JPH04107075A - Clamp circuit - Google Patents

Abstract

PURPOSE:To stably clamp the prescribed level even of signals into which pulse noises are intruded into a reference level by allowing a clamp circuit to have a negative-feedback action and a median filter processing, and clamping the prescribed level of the input signals into the reference level. CONSTITUTION:An input video signal (g) into which the pulse noises are intruded is inputted to a subtracting circuit 1. An output signal (h) of the circuit 1 is performed feedback of through a median circuit 2, subtractor 3, and a memory circuit 4 to the other input of the circuit 1 as a signal (m). The signal (h) inputted to the circuit 2 is operated for sampling by a clamp pulse (i) in the prescribed period, and converted into a median value (j) by the median filter of the circuit 2. A difference between the median value (j) and the reference level is calculated by the subtractor 3, and a control value (l) is outputted. The control value (l) is operated for sampling by the next clamp pulse, and the previous control value is held in the memory circuit 4 until the next control value is obtained, and continues to be the signal (m) inputted to the other input of the subtracting circuit 1.

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 signal clamp circuit, and by providing this circuit with a negative feedback effect and median filter processing, it is possible to control a predetermined level of an input signal, such as a television signal. The back porch pedestal level of the horizontal synchronization signal is clamped to the 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 forcibly adjust a predetermined level of an input signal, especially a video signal, to a reference level. This is a processing-specific circuit.

FIG. 3 shows some conventional examples of clamp circuits. figure(
A) is the simplest lamp circuit and uses one diode. In the circuit shown in Figure (a), when the potential on the output side becomes -0.6V or less, the diode becomes conductive and charges the capacitor, and the input signal DC level is clamped to the potential of 10.6V. However, in this circuit, the forward characteristics of the diode have a significant influence, so Figures (b) and (
Something like C) is used to cancel it. The clamp level is Ov in figure (b) and +VN in figure (C).
It is.

On the other hand, in the low frequency range, a clamp circuit may be constructed using an OP amplifier (actually used as a comparator) as shown in Figure (d). This circuit has good accuracy, but when handling high-speed signals with low load resistance such as video signals,
Since it is necessary to charge the capacitor in a short period of time, a high-speed and high-power op amp must be used. figure(
In the case of e) and (f), the bias of the high-speed, high-input impedance amplifier A1 is controlled from the clamping comparator A2.

(Problem to be solved by the invention) The conventional examples described above are typical examples, but
In either case, when pulse noise mixes into the position of the video signal to be clamped, the potential of the signal is disturbed by the pulse noise, resulting in the creation of white or black horizontal lines on the screen.

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

(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 cascade-connected median circuit for obtaining the fed back signal level from the output signal of the subtraction circuit; In the median circuit, the output signal of the subtraction circuit is repeatedly sampled by a clamp pulse for a predetermined period, and the sampled signal is passed through a median filter to obtain a median value. The subtracter subtracts the clamp level from this median value and outputs the control value, and the memory circuit holds the previous control value until sampling is performed by the next clamp pulse and the next control value is obtained. It is characterized by this.

(Function) According to the present invention, the clamp circuit is provided with a negative feedback function and median filter processing, and a predetermined level of an input signal, particularly a video signal, is clamped stably and reliably to a reference level. Therefore, 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 signal is a television video signal, and the pedestal level of this is! shall be clamped to a bell.

In FIG. 1, an input video signal g containing pulse noise is input to one input of a subtraction circuit 1. In FIG.

The input video signal g 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 the television signal, and only a small part of the video signal. ing. 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 applied to the other input of the subtraction circuit 1 through the cascade connection of the median circuit 2, the subtracter 3 and the memory circuit 4. It is fed back as signal m.

The output signal of the subtraction circuit l inputted to the input terminal of the median circuit 2 is repeatedly sampled by the clamp pulse i during its predetermined period, that is, when the input signal is a television signal, the horizontal synchronization bank porch period. Ru.

In FIG. 2, the position of the clamp pulse during the sampling period of signal i is shown corresponding to the position of input signal g. That part of the sampled output signal is converted into a median value j by a median filter (not specifically shown) in the median circuit 2.

The median value is when focusing on one sampling clamp pulse, dividing the sampling period into several sub-sampling periods, arranging the sub-sampled variables in order of magnitude, and calculating the median value. This value is considered to be a representative value of this sampling group, and this value is called a median number or median value. Outputting this median value is called converting it to a median value j, and this calculation is executed by a median filter. This median value is a well-known technical term used in statistics, and for details, please refer to the literature, for example, Takubu Kishine, "Theoretical and Applied Statistics," published by Yokendo, 1975.

In the above-mentioned embodiment, the median value j output from the median circuit 2 is then converted into a clamp level having a constant DC level in the subtracter 3, that is, the difference from the reference level is determined and the difference is determined as the control value! is output. Therefore, this control value l is sampled by the next clamp pulse, and the previous control value is held in the memory circuit 4 until the next control value is obtained, and the previous control value is input to the other input of the subtraction circuit 1. The input signal continues to be m.

The embodiments described in detail above are not limited thereto, and various modifications and changes can be made within the scope of the claims.

(Effects of the Invention) As described above in detail, according to the clamp circuit of the present invention, this circuit is provided with a negative feedback effect and a median filter process, so that a predetermined level of the input signal can be adjusted. For example, since the bank porch level of the horizontal synchronization signal in the television signal is clamped to the reference level, even if pulse noise is mixed in at the clamp position, the pulse noise will be almost eliminated by the circuit's median filter processing. This has the effect of providing a stable screen after clamping.

[Brief explanation of drawings]

FIG. 1 shows a linear diagram of an embodiment of the present invention, FIG. 2 shows signal waveforms at various parts thereof, and FIG. 3 shows some conventional examples of clamp circuits.

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. the subtraction circuit, a cascade-connected median circuit for obtaining the fed back signal level from the output signal of the subtraction circuit, a subtracter, and a memory circuit; The output signal of the circuit is repeatedly sampled by a clamp pulse for a predetermined period, and the sampled signal is converted to a median value through a median filter, and a clamp level is subtracted from this median value in a subtracter. 1. A clamp circuit characterized in that a control value is outputted in a memory circuit, and a previous control value is held in a memory circuit until sampling is performed by a next clamp pulse and a next control value is obtained. 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 sampled by the clamp pulse. A clamp circuit characterized in that a period during which the period is the back porch period. 4. The circuit according to claim 3, wherein the period sampled by the clamp pulse is a horizontal line period of a television signal.