JP3356297B2 - Image noise reduction circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image noise reduction circuit used in a video disk player, a video tape recorder, a television receiver and the like.

[0002]

2. Description of the Related Art A video signal in a video disk player, a video tape recorder, a television, or the like is composed of frames. Generally, an image has high autocorrelation between frames, and a noise component has almost no correlation between frames. Therefore, if the image is temporally averaged between frames using the presence or absence of the autocorrelation, it is possible to reduce only the noise component without substantially changing the energy of the signal component of the image itself.

FIG. 8 shows a conventional noise reduction circuit constructed based on such a principle. This circuit delays a video signal by one frame period in a frame memory 1, subtracts the delayed video signal B from an input video signal A in a subtraction circuit 2 to obtain a difference signal (AB), and calculates a compensation signal. The difference signal (A−
A noise reduction compensation signal K (AB) is calculated from B). Then, the compensation signal K obtained in the subtraction circuit 4
By subtracting (AB) from the input video signal A, the noise component in the input video signal is reduced.

[0004] Such a noise reduction circuit is extremely effective for a still image, but has a disadvantage that an afterimage occurs in a moving image portion and the image is blurred. Therefore,
Normally, the compensation signal K (AB) depends on the motion of the image.
The mixture ratio K in the medium is changed in the range of 1 ≧ K ≧ 0, and K is increased in a portion where motion is small to increase the noise reduction effect. It is adaptively controlled to eliminate blur.

Since the motion of the image is proportional to the difference signal (AB), the mixture ratio K is also a function of the difference signal (AB). Therefore, the compensation signal K for noise reduction
Since (AB) can be expressed as a function of the difference signal (AB) as a whole, the relationship between the difference signal (AB) and the compensation signal K (AB) is stored in advance in a table such as a ROM. By referring to this table according to the input difference signal (A-B), noise reduction according to the motion of the image can be performed.

FIG. 9 shows an example of a difference signal-compensation signal characteristic table for performing a noise reduction action according to the motion of the image. As is clear from this figure, the difference signal (A−
When B) exceeds a certain value, that is, when the motion of the image exceeds a certain limit, the compensation signal K sent to the subtraction circuit 4
The value of (AB) becomes 0, the noise reduction effect is lost, and blurring of the image is prevented.

[0007]

As described above, the conventional noise reduction circuit changes the magnitude of the compensation signal in accordance with the motion of the image to obtain a noise reduction effect in accordance with the motion of the image. . By the way, according to experiments by the present inventors, when a processed image is a color image, color noise is more conspicuous than image blur in an image portion having a high color signal level, and in an image portion having a low color signal level in an image portion having a high color signal level. It was recognized that the blur of the image tended to be more noticeable than the color noise. Therefore, in order to obtain a higher quality image, it is desirable to variably control the noise reduction effect not only according to the above-mentioned image movement but also according to the level of the signal level of the color signal component.

The present invention has been made in view of the above circumstances. It is an object of the present invention to suppress the occurrence of color noise in an image portion having a high color signal level, and to reduce blurring of an image in an image portion having a low color signal level. An object of the present invention is to provide an image noise reduction circuit that can be made inconspicuous.

[0009]

In order to achieve the above object, according to the present invention, there is provided an image delay means for delaying an original image signal by one frame, a delay image signal output from the image delay means, and Compensation signal generating means for performing non-linear processing on a difference signal from an original image signal to generate a noise-reducing compensation signal having an output of 0 when the value of the difference signal is equal to or greater than a predetermined value; A mixing means for mixing the compensation signal obtained in (1) with the original image signal to cancel noise components in the original image signal, wherein any of the original image signal or the delayed image signal Color signal extraction means for extracting a color signal component from one of the image signals; and level detection means for detecting a signal level of the color signal component extracted by the color signal extraction means. Means for variably controlling the level of the compensation signal according to the signal level of the color signal component detected by the step
Is provided downstream of the compensation signal generating means . In the invention according to claim 2, the compensation signal is set to 0 when the signal level of the color signal component detected by the level detecting means is equal to or lower than a predetermined level. Further, in the invention according to claim 3, the signal level detection of the color signal component by the level detecting means is performed by detecting which of a plurality of predetermined level areas is, and corresponding to the detected area. Then, the level of the compensation signal is varied stepwise.

[0010]

In the present invention, when the signal level of the color signal component is high, the compensation signal increases accordingly, and when the signal level of the color signal component is low, the compensation signal decreases accordingly. Therefore, in an image portion having a high color signal level, color noise is suppressed more than in image blur, and in an image portion having a low color signal level, image blur is suppressed more than color noise.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of the noise reduction circuit of the present invention. In the first embodiment, whether or not the signal level of the color signal component is equal to or higher than a predetermined threshold value is determined.
The noise reduction function is turned on / off. The same parts as those in the conventional example (FIG. 8) are denoted by the same reference numerals.

