JPH05336436A - Automatic gain control device - Google Patents

Abstract

(57) [Abstract] [Purpose] When high gain is required, low power consumption and quantization noise suppression are possible. [Arrangement] The analog signal from the sensor 2 is the AGC circuit 3
Is amplified by the A / D converter 4, converted into a digital signal by the A / D converter 4, amplified by the AGC circuit 5, and processed by the signal processing circuit 6. The level of the output luminance signal Y of the signal processing circuit 6 is detected by the level detection circuit 9, and the microcomputer 7 compares the detected value with a reference value, and according to the result, the aperture control signal of the iris 1 and the AGC circuit 3 Gain control signal 73, AGC
The gain control signal 75 of the circuit 5 is generated. At low illumination, AG
The gains of the C circuits 3 and 5 are minimized, the detected value is made to match the reference value by controlling the aperture amount of the iris 1, the iris 1 is fully opened, the gain of the AGC circuit 5 is minimized, and the AGC circuit 3 Match the detected value with the reference value by gain control,
Fully open the iris 1 and maximize the gain of the AGC circuit 3,
The gain control of the GC circuit 5 matches the detected value with the reference value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic gain control device suitable for use in a video camera device or the like.

[0002]

2. Description of the Related Art Generally, in a video camera device,
For example, Technical Report Vol.15, No.7 of the Television Society of Japan
(Jan, 1991) As described in pp.37-42, digitalization of signal processing is being promoted for the purpose of downsizing / lightening, high functionality, low power consumption, etc.
Even in this case, an automatic gain control circuit for keeping the camera output signal level constant is provided regardless of the output level of the sensor that converts the light information from the subject into an analog electric signal, as in the case of the analog video camera device. Has been.

[0003]

However, in the above-mentioned conventional video camera apparatus, the automatic gain control circuit is provided between the sensor and the analog-digital converter, and the automatic gain control is performed by analog processing. Therefore, for example, in the case of shooting in low illuminance, the gain of the automatic gain control circuit must be sufficiently increased to obtain a constant output signal, and in order to obtain high gain,
The scale of the automatic gain control circuit increases, and power consumption also increases in proportion to the circuit size. This poses a serious problem for the video camera device whose low power consumption is one purpose.

At present, the reduction in power consumption of devices is approaching the limit in analog, whereas it is progressing more and more in digital. Therefore, in consideration of the above problems and technological trends, it is conceivable to digitize the automatic gain control circuit in order to reduce the power consumption of the video camera device. However, if all the automatic gain control circuits are digitized, there is a problem that the quantization noise increases at high gain and the S / N of the camera output signal deteriorates.

The object of the present invention is to eliminate such problems and to solve
An object of the present invention is to provide an automatic gain control device having a good / N ratio, a small circuit scale, and low power consumption.

[0006]

In order to achieve the above object, the present invention provides a first variable gain amplifier for amplifying an input signal as an analog signal with a gain according to a first control signal. Circuit, an analog-digital converter for converting the output signal of the first variable gain amplifier circuit into a digital signal, and the digital signal output from the analog-digital converter with a gain according to a second control signal. A second variable gain amplification circuit, a signal processing circuit for processing the output digital signal of the second variable gain amplification circuit, a level detection circuit for detecting the level of the output signal of the signal processing circuit, A control circuit for generating the first and second control signals for setting the gains of the first and second variable gain amplifier circuits so that the value detected by the level detection circuit becomes equal to a preset reference value. Equipped with a.

[0007]

A part of the maximum gain required by the automatic gain control device is used as the gain of the first variable gain amplifier circuit, and the rest is compensated by the gain of the second variable gain amplifier circuit. The load on the first variable gain amplifier circuit for amplification is reduced. Further, the gain range of the second variable gain amplifying circuit for amplifying the digital signal is also smaller than the gain range of the variable gain amplifying circuit for amplifying after converting the analog input signal into the digital signal. As a result, the circuit scale of the first variable gain amplifier circuit can be reduced, power consumption can be reduced, and quantization noise generated in the second variable gain amplifier circuit can be reduced.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an automatic gain control device according to the present invention used in a video camera device.
Is an iris, 2 is a sensor, 3 is a variable gain amplification circuit for analog processing (hereinafter referred to as analog AGC circuit), 4 is an A / D (analog-digital) converter, and 5 is a variable gain amplification circuit for digital processing (hereinafter, Digital AGC
Circuit), 6 is a signal processing circuit, 7 is a microcomputer (hereinafter referred to as a microcomputer), 8 is a drive circuit, and 9 is a level detection circuit.

