JP2545373B2 - Noise cancellation circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a noise canceling circuit suitable for use in a reproduction signal processing circuit in a video reproducing device such as a video disc player.

[Conventional technology]

Generally, a video reproducing apparatus is provided with a means for reducing a noise component mixed in a reproduction signal. Especially, in a video reproducing apparatus such as a video disc player (hereinafter, referred to as VDP), a means for reducing the noise component is provided. A noise cancel circuit is widely used as a means.

A typical conventional noise canceling circuit is, for example, “VTR” by Kenichi Sawasaki, published by Corona Publishing Co., Ltd. (Showa 4
6 years) A circuit as shown in FIG. 2 as described on pages 175 to 177 is known.

FIG. 2 is a block diagram showing a conventional noise canceling circuit.

In FIG. 2, 1 is a video signal input terminal, 2 is a high pass filter for extracting high frequency components, 3 is an amplifier, 4 is a limiter for limiting the amplitude of the input signal, and 5 is polarity inversion for inverting the polarity of noise components. A circuit, 6 is an attenuator for adjusting the level, 9 is an adder for adding a noise component whose polarity is inverted to the original video signal, and 10 is a video signal output terminal.

The operation will be described below with reference to FIGS. 3 (A) to 3 (D).

FIG. 3 is a waveform diagram showing the waveforms of essential signals in FIG. 2 and FIG. 1 which will be described later.

The video signal input to the video signal input terminal 1 includes the third
Noise component (jagged portion in the figure) as shown in the figure (A)
Are superimposed. Since the noise component is a high frequency component,
In order to take it out, first, the video signal is passed through the high-pass filter 2 to remove the DC component. Then, as shown in FIG. 3B, a noise component and a high-level differential pulse corresponding to the edge portion of the video signal (because the high-pass filter operates as a kind of differentiating circuit) are obtained. .

Then, after amplifying the obtained signal with the amplifier 3,
Since the higher level differential pulse is unnecessary, the level of the higher level differential pulse is set to the third level through the limiter 4.
The waveform of FIG. 3 (C) is obtained by limiting the broken line portion shown in FIG.

After that, the polarity of the signal is inverted through the polarity inversion circuit 5, the attenuator 6 adjusts it to an appropriate level, and the adder 9 adds it to the original video signal. A waveform with reduced noise is obtained as shown in FIG.

[Problems to be solved by the invention]

In the above-described conventional technique, the noise component in the video signal is extracted by passing through the bypass filter 4. However, as the signal obtained, as shown in FIG. High-level differential pulses corresponding to the trailing edge portion and the trailing edge portion (that is, the edge portion) are also obtained. Therefore, the level of the differentiated pulse is limited through the limiter 4, and the pulse waveform (waveform similar to a square wave) corresponding to the edge portion is also obtained through the limiter 4 as shown in FIG. 3 (C). Therefore, when the video signal is subtracted from the original video signal, the edge portion of the video signal is blunted as shown in FIG. 3D, and as a result, when the video signal is reproduced, the contour of the screen is There was a problem that it became blurry.

Therefore, an object of the present invention is to solve the above-mentioned problems of the conventional technology, eliminate the rounding of the edge portion of the video signal due to the differential pulse, and prevent the contour of the reproduced screen from being blurred.
It is an object of the present invention to provide a noise canceling circuit that can remove noise components and that does not malfunction even when excessive noise is superimposed on a video signal at a high frequency.

[Means for solving problems]

In order to achieve the above object, in the present invention, a high-pass filter that removes a DC component from an input video signal to extract a high-frequency component, and a limiter that limits the amplitude of the extracted high-frequency component. A polarity inverting circuit for inverting the polarity of the output signal from the limiter, adjusting means for adjusting the level of the polarity-inverted signal, and an adder for adding the output signal from the adjusting means to the video signal. In a noise canceling circuit for removing a noise component included in the video signal, a signal whose band is limited by passing the output signal from the limiter through a low-pass filter is input, and a section whose amplitude is limited by the limiter of the signal is input. A first means for detecting and a second means for switching the level of the output signal from the adjusting means input to the adder by the detection signal from the first means. And, it was to provide a.

[Action]

The first means inputs the output signal from the limiter and detects the amplitude-limited (limiting) section of the signal. As a result, the portion of the differential pulse corresponding to the edge portion of the video signal can be detected.

Next, the second means switches the level of the output signal from the adjusting means that is added to the video signal according to the detection signal from the first means. For example, the level of the output signal from the adjusting means to be added to the video signal is switched to zero only when the detection signal indicates that the amplitude is limited.

As a result, in the portion of the differential pulse corresponding to the edge portion, the output signal from the adjusting means is not added to the original video signal and the noise cancel operation is not performed, so that the edge portion of the original video signal is blunted. Therefore, the outline of the playback screen will not be blurred.

〔Example〕

FIG. 1 shows a circuit useful for understanding a noise canceling circuit as an embodiment of the present invention.

