Black-level correction circuit for video camera

Claim 1

A black-level correcting circuit for a video camera having image pick-up means (1, 2, 3R, 3G, 3B) for generating a video signal corresponding to an image, video signal amplifying means (6) for amplifying the video signal from said image pick-up means (1, 2, 3R, 3G, 3B), the gain of said video signal amplifier means (6) being switch­able between at least two values and video signal processing means (7) supplied with the output of said video signal amplifying means (6) for processing the same, said video signal processing means (7) including a black-level clip circuit for clipping a black-level signal below a pred­termined level, said black-level correcting circuit being characterized by: control means (12) for controlling said image pick-up means (1, 2, 3R, 3G, 3B) such that no light enters said image pick-up means (1, 2, 3R, 3G, 3B) in a black-level correcting mode; detecting means (9, 10) for detecting the level of the output of said video signal processing means (7) when no light enters said image pick-up means; means (11, 12, 13) for judging whether a difference between the outputs of said detecting means (9, 10) detected at two time points with a predetermined interval of time therebetween is below a threshold value and determining therefrom whether the black level is stabilized; correcting means (13, 14) for generating a correcting signal upon a change in said stabilized black levels before and after switching the gain of said video signal amplifying means (6) and for supplying said correcting signal to said video signal amplifying means (6) for decreasing said change; means (12) for converging said correcting signal by repeat­edly operating said detecting means (9, 10), said means (11, 12, 13) for judging, and said correcting means (13, 14); and clip detecting means for detecting that the corrected black level is below the predetermined level by comparing said correcting signal and a predetermined reference level and for causing a fixed value to be output instead of the correcting signal and said converging means to be operated again.

Claim 7

A method of correcting a black-level signal in a video camera having image pick-up means (1, 2, 3R, 3G, 3B) for generating a video signal corresponding to an image, a video signal amplifier (6) for amplifying the video signal from the image pick-up means (1, 2, 3R, 3G, 3B) the gain of said video signal amplifier (6) being switchable between at least two levels, and a video signal processor (7) supplied with the output of said video signal amplifier (6) for processing the same, said video signal processor (7) including a black-level clip circuit for clipping the black-level signal below a predetermind level, the method for correcting a black-level signal being charac­terized by the steps of: controlling the image pick-up means (1, 2, 3R, 3G, 3B) such that no light enters therein to place the video camera in a black-level correcting mode; detecting the level of the output of the video signal processor (7) when no light enters the video image pick-up means (1, 2, 3R, 3G, 3B); judging whether a difference between the output of the video signal processor (7) detected at two time points with a predetermined interval of time therebetween is below a threshold value and determining therefrom whether the black level has been stabilized; generating a correcting signal upon a change in said stabilized black levels before and after the switching of the gain of video signal amplifier (6) and supplying said correcting signal to the video signal amplifier (6) to decrease said change; converging said correcting signal by repeatedly performing said detecting operations, said judging operation, and said generation of a correcting signal; and detecting a clip condition by determining that the corrected black-level signal is below the predetermined level and comparing said correcting signal and a predetermined reference level and causing a fixed value to be output in place of the correcting signal, and performing said step of converging once again.

Claim 8

A method of correcting a black-level signal in a video camera having image pick-up means (1, 2, 3R, 3G, 3B) for generating a video signal corresponding to an image, a video signal amplifier (6) for amplifying the video signal from the image pick-up means (1, 2, 3R, 3G, 3B) the gain of said video signal amplifier (6) being switchable between at least two levels, and a video signal processor (7) supplied with the output of said video signal amplifier (6) for processing the same, said video signal processor (7) including a black-level clip circuit for clipping the black-level signal below a predetermind level, the method for correcting a black-level signal being charac­terized by the steps of: controlling the image pick-up means (1, 2, 3R, 3G, 3B) such that no light enters therein to place the video camera in a black-level correcting mode; detecting the level of the output of the video signal processor (7) when no light enters the video image pick-up means (1, 2, 3R, 3G, 3B); judging whether a difference between the output of the video signal processor (7) detected at two time points with a predetermined interval of time therebetween is below a threshold value and determining therefrom whether the black level has been stabilized; generating a correcting signal upon a change in said stabilized black levels before and after the switching of the gain of video signal amplifier (6) and supplying said correcting signal to the video signal amplifier (6) to decrease said change; converging said correcting signal by repeatedly performing said detecting operations, said judging operation, and said generation of a correcting signal; and detecting a clip condition by determining that the corrected black-level signal is below the predetermined level and comparing said correcting signal and a predetermined reference level and causing a fixed value to be output in place of the correcting signal, and performing said step of converging once again.

Claim 11

A method of correcting a black-level signal according to any one of claims 6 to 8, characterized in that said step of converging includes detecting the polarity of the correcting signal and amending the correcting signal by a predetermined amount in a polarity opposite to the detected polarity.