JPH06105223A - Beam current control circuit for image pickup tube - Google Patents

Abstract

PURPOSE:To obtain the more suitable system for a beam current optimizing circuit of an image pickup tube of a television camera by providing a low pass filter and a non-additive mixing circuit(NAM circuit) mixing signals before and after the low pass filter. CONSTITUTION:A low pass filter 10 and a NAM circuit 11 are newly provided between a nonlinear processing circuit 7 and a signal amplifier circuit 8. An output signal waveform Vs1 of the nonlinear processing circuit 7 is inputted to one input terminal of the NAM circuit 11 and inputted also to the low pass filter 10. A signal waveform Vs2 after the low pass filter 10 is inputted to other input terminal of the NAM circuit 11. The Vs1, Vs2 are mixed in the NAM circuit 11, from which a signal waveform V's is obtained. The V's is amplified to a level of several tens Vp-p by the signal amplifier circuit 8 and fed to a 1st grid of an image pickup tube. Since a tail end of the signal input waveform V's is smoothed, occurrence of waveform distortion and a false signal due to undershoot of the 1st grid voltage waveform EC1 is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television camera using an image pickup tube, and more particularly to an improvement of a circuit for controlling a current amount of an electron beam of the image pickup tube according to an illuminance of an object.

[0002]

2. Description of the Related Art FIG. 3 schematically shows a so-called photoconductive type image pickup tube used in a television camera. As shown in the figure, in the photoconductive image pickup tube 1, a charge pattern 13 is generated on the surface of the photoconductive target 2 in accordance with the image intensity of the subject optical image 12 formed on the back surface of the target.
The same charge pattern is sequentially scanned by scanning with the electron beam 3 emitted from the cathode 4, and the charging current Is corresponding to this discharge is taken out as a video signal current to the outside. Therefore, the signal current Is that can be taken out from the image pickup tube 1
The maximum value of is limited by the beam current Ib of the electron beam 3, and Is ≦ Ib (1). If Ib is insufficient, the level of Is will reach a peak, and a false signal called a comet tail will be generated, which will significantly deteriorate the image quality.

The beam current Ib is controlled by a first grid 5 which is normally given a positive voltage on the cathode 4. FIG. 4 shows the relationship between the first grid voltage Ec ₁ and the beam current Ib (Ib-Ec ₁ characteristic). As shown in Figure 4,
Generally, the beam current Ib increases as the first grid voltage Ec ₁ increases. By the way, according to the equation (1), for example, when an image of a high-brightness object is picked up, in order to obtain a high level signal current Is, the beam current Ib needs to be sufficiently large. . However, generally, when the beam current Ib is increased, the dynamic range of the signal current Is is widened, but on the other hand, the effective electron beam diameter for scanning the target 2 is increased due to the beam diffusion, and the resolution is deteriorated. I will end up. Therefore, in a recent television camera, the beam current Ib is kept at a relatively low level which is about twice the rated output signal current (Isj) of the normal image pickup tube, and only when a signal current higher than that is required, Beam current I according to signal current Is
A circuit for dynamically controlling and optimizing b is provided. This is generally called a beam current optimization circuit, and various methods have been proposed in the past.

An example of the beam current optimization circuit is shown in FIG. In the circuit of FIG. 5, the first grid voltage Ec ₁ of the image pickup tube ₁ is maintained while the output signal current Is of the image pickup tube is at a low level below Ib ₀ on the Ib-Ec ₁ characteristic diagram shown in FIG. , A DC voltage E ₀ that gives a beam current Ib ₀ by the constant voltage source 9. Here, Ib ₀ is generally set to twice the rated output current Isj of the image pickup tube, that is, Ib ₀ = 2 × Isj (2). If Is exceeds the above Ib ₀ , the required Ib is obtained by a series of circuits including the preamplifier 6, the non-linear processing circuit 7, and the signal amplification circuit 8 based on the Is obtained at that time.
A voltage waveform E (Is) giving (= Is) is created, which is superimposed on the DC voltage E ₀ and applied to the first grid 5. That is, the signal current Is is converted into the voltage waveform Vs as shown in FIG. 6 in the preamplifier 6 and input to the non-linear processing circuit 7 in the next stage. In the non-linear processing circuit 7, the black side portion of the waveform of Vs shown in FIG. 6 corresponding to the above Ib ₀ and below the level V ₀ is clipped and removed, and in accordance with the curve of the Ib-Ec ₁ characteristic of FIG. Non-linear correction is performed in the amplitude direction of Vs, and the white side portion of level V ′ or higher corresponding to the control limit Ib ′ shown on the characteristic diagram of FIG. 4 is clipped and removed. As a result, shown in FIG. Obtain the waveform Vs'. This Vs' is input to the signal amplification circuit 8 at the next stage and amplified to a level commensurate with the absolute value of the characteristic of FIG. Signal voltage waveform E (Is) obtained by the above processing
Is superimposed on the DC voltage E ₀ to obtain a waveform as shown in FIG. This is applied as Ec ₁ to the first grid 5 to control the beam current Ib.