In FIG. 1, light from a subject (not shown) is incident on the sensor 2 through the iris 1. Sensor 2
Scans the subject image generated on the light receiving surface with a control signal supplied from the drive circuit 8, generates an analog electric signal having the subject image as information content, and supplies the analog AGC circuit 3. This electric signal is amplified by the analog AGC circuit 3 with a gain according to the digital control signal 73 supplied from the microcomputer 7, and the A / D converter 4 digital signal with the quantization bit number m (m is a positive integer). After being converted into 501 (hereinafter referred to as m-bit digital signal), it is supplied to the digital AGC circuit 5.

A specific example of the digital AGC circuit 5 will be described with reference to FIG. However, 51 is a gain control unit,
511 is a latch circuit, 512 is a serial-parallel converter (hereinafter referred to as S / P converter), 513 is a latch circuit, 514 is a multiplier, 515 is a latch circuit, and 52 is a limiter.

In FIG. 1, the digital AGC circuit 5 comprises a gain control section 51 and a limiter 52,
The gain controller 51 includes a multiplier 514 and an S / P converter 512.
And latches 511, 513, 515.

M output from the A / D converter 4 (FIG. 1)
The bit digital signal 501 is latched by the latch circuit 511 for each latch pulse 75b of the control signal 75 supplied from the microcomputer 7 (FIG. 1), and the latch output is supplied to the multiplier 514. The gain control signal 75a of the control signals 75 supplied from the microcomputer 7 (FIG. 1)
Is converted into a p (p is a positive integer) bit gain control signal by the S / P converter 512, and latched by the latch circuit 513 every latch pulse 75b. This latch circuit 5
The gain control signal output from 13 is multiplied by the digital signal output from the latch circuit 511 by the multiplier 514. Then, the multiplier 514 outputs the upper q-bit portion of these multiplication values, and the latch circuit 515
Is latched every latch pulse 75b. The output signal of the latch circuit 515 is supplied to the limiter 52.

The limiter 52 sets the level of the output signal of the latch circuit 515 to n according to a predetermined input / output characteristic.
(N is a positive integer less than or equal to q) bits, and the limited n-bit output signal 502 is limited to the signal processing circuit 6 of FIG.
Supply to.

FIG. 3 shows an example of the input / output characteristics of the limiter 52. When the level of the value of {(2 to the nth power) -1-x} or more, the value of {(2 to the nth power) -1} is shown. The level of the output signal is limited as the level. However, x is an arbitrary integer.

Returning to FIG. 1, in the signal processing circuit 6, the above-mentioned technical report of the Television Society of Japan, Vol. 15, No. 7 (Ja.
n, 1991) Similarly to the signal processing circuits described in pp. 37 to 42, the n-bit output digital signal 502 of the digital AGC circuit 5 is supplied, and 2 or 3 lines (1 line is 1 horizontal scanning) The luminance signal Y and the color signal C are generated by using the digital signal 502 for (period). The luminance signal Y and the color signal C are supplied to a processing circuit in the next stage (not shown) to generate a color video signal.
The luminance signal Y is also supplied to the level detection circuit 9 to detect the level of the luminance signal Y in a predetermined area in the image display screen, and the obtained detection value is supplied to the microcomputer 7.

The microcomputer 7 compares the detection value from the level detection circuit 9 with a reference value preset in the microcomputer 7 to determine how much the illuminance of the object is, and the diaphragm control signal 71 corresponding to this illuminance. And gain control signals 73 and 75 are generated to generate iris 1 and analog A respectively during the blanking period.
Supply to the GC circuits 3 and 5. The control operation of the microcomputer 7 will be described more specifically with reference to FIGS.

