JPH0548961A - Image pickup device capable of image pickup under low illuminance - Google Patents

Abstract

PURPOSE:To enable photographing under low illuminance without deteriorating image quality by increasing the maximum gain range of an AGC circuit and synthesizing an adjacent signal. CONSTITUTION:When a low illuminance detection circuit 7 detects the low illuminance while a changeover switch 9a is located at a position S, a gain-up control circuit 8a selects a gain equivalent to twice the normal gain of a gain variable amplifier 5 and selects the MAX gain range equivalent to twice the MAX gain range of an AGC circuit 6a. At this time, an adjacent signal synthesization circuit 11 is operated to prevent the deterioration of an S/N in a gain-up mode. Thus, in the low illuminance photographing, the sensitivity can be improved by four times without deteriorating the S/N and an unnecessary wide band noise component can be eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image pickup apparatus used for a video camera or the like, and more particularly to an image pickup apparatus used for a video camera or a surveillance video camera which enables low-illuminance shooting.

[0002]

2. Description of the Related Art FIG. 7 is a block diagram showing an image pickup apparatus having a sensitivity increasing (gain increasing) function according to the prior art. In this imaging device, the incident light from the subject is
A lens 1 including an iris (aperture) forms a photographed image on the CCD image pickup device 2. CCD element 2 is
The captured image is photoelectrically converted and output as an electrical video signal. The output from the CCD element 2 has a low frequency noise removed by a CDS (correlation double sampling) circuit 3.
That is, since the output of the CCD element 2 includes noise components such as a clock signal for driving the CCD element 2 and a reset pulse, the CDS circuit 3 performs sampling for each pixel and focuses only on the video signal. To do. The output signal of the CDS circuit 3 is input to the variable gain amplifier 5 and also to the ALC (automatic light amount control) circuit 4 and the low illuminance detection circuit 7, which are also called AE (auto iris). The ALC circuit 4 controls the aperture diameter of the iris in the lens 1 so as to stabilize the level of the output signal from the CDS circuit 3. Low illuminance detection circuit 7
Can be composed of a comparator. That is,
If the average level of the output of the CDS circuit 3 falls below a predetermined level, the output of the low illuminance detection circuit 7 is, for example, "L".
The (low) level changes to the “H” (high) level.

The output of the low illuminance detection circuit 7 is input to the gain-up control circuit 8. The variable gain amplifier 5 selects either a standard gain or a gain (gain at the time of gain increase) that is about twice the standard gain (plus 6 dB) depending on the control signal from the gain increase control circuit 8. That is, when the gain up switch 9 is off, the gain up control circuit 8 causes the variable gain amplifier 5 to select the standard gain regardless of the output of the low illuminance detection circuit 7. However, when the gain up switch 9 is turned on, the low illuminance detection circuit 7 detects the low illuminance and the above-mentioned "H"
When the level signal is output, the gain-up control circuit 8 causes the variable gain amplifier 5 to select the gain at gain-up (twice the standard gain).

The output of the variable gain amplifier 5 is input to an AGC (automatic gain control) circuit 6, and even if the amount of light is such that the iris is fully opened, the variable gain amplifier 5 is turned on if the gain up switch 9 is turned on. A stable video signal level can be obtained from the AGC circuit 6 up to the amount of light that can be compensated by increasing the gain by 2 times.

The output of the AGC circuit 6 is input to the signal processing circuit 10, where various signal processing such as γ correction, color separation and white balance is performed. Signal processing circuit 1
A luminance (Y) signal and a color difference signal are output from 0 and input to the encoder circuit 12. The encoder circuit 12 performs processing conforming to TV signal standards such as blanking, synchronization signal load, and modulation by subcarriers, and as a composite (composite video) signal or separation of a luminance (Y) signal and a chroma (C) signal. A video signal is output as the S signal.

FIG. 8 shows the ALC circuit 4 with respect to the amount of incident light.
6 is a graph for explaining a region where the AGC circuit 6 can effectively work. In this graph, the horizontal axis represents the amount of incident light and the vertical axis represents the output signal level. The solid line A in the graph represents the output signal level when only the ALC circuit 4 is working effectively. Also, the solid line B
Indicates the output signal level when the gain up switch 9 is off. Further, the solid line C represents the output signal level when the gain up switch 9 is on.

