JP2000196916A - Noise reducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a noise reducing device which does not increase after images when the gain of an AGC is small and can increase its noise reducing effect when the gain is large. SOLUTION: A noise reducing device is provided with an automatic gain control circuit 3 which automatically controls the amplitudes of video signals outputted from an image pickup element 1. In addition, the device is constituted in such a way that, when the device reduces the noise of the video signals by adding a signal obtained by multiplying the signal obtained from a subtractor 10 which makes subtraction on the present video signal and the vide signal of one frame or field before by a prescribed coefficient after limiting the signal with a prescribed value, the device controls the limit value in accordance with the value of the gain of the control circuit 3. The device controls the limit value so that the value may become larger as the gain of the AGC becomes larger.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal noise reduction apparatus for a video camera.

[0002]

2. Description of the Related Art Generally, a video camera is provided with an automatic gain control circuit (hereinafter referred to as an AGC circuit) for automatically controlling the amplitude of a video signal output from an image pickup device. Then, the gain of the AGC circuit is automatically increased so that a sufficient video output level is obtained. As a result, the video camera can capture an image in a dark place.

However, when the gain of the AGC circuit increases, the amplitude of the noise component increases together with the video signal, and the ratio of the noise component to the video signal increases (so-called S / N ratio deteriorates), resulting in an unsightly image.

Therefore, a conventional video camera is configured so that noise is not noticeable by inserting a circuit for reducing a noise component.

FIG. 7 is a block diagram showing a configuration of a noise reduction device in a conventional video camera.

As shown in FIG. 7, an imaged video signal is converted into an electric signal by an image sensor 1 and input to an AGC circuit 3 via a sample / hold circuit (hereinafter, S / H circuit) 2. The S / H circuit 2 samples and holds the electric signal for each pixel generated by the photoelectric conversion in the image sensor 1 using a clamp pulse to extract a signal portion. The extracted signal component is amplified by an AGC circuit 3 with a predetermined gain, then converted into a digital signal by an A / D converter 4, subjected to signal processing such as gamma and detail processing by a signal processing circuit 5, and processed into an image. Output as a signal.

The control circuit 13 determines the gain of the AGC circuit 3 based on the brightness detection signal S1 output from the signal processing circuit 5, and controls the gain of the AGC circuit 3 with the gain control signal S2. Generally, the gain of the AGC circuit 3 is controlled so that the brightness detection signal S1 becomes a certain level.
That is, control is performed so that the gain of the AGC circuit 3 increases as the brightness detection signal S1 decreases.

The video signal output from the signal processing circuit 5 passes through an adder 6 in the noise reduction circuit
The signal is converted into an analog video signal by the / A converter 7, NTSC encoded by the encoder 8, added with various signals such as a synchronization signal, and output.

The noise reduction circuit block 14 is a block for reducing a noise component by utilizing that a video signal has a correlation between frames and a noise component has no correlation between frames. A frame memory 9 as a delay means, a subtractor 10, a limit circuit 11, and a multiplication circuit 12.

The current video signal and the video signal one frame before (that is, the video signal delayed by one frame by the frame memory 9) are subtracted by a subtractor 10 and the subtracted signal is passed through a limit circuit 11 and a multiplication circuit 12 to be an adder. In step 6, the noise is reduced by being added to the current video signal.

The limit circuit 11 has the input / output characteristics shown in FIG. 8, and when the output signal of the subtracter 10 is higher than a certain level, the signal is limited to a predetermined level (limit value D) so that a moving picture (limit value D) is obtained. In this case, the feedback amount is limited for the output of the subtracter 10 for the output of the subtracter 10 when the image of the still image is completely stationary. Indicates that only a noise component appears, but 1 indicates that a moving object is being imaged.
It also includes the difference between the video components of the video signal before the frame and the current video signal. An afterimage occurs because a signal including the video difference is added to the current video signal. ) Is prevented from increasing.

