JPS637072B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a preamplifier circuit for amplifying the output signal of an image pickup tube used in a television camera.

Before converting the video signal from the television camera into a transmitted television signal, the output of the image pickup tube is once amplified by a preamplifier and subjected to signal processing such as white compression or gamma correction. The coupling between the image pickup tube and the preamplifier is known to take into account problems in the design of the amplifier, and there are two known methods: capacitor coupling and direct coupling. In the capacitor coupling, when a target voltage is applied to the target of the image pickup tube from a DC power source, a capacitor blocks the DC component so that the DC voltage is not directly applied to the preamplifier. In the case of capacitor coupling, the low frequency components of the signal output are blocked, resulting in sag, which is low-frequency distortion, in the amplified output signal, and the operation of the preamplifier becomes unstable due to fluctuations in the video average level of the signal current. cause

Therefore, when performing video signal processing such as white compression or white clipping on the output of the preamplifier, it is necessary to perform DC reproduction using a clamp circuit. accompanied by the occurrence of

On the other hand, with the direct-coupling method, the output signal from the target is directly amplified, so the DC component of the signal is not lost, so the output signal of the amplifier contains everything from DC components to high frequency components, and is sensitive to changes in the average video level. Therefore, the operation of the amplifier is not adversely affected. Therefore, the output signal of the amplifier can be subjected to signal processing related to the video level, such as white compression, white clipping, and gamma, without being clamped.

However, the above-mentioned preamplifier and signal processing circuit are usually composed of semiconductors, and are affected by the operating temperature, so that the DC level (operating point) tends to fluctuate. In some cases, the value of this variation cannot be ignored compared to the signal level from the image pickup tube. On the other hand, in a signal processing circuit, it is necessary to fix the operating point, that is, the black level. In conventional systems, in order to eliminate black level fluctuations (offsets) caused by temperature fluctuations, a preamplifier and a signal processing circuit are coupled with a capacitor, and the signal processing circuit regenerates the DC current.
However, this method is effective when the output signal of the preamplifier is small compared to the dynamic range of the signal processing circuit, but when the output signal is large, especially the image pickup tube using the newly developed beam current control method. In the case of the output signal, the value is extremely large, so if the video signal level suddenly changes, an undershoot portion of the waveform based on the coupling capacitor will occur, causing streaking.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to realize a circuit arrangement which keeps the output voltage of a preamplifier for a television camera precisely constant over the entire temperature range in which the television camera is used.

Another object of the present invention is to realize a television preamplifier circuit that does not cause clamp noise or streaking even when the output voltage of an image pickup tube is large.

In order to achieve the above object, the present invention provides a preamplifier that amplifies the output of an image pickup tube and a video processing circuit that processes the output signal. Detect the output voltage during the retrace period, compare this detected voltage with the reference voltage, and calculate the difference voltage (offset).
is configured to be negatively fed back to the input section of the preamplifier via a high gain direct-coupled amplifier.

Hereinafter, the present invention will be explained in detail with reference to Examples.

FIG. 1 is a block diagram showing the structure of an embodiment in which the present invention is implemented in a direct-coupling television preamplifier circuit. In the figure, 1 is an image pickup tube such as a videocon, 2 is a target electrode, 5 is a cathode electrode,
Reference numeral 13 denotes a DC power supply. In this embodiment, a direct connection system is adopted, and the DC power supply is configured to provide a cathode voltage and a target voltage in order to maintain a zero potential when there is no output signal from the target electrode. 6 is a preamplifier whose input is directly connected to the target electrode, and whose configuration is a normal amplifier. 8 is a video processing circuit,
This circuit performs white compression, white clipping, gamma correction, etc., and its input terminal is directly connected to the preamplifier, and its output is output from output terminal 4.
converted into a television transmission signal. A sampling circuit 9 samples the output of the preamplifier 6 using the gate signal from the sampling gate pulse generating circuit 10, and holds the sampling signal. Sampling gate pulse generation circuit 1
0 is configured to generate a pulse synchronized with the retrace period of the video signal. Therefore, it is configured to be used in common with the scanning drive pulse of the image pickup tube 1.
11 is a reference voltage generator, which is set to a desired value as the black level potential of the output of the preamplifier. Reference numeral 12 is a high gain direct-coupled amplifier 12, specifically a differential amplifier. This amplifier compares the black level signal voltage of the output signal of the preamplifier 6 detected by the sampling circuit 9 with the reference voltage from the reference voltage generator 11, amplifies the difference signal, and converts the signal into Negative feedback is provided via the resistor 7 to the target electrode and the input terminal of the preamplifier. As is clear from the above configuration, when the operating point of the video signal, that is, the black level, changes due to changes in the operating temperature due to changes in the operating point of the preamplifier, the fluctuation (offset) is compensated for by the sampling circuit. 9. It is easily detected by the reference voltage generator 11 and differential amplifier 12. Since the detection signal is negatively fed back, the offset is improved to 1/the gain of the feedback loop. In particular, since the gain of the differential amplifier 12 is extremely large, the black level of the video signal based on temperature fluctuations can be fixed at an extremely stable value.

