JPH04301974A - Noise reducing circuit - Google Patents

Abstract

PURPOSE:To improve an S/N near a black level and to make noise inconspicuous in the black area of an image even when gamma correction having a high amplification factor near the black level of a video signal is executed by a digital signal processing. CONSTITUTION:An HPF 11 extracts the high frequency component of an input video signal from an A/D converter 4. A level detector 12 detects the level of the input video signal. A multiplier 13 amplifies the output of the HPF 11 while being controlled so as to enlarge the amplification factor near the black level corresponding to the level of the input video signal from the level detector 12. An adder 14 subtracts the output of the multiplier 13 from the input video signal and outputs the added result to a camera process circuit 5 composed of a gamma correction circuit or the like.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise removal circuit, and more particularly to a noise removal circuit for a video camera or the like having a so-called gamma correction function.

0002

2. Description of the Related Art A video camera includes an image sensor such as a CCD image sensor (hereinafter referred to as CCD), and a so-called AGC (Automatic Gain Control) applied to a video signal from the image sensor.
It is composed of an AGC circuit that performs control) and a so-called camera processing circuit that performs various signal processing on the video signal from the AGC circuit. The camera process circuit is provided with various signal processing circuits such as so-called contour compensation (aperture distortion compensation), pedestal clamp, white clip, knee, gamma correction, and black clip. This video camera applies AGC to a video signal from an image sensor obtained by photographing a subject, and also performs various signal processing such as gamma correction and outputs the signal.

[0003] The gamma correction described above is for correcting the nonlinear characteristic of the cathode ray tube of a monitor receiver, for example, since the electric/optical conversion characteristic has a nonlinear characteristic of γ = 2.2. In order to reduce the cost on the side, gamma correction is now provided on the transmitter side, that is, on the video camera.

Conventional video cameras (hereinafter referred to as analog video cameras) perform signal processing such as gamma correction on the analog video signal from the image sensor in the analog signal state.
This gamma correction was performed using the so-called forward characteristic of the diode.

[0005]

Incidentally, in recent years, digital signal processing technology has developed, and video cameras (hereinafter referred to as digital video cameras) that perform signal processing such as gamma correction using digital signals have been provided. Since this digital video camera performs the gamma correction described above using digital signal processing, a problem that did not need to be considered with analog video cameras has arisen: noise is noticeable near the so-called black level of the video signal, that is, in areas where the image is black. .

In other words, the gamma correction characteristic has the characteristic of amplifying, for example, 4 to 5 times near the black level of the video signal, but analog video cameras utilize the forward characteristics of diodes, so the black level of the video signal is amplified by a factor of 4 to 5. In the vicinity, the characteristic is a gentle one where the amplification factor gradually increases from zero. On the other hand, with digital video cameras,
Digital signal processing, that is, four arithmetic operations are performed using amplification factors that approximate the gamma correction characteristics prepared in advance, so there is a limit to the number of amplification factors that can be prepared in advance (generally depending on the capacity of the memory that stores various amplification factors). ), its characteristics are steep, with an amplification of 4 to 5 times, for example, even near the black level of the video signal. Therefore, in digital video cameras, noise is also amplified near the black level of the video signal due to the high amplification factor, resulting in the so-called S/N (Signal
A problem has arisen in which noise is noticeable in black areas of the image. Furthermore, noise near the black level of the video signal is easily noticeable and unsightly.

The present invention was made in view of the above circumstances, and an object of the present invention is to provide a noise removal circuit that can make noise inconspicuous near the black level of a video signal in, for example, a video camera. shall be.

[0008]

[Means for Solving the Problems] In order to solve the above problems, the present invention provides a filter for extracting high-frequency components of an input video signal, a level detection means for detecting the level of the input video signal, and a level detecting means for detecting the level of the input video signal. an amplification means for amplifying the output of the filter, the amplification factor being controlled so that the amplification factor becomes large near the black level according to the level of the input video signal from the detection means; and an amplification means for subtracting the output of the amplification means from the input video signal. and a subtraction means for outputting the result.

[0009]

[Operation] The noise removal circuit according to the present invention removes high-frequency components of the input video signal from the input video signal near the black level of the input video signal and outputs the removed signal.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the noise removal circuit according to the present invention will be described below with reference to the drawings. In this embodiment, the present invention is applied to a so-called video camera, and FIG. 1 is a block circuit diagram showing the circuit configuration of this video camera.

[0011] As shown in FIG. 1, this video camera has the following features.
CCD image sensor (hereinafter referred to as CCD) 1 and the CCD image sensor (hereinafter referred to as CCD) 1;
The so-called AGC (Automation) is applied to the imaging signal from CD1.
AGC circuit 2 that applies (Gain Control)
, an S/H circuit 3 that samples the video signal from the AGC circuit 2, and an analog/digital (hereinafter referred to as A/D) conversion that converts the sampled video signal from the S/H circuit 3 into a digital signal. a noise removal circuit 10 that removes high-frequency components of the video signal from the video signal converted into a digital signal from the A/D converter 4 according to the level; A camera process circuit 5 that performs various signal processing such as contour compensation, pedestal clamp, white clip, knee, gamma correction, and black clip on the signal, and converts the digital video signal from the camera process circuit 5 into an analog signal. Digital/analog (hereinafter referred to as D/A)
It is composed of a converter 6.

