JPS5857018B2 - Image tube dark current compensation device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a dark current compensation device for an image pickup tube, which compensates for dark current that occurs due to the characteristics of an image pickup tube and is a factor that causes deterioration in the image quality of an image pickup screen.

For example, in a photoconductive image pickup tube such as a vidicon, the signal extracted from the target electrode of the image pickup tube consists of a so-called signal current that changes depending on the intensity of light irradiated onto the photoconductive surface, and a signal current that changes depending on the intensity of light irradiated onto the photoconductive surface. Even if there is no dark current, it is extracted as the sum of the dark current determined by the target voltage and face plate temperature.

That is, even in a dark state where no optical signal is incident, the surface potential of the target increases due to the dark current flowing through the target node, and the scanning of the electron beam generates a dark current.

When this dark current increases as the temperature rises, the proportion of the signal current that should originally be extracted as a video signal in a constant video signal amplitude decreases, leading to a decrease in the S/N ratio. be.

Traditionally, to compensate for the above-mentioned dark current in image pickup tubes used in color video cameras, a temperature-sensitive element (such as a thermistor) has been placed near the face plate to compensate for the dark current caused by temperature changes. Methods of making compensation are known.

In this method, since the temperature characteristics of the temperature sensitive element and the dark current are different from each other, it is difficult to match the temperature characteristics of both.

In addition, near the effective scanning area of the electron beam on the target (effective scanning area of the video signal), an area called an optical blank is provided to block the incident light, and this area is scanned with the electron beam. A video processing method is also known in which the signal is set as a reference black level and the potential of the black level is subtracted from the potential of the video signal to obtain a regular video signal.

However, with this method, if the ambient temperature changes,
As is well known, as the black level potential changes, the potential due to the dark current in the video signal portion also changes at approximately the same rate. Even if the screen is projected, the difference between the black level potential and the video signal potential will change as the temperature changes, as described above, and the image quality of the central portion of the screen will change significantly.

The present invention provides a dark current compensator that sets the central part of the screen where good image quality is most needed at the potential of the reference black level, thereby minimizing changes in the image quality of the central part of the screen due to temperature changes. It provides:

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a waveform diagram of each part in the dark current compensation circuit shown in FIG. 3, where ■ is the output waveform of the preamplifier 3, ■
is the drive pulse waveform that drives the analog switch S2, ■
is the drive pulse waveform that drives the analog switch S1,
■ is the drive pulse waveform that drives transistor Q1, ■ is the output of transistor Q2, that is, the waveform at point ■, and ■ is the amplifier A
, each one horizontal period part of the output waveform of is shown.

FIG. 2 shows the temperature characteristics of dark current.

The dark current Id is plotted on a logarithmic scale on the vertical axis, and the temperature tCO is plotted on the horizontal axis. 1 shows the change in dark current at the center of the image pickup tube shown at 4 in FIG. 1 with respect to temperature.

From this, it can be seen that the rate of change of the dark current with respect to temperature is approximately the same in the periphery and the center of the image pickup tube, although the absolute amount is different, as seen from the slopes of I and (2).

Therefore, by making the potential of the reference black level the same as the potential caused by the dark current in the center of the image pickup tube, the image quality in the center will remain approximately constant even when the temperature changes, and the best condition can be obtained. Recognize.

FIG. 3 is an example of a compensation circuit for dark current used to implement the present invention described above.

In the figure, numeral 1 indicates the photoconductive surface of the image pickup tube, and the shaded area is provided with an optical blank (blackened area).

Further, 2 indicates an effective scanning area of the electron beam, and the electron beam is configured to scan all of the effective scanning area 2 and at least a part of the optical black portion.

The video signal taken out from the image pickup tube is first amplified by the pre-amplifier 3, then introduced into the amplifier A4 via the resistor R6, where it is further amplified, and finally It is taken out as a video signal output from terminal T4.

The video signal from the preamplifier 3 is inverted by the amplifier A4, and its polarity becomes negative.

Further, the circuit within the frame indicated by the dotted line in FIG. 3 is a reference pulse generation circuit for generating a pulse at the reference level of the video signal (video signal voltage value when there is no signal).

The reference pulse created by the reference pulse creation circuit is connected to a resistor R.
7 to the amplifier A4, where it is superimposed on the video signal applied from the preamplifier 3 via the resistor R6.

Among the above reference pulse generation circuits, amplifiers A1 and A
2 constitutes a pulse clamp circuit, and is connected to the amplifier A, via the resistor R1 from the preamplifier 3.
Among the video signals applied to the horizontal synchronizing signal, a gate pulse (■ in Fig. 1) for turning on the analog switch S2 is applied during the certain period of the horizontal synchronizing signal so that the potential during the certain period of the horizontal synchronizing signal shifts to Ov. is applied to terminal T, interposing the amplifier A2 in the feedback loop;
Operate as a feedback pulse clamp circuit.

Therefore, the potential during a certain period of the horizontal synchronizing signal is clamped to OV at the output of the amplifier A1, and a video signal having a polarity different from that of the input of the amplifier A1 is output.

The output signal from this amplifier A is the analog switch S.
1 is applied to the terminal T2 only during the optical blank portion of the video signal period, and the potential of the optical black portion is changed to the amplifier A3, the resistor R9, and the potential of the optical black portion. The signal is averaged by an active filter composed of a capacitor C2, and is held by the capacitor C2.

Here, since the input terminal of the amplifier A3 is grounded, its output appears with a polarity opposite to that of the input signal.

Next, when a gate pulse (■ in Figure 1) is applied to the input terminal A3 of the gate pulse that operates the transistor Q1, the transistor Q1 is turned off only during the period when the horizontal synchronization signal is present, and therefore the transistor Q2 is also turned on. The potential held in the capacitor C2 is amplified and appears at point ■ in FIG. 3 (■ in FIG. 1).

