JPH05268534A - Black level clamp circuit - Google Patents

Abstract

PURPOSE:To provide the circuit not requiring troublesome adjustment by giving shift data to a circuit input digital signal so as to average the circuit input digital signal. CONSTITUTION:Circuit input digital signals X, Y are inputted to a subtractor circuit 18 being a shift data provision means, the subtractor circuit 81 give shift data (a), (b)to the circuit input digital signals X, Y and outputs arithmetic operation data X-a, X-b. An alternate input of the shift data (a), (b)to the subtractor circuit 81 is implemented switching a changeover switch 82 receiving the shift data (a), (b)at a period being twice that of an element clock of a CCD. Subtracted data are inputted to a limit circuit 83, output data of the limit circuit 83 are inputted to a low pass filter 84 as an averaging means and when the subtraction data X-a, Y-b are inputted, averaged data (X-a+Y-b)/2 are outputted at the point of time when the data Y-b are inputted, A rounding circuit 85 implements rounding processing of the average data.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a black level clamp circuit for clamping a black level of an image sensor output signal to a black reference level.

[0002]

2. Description of the Related Art FIG. 6 is a block diagram showing an image processing apparatus using a conventional black level clamp circuit. An image signal (for example, an image signal as shown in FIG. 7) output from the CCD 1 as the image sensor is input to the AGC circuit 2, and the AGC circuit 2 outputs an image signal of a constant level. The black signal clamp BL 3 (see FIG. 7) clamps this image signal to the black reference level, and the A / D conversion circuit 4 performs A / D conversion. This A /
The image signals output from the D conversion circuit 4 are, for example, color signals (Mg + Cy) and (Ye + G).

[0003]

In the conventional black level clamp circuit 3 shown in FIG. 6, the clamp level, the time constant of the clamp, etc. are set in an analog manner, for example, by a variable resistor, and the adjustment is delicate and complicated. It was a thing.

The present invention has been made in consideration of the above circumstances, and an object thereof is to provide a black level clamp circuit which does not require complicated adjustment.

[0005]

In order to solve the above-mentioned problems, according to the present invention, the resolution of a circuit input digital signal is the same as that of a circuit output digital signal, and the black level of an image signal output from an image sensor is black. In a black level clamp circuit for clamping to a reference level, shift data giving means for giving shift data to a circuit input digital signal, averaging means for averaging the circuit input digital signal and outputting averaged data, And rounding means for rounding the averaged data.

[0006]

In the black level clamp circuit according to the present invention, as shown in FIG. 1, the subtraction circuit (shift data adding means) 81 is -a, for two types of 10-bit circuit input digital signals X and Y. -B is added and 11-bit shift data Xa and Yb are output. The shift data a and b are supplied from a microcomputer (not shown) and are alternately supplied to the subtraction circuit 81 by the changeover switch 82 which is switched at a cycle of twice the pixel clock of the CCD 1, and the subtraction circuit 81 outputs subtraction data which is subtraction result data. It outputs X-a and Y-b.

The subtracted data X-a and Y-b are input to the limiter circuit 83. The limiter circuit 83 removes the negative subtraction data from the input subtraction data, and the subtraction data output from the limiter circuit 83 has a 10-bit configuration.

A low-pass filter (averaging means) 84 averages the subtracted data Xa and Yb,
Averaged data (X-a +) output from the averaging means 84.
Y-b) / 2 is rounded because the circuit output digital signal of the black level clamp circuit 8 (FIG. 1) has a 10-bit configuration. The rounding process is, for example, a process of rounding up if there is a numerical value below the decimal point. A rounding circuit (rounding means) 85 performs the rounding process and outputs circuit output digital signals X ′ and Y ′ having the same number of bits (10 bits) as the circuit input digital signals X and Y.

In this way, subtraction data Xa and Yb obtained by shifting the circuit input digital signals X and Y by the shift data a and b provided from the microcomputer are obtained.
By performing a rounding process after averaging the subtracted data, the black level of the circuit input digital signal can be automatically clamped to the black reference level, and complicated adjustment for clamping is unnecessary.

