JP3487041B2 - Video camera signal processor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention converts an output of an image pickup device such as a CCD or an image pickup tube into a digital signal, and processes the image pickup signal by a digital signal processing circuit. And a video camera signal processing device. 2. Description of the Related Art It is known that an image pickup signal obtained by a CCD is converted into a digital signal and signal processing such as gamma correction is performed by a digital circuit. In the first stage of the digital signal processing circuit, the black level of the digital image signal is usually clamped to a predetermined level by a digital clamp circuit. That is, an optical black level detection area is provided on a part of the imaging surface of the image sensor, and the black level of the digital imaging signal corresponding to the optical black level detection area is detected. . The digital clamp circuit controls the level in units of one bit of the digital image signal. However, the black level is detected using an integration process for noise elimination, and even if the black level is detected correctly on average, it cannot completely follow the fluctuation of the digital imaging signal, and the black level of the digital clamp output is not detected. The level fluctuates. The minimum value of the change in the black level corresponds to a change in the LSB. [0003] Such a change in the level of a black portion may be visually recognized in a reproduced image.
For example, when the inclination of the γ correction curve in the signal processing circuit is steep or when the setup is large, the change in the black level due to the change in the LSB is easy to notice. [0004] In order to solve this problem, it is conceivable to increase the number of bits of the digital imaging signal and reduce the width of the level change due to the change of the LSB. This method
It is not preferable because the scale of hardware increases with an increase in the number of bits. As another method, it is conceivable to intentionally prohibit the control of the LSB according to the condition of the change of the LSB, that is, under the condition that the level change is easily visible. In this method, there is a possibility that the black level cannot be accurately clamped due to an erroneous determination of the condition. As still another method, the slope of the γ curve is made gentle to visually
There is also a method of making a change in black level due to a change in SB less noticeable. This method has a problem that black gradation is lost. Accordingly, an object of the present invention is to control the black level without increasing the number of bits, without intentionally prohibiting the control of the black level, and without changing the γ level.
An object of the present invention is to provide a signal processing device of a video camera capable of preventing a change in a black level from being visually recognized without making a slope of a curve gentle. SUMMARY OF THE INVENTION The present invention relates to a video camera signal processing device which converts an image pickup signal from an image pickup device into a digital image pickup signal and supplies it to a digital signal processing circuit. Black level detecting means for detecting the black level of the image, and averaging the detected black levels.
Reduction and performs noise removal, on the basis of the average value of the black level, and clamping means for clamping the black level of the digital imaging signal to a predetermined level, LSB non average value of the black level
Full level fluctuation component is referred to A / D converter reference voltage
And a control means for controlling the level of the black level below the LSB by synthesizing the video signal. [0007] The detected black level is integrated, the black level of the digital image signal is shifted by a minute component of the integrated black level, and when the digital clamp is performed, the clamp output can be prevented from fluctuating in LSB units. An embodiment of the present invention will be described below. FIG. 1 shows the overall configuration of this embodiment. Reference numeral 1 denotes a CCD image pickup device. An image pickup signal from the CCD image pickup device 1 is supplied to an A / D converter 2, and one sample is converted into a 10-bit digital image pickup signal, for example. The digital imaging signal is supplied to the signal processing circuit 3. The signal processing circuit 3 includes a clamp circuit, a γ correction circuit, a matrix circuit, and the like, as will be described later, and a digital component signal (a luminance signal and two color difference signals) is extracted from an output thereof. [0009] The output of the A / D converter 2 is also supplied to a black level detection circuit 4 to detect the black level of the digital image signal. An optical black level detection area is provided in a partial area of the imaging surface of the imaging element 1. The section (black level) in the image signal corresponding to this area is detected by the detection circuit 4. The detected black level is the microcontroller 5