The potential appearing at the point (■) is divided by the semi-fixed resistor R2 to set the reference black level potential, and then applied to the amplifier A.
1 and is also applied to the amplifier A40 input via resistor R7.

Therefore, the output of the amplifier A1 is connected to the preamplifier 3.
During a certain period of the above horizontal synchronization signal of the output signal from
A video signal on which the pulse signal from the point is superimposed is output.

Then, the period during which the analog switch S2 is in the on state, that is, the period of the pulse width of the pulse signal from point 1) is O.
The feedback pulse clamp circuit operates to shift the value to v, and the series of operations described above is repeated until it converges to a certain value and becomes stable (state 2 in FIG. 1).

Now, if the value of dark current increases due to temperature change,
The potential of the optical blank section also increases.

Then, since the potential at the point ■ has not changed yet, even if the analog switch S is turned on, there is no change in the potential during the period when it is clamped to OV, and the potential at the optical black part is lower than before. Shifts to negative potential.

Next, when the analog switch S is turned on, a potential higher than the previous potential is held in the capacitor C2.

When the transistor Q+ is turned on during a certain period of the next horizontal synchronizing signal, the potential Ec of the reference pulse at the point 2 becomes higher than the previous one.

At this time, the analog switch S2 is also in the on state, so the potential that should be clamped to the Ov tries to move in the negative direction, but as described above, the entire video signal waveform changes due to the action of the feedback pulse clamp circuit. Since it is moved in the positive direction, the potential of the optical black portion is also moved in the direction of Ov.

Next, when the analog switch S is turned on, the potential held in the capacitor C2 becomes smaller than the previous potential, so when the transistor Q1 is turned off the next time, the potential Ec at the point ■ is smaller than the previous value. and is applied to the input of the amplifier A.

These series of operations are repeated, and the potential at the point (2) converges to a certain value and becomes stable, as is well known.

Therefore, there is a causal relationship between the potential Ec of the point (■) and the potential of the optical blank section, so if the potential Ec is adjusted, that is, the semi-fixed resistor R2 is adjusted, the reference black The relationship between the level potential and the potential at the center of the screen due to the dark current can be appropriately set.

On the other hand, as described above, the video signal from the preamplifier 3 and the reference pulse from the above point (■) are mixed and applied to the input of the amplifier A4, so the peak value is applied to the output terminal T4 at the center of the screen. A video signal containing a reference pulse having approximately the same potential as the non-signal potential of the portion is output.

Therefore, even if there is a change in dark current due to a change in temperature, the image quality at the center of the screen can be maintained at its best.

As described above, in the present invention, an optical blank part that enables optical shielding is provided around the electron beam effective scanning area 20 on the photoconductive surface 1 of the image pickup tube, so that the optical blank part that can be optically shielded is provided in the vicinity of the electron beam effective scanning area 20 on the photoconductive surface 1 of the image pickup tube. In a dark current compensating device for compensating for dark current contained in a video signal transmitted from the image pickup tube, a clamping means (clamping means) for clamping the video signal so that the potential during the horizontal synchronization signal period of the video signal from the image pickup tube becomes a predetermined potential. analog switch S2 and amplifier A2), detection means (analog switch S1 and amplifier A3) for detecting the potential during the period of the optical blank section in the video signal clamped by the clamping means, and a detection means (analog switch S1 and amplifier A3) related to the detected potential. holding means (amplifier A3 and capacitor C2) for holding the potential held by the holding means;
adding means (transistor Q+, Q2 and resistor R2)
and second addition means (transistors Q+, Q2 and resistor R7) for adding the potential held by the holding means to the horizontal synchronization signal period of the video signal output from the image pickup tube. Since the potential held by the holding means is added by the second addition means to the horizontal synchronization signal period of the video signal output from the image pickup tube, the video signal is clamped by this potential. As a result, it was possible to compensate for changes in image quality due to dark current in the center of the screen, and to minimize the effects of changes in dark current due to temperature changes.

[Brief explanation of drawings]

FIG. 1 is a waveform diagram of each part of a dark current compensation circuit according to an embodiment of the present invention, and FIG. 2 is a diagram showing the temperature characteristics of dark current.
FIG. 3 is a diagram showing a dark current compensation circuit according to an embodiment. AI 2 A2) A32 A4: Amplifier, S,
, S2: analog switch, Q+, Q2: l'
Silan Star, T1: Drive pulse input terminal of analog switch S2, T2: Drive pulse input terminal of analog switch Sl, T3 Drive pulse input terminal of Nidoran Sister Q, T4: Video output terminal, R1, R3 + R4t R
5+ R6tR, 7, R8, R9: Resistor, R2: Semi-fixed resistor, C1°C2: Capacitor.

Claims (1)

[Claims] 1. An optical black section that enables optical shielding is arranged around the electron beam effective scanning area on the photoconductive surface of the image pickup tube, so that the dark contained in the video signal obtained by the scanning of the electron beam is removed. A dark current compensator configured to compensate for current includes a clamping means for clamping the video signal so that the potential during the horizontal synchronization signal period of the video signal from the image pickup tube becomes a predetermined potential; detection means for detecting the potential during the period of the optical black portion in the video signal;
a holding means for holding a potential related to the detected potential; and a first holding means for adding the potential held by the holding means to the horizontal synchronization signal period of the video signal from the image pickup tube input to the clamping means. An image pickup tube comprising: an addition means; and a second addition means for adding the potential held by the holding means to a horizontal synchronization signal period of a video signal output from the image pickup tube. Dark current compensator.