[0010]

Next, an embodiment of a black level clamp circuit according to the present invention will be described in detail with reference to the drawings.

FIG. 3 is a block diagram showing an image processing apparatus using a black level clamp circuit according to an embodiment of the present invention.

An image signal output from the CCD 1 as an image sensor, for example, color signals (Mg + Cy), (Ye)
+ G) is input to the AGC circuit 2, and the AG of the AGC circuit 2
Color signals (Mg + Cy) of a constant level, (Y
e + G) is output. This color signal (Mg + Cy),
(Ye + G) is A / D converted by the A / D conversion circuit 4 and input to the defect correction circuit 5.

If the color signal has a missing portion, for example, the defect correcting circuit 5 complements the missing portion with the preceding and following color signals. In this way, the defect correction circuit 5 outputs a defect-free color signal.

The color signal (M
g + Cy) and (Ye + G) are delayed by 1 line by the 1-line (1H) delay circuit 6, and further delayed by 1 line by the 1-line delay circuit 7.

The color signals (Mg + Cy) and (Ye + G) output from the 1-line delay circuit 6 are clamped by the black level clamp circuit 8 so that the black signal portion (see BL in FIG. 7) becomes the black reference level. .. The shift data described below that determines the black reference level is supplied from a microcomputer (not shown). The black level clamp circuit 8 will be described in detail later.

H to which the color signal delayed by one line is input
The aperture control circuit 9 uses the horizontal contour signal H.
Output C. The V aperture control circuit 10 to which the three color signals of no delay, 1 line delay, and 2 line delay are input, outputs a vertical contour signal VC.

The clamped color signal (Mg + C
y), (Ye + G) and the contour signals HC and VC are added by the adder circuit 11, and then the low-pass filter (LPF) 1
It is filtered by 2 and output as a luminance signal Y. The luminance signal Y is gamma-corrected by the gamma-correction circuit 13, and D / A
It is converted into an analog signal by the conversion circuit 14 and output as a gamma-corrected analog luminance signal Y '.

The above three color signals without delay, 1 line delay and 2 line delay are also input to the color signal processing circuit 15. The color signal processing circuit 15 processes the above three color signals and outputs a chroma signal CRM obtained by modulating the color subcarrier with the color difference signals.

Color signals (Mg + Cy), (Ye +)
G) is input to the OPB signal extraction circuit 16. The OPB signal extraction circuit 16 detects the black level of the color signal output from the A / D conversion circuit 4. As shown in FIG. 4, the black level is detected by optical black (OP
B) It is performed by extracting the black level of the specific area 19 of the area 18. The circuit for extracting the black level of the specific area 19 is the OPB signal extraction circuit 16. 20 is a CCD effective area.

The OPB signal which is the extracted black level signal of the specific area 19 is output from the OPB signal extraction circuit 16 to the average value calculation circuit 17 as described above, and the average value calculation circuit 17 is supplied.
Calculates the average value of the black level of the specific area 19 and outputs the calculated value to the microcomputer. The microcomputer clamps the black level of the circuit input digital signals which are the color signals (Mg + Cy) and (Ye + G) input to the black level clamp circuit 8 to the black reference level. Therefore, the calculated black level average value and the black reference value are used. The result of comparison with the level is output to the black level clamp circuit 8 as shift data described later.

Next, the black level clamp circuit 8 of FIG. 3 will be described in detail. FIG. 1 is a block diagram showing an embodiment of a black level clamp circuit according to the present invention. Figure 1
Then, two types of 10-bit circuit input digital signals X and Y are input to the terminal T1. X is, for example, a color signal (Mg +
Cy), and Y is, for example, a color signal (Ye + G).
Therefore, the circuit input digital signals input to the terminal T1 are, for example, X1, Y1, X2, Y2, ..., Xn, Y.
It can be expressed as n.

