JPH0438076A - Clamping circuit - Google Patents

Abstract

PURPOSE:To prevent a screen from generating white or black horizontal stripes even if a pulse noise is mixed into a clamping position by providing the clamping circuit with a negative feedback action and a fly wheel effect and clamping a previously determined prescribed level in an input signal at a reference level. CONSTITUTION:When a clamp pulse (b) switches a switching circuit 2 to the side of a video output signal (e), a negative feedback circuit is constituted of a subtractor 1, the circuit 2, an amplifier circuit 4, and a loop filter circuit and negative feedback operation is executed so that a difference between the pedestal level of the signal (e) and the reference level becomes zero. When the circuit 2 is switched to the side of a reference level feeding source 3 by the pulse (b), the circuit 5 holds the potential of the negative feedback operation until the circuit 2 is switched to the signal (e) side. Since the time constant of the circuit 5 can be set up to a value larger than the repeating period of a horizontally synchronizing signal, the stable operation of the circuit 5 can be attained by providing the circuit 5 with a fly wheel effect.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a clamp circuit for signals, particularly video signals.

(Summary of the Invention) This invention relates to a clamp circuit, and by giving this circuit a negative feedback effect and a flywheel effect,
a predetermined level in the input signal,
For example, the vedestal level of the back porch of a horizontal synchronizing signal like a television signal is clamped to a reference level.

In this way, even video signals containing pulse noise can be stably clamped.

(Prior Art) A clamp circuit is a circuit that forcibly adjusts a predetermined level of an input signal, especially a video signal, to a reference level, and is used to adjust the bias reference level or to adjust an AC amplified image. This is a circuit specific to video signal processing such as DC signal reproduction.

Conventional examples of clamp circuits include those described below, which can be broadly divided into methods that do not use clamp pulses and methods that use clamp pulses.

An example of a method that does not use a clamp pulse is to adjust the peak of a synchronizing signal in a television signal to a reference level. The circuit consists mainly of a combination of a diode and a capacitor, and when the voltage value contained in the input signal becomes low and becomes lower than the forward voltage of the diode, the diode becomes conductive, a current is generated in the capacitor, and the voltage across both ends changes. The peak value of the input signal (in this case, the lowest potential and the leading edge of the synchronizing signal) is always clamped to the reference potential.

Next, an example of a method using a clamp pulse will be described.

The best place to clamp the television video signal is the black reference (where the color burst signal is inserted).
is at the pedestal level). The input video signal is clamped to a reference level by an external clamp pulse. The clamp circuit consists of a capacitor, a reference power supply, and a switching circuit.During the clamp period, the switching circuit charges the capacitor with the voltage difference between the input level and the reference power supply, and then opens the switch to maintain that voltage and perform DC regeneration. It is. As this clamp pulse, a pulse obtained by delaying a pulse obtained by synchronous separation is often used. Also, when clamping with this circuit, care must be taken to ensure that the conduction resistance of the switching circuit is too low and will not damage the color burst simultaneously transmitted to the black reference.

(Problems to be Solved by the Invention) The conventional examples described above are typical examples, but in any of the conventional examples, when pulse noise mixes at the position of the video signal that should be clamped, the potential of the signal changes to the pulse noise. The problem was that white or black horizontal lines appeared on the screen.

SUMMARY OF THE INVENTION An object of the present invention is to provide a clamp circuit which can eliminate the above-mentioned drawbacks and efficiently clamp a predetermined level of a signal, particularly a video signal, mixed with pulse noise to a reference level.

(Means for Solving the Problems) In order to achieve this object, the clamp circuit according to the present invention is a clamp circuit that clamps a predetermined level of an input signal. and a signal level fed back by the feedback action of the clamp circuit, a switching circuit that switches between the output of the subtraction circuit and a reference level using a clamp pulse, and the signal level after passing through the amplifier circuit. The present invention is characterized in that it includes a loop filter circuit that integrates the output of the switching circuit and uses the output as the signal feedback level, and the output of the subtraction circuit is used as the output of the clamp circuit.

Further, in a preferred embodiment of the clamp circuit according to the present invention, the input signal is a transmitted television signal, and the predetermined level is a back porch of a transmitted television signal horizontal synchronizing signal. pedestal level, the period during which the output of the subtraction circuit is fed back by the switching circuit is the back porch period, and the time constant of the loop filter circuit is longer than the repetition period of the horizontal synchronization signal. That is.

