Device for adjusting the black level of a video signal
AssigneeSGS-Thomson Microelectronics S.A.
InventorThierry Meunier
Life of this patent
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Abstract
A device for adjusting a video signal so that its black level is in coincidence with a predetermined reference level includes a capacitor having a first terminal that receives the video signal and a second terminal that provides the adjusted video signal, and means for discharging the capacitor at a constant current when the adjusted video signal exceeds the reference level, and for charging the capacitor at a constant current when the adjusted video signal is below the reference level, the ratio between the charging and discharging currents ranging from 4.3 to 12.6. The device further includes means for significantly reducing the current ratio at least during a portion of a frame synchronization pulse train.
Claims26
A device for adjusting a video signal so that its black level is in coincidence with a predetermined reference level, comprising:
- a capacitor having a first terminal that receives the video signal and a second terminal that provides an adjusted video signal;
- means for discharging the capacitor with a first constant current when the adjusted video signal exceeds the reference level, and means for charging the capacitor with a second constant current when the adjusted video signal is below the reference level, a ratio between the first and second constant currents ranging from 4.3 to 12.6;
- the video signal having a frame synchronization pulse train;
- and
- means for reducing said ratio at least during a portion of an occurrence of the frame synchronization pulse train.
A circuit for adjusting the voltage level of an electrical signal to coincide with a predetermined reference level, comprising:
- a capacitor having a first terminal that receives the electrical signal and a second terminal that provides an adjusted electrical signal having a voltage;
- a charging current source, providing a charging current, that charges the voltage at the second terminal of the capacitor;
- a discharging current source, providing a discharging current, that discharges the voltage at the second terminal of the capacitor;
- a differential amplifier that accepts the adjusted electrical signal and the predetermined reference level, and the generates a first control signal that controls at least one of the charging and discharging current sources to generate at least two different values of current;
- and
- a circuit that provides a second control signal in response to a pattern in the electrical signal, the second control signal controlling at least one of the charging and discharging current sources to generates at least two different values of current.
A device for adjusting a video signal so that its black level is in coincidence with a predetermined reference level, comprising:
- means for AC-coupling the video signal to generate an adjusted video signal having a voltage;
- means for increasing the voltage of the adjusted video signal at a first rate;
- means for decreasing the voltage of the adjusted video signal at a second rate;
- means for generating a first control signal having a value that is determined by a difference between the adjusted video signal and the reference level;
- first means, operative in response to a change in the value of the first control signal, for changing one of the first rate and the second rate;
- means for detecting a frame retrace pulse train in the video signal;
- second means, operative in response to a detection of a frame retrace pulse train, for changing one of the first rate and the second rate.
A method for adjusting the black level of a video signal to be coincident with a predetermined reference level, comprising the steps of:
- AC-coupling the video signal to produce an adjusted video signal;
- comparing the adjusted video signal to the reference signal;
- charging the adjusted video signal to a higher voltage when the adjusted video signal exceeds the reference signal;
- discharging the adjusted video signal to a lower voltage when the reference signal exceeds the adjusted video signal;
- and
- varying a rate of the discharging in response to an occurrence of a frame retrace pulse train.
A device for adjusting a video signal so that its black level is in coincidence with a predetermined reference level, comprising:
- a capacitor having a first terminal that receives the video signal and a second terminal that provides an adjusted video signal;
- a first current source providing a first constant current that discharges the capacitor when the adjusted video signal exceeds the reference level, and a second current source providing a second constant current that charges the capacitor when the adjusted video signal exceeds the reference level, a ratio between the first and second constant currents ranging from a first value to a second value;
- the video signal having a frame synchronization pulse train;
- and
- a circuit that adjusts the second current source to reduce said ratio from said first value to said second value at least during a portion of a frame synchronization pulse train.
Dependencies are inferred from the claim text.