JPH07154645A - Clamp circuit - Google Patents

Abstract

PURPOSE:To provide a clamp circuit by which high quality clamping is attained without sacrificing the performance in the transient state and the steady-state. CONSTITUTION:A signal processing circuit 2 is used to process a video signal obtained through a level shifter 1. An output of the signal processing circuit 2 and a reference voltage from a reference voltage source 4 are given to an offset detection circuit 3, and a time constant is decided based on an output of the offset detection circuit 3 and a capacitor 5 depending on the voltage difference. A level shift adjustment circuit 6 decides the shift quantity of a level shifter 1 based on a voltage across the capacitor 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a clamp circuit for a television signal processing IC.

[0002]

2. Description of the Related Art A conventional clamp circuit will be described with reference to FIG. Supply the video signal from the previous stage to the input IN,
This is supplied to the input of the level shifter 1. The video signal that has passed through the level shifter 1 is level-shifted, gamma-corrected by the video signal processing circuit 2, and then output from the output OUT to the next stage circuit. The output of the video signal processing circuit 2 is supplied to one input of the offset detection circuit 3, and in the offset detection circuit 3, the reference voltage supplied to the other input of the offset detection circuit 3 during the gate pulse period for extracting the burst signal. The voltage difference between the video signal input to one input of the source 4 and one input of the offset detection circuit 3 is detected and the capacitor 5 is charged. The capacitor end voltage of the capacitor 5 is supplied to the bell shift amount adjusting circuit 6. The output of the level shift amount adjusting circuit 6 is supplied to the controlled input of the level shifter 1.

The offset detection output of the offset detection circuit 3, that is, the capacitance end voltage of the capacitance 5 is set so that the output of the video signal processing circuit 2 becomes equal to the reference voltage source 4 during the gate pulse period. The output of the level shift amount adjusting circuit 6 is set, and the shift amount of the level shifter 1 is determined. By such a feedback action, the video processing circuit 2
Will be clamped to the reference voltage source 4 during the gate pulse period.

In such a clamp circuit, in order to make the black level of the video signal output of the video signal processing circuit 2 exactly equal to the reference voltage source 4, the offset detection circuit 3
It is necessary to increase the gain of the feedback section from to the level shift amount adjusting circuit 6 to some extent. This gain needs to be set high in order to quickly end the clamp operation when the power is turned on. This means that it is necessary to increase the offset detection sensitivity of the feedback section. The offset detection sensitivity is roughly divided into two factors, the gain of the circuit existing in the feedback section and the offset detection speed. If the gain of the feedback section, that is, the offset detection sensitivity, is set high, the clamp operation at power-on can be terminated quickly, but if the sensitivity is set high, unnecessary components such as noise mixed in the gate pulse period will be generated. The peak of is detected, and an error occurs due to noise at the time of clamping. Therefore, it is necessary to lower the offset detection sensitivity contrary to the above. Generally, the detection sensitivity is obtained by changing the time constant determined by the output of the offset detection circuit 3 and the value of the capacitor 5, and this time constant is selected to be a suitable place by experiments and experience by the designer. It was

As described above, there is a conflict between the clamp operation required in the transient state when the power is turned on and the clamp operation required in the steady state. Both performances have to be sacrificed to some extent.

[0006]

In the above-mentioned conventional clamp circuit, the clamp operation required in the transient state at the time of power-on and the clamp operation required in the steady state are contradictory to each other. Yes, there is no choice but to sacrifice both power-on stability and noise performance to some extent.

The present invention provides a clamp circuit that enables a high-quality clamp operation without sacrificing the performance in the transient state and the steady state.

[0008]

According to the present invention, a comparator for comparing a reference voltage with an output voltage of a video signal processing circuit is provided in a feedback section of a clamp circuit, and an output of the comparator is an offset detection circuit or a level. By inputting to the shift amount adjustment circuit or both circuits, the offset detection circuit is set so that the time constant changes according to the output of the comparator, and the level shift amount adjustment circuit outputs the comparator output. The level shift amount change rate of the level shifter is set so that it can be changed.

[0009]

By the above means, the clamp operation is quickly terminated in a transient state such as when the power is turned on, and in the steady state, the image is always consistent with the reference voltage without being influenced by noise in a certain pulse period. The signal output voltage can be obtained.

[0010]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention. In FIG. 1, the same parts as those in FIG.