In summary of the above, according to the present beam current optimization circuit, the waveform of the first grid voltage Ec ₁ is 2 when the output signal current Is of the image pickup tube is the rated signal current Isj.
In the normal use range where the beam current Ib ₀ is doubled or less
= 2 × Isj, which is a constant value of the DC voltage E ₀ , whereby stable image quality without deterioration of resolution can be obtained. When a high-luminance object is imaged and Is becomes 2 × Isj or more, a signal component E (Is) corresponding to the waveform of the portion of Is exceeding 2 × Isj appears on E ₀ and appears. A change is made between E ₀ and the control limit level E ′ according to the change, so that the required beam current Ib is always supplied without excess or deficiency.
The signal current Is, that is, the dynamic range of the obtained image is widened.

[0006]

The required level of the above-mentioned first grid voltage waveform Ec ₁ is several tens V depending on the image pickup tube.
The amplitude is as large as pp. In order to obtain this large amplitude signal, in the conventional beam current optimization circuit shown in FIG. 5, a large gain is required for the signal amplification circuit 8, but
Therefore, sufficient frequency response performance cannot be obtained, and
(A), the 8 to the rear edge of the waveform of Ec ₁ (a)
Undershoot was likely to occur as shown in. Then, the beam current is locally insufficient in the image portion in which the undershoot occurs, which causes the image pickup tube output signal Is.
Waveform distortion, or an undershoot component originally unrelated to the image is mixed into Is through capacitive coupling between the electrodes of the image pickup tube to cause a false signal, resulting in image quality deterioration. As a measure to prevent the above-mentioned undershoot, a method of smoothing the waveform of FIG. 8A through a low-pass filter as a waveform B of FIG. 8B can be considered. However, this waveform B lacks the portion of the waveform A that is originally required as Ec ₁ and inevitably causes rounding and delay of the waveform as compared with the image pickup tube output signal Is. The beam current becomes insufficient at the leading edge of the signal indicated by the diagonal line in FIG. An object of the present invention is to eliminate the above-mentioned drawbacks of the conventional method and to provide a more suitable method as a beam current optimizing circuit for an image pickup tube in a television camera.

[0007]

In order to achieve the above object, the present invention provides any one of the signal waveform processing steps from the input of the nonlinear processing circuit to the output of the signal amplification circuit in the conventional beam current optimization circuit, Smooth the signal waveform, and
A low-pass filter for delaying and a non-adding mixing circuit (NAM circuit: non-additiv) for mixing a signal waveform after passing through this low-pass filter and a signal waveform before passing through the same low-pass filter.
e mixing circuit) is newly added, and the waveform shaping processing function of the signal by these circuits is added.

[0008]

As described above, by mixing the two signal waveforms before and after passing through the low-pass filter in the NAM circuit, there is no waveform rounding or delay at the leading edge of the output signal waveform of the NAM circuit. The signal waveform before passing through the filter appears, while the smoothed signal waveform after passing through the filter appears at the trailing edge. That is, only the trailing edge of the signal waveform can be smoothed by the above circuit. Therefore, by using the waveform shaping processing circuit including the low-pass filter and the NAM circuit of the present invention in any of the signal waveform processing processes including the nonlinear processing circuit and the signal amplification circuit of the beam current optimization circuit of the image pickup tube, As the first grid voltage waveform Ec ₁ of the tube, only the trailing edge of the tube is smoothed without causing the waveform to be blunted or delayed such that the beam current becomes insufficient at the leading edge thereof. It is possible to supply a waveform without undershoot. As a result, it is possible to realize a good beam current optimization circuit that does not cause waveform distortion or spurious signals in the image pickup tube output signal Is.