The multiplication circuit 12 has a feedback coefficient K (0 ≦ K <1)
The effect of noise reduction increases as K approaches 1, but the afterimage of a moving image increases. Therefore, the feedback coefficient K is set together with the limit value according to the camera performance and usage conditions.

As described above, in the above-described conventional noise reduction device for a video camera, an increase in noise components due to an increase in gain in the AGC circuit is made inconspicuous in the noise reduction circuit.

However, in the above-described conventional noise reduction device, when the limit value of the limit circuit is increased to obtain the noise reduction effect when the gain in the AGC circuit is large, the amount of feedback increases and the noise reduction effect increases, but the noise reduction effect increases. The afterimage when a certain subject is imaged becomes large,
Even when the gain in the C circuit is small and the noise component is not a concern, increasing the limit value has a problem that an afterimage is conspicuous.

[0015]

As described above, in the conventional noise reduction device, when the limit value of the limit circuit is increased in order to obtain a noise reduction effect when the gain of the AGC is large, a moving object is imaged. Has a problem that even if the gain is small and the noise component is not a concern, increasing the limit value makes the afterimage noticeable.

The present invention has been made in view of the above problems, and
It is an object of the present invention to provide a noise reduction device capable of increasing the noise reduction effect when the gain of C is small without increasing the afterimage when the gain of C is small.

[0017]

According to the first aspect of the present invention,
In the photoelectric conversion unit of the image sensor, every one field period or one
A noise reduction device in a video camera for reducing a noise component from a video signal obtained based on charges accumulated and exposed every n times (n is a natural number) of a frame period, wherein a video output from the image sensor is provided. An automatic gain control circuit that automatically controls the amplitude of the signal, and an adder that uses the video signal that has passed through the automatic gain control circuit as one of the addition inputs,
Delay means for delaying the output of the adder by one frame;
A subtractor for calculating a difference between the video signal delayed by one frame from the delay unit and the video signal input to the adder; a limit circuit for limiting the output from the subtractor to a predetermined value and outputting the output; Multiplying an output from the limit circuit by a predetermined coefficient, and controlling a limit value of the limit circuit according to a gain value of the automatic gain control circuit and a multiplication circuit serving as the other addition input of the adder. Means.

According to the first aspect of the present invention, there is provided an automatic gain control circuit for automatically controlling the amplitude of the video signal output from the image pickup device, and the current video signal passed through the automatic gain control circuit and one frame before the current video signal. When reducing the noise of the video signal by adding a signal obtained by limiting the signal obtained by subtracting the video signal to a predetermined value and multiplying the signal by a predetermined coefficient to the current video signal, the gain of the automatic gain control circuit is reduced. And the limit value is controlled in accordance with the value of.

Thus, in the normal sensitivity mode of the video camera (in the case where a video signal is obtained based on the charges accumulated and exposed in the photoelectric conversion unit of the image sensor every one field period) or in the high sensitivity mode (in the case of the image sensor) 1 for photoelectric converter
The noise component can be reduced both in the case where the video signal is obtained based on the charge accumulated by exposure every n times (n is a natural number) of the frame period.

According to a second aspect of the present invention, a noise component is obtained from a video signal output obtained on the basis of charges accumulated and exposed in the photoelectric conversion unit of the image sensor every n times (n is a natural number) of one frame period. A noise reduction device in a video camera for reducing, an automatic gain control circuit that automatically controls the amplitude of a video signal output from the image sensor, and one of the addition inputs the video signal that has passed through the automatic gain control circuit. An adder for performing the above operation; a delay unit for delaying the output of the adder by one field; a subtractor for obtaining a difference between the video signal delayed by one field from the delay unit and the video signal input to the adder; A limit circuit for limiting the output from the subtractor by a predetermined value and outputting the result; and a multiplication unit for multiplying the output from the limit circuit by a predetermined coefficient and using the result as the other addition input of the adder. And road, in which and means for controlling the limit value of the limit circuit in accordance with the value of the gain of the automatic gain control circuit.