In FIG. 1, the video signal from the preceding stage is supplied to the input IN, and this is supplied to the input of the level shifter 1. The video signal that has passed through the level shifter 1 is level-shifted, subjected to, for example, gamma correction by the video signal processing circuit 2, and then output from the output OUT to the next stage circuit. Also,
The output of the video signal processing circuit 2 is supplied to one input of the offset detection circuit 3 and is supplied to one input of the comparator 11 whose reference voltage source 4 is supplied to the other input. The offset detection circuit 3 detects the difference between the reference voltage source 4 supplied to the other input of the offset detection circuit 3 and the voltage of the video signal input to one input of the offset detection circuit 3 during the gate pulse period, Charge to capacity 5. Level shift amount adjustment circuit 6 based on the voltage across the capacitance of the capacitance 5
Controls the level shift amount of the level shifter 1. The clamp operation is performed by such a loop.

In this embodiment, a comparator 11 is provided in the clamp loop and the comparison output is supplied to the offset detection circuit 3. Thereby, in the offset detection circuit 3, the time constant determined by the capacitance 5 is controlled by the output of the comparator 11, and in the level shift amount adjustment circuit 6, the gain is controlled to compare the level shift amount of the level shifter 1. It is controlled according to the output of the container 11.

An example will be described below. FIG. 2 shows the relationship between the difference voltage Va between the output of the video signal processing circuit 2 and the reference voltage source 4 and the comparison output voltage Vb of the comparator 11. 3 shows the relationship between the output Vb of the comparator 11, the offset detection circuit 3, and the time constant τε determined by the capacitance 5. Assuming the respective relationships as shown in FIGS. 2 and 3, the voltage difference between one input and the other input of the comparator 11, as shown in FIG.
That is, if | Va | is large, the output of the comparator 11 becomes large, and as a result, the time constant τε of the clamp loop becomes short as shown in FIG. Conversely, if | Va | is small, τε
Becomes longer.

By controlling the time constant in this way,
When the power is turned on, the video signal processing output quickly shifts to the same voltage as the reference voltage source 4 even if | Va | is large. Further, when the output of the video signal processing circuit 2 is equal to the reference voltage source 4 in the stable state after the power is turned on, the time constant is very long, and the video signal output fluctuates due to noise or the like. However, since it does not follow the change, it is possible to eliminate the adverse effects such as horizontal streak noise that the brightness slightly changes every 1H and the screen looks noisy.

Next, another embodiment of the present invention will be described with reference to FIG. In this embodiment, the output of the comparator 11 is supplied to the level shift amount adjusting circuit 6.
The operation will be described below with reference to FIGS. 4 and 5.

FIG. 4 shows the comparison voltage Vb of the comparator 11,
The relationship of the output voltage Vc of the level shift amount adjusting circuit 6 is shown. FIG. 5 shows the relationship between Vb and the level shifter output Vc.
Shown in. According to the relationships of FIGS. 4 and 5, Vb increases when | Va | is large, so the level shift amount Vd of the level shifter 1 controlled by Vc increases. On the contrary, when | Va | is small, the comparison voltage Vb is small, and as a result, the level shift amount Vd controlled by Vb is small.

Also in the case of this embodiment, when the power is turned on, | Va
Even if | is large, the video signal processing output quickly shifts to the same voltage as the reference voltage source 4. Further, when the output of the video signal processing circuit 2 is equal to the reference voltage source 4 in a stable state after power-on, even if the video signal output fluctuates due to noise or the like, it does not follow the fluctuation. .

In each of the above embodiments, the output of the comparator 11 is used to control the time constant by the output of the offset detection circuit 3 and the capacitor 5, and the output of the comparator 11 is used to adjust the level shift amount. Although the control of the level shift amount of 6 has been described, the present invention is not limited to this, and the output of the comparator 11 may control the offset detection circuit 3 and the level shift amount adjusting circuit 6.

FIG. 6 is for more specifically explaining the embodiment of FIG. 1 in which the offset detection circuit 3 is controlled by the output of the comparator 11. A differential input / differential output amplifier 61 corresponds to the comparator 11, and includes an amplifier 62 and a resistor 63 that are operated only during the period of the gate pulse GP, and a reverse conductivity type MOS that inputs the differential output of the amplifier 61. The transistors 64 and 65 correspond to the offset detection circuit 3. Further, the current output amplifier 66 is the level shift amount adjusting circuit 6
The resistor 67 and the amplifier 68 correspond to the level shifter 1 and the video signal processing circuit 2. Video signal processing circuit 2 of this circuit
Simply exemplifies a case where the video signal from the video signal source 69 obtained via the level shifter 1 is amplified.