As shown in FIG. 4, the microcomputer 7 divides the illuminance of the object into areas 1, 2, 3 and a reference value indicating the boundary of each area and a detection value of the level detection circuit 9 (FIG. 1). It is determined which of the areas 1, 2 and 3 the illuminance of the subject is in by comparing with the iris 1 and the analog A so that the reference value and the detected value may match according to the determination result.
It controls the GC circuit 3 and the digital AGC 5.

When the illuminance is within the area 1, the respective control signals 7 are set so that the gains of the AGC circuits 3 and 5 are minimized.
After setting 3,75, the diaphragm control signal 71 for adjusting the incident light quantity of the sensor 2 by the diaphragm quantity of the iris 1 is generated so that the output signal of the camera becomes constant. As a result, the light amount of the incident light of the sensor 2 is adjusted so that the above detection value matches the above reference value. When the illuminance is within the area 2, the microcomputer 7 sends the diaphragm control signal 71 so that the iris 1 is fully opened, and the digital AGC circuit 5
The gain control signal 75 is set so as to minimize the gain, and the gain control signal 73 is generated so that the signal level is adjusted by the analog AGC circuit 3. That is,
The gain control signal 73 controls the gain of the analog AGC circuit 3 so that the detected value matches the reference value. Furthermore, when the illuminance is within the area 3,
The aperture control signal 71 is set so that the iris 1 is fully opened, and the gain control signal 75 is set so that the gain of the digital AGC circuit 5 is maximized.
The gain control signal 75 is generated so that the signal level is adjusted by the C circuit 5. That is, the gain control signal 75 causes the detected value to match the reference value,
The gain of the digital AGC circuit 5 is controlled.

The above series of operations constitutes a feedback loop, which allows the output signal of the camera to be kept constant at all times.

The operation of the microcomputer 7 will be described in more detail with reference to FIG. 5. First, the gains of the analog AGC circuit 3 and the digital AGC circuit 5 of FIG. 1 are initialized (step S1), and the level detection circuit 9 is detected. Is detected (step S2), and it is determined whether the level of the luminance signal Y matches the reference value. This determination is made in steps S3 and S10. When they match, the aperture amount of the iris 1, the analog AGC circuit 3 and the digital AGC circuit 5
The gain of is maintained as it is, and steps S3 and S1 are performed.
The operation of the loop of 0 and S2 is repeated. Analog AG
The initial setting value of the gain of the C circuit 3 and the digital AGC circuit 5 is the minimum value of the above gain.

When the level of the luminance signal is smaller than the reference value (step S3), it is determined whether or not the iris 1 is fully opened (step S4), and steps S7, S17, S2, S3 and S4 are performed until the iris 1 is fully opened. Repeat a series of operations. Of course, if the level of the luminance signal coincides with the reference value during this period, the operation is stopped and the above steps S2 and S are performed.
3, a series of operations of S10 is performed. That is, the operation is
It is determined whether or not the illuminance of the subject is in the area 1 in FIG. 4, and if it is in the area 1, the brightness signal level is made to match the reference value by the diaphragm amount of the iris 1. To do.

When the level of the luminance signal does not match the reference value even when the iris 1 is fully opened (steps S3 and S).
4), it is determined whether or not the gain of the analog AGC circuit 3 is maximum (step S5).
A control signal 710 for increasing the gain of the C circuit 3 is generated (step S8), and this is used for the analog AGC in the blanking period.
It is sent to the circuit 3 to increase its gain (step S1).
6, 17). This operation is performed by the steps S8 and S1.
6, S17, S2, S3, S4, S5
This is repeated until the level of the luminance signal matches the reference value or the gain of the analog AGC circuit 3 becomes maximum. During this time, when the level of the luminance signal matches the reference value, the operation is stopped and the above steps S2, S3 and S10 are performed.
Move to a series of operations. That is, this operation is for determining whether or not the illuminance of the subject is in the area 2 in FIG. 4, and if it is in this area 2, the level of the luminance signal is controlled by the gain control of the analog AGC circuit 3. To match the reference value.

