Beam Control Circuit for Diode-Gun Type Camera Tubes

1. Field

The invention relates to highlight suppression circuits, and particularly to a circuit for controlling the beam current of a diode-gun camera tube and the like.

2. Prior Art

In camera pickup tubes typically exemplified by those tubes known as vidicons or plumbicons, the beam current of the electron beam which scans the target electrode thereof, is generally maintained within its normal operating range corresponding to the normal operating range of the video signal, in order to insure optimum tube resolution. That is, it is generally known that increasing the beam current of an electron beam which scans a target electrode above its given normal operating range generally causes a defocusing of the electron beam and thus a loss of resolution of the video picture.

In the event of the occurrence of highlights in the viewed scene, it is necessary that the photoelectric conversion surface of the target electrode be neutralized, generally by increasing the beam current to discharge the excess accumulation of charges on the target electrode of a pickup tube in the area of the highlight. However, one of the disadvantages inherent in pickup tubes when suppressing highlights by an increase in the beam current is that the use of a high intensity beam to discharge the excessive charges caused by the highlight also generates excessive soft x-rays, which bombard the target thereby significantly reducing the lifetime of the pickup tube. Thus, it is generally necessary to compromise the increase in beam current in order to provide the maximum highlight suppression possible while simultaneously maintaining the optimum lifetime of the pickup tube.

There are various techniques presently known which provide for highlight suppression in pickup tubes, wherein the techniques involve some means for increasing the beam current to discharge the excessive charges on the target electrode of the pickup tube caused by the presence of a highlight. The circuits corresponding to such techniques are commonly known by the general terms of anti-comet tail, automatic beam optimization, and automatic beam control, etc., circuits. Typical of the anti-comet tail circuits are those which increase the beam current during either the active or the flyback periods of the electron beam scan, and exemplified by way of example in U.S. patent application Ser. No. 802,839, to R. Salem, et al., and assigned to the same assignee as this application. This anti-comet tail circuit provides an increase in beam current only during the flyback period, and only "on demand", i.e., upon the occurrence of a highlight. Typical of automatic beam optimization and/or control circuits, are those described in U.S. Pat. Nos. 3,999,011 to K. Sato, et al., and 2,930,929 to C. Sheldon. Such circuits provide highlight suppression during the active period of the electron beam scan and in response to the presence of a highlight in the video picture being viewed. The circuits variously include some means by which the beam and/or cathode current is monitored, and the resulting control voltage signal is fed back to the grid of the tube via a generally complex negative feedback circuit which in turn provides the desired controlled increase in the beam as required to suppress the highlight effects.

All the above techniques are utilized with standard pickup tubes of the types known as vidicons, plumbicons, etc., which possess the characteristic that no current flows in the control grids between the grid voltage and the beam current. Whereupon the beam optimization circuits of previous mention utilize the generally linear relationship between the cathode current and the beam current to provide the negative feedback of control voltage to control the grid and thus the beam current.

Very recently, an improved camera pickup tube has been developed which is generally known as a diode-gun tube utilizing lead oxide target plumbicons, saticons, etc., wherein increased beam currents may be applied without unduly deteriorating the lifetime of the tube. Further, such diode-gun tubes have the characteristic that current flows in the control grid thereof, and a substantially linear relationship exists between the grid current and the available beam current.

The present invention provides a beam control circuit for use with diode-gun pickup tubes which instantaneously increases the beam current in response to a highlight, by an amount which is only sufficent to discharge the highlight, while insuring that the beam current is held at a level consistent with minimum picture degradation and maximum tube life in the absence of a highlight. The system does not require beam or cathode current monitoring, nor the associated feedback of corresponding voltages to control the levels of the beam current during the existence of highlights.

To this end, a tube grid control circuit is coupled to the grid of a diode-gun camera tube, with a highlight enable circuit coupled in turn to the tube grid control circuit. The highlight enable circuit is pre-biased such that the pickup tube operates in its normal range consistent with the normal range of video signal current in the absence of highlights. Upon the occurrence of a highlight, the highlight enable circuit enables the tube grid control circuit, which drives the grid of the pickup tube to generate a correspondingly increased beam current. The grid control circuit is calibrated to provide a beam current which increases in a manner such that a substantially constant, preselected margin is maintained above the incoming video signal current as the latter increases due to the presence of highlights.

FIG. 1 is a schematic diagram of the beam control circuit of the invention.

FIG. 2 is a graph depicting the relationship, i.e., the preselected constant margin, between the video signal current and the available beam current, as maintained by the circuit of FIG. 1.

The beam control circuit of description herein, utilizes the fact that a substantially linear relationship exists between the grid current and the available beam current of a diode-gun camera tube. Thus, by way of example, if one milliamp (mA) of grid current provides 300 nanoamps (nA) of beam current, then two mA of grid current will provide 600 nA of beam current, etc. However, the magnitude of the proportionality constant is generally not known and will vary between different tubes. Accordingly, the beam control circuit employs means for its calibration such that the proportionality constant is of no concern but is compensated for automatically.

Referring initially to FIG. 2, a substantially linear relationship is desired between the video signal current and the beam current of the tube, as depicted by the curve 10.

If the signal current were to be plotted on the graph, it would appear as linear curve 12 which extends substantially parallel to the curve 10. Thus, as the video signal current increases above, for example, 300 nA (the upper limit of the normal video signal current operating range in the absence of highlights), the beam current also increases linearly to maintain substantially the same difference margin therebetween.