In FIG. 1, 1 is a frame memory, and 2 and 4 are subtraction circuits. The frame memory 1 outputs the video signal delayed by one frame period. The subtraction circuit 2 obtains a difference signal (A−B) between the input video signal A and the delayed video signal B output from the frame memory 1 and delayed by one frame period. Further, the subtraction circuit 4 subtracts the compensation signal K (AB) from the input video signal A to reduce a noise component in the input video signal.

Reference numeral 5 denotes a compensation signal operation circuit, which outputs a difference signal
It comprises a compensation signal characteristic table 501 and an ON / OFF switch 502. The difference signal-compensation signal characteristic table 501 is, for example, a characteristic table similar to the conventional one as exemplified in FIG. 9, and receives a difference signal (AB) as an input and outputs a noise reduction compensation signal K (AB). What you get. The switch 502 is turned on / off by a switch ON / OFF control signal sent from the level detection circuit 7 described later.

6 is a color signal band pass filter (BP)
F), in which only the color signal component is extracted from the delayed video signal output from the frame memory 1.
As the color signal BPF 6, for example, a comb filter for YC separation can be used.

Reference numeral 7 denotes a level detection circuit for detecting a signal level of a color signal component. As shown in FIG. 2, the level detection circuit 7 outputs a switch ON signal when the color signal level is equal to or higher than a predetermined threshold _CTH, and outputs a switch OFF signal when the color signal level is equal to or lower than the threshold _CTH. Output.

Next, the operation of the first embodiment will be described.
The frame memory 1 delays the video signal by one frame period, and sends the delayed video signal B to the subtraction circuit 2. The subtraction circuit 2 converts the delayed video signal B into the input video signal A
, A difference signal (A−B) is obtained and sent to the compensation signal calculation circuit 5.

The compensation signal calculation circuit 5 calculates the difference signal (A−
B), the difference signal-compensation signal characteristic table 50
1 (see FIG. 9), a corresponding compensation signal K (AB) is obtained, and this compensation signal K (AB) is sent to the switch 502 as a noise reduction signal.

On the other hand, in parallel with the processing operation, the color signal BPF 6 extracts only the color signal component from the delayed video signal output from the frame memory 1 and sends it to the level detection circuit 7. The level detection circuit 7 detects the signal level of the extracted color signal component, and outputs a switch ON signal when the signal level is equal to or higher than a predetermined threshold value C _TH as shown in FIG. If the threshold value is equal to or less than the threshold value C _TH , a switch OFF signal is output.

As a result, when the signal level of the color signal component is _{equal to} or higher than the threshold value C _TH , the switch 502 of the compensation signal calculation circuit 5 is turned on (conducting), and the difference signal-compensation signal characteristic table 501 is obtained. Compensation signal K (A for noise reduction)
-B) is sent to the subtraction circuit 4 as it is. On the other hand, when the signal level of the color signal component is equal to or less than the threshold value C _TH , the switch 502 of the compensation signal calculation circuit 5 is turned off (disconnected), and the compensation signal K (AB) is sent to the subtraction circuit 4. Can not be.

Therefore, in the case of the first embodiment, a noise reduction effect is effected in an image portion where the color signal level is larger than the threshold value _CTH , and the color noise is reduced more than the image blur. On the other hand, in the image portion where the color signal level is smaller than the threshold value _CTH , the noise reduction effect is cancelled,
The blur of the image is suppressed more than the reduction of the color noise.

For the specific value of the threshold value C _TH , the most effective value was experimentally searched for while considering the visual characteristics.
You only have to decide. Also, in the example of FIG.
Is connected to the rear side of the difference signal-compensation signal characteristic table 501, but it can be similarly implemented by connecting it to the front side of the difference signal-compensation signal characteristic table 501.

FIG. 3 shows a second embodiment of the noise reduction circuit of the present invention. In the second embodiment, the noise reduction effect is changed stepwise according to the signal level of the color signal component. The same parts as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.

In the case of the second embodiment, the level detection circuit 7
Window comparator 701 and multiple multiplier coefficients 702
To 706. Further, a multiplier 503 is connected to the rear side of the difference signal-compensation signal characteristic table 501 in the compensation signal calculation circuit 5, and the multipliers 702 to 70
6 is used as the compensation signal K
(A-B).

The window comparator 701 has a color signal generator.
The signal level is detected as shown in FIG.
Yeah, level C₀Below, level C₀~ C₁, Level C₁
~ C _Two, Level C_Two~ C_Three, Level C_ThreeThe above five
Level, and detects the color signal level.
Outputs the selection signal "1" to the corresponding output terminal,
One of the arithmetic coefficient units 702 to 706
You.

Each of the multiplication coefficient units 702 to 706 has
A unique multiplication coefficient α (for example, 0, α ₁, Α_Two, Α_Three,
1) is stored in the window comparator 70
The multiplication coefficient α of the multiplication coefficient unit selected by 1 is multiplied by
It is sent to the road 503.