Even if the gain of the analog AGC circuit 3 becomes maximum, if the level of the luminance signal does not match the reference value (step S5), then steps S6, S9, S1.
A series of operations S6, S17, S2 to S5 are repeated, and the control signal 75 is sent to the digital AGC circuit 5 during the blanking period so that the gain of the digital AGC circuit 5 is maximized. Of course, if the level of the luminance signal coincides with the reference value during this period, the operation is stopped and the above steps S2 and S are performed.
3, a series of operations of S10 is performed. That is, the operation is
It is determined whether the illuminance of the subject is in the area 3 in FIG. 4, and if it is in this area 3, the digital A
The gain control of the GC circuit 5 causes the level of the luminance signal to match the reference value.

The above is the operation at the start of the operation after the power is turned on. However, when the level of the luminance signal becomes smaller than the reference value due to the change of the subject or the change of the shooting scene during the operation. This is determined in steps S3 and S4, and as a result of this determination, the microcomputer 7 performs the following control operation.

(1) When the illuminance of the subject changes from the area 1 to the area 3 in FIG. 4; as described above, in the area 1, the gains of the analog AGC circuit 3 and the digital AGC circuit 5 are minimum values, and the iris 1 is 1. Is between the fully open state and the minimum open state. In this case, it is similar to the case where the subject illuminance is within the region 3, and first, when it is determined that the level of the luminance signal is smaller than the reference value (step S3), the iris 1 is not in the fully open state. Therefore (step S4), as described above, the loop operation including step S7 is performed, the diaphragm control signal 71 is generated to fully open the iris 1, and then the loop operation including step S8 is performed to perform the gain control. The signal 73 is generated to maximize the gain of the analog AGC circuit 3. When the gain of the analog AGC circuit 3 reaches the maximum value, next, the loop operation including step S9 is performed to generate the gain control signal 75 to control the gain of the digital AGC circuit 5 and thereby the level of the luminance signal. Will match the reference value.

(2) When the illuminance of the subject changes from area 1 to area 2 in FIG. 4; first, when it is determined that the level of the luminance signal is smaller than the reference value (step S3), the above (1)
Similarly to the case of, the loop operation including step S7 is performed, the diaphragm control signal 71 is generated to fully open the iris 1, and then the loop operation including step S8 is performed.
The gain control signal 73 is generated to control the gain of the analog AGC circuit 3. As a result, the level of the luminance signal matches the reference value.

(3) When the illuminance of the subject changes from the area 2 to the area 3 in FIG. 4; as described above, in this area 2, the gain of the digital AGC circuit 5 is the minimum value, and the iris 1 is in the fully open state. The gain of the analog AGC circuit 3 is between the maximum value and the minimum value. Therefore, first, when it is determined that the level of the luminance signal is smaller than the reference value (step S
3), the iris 1 is held in the fully open state as it is (step S4), and as described above, the loop operation including step S8 is performed to generate the gain control signal 73 to generate the analog AG.
The gain of the C circuit 3 is maximized. Analog AGC circuit 3
When the gain of is equal to the maximum value, the loop operation including step S9 is performed to generate the gain control signal 75 to control the gain of the digital AGC circuit 5, whereby the level of the luminance signal becomes the reference value. Will come to match.

The above is the case where the level of the luminance signal becomes smaller than the reference value due to the change of the subject or the change of the imaged scene. However, when the level of the luminance signal becomes larger than the reference value, this is step S3. , S10. As a result of this determination, the microcomputer 7 performs the following control operation.

(1) When the illuminance of the subject changes from the area 3 to the area 1 in FIG. 4; as described above, in the area 3, the gain of the analog AGC circuit 3 is the maximum value, the iris 1 is in the fully open state, and the digital value is digital. The gain of the AGC circuit 5 is between the maximum value and the minimum value. Therefore, first, when it is determined that the level of the luminance signal is higher than the reference value (step S10),
It is determined whether the gain of the digital AGC circuit 5 is the minimum value (step S11), and the gain control signal 75 for reducing the gain of the digital AGC circuit 5 is generated (step S1).
4) Send to the digital AGC circuit 5 during the blanking period. This operation is performed in steps S10, S11, S
A series of operations of S14, S16, S17, S2, and S3 are repeated until the gain of the digital AGC circuit 5 becomes the minimum value.