FIG. 1 depicts, by way of example only, one implementation of the beam control circuit in accordance with the invention combination. Although a single channel of electronics is shown, e.g., the green channel, two additional channels are utilized for the red and blue video signal channels. Thus, the video signal generated by the preamplifier of the video processing circuitry (not shown) of a conventional camera system is introduced at input 16 of a highlight enable circuit 14, and particularly to the emitter electrode of a first stage 18 of a dual transistor 20. The collector of stage 18 is coupled via a resistor 22 to a positive voltage, and the base thereof is coupled to the collector and thence to the base of a second stage 24 of dual transistor 20. Transistor stage 18 provides means for compensating for temperature variations. The emitter of stage 24 is coupled to ground via a resistor 26 while the collector thereof generates the output of the highlight enable circuit 14 at a junction 27.

The junction 27 of the highlight enable circuit 14 corresponds to the input to a tube grid control circuit 28. The junction 27 is coupled to one side of a current source 30, whose other side is grounded. The junction 27 is coupled to the base of a transistor 32, and to one side of a variable resistor 34 whose other side is coupled across the base and collector of a transistor 36. The emitter of transistor 36 is coupled to a positive voltage, and also via a resistor 38 to the emitter of the transistor 32. The collector of transistor 32 extends to the control grid 40 of a diode-gun tube 42, whose cathode 44 is coupled to ground.

The control grid 40 is also coupled to a collector of a transistor 46, whose base is coupled to a conventional blanking signal of the video camera system via a resistor 48 and input line 50. The base thereof is also coupled via a resistor 52 to the emitter of transistor 46 and thence to a negative voltage source.

The transistors 36 and 32, in conjunction with the resistors 34 and 38, form a multiplying current mirror; i.e., a signal current I at junction 27 results in a collector current in transistor 32 which equals the current I multiplied by the ratio of the values of the resistor 34 to the resistor 38. Thus, the variable resistor 34 provides, inter alia, means for calibrating the tube grid control circuit 28, and particularly for establishing the magnitude of the proportionality between the current at junction 27 and the available beam current of tube 42.

The calibration is accomplished by applying an arbitrary reference current of, for example, 2 mA to the junction 27 via the current source 30. The camera iris (not shown) of the video camera system is adjusted utilizing the conventional test chart to provide a peak signal current of the order of 600 nA, wherein 600 nA is selected as being twice the upper limit of the normal video signal current in the absence of highlights. The resistor 34 is then adjusted to reduce its resistence, thereby reducing the beam current in tube 42 until the 600 nA signal begins to collapse. The condition now exists that a current of 2 mA entering junction 27 to the tube grid control circuit 28 produces the equivalent of 600 nA of beam current. Now if an increase of signal over 2 mA current occurs, as due to the occurrence of a highlight, the beam current is correspondingly increased.

The transistor 20 of the highlight enable circuit 14 converts the video signal from the preamplifier into a proportional current entering the junction 27. The preamplifier generally has a sensitivity of one millivolt per one nanoamp of video signal current and, in the absence of signal current, the output of the preamplifier is set at -0.3 volts by, for example, a clamping circuit (not shown). The pre-biasing arrangement of transistor 20 is selected such that no collector current flows into the junction 27 until the output of the preamplifier reaches the zero volt level, corresponding to a video signal current of 300 nA. Thus, the beam current of the tube 42 remains at the preselected constant value of 600 nA during the normal video signal operating range of from zero to 300 nA. However, when the video signal current increases above the 300 nA level, as in the presence of highlights, the transistor 24 conducts and causes the beam current of the tube 42, via the tube grid control circuit 28, to correspondingly increase, whereby the beam current maintains a substantially constant margin of 300 nA above the value of the video signal current, as depicted in FIG. 2 by curves 10, 12. Thus, if the video signal current increases to 400 nA or 500 nA due to respective highlights, the beam current increases to values of 700 nA and 800 nA, respectively, to efficiently suppress the effects of the specific highlights. The value of the resistor 26 insures the constant difference margin of 300 nA, herein selected by way of example only, between the beam current and the video signal current during the presence of highlights.

During the active portion of the scanning time, the transistor 46 is turned off in the absence of a blanking signal via terminal 50, and all of the current from the collector of transistor 32 flows into the control grid 40 of tube 42. During the blanking interval of the signal on terminal 50, the transistor 46 is saturated, which reverse-biases the grid 40 and absorbs all of the collector current from transistor 32, thereby shutting off the pickup tube 42.

It is understood that although specific values of currents, voltages, resistance, etc., have been employed herein, other values commensurate with the operating parameters, types of camera systems, circuit components, etc., may be employed, and that the specific values have been used only by way of clarifying the description of the instant beam control system.

Likewise, although the curves 10, 12 of FIG. 2 are depicted as parallel throughout their values shown here, there are practical limits to the extent to which the pre-calibrated (300 nA) margin is maintained due to the operating parameters of the diode-gun tube 42.

As previously mentioned, only one channel of the instant beam control circuit is described herein, whereas three similar channels are employed in a three tube camera, one for each tube. In the event the highlight affects only a specific color, only the respective color channel is affected. However, most highlights are specular and all three color channels operate a described above.