(Function) According to the present invention, the difference between the input signal level and the signal level fed back by the feedback action of the clamp circuit is taken, and the feedback circuit uses a switching circuit to connect the output of the clamp circuit and the reference signal level. Since the signals are alternately fed back through appropriate period integration circuits, pulse noise mixed into the video signal can be effectively eliminated.

(Examples) The present invention will be described in detail below by way of examples with reference to the accompanying drawings.

FIG. 1 shows a configuration block diagram of an embodiment of the present invention which is not limited thereto, and FIG. 2 shows signal waveforms of each part thereof. In this case, the input is a television video signal, and the case where this is clamped at the pedestal level will be explained.

In FIG. 1, an input video signal a is input to one input of a subtraction circuit 1. In FIG. The input video signal a shown in the second figure is a necessary part according to the embodiment of the present invention, that is, a back porch part including a horizontal synchronizing signal and a burst signal in a television signal, and only a small part of the video signal. The output signal of the subtraction circuit 1 is the output signal of the clamp circuit of the present invention, but this signal is also fed back to the other input of the subtraction circuit 1 via the switching circuit 2, the amplifier circuit 4, and the loop filter circuit 5. .

The video output signal e input to one input of the switching circuit 2 and the voltage based on the reference level 3 input to the other input are switched by a clamp pulse b, and the mode thereof is pulse b shown in the second diagram. The back porch portion containing the burst signal of the horizontal synchronization signal is cut out, and the switch of the switching circuit 2 is connected to the reference level supply source 3 when not in operation.

The removed burst signal portion is output as a pulse train C, and a pulse train d amplified by the amplifier circuit 4 is obtained. The loop filter circuit 5 has an integrating action, and its output becomes a DC potential and is input to the other input of the subtraction circuit 1.

In this circuit, when the clamp pulse b switches the switching circuit 2 to the video output signal e side, a negative feedback circuit is configured by the subtraction circuit 1, the switching circuit 2, the amplifier circuit 4, and the loop filter circuit 5, and the video output signal e is A negative feedback operation is performed so that the difference between the hedestal level and the reference level becomes zero. When the switching circuit 2 is switched to the reference level supply source 3 side by the clamp pulse b, the loop filter circuit 5 holds the potential during the negative feedback operation until the switching circuit 2 is switched to the video output signal e side again. Here, the amplifier circuit 4 is used to increase the loop gain of the negative feedback circuit. In this circuit, since the time constant of the loop filter circuit 5 can be made larger than the repetition period of the horizontal synchronizing signal, this circuit is given a flywheel effect and stable operation is possible.

It is obvious that the embodiments of the present invention are not limited to these, and that various modifications and changes can be made within the scope of the claims.

(Effects of the Invention) As described above in detail, the clamp circuit of the present invention has a negative feedback effect and a flywheel effect, and is capable of adjusting the length of the input signal to a predetermined level. For example, since the pedestal level of the back porch of the TV Shore (No. 3 horizontal synchronization signal) is clamped to the reference level, even if pulse noise is mixed in at the clamp position, the loop filter circuit that has a flywheel effect reduces the average pulse noise. This has the advantage that white or black horizontal stripes do not appear on the surface after clamping.

[Brief explanation of the drawing]

FIG. 1 shows a configuration Bronk diagram of an embodiment of the present invention, and FIG. 2 shows signal waveforms at various parts of the circuit configuration shown in the first diagram. 1...Subtraction circuit 2...Switching circuit 3...Reference level supply 'a 4...Amplification circuit 5
...Loop filter circuit. Figure 1: Nuihome and the reception of the guests Figure 21

Claims (1)

[Claims] 1. In a clamp circuit that clamps a predetermined level in an input signal, the clamp circuit detects the difference between the input signal level and the signal level fed back by the feedback action of the clamp circuit. a switching circuit that switches between the output of the subtraction circuit and a reference level using a clamp pulse; and a switching circuit that integrates the output of the switching circuit after passing through an amplifier circuit and uses the output as the feedback signal. and a loop filter circuit that uses the subtraction circuit as the output of the clamp circuit. 2. The clamp circuit according to claim 1, wherein the input signal is a transmitted television signal. 3. The circuit according to claim 2, wherein the predetermined level is a back porch pedestal level of a transmitted television signal horizontal synchronizing signal, and the output of the subtraction circuit is fed back by the switching circuit. A clamp circuit characterized in that a period during which the period is the back porch period. 4. The clamp circuit according to claim 3, wherein the time constant of the loop filter circuit is longer than the repetition period of the horizontal synchronizing signal.