JPS5857019B2 - 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, 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, causing a deterioration of the S/N ratio. This causes a deterioration in the image quality of the imaging screen.

Conventionally, in order to compensate for the above-mentioned dark current of image pickup tubes used in color video cameras, in particular, there has been a method of blocking incident light around the effective scanning area of the electron beam (effective scanning area of the video signal) on the target surface. There is a known method in which a so-called optical black area is provided, and a signal obtained by scanning this area with an electron beam is used as a reference black level to process a video signal.

However, in this method, the signal detected in the above-mentioned optical black area provided at the periphery of the effective scanning area of the electron beam is used as the reference black level, so the center and periphery of the image pickup tube are dark. If there is a shading phenomenon in which the current is non-uniform, the amount of change due to temperature will also be different, so compensation for the center of the dark current cannot be achieved.

Further, when non-uniformity of dark current is corrected using a shading correction waveform, the amount of correction changes depending on the temperature, so there is a drawback that the amount of correction must be changed each time.

The present invention proposes an improvement to the conventional dark current compensation method described above, by detecting the dark current in the optical black part arranged around the electron beam effective scanning area of the image pickup tube. Even if the dark current is non-uniform between the periphery and the center of the effective scanning area (so-called shading phenomenon), a shading correction waveform controlled based on the dark current is added to correct the non-uniformity. In addition, it is an object of the present invention to provide a method for compensating for dark current in an image pickup tube, which compensates for dark current and makes it possible to reproduce colors accurately and stably even with changes in temperature.

Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit configuration diagram showing an example of the present invention. In the figure, 1 indicates the photoconductive surface of the image pickup tube, and the shaded area 1 has optical black (shielding the incident light). The melanized area (with the effect of blackening) is affected by the disease.

Further, 2 indicates an effective scanning area of the electron beam, and the electron beam is configured to scan all of the above-mentioned effective scanning area 2 and at least a part of the above-mentioned optical black section 1A.

FIG. 2 is a gain characteristic diagram of the gain control amplifier A4 (described later), and FIG. 3 is a waveform diagram comparing the waveforms of each part in FIG. 1 with one horizontal period and one vertical period.

Next, to explain the operation of each part in FIG. 1, the image pickup tube 1
The video signal (which includes the dark current Ed) passed through the preamplifier 3 is applied to the control signal generating circuit a that generates the dark current Ed based on the dark current Ed.

The control signal Es obtained by the control signal generation circuit a is applied to the gain control amplifier A4.

On the other hand, a correction signal Ec' from the shading correction waveform generation circuit is added to the gain control amplifier A4, and in the gain control amplifier A4, the control signal Es from the control signal generation circuit a is applied. Based on this, the correction signal Ec' of the shading correction waveform generation circuit is amplified to obtain a correction signal Ec that follows the temperature characteristics of the dark current of the optical black single part, thereby canceling the dark current. Compensation is made for current.

The above-mentioned shading correction waveform generation circuit generates a sawtooth waveform, a parabolic waveform, etc. in synchronization with each of the horizontal synchronization pulse HD and the vertical synchronization pulse VD, and mixes the waveforms as appropriate to create a correction signal Ec'. It is also configured to generate a correction signal (■ in FIG. 3) mixed with a reference pulse for setting the black level potential during the horizontal synchronizing signal period.

In the above configuration, the video signal (including the dark current Ed) passed through the preamplifier 3 and the resistor R1 and the correction signal Ec passed through the gain control amplifier A4 are both transmitted through the amplifier A1. A mixed output signal of opposite polarity is output from the amplifier A1.

The loop of the amplifier A1, resistor R3, amplifier A2, and resistor R4 turns on the analog switch S2 during the horizontal synchronization signal period by the gate pulse (■ in Figure 3) applied to the terminal T1, and generates a feedback pulse. It operates as a clamp circuit and clamps the potential of the output signal of the amplifier A1 during the horizontal synchronizing signal period by shifting it to 0.

