JPH1188776A - Digital clamp circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain black level correction proper to a digital video signal by setting a black level of a digital video signal based on a signal level for an optical black period of the digital video signal resulting from digitizing a video signal from an image pickup element and controlling a frequency of a black level setting operation in response to a timewise change of the signal level. SOLUTION: A microcomputer 6 cuts off low-order 4-bits from a signal level object value in 12-bit for an optical black period detected by a detection circuit 4a into 8-bit in total to calculate a difference from a black level sent precedingly. In the case that a difference is small, the value differs from a preceding value succeedingly for 60 times, the black label to be sent is updated and when the difference is not small and the value differs from a preceding value consecutively twice, the black level to be sent is updated and the updated black level is set to a setting circuit 4b. Thus, a change in a hue caused by updating frequently the black level is suppressed when the hue of the object is unchanged.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a digital clamp circuit used in a digital camera.

[0002]

2. Description of the Related Art In a video camera, especially a digital video camera for digitally processing a signal, a digital clamp circuit is provided to stabilize a reference level of a video signal taken out. In a conventional digital clamp circuit, a signal level in an optical black period of a digital video signal obtained by digitizing a video signal output from an image sensor is detected, and a black level in subsequent signal processing is determined in accordance with the signal level. Was set.

[0003]

However, in the conventional digital clamp circuit, the signal level of the digital video signal obtained by digitizing the video signal output from the image sensor during the optical black period is determined by the brightness of the subject. In the case where the black level changes due to noise or the like in a state where it does not change, the set black level is frequently updated, so that the hue changes and a false color occurs.

An object of the present invention is to provide a digital clamp circuit that performs appropriate black level correction on a digital video signal obtained by digitizing an output from an image sensor.

[0005]

In order to solve the above-mentioned problems, the present invention provides an analog-to-digital converter as a digital clamp circuit, which digitizes a video signal output from an image sensor to obtain a digital video signal; Detecting means for detecting a signal level of an optical black period of a digital video signal, and setting provided at a subsequent stage of the detecting means and setting a black level of the digital video signal based on the signal level detected by the detecting means Means, a change amount detecting means for detecting a magnitude of a temporal change of the detected signal level, and a signal level by the setting means in accordance with the magnitude of the temporal change detected by the change amount detecting means. Control means for controlling the frequency of the setting operation.

Further, the control means increases the frequency of the signal level setting operation by the setting means as the magnitude of the temporal change detected by the change amount detecting means increases.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a functional block diagram showing an embodiment of the present invention, and FIG. 2 is a flowchart showing the operation.

The output from the image pickup device 1 is adjusted in level by an AGC circuit 2 and then converted into a digital video signal by an A / D conversion circuit 3. Then, the black level is corrected by the digital clamp circuit 4 and output from the output terminal 5.

In the digital clamp circuit 4, the detection circuit 4
a, the signal level during the optical black period is 12
It is detected as a bit and is output as a candidate value to the microcomputer 6 that controls the digital clamp circuit.

In the microcomputer 6, as shown in FIG. 2, first, candidate values are inputted (S1).
It is cut to 8 bits (S2). Next, a difference from the previously transmitted black level is calculated (S3). If the difference is small, the black level to be transmitted is updated if it is different from the previous value 60 times in a row (S4, S6), otherwise, it is not updated (S7). On the other hand, if the difference is not small, the black level to be transmitted is updated twice if it is different from the previous value (S5, S5).
6) If not, update is not performed (S7). Finally, the 8-bit black level determined in this way is transmitted to the setting circuit 4b of the digital clamp circuit 4 (S8).

The black level (8 bits) transmitted from the microcomputer 6 is set as the black level of the digital video signal by the setting circuit 4b of the digital clamp circuit 4.

[0012]

According to the present invention, the setting of the black level, which is the reference of the color signal, is controlled in accordance with the level of the level change of the digital video signal. Changes in hue caused by frequent updating of the black level can be suppressed, and the effect is significant.

[Brief description of the drawings]

FIG. 1 is a functional block diagram showing an embodiment of the present invention.

FIG. 2 is a flowchart showing the operation of the embodiment of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Image sensor 3 A / D conversion circuit 4 Digital clamp circuit 6 Microcomputer

Claims (2)

[Claims] An analog-to-digital converter for digitizing a video signal output from an image sensor to obtain a digital video signal; a detector for detecting a signal level of the digital video signal during an optical black period; Setting means provided at a subsequent stage for setting a black level of the digital video signal based on the signal level detected by the detection means; and a change amount for detecting a magnitude of a temporal change in the detected signal level A digital clamp comprising: a detecting unit; and a control unit that controls a frequency of a signal level setting operation performed by the setting unit in accordance with a magnitude of a temporal change detected by the change amount detecting unit. circuit. 2. The control unit according to claim 1, wherein the frequency of the signal level setting operation by the setting unit increases as the magnitude of the temporal change detected by the change amount detecting unit increases. 2. The digital clamp circuit according to 1.