When this gain becomes the minimum value, it is next judged whether or not the gain of the analog AGC circuit 3 is the minimum value, and a gain control signal for reducing this gain is generated and the analog AGC circuit 3 is supplied to the analog AGC circuit 3 during the blanking period. Sending steps S10 and S1
The series of operations of 1, S12, S15, S16, S17, S2 and S3 is repeated. When this gain becomes the minimum value, next, the diaphragm control signal 71 for diaphragming the iris 1 is generated and sent to the iris 1 in the blanking period (step S1).
0, S11, S12, S13, S16, S17, S2
Then, the series of operations in S3 is repeated. By such an operation, that is, the control of the diaphragm amount of the iris 1, the level of the luminance signal becomes equal to the reference value.

(2) When the illuminance of the subject changes from the area 3 to the area 2 in FIG. 4; first, when it is determined that the level of the luminance signal is higher than the reference value (step S10), the gain of the digital AGC circuit 5 is obtained. Is determined to be the minimum value (step S11), a gain control signal 75 for reducing the gain of the digital AGC circuit 5 is generated (step S14), and is sent to the digital AGC circuit 5 during the blanking period. This operation is performed in steps S10, S11, S14, S1.
A series of operations of S6, S17, S2 and S3
This is repeated until the gain of the GC circuit 5 reaches the minimum value.

When this gain becomes the minimum value, it is next determined whether the gain of the analog AGC circuit 3 is the minimum value and a gain control signal 73 for reducing this gain is generated to generate the analog AGC circuit 3 in the blanking period. Send to S10, S
A series of operations of 11, S12, S15, S16, S17, S2 and S3 are repeated. During the repetition of this operation, the level of the luminance signal becomes equal to the reference value.

(3) When the subject illuminance changes from the area 2 to the area 1 in FIG. 4; as described above, in the area 2, the gain of the digital AGC circuit 5 is the minimum value, the iris 1 is in the fully open state, and the analog The gain of the AGC circuit 3 is between the maximum value and the minimum value. Therefore, first, when it is determined that the level of the luminance signal is higher than the reference value (step S10) and the gain of the digital AGC circuit 5 is determined to be the minimum value (step S11), it is determined whether or not the gain of the analog AGC circuit 3 is the minimum value. It is determined (step S12), the gain control signal 73 for reducing the gain of the analog AGC circuit 3 is generated (step S14), and the analog AGC circuit 3 is generated during the blanking period.
Send to. This operation is performed in steps S10 and S1.
A series of operations S1, S12, S15, S16, S17, S2, S3 are repeated until the gain of the analog AGC circuit 3 reaches the minimum value. During the repetition of this operation, the level of the luminance signal becomes equal to the reference value.

It should be noted that the control in the same region in FIG. 4 is performed by controlling the aperture amount of the iris 1 by repeating the loop operation including step S7 or S13 for the region 1 S8 or S15
Repeat the operation of the loop including the analog AGC circuit 3
By controlling the gain of the digital AGC circuit 5 by controlling the gain of the digital AGC circuit 5 by repeating the operation of the loop including step S9 or S14,
Each is done.

Next, the influence of quantization noise in this embodiment will be described. Now, the signal amplification factor of the analog AGC circuit 3 is p, the signal amplification factor of the digital AGC circuit 5 is q, and the sum of the signal amplification factors of these AGC circuits 3 and 5 is k,
The noise generated by the sensor 2 is Ns, the quantization noise generated by the A / D converter 4 is Nq, the noise Ns and the quantization noise N.
Let α be the ratio to q, Na be the noise in the automatic control device that has only the analog AGC circuit 3 as the AGC circuit and amplifies the signal k times, and the analog AGC circuit 3 amplifies the analog input signal p times If the noise in the automatic control device (hereinafter referred to as a hybrid automatic gain control device) of this embodiment that amplifies a digital signal q times by the digital AGC circuit 5 is Nd, the following equations (1) to (4) are obtained. Be done. ^{Na 2 = (k · Ns)} 2 + Nq 2 ········ (1) Nd 2 = (k · Ns) 2 + (q · Nq) 2 ···· (2) Ns = α · Nq (3) k = p.q (4) Also, the ratio of the magnitudes of the noises Na and Nd. N is represented by the following equation (5). N = 10 · log (Nd ² / Na ² ) [dB] (5) Further, from the formulas (1), (2) and (3), the following formula (1-
2) and formula (2-2), and also formulas (1-2) and (2-
The following equation (5-2) is obtained from 2) and (5). Na ² = (k ² · α ² +1) Nq ² ··· (1-2) Nd ² = (k ² · α ² + q ² ) Nq ² ··· (2-2) N = 10 · log {(k ² · α ² + q ² ) / (k ² · α ² +1)} (5-2).