[0012] This video camera captures the subject at C.
AGC is applied to the video signal obtained by shooting with CD1, and after converting to a digital signal, various signal processing such as gamma correction is performed, and the video signal that has been subjected to this signal processing is sent to terminal 7 as a digital signal. It is designed to be output via . Further, if necessary, the signal is converted into an analog signal by a D/A converter 6 and outputted via a terminal 8.

The noise removal circuit 10 includes a high pass filter (hereinafter referred to as HPF) 11 that passes high-frequency components of the video signal, a level detector 12 that detects the level of the video signal, and a level detector 12 that detects the level of the output of the HPF 11. vessel 12
a multiplier 13 that amplifies the output of the multiplier 13 according to the detection result of the multiplier 13, an adder 14 that subtracts the output of the multiplier 13 from the video signal, and an adder 14 that subtracts the output of the multiplier 13 from the video signal;
and a switch 15 for disconnecting the adder 14, and the video signal from the D/A converter 4 is transmitted to the HPF 11.
, a level detector 12 and an adder 14, and the output of the adder 14 is supplied to the camera process circuit 5.

The operation of the noise removal circuit 10 will now be explained. The HPF 11 extracts the high frequency component of the video signal from the A/D converter 4 and transmits this high frequency component to the multiplier 1.
Supply to 3.

On the other hand, the level detector 12 detects the level of the video signal from the A/D converter 4 and supplies the multiplier 13 with a multiplication coefficient K corresponding to the level. Specifically, as shown in FIG. 2, for example, the coefficient K is 1 when the level is zero, and gradually decreases as the level increases until the level reaches 4 to 5% of the so-called white level. It has the characteristic that it sometimes becomes zero.

Multiplier 13 multiplies the output of HPF 13, that is, the high frequency component of the video signal, by a coefficient K, and supplies the multiplication result to adder 14 via switch 15. As a result,
The multiplier 13 outputs a high-frequency component of the video signal, that is, a component containing a lot of noise (hereinafter referred to as a noise component) near the black level of the video signal, and the level of the video signal is 4 to 5% of the white level. Up to this point, a signal obtained by multiplying the noise component by a coefficient K that gradually decreases is output, and when the level of the video signal is 4 to 5% or more of the white level, a signal of zero level is output.

Adder 14 subtracts the output of multiplier 13 from the video signal from A/D converter 4. Therefore, the output of the adder 14 becomes a video signal with noise components removed near the black level, and high-frequency components are not removed in a large level range, that is, there is no deterioration in resolution or sharpness. It becomes a video signal.

The video signal from which noise has been removed near the black level in the noise removal circuit 10 in this manner is supplied to the camera processing circuit 5. The camera processing circuit 5 performs various signal processing such as gamma correction on the video signal from the noise removal circuit 10 by digital signal processing.

By the way, as described in the prior art, this gamma correction has a large amplification factor of 4 to 5 times in the vicinity of the black bell of the video signal, and also because the gamma correction is performed by digital signal processing. Although it is not possible to have a smooth characteristic near the black level due to limitations such as the capacity of the memory that stores the amplification factor, the video signal from the noise removal circuit 10 has noise components removed near the black level as described above. Therefore, even if large amplification is performed by gamma correction, the S/N (Signal to Noise)
ratio). In other words,
By applying signal processing to the video signal to remove high-frequency components near the black level, it is possible to improve the S/N near the black level, and perform gamma correction that makes noise less noticeable in areas where the image is black. Can be done.

Note that the noise removing circuit 10 is designed so that the user can turn off the effect of the noise removing circuit 10 by turning off the switch 15.

It should be noted that the present invention is not limited to the above-described embodiments, and it goes without saying that the present invention can be applied to so-called process circuits such as video tape recorders.

[0022]

[Effects of the Invention] As is clear from the above explanation, the present invention extracts the high frequency component of the input video signal and subtracts this high frequency component from the input video signal according to the level of the input video signal. This makes it possible to improve the S/N ratio near the black level and eliminate noise in the black areas of the image, even when digital signal processing performs gamma correction that has a large amplification factor near the black level of the video signal. It can be done inconspicuously.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram showing the circuit configuration of a video camera to which the present invention is applied.

FIG. 2 is a diagram showing noise removal characteristics of a noise removal circuit that constitutes the video camera.

[Explanation of symbols]

11...High pass filter 12...Level detector 13... Multiplier 14... Adder

Claims (1)

[Claims] 1. A filter for extracting a high frequency component of an input video signal, a level detection means for detecting the level of the input video signal, and a filter for detecting a high frequency component near the black level according to the level of the input video signal from the level detection means. The amplification means is controlled such that the amplification factor is large, and includes an amplifying means for amplifying the output of the filter, and a subtracting means for subtracting the output of the amplifying means from the input video signal and outputting the result. Noise removal circuit.