Control circuitry, in an AKB sample pulse generator, enables charging of a capacitor upon initiation of a vertical retrace pulse, initiates discharge of the capacitor in response to termination of the retrace pulse and concurrently produces an AKB output pulse having a width (W) determined by an RC time constant and having a delay time (Td) relative to initiation of the retrace pulse that is equal to the length (L) of the retrace pulse and that is independent of the value of the RC time constant. Advantageously, the AKB sample pulse delay time tracks variations, if any, of the retrace pulse and is unaffected by parameter variations of the RC components. Additionally, the sample pulse width can be controlled with analog simplicity and can be increased, for the same amount of overscan as compared with systems in which both pulse delay and width exhibit component sensitive delay times, thereby providing improved AKB control accuracy due to the increased "aperture" or sampling time.
A black level detecting circuit compares an input video signal with a current minimum value of input video signals, then detects a difference between the input signal and the current minimum value when the input value is smaller than the current minimum value, and then outputs the detected difference only during a detecting period of the black level, then corrects the minimum value by using this output for renewing the current minimum value, and finally outputs this renewed minimum value as a new minimum value of the input video signal. The black level detecting circuit also latches the minimum value during the vertical blanking pulse and initializes the minimum value during a vertical blanking period so as to detect a black level of the input signal. As such, the black level detecting circuit measures the black level only during a video period, thereby reducing detection errors due to noises, and also an automatic initialization of the minimum value during a blanking period can contribute to a more stable detection of the black level. The black level detected can be used effectively as a control signal over picture quality for each field or each frame.
A video imaging apparatus includes a source of a blanking signal that is indicative when a blanking interval occurs in a video signal. A delay circuit including a horizontal line counter is responsive to the blanking signal and to a signal at the horizontal rate for delaying the blanking signal by a multiple number of horizontal periods to generate a delayed signal. A dynamic focus voltage generator includes a switch responsive to the delayed signal for applying a dynamic focus voltage to a focus electrode, when the switch is at a first state, and for disabling the application of the dynamic focus voltage, when the switch is at a second state. An end time of the interval, during which the dynamic focus voltage is disabled, is determined in accordance with an output signal of the counter.
An amplifier (60) provides an amplified video signal in response to a video input signal supplied thereto. A cathode current sensor (Q3), couples an output of the amplifier to the cathode of a kinescope (20) and also provides an output current (Ik) proportional to the kinescope cathode current. A feedback path (82) applies a portion of the sensed cathode current provided by the cathode current sensor to a circuit node (65 or 63) in the amplifier for imparting gamma correction to images produced by the kinescope. In one application the cathode current is apportioned (Q3, Q5) to provide gamma correction for the kinescope and to provide automatic kinescope bias (AKB) control.
A pump-up circuit includes a first capacitor C1 connected to the power supply Vcc via a first change-over switch SW1 to the reference potential point or a second capacitor C2. The second capacitor C2 is charged by the voltage control circuit 45 under the control of the pump-up voltage control signal so that the capacitors C1 and C2 are charged respectively during the scanning period. Upon input of a NTSC signal, the change-over switch SW1 is connected to the reference potential point and the charging voltage of the first capacitor C1 is used as the pump-up voltage. Upon input of a signal having a short vertical flyback period such as the signal from personal computers, the total of the charging voltages at the first and second capacitors C1 and C2 is used as the pump-up voltage.