Next, the operation of the second embodiment will be described.
The window comparator 701 detects the signal level of the color signal component extracted by the color signal BPF 6, and selects one of the multiplication coefficients 702 to 706 corresponding to the signal level.

Any one of the selected multiplication coefficient units 702 to 706 multiplies the stored multiplication coefficient α by a multiplication circuit 503.
Send to The multiplication circuit 503 multiplies the received multiplication coefficient α by the compensation signal K (A−B) sent from the difference signal / compensation signal characteristic table 501, and outputs the compensation signal αK ( AB) is sent to the subtraction circuit 4 as a final noise reduction signal.

The compensation signal α output from the multiplication circuit 503
If the comprehensive characteristics of K (AB) are illustrated, as shown in FIG. 5, a plurality of curve groups having the multiplication coefficient α as a parameter are obtained. Therefore, even if the value of the difference signal (AB) sent from the subtraction circuit 2 is the same, the value of the compensation signal αK (AB) is the value of the multiplication coefficient α, that is, the signal of the color signal component. It changes adaptively according to the level. As a result,
The effect of reducing noise according to the signal level of the color signal is realized.

Although the value of the multiplication coefficient α changes stepwise as shown in FIG. 4 in the second embodiment, it may be changed continuously as shown in FIG. . In this case, the value of the compensation signal αK (AB) can be changed continuously according to the change of α.

FIG. 7 shows a third embodiment of the noise reduction circuit of the present invention. The third embodiment is different from the first and second embodiments in that the delayed video signal is replaced with a color signal BP.
An input video signal is directly input to F6, and the control is performed according to the color signal level of the input video signal. The other circuit parts are the same as those shown in FIGS.
Since the circuit is exactly the same as that of FIG.

In each of the above embodiments, the difference signal shown in FIG.
Although the operation has been described using the case where the compensation signal characteristic table 501 is used as an example, the present invention is not limited to this table, and it goes without saying that a characteristic table having an arbitrary curve can be adopted according to the purpose. Also,
Instead of storing the relationship between the difference signal and the compensation signal in the ROM as a table as shown in FIG. 9, the input / output characteristics of FIG. 9 may be configured by a dedicated hardware circuit.

[0032]

As is apparent from the above description,
According to the noise reduction circuit of the present invention, the compensation signal for noise reduction is variably controlled in accordance with the signal level of the color signal component. Generation of noise can be suppressed, and the noise reduction effect can be reduced in the image portion where the color signal level is small, so that blurring of the image is less noticeable, and a higher quality image can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of a noise reduction circuit according to the present invention.

FIG. 2 is a diagram showing color signal levels and ON / OFF characteristics of switches.

FIG. 3 is a diagram showing a second embodiment of the noise reduction circuit of the present invention.

FIG. 4 is a color signal level-multiplication coefficient characteristic diagram.

FIG. 5 is a characteristic diagram of a difference signal-compensation signal according to the present invention.

FIG. 6 is a diagram illustrating another example of a color signal level-multiplication coefficient characteristic.

FIG. 7 is a diagram showing a third embodiment of the noise reduction circuit of the present invention.

FIG. 8 is a diagram showing a configuration of a conventional noise reduction circuit.

FIG. 9 is a characteristic diagram of a conventional difference signal-compensation signal.

[Explanation of symbols]

 Reference Signs List 1 frame memory 2 subtraction circuit 4 subtraction circuit 5 compensation signal operation circuit 6 band-pass filter for color signal 7 level detection circuit

Continuation of front page (56) References JP-A-1-126090 (JP, A) JP-A-2-186890 (JP, A) JP-A-63-64494 (JP, A)

Claims (3)

(57) [Claims] An image delay means for delaying an original image signal by one frame; and a non-linear process for a difference signal between the delayed image signal output from the image delay means and the original image signal to perform the non-linear processing on the difference signal. A compensation signal generating means for generating a noise reduction compensation signal whose output becomes 0 when the value is equal to or more than a predetermined value;
A mixing means for mixing the compensation signal obtained by the compensation signal generation means with the original image signal to cancel noise components in the original image signal. Color signal extraction means for extracting a color signal component from one of the delayed image signals; and level detection means for detecting a signal level of the color signal component extracted by the color signal extraction means, the compensation signal producing means for variably controlling the level of the compensation signal according to the signal level of the color signal component detected by the detection means
An image noise reduction circuit, which is provided at a stage subsequent to the generating means . 2. The image noise reduction circuit according to claim 1, wherein the compensation signal is set to 0 when the signal level of the color signal component detected by the level detection means is equal to or lower than a predetermined level. 3. The signal level of a color signal component detected by the level detecting means is detected in which of a plurality of predetermined level areas, and the compensation signal is detected in accordance with the detected area. 3. The image noise reduction circuit according to claim 1, wherein the level of the image noise is varied stepwise.