Supplied to The microcontroller 5 integrates (averages) the detected black levels and removes noise components. The microcontroller 5 supplies the black level obtained by the integration to the digital clamp circuit of the signal processing circuit 3 as a digital clamp value. The control of the black level uses the change of the LSB as a minimum unit. At the same time, the microcontroller 5, as described later,
In order to perform control below LSB, the processed black level is
The signal is supplied to an RB (reference bottom voltage of the A / D converter 2) modulation circuit 6. [0010] The output of the VRB modulation circuit 6 is the A / D converter 2
Is supplied as VRB. A / D converter 2
For example, a plurality of resistors are connected in series between a reference top voltage and a reference bottom voltage, and a plurality of voltages divided by the resistors are compared with an input voltage,
It is configured to output a 10-bit digital signal. Therefore, by varying the reference bottom voltage, it is possible to perform control at a level smaller than a voltage change due to a change in LSB. In other words, while the smallest unit is LSB, the level of a value smaller than 1 can be controlled within the range of 0 to 1. The configuration surrounded by a broken line in FIG. 1 is provided for controlling the LSB or lower. FIG. 2 shows a configuration for processing the output of the image sensor 1 more specifically. 11G, 11R, and 11B are CCD image sensors that output G (green), R (red), and B (blue) image signals, respectively. An output signal of the CCD image pickup device 11G is sampled by a sample hold circuit (for example, a correlated double sampling circuit) 12G, and the output is supplied to an AGC (automatic gain control) circuit 13G. The gain of the AGC circuit 13G is controlled by a gain control signal. The output signal of the AGC circuit 13G is supplied to the offset adding circuit 14G. In the offset adding circuit 14G, an offset value (analog voltage) for raising the black level to a predetermined digital value is added. The output signal of the offset adding circuit 14G is supplied to the A / D converter 2G, and one sample is converted into, for example, a 10-bit digital image signal. As with the processing for the G signal, the sample and hold circuit 12 for the R and B signals
R, 12B, AGC circuits 13R, 13B, offset adding circuits 14R, 14B, and A / D converters 2R, 2B are provided. Offset adding circuits 14G, 14R, 14
In B, an offset value of 50 is added when, for example, the image has gradation levels of 0 to 1023. FIG. 3 shows an example of the signal processing circuit 3 connected to the A / D converters 2G, 2R and 2B. A / D
The digital imaging signal from the converter 2G is applied to the clamp circuit 2
1G. As described above, the clamp circuit 21G clamps the black level to which the offset is added to a predetermined value, for example, a value of 16. The digital clamp circuit 21G is configured to add / subtract a digital clamp value to / from a black level of a digital image signal. If the black level of the image signal output from the image sensor 1G is 0, the black level is shifted to a value of 50 by adding an offset. In this case, the digital clamp circuit 21G subtracts the 34 digital clamp values to form an image signal having a black level of 16. On the other hand, when the gain is higher and the black level is 3, the black level becomes 53 after the offset is added. In this case, the digital clamp value of 37 is subtracted to obtain the black level. Is a value of 16. The digital clamp value is supplied from the microcontroller 5 in FIG. The control by the clamp circuit 21G is a control in LSB units. A white balance control circuit 22G is connected to the clamp circuit 21G. The gamma correction circuit 23G is connected to the white balance control circuit 22G,
An aperture control (contour enhancement) circuit 24G is connected to the γ correction circuit 23G, and a white clip circuit 25G is connected to the aperture control circuit 24G. These signal processing circuits are the same as those conventionally used for processing an image pickup signal of a video camera. A signal processing circuit similar to the above-described signal processing circuit for the G signal is provided for each of the R signal and the B signal. That is, the clamp circuits 21R,
1B, white balance control circuits 22R and 22B, gamma correction circuits 23R and 23B, aperture control circuit 2
4R, 24B and white clip circuits 25R, 25B are provided. White clip circuit 25G, 25
The three primary color signals from R and 25B are subjected to a matrix operation by the matrix circuit 26, and a luminance signal Y, a color difference signal RY,
BY is formed. Although omitted in FIG. 2, FIG.
The configuration for controlling the black level shown in FIG.
