JPS5832832B2 - Image tube dark current compensation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for compensating for dark current in an image pickup tube, which compensates for dark current that occurs due to the characteristics of the image pickup tube and is a factor in degrading 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, leading to a decrease in the S/N ratio. be.

Conventionally, in order to compensate for the above-mentioned dark current of image pickup tubes used in color video cameras, a temperature-sensitive element (for example, a thermistor, etc.) has been placed near the face plate to compensate for the dark current caused by temperature changes. Methods of current 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 target electron beam (effective scanning area of the video signal), a so-called optical black area is provided to block the incident light, and this area is scanned with the electron beam. A method is also known in which a video signal is processed using the signal as a reference black level.

However, in this method, the area detected in the above-mentioned optical black area provided near the effective scanning area of the electron beam is used as the reference black level, so dark current is generated in the center and periphery of the image pickup tube. If there is a shading phenomenon that is non-uniform, the amount of change due to temperature will also differ, so it will not be possible to compensate for the dark current in the center.

The conventional method described above has many disadvantages, especially as a method for compensating for dark current in a multi-tube color video camera.

The present invention proposes an improvement to the conventional dark current compensation method described above, and uses the dark current to compensate for the dark current at the periphery and center of the effective scanning area of the electron beam of the target of the image pickup tube. By creating a correlation based on the relationship obtained from the temperature characteristics of Furthermore, it is an object of the present invention to provide a dark current compensation method for an image pickup tube, which is excellent in accuracy and stability without complicating adjustment, and is particularly suitable for use in a multi-tube type color video camera. do.

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

The figure shows an example of a compensation circuit for compensating for dark current in an image pickup tube, and shows an example of a two-tube configuration.

In the figure, 1 is an image pickup tube for green signals, and an optical black 1B that can be optically shielded is arranged around the area of one person (shaded area) in the effective scanning area of the electron beam. .

Further, 2 is a two-electrode power type image pickup tube, which is an image pickup tube for red and blue signals.

The two figures show the effective scanning area of the electron beam.

The above-mentioned image pickup tubes 1 and 2 are isolated from each other by a metal case or the like as shown by the dotted line S, and both of the image pickup tubes 1 and 2 are designed so that their face plates maintain a thermally balanced state even when the temperature changes. ing.

In addition, in the green signal image pickup tube 1, the electron beam covers all of the effective scanning area 1A and the optical
Scan at least a partial area of Black 1B.

suru-like (composed of)

The video signal taken out from the image pickup tube 1 for the green signal is first amplified by the pre-amplifier 3 and passed through the resistor R1 as described below (compensation pulse surrounded by a dotted line in the circuit diagram). It is sent to the generation circuit P to generate a compensation pulse Ec for compensating for dark current, and is also sent to the amplifier A via the resistor R6.
4, further amplified here, and finally a green signal (hereinafter referred to as G signal) from terminal T4 as a signal compensated in line with the dark current by the above-mentioned compensation pulse Ec.
retrieved as output.

In addition, the red signal (hereinafter referred to as R) from the image pickup tube 2 for red and blue signals is
signal) is sent via a preamplifier 4 via a resistor R10 to an amplifier A5, where it is summed with the compensation pulse Ec applied via a resistor R11 and output to an output terminal. It is taken out as an R signal output from T5.

Similarly, the blue signal (hereinafter referred to as B signal) is also transmitted through the preamplifier 5.
is further sent to the amplifier A6 via the resistor R13,
The amplifier A6 adds this signal to the compensation pulse Ec applied via the resistor R14, and outputs the B signal from the output terminal T6.

In addition, the video signal component G from the above preamplifiers 3, 4, and 5
, R, B are amplifiers A4. A5. It is inverted by A6, and its polarity becomes negative.

The compensation pulse Ec (taken out from the point in the figure) created by the above-mentioned compensation pulse generation circuit P (within the dotted line frame in the figure) is
The amplifier A4. A5. Since the operations in A6 are all similar, only the case where it is applied to the amplifier A4 will be explained.

The compensation pulse Ec applied to the amplifier A4 is added to the G signal applied via the resistor R6 from the preamplifier 3 in the amplifier A4, and is superimposed on the G signal. It acts to cancel out the dark current generated by the current.

In the above amplifier A4, when its gain is sufficiently large, its output is as follows, and it operates as a summing amplifier for two signals EdG and Ec.

Note that EdQ is a G signal component containing dark current, and Ec is a compensation pulse at point 2 in the figure created by the compensation pulse creation circuit P for canceling the dark current.

Amplifier A5. The output at A6 is obtained in exactly the same manner as follows.

However, EclR and EdB are an R signal component and a B signal component, respectively, including dark current.

Of the compensation pulse generation circuit P described above, the loop composed of the amplifiers A1 and A2 constitutes a pulse clamp circuit, and as described above, the amplifier is connected to the amplifier via the resistor R1 from the preamplifier 3. Among the G signal components applied to A1, the video signal period during which dark current is detected is clamped by the clamp pulse from terminal T1, so that switch S2 is closed and the amplifier A2 is interposed in the feedback loop. - Operate as a pulse clamp circuit.

Thus, the video signal period is clamped to the output of the amplifier A1, and for example, at the beginning of one vertical scanning period, the image pickup tube 1
A detection signal corresponding to the dark current detected in the optical black 1B portion is obtained.

That is, the retrace period (blanking period) of the output signal of the amplifier A1 is always clamped to Ov.