The circuit input digital signals X and Y are input to a subtraction circuit 81 as shift data adding means, and the subtraction circuit 81 adds shift data a and b to the circuit input digital signals X and Y and subtracts the subtraction data. It outputs X-a and Y-b. Alternate input of the shift data a and b to the subtraction circuit 81 is performed by the changeover switch 8 to which the shift data a and b are input.
2 is switched at a cycle twice as long as the pixel clock of the CCD 1. Therefore, the subtraction circuit 81 has the circuit input digital signals X1, Y1, X2, Y2, ...
When Xn and Yn are input, subtraction data X1-a and Y1
-B, X2-a, Y2-b, ..., Xn-a, Yn-
Output b. The shift data a and b are constant for at least one field.

The subtraction data X-a and Y-b can take positive and negative values. Therefore, the subtraction data X-a and Y-b are 11-bit data. This subtraction data is the limit circuit 83
The negative data of positive and negative is removed. Therefore, the output data of the limit circuit 83 is only positive data and 10-bit data.

The output data of the limit circuit 83 is input to a low pass filter (LPF) 84 as an averaging means. The low-pass filter 84 is a filter for averaging the input digital signal, and when the subtraction data X-a and Y-b are input, the averaged data (X-a + Y-b) at the time when Y-b is input. Outputs / 2. Therefore, assuming that the data input to the terminal T1 is X1, Y1, X2, and Y2, the low-pass filter 84 receives (X1-a + Y1-b) / 2 at the time of Y1 input and (Y1 at the time of X2 input).
-B + X2-a) / 2, at the time of Y2 input, (X2-a +
Y2-b) / 2 is output. The averaged data may include a numerical value of a decimal number or less, and the averaged data has an 11-bit configuration.

A rounding circuit (rounding means) 85 is a circuit for rounding the averaged data. The rounding process is to round up or round down the averaged data to a decimal point or less so that the circuit output digital data X ′, Y ′ has the same 10 bits as the number of bits of the circuit input digital signals X, Y. is there. In this embodiment, rounding up processing is performed.

Next, examples in which the subtraction circuit 81 does not add shift data and in which the shift data is added are shown.

First, the case where no shift data is added will be described. Here, the value of the circuit input digital signal is
When 10, 10, 10, 10, ..., 9, 10,
The case of 9, 10, ... Will be described. When the input digital signal values are 10, 10, 10, 10, ..., Since the averaged data output from the low-pass filter 84 is 20/2 = 10, the output data of the rounding circuit 85, that is, the circuit output digital. The signals are 10, 10,
10, 10, ... Further, when the circuit input digital signal value is 9, 10, 9, 10, ..., The averaged data output from the low-pass filter 84 is 19/2.
= 9.5, the output data of the rounding circuit 85 becomes 10, 10, 10, 10, ... By rounding up.

The circuit input digital signal value is a random value, and the circuit input digital signal value is 10, 10,
It is considered that the probability that the sum of two preceding and following data is an even number is equal to the probability that the sum of two preceding and following data is an odd number. In the case of, since the output data is 10, the average value of the output data (the average value of the output data when the sum of two preceding and following data is even and the output data when it is odd) is 1
It becomes 0.

Next, the case where the shift data is added will be described with the shift data a = 0 and b = 1. As with the case without addition, the value of the circuit input digital signal is 1
When 0, 10, 10, 10, ..., 9, 10,
The case of 9, 10, ... Will be described. When the circuit input digital signal value is 10, 10, 10, 10, ..., The averaged data output from the low pass filter 84 is 19/2 = 9.5, and the output data of the rounding circuit 85 is rounded up. 10, 10, 10, 10, ...
Becomes This is the same as the case of no addition. When the circuit input digital signal value is 9, 10, 9, 10, ..., The averaged data output from the low pass filter 84 is 18/2 = 9, and the output data of the rounding circuit 85 is 9. .. This is a value from 10 without a grant to -1
Only shifts.