(■ in Figure 3) The output signal clamped to O■ turns on the analog switch S1 for a certain period of time in the optical black section due to the gate pulse (■ in Figure 3) applied to the terminal T2. The difference between the potential of the optical black portion and the O potential is averaged by an active filter composed of an amplifier A3, a resistor R6, and a capacitor C2, and the capacitor c2 is gradually charged or discharged.

Since the charging/discharging time constant at this time is set to be sufficiently longer than one vertical period, the difference between the potential of the optical black section and the O-electromotor immediately changes to the output potential of the amplifier A3. This does not occur, and the output potential is almost constant when viewed over a period of l vertical cycles.

The output of the amplifier A3, that is, the control signal Es, is applied to the gain control amplifier A4, which amplifies the correction signal EC' from the shading correction waveform generation circuit based on the control signal Es to generate a correction signal. Ec, and the correction signal Ec is applied to the amplifier A1 via the resistor R6, and the dark current is compensated by a series of feedback systems.

FIG. 2 shows the characteristics of the gain control amplifier A4, which follows the change in the control signal Es as described above.
Shading correction waveform generation c The correction signal Ec' from the circuit is amplified to Ec (A=, 3.
) and added to the amplifier A1, and finally taken out from the terminal T6 as a dark current corrected video signal and supplied to the signal processing circuit.

As a result, the output voltage of amplifier A, optical
The detected signal corresponding to the dark current in the black portion gradually charges or discharges the capacitor C2 as described above.

That is, until the potential of the control signal Es is stabilized by the above-described series of feedback systems, the amplitude of the correction signal Ec obtained by amplifying the correction signal Ec' from the shading correction waveform generation circuit is based on the control signal Es. controlled by

Therefore, the gain control amplifier A4 corrects the change in shading caused by a temperature change that is longer than one vertical cycle, and the correction waveform Ec' corrects the shading within a vertical cycle, that is, within one screen. .

The above correction signal Ec' is generated by the shading correction waveform generation circuit as explained below.

In the above shading correction waveform generation circuit, the terminal T
3 and a horizontal sawtooth waveform and a parabolic waveform, respectively, from a horizontal sawtooth generator 4 based on the horizontal synchronization pulse HD from terminal T4, and a horizontal sawtooth waveform and a parabolic waveform from the horizontal parabolic wave generator 5, respectively, and based on the vertical synchronization pulse VD from terminal T4. A vertical sawtooth waveform and a parabolic waveform are generated by a vertical sawtooth wave generator 6 and a vertical parabolic wave generator 7, respectively.

Each waveform in the waveform generators 4, 5, 6.7 is generated by variable resistors VR, VB2, . Analog switch S3 which is appropriately adjusted by VB3 and VB4, mixed through resistors R7, R8, and R9°Rto, and is turned on only during the period excluding the horizontal synchronization signal period and the vertical synchronization signal period.
is applied to amplifier A5 via.

Further, while the analog switch S3 is in the on state, the analog switch S4 is in the off state, and a certain DC potential is applied to the amplifier A through the analog switch S4.

Therefore, the horizontal synchronizing signal period and the vertical synchronizing signal period of the output waveform of the amplifier A5 are constant values determined by the DC potential, and the other periods are the above-mentioned sawtooth waves, parabolic waves, or their combinations. It has a composite waveform and is applied to the gain control amplifier A4 as a correction signal Ec'.

The analog switches S3 and S4 are driven by a gate pulse applied to the terminal T5, and the on/off states of the analog switches S3 and S4 are reversed by the inverter 8.

As described above, the output of the amplifier A5 can have a waveform substantially similar to the dark current waveform in order to correct dark shading and bring the optical black portion to a black level potential.

Therefore, by adjusting the amplitudes of the horizontal synchronization period and periods other than this similar waveform, the correction signal Ec'
By applying the signal to the gain control amplifier A4 as a signal, dark shading correction and black level setting can be made most appropriate.