In the equation (5-2), in order to bring the value of the ratio N closer to 0, the value of k or α must be made sufficiently larger than 1. In the equation (3), if the noise Ns is a fixed value, the value of the noise Nq must be reduced in order to increase the value of α. That is, the quantization noise Nq of the A / D converter 4 needs to be sufficiently smaller than the noise Ns of the sensor 2.

However, at present, there is a quantization noise in a high-speed and low power consumption A / D converter suitable for use in a video camera device aiming at size reduction, weight reduction and power consumption reduction. There is no one with the number of bits as small as possible. Therefore, since the value of α never becomes sufficiently larger than 1, the influence of the signal amplification factor q of the digital AGC circuit 5 on N in Expression (5-2) becomes large. That is, the influence of the quantization noise Nq appears in the output image according to the signal amplification factor q of the digital AGC circuit 5.

However, if the signal amplification factor q of the digital AGC circuit 5 is suppressed to some extent, the deterioration of the output image can be kept within a visually permissible range. In Table 1 below, it is assumed that the noise Ns of the sensor 2 and the quantization noise Nq of the A / D converter 4 are almost the same (α = 1), and further the threshold value, that is, the signal amplification of the analog AGC circuit 3 is performed. Digital AGC, assuming the maximum value of rate p is 4
A specific example showing the comparison result of the noise when amplifying only by the circuit 5 and the noise when amplifying by the hybrid automatic gain control device is shown.

[0039]

[Table 1]

As is clear from Table 1, when the digital AGC circuit 5 alone is used for amplification, the noise is almost twice the noise when the analog AGC circuit 3 is used for amplification. Absent. However, the noise in the case of amplification by the hybrid automatic gain control device is analog A when the required amplification factor is less than the threshold value (k ≦ 4).
Since it is amplified only by the GC circuit 3 and not by the digital AGC circuit 5 (q = 1), there is no influence of quantization noise due to amplification, and when k> 4, the digital AGC circuit 5 also amplifies (q> 1). , The effect of quantization noise due to amplification occurs, but in analog AGC circuit 3 to some extent (Table 1
By amplifying up to 4 times) and amplifying the lacking amount by the digital AGC circuit 5, noise is greatly reduced as compared with the case of amplifying only by the digital AGC circuit 5, and the analog The difference from the case of amplifying only by the AGC circuit 3 becomes small, and it can be said that it is within an allowable range in consideration of the effect of reducing power consumption.

Furthermore, the difference between N in cases (i) and (ii) in Table 1 is only 0.05 dB, and the rate of deterioration due to quantization noise does not change much even if the amplification factor k is increased. Therefore, above a certain threshold, the digital AGC
It can be said that there is no problem even if it is amplified by the circuit 5.

As described above, in this embodiment, the conventional video camera device is provided with the digital processing AGC circuit, and part of the gain control operation which is conventionally performed according to the analog processing AGC circuit is converted to the analog processing. This is performed by a digital processing AGC circuit which consumes less power than that of the digital camera. As a result, low power consumption of the video camera device can be realized.

FIG. 6 is a block diagram showing another embodiment of the automatic gain control circuit according to the present invention used in a video camera device. 5'is a digital AGC circuit, 10 is a clamp circuit, and corresponds to FIG. The same reference numerals are given to the portions to be described, and the duplicated description will be omitted.