In FIG. 1, a limiter detection circuit 8 detects a section in which the level of the differential pulse or the like corresponding to the edge portion of the video signal is limited. Further, 7 is a changeover switch, which is limited by the output signal of the limiter detection circuit 8.

The parts corresponding to those in FIG.
A duplicate description will be omitted.

 Hereinafter, the operation of FIG. 1 will be described again with reference to FIG.

As shown in FIG. 1, an output signal from the limiter 4 is input to the limiter detection circuit 8, and the section of the input signal that has been limited (amplitude limited) by the limiter 4 is detected therein.

As a detection method, for example, the detection levels v ₁ ′ and v ₂ ′ are set to almost the same levels as the limiting levels v ₁ and v ₂ at the time of limiting, which are shown by broken lines in FIG. However, v ₁ > v
₁ ′, v ₂ > v ₂ ′. ), The input signal (FIG. 3 (C)) is compared with the detection levels v ₁ ′ and v ₂ ′, and when the input signal is larger than the detection level v ₁ ′ or smaller than v ₂ ′, Determines that the section is a limited section and outputs "H" level.
When it is smaller than v ₁ ′ and larger than v ₂ ′, it is determined that the section is not limited and the “L” level is output.

Therefore, the output signal from the limiter circuit 8 has a waveform as shown in FIG. That is, as the waveform of FIG. 3 (E), when there is a differential pulse corresponding to the edge portion of the video signal or when there is excessive noise, it becomes "H" level, and in the normal noise component portion, " It becomes L "level.

Therefore, by the output signal from the limiter detection circuit 8,
When the changeover switch 7 is controlled, the output from the attenuator 6 is directly input to the adder 9 when the waveform in FIG. 3 (E) is at the “L” level, and the output from the attenuator 6 is at the “H” level. The changeover switch 7 is switched so that it is not input to the adder 9.

As a result, the video signal input to the adder 9 from the input terminal 1 is subjected to the noise canceling operation when the waveform of FIG. 3 (E) is at the “L” level, but is at the “H” level. Is output from the video signal output terminal 10 without performing the noise canceling operation. Therefore, by this switching, it is possible to obtain a waveform having no waveform rounding at the edge portion at the video signal output terminal 10 as shown in FIG. 3 (F).

In this circuit, the changeover switch 7 receives the output signal from the limiter detection circuit 8 and outputs the attenuator 6
It is also possible to use a switch for switching the signal level from the output.

FIG. 4 is a block diagram showing an embodiment of the present invention realized based on the above.

 The operation of FIG. 4 will be described below with reference to FIG.

The video signal containing the noise component shown in FIG. 5 (A) is passed through the high-pass filter 2 and the amplifier 3 to obtain the fifth signal.
The waveform shown in FIG. And the waveform (B)
Is further passed through a limiter 4 having a limiter voltage V _S1 (difference between the above-mentioned limit levels v ₁ and v ₂ ).

Here, when the output signal of the limiter 4 is directly input to the limiter detection circuit 8 as shown in FIG. 1, the limiter detection circuit 8 receives the differential pulse corresponding to the edge portion of the video signal as described above. , Even if a noise component having a peak value higher than the limiter voltage of the limiter detection circuit 8, that is, excessive noise is input, the "H" level is output as a detection signal, so that the portion corresponding to the excessive noise is An erroneous pulse occurs, and the output signal of the limiter detection circuit 8 has a waveform including the erroneous pulse as shown in FIG. 5 (C).
As a result, if noise is canceled by controlling the changeover switch 7 with the signal shown in FIG. 5 (C), noise cancellation is not performed in the erroneous pulse portion, and the reproduced screen becomes a screen in which noise is conspicuous. .

Therefore, in the embodiment shown in FIG. 4, a noise component having a high peak value that causes such an erroneous pulse,
That is, paying attention to the fact that the excessive noise is a waveform of a high frequency, for example, between the limiter 4 and the limiter detection circuit 8 so that the noise cancel operation is not turned off when the output pulse width from the limiter is narrow, that is, when the frequency is high. By inserting the low-pass filter 11 in, the level of the high frequency waveform is lowered to prevent the generation of erroneous pulses. That is, since the level of the high-frequency waveform is lowered by passing through the low-pass filter 11, even if the detection level of the limiter detection circuit 8 is set to substantially the same level as the limiter voltage V _{S1 of the} limiter 4, it is substantially the same. Specifically, as shown in FIG. 5 (B), the detection level of the limiter detection circuit 8 becomes equal to the limiter voltage V _S1 of the limiter 4 set to the voltage V _S2 of the waveform (B). Therefore, it is possible to prevent the occurrence of the above-mentioned erroneous pulse, detect only the edge portion of the video signal, and operate the noise cancel circuit properly. In addition, as a method of detecting a narrow pulse width, there is a method of limiting the pulse width by using a mono-multivibrator.