To create the similar correction waveform, adjust the DC potential applied to VR1 and VB2 and the analog switch S4 so that it is most similar to the shading waveform of one horizontal period in the center of the screen (■ in Figure 3). Then, the envelope of the maximum value in one horizontal period of the similar shading correction waveform seen in one vertical period in the center of the screen is the maximum value in one horizontal period of the shading waveform to be corrected in one vertical period. The above ■R so that it is most similar to the envelope (■ in Figure 3)
3 and VB4.

Therefore, the video output signal waveform corrected by the above correction waveform is almost completely corrected at the center of the screen, and the curvature of the correction waveform is larger than that of the shading waveform at the periphery of the screen (the correction waveform at the center of the screen ), so it is overcorrected.

However, since the result of this overcorrection is sufficiently smaller than the result before correction, the entire screen is corrected.

(■ in Figure 3) Next, if the amount of shading increases due to temperature change, the potential of the optical black section will also increase in proportion to this, so the potential will be applied to the amplifier A3 that intercepts the active filter. Since the potential of the control signal Es changes to a more negative direction than the previous one, its output, that is, the control signal Es, changes to a positive direction.

Therefore, the amplitude of the correction signal Ec is increased compared to the previous one and is mixed with the video signal from the preamplifier 3, so that a part of the increased amount of shape ink is canceled out.

This repetition is gradually repeated until the control signal Es becomes stable as described above, so that the increase in the amount of shading caused by the temperature change is corrected and the amount of shading becomes the minimum amount.

As mentioned above, FIG. 3 is a waveform diagram of each part in FIG. 1, where ■ is the output waveform of the amplifier 3, ■ is the gate pulse applied to the terminal T1, ■ is the waveform of the correction signal Ec, ■
is the waveform of the output terminal T6, and ■ is the gate pulse applied to the terminal T2.

As described above, the present invention provides an optical black section that enables optical shielding at the periphery of the electron beam effective scanning area 2 on the photoconductive surface 1 of the image pickup tube. In the dark current compensator, the video signal is clamped to a terminal so that the potential during the horizontal synchronization signal period of the video signal from the image pickup tube becomes a predetermined potential. T6
Clamping means (amplifiers A1 and A
2), and a detection means (analog switch S1 and amplifier A
3), holding means (amplifier A3 and capacitor C2) that holds a potential related to this detected potential, amplification means (amplifier A4) whose amplification degree is controlled according to this held potential, and the above-mentioned comprising means (correction waveform generation circuit b) for obtaining a correction waveform substantially similar to the electromagnetic waveform caused by dark current;
The correction waveform from the means is applied to the amplifying means, and the output signal of the amplifying means is added to the video signal manually input to the clamping means to compensate for the dark current.
It was possible to compensate for the shading of the image pickup tube by setting the potential of the horizontal synchronizing signal portion to a black level potential, and also to track and compensate for temperature changes.

[Brief explanation of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention, FIG. 2 is a characteristic diagram of a gain control amplifier, and FIG. 3 is a waveform diagram of each part in FIG. 1. 1A Niobthetical Black, A1. A2. A3: Amplifier, A4: Gain control amplifier, Sl, S2: Analog switch, C1, C2: Capacitor, R1゜R2, R3
r R4r R5+ R6: Resistance.

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 and outputting the video signal so that the potential of the video signal from the image pickup tube during the horizontal synchronization signal period becomes a predetermined potential; a detection means for detecting the potential during the period of the optical black portion in the video signal clamped by the clamping means; a holding means for holding a potential related to the detected potential; and a holding means for holding a potential related to the detected potential; The amplification means is provided with an amplification means whose amplification degree is controlled accordingly, and means for obtaining a correction waveform substantially similar to the voltage waveform due to the dark current, and the correction waveform from the means is applied to the amplification means, and the correction waveform of the amplification means is applied to the amplification means. A dark current compensation device for an image pickup tube, characterized in that the output signal is added to the video signal input to the clamping means to compensate for the dark current.