JPH03120967A - Noise reduction circuit - Google Patents

Abstract

PURPOSE:To eliminate the noise component of a large amplitude and of a moving picture part by providing a high pass filter extracting a high frequency component from an output signal of a 1st adder circuit and a 2nd limiter extracting a noise component from an output signal of the high pass filter. CONSTITUTION:Since a noise equivalent signal (a) outputted from a 2nd adder circuit 3, when it is a noise component of a moving picture or a large amplitude, exceeds a prescribed level, the level is limited by a 1st limiter 5 and the noise is not fed to a 1st adder circuit 2. The noise component of an input video signal (c) from the 1st adder circuit 2 is extracted by a high pass filter 8 and a 2nd limiter 9, and the extracted noise component is fed to a 3rd adder circuit 7 via a switch circuit 10. Thus, even the noise component or the large amplitude noise component in existence in mixture in the moving part when the input video signal is moving is eliminated surely.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a noise reduction circuit used in a video tape recorder (VTR) or the like to reduce noise components in a video signal.

(Prior Art) In a VTR, several noise reduction methods are employed to improve the apparent S/N ratio of a reproduced video signal. For example, there are methods that treat high-frequency low-level signals as noise components and use a noise canceller to remove these noise components, and methods that use a comb filter and utilize the line correlation of the video signal to reduce noise components. It is.

In addition, in recent years, with the decline in the price of digital memory, it has become possible to use large-capacity memory as a 1-field delay line or 1-frame delay line. Filters have also been used to reduce noise components. FIG. 3 shows a conventionally known noise reduction circuit using such a comb filter.

In this figure, a video signal input to an input terminal 1 is supplied to adder circuits 2 and 3, and the output signal of the adder circuit 2 is delayed by a delay circuit 4 consisting of, for example, a 1-field delay line, and then sent to an adder circuit 3. and subtracted from the input video signal. The output signal of this addition port 83 is supplied to the addition port 82 via the limiter 5, and is subtracted from the input video signal.

In this configuration, the output signal a of the adder circuit 3 is an inter-field difference signal, which is a signal obtained by extracting a component equivalent to noise from the input video signal, and a signal obtained by passing this addition output signal a through the limiter 5. By subtracting the noise from the input video signal in the adding circuit 2, a video signal output with reduced noise components can be obtained. By repeating this operation cyclically in the noise reduction loop L, a video signal from which noise components have been removed can be extracted from the output terminal 6.

Here, the limiter 5 is provided to prevent movement in the input from being mistaken as noise and causing an afterimage phenomenon in the output image. As shown in Fig. 4, when the amplitude of limiter power a (output signal a of addition port I43) exceeds A, the limiter output decreases, and when the amplitude of limiter power a exceeds B, the limiter output decreases. becomes zero. Therefore, for example, when the limiter input a, which is the inter-field difference signal, exceeds the amplitude B in a moving part of the image, the limiter output becomes no signal, and no afterimage component is output to the output terminal 6.

(Problem to be Solved by the Invention) By the way, as is clear from the explanation of the operation of the limiter 5, the conventional noise reduction circuit described above is not effective in removing noise for noise components higher than the limiter level, and it also reduces the image quality. There was a problem in that there was no noise removal effect at all even for moving parts (video parts).

The present invention was proposed to solve these problems, and an object of the present invention is to provide a noise reduction circuit that can effectively remove large amplitude noise components and noise components in video parts. shall be.

[Structure of the Invention] (Means for Solving the Problem) In order to achieve the above object, a noise reduction circuit according to the present invention includes a first addition circuit to which an input video signal is supplied, and a first addition circuit of the first addition circuit. a delay circuit that delays an output signal; a second addition circuit that subtracts the output signal of the delay circuit from the input video signal to extract a signal equivalent to noise; and a signal of a predetermined level or higher among the output signals of the second addition circuit. a first limiter that prevents passage of the noise signal, and the noise reduction circuit performs noise removal by subtracting the noise signal output from the first limiter from the input video signal with the first addition circuit, 1st above
a third adder circuit to which the output signal of the adder circuit is supplied;
a level detection circuit that detects the level of the output signal of the second addition circuit and outputs a switching control signal when the output signal is above a predetermined level; and a level detection circuit that extracts a high frequency component from the output signal of the first addition circuit. A high-pass filter and a second filter that extracts noise components from the output signal of this high-pass filter.
a limiter, and a switch circuit inserted in series between the second limiter and the third adder circuit, the switch being closed by the switching control signal;
The adder circuit is characterized in that the output signal of the limiter supplied via the switch circuit is subtracted from the output signal of the first adder circuit.

(Function) According to the above-described configuration, the noise-equivalent signal output from the second addition circuit exceeds a predetermined level when it is a moving image part or large-amplitude noise, so it is blocked by the first limiter and It is not supplied to the adder circuit. in this case,
The input video signal is directly output from the first adder circuit,
A switching control signal is supplied to the third adder circuit and the high-pass filter, and a switching control signal is supplied from the level detection circuit to the Swiss-Sochi circuit to close the switch.

A noise component is extracted from the input video signal from the first adder circuit by a high-pass filter and a second limiter, and the extracted noise component is passed through the Sui-Sochi circuit to the third
is supplied to the adder circuit.

In the third adder circuit, this noise component is subtracted from the input video signal from the first adder circuit, so noise components and large-amplitude noise components that are mixed when there is movement in the input video signal (moving image part) are also subtracted. Noise removal can be performed reliably.