According to the second aspect of the present invention, there is provided an automatic gain control circuit for automatically controlling the amplitude of the video signal output from the image pickup device, and the current video signal having passed through the automatic gain control circuit and one field before. When reducing the noise of the video signal by adding a signal obtained by limiting the signal obtained by subtracting the video signal to a predetermined value and multiplying the signal by a predetermined coefficient to the current video signal, the gain of the automatic gain control circuit is reduced. And the limit value is controlled in accordance with the value of.

Thus, in the high sensitivity mode of the video camera (in the case where a video signal is obtained based on the charges accumulated and exposed in the photoelectric conversion unit of the image sensor every n times (n is a natural number) of one frame period). In addition, noise components can be reduced.

According to the second aspect, in the high-sensitivity mode, it is not necessary to use a frame memory as the delay means, and it is sufficient to use a field memory for one-field delay. In this case, the memory capacity is small and the cost is advantageous, for example, when configuring a long-exposure video camera with dropped frames.

According to a third aspect of the present invention, in the noise reduction device according to the first or second aspect, the means for controlling the limit value includes a value of the limit value as the gain of the automatic gain control circuit increases. Is set to be large.

According to the third aspect of the present invention, the limit value is set to be large as the gain of the AGC increases,
By controlling the limit value to be set smaller as the gain becomes smaller, the limit value is set smaller when the gain is smaller, the video signal with less afterimage is suppressed by suppressing the noise reduction effect, and the limit value is set when the gain is larger. Is set to be large, and the noise reduction effect is increased, so that a video signal with less noise can be obtained.

[0026]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram illustrating a configuration of a noise reduction device according to an embodiment of the present invention. The same parts as those in FIG. 7 are described with the same reference numerals.

The noise reduction device according to the present embodiment
An image sensor 1, an S / H circuit 2, an AGC circuit 3, an A /
It comprises a D converter 5, an adder 6, a D / A converter 7, a frame memory 9 as delay means, a subtractor 10, a limit circuit 11A, a multiplication circuit 12, and a control circuit 13A. ing. Since the block configuration is almost the same as the configuration in FIG. 7, only the portions different from FIG. 7 will be described.

The control circuit 13A determines the gain of the AGC circuit 3 based on the brightness detection signal S1 output from the signal processing circuit 5 and generates a gain control signal S2.
In addition to controlling the gain of the GC circuit 3, a limit value control signal S3 is generated in accordance with the determined gain of the AGC circuit 3, thereby controlling the limit value of the limit circuit 11.
The limit circuit 11 has the input / output characteristics shown in FIG. 2 and limits the signal to a predetermined level when the output signal of the subtractor 10 is higher than a certain level.
The limit value is controlled by the limit value control signal S3 from A (according to the gain of the AGC as shown in FIG. 2).

The difference from FIG. 7 is that the control circuit 13A can control the limit value of the limit circuit 11A according to the gain of the AGC circuit 3, as shown in FIG. Other configurations are the same as those in FIG.

In FIG. 1, the control circuit 13A determines the gain of the AGC circuit 3 based on the brightness detection signal S1, and at the same time, controls the limit value of the limit circuit 11A according to the gain by the limit value control signal S3.

In the present embodiment, as in the input / output characteristics of the limit circuit 11A (FIG. 2), the limit value is switched between A, B and C in three stages according to the magnitude of the gain of the AGC circuit 3. .

For example, when the gain of the AGC is changed from 0 dB to 18 d
When the range up to B (decibel) can be changed in increments of 1 dB, the limit value is set to C when the gain setting is 0 to 6 dB, and the limit value is set to B when the gain setting is 7 to 12 dB. Is set to 13 to 18 dB, the limit value is set to A. The relationship between the gain and the limit value depends on the camera performance,
The noise reduction effect and the afterimage are determined so as to be optimal depending on the use conditions. In general, the noise increases as the gain increases, so that it is easier to view the image if the noise is reduced than the afterimage. Therefore, the limit value may be set to a value that increases as the gain increases.

In FIG. 2, the limit value is switched stepwise according to the magnitude of the gain. However, the limit value may be switched continuously according to the magnitude of the gain.

