JPH0666897B2 - Video signal processor - Google Patents

Description

The present invention relates to a device for automatically controlling a bias of an image display device of a video processing display device.

[Background of the Invention]

In a video signal processing display device such as a television receiver or a video monitor, in order to automatically maintain an appropriate black current level for each electron gun of an associated video display video tube,
Automatic picture tube bias (AKB) control schemes are often used and as a result of this behavior, the color of the displayed image and the grayscale tracking of the picture tube vary from the required level of picture tube bias, especially with age and temperature. Because it is not harmed. Various AKB systems are known, such as those described in US Pat. Nos. 4,362,622 and 4,387,405.

The AKB system normally operates during an image erasing period when the black level display current flowing through the picture tube is small, and senses this current to generate a bias control signal representing the difference between the current level and the required black current level. This control signal is applied to the video signal processing circuit in a direction to reduce the difference. In the conventional AKB system, a bias control signal is applied to the video output drive stage of the picture tube to change the output bias of the drive stage, and thereby the signal input electrode of the picture tube is maintained so that the bias of the picture tube is properly maintained. The picture tube bias is adjusted to an appropriate level by changing the bias (for example, the cathode), but such a method requires the picture tube to be DC coupled to the output of the drive stage.

However, it is desirable to AC couple the drive stage to the picture tube (capacitively coupled), such as in wideband video monitors and devices where it is desirable to maintain a substantially constant DC output bias to the drive stage. There are many things. The advantage of AC coupling the drive stage with the picture tube is that the operating point of the drive stage is optimized, allowing the use of lower operating voltages and reducing power consumption. A low operating voltage allows the use of a lower load resistance in the drive stage, which is preferable for wideband video devices, and is particularly advantageous for consumer television receivers because it allows the use of cheaper output transistors that do not have a heat sink. Is.

Accordingly, it is an object of the present invention to provide an AKB controller in an AC coupled video device in which the above-mentioned advantages of both AKB control and AC coupling can be realized.

[Background of the Invention]

The video signal processing device having the features of the present invention will be described below with reference to the reference numerals used in the embodiments described later. The intensity control means (22) formed by the cathode (22) and the grid (24) will be described below. , 24), and an image display device (25) for displaying video information according to a video signal applied to the control means. In addition, this video signal processing device has a drive amplifier stage.
(12) and a coupling including a capacitor (20) for AC coupling the video signal from the output circuit (16, 18) of the amplifier stage (12) to the cathode (22) of the intensity control means of the image display device (25). Means (20, 47
0Ω resistance), and bias control means (32 to 40) for maintaining the required bias state for the image display device. More specifically, the bias control means (32-40) is coupled to the intensity control means (22) of the image display device (25) and generates a signal representing the black current flowing through the image display device during the image erasing period. Means (40, G) and means (32 to 39) for generating a bias control signal related to the magnitude of the black current signal in response to the signal representing the black current. Is the above capacitor (20)
And the cathode (22) of the intensity control means are coupled to the coupling means, and serve to maintain the required bias state of the image display device (25).

Another feature of the present invention is that the video output signal is AC-coupled from the output circuit (16, 18) of the video drive amplifier stage (12) to the intensity control means (22) of the image display device (25). It is in an AKB control device that can be used in a video signal processing device. According to this AKB control device, the image display device is displayed during the periodic AKB control period.
A signal representing a black current flowing through the intensity control means (22) of (25) is generated, and a DC recovery circuit (30) having an output coupled to the intensity control means (22) responds to the signal representing the black current. To generate a bias control signal related to the magnitude of the signal. This bias control signal serves to keep the black level of the intensity control means (22) at a required value. Strength control means (22, 2
4) is energized during the periodic image erasing period to generate an output pulse having a magnitude related to the magnitude of the black current flowing through the means, and the DC recovery circuit (30) responds to this output pulse. ,
A bias control signal is supplied to the AC coupling capacitor in the signal path for coupling the video output signal from the video drive amplifier stage (12) to the intensity control means (22).

[Detailed description of the preferred embodiment]

In FIG. 1, a video signal from the signal source 10, for example, a red component of a color video signal is amplified by a video output drive amplifier stage 12. The amplifier 12 includes a high voltage transistor 15 whose collector is connected to the operating voltage (+ 150V) power supply of the transistor 15 via series load resistors 16 and 18 of different values. The normally non-conducting diode 17 limits excessive amplitude of the video output signal during the normal image period. The amplified video signal generated at the collector output of the transistor 15 is the cathode which constitutes the intensity control means together with the grid of the color video tube 25.
It is of sufficient size to drive 22a and is AC coupled to its cathode 22a via capacitor 20. In this example, the picture tube 25 is of the self-concentrating in-line electron gun type and has separate cathodes 22a, 22b, 22c and a grid 24 common thereto. Each cathode together with the grid 24 forms part of the electron gun assembly of the picture tube.

An automatic picture tube bias (AKB) control circuit 30 operates as a DC recovery circuit that maintains the black current level of the picture tube electron gun assembly including the cathode 22a at a required value, and a similar AKB circuit (not shown).
Is also attached to each electron gun including cathodes 22b, 22c that receive green and blue signal components, for example, as shown for the red signal component. The operation of the AKB circuit 30 will be described with reference to the AKB timing signal waveform of FIG. This timing signal is generated from the signal source 40 in response to the horizontal image synchronizing signal H and the vertical image synchronizing signal V extracted from the deflection circuit of the receiver.

Next, in FIG. 2, the AKB operation period includes a sensing period without image information of the video signal which occurs shortly after the end of the vertical blanking period of each video signal within the vertical erase period. In the example, a positive grid drive pulse G and a positive sensing enable pulse S each having a length of approximately one horizontal image line period are generated. Further, there is a clamp period in which the clamp keying pulse K is generated before the pulses G and S related to the AKB operation. The blanking pulse B extends over the AKB operation period in which the pulses K, G, and S occur, and functions to set the reference condition of the video signal during the AKB operation period.

Returning to FIG. 1, the blanking signal B is applied to the erase input terminal of the signal source 10 and blocks the video signal output of the signal source 10 during the AKB operation period. During each AKB black current sensing period, a positive grid drive pulse G forward biases the grid 24 of the picture tube,
The conduction of the electron gun structure including the cathode 22a and the grid 24 is increased. Cathode 22a according to the pulse G during the grid pulse period
A positive current pulse is induced on the cathode, but the amplitude of this cathode output current pulse is determined by the black current level (typically several μm) flowing through the cathode.
Related to A).

This induced cathode output pulse is an AC coupling capacitor 20
1μA to the collector output circuit of transistor 15 via
Are combined as a current pulse P1. The current pulse P1 produces a positive voltage pulse P2 across the sensing resistor 18 with a peak-to-peak amplitude of about 1 mV. The pulse P2 is AC-coupled via a capacitor 32 to an inverting operational amplifier 33 having a voltage gain of about 500,
Pulse P2 shown as voltage pulse P3 at the output of amplifier 33
Produces an inverted version of. This pulse P3 exhibits a peak-to-peak amplitude of approximately 0.5 volts with respect to the positive peak pedestal reference level of +6.0 volts stabilized by the feedback clamp circuit.

The feedback clamp circuit includes an operational transconductance amplifier 34 and a capacitor 32, the inverting input (-) of the amplifier 34 is coupled to the positive reference voltage VR1 (+ 6.0V) source, and the non-inverting input (+) is coupled to the output of the amplifier 33. The output is coupled to the inverting input of amplifier 33 and capacitor 32. As shown in FIG. 2, the clamp amplifier 34 is keyed so as to conduct in response to the keying signal K immediately before the AKB sensing period that the pulse S extends, and the feedback action changes the charge of the coupling capacitor 32 to change the signal. P3 positive peak pedestal reference level
Keep clamped to 6.0 volts.

The clamped signal P3 is applied to the non-inverting input (+) of the operational transconductance amplifier 35 which acts as a keyed comparator, to its inverting input (-) a positive reference voltage VR2 of + 5.5V in this example is applied. To be done. The amplifier 35 is keyed to conduct by the sensing enable pulse S to sense the peak amplitude of the black current representing the input signal P3 and compare it to the input reference voltage VR2, the negative peak amplitude of the signal P3 being the reference. If it is substantially equal to the voltage VR2, its output current is unchanged. This corresponds to a correct picture tube cathode bias condition in which the picture tube bias is unchanged by the AKB method.

As will be described below, circuit 30 acts to restore the DC level to cathode 22a via terminal A, thereby setting the DC bias of cathode 22a as a function of the magnitude of signal P3.

The negative peak amplitude of the signal P3 differs from the reference voltage VR2 by the amount that the cathode bias of the picture tube is incorrect. For example, if the amplitude of signal P3 increases the output current of amplifier 35, then the charge on capacitor 36 will increase accordingly. Due to the increase of the electric charge of the capacitor 36, the conductivity of the transistor 37 is increased and the current flowing through the resistors 38 and 39 is increased. This increase in current reduces the DC bias voltage at node A of the cathode signal path to reach the correct bias condition as sensed by amplifier 35. In this way, the capacitors 20, resistors 16, 1
VR1- that supports the correct black level picture tube bias condition due to the closed loop control action of the AKB feedback path including the capacitor 32, the amplifiers 33 and 35, the transistor 37 and the resistors 38 and 38.
The peak amplitude of the signal P3 is stabilized at the level of VR2. In this example, the correct bias conditions correspond to a current bias P1 with a peak amplitude of about 1 μA, a voltage pulse P2 with a peak amplitude of about 1 mV and a voltage pulse P3 with a peak amplitude of about + 0.5V. Feedback action circuit 30 maintains this amplitude level by varying the voltage at node A to compensate for the sensed bad bias condition. Resistor 3 with low parasitic capacitance such as metal thin film resistor
By selecting 8 and 39, the load of the DC recovery circuit 30 at the video signal connection point A can be made negligible.

Suppressing the video tube beam spot during the periodic horizontal and vertical image blanking period by applying a negative pulse of -150 V to the video tube grid 24 instead of erasing the video signal prior to the video tube drive amplifier. Although a blacker than normal black erase effect is obtained, the blackline blanking method does not require the picture tube driver to otherwise adapt to the associated movement of the operating point.

The manner in which the black current indication signal P2 is sensed via the relatively low series sampling resistor 18 of the collector output circuit of the drive transistor 15 significantly reduces the capacitive load of the picture tube drive stage. This point of the above AKB control device is particularly advantageous for a wide band system such as a video monitor.
And British Patent Application No. 8420537 (both corresponding to JP-A-61-58382). Simply put, the fact that a capacitive load is detrimental to the frequency response of a picture tube driver means that it is at the output of the driver (eg the collector output of transistor 15) or between the output of the driver and the picture tube input. At some point on the signal path of the, the capacitive load is actually affecting the response. However, the input capacitance of this AKB circuit 30 has virtually no effect on the high frequency response of the drive stage. The reason is that the load resistor 16 having a large value exhibits a separating effect as compared with the sense resistor 18 having a small value, which provides a very small impedance to the input capacitance of the AKB circuit 30.

FIG. 3 is a partial modification of the AKB method of FIG. 1, in which the corresponding elements are indicated by the same reference numerals, and the conducting wires indicated by broken lines are connected at the same positions as the corresponding conducting wires of FIG. The circuit shown in Fig. 3 amplifies the signal path P2 and the clamp specifications and signal P2 shown in Fig. 1 to obtain a signal.
The specifications for generating P3 are different.

In FIG. 3, the two black current indicating signals P2 and P3 are clamped by a keyed feedback clamp circuit including a keyed amplifier 34 and a video signal input capacitor 22 'for AC coupling the video signal to the base input of the driving transistor 15. The signal gain applied to the signal P2 to generate the signal P3 is obtained by a circuit including a PNP amplification transistor having collector output resistors 42, 43 and an emitter resistor 44. The required signal gain of about 500 is determined by the ratio of resistor 43 to resistor 44, which reduces power consumption by lowering the collector-emitter voltage of transistor 31, and thereby transistor 31.
It enables the use of a small and inexpensive plastic type.

In the circuits of FIGS. 1 and 3, the gray scale balance of the image can be manually pre-adjusted by changing the reference voltage VR2 while keeping the reference voltage VR1 constant. This allows black level preconditioning in a narrow range that may be required in some applications such as precision video monitors. Further, since the high frequency response of the driver 12 is increased, the values of the collector resistance and the emitter resistance of the transistor 15 can be reduced, and the cascode amplifier configuration is used as a transistor.
It can be used instead of 15.

[Brief description of drawings]

FIG. 1 is a diagram showing a part of a television receiver including an AC coupled video output drive stage and an AKB controller according to the present invention, and FIG. 2 is a timing signal waveform diagram used for explaining the operation of the system of FIG. FIG. 3 is a diagram showing another form of the apparatus shown in FIG. 12 …… Drive amplifier stage, 16,18 …… Two resistors (load resistor and sense resistor) that make up the output circuit of the drive amplifier stage, 20 ・ ・ ・
... Capacitors in AC coupling means, 22, 24 ... Cathodes and grids constituting intensity control means, 25 ... Image display device (video tube), 32 to 39 ... Capacitors constituting bias control signal generating means (32 ), An inverting operational amplifier (33), an operational transconductance amplifier (34, 35), a capacitor (36), a transistor (37), a resistor (38, 39), 40 ... means for generating a signal representing the black current (timing) Signal source).

Claims (1)

[Claims] 1. A video signal processing device comprising an image display device having intensity control means comprising a cathode and a grid, and displaying image information in accordance with a video signal applied to the intensity control means: An amplifier stage; means for AC coupling an image signal from the output circuit of the amplifier stage to the cathode of the intensity control means; and bias control means for maintaining a required bias state for the image display device. The bias control means is coupled to the intensity control means and operates during the image erasing period to generate a signal representative of a black current flowing through the image display device during the image erasing period. Means for generating a bias control signal related to the magnitude of the signal representative of the black current in response to the signal representative of the black current. Is connected to the means for AC coupling between the capacitor and the cathode to maintain the required bias state.