The MOS transistors 64 and 65 are used as variable resistors and combined with the capacitor 5 to form a variable time constant circuit. When the MOS transistors 64 and 65 are used as resistors, the resistance value R is

[Equation 1] Can be represented. Here, L is the gate length and W is the gate width. Further, Rs is a sheet resistance, which can be expressed as follows.

[0021]

[Equation 2] Where μ is the electron mobility, Cox is the gate capacitance, Vg
Is the gate voltage, and Vt is the threshold voltage of the MOS transistor. From equations (1) and (2), it can be seen that the MOS transistor is a variable resistor that changes depending on the voltage of the gate terminal. The time constant τ is

[Equation 3] It is represented by. Here, C is the capacitance value of the capacitor 5.

In this way, when the gate pulse GP is input, the output of the comparator 11 is applied to the gates of the MOS transistors 64 and 65, so that the time constant of the offset detection circuit 3 is changed to the video signal processing circuit. It can be changed according to the difference between the output of the amplifier 68 corresponding to 2 and the reference voltage source 4. That is, the clamp operation can be performed during the period when the gate pulse GP is input.

FIG. 7 is for explaining another example in which FIG. 1 is embodied. The output of the absolute value circuit 71 for taking the absolute value of the video signal obtained through the amplifier 68 and the reference voltage source 4 are shown. The output of the comparator 11 composed of the differential amplifier 72 that outputs the difference between the two is supplied to the offset detection circuit 3 composed of the variable current output type amplifier 73 that operates during the period when the gate pulse GP is input, The current output of the amplifier 73 is variable. Also in this example, the clamp operation can be performed during the period in which the gate pulse GP is input.

The present invention is not limited to the above-described embodiment, and for example, the processing circuit for the video signal to be clamped may be for noise removal for removing a minute signal near 4 MHz. . Also, level shifter 1
The output of the level shift amount adjusting circuit 6 that controls the current may be current or voltage.

[0025]

As described above, according to the clamp circuit of the present invention, by changing the detection sensitivity of the loop according to the difference between the output of the video signal processing circuit and the reference voltage,
The clamp operation can be quickly terminated when the power is turned on, and in the stable state, it is possible to accurately clamp the reference voltage without being affected by disturbance such as noise, and it is possible to eliminate the adverse effect of horizontal streak noise.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram for explaining an embodiment of the present invention.

FIG. 2 is a characteristic diagram for explaining the operation of FIG.

FIG. 3 is a characteristic diagram for explaining the operation of FIG.

FIG. 4 is a characteristic diagram for explaining the operation of another embodiment of the present invention.

FIG. 5 is a characteristic diagram for explaining the operation of another embodiment of the present invention together with FIG.

FIG. 6 is a circuit configuration diagram for explaining a specific example of FIG.

7 is another circuit configuration diagram for explaining the specific example of FIG. 1. FIG.

FIG. 8 is a circuit configuration diagram for explaining a conventional clamp circuit.

[Explanation of symbols]

 1 ... Level shifter 2 Video signal processing circuit 3 Offset detection circuit 4 Reference voltage source 5 Capacitance level adjustment level adjustment circuit 11 Comparator

Claims (2)

[Claims] 1. A level shifter for shifting the level of an input video signal, a video signal processing circuit for performing signal processing on the video signal obtained through the level shifter, and outputting the video signal, and an output of the video signal processing circuit. An offset detection circuit that detects an offset amount based on a voltage difference from a reference voltage, a time constant circuit that determines a time constant between the offset amount of the offset detection circuit, and an output of the time constant circuit, A level shift amount adjusting circuit that determines a shift amount of the level shifter, and a time constant of the time constant circuit is changed based on a comparison result output based on a voltage difference between an output of the video signal processing circuit and a reference voltage. A clamp circuit comprising a comparator. 2. A level shifter for shifting the level of an input video signal, a video signal processing circuit for performing signal processing on the video signal obtained through the level shifter, and outputting the video signal, and an output of the video signal processing circuit. An offset detection circuit that detects an offset amount based on a voltage difference from a reference voltage, a time constant circuit that determines a time constant between the offset amount of the offset detection circuit, and an output of the time constant circuit, A level shift amount adjusting circuit that determines the shift amount of the level shifter, and a comparator that controls the level shift amount of the level shift amount adjusting circuit based on the voltage difference between the output of the video signal processing circuit and a reference voltage. Clamp circuit characterized in that