In the embodiment of FIG. 1, a frame memory for delaying one frame is used as the delay means in the noise reduction circuit block 14. However, when the video camera is used in the high sensitivity mode, As the delay means, a field memory for delaying one field may be used instead of the frame memory.

As described above, in the high sensitivity mode of the video camera, a circuit using a field correlation may be used as the noise reduction circuit block 14. Therefore, the normal sensitivity mode and the high sensitivity mode will be described below.

That is, in the noise reduction circuit of the video camera, when the noise component is reduced from the video signal obtained based on the charge accumulated and exposed in the photoelectric conversion unit of the image sensor every field period (in the normal sensitivity mode) In
The video signal has a correlation between frames (this is because the odd field and the even field constituting one frame are interlaced and have a small correlation and a relatively large correlation between the frames), and a noise component Is used to extract only the noise component by taking the difference between the currently input video signal and the video signal delayed by one frame by utilizing the fact that there is no correlation between the frames. Noise can be reduced by subtracting from the signal.

In a noise reduction circuit in a video camera, a field based on charges accumulated in the photoelectric conversion unit of the image sensor every n times (n is a natural number) of one frame period and no exposure charge is applied to a field. When the noise component is reduced from the video signal obtained by performing interpolation (in the high sensitivity mode), the video signal is always fixed to one of the odd field and the even field. Utilizing the fact that the noise component has no correlation between the fields,
By taking the difference between the currently input video signal and the video signal delayed by one field, only the noise component is extracted, and by subtracting this from the current video signal, noise can be reduced. In the case of the high-sensitivity mode, it is also possible to extract a noise component by calculating a difference between the current video signal and the video signal delayed by one frame.

Generally, in a video camera, a charge coupled device (CCD) is used as a solid-state imaging device. The normal sensitivity mode and the high sensitivity mode will be described with reference to FIGS.

FIG. 3 is a diagram showing an example of the configuration of a CCD. The CCD shown in FIG. 3 includes a photoelectric conversion unit 21 that receives light from a subject and converts the light into an electric signal.
The signal charge accumulated in the field shift pulse (FS)
Vertical transfer unit 22 which is transferred in synchronization with
2 includes a horizontal transfer unit 23 in which signal charges for one screen transferred to one line are transferred for each line, and an output unit 24 for outputting the horizontally transferred signal charges from a terminal 25 for each pixel.

FIG. 4 shows a timing chart of the CCD output and the video output in the normal sensitivity mode of the video camera. The exposure time of the CCD video camera in the normal sensitivity mode is one field (1/60 s).

FIG. 4A shows an exposure time (one field), FIG. 4B shows a field shift pulse (FS) for reading signal charges from the photoelectric conversion unit 21 to the vertical transfer unit 22, and FIG.
4 shows a CCD output signal, and FIG. 4D shows a video output signal.

As shown in FIG. 4B, a field shift pulse is output to the CCD for each field.
Thus, the charges accumulated in the photoelectric conversion unit 21 for one field period (that is, the periods A, B, C, and D shown in FIG.
.. Are output as CCD output signals delayed by one field period as shown in FIG. 4 (c), which are further processed by signal processing means (signal processing circuit 5, encoder 8, etc.). The video signal is converted and output as shown in (d).

On the other hand, in a CCD video camera (or a frame dropping long exposure video camera) in the high sensitivity mode,
By increasing the exposure time from normal one field to 2, 4, 6,... Fields by thinning out the field shift pulse, high sensitivity is realized. Note that the change in the exposure time is made every two fields.
This is to prevent the jitter of the video output by fixing the odd field / even field (ODD / EVEN) of the signal charge to be read to any one.

Next, with reference to FIG. 5, the high sensitivity mode operation by long time exposure will be described. FIG. 5 shows a timing chart of the CCD output and the video output in the high sensitivity mode of the video camera.

