduced. Consequently, such a digital image signal after the correction makes it possible to obtain an image whose image quality is prevented from being lowered by quantization whose level is low.
In the conventional false contour correction circuit, the digital image signal is subjected to a variation corresponding to a change in the most significant bit without being correlated with an image so that the position on the screen where the quantization level changes is not correlated with the image signal. Therefore, even when a signal representing an image whose brightness is constant is inputted, an image which contains noises roughing the display on the screen is obtained by the variation of the image signal corresponding to the change in the most significant bit.
Therefore, an object of the present invention is to provide a false contour correcting apparatus capable of reducing a false contour in an image based on a digital image signal while avoiding lowering the image quality by such a side effect or the like caused by false contour correction in which the above-mentioned noises occur.
SUMMARY OF THE INVENTION
A first aspect of the present invention is directed to a false contour correcting apparatus for reducing a false contour in an image based on a digital image signal, comprising:
a double bit change detection circuit for detecting in the digital image signal a signal value change which is twice the minimum quantization unit of the digital image signal as a double bit change and outputting a signal representing the results of the detection as a double bit change detection signal; and
a signal correction circuit for subjecting a double bit change portion, which is a portion where the double bit change exists, in the digital image signal to correction for reducing the false contour on the basis of the double bit change detection signal.
According to the first aspect of the present invention, the double bit change in the digital image signal is detected, and the double bit change portion in the digital image signal is subjected to the correction for reducing the false contour. Therefore, it is possible to reduce the false contour while avoiding lowering the image quality by such a side effect or the like in conventional false contour correction where noises occur.
According to a second aspect of the present invention, in the first aspect of the present invention,
the signal correction circuit corrects the double bit change portion in the digital image signal into a portion where there exist two one-bit changes each of which is a signal value change corresponding to the minimum quantization unit on the basis of the double bit change detection signal.
According to the second aspect of the present invention, the double bit change in the digital image signal is converted into the two one-bit changes, so that the false contour corresponding to the double bit change is removed or reduced.
According to a third aspect of the present invention, the false contour correcting apparatus in the first aspect of the present invention further comprises
a front and rear flatness detection circuit for judging whether or not there exists a signal value change which is not less than twice the minimum quantization unit in predetermined sections before and after the double bit change in the digital image signal, and outputting a signal representing the results of the judgment as a flatness detection signal,
the signal correction circuit subjecting, only when there exists no signal value change which is not less than twice the minimum quantization unit in the predetermined sections before and after the double bit change, the double bit change portion to the correction for reducing the false contour on the basis of the flatness detection signal.
According to the third aspect of the present invention, only when there is no change which is not less than the double bit change in the predetermined sections before and after the double bit change in the digital image signal, that is, only when the value of the digital image signal is approximately constant in the predetermined sections, the double bit change is corrected. Therefore, it is possible to reliably correct only the false contour, and prevent an image from being adversely affected by the correction for the false contour.
According to a fourth aspect of the present invention, in the third aspect of the present invention, the predetermined sections are sections respectively corresponding to five adjacent pixels in the digital image signal.
According to a fifth aspect of the present invention, the false contour correcting apparatus in the second aspect of the present invention further comprises
a front and rear flatness detection circuit for judging whether or not there exists a signal value change which is not less than twice the minimum quantization unit in predetermined sections before and after the double bit change in the digital image signal, and outputting a signal representing the results of the judgment as a flatness detection signal,
the signal correction circuit subjecting, only when there exists no signal value change which is not less than twice the minimu
Kageyama Atsuhisa
Kawabata Minoru
Takeshima Masahiro
Lee Michael H.
Matsushita Electric - Industrial Co., Ltd.
Nathael Paulos
Wenderoth , Lind & Ponack, L.L.P.
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