JPH0628469B2 - Color noise reduction circuit - Google Patents

Description

The present invention relates to a circuit for reducing color noise superimposed on a color signal at a color carrier signal stage in a television receiver.

[Prior Art] A conventional television receiver has proposed a contour correction means for emphasizing a color signal of a video signal to reduce seemingly color noise, but the present invention does not impair the characteristics of an input color carrier signal itself. The present invention is intended to remove color noise and transmit it to the ceiling, and is an invention relating to color noise reduction in a color carrier signal stage before demodulation into a color signal.

Conventionally, this type of circuit has been shown in FIG. A video signal detection circuit (not shown) of the television receiver separates the luminance signal and the color carrier signal, and the latter is supplied to the input of FIG. In the figure, the color carrier signal is supplied to the coefficient unit 7 and the subtractor 4, the signal level is multiplied by (1-K) by the coefficient unit 7, and the result is supplied to the adder 3 which adds this output and the output of the coefficient unit 6. It Next, the output of the adder 3 is set to one horizontal scanning period (1
H) The delay element 1 for delaying, the subtracter 4 and the adder 5 are supplied. The output of the delay element 1 is supplied to a coefficient unit 6 having a coefficient of K smaller than 1, the signal level is multiplied by K, and the result is supplied to an adder 3. The output of the subtractor 4 passes through the noise clip circuit 2 which passes only a large signal level portion, and then the adder 5
Added to.

Next, this operation will be described with reference to FIG. 5 which shows a waveform chart of each part of the block diagram of FIG. A waveform diagram of one vertical scanning period (1 V) of the input color carrier signal is shown as waveform A in FIG.

Here, if the signal passed through the delay element 1 of 1H and the coefficient unit 6 multiplied by K and the signal from the coefficient unit 7 that multiplies the input color carrier signal by (1-K) are added by the adder 3, The output waveform diagram of the adder 3 is the waveform B. That is, the displayed image has a waveform that looks like it has passed through a low-pass filter in the vertical direction. Therefore, this input is used as a color carrier signal, and (1-
K) times coefficient unit 7, adder 3, delay element 1 of 1H and K
A system composed of the double coefficient unit 6 is called a cyclic filter system, and when the output signal of the adder 3 is output, the transfer function A (w) of this cyclic filter system is generally expressed by the following equation. become.

A (w) = (1-K) / [1-Kexp (-jwT)] where T = 1H (63.5 μs), K = 0.3 to 0.7 w is the angular frequency of the color carrier signal, and is there.

With this transfer function A (w), for example, if the value of this coefficient K is 0.7, the frequency component in the vertical direction is 2 to 3 kHz.
[(525/15 to 525/10) CPh (cycle
The color noise (also referred to as hue unevenness) equal to or higher than the "per height"] is removed, and the waveform B in FIG. 5 is obtained. Also,
When the value of K is set to 0.3, the waveform becomes sharper in the rising vertical contour portion than the waveform B. However, since only the recursive filter system has a slow rise and a fall, the vertical resolution is deteriorated. Therefore, the following means are provided.

That is, as shown in FIG. 4, if the subtractor 4 subtracts the waveform B signal from the input color carrier signal of the waveform A, the waveform C
It becomes the signal shown in. When the signal of the waveform C is clipped by the noise clipping circuit 2 at the clipping level a, this output becomes the signal shown in the waveform D. If the signals of the waveforms B and D are added by the adder 5, the signal shown by the waveform E is obtained. Here, the times t ₁ and t ₂ are vertical contour portions, and a temporal difference occurs before and after clipping. By adding the noise clip circuit 2 and the adder 5, in comparing the waveform E and the waveform A, it is possible to restore a signal having a certain vertical contour portion from which noise is removed.

However, as shown by the waveform E, there are missing signals and trailing. That is, this occurs based on the difference between the times t ₁ and t ₂ shown in FIG.

When the video signal is of the NTSC system, noise is reduced by utilizing the polarity of the color carrier signal inverted every 1H, but naturally the coefficient K of the coefficient unit becomes negative every 1H.

[Problems to be Solved by the Invention] Since the conventional color noise reduction circuit is configured as described above, a part of the contour is lost together with the noise component due to the force of the noise clip circuit 2, and the vertical direction is reduced. However, there are drawbacks such that the color signal is lost at the rising portion of the color due to the lightening of the color, and the trailing portion of the image is tailed downward.