FIG. 5A shows an exposure time (two fields), FIG. 5B shows a field pulse for reading signal charges from the photoelectric conversion unit 21 to the vertical transfer unit 22, and FIG. 5C shows a CCD output signal. 5 (d) shows a video output signal.

FIG. 5B shows an example in which a field shift pulse is output every two fields instead of every field. The CCD output signal is output every two fields as shown in FIG. This CCD output signal is 2
Since the charges accumulated and exposed during the field period 33 are output during the one-field period 31, a signal exposed for a long time can be obtained.

However, since there is no exposure signal output in the field period 32 following the field period 31, it is necessary to interpolate the field period 32 with the exposure signal of the field period 31 using the memory of the signal processing circuit 5 at the subsequent stage. In this manner, long-time exposure is performed by the photoelectric conversion unit 21, and a video output signal shown in FIG. 5D with high sensitivity is obtained. The memory interpolation is performed by using a known method. For example, the signal immediately before the output of the exposure signal is lost is used as it is, or the signal before and after the output of the exposure signal is lost is obtained by interpolation (using the average value before and after).

Automatic control of the exposure time according to the brightness of the video output as shown in FIGS. 4 and 5 is called automatic sensitivity switching. In the automatic sensitivity switching, for example, the level of the integrated value or the peak value of the luminance signal is compared with two threshold values of light and dark, and when it is determined that the image is dark, the exposure time is set to 2 fields (1/30 s) and 4 fields. ,..., And when it is determined that the image is bright, the exposure time is reduced by two fields or only one field. That is, when the image is dark, a field shift pulse having an extended period of 2 field periods, 4 field periods,..., And when the image is bright, a field shift pulse having a period shortened by 2 field periods or a field shift pulse of 1 field period. Is supplied to the CCD.

FIG. 6 shows another embodiment of the present invention.
FIG. 2 shows a block diagram of a noise reduction device in a video camera having an automatic sensitivity switching function.

FIG. 6 is different from FIG. 1 in that an exposure time control circuit 15 and a CCD drive circuit 16 are provided. The luminance signal from the signal processing circuit 5 is led to an exposure time control circuit 15, and the level of the integrated value or the peak value of the luminance signal is compared with two threshold values of light and dark, and an exposure time control signal indicating the result of the comparison is shown. S4 is generated and CCD driving circuit 1 is generated.
6 The CCD drive circuit 16 generates a CCD drive pulse S5 including a field shift pulse (FS) to drive the image pickup device 1. The pulse cycle of FS in the CCD drive pulse S5, that is, the FS pulse thinning amount Is controlled by the exposure time control signal S4.
That is, when the exposure time control circuit 15 determines that the image is bright (in the normal sensitivity mode), the cycle of the FS is, for example, 1
The field is set to be short, the charge accumulation time of the photoelectric conversion unit of the image sensor 1 is set short, and when it is determined that the image is dark (in the high sensitivity mode), the cycle of FS is set to, for example, two fields, and the photoelectric conversion of the image sensor 1 is performed. The charge storage time of the unit is set long. Note that the exposure time (F
As described above, the setting of the S-pulse period is shortened by 2 fields, 4 fields,... When the image is dark, and is shortened by 2 field periods when the image is bright. A field-shift pulse having a period or a one-field period is supplied to an image sensor (CC
D). Alternatively, the sensitivity may be switched by enabling switching between two threshold values of light and dark. further,
If the exposure time control signal S4 from the exposure time control circuit 15 indicates that the image is dark, the frame memory 9
May be switched to a field memory.

In the above embodiment, the noise reduction device in a video camera has been described. However, the present invention is not limited to this, and is applied to a noise reduction device in a video device such as a VTR or a TV receiver. It is possible.

[0052]

As described above, according to the present invention, when the signal obtained by subtracting the current video signal passed through the automatic gain control circuit and the video signal one frame before or one field before is subjected to amplitude limitation by the limit circuit. When the gain of the automatic gain control circuit is small, the limit value is set small, and when the gain of the automatic gain control circuit is large, the limit value is set large.When the gain is small, the noise reduction effect is suppressed and the afterimage is reduced. When a small video signal is obtained and the gain is large, it is possible to obtain a noise-free video signal by increasing the noise reduction effect.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a noise reduction device in a video camera according to an embodiment of the present invention.

