JPH0357676B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise removal circuit that removes noise components from an input video signal.

FIG. 1 is a block circuit diagram of a conventionally known noise removal circuit, and FIG. 2 is a waveform diagram for explaining the operation of FIG. 1.

In FIG. 1, an input video signal (FIG. 2A) is applied from an input terminal 1 to a high-pass filter 2, where high-frequency components including noise are extracted. As shown in FIG. 2B, the output of the high-pass filter 2 also includes a large amplitude component (differential component) other than noise at the sudden rise or fall of the input signal. threshold level (LIM) or less. The output of the limiter 3 and the original input video signal are added in an adder 4 with opposite polarities, thereby eliminating noise in the input video signal as shown in FIG. 2C.

In such a conventional noise cancel circuit, if there is a large phase difference between two signals of opposite polarity supplied to the adder 4, the noise cancel operation cannot be performed correctly. For this reason, the high-pass filter 2 for noise extraction shown in FIG. 1 is usually a simple differential circuit with one stage of CR. In order to remove even low-frequency noise, the time constant of the high-pass filter 2 is increased.
A relatively wide portion containing residual noise remains at the rising or falling portion of the large amplitude signal, and the video signal waveform is distorted as shown in FIG. 2C, resulting in smearing at the edges of the image, resulting in a loss of image quality.

When the time constant of the high-pass filter 2 is made small as shown in FIG. 2D, waveform distortion and residual noise are reduced, but the noise removal effect at relatively low frequencies is reduced.

The present invention provides a novel noise removal circuit that eliminates the above-mentioned contradictory disadvantages.

The present invention will be explained below based on the drawings.

3 and 4 are principle circuit diagrams for explaining the present invention. In FIG. 3, a video signal applied to an input terminal 1 is separated into high and low frequencies by a high-pass filter 2 and a low-pass filter 6.
They are added together by an adder 7. high pass filter 2
and the cutoff frequency of the low-pass filter 6, and add their respective outputs at the same level, we get
The input signal of terminal 1 and the output signal of adder 7 match.

FIG. 4 shows an application of FIG. 3 to a noise removal circuit. Between the high-pass filter 2 and the adder 7,
A noise cancel circuit consisting of the limiter 3 and adder 4 shown in FIG. 1 is inserted. According to the configuration shown in FIG. 4, waveform distortion and differential signal loss are reduced by adding the high-frequency component (on the high-pass filter 2 side) with the noise component canceled to the output of the low-pass filter 6 in the adder 7. Reduced.

In the present invention, based on the configuration shown in FIG. 4, the time constant of the high-pass filter 2 is made small (the cutoff frequency is made large) to reduce waveform distortion and to reduce residual noise in the rising and falling parts of the video signal. We are trying to reduce this.

FIG. 5 is a block circuit diagram of a noise removal circuit showing an embodiment of the present invention, which is basically the circuit shown in FIG. 4 with a double time constant low-pass filter 8 inserted after the adder 4. It is. 6th A,
B shows the frequency characteristics of the high-pass filter 2 and the low-pass filters 6 and 8. high pass filter 2
is, for example, a primary filter with a cutoff frequency of _f2 , as shown in FIG. 6A. The low-pass filter 6 is a primary filter with a cutoff frequency of _f1 , as shown by the dotted line in FIG. 6B, and is set to satisfy _f1 < _f2 .

A dual time constant low pass filter is a filter having a cutoff frequency _f1 and a rolloff frequency _f2 , as shown in FIG. 6A.

The overall frequency characteristics of the filters 2 and 8 on the noise canceling circuit side are equivalent to a high-pass filter with a cutoff frequency of _f1 , as shown in FIG. 6B.
Therefore, as shown in FIG. 6B, in the circuit of FIG. 5, the original signal is reproduced in the same way as in FIG.

In this case, even if the time constant of the high-pass filter 2 is small, the total frequency characteristic shown in FIG. 6B is obtained, so
Waveform distortion due to the rising and falling edges (differential signal) of a large amplitude input video signal being removed by the limiter 3 is reduced, and the width of residual noise due to the loss of the noise canceling effect in this area is reduced. be able to.

In addition, signal components other than noise included in the output of the high-pass filter 2 in Fig. 5 (high-frequency small amplitude components)
However, the loss lost by the noise cancel circuit can be compensated to some extent by adjusting the addition level in the final stage adder 7, as explained in FIG. Characteristics can be improved.

In addition, in the conventional circuit shown in FIG. 1, a double time constant low-pass filter 8 as shown in FIG. 5 is installed after the limiter 3.
It may _be possible _to insert The smear becomes large and the image quality is degraded.

As described above, the present invention separates the high-frequency noise of the input video signal using the high-pass filter ₂ with the cutoff frequency f 2 and supplies it to the noise canceling circuit, and the noise-cancelled signal has the cutoff frequency f ₁ and the roll-off frequency f ₂ ( The input video signal is passed through a low-pass filter 8 with a double time constant (f ₁ <f ₂ ) and added to the input video signal passed through a low-pass filter 6 with a cutoff frequency f ₁ .

Therefore, according to the present invention, by adding the passing signal on the high-pass side (noise canceling circuit side) and the passing signal on the low-pass side at an appropriate ratio, the high-frequency minute signal lost due to noise canceling can be compensated to a certain extent. It is possible to reduce waveform distortion and make transmission frequency characteristics flatter.

In addition, by inserting a double time constant low pass filter, the time constant of the high pass filter for noise extraction can be made small (the cutoff frequency is large) without losing the above-mentioned effect.
Waveform distortion and residual noise at the edge portions of a large amplitude input video signal caused by noise canceling are reduced, and high quality images with less smear or streaking can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram of a conventionally known noise removal circuit, FIG. 2 is a waveform diagram for explaining the operation of FIG. 1, and FIGS. 3 and 4 are diagrams for explaining the present invention. FIG. 5 is a block circuit diagram of a noise removal circuit showing an embodiment of the present invention, and FIGS. 6A and 6B are graphs showing frequency characteristics of the high-pass filter and low-pass filter of FIG. 5. In addition, in the symbols used in the drawings, 2...high-pass filter, 3...limiter, 4...adder, 6
. . . low pass filter, 7 . . . adder, 8 . . . low pass filter.

Claims (1)

[Claims] 1 a first low pass filter having a first cutoff frequency and to which the input video signal is supplied; and a second cutoff frequency higher than the first cutoff frequency and to which the input video signal is supplied. a high-pass filter; a noise cancel circuit to which the output of the high-pass filter is supplied;
each having a cutoff frequency substantially equal to the first cutoff frequency and a rolloff frequency substantially equal to the second cutoff frequency;
A noise canceling circuit comprising: a second low-pass filter to which the output of the noise canceling circuit is supplied; and an adding circuit that adds the outputs of the first and second low-pass filters.