That is, in the region where the amount of incident light is sufficiently large, the aperture diameter of the ALC circuit 4 or the iris is adjusted,
This allows the output signal level to be kept constant. However, when the amount of incident light decreases to point LA on the horizontal axis, the iris is fully opened. Therefore, when the light amount further decreases from the LA point, the AGC circuit 6 electrically automatically adjusts the gain amount to stabilize the output signal level.

When the amount of incident light further decreases to the point LB on the horizontal axis, the gain amount of the AGC circuit 6 becomes the maximum value. Here, when the light amount further decreases from the LB point, the gain of the variable gain amplifier 5 is increased to about twice if the gain up switch 9 is turned on, so that the AGC circuit 6 outputs the output signal level. Can be stabilized at a constant level.

However, if the amount of incident light further decreases to the LC point on the horizontal axis, the gain amount of the AGC circuit 6 reaches the maximum value again even when the variable gain amplifier 5 is in the gain up state. Therefore, if the amount of light decreases further than the LC point,
The output signal level cannot be kept constant and changes along the solid line C.

[0010]

As described above, in the prior art, when the light quantity drops below the LC point, it is no longer possible to maintain the signal output level constant. However, if an attempt is made to further increase the gain in order to enable shooting even under low illuminance, the noise component from the CCD element remaining in the video signal will be amplified together with the signal component, making it difficult to see and practically useless. It becomes a video signal.

Therefore, the present invention has an object to provide an image pickup apparatus capable of further increasing the gain without lowering the image quality of a picked-up image to a practical level or lower, thereby making it possible to take an image even in a low illuminance. I am trying.

[0012]

According to the present invention, there is provided an image pickup apparatus capable of photographing with low illuminance, a gain variable amplifier for amplifying a video signal, and a video signal amplified by the gain variable amplifier is further amplified to a predetermined level. An automatic gain control circuit for controlling, an adjacent signal synthesizing circuit for synthesizing two video signals temporally adjacent to each other to reduce a noise component, and a low signal for detecting a low illuminance of a subject from an input signal to the variable gain amplifier. Illuminance detection circuit, gain up mode setting switch for setting gain up mode, and gain up for controlling gain variable amplifier means, automatic gain control means, and adjacent signal synthesizing means according to the set gain up mode In one of the gain up modes set by the gain up mode setting switch, including the control means, the low illuminance detection is performed. When the circuit detects a low illuminance, the variable gain amplifier selects the second gain larger than the standard gain and the automatic gain control circuit selects the second maximum gain range larger than the standard maximum gain range. The feature is that the means is also operated.

[0013]

In the image pickup apparatus capable of photographing with low illuminance according to the present invention, the gain-up control circuit also increases the maximum gain range of the AGC circuit, and the S signal is increased by the adjacent signal synthesizing circuit.
Since the / N (signal / noise) ratio is improved and the high-frequency noise component is also cut, it is possible to shoot with a lower illuminance without degrading the image quality of the shot image.

[0014]

1 is a block diagram showing an image pickup apparatus according to an embodiment of the present invention. The image pickup apparatus of FIG. 1 is similar to that of FIG. 7, except that the AGC circuit 6a, the gain-up control circuit 8a, and the changeover switch 9a are changed, and further, between the signal processing circuit 10 and the encoder circuit 12. The adjacent signal synthesis circuit 11 is newly provided.

The gain up control circuit 8a is mode-selected by the changeover switch 9a. When the changeover switch 9a is in the position N, it is in the standard gain mode, when it is in the position C, it is the first gain-up mode of about 2 times (+6 dB), and when it is in the position S, it is about 4 times (+12 dB). The second gain-up mode is set.

That is, the changeover switch 9a position N
, The gain up circuit 8a causes the variable gain amplifier 5 to select the standard gain by the variable gain amplifier control signal regardless of the output of the low illuminance detection circuit 7,
The C-MAX gain control signal causes the AGC circuit 6a to select the normal first MAX (maximum) gain range. The operation at this time corresponds to the case where the changeover switch 9 in FIG. 7 is in the off state.

Next, when the change-over switch 9a is in the position C and in the first gain-up mode, the gain-up circuit 8a is standard for the variable gain amplifier 5 when the low illuminance detection circuit 7 detects low illuminance. The gain at the time of gain increase, which is twice the gain, is selected, but the AGC circuit 6a is made to maintain the normal first MAX gain range. That is, the gain increasing operation at this time corresponds to the case where the changeover switch 9 in FIG. 7 is in the ON state.