This embodiment differs from the embodiment shown in FIG. 1 in that, as shown in FIG. 6, a certain offset indicating the black level is detected in order to prevent black underexposure from occurring during the automatic gain control process. In order to remove the black level due to this offset from the signal after the automatic gain control processing, the digital A
This is the point that the GC circuit 5'subtracts.

FIG. 7 is a block diagram showing a specific example of the digital AGC circuit 5'in FIG. 6, in which 516 is a subtractor, 517 is a latch circuit, and the portions corresponding to those in FIG. ing.

In FIG. 7, this digital AGC circuit 5'is similar to the digital AGC circuit 5 shown in FIG.
The subtractor 516 for removing the black level added by the clamp circuit 10 and the latch circuit 517 for latching the output of the subtractor 516 with the latch pulse 75b are added.

Now, assuming that the value of the black level is k (k is an integer equal to or greater than 0), the subtractor 516 operates as the A / D converter 4 of FIG.
The black level k is subtracted from the m-bit digital signal 501 supplied from the above, and an m ′ (m ′ is a positive integer) bit digital signal 514 is output. This digital signal 5
14 is supplied to the multiplier 514 via the latch circuit 517 similarly to the digital AGC circuit 5 shown in FIG. 2. The digital signal 514 is a signal indicating the difference between the digital signal 501 and the black level k. Therefore, this is also the sign bit signal in the difference signal, that is,
It is also supplied to the clamp circuit 10 as a control signal 503 indicating the magnitude comparison result of the level of the digital signal 501 and the black level k.

The microcomputer 7 also generates a control signal 710 indicating the timing at which the sensor 2 reads out pixel data in the optical black section and supplies it to the clamp circuit 10. The clamp circuit 10 receives the control signal 710 from the microcomputer 7.
, The level of the electric signal output from the sensor 2 is controlled according to the control signal 503 from the digital AGC circuit 5, and the digital signal 50 output from the subtractor 516 is controlled.
The level is 0 during the period of the control signal 710 every 4 horizontal cycles.
Adjust so that

Here, the subtractor 516 is provided before the multiplier 514 because the reference level of the digital signal is set to 0 before the digital signal is amplified by the multiplier 514.
If it is not set to, the black level will fluctuate when the signal amplification factor changes, and it will be necessary to correct it. The actual setting of the black level is performed by setting the input / output characteristic of the limiter 52 to x = k in FIG. That is, the limiter 52 adds the component k subtracted by the subtractor 516 to the input signal.

The other operation is the same as that of the digital AGC circuit 5 shown in FIG.

Therefore, in this embodiment, the same effect as that of the embodiment shown in FIG. 1 is obtained, and by the operation of the clamp circuit 10 and the digital AGC circuit 5 ',
When adjusting the black level during the period of the video signal corresponding to the optical black section of the sensor 2, the digital AGC shown in FIG.
In the circuit 5 ′, since the reference level of the digital signal before the signal amplification by the multiplier 514 is set to 0, the fluctuation of the black level due to the signal amplification can be prevented.

FIG. 8 is a block diagram showing still another embodiment of the automatic gain control device, in which parts corresponding to those in FIG. 1 are designated by the same reference numerals.

This embodiment is not limited to the video camera device as in the above embodiments, but can be used in any device that keeps the output signal level of the signal processing circuit by digital processing constant. .. The basic operation is the same as that of the embodiment shown in FIG.

That is, in this embodiment, since the gain control is performed by the two AGC circuits for analog processing and digital processing, it is possible to obtain a high gain with lower power consumption as compared with the case where the gain control is performed only by the analog processing AGC circuit. ,further,
Since the distribution of the gain of these AGC circuits is automatically controlled by the microcomputer, it is possible to obtain an output signal that is not easily affected by the quantization noise.

In each of the embodiments described above, the A / D
In order to make it inconspicuous that the output image is deteriorated by the quantization noise generated in the converter 4 being amplified by the digital AGC circuit 5, the analog A
If the gain of the GC circuit 3 is increased and the required gain is equal to or higher than the threshold value, the control method is used to compensate the shortage with the gain of the digital AGC circuit 5. If the noise is sufficiently smaller than the sensor noise generated from the sensor 2, the deterioration of the output image due to the amplification of the quantization noise by the digital AGC circuit 5 becomes inconspicuous. Therefore, the positions of the analog AGC circuit 3 and the digital AGC circuit 5 are reversed. Then, in low illuminance, digital AG
The control method may be such that the gain of the C circuit 5 is increased, and if the required gain is equal to or higher than the threshold value, the shortage is compensated by the gain of the analog AGC circuit 3.