As explained in the above embodiment, the operating point (black level potential) of the video signal can also be adjusted by direct coupling.
can be easily stabilized without using a clamp circuit, so clamp noise caused by a clamp circuit does not occur. Especially white compression, white clip,
Video processing circuit 8 related to video levels such as gamma
Therefore, fixing the black level is extremely important, and being able to fix the black level without using a clamp circuit is actually an extremely effective means for improving image quality.

In the above embodiment, the black level of the video signal is detected from the output signal of the preamplifier, but the black level may be detected from the output signal of the video processing circuit 8.

Furthermore, if the black level (DC level) of the output signal of the preamplifier or video processing circuit is not sufficiently stabilized even with the configuration of the above embodiment, the spike clamp circuit shown in FIG. position amplifier 6
Alternatively, further improvement can be achieved by using the output terminal of the video processing circuit 8 in combination. In this spike clamp circuit, a gate pulse from a sampling gate pulse generator is applied to the base of the transistor 15 in synchronization with the blanking period of the video signal, and a constant voltage is constantly applied to the emitter from the reference voltage generator. A video signal is applied to the collector from the preamplifier 6 or the video processing circuit 8 via the resistor element 14. Therefore, transistor 15
performs a switch operation, and the transistor is in a cut-off state during the video signal period, and is in a conductive state during the retrace period. Therefore, during the conduction period, the collector potential, that is, the output potential, is the potential difference between the output voltage of the preamplifier 6 (or video processing circuit 8) and the reference voltage of the reference voltage generator, divided by the ratio (gain) of the following formula. That means you did it.

R _S /R ₁₄ +R _S where R ₁₄ is the resistance value of the resistor element 14,
R _S is the saturation resistance value of the transistor. As a result, noise during the retrace period, intrusion signals from the deflection circuit, etc. are also compressed at the rate of the above formula. Therefore, in combination with the compression of spike noise by this circuit and the effects of negative feedback described above, further improved stabilization of the DC level is achieved.

Since spike noise is compressed, generation of clamp noise can be suppressed even when a clamp circuit is further added to the circuit of the present invention.

Although the above embodiments have been described in the case of a direct-coupling type preamplifier circuit in which the effects of the present invention are remarkable, it is obvious that the present invention can also be implemented in a capacitor-coupling type preamplifier circuit for a television camera.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of one embodiment of a preamplifier circuit for a television camera according to the present invention, and FIG. 2 is a circuit diagram of a spike clamp circuit added to the above embodiment. 6: Preamplifier, 8: Video processing circuit, 9: Sampling circuit, 10: Sampling gate pulse generation circuit, 11: Reference voltage generator, 12: High gain differential amplifier.

Claims (1)

[Claims] 1. A preamplifier that amplifies the output signal of the image pickup tube;
In a preamplifier circuit having a video processing circuit that processes the output signal, the offset of the retrace period of the output of either the preamplifier or the video processing circuit is detected, and a DC current is applied to the input terminal of the preamplifier. A preamplifier circuit for a television camera, characterized in that it is provided with a feedback circuit. 2. In the circuit described in claim 1,
The feedback circuit includes a sampling circuit that samples a signal during the retrace period of the output signal of the preamplifier or the video processing circuit, a circuit that detects a potential difference between the output voltage of the sampling circuit and a reference voltage, and the detection A preamplifier circuit for a television camera configured to include a high gain amplifier that amplifies the output of the circuit. 3. In the circuit described in claim 2,
A preamplifier circuit for a television camera, in which the above-mentioned image pickup tube and preamplifier are directly coupled. 4. In the circuit described in claim 2,
A preamplifier circuit for a television camera, in which the above-mentioned image pickup tube and preamplifier are capacitor-coupled. 5. In a preamplifier that has a preamplifier that amplifies the output signal of the image pickup tube and a video processing circuit that processes the output signal of the preamplifier, an image is connected to the output side of either the preamplifier or the video processing circuit. A noise clamp circuit that compresses the signal only during the retrace period of the signal is provided, and an offset between the retrace effects of the output of the preamplifier or the video processing circuit is detected and negative DC feedback is provided to the input terminal of the preamplifier. 1. A preamplifier circuit for a television camera, comprising a circuit. 6. In the circuit described in claim 5,
The feedback circuit includes a sampling circuit that samples the output signal of the preamplifier or the video processing circuit, a detection circuit that detects a voltage difference between the output voltage of the sampling circuit output and a reference voltage, and amplifies the output of the detection circuit. The noise clamp circuit has a collector connected to either the preamplifier or the output terminal of the video processing circuit via a resistive element, and an emitter connected to a reference voltage source. A preamplifier circuit for a television camera, which is composed of a transistor whose base is connected to a gate pulse circuit for making the transistor conductive only during the retrace period of the video signal.