However, the changeover switch 9a moves to the position S.
Therefore, when in the second gain-up mode, the gain-up circuit 8a not only causes the variable gain amplifier 5 to select the gain at the time of gain-up when the low illuminance detection circuit 7 detects low illuminance, but also the AGC. The circuit 1a has a normal first
The second MAX gain range, which is about twice the MAX gain range of (6 dB), is selected. In other words, the sensitivity of the prior art image pickup device was only doubled, but the sensitivity of the image pickup device according to the present embodiment was increased fourfold.

FIG. 2 is a graph showing the relationship between the amount of incident light and the output signal level in the image pickup apparatus of FIG. The graph of FIG. 2 is similar to the graph of FIG. 8, but with the solid line D added. That is, the changeover switch 9a is in the position N
Therefore, when in the standard gain mode, the output signal level can be kept constant when the changeover switch 9 in FIG. 7 is in the off state and the incident light quantity is equal to or higher than the LB point. Further, when the changeover switch 9a is in the position C and in the first gain mode, it corresponds to the case where the changeover switch 9 in FIG. 7 is in the ON state, and the output signal level is constant when the incident light amount is equal to or higher than the LC point. Can be held at.

However, in the image pickup apparatus according to this embodiment, the second gain-up mode can be set by the position S of the changeover switch 9a, and the output signal level is kept constant up to the LD point where the amount of incident light is lower. obtain. That is, the image pickup apparatus of the present embodiment is capable of shooting even under a lower illuminance than the image pickup apparatus of the prior art. However, when the light amount further decreases below the limit of the LD point, the output level changes along the solid line D.

By the way, the normal MAX of the AGC circuit 6a
The gain range is set to such a range that deterioration of the S / N ratio does not pose a practical problem. Therefore, if the MAX gain range of this AGC circuit is controlled and the MAX gain range is simply expanded, the S / N ratio may be deteriorated to a level not practical. Therefore, S in the second gain-up mode for expanding the MAX gain range of the AGC circuit
In order to prevent the deterioration of the / N ratio, the image pickup apparatus of FIG. 1 is provided with the adjacent signal synthesis circuit 11.

FIG. 3 shows a concrete example of the adjacent signal synthesizing circuit 11.
It is a circuit diagram which shows an example. This adjacent signal synthesis circuit 11
Is two transistors Tr₁, Tr₂; Four resistors R
₁, R ₂, R₃, R_FourOne capacitor C₁One
Delay line DL and two positions
One analog switch A-SW with 3A, 3B
Contains.

A control signal from the gain-up control circuit 8a is applied to the analog switch A-SW. When the changeover switch 9a of FIG. 1 is in the position N or C and is in the standard gain mode or the first gain-up mode, the analog switch A-SW is set to the position 3A, and the adjacent signal synthesis circuit 11 passes the input signal as it is. I will let you. This is standard gain mode or first
This is because the MAX gain range of the AGC circuit is not expanded to the second range in the gain-up mode of (1) and it is not necessary to improve the S / N ratio.

However, the changeover switch 9a moves to the position S.
And the low illuminance detection circuit 7 detects low illuminance, the analog switch A-SW is set to the position 3B. As a result, the adjacent signals are combined and the deterioration of the S / N ratio due to the expanded second MAX gain range of the AGC circuit 6 is improved. At this time, if the capacitor C ₁ is omitted, the signal flow is shown in FIG. The configuration is similar to that of the comb filter. However, the delay line DL in this case is not a 1H (horizontal) delay line but 10
It is a delay line of about 0 to 150 ns. That is, the input signal X _n is added to the immediately preceding input signal X _n -1 delayed by the delay line DL, and the output signal Y _n = (X _n + X
It is output as _n-1 ) / 2. At this time, since the noise components have no correlation between the adjacent signals, the noise components tend to cancel each other, and the ratio to the signal components is reduced. That is, this improves the S / N ratio.

FIG. 5 shows the frequency characteristics of the adjacent signal combining circuit of FIG.
It is a graph showing sex. The horizontal axis represents frequency (Hz)
However, the vertical axis represents the input / output ratio (dB). Curve in graph
5A, the analog switch A-SW in FIG. 3 is in position 3
Corresponding to the case of A, the input signal is
You can see that you are passing through the road. On the other hand, the curve 5B is shown in FIG.
The analog switch A-SW is in position 3B
Capacitor C₁If is omitted,
(Corresponding to the signal flow in FIG. 4). This and
Then, the delay amount of the delay line DL is set to T = 150 nS, for example.
If so, the trap frequency f_p= 1 / 2T, so f_p≒
It becomes 3.3 MHz. Where capacitor C ₁Insert
If so, the frequency characteristic becomes like curve 5C, and
Wave number f_pNoise components in the high frequency range above and near are even lower
You can see that it will be reduced. This will further increase the S / N ratio
Will be improved.