[0056]

As described above, according to the present invention,
Since the maximum gain of the first variable gain amplifying circuit that amplifies the analog signal can be reduced, the circuit scale of the first variable gain amplifying circuit can be reduced, so that the power consumption thereof can be suppressed to a low level. Since the maximum gain of the second variable gain amplifier circuit that amplifies the digital signal can also be reduced, the quantization noise generated therein can be suppressed to a small level. As a result, since the power consumption of the second variable gain amplifier circuit is low, even when high gain is required as in shooting with low illuminance, that is, the gain of the first and second variable gain amplifier circuits is sufficient. Even if it is necessary to increase the size, automatic gain control can be performed with low power consumption and less quantization noise is generated.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an automatic gain control device according to the present invention used in a video camera device.

FIG. 2 is a block diagram showing a specific example of a digital AGC circuit in FIG.

3 is a diagram showing input / output characteristics of a limiter in the digital AGC circuit shown in FIG.

FIG. 4 is a characteristic diagram showing illuminance / AGC gain characteristics in the embodiment shown in FIG.

5 is a flowchart showing the operation of the embodiment shown in FIG.

FIG. 6 is a block diagram showing another embodiment of the automatic gain control device according to the present invention used in a video camera device.

7 is a block diagram showing a specific example of a digital AGC circuit in FIG.

FIG. 8 is a block diagram showing still another embodiment of the automatic gain control device according to the present invention.

[Explanation of symbols]

 1 Iris 2 Sensor 3 Analog AGC circuit 4 A / D converter 5 Digital AGC circuit 6 Signal processing circuit 7 Microcomputer 8 Drive circuit 9 Level detection circuit 10 Clamp circuit

Claims (4)

[Claims] 1. A first variable gain amplifier circuit which uses an input signal as an analog signal and amplifies the input signal with a gain according to a first control signal, and a digital output signal of the first variable gain amplifier circuit. An analog-digital converter for converting the signal into a signal, and a second for amplifying the digital signal output from the analog-digital converter with a gain according to the second control signal
Variable gain amplifier circuit, a signal processing circuit that processes the output digital signal of the second variable gain amplifier circuit, a level detection circuit that detects the level of the output signal of the signal processing circuit, and a detection by the level detection circuit And a control circuit for generating the first and second control signals for setting the gains of the first and second variable gain amplifier circuits so that the value becomes equal to a preset reference value. Automatic gain control device characterized by. 2. The control circuit according to claim 1, wherein the gain required to keep the detected value by the level detection circuit equal to the reference value is smaller than a first threshold value. The gain of one of the first and second variable gain amplifier circuits is adjusted to fix the other gain to the minimum value so that the detected value by the level detection circuit is always equal to the reference value. When the gain required for the operation is larger than the first threshold value, one of the first and second variable gain amplifier circuits has the first and second gain values equal to the first threshold value. One of the control signals is fixed, and the first and first control signals are adjusted so that the lacking gain is compensated by the other gain of the first and second variable gain amplifier circuits. Characteristic automatic gain control device. 3. The automatic gain control device according to claim 1, which is a video camera device provided with a sensor for converting optical information into an analog electric signal and outputting the analog electric signal, wherein the electric signal output from the sensor is the first variable gain. As an input signal of an amplifier circuit, the signal processing means generates a luminance signal and a color signal from an output signal of the second variable gain amplifier circuit, and the level detection circuit has a level of the luminance signal in a predetermined constant area. And the control circuit adjusts the gains of the first and second variable gain amplification circuits so that the reference value is equal to the detection value of the level detection circuit. Two
And an automatic gain control device for generating the control signal. 4. The automatic gain control device according to claim 3, wherein the second variable gain amplifier circuit includes a subtractor for subtracting a black level from the digital signal from the analog-digital converter. ..