As described above, the output data value of the rounding circuit 85 becomes the same value for one input digital signal value 10, 10, ... .. differ by −1 depending on the presence or absence of addition. As described above, since it is considered that the respective circuit input digital signal values are generated with the same probability, the average value of the output data of the rounding circuit 85 is 9.5 with the addition, and compared with the case without the addition. Shift amount is −0.5, and −0.
This is equivalent to a shift of 5 occurring. That is, switch 8
If either of the shift data a and b input to 2 is 1, the circuit input digital signal value is shifted by -0.5, and the average value of the shift data is the shift amount.

The values of the shift data a and b are determined by the microcomputer inputting the average value of the detected black levels output from the average value calculation circuit 17 of FIG. 3. For example, as shown in FIG. When the difference between the detected black level and the reference black level is within -3, the shift amount of the circuit input digital signal is set to -0.5.

In the embodiment shown in FIG. 1, the case where there are two types of input digital signals has been described.
Even if there are four types, if the input digital signal value is a random value, the present invention can be similarly applied and the same effect can be obtained. Also, only the negative shift amount is shown,
The same can be applied to the case of positive as shown in FIG.

FIG. 2 is a block diagram showing another embodiment of the present invention. Two types of 10-bit circuit input digital signals X and Y are input to a low-pass filter (averaging means) 86 and averaged data (X
+ Y) / 2 is 11-bit data, since it may have a numerical value below the decimal point due to averaging. This averaged data is input to the subtraction circuit (shift data addition means) 87, and the subtraction circuit 87 subtracts the shift data c from the averaged data and outputs the data {(X + Y) / 2-c}. The data {(X + Y) / 2-c} is 12-bit data having positive and negative values.

The limit circuit 88 removes the negative value of the 12-bit output data {(X + Y) / 2-c} into 11-bit data, and the rounding circuit 89 outputs the input data {(X
+ Y) / 2-c} is rounded up, such as rounding up to the right of the decimal point, and 10-bit circuit output digital data X'and Y'having no digits below the decimal point are output from the terminal T2.

In this way, subtraction data Xa, Yb obtained by shifting the circuit input digital signal by the shift data a, b supplied from the microcomputer are obtained, and the rounding process is performed after averaging the subtraction data. As a result, it is possible to obtain a circuit output digital signal in which the black level of the circuit input digital signal is automatically clamped to the black reference level, and complicated adjustment for clamping is unnecessary.

[0036]

As described above, in the black level clamp circuit according to the present invention, the shift data is added to the circuit input digital signal and the circuit input digital signal is averaged. The level shift amount can be automatically set so that the black level matches the black reference level, and the complexity of the conventional black level adjustment can be prevented. Further, since the shift amount is set by averaging the circuit input digital signals, the shift amount is obtained by averaging the shift data, and the circuit output digital signal obtained by finely shifting the circuit input digital signal can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a black level clamp circuit according to the present invention.

FIG. 2 is a block diagram showing another embodiment of the black level clamp circuit according to the present invention.

FIG. 3 is a block diagram showing an image processing apparatus using an embodiment of the present invention.

FIG. 4 is an image area diagram showing an optical black area.

FIG. 5 is a graph showing a shift amount corresponding to a difference between a calculated black level average value and a black reference level.

FIG. 6 is a block diagram showing a conventional image processing apparatus.

FIG. 7 is a waveform diagram showing a waveform of a CCD output image signal.

[Explanation of symbols]

 81 subtraction circuit (shift data giving means) 82 changeover switch 83 limit circuit 84 low-pass filter (averaging means) 85 rounding circuit (rounding means)

Claims (1)

[Claims] 1. A black level clamp circuit for clamping the black level of an image signal output from an image sensor to a black reference level, wherein the circuit input digital signal and the circuit output digital signal have the same resolution. Shift data giving means for giving shift data to the signal, averaging means for averaging the circuit input digital signals and outputting averaged data, and rounding means for rounding the averaged data. A black level clamp circuit characterized in that