[0009]

EXAMPLES The present invention will be described below with reference to examples. FIG. 1 shows an embodiment of a beam current optimization circuit for an image pickup tube to which the present invention is applied. The basic method and operation of the circuit of FIG. 1 are the same as those of the conventional circuit shown in FIG. The difference is that, in the circuit of FIG. 1, a low pass filter 10 and a NAM circuit 11 are newly provided between the nonlinear processing circuit 7 and the signal amplification circuit 8, and this is also the main point of the present invention. . In the circuit of FIG. 1, the output signal waveform Vs ₁ of the non-linear processing circuit 7 is input to one input terminal of the NAM circuit 11 and at the same time is input to the low pass filter 10. The signal waveform Vs ₂ after passing through the low-pass filter 10 is input to the other input terminal of the NAM circuit 11. These Vs ₁ and V
s ₂ is mixed in the NAM circuit 11, and the signal waveform Vs ′
To get

The waveforms of Vs ₁ , Vs ₂ and Vs' are shown in FIG. Respect Vs ₁ shown in FIG. ₂ (a), Vs 2 shown in FIG. (B), the entire signal by passing the low-pass filter is delayed, leading and trailing edges are smoothed waveform Has been done. When these Vs ₁ and Vs ₂ are mixed in the NAM circuit 11, the respective parts of both waveforms are compared and the one with the higher level is output. Therefore, in the output signal waveform Vs ′, as shown in FIG. At the leading edge, Vs ₁ without waveform delay and rounding appears, and at the trailing edge, smoothed Vs ₂ appears. That is, Vs' can be regarded as a waveform in which only the trailing edge of Vs ₁ is smoothed. In the circuit of FIG. 1, Vs' is amplified to a level of several tens Vp-p by the signal amplifier circuit 8 and applied to the first grid of the image pickup tube. At this time, even if the frequency response characteristic of the signal amplifying circuit 8 which becomes a large signal operation is slightly bad,
Since the trailing edge of the input signal waveform Vs' is smoothed, the output signal waveform E (Is) of the circuit, and by extension the first signal
Undershoot is less likely to occur at the trailing edge of the grid voltage waveform Ec ₁ . As a result, it is possible to prevent waveform distortion and spurious signals in the output signal Is of the image pickup tube caused by the undershoot of the first grid voltage waveform Ec ₁ , which is a drawback of the above-mentioned conventional circuit, and is favorable as a beam current optimization circuit. Excellent performance. The waveform shaping circuit configured by the low pass filter 10 and the NAM circuit 11 according to the present invention is not limited to the position shown in FIG. It goes without saying that it may be placed anywhere in the waveform processing process.

[0011]

According to the present invention, in the beam current optimizing circuit of the image pickup tube of the television camera, the deterioration of the frequency response performance, which is likely to occur when the signal gain is increased, is eliminated by the waveform shaping process by the extremely simple circuit. This can be compensated for, and the occurrence of distortion in the output signal of the image pickup tube due to the above-mentioned performance deterioration can be prevented. As a result, it is possible to realize a beam current optimization circuit with a high gain and a large beam current of the image pickup tube, and thus to provide a high-performance television camera with a wide dynamic range of the image pickup tube output signal.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention.

FIG. 2 is a schematic waveform diagram of each part of the circuit of FIG.

FIG. 3 is a schematic explanatory view of an operation principle of a photoconductive type image pickup tube.

FIG. 4 is a characteristic diagram of the beam current (Ib) of the image pickup tube versus the first grid voltage (Ec ₁ ).

FIG. 5 is a schematic configuration diagram showing a conventional example.

6 is a schematic waveform diagram of each part of the circuit of FIG.

7 is a schematic waveform diagram of each part of the circuit of FIG.

8 is a schematic waveform diagram of each part of the circuit of FIG.

[Explanation of symbols]

 1 Imaging Tube 5 First Grid 6 Preamplifier 7 Nonlinear Processing Circuit 8 Signal Amplifying Circuit 9 Constant Voltage Source 10 Low-Pass Filter 11 NAM Circuit

Claims (1)

[Claims] 1. In a television camera using an image pickup tube, the output signal of the image pickup tube is subjected to a non-linear waveform shaping process and then applied to an electron beam control electrode of the image pickup tube to obtain the image pickup image. A low-pass filter for smoothing and delaying the signal waveform in the beam current control circuit of the imaging tube that dynamically controls the electron beam current of the tube, and the signal waveform after passing through this low-pass filter And a non-additive mixing circuit (NAM circuit: non-additive mixing circuit) for mixing the signal waveform before passing through the low-pass filter, and in the waveform shaping process of the image pickup tube output signal,
A beam current control circuit for an image pickup tube, which has a function of performing a waveform shaping process of the signal using a waveform shaping circuit including the low pass filter and a non-adding mixing circuit.