JPH0134455Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a black level fixing circuit for an ITV (industrial television) receiver, and a blanking circuit is connected after the black level fixing circuit to provide the black level fixing circuit necessary for the black level fixing circuit. This is intended to simplify the black level fixing circuit by combining the ranking function and the original blanking function.

ITV receivers are generally used to monitor automatic machinery, etc., and are required to clearly distinguish between the monitored object and the background. Conventionally, the black level fixing circuit of such an ITV receiver has a circuit for clamping to the black level and a circuit for blanking. was a separate dedicated circuit.
Therefore, two circuits with the same function are required, resulting in high cost.

The present invention was made to improve these conventional drawbacks, and by connecting the original blanking circuit after the black level fixing circuit, the blanking effect in the black level fixing circuit is replaced by the original blanking circuit. This is done using a circuit to simplify the circuit.

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

1 is an input terminal for a composite video signal (a signal obtained by superimposing a video signal and a synchronization signal), and 2 is an impedance conversion transistor. A capacitor 4, a clamp circuit 7 mainly composed of an npn type transistor 5, and a transistor 6 for impedance conversion are connected to the emitter resistor 3 of the transistor 2 in this order. The emitter resistor 8 of the transistor 6 is connected to an original blanking circuit 12 for blanking, which is made up of a resistor 9 and a transistor 10 and has a blanking pulse input terminal 11.
3 is connected to the output terminal 16 via an emitter resistor 14 and a black clip circuit 15.

A transistor 17 and a resistor 1 are connected to the collector side of the transistor 6, that is, to the inverter output side.
8, a comparator circuit 23 consisting of a capacitor 19, a diode 20, a variable resistor 21, and a resistor 22 is connected to the collector of the transistor 17 of the comparator circuit 23, a transistor 24, resistors 25, 2
6, 27, and a level fixing circuit 29 consisting of a capacitor 28, the capacitor 28 being connected to the emitter of the transistor 5 of the clamp circuit 7. In addition, 30 is a clamp pulse input terminal, 31 is a capacitor, 32 is a base resistor, and 33
is the power terminal.

Next, the operation of the circuit according to the present invention will be explained.

It is assumed that the composite video signal input to the input terminal 1 passes through the transistor 2 and a signal having a waveform as shown in FIG. 2a is obtained at the emitter. This signal is applied to the base of the transistor 6 via the clamp circuit 7, so the emitter of the transistor 6 is clamped at the black level L with the waveform signal shown in FIG. A signal with a waveform like that shown in FIG. 2B is obtained by blanking the synchronizing signal with the ranking pulse. That is, in the video period without blanking pulses, the blanking circuit 1
Since the transistor 10 of No. 2 is off and the only resistance between the emitter of the transistor 6 and the ground is the resistor 8, the emitter of the transistor 6 receives a waveform obtained by clamping the waveform of the video signal portion in FIG. 2a at the black level L. During the retrace period with a blanking pulse, the transistor 10 of the blanking circuit 12 is turned on, and the resistance between the emitter of the transistor 6 and the ground is approximately the sum of the resistors 8 and 9 connected in parallel. Since the collector current of transistor 6 becomes larger than that of resistor 8, the potential appearing at the emitter becomes lower, and the emitter of transistor 6 receives a signal with a waveform obtained by blanking the synchronizing signal part of a in the same figure. Because you can get it.

The signal having the waveform shown in FIG. A signal with a waveform as shown in f is obtained.

On the other hand, the transistor 6 for impedance conversion
As shown in Figure 2c, the collector receives a signal whose waveform is obtained by inverting the video signal portion of the waveform shown in Figure 2b during the video period, and blanking the waveform shown in Figure 2B with a blanking pulse during the retrace period. appears as an output and is applied to the base of transistor 17 of comparator circuit 23. At this time, the video signals A and B in the black portions as shown in FIG. 2a are inverted to become upward signals A' and B'. When the base voltage at this time becomes higher than the voltage obtained by adding the base-emitter voltage of the transistor 17 to the voltage adjusted by the variable resistor 21, that is, the reference level X, the transistor 17 is turned on. Then, a pulse as shown in FIG. 2e is obtained at the collector of this transistor 17. This pulse turns on the transistor 24 of the level fixing circuit 29, charges the capacitor 28 through the resistor 26, and charges the transistor 5.
increases the emitter voltage. That is, the clamp voltage of the clamp circuit 7 increases and the collector voltage of the transistor 6 decreases.

Next, when the upper end voltage of the upward signals A' and B' becomes lower than the voltage X, the transistor 1
7 and 24 are turned off, charging of the capacitor 28 is stopped, and the charge of this capacitor 28 is transferred to the resistor 27.
discharge through. Then, the clamp voltage falls, and the collector voltage of transistor 6 rises again, turning on transistors 17 and 24, and turning on resistor 2.
6 to charge the capacitor 28. In this way, when the charging current through the resistor 26 and the discharging current through the resistor 27 are balanced, the emitter voltage of the transistor 5 is fixed.

Here, if the tip voltages of the video signals A and B in the black part in FIG. The reference level X of is always maintained at a constant voltage, and therefore acts as a black level fixing circuit. In addition, the second
In the figure, L indicates the black level, E indicates the ground line level, and X indicates the reference level of the comparator circuit 23.

As mentioned above, this invention combines the black level fixing circuit with the original blanking circuit for blanking, so only one blanking circuit is required, reducing the number of components such as transistors, and reducing the cost. This is something that can be provided.

[Brief explanation of drawings]

FIG. 1 is an electrical circuit diagram showing an embodiment of the black level fixing circuit according to the present invention, and FIG. 2 is a waveform diagram of each part. 1...Video signal input terminal, 7...Clamp circuit, 11...Blanking pulse input terminal, 12
... Blanking circuit, 15 ... Black clip circuit, 16 ... Output terminal, 23 ... Comparison circuit, 29
... Level fixing circuit, 30 ... Clamp pulse input terminal, L ... Black level, E ... Ground wire level, X ... Reference level of comparison circuit 23.

Claims (1)

[Claims for Utility Model Registration] (1) A blanking circuit is coupled to the rear stage of the video signal clamp circuit via an impedance conversion transistor, and the inverter output is connected to the inverter output side of the impedance conversion transistor. An ITV characterized in that a comparison circuit for comparison with a reference level is coupled to the output side of the comparison circuit, and a level fixing circuit is coupled to the output side of the comparison circuit for holding the operating voltage of the clamp circuit constant based on the comparison output. Receiver black level fixing circuit. (2) The black level fixing circuit for an ITV receiver according to claim 1, wherein the clamp circuit comprises an NPN transistor that is turned on and off by a clamp pulse. (3) The comparator circuit couples the reference voltage to the emitter,
A black level fixing circuit for an ITV receiver according to claim 1 or 2, which is comprised of a transistor whose base is connected to an inverter output voltage from an impedance conversion transistor connected to a downstream stage of a clamp circuit. (4) The level fixed circuit has a transistor that turns on and off depending on the presence or absence of the comparison output of the comparison circuit, and when this transistor is on, it is charged through a resistor, and when it is off, it is discharged through another resistor for balance. 4. A black level fixing circuit for an ITV receiver as claimed in claim 2 or 3, comprising a capacitor coupled to the emitter of a transistor of a clamp circuit.