Further, since the noise clipping level of the noise clipping circuit is constant, noise clipping is performed regardless of the level of the input color carrier signal, and distortion such as tailing occurs particularly in the vertical contour (edge) portion or a minute color signal is lost. There was a drawback.

[Means for Solving the Problems] The present invention has been made to eliminate the above-mentioned conventional drawbacks, and is a cyclic filter type adder 3 including a color carrier signal and a delay element 1. The difference from the output is taken, and the noise clip level of the noise clip circuit that receives this difference output is variably controlled by the luminance signal level and K ₁
Of the input color carrier signal and the adder, as well as the cyclic filter system including the delay element 1, and the output of the adder 3 is a signal output with the color noise reduced. Depending on the level of the brightness signal (brightness and darkness on the screen)
It is an object of the present invention to provide a color noise reduction circuit that can perform optimum noise clipping according to the above, and thereby does not cause a missing color contour portion or tailing.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, 8 is a coefficient unit having a coefficient of K ₁ using an operational amplifier and the like, 9 is brightness after video detection of a television receiver, and level of a brightness signal from a color carrier signal separation circuit (not shown). , Which is a terminal for controlling the noise clip circuit 2, and the same parts as those in FIG. 4 are denoted by the same reference numerals, and the description of the overlapping parts will be omitted.

Next, a signal transmission process will be described. The input color carrier signal is supplied to the adder 5 and the subtractor 4. The other input of the adder 5 is a noise clip circuit 2 whose clip level is variably controlled according to the level of a luminance signal having a strong correlation with the input color carrier signal from the subtractor 4, and a noise component is removed.
The signal is introduced by the coefficient unit 8 as a signal multiplied by K ₁ which becomes a predetermined noise. The output of the adder 5 is the (1-K) times coefficient unit 7, the adder 3, the delay element 1 for delaying the delay time of 1H, and K, as in the conventional example shown in FIG. It is input to a recursive filter system composed of a double coefficient unit 6. The outputs of this recursive filter system and this color noise reduction circuit are taken out from the output of the adder 3 and used as the other input of the subtractor 4.

Next, the operation of the present invention will be described with reference to the waveform diagrams of the respective parts in FIG. When the input color carrier signal is waveform A, the output of the recursive filter system is the signal of waveform B filtered in the vertical direction of the image.

Here, the transfer function A (w) of the recursive filter system is, as described above, A (w) = (1-K) / [1-Kexp (-jwT)].

The signal of the output waveform B of the adder 3 is supplied to the subtractor 4 and the difference from the input color carrier signal is taken. This difference signal passes through the noise clipping circuit 2 which passes a signal of a predetermined level or higher, reaches a predetermined level by the coefficient unit 8 of K ₁ and is supplied to the adder 5.

Here, the level of the signal input from the subtractor 4 will be discussed assuming that the signal level from the luminance signal input terminal 9 is constant and the noise clip level of the noise clip circuit 2 is constant. When the level of the output signal of the subtractor 4 is lower than the noise clip level of the noise clip circuit 2,
The noise clip circuit 2 is off, and the transfer function A '(w) of this embodiment in this case is equal to the transfer function (w) of the recursive filter system and A' (W) = A (W). .

However, when the signal level is higher than the noise clip level, the noise clip circuit 2 is on and the transfer function A ′ (w) in this case is A ′ (w) = (1 + K ₁ ) / [K ₁ +1 / A (w)].

This equation will be described with reference to FIG. 3 which replaces the case where the noise clip circuit of FIG. 1 is on. Here, 12 is the cyclic filter system having the transfer function A (w),
Since the noise clip circuit 2 is on, it is through. Assuming that the two signal inputs and outputs to the subtractor 4 are V ₁ , V ₂ and V ₃ , respectively, as shown in the figure, V ₃ = V ₁ −V ₂ V ₂ = (V ₁ + K ₁ · V ₃ ) · A (W) and from these two equations, V ₂ = [V ₁ + K ₁ · (V ₁ −V ₂ )] A (w)), and thus the output V ₂ is V ₂ = {(1 + K ₁ ) / [ K ₁ + 1 / A (w)]} V ₁ and thus the transfer function A ′ (w) is as described above.

