JPS61161873A - Clamping circuit of video signal - Google Patents

Abstract

PURPOSE:To nullify the deviation component in the pedestal level for 1-horizontal cycle to obtain a stable output signal by making the delay time difference between the signals from 2 sample-holding circuits to be approximately 1 horizontal cycle to mix the two outputs. CONSTITUTION:The video signal (a) supplied to an input terminal 1 is supplied to an adder 10 and to a synchronizing separating circuit 11. The synchronizing signal (b) separated by the circuit 11 is supplied to a sampling pulse generating circuit 12, so that timing pulses (c) and (d) are supplied to first and second sample-holding circuits 5 and 6. The output signals of the circuit 5 and 6 are so made as the time difference between their phases is approximately 1H, and these outputs are added by an adder 7. In consequence, the phase of color burst inverses at every 1H. Accordingly, the component that causes the pedestal level deviation per 1H which can not be removed even by an LPF4 and hence remains, is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] Akihiro Kinoe This invention relates to an improvement in a clamp circuit for video signals.

[Conventional technology]

When performing DC reproduction of a composite color signal, a clamp circuit is used.This clamp circuit detects the voltage at the back porch, amplifies the signal with the opposite polarity, and connects the input and A clamp circuit was used to fix the pedestal voltage by mixing it with the composite color signal.0 Also, since a color burst is added to the pack pouch of the color video signal, the color burst is generated before detecting the voltage of the puncture pouch. A filter is provided to remove the components.

[Problem that the invention seeks to solve]

In the conventional clamp circuit described above, when adding a color burst, the distortion of the color burst may include a component that cannot be completely removed by the filter, and the pedestal voltage is affected by the force 2-burst. , N.T.
In the case of the SC method, there is a problem in that it may vary every horizontal period.

This means that when a color burst is generated by modulating a subcarrier, if the subcarrier is modulated with a signal that also contains high frequency components, the low frequency components will be generated. Since the phase of the subcarrier is inverted between IH and IH, a component whose pedestal level fluctuates between IH and IH is generated. In particular, a waveform in which the envelope of a color burst rises quickly tends to generate low frequency components that cannot be removed by a low-pass filter.

[Means for solving problems]

In view of the above points, the present invention has been made to solve the table problem and eliminate such drawbacks.The purpose of the present invention is to cancel the component in which the pedestal level fluctuates every horizontal period through a simple configuration. An object of the present invention is to provide a video signal clamp circuit that can obtain a stabilized output signal.

In order to achieve such an object, the present invention includes a first sampling and holding circuit that removes color burst components from an output composite color signal using a filter and obtains a bank pouch voltage of a post-digital video signal; a second sampling and holding circuit that receives a signal from the sampling and holding circuit and outputs a signal that is delayed by a predetermined horizontal period with respect to the output of the first sampling and holding circuit;
and means for adding the output signals of the second sampling and holding circuits and mixing them with the input composite color signal via a loop filter to obtain a clamped output signