Such as figure 1 The display device shown has a cathode ray tube T, a video amplifier VA, a video processing circuit VP, a high voltage generator H, and a detection circuit S. The tube T has an electron gun (G) for generating at least one electron beam. The cathode ray tube T may be a monochromatic tube whose electron gun G has one cathode or a color tube whose electron gun G has three cathodes G. in figure 1 Here, the tube T shown has a gun (G) including three cathodes CR, CG, CB for generating, for example, three electron beams corresponding to red, green, and blue colors. The tube T also has an internal high-voltage capacitor C formed by the conductive layer at the inner side of the tube connected to the anode AN of the tube T and the outer conductive layer AD at the outer side of the tube opposite to the inner conductive layer. The video amplifier VA includes three amplifier stages AR, AG, AB, which receive the corresponding color components VoR, VoG, and VoB in the video signal Vo. The respective amplifier stages AR, AG, AB of the video amplifier VA drive respective cathodes CR, CG, CB to modulate the corresponding electron beams flowing from the respective cathodes CR, CG, CB to the anode AN. The video processing circuit VP includes a black level control circuit BL for controlling the black level of the video signal Vo. in figure 1 In the illustrated embodiment, each color component VoR, VoG, VoB has a corresponding black level BLR, BLG, BLB that can be controlled by the black level control circuit BC. The video processing circuit VP supplies the video signal Vo to the video amplifier VA. The high-voltage generator H has two output terminals: one end is connected to the anode AN, and the other end is connected to the outer conductive layer AD through the node X. Therefore, the output terminal of the high-voltage generator H and the high-voltage capacitor C are coupled in parallel. The detection circuit S is connected in series with a parallel coupling at the node X. The detection circuit is also connected to node Y. The node Y is connected to the reference voltage V1 through a resistor R1. Connected in parallel with the series connection of the resistor R1 and the reference voltage V1 is the capacitor C1. The output of the detection circuit S is connected to the feedback input FI of the black level control circuit BL.
 Such as figure 1 As shown, the three electron beams originating from the cathodes CR, CG, and CB all reach the same anode AN of the tube T. Therefore, in other words, the beam current IB flowing on the anode side is the sum of three electron beam currents. This current flows from the ground potential via the node Y through the detection current S and the high-voltage capacitor C to the anode AN. The recharging of the high-voltage generator H compensates for any discharge of the high-voltage capacitor C by the beam current IB. The generated recharging current flows in the loop formed by the parallel coupling of the high-voltage generator H and the high-voltage capacitor C, and does not flow into the node X. Therefore, since only the beam current IB flows between the nodes Y and X, the detection circuit S can be provided between these nodes X and Y.
 The black level control circuit BL receives the video input signal Vi, which is composed of red, green, and blue color components in the example where the tube T has electron beams that generate red, green, and blue colors. According to the feedback signal FS received through the feedback input FI of the black level control circuit BL, the DC level of each color component can be moved by the black level control circuit BL. The purpose of the movement is to ensure the black level BLR, BLG of each color component VoR, VoG, VoB of the video signal Vo present at the output of the black level control circuit BL and applied to the cathode ray tube T via the video amplifier VA , BLB, matches the corresponding black level of tube T. In this way, the video signal Vo is presented at the correct black level on the screen of the tube T. In addition, by matching the relative black levels of the color components VoR, VoG, and VoB of the video signal Vo, the correct presentation of colors is also ensured.
 In addition, during the vertical flyback period VF, for example, during multiple consecutive lines, the black level control circuit BL sequentially inserts the black level into each color component of the video input signal Vi, resulting in figure 2 The color components VoR, VoG, and VoB of the video signal Vo shown are waveforms as a function of time t. The inserted black level of each color component VoR, VoG, and VoB is represented by BLR, BLG, and BLB, respectively. Thus, during the time interval, in each of the color components VoR; VoG; VoB, insert the black level BLR; BLG; BLB, other color components VoR; VoG; VoB has a lower than the black level BLR; BLG; BLB blanking level (blanking level) BO, so as to ensure that the corresponding electron beam is completely cut off. Therefore, the generated beam current IB flowing into the detection circuit S through the node X sequentially includes black current levels IBR, IBG, and IBB, which can be sequentially detected by the detection circuit S, resulting in figure 2 The beam current IB shown has a similarly shaped feedback signal FS. The black level control circuit BL uses the feedback signal FS to stabilize the black current levels IBR, IBG, IBB at a predetermined level by adapting to the black levels BLR, BLG, and BLB of the respective color components VoR, VoG, and VoB of the video signal Vo Level, until each color component VoR, VoG, VoB obtains a predetermined level.
 Since it is also desired to detect the average beam current value and/or the peak beam current value, a resistor R1 is provided between the node Y and the reference voltage V1, such as figure 1 Shown. Thus, the voltage on node Y can fluctuate according to the beam current. This voltage can be used in another feedback loop to limit the average and/or peak beam current. The time constant used for averaging is determined by the RC time of resistor R1 and capacitor C1.
 Since the detection circuit S is connected to the fluctuating voltage through the node Y, the detection circuit S requires a special circuit configuration to detect the beam current IB. in image 3 An example of a suitable detection circuit S is shown in. The first transistor Q1 has a first main terminal, that is, an emitter and a collector connected to the node Y and the node X, respectively. The first transistor Q1 also has a first control terminal, that is, a base connected to the node X. The second transistor Q2 has a second main terminal, that is, an emitter and a collector connected to the node Y and the feedback input FI, respectively. The second transistor Q2 also has a second control terminal, that is, a base connected to the first control terminal. The configuration of the first Q1 and the second transistor Q2 is a current mirror. In the second transistor Q2, the beam current IB flowing through the main current path via the collector and emission collector of the first transistor Q1 is "mirrored". Thus, through the second transistor Q2, current flows from its emitter to its collector as the output of the current mirror circuit to the feedback input FI, which is equal to the beam current IB.
 It should be understood that the above-mentioned embodiments are intended for illustrative purposes and do not limit the present invention. Those skilled in the art should understand that many other embodiments can be designed without departing from the scope of the appended claims. For example, the present invention can also be applied to a projection television system in which an independent monochromatic cathode ray tube is applied to each color, so that the high-voltage capacitor C of each tube can be connected to the common high-voltage generator H in parallel.
 In the claims, any reference signs between parentheses do not constitute a limitation on the claims. The verb "comprise" and its conjugations do not exclude the presence of elements or steps other than those stated in the claims. The article "a" before an element does not exclude a plurality of such elements. The invention can be implemented by hardware comprising several different elements, and by a suitably programmed computer. In the equipment claims that list several devices, several of these devices can be embodied by the same hardware item. The only fact is that certain measures mentioned in different dependent claims do not indicate that a combination of these measures cannot be used to obtain benefits.