FIG. 2 is a diagram showing input / output characteristics of the limit circuit shown in the embodiment of FIG.

FIG. 3 is a diagram showing an example of a configuration of a CCD.

FIG. 4 is a CCD in a normal sensitivity mode of a video camera.
6 is a timing chart showing the timing of output and video output.

FIG. 5 is a timing chart showing timings of a CCD output and a video output in a high sensitivity mode of the video camera.

FIG. 6 is a block diagram showing a noise reduction device in a video camera according to another embodiment of the present invention.

FIG. 7 is a block diagram showing a conventional noise reduction device.

8 is a diagram showing input / output characteristics of the limit circuit shown in the conventional example of FIG. 7;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Image sensor 3 ... Automatic gain control circuit 6 ... Adder 9 ... Frame memory (delay means) 10 ... Subtractor 11A ... Limit circuit 12 ... Multiplier circuit 13A ... Control circuit 14 ... Noise reduction circuit block S1 ... Brightness detection Signal S2: Gain control signal S3: Limit value control signal

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masatoshi Okubo 1-9-2 Hara-cho, Fukaya-shi, Saitama Prefecture Inside the Toshiba Fukaya Plant (72) Inventor Akira Nakao 1-9-1-2 Harara-cho, Fukaya-shi, Saitama No. Inside the Toshiba Fukaya Plant (72) Inventor Kei Tashiro 1-9-2 Hara-cho, Fukaya-shi, Saitama Prefecture Inside the Toshiba Fukaya Plant (72) Inventor Tetsuo Sakurai 3-3-1-9 Shimbashi, Minato-ku, Tokyo F-term (reference) in Toshiba AE Corporation 5C021 PA02 PA12 PA17 PA53 PA64 PA67 PA72 PA78 PA82 PA83 PA85 PA89 PA92 RA16 RB01 SA24 XA03 XA13 YA01 YC03 ZA02 5C026 BA05 BA11 BA20

Claims (3)

[Claims] 1. A noise component is reduced from a video signal obtained on the basis of electric charges accumulated and exposed in a photoelectric conversion unit of an image sensor every one field period or every n times (n is a natural number) of one frame period. A noise reduction device for a video camera, comprising: an automatic gain control circuit that automatically controls the amplitude of a video signal output from the image sensor; and a video signal that has passed through the automatic gain control circuit as one of the additional inputs. An adder; delay means for delaying the output of the adder by one frame; a subtractor for calculating a difference between the video signal delayed by one frame from the delay means and the video signal input to the adder; A limit circuit for limiting the output from the subtracter by a predetermined value and outputting the result; and multiplying the output from the limit circuit by a predetermined coefficient to obtain the other input of the adder. Noise reducing apparatus characterized by comprising a circuit, and means for controlling the limit value of the limit circuit in accordance with the value of the gain of the automatic gain control circuit. 2. The noise reduction in a video camera for reducing a noise component from a video signal obtained on the basis of charges accumulated and exposed in a photoelectric conversion unit of an image sensor every n periods (n is a natural number) of one frame period. An automatic gain control circuit that automatically controls the amplitude of a video signal output from the image sensor; an adder that receives the video signal that has passed through the automatic gain control circuit as one addition input; Delay means for delaying the output of the adder by one field, a subtractor for taking the difference between the video signal delayed by one field from the delay means and the video signal input to the adder, and an output from the subtractor A limit circuit for limiting and outputting a predetermined value, a multiplication circuit for multiplying an output from the limit circuit by a predetermined coefficient and using the output as the other addition input of the adder, Means for controlling a limit value of the limit circuit according to a gain value of a control circuit. 3. The noise reduction apparatus according to claim 1, wherein said means for controlling the limit value sets the limit value to be larger as the gain of the automatic gain control circuit increases. apparatus.