The adjacent signal synthesizing circuit 11 can also be constructed by using the low pass filter LPF itself as shown in FIG. However, in this case, a double amplifier is required, which is more disadvantageous than the case of the adjacent signal combining circuit of FIG. 3 in that the circuit configuration becomes complicated. That is, in the adjacent signal synthesizing circuit of FIG. 6, three transistors Tr _{3 to} Tr ₅ and seven resistors R _{5 to} R are additionally provided as compared with those of FIG.
_{1 1} is required (however, the capacitor C ₁ is unnecessary). In FIG. 6, R ₂ = R ₃ , R ₄ = R ₅ ,
R ₈ = R ₉ .

To summarize the characteristic functions of the image pickup apparatus of FIG. 1, if the low illuminance detection circuit 7 detects low illuminance when the changeover switch 9a is in the position S, the gain up circuit 8a changes the gain. Double the standard gain of amplifier 5 (+
6 dB), the AGC circuit 6a is made to select the second MAX gain range which is twice the normal MAX gain range (+6 dB), and the adjacent signal synthesis circuit 11 is operated. To output each control signal (otherwise, the adjacent signal synthesis circuit 11 does not operate). As a result, the gain is increased by 12 dB without reducing the S / N ratio as compared with the case where the changeover switch 9a is in the position N in the standard state, that is, the cand up is increased by 4 times, and unnecessary wide area noise components are generated. More reliably removed.

[0028]

As described above, according to the present invention, it is possible to provide an image pickup apparatus capable of photographing without significantly deteriorating the image quality even in a very low illuminance. As a result, it is possible to shoot a subject with extremely low illuminance such as 1 to 3 lux, and it becomes possible to reveal a subject that has been buried in the darkness. That is,
The subject can be photographed sufficiently brightly in a scene of a candle service, when the light of the candle on the cake is the only light source, and when a surveillance camera such as a door camera photographs at night.

[Brief description of drawings]

FIG. 1 is a schematic block diagram showing a main part of a connection device according to an embodiment of the present invention.

FIG. 2 is a graph showing a relationship between an incident light amount and an output signal level in the image pickup apparatus of FIG.

FIG. 3 is a circuit diagram showing a specific example of an adjacent signal synthesizing circuit.

FIG. 4 is a diagram showing a signal flow when a capacitor in the adjacent signal combining circuit of FIG. 3 is omitted.

FIG. 5 is a graph showing frequency characteristics of an adjacent signal synthesis circuit.

FIG. 6 is a circuit diagram showing another specific example of the adjacent signal synthesis circuit.

FIG. 7 is a block diagram showing an imaging device according to the prior art.

8 is a graph showing the relationship between the amount of incident light and the output signal level in the imaging device of FIG.

[Explanation of symbols]

 1 lens 2 CCD image sensor 3 CDS circuit 4 automatic light amount control circuit 5 gain variable amplifier 6a automatic gain control circuit 7 low illuminance detection circuit 8a gain up control circuit 9a gain up mode setting switch 10 signal processing circuit 11 adjacent signal synthesis circuit 12 encoder circuit

Claims (1)

[Claims] 1. A variable gain amplifier means for amplifying a video signal, an automatic gain control means for amplifying the video signal to a predetermined level, and two time-adjacent video signals are combined. Adjacent signal synthesizing means for reducing noise components, low illuminance detecting means for detecting low illuminance of the subject from the video signal, gain up mode means for setting a gain up mode, and the set gain up mode The gain variable amplifier means, the automatic gain control means, and the gain up control means for controlling the adjoining means synthesizing means according to the above, in one of the gain up modes set by the gain up mode setting means. The gain-up control means controls the gain variable amplifier means when the low-illuminance detecting means detects low illuminance. A low gain characterized by selecting a second gain larger than a standard gain and causing the automatic gain control means to select a second maximum gain range larger than a standard maximum gain range, and also operating the adjacent signal combining means. Imaging device capable of illuminance photography.