From the above, as can be seen from the transfer function A ′ (w), the coefficient K
_{When 1} is close to 0, A '(W) = A (W) is obtained as in the case where the noise clip circuit is off, which is the characteristic of the recursive filter, but when the coefficient K ₁ is close to ∞, A' (W) = A (W) ′ (W) = 1
The transmission system will be perfect. In practice, the noise clip circuit is turned on to increase the gain of the coefficient unit 8 to some extent, and the coefficient of the cyclic filter system is changed in the vertical contour portion of the color carrier signal to reduce the time constant of the transfer function. The coefficient K _{1 of the} device 8 is activated.

Next, a case where the luminance signal of the input terminal 9 is added will be described. The clipping level of the noise clipping circuit 2 is varied in inverse proportion to the level of the luminance signal correlated with the color carrier signal. That is, the noise clip level is increased in the low luminance signal portion and decreased in the high luminance signal portion, and the noise clip level is always set to the optimum setting for noise-free transmission.

Therefore, since the noise clip circuit 2 controlled by the level of the luminance signal level adds a difference signal to the blunted signal in the cyclic filter system through the coefficient unit 8, the signal in the signal transmission in this embodiment is added. There is no omission or trailing, and the transmission system has no distortion.

Waveform B in FIG. 2 indicates that the noise clip circuit 2 is off, waveform C indicates that the noise clip circuit is on and the luminance signal is extremely high and the clip level is small, and waveform D indicates that the noise clip circuit 2 is on. It is an output waveform diagram of the adder 3 in the case.

As described above, the actual output of the adder 3 has a clear waveform as shown by the waveform D without any signal drop or signal tailing in the vertical contour portion.

In the above embodiment, the case of the television receiver has been described. However, even in the case of a video tape recorder or a video disc player that handles a color carrier signal, a color noise removing circuit using this noise clip circuit may be used. The same effective effect as can be obtained.

Further, in the above embodiment, the case of the color carrier wave signal has been described, but it may be the case of the luminance signal, and the same effect can be obtained in this case as well.

As described above, according to the present invention, the vertical contour component of the color carrier signal is returned to the first stage of the cyclic filter, and the clip level of the noise clip circuit is variably controlled in inverse proportion to the luminance signal level. , Since it is configured to be added to the color carrier signal at a predetermined level, it is possible to remove the lack of color and the trailing in the vertical contour portion, and the color contour is reproduced faithfully,
Only color noise (hue unevenness / color noise) can be reduced.

In particular, since the noise clipping circuit is made harder to turn on by increasing the clip level in the low-brightness area where the color noise is noticeable, the noise can be sufficiently removed by the color noise removal of the recursive filter system, and the minute color signal is not lost and input. It does not damage the color carrier signal.

[Brief description of drawings]

FIG. 1 is a block diagram of a color noise reduction circuit according to an embodiment of the present invention, FIG. 2 is a waveform diagram of an essential part of FIG. 1, and FIG. 3 is a block diagram of a part of the color noise reduction circuit of FIG. FIG. 4 is a block diagram showing a state in which the drawings are combined and replaced, FIG. 4 is a block diagram of a conventional color noise reduction circuit, and FIG. 5 is a waveform diagram in a main part of the block diagram of FIG. 1 ... Delay element, 2 ... Noise clip circuit, 3, 5 ... Adder, 6, 7, 8 ... Coefficient unit

Claims (1)

[Claims] 1. A recursive filter system receives an input signal as (1-K).
A first coefficient multiplier for multiplying, a first adder for adding the output of this coefficient multiplier and the output of the second coefficient multiplier, a delay element for delaying the output of this adder for one horizontal scanning period, and this delay A subtractor for extracting the output from the first adder, the subtractor comprising the second coefficient multiplier for multiplying the signal from the element by K, and subtracting the input signal from the output of the cyclic filter system And a noise clipping circuit connected to the output of the subtractor and whose clip level is variably and inversely controlled by the luminance signal of the video signal, and a second addition that sets the signal level from the noise clipping circuit to a predetermined level. A third coefficient unit to be supplied to one input of the filter, and a second adder for adding the signal of the input and the output of the third coefficient unit and supplying the sum to the input of the cyclic filter system. And the cyclic filter Color noise reduction circuit, characterized in that retrieving the from output.