Television – Image signal processing circuitry specific to television – Combined noise reduction and transition sharpening
Reexamination Certificate
2001-01-12
2003-10-14
Hsia, Sherrie (Department: 2614)
Television
Image signal processing circuitry specific to television
Combined noise reduction and transition sharpening
C348S607000, C348S625000, C382S261000, C382S264000, C382S266000
Reexamination Certificate
active
06633342
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for compensating picture quality of an image signal, and more particularly to an apparatus and method for eliminating noise of a received image and improving its sharpness.
2. Description of the Related Art
Generally, a moving picture receiving system such as a TV includes a transmitter and a receiver. The transmitter converts a light energy sensed through a camera to an electrical signal and transmits the signal through analog or digital signal processing and channel coding. The receiver processes inverse steps of the transmitter to display a moving picture in a display device. The image displayed through a series of steps has noise due to various reasons and sharpness of the image is lowered due to limited bandwidth of transmission channels and physical characteristics of devices. Those noise and low sharpness cause a deterioration of picture quality. In order to solve the problem as described above, various methods for eliminating noise and improving sharpness have been developed. In general, it is known that noise in the natural world has white Gaussian characteristic. The noise in a frequency domain is uniformly distributed in a whole frequency band. On the other hand, the image signal is characterized in that most of power is concentrated in a low frequency area and power is rapidly reduced in a high frequency area. Accordingly, noise can be eliminated by low pass filtering as shown in FIG.
1
.
However, as shown in
FIG. 1
, it is noted that high frequency components of the image signal are eliminated when the noise is eliminated. This deteriorates sharpness of the image.
Therefore, methods for eliminating noise while preserving high frequency components of the image have been suggested. Of them, there is a double smoothing method which is a low pass filtering method through two steps as shown in FIG.
2
. That is, an input signal X(t) is input to a first delay
201
and a first low pass filter (LPF)
202
so that it is delayed by the first delay
201
and at the same time primarily low pass filtered by the first LPF
202
. The primarily low pass filtered signal L(t) is output to a subtractor
203
and a second delay
205
.
The subtractor
203
outputs a difference signal E(t) between a signal delayed by the first delay
201
and a signal primarily low pass filtered by the first LPF
202
to a second LPF
204
. The second LPF
204
low pass filters the difference signal E(t) and outputs the low pass filtered signal to an adder
206
. The adder
206
adds the low pass filtered signal of the second LPF
204
to a signal delayed by the second delay
205
.
In other words, the double smoothing method of
FIG. 2
is performed in such a manner that an image signal is primarily low pass filtered and a difference signal E(t) between an original image signal and the primarily low pass filtered signal is secondarily low pass filtered. A final output signal Y(t) is obtained by adding the secondarily low pass filtered signal N(t) to the primarily low pass filtered signal L(t). The first and second delays
201
and
205
respectively delay data as much as latency of the first LPF
202
and the second LPF
204
.
FIGS. 3
a
to
3
e
show a method for eliminating noise when a unit step signal having noise is processed by the double smoothing method.
As shown in
FIGS. 3
a
to
3
e,
the double smoothing method has a problem in that sharpness is deteriorated because the edge of the image is smoothed when eliminating noise.
Meanwhile, sharpness of the image in view of visual characteristic of human being is greatly varied depending on contrast of an outline portion. That is to say, sharpness is high as contrast of the edge region is great. This is obtained by amplifying high frequency components of the image. For example, if the high frequency components are amplified in a signal having abrupt brightness change, over shoot and under shoot occur around the outline, thereby increasing contrast.
There are methods for making the outline sharp, such as a second derivative method, a high pass filtering method, and an unsharp masking method.
Of them, the unsharp masking method has a structure of FIG.
4
. That is, an original signal X(t) is delayed by a first delay
401
and at the same time low pass filtered by an LPF
402
. A subtractor
403
extracts a difference signal E(t) between the delayed original signal and the low pass filtered signal L(t) and outputs the difference signal E(t) to a multiplier
404
. At this time, the difference signal corresponds to high frequency components of the image. The multiplier
404
multiplies the high frequency components by &agr; and then amplifies the resultant value. The amplified value is added to the original signal delayed by the first delay
401
by an adder
405
. Thus, an image having fine sharpness can be obtained.
FIGS. 5
a
to
5
e
show a method for contrasting the outline by processing a unit step signal by the unsharp masking method to improve sharpness of the image.
The unsharp masking method has a problem that while improving sharpness, it also amplifies noise.
As described above, it can be recognized that the method for eliminating noise and the method for improving sharpness require contrary functions. That is, if noise is eliminated, the outline of the image is smoothed too, thereby deteriorating sharpness. If filtering is performed to improve sharpness, sharpness is obtained but noise is also amplified. The related art methods suggest algorithms targeting only one of the two missions, i.e., sharpness improvement or noise reduction. Accordingly, the two algorithms for sharpness improvement and noise reduction are sequentially applied, there is a problem that noise reduction performance and sharpness improvement performance are attenuated. Particularly, in case of deteriorating picture quality for the edge region, visual quality is deteriorated.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to an apparatus and method for compensating an image signal that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an apparatus and method for compensating an image signal in which noise of an image signal is eliminated and an outline becomes sharp using one parameter by unifying algorithms for eliminating noise and making an outline sharp.
Another object of the present invention is to provide an apparatus for compensating an image signal in which the cost for implementing hardware is reduced by sharing a common portion in algorithms for eliminating noise and making an outline sharp.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the scheme particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an apparatus for compensating an image signal according to the present invention is characterized in that noise is eliminated and an outline becomes sharp by sharing a common portion in a structure for eliminating noise by a double smoothing method and a structure for making an outline sharp in an unsharp masking method.
The apparatus for compensating an image signal according to the present invention includes a first filter for low pass filtering an input signal, a subtractor for obtaining a difference signal between the input signal and the low pass filtered signal, a noise eliminating unit for eliminating noise while preserving high frequency components of an image from the difference signal, a multiplier for multiplying an output of the noise eliminating unit by a constant (&agr;+1), and an adder for adding th
Hsia Sherrie
Lee Hong Degerman Kang & Schmadeka
LG Electronics Inc.
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