Therefore, the dark current supplied from the preamplifier 33 is amplified by a factor of 1, and a signal clamped to OV during the blanking period is taken out as the output of the amplifier A1.

The signal from this amplifier A1 is transmitted from terminal T2 to switch S.
1, only the detection signal corresponding to the dark current obtained when the electron beam scans the optical black 1B portion provided on the photoconductive surface is extracted, and the amplification A3, resistor R9, and capacitor C2 are averaged by an active filter, and the capacitor C2 holds the signal for one vertical scanning period.

Therefore, dark current detection is performed over one vertical scanning period, for example, over a period of 1H to 2H at the beginning of the one vertical scanning period.
Just go around.

The above amplifier A3 is composed of a well-known operational amplifier, the non-inverting input terminal -R+) is grounded, and the detection signal corresponding to the dark current from the above-mentioned amplifier A1 is sent to the inverting input terminal -TR→ through the resistor R9. Therefore, as the dark current increases, the output voltage of the amplifier A3 increases in the positive direction.

The output voltage of the amplifier A3 above is the same as that of the transistor Q1,
The transistor Q2 is introduced into the switching circuit composed of the transistor Q2, is added to the base of the transistor Q1, and is switched by, for example, a blanking signal or a horizontal synchronizing signal, etc., which is supplied to the terminal T3, and is synchronized with the blanking signal or horizontal synchronizing signal, etc. It is converted into a compensation pulse Ec and taken out at the point (■) shown in the figure.

The compensation pulse EC obtained here is applied to the amplifier A4. A5. Resistors R7, R11, R1 to A6 respectively
4, and is further applied to the above-mentioned amplifier A1 via a variable resistor R2.

At this time, it goes without saying that the amplifier A1 operates as a summing amplifier similarly to the amplifier A4 as described above.

Therefore, the output of amplifier A1 is optical black 1
The compensation pulse Ec acts so that the value of the dark current corresponding to part B is always zero, and the compensation pulse Ec
, that is, the DC voltage that is the output of the amplifier A3 described above is determined.

Now, if Edo is the signal output that corresponds to the dark current in the optical black 1B section that is input to the amplifier A1, the above-mentioned compensation pulse Ec at this time can be expressed as follows, including the temperature change. .

In the above equation, T: Temperature of the face plate of the image pickup tube a: A constant determined by the characteristics of the image pickup tube Edo Signal output according to the dark current of the niobtical black part 0: A constant phantom to Edo determined by the target bias voltage , and the negative sign - means the polarity of the pulse.

Next, let Edo, EdR, and EdB be the values of the dark currents contained in each video signal component at the center of the effective scanning area of the electron beam to be compensated, respectively.

In addition, the temperature coefficient in equations (2), (3), and (4) is G
, R, and B signals are considered to be the same.

Therefore, terminal T4. T5. Each signal output at T6 -e
G and -eR2-eB are respectively as follows.

The dark current included in the output component is canceled out, and compensation is performed in accordance with the current.

Therefore, the dark current contained in the G signal component can be compensated for by adjusting the variable resistor R2 so as to satisfy the above equation (8).

Furthermore, regarding the dark current contained in the R signal and B signal components, since 2 and 3 in equations (9) and (10) above are functions of the target bias voltage of the image pickup tube, Compensation can be achieved by appropriately adjusting the target bias voltage so as to satisfy the above equations (9) and 00).

Incidentally, as explained in the above embodiment, when the image pickup tube 2 is of a two-electrode type, 2=3.

Furthermore, when the present invention is applied to a three-pipe type, two
Since 3 and 3 are independent constants, the target bias voltage of each image pickup tube can be calculated as shown in (9) above.
The dark current can be compensated by adjusting so as to satisfy the equation (10).

In the above explanation, the dark current detection period in the optical black portion is performed in the initial period of the vertical scanning period, but it is also possible to perform this in a part of the period at the end of the vertical scanning period. Needless to say.

In addition, dark current in the optical black area may be detected during the initial period or a part of the end of the horizontal scanning period, and in this case, the sampling gate pulse is synchronized with the horizontal scanning pulse. You can use it as a gate pulse.

As described above (according to the present invention, particularly in a multi-tube color video camera), the face plate portion of each image pickup tube is maintained in a thermally balanced state, and the dark current value of each image pickup tube is The target bias voltages of the image pickup tubes are kept the same by adjusting them, and an optical black is arranged around the electron beam effective scanning area of one image pickup tube, and the dark detected by the optical black portion is Compensation pulses created based on the current value are used to compensate for the dark current of each of the image pickup tubes mentioned above, so that compensation can be performed in accordance with the dark current with excellent accuracy and stability through simple adjustment. It is possible to provide a method for compensating for dark current in an image pickup tube.

[Brief explanation of drawings]

The figure shows an embodiment of a dark current compensation circuit according to the present invention. 1.2: Image pickup tube, 1B niobacterial black, 3,
4, 5: Preamplifier, P: Compensation pulse generation circuit, A4.
A5. A6: Amplifier.

Claims (1)

[Claims] 1. In a multi-tube color video camera, each face plate is maintained in thermal equilibrium, and
The dark current values of each of the above image pickup tubes are made the same by adjusting the target bias voltage of each of the image pickup tubes, and each of the above image pickup tubes (05) By arranging an optical black in the peripheral area, generating a compensation pulse based on the dark current value detected in the optical black, and adding the compensation pulse to the video output signal of each image pickup tube. A dark current compensation method for an image pickup tube, which compensates for the dark current of each of the image pickup tubes described above.