By the way, in the conventional noise canceling circuit shown in FIG. 2, the input video signal is a video signal such as a television signal (direct NTSC) in which a luminance signal component and a chrominance signal component are multiplexed. , The chrominance signal component has a higher frequency, and therefore passes through the high-pass filter 2 and is input to the limiter 4. At this time, if the amplitude of the chrominance signal component is larger than the limiter voltage of the limiter 4, The color signal component is limited by the limiter 4, and the output from the limiter 4 has a waveform shaped like the edge portion shown in FIG. 3C, and noise in that portion cannot be extracted. Therefore, even if the noise canceling operation is performed thereafter by the polarity inverting circuit 5, the adder 9, etc., the noise existing in the part of the color signal component cannot be removed like the edge part, so that the noise canceling is performed. The effect is reduced.

Therefore, in such a case, conventionally, as shown in FIG. 6, in general, the Y / C separation circuit 12 separates the luminance signal component and the chrominance signal component so that only the luminance signal component is noise-canceled. The method of applying the action is adopted.

As the Y / C separation circuit 12, a comb filter, a trap filter, or a bandpass filter (hereinafter, BP
It is known that a comb filter is more expensive than others, so a trap filter or
BPF is generally used.

However, the Y / C separation circuit 12 has a trap filter and B
In a noise canceling circuit using a PF as shown in FIG. 6, when a single color waveform is input as a direct NTSC signal, as shown in FIG. 7 (however, the color burst part is related to the explanation of the present invention). Since it is not included), in the luminance signal component (Fig. 7 (A)) and chrominance signal component (Fig. 7 (B)) after Y / C separation, high frequency components (near 3.58MHz) such as edges are Ringing occurs in the portion that has. This is because the phase characteristics of the luminance signal component and the chrominance signal component near 3.58 MHz change abruptly by passing through the trap filter and BPF.

When ringing occurs in this way and the generated ringing has a relatively large amplitude, the output waveform of the high-pass filter 2 becomes as shown in FIG. 7 (C), and if it is passed through the limiter 4 via the amplifier 3. As shown in FIG. 7D, the ringing portion has a pulse waveform as in the edge portion. Then, the signal is applied to the polarity inversion circuit 5 and the attenuator 6
When input to the adder 9 via, the output waveform of the adder 9 becomes a signal whose ringing is smaller than the luminance signal component after Y / C separation (FIG. 7 (A)) as shown in FIG. 7 (E), When the color signal components of FIG. 7 (B) are added by the adder 13 for Y / C mix, as shown in FIG. 7 (F), the ringing generated in Y / C separation is opposite in phase and the amplitude is small. The waveform has ringing, which is a problem.

Therefore, if the present invention is applied to the noise canceling circuit shown in FIG. 6, the changeover switch 7 operates in the ringing portion as in the edge portion described in FIG.
Since the signal from the limiter 4 is not input to the adder 13, the luminance signal component and the chrominance signal component input to the adder 13 have opposite phases and have the same amplitude ringing. No signal is output.

〔The invention's effect〕

According to the present invention, it is possible to eliminate the problem that the edge portion of the video signal is rather dull due to the noise canceling operation and to make the outline of the reproduction screen clear without blurring.

[Brief description of drawings]

FIG. 1 is a block diagram showing a noise canceling circuit as a circuit useful for understanding the present invention, FIG. 2 is a block diagram showing a conventional noise canceling circuit, and FIG. 3 is a block diagram showing the noise canceling circuit shown in FIGS. FIG. 4 is a waveform diagram showing the waveform of the essential signal,
FIG. 5 is a block diagram showing an embodiment of the present invention, FIG. 5 is a waveform diagram for explaining the generation of an erroneous pulse due to excessive noise, and FIG. 6 is a conventional noise canceling circuit having a Y / C separation circuit. FIG. 7 is a block diagram showing the waveform of the main signal of FIG. Explanation of symbols 1 ... video signal input terminal, 2 ... high-pass filter, 3
...... Amplifier, 4 …… Limiter, 5 …… Polarity inversion circuit, 6
…… Attenuator, 7 …… Changeover switch, 8 …… Limiter detection circuit, 9 …… Adder, 10 …… Video signal output terminal,
11 …… Low-pass filter

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Noritaka Narita 292 Yoshida-cho, Totsuka-ku, Yokohama City, Hitachi Video Engineering Co., Ltd. (56) References JP-A-60-167575 (JP, A)

Claims (1)

(57) [Claims] 1. A high-pass filter for removing a DC component from an input video signal to extract a high-frequency component, a limiter for limiting the amplitude of the extracted high-frequency component, and an output signal from the limiter. A polarity reversing circuit for reversing the polarity, adjusting means for adjusting the level of the polarity-inverted signal, and an adder for adding the output signal from the adjusting means to the video signal,
In a noise canceling circuit for removing a noise component included in the video signal, a signal whose band is limited by passing the output signal from the limiter through a low pass filter is input, and the amplitude is limited by the limiter of the signal. A first means for detecting the section, and a second means for switching the level of the output signal from the adjusting means input to the adder according to the detection signal from the first means.
And a noise canceling circuit.