(Example) Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

The block diagram of FIG. 1 shows one embodiment of a noise reduction circuit according to the present invention. Note that parts similar to those of the conventional example are given the same reference numerals, and explanations of overlapping parts will be omitted.

In this figure, the configuration from the input terminal 1 where the video signal is input to the part where the output signal C of the adder circuit 2 is taken out (the configuration of the noise reduction loop L) is the same as that of the conventional one.

In the noise reduction circuit according to the present invention, the adder circuit 2
The output terminal of the adder circuit 2, that is, the connection point between the adder circuit 2 and the delay circuit 4, is connected to each input terminal of the adder circuit 7 and the high-pass filter (HPF) 8, and the output signal C of the adder circuit 2 is connected to the adder circuit 7 and the high-pass filter (HPF) 8. supplying.

HPF8 extracts the high frequency component of the addition output signal C,
Supplied to limiter 9. This limiter 9 has input/output characteristics similar to those of the limiter 5, allowing very small level signals to pass through and blocking large amplitude signals. The output of the limiter 9 is a signal component having a high frequency and a small amplitude, that is, a noise component extracted from the addition output signal C.

The output signal of the limiter 9 is supplied to the addition port 87 via the switch circuit 10, and is subtracted from the addition output signal C. Note that this switch circuit 10 is normally in an open state unless a high level ("°H level") switching control signal, which will be described later, is input.

Further, the output signal a of the adder circuit 3 is output to the level detection circuit 81.
1, and the level of the output signal (interfield difference signal) a is detected. The output of this level detection circuit 11 is normally at a low level ("L, +t level"), and when a large level interfield difference signal a is detected, the output d is as shown in FIG. 2(b). is at a high level
l Lebel).

Note that FIG. 2(a) shows the input/output characteristics of the limiter 5.

The "°H°" level output signal d of the level detection circuit 11 is supplied as a switching control signal to the control input terminal of the switch circuit 10, and the switch circuit 10 is closed by this "H" level switching control signal d.

Next, the operation of the noise reduction circuit configured as described above will be explained.

As described above, by subtracting the delayed output of the delay circuit 4 from the input video signal in the adder circuit 3, the inter-field difference signal a corresponding to the noise component can be extracted as the output of the adder circuit 3.

This inter-field difference signal a is supplied to a limiter 5 and a level detection circuit 11. The level detection circuit 11 detects the level of the inter-field difference signal a and outputs "L" when the signal level is a small level.

Since the level is output, the switch circuit 10 remains open. Therefore, the output signal of limiter 9 is not supplied to adder circuit 7.

A small-level interfield difference signal a corresponding to a noise component
is supplied to the adder circuit 2 through the limiter 5 and subtracted from the input video signal, thereby reducing noise components from the input video signal. By repeating this operation cyclically in the noise reduction loop L, a video signal from which noise components have been removed is output from the output terminal 6.

On the other hand, when the level of the inter-field difference signal a is a large level, this large level signal is detected by the level detection circuit 11, and the switching control signal d of IIHI+ level is supplied to the switch circuit 10. is closed. When the inter-field difference signal a is a high level signal, the limiter 5 prevents the signal from passing, as described above, so the noise reduction function in the noise reduction loop does not work.

However, the input video signal that has passed through the adder circuit 2 (corresponding to the output signal C of the adder circuit 2) is supplied to the HPF 8, and the filter output is supplied to the limiter 9, so that noise components are removed from the input video signal C. Extracted. This noise component is supplied to the addition circuit 7 via the switch circuit 10, so the noise component is subtracted from the input video signal C in the addition circuit 7, and the video signal from which the noise component has been removed is output from the terminal 6. .

In this way, the above-mentioned noise reduction circuit can effectively remove noise components in the video portion and large-amplitude noise components that could not be removed by the noise removal loop L in which the limiter 5 is inserted to reduce afterimages. I can do it.

[Effects of the Invention] As explained above, according to the present invention, noise components in video parts and large amplitude noise components, which were difficult to remove in the past, can be successfully removed, so the S/N ratio can be improved. Can provide high quality images.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the noise reduction circuit according to the present invention, Fig. 2 is a waveform diagram for explaining the operation of the noise reduction circuit of Fig. 1, and Fig. 3 is a block diagram showing an example of the noise reduction circuit of the present invention. The block diagram shown in FIG. 4 is a waveform diagram for explaining the operation of a conventional noise reduction circuit. 2.3.7...Addition circuit 4...Delay circuit 5.9...Limiter 8...High pass filter 11...Level detection circuit

Claims (1)

[Claims] A first adder circuit to which an input video signal is supplied, a delay circuit that delays an output signal of the first adder circuit, and a delay circuit that subtracts the output signal of the delay circuit from the input video signal and eliminates noise. It has a second adder circuit that takes out the corresponding signal, and a first limiter that blocks the passage of signals of a predetermined level or higher among the output signals of the second adder circuit, and the first adder circuit extracts the corresponding signal. In the noise reduction circuit that performs noise removal by subtracting the noise signal output from the first limiter from the input video signal, a third addition application to which the output signal of the first addition circuit is supplied; a level detection circuit that detects the level of the output signal of the first addition circuit and outputs a switching control signal when the output signal is equal to or higher than a predetermined level; and a high-pass filter that extracts high-frequency components from the output signal of the first addition circuit. ,
a second limiter for extracting noise components from the output signal of the high-pass filter;
a switch circuit inserted in series between the adder circuit and the switch circuit whose switch is closed by the switching control signal,
A noise reduction circuit characterized in that the third adder circuit subtracts the output signal of the limiter supplied via the switch circuit from the output signal of the first adder circuit.