It is provided for each of the R signal and the B signal. FIG. 4 shows an example of a configuration for control below the LSB indicated by a broken line in FIG. Microcontroller 5
, The average value is calculated by integrating the black level detected for each field for, for example, 60 fields. The averaging detects a black level having a component below the decimal point. This black level is supplied to the D / A converter 31. The D / A converter 31 outputs a voltage value ΔV corresponding to a component below the decimal point. This voltage value ΔV
Is smaller than the voltage value generated by the change of the LSB of the digital imaging signal. The voltage value ΔV from the D / A converter 31 and the reference voltage _Vref are added by resistors 32 and 33. The voltage value (V _ref + ΔV) corresponds to the buffer amplifier 3
4 to the A / D converter 2 as its VRB (reference bottom voltage). By changing the VRB, the reference voltage for generating the digital output of the A / D converter is also shifted by ΔV, and the change in the level of ΔV can be canceled. In this way, the black level can be controlled with a value smaller than the voltage value caused by the change in LSB. By controlling the black level, a minute change in the black level is canceled on average, and it is possible to prevent the black level from changing with the LSB as the minimum unit when digital clamping is performed. FIG. 5 is a flowchart showing the flow of the black level control process according to one embodiment of the present invention. In step ST1, when the analog imaging signal is received, A /
A reference value for D conversion is set (step S
T2). The setting of the reference value is performed by the control below the LSB as described above. A / D conversion of the analog imaging signal is performed based on the set reference value (step ST3). The digital image signal obtained by the A / D conversion is supplied to a black level detection circuit 4, and a black level generated corresponding to an optical black level area is detected (step ST4). As shown in step ST5, the detected black level is supplied to the microcontroller 5, and the noise component is removed by integration (averaging). FIG.
In the example shown in (1), a fixed value, for example, an analog black level offset of 50 is added. Even if an imaging output of 0 is generated in correspondence with the optical black level area, the black level after adding the offset is 50 due to the influence of the characteristics of the imaging element, the gain and noise in the previous stage of the offset adding circuit. To fluctuate. In the case where the control below the LSB is not performed, which is a feature of the present invention, the digital clamp circuit adds a digital clamp value to the detected black level.
By subtraction, the black level is clamped to a predetermined value, for example, 16. However, since the averaged black level is used, the black level of the clamp output has a change in LSB. Changes can be noticeable. According to the present invention, a component below the decimal point of the detected black level is synthesized with the reference voltage of the A / D converter 2, and the output of the A / D converter 2 averages minute fluctuations due to noise or the like. Can be canceled. Therefore, when the digital clamp is performed, the black level becomes L
Having a change in SB can be suppressed. In the above-described embodiment, the LSB is varied by changing the reference voltage of the A / D converter.
Although the following control is performed, control other than LSB may be performed by an analog level control circuit. In addition, the present invention is not limited to the CCD, and can be applied to a case where another solid-state imaging device or an imaging tube is used. According to the present invention, the black level is digitally detected and averaged to detect a change in the black level less than the LSB, and this minute change in the black level is detected by the A / D converter.
Before conversion, it can be canceled by the analog circuit,
Thereby, the fluctuation of the black level can be suppressed.
Therefore, the present invention can improve the image quality. According to the present invention, there is no need to increase the number of quantization bits for digitization, and the circuit scale does not increase. Further, according to the present invention, there is no possibility of malfunction due to an erroneous condition determination, as compared with the case where the control of the black level is stopped under a certain condition. Further, the present invention has an advantage that there is no problem of losing the black gradation in the process of making the slope of the γ correction curve gentle.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an overall configuration of an embodiment of the present invention. FIG. 2 is a block diagram of an example of an analog signal processing circuit for an image pickup signal according to an embodiment of the present invention. FIG. 3 is a block diagram of an example of a digital signal processing circuit of an image pickup signal according to an embodiment of the present invention. FIG. 4 is a block diagram showing an example of a configuration for controlling a black level lower than LSB according to an embodiment of the present invention; FIG. 5 is a flowchart showing the flow of processing according to one embodiment of the present invention. [Description of Signs] 1, 1G, 1R, 1B CCD imaging device 2, 2G, 2R, 2B A / D converter 4 Black level detection circuit 5 Microcontroller 6 VRB modulation circuit 21G, 21R, 21B Digital clamp circuit

Continuation of the front page (56) References JP-A-6-86095 (JP, A) JP-A-4-299670 (JP, A) JP-A-1-171382 (JP, A) JP-A-61-257077 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H04N 5/14-5/217 H04N 5/335

Claims (1)

(57) [Claim 1] In a signal processing device of a video camera which converts an image pickup signal from an image pickup device into a digital image pickup signal and supplies it to a digital signal processing circuit, a black level detecting means for detecting the black level, the detected black level averaging, performs noise removal, on the basis of the average value of the black level to clamp the black level of the digital imaging signal to a predetermined level clamp Means and a level variation component less than the LSB of the average value of the black level.
By combining it with the reference voltage of the A / D converter,
And a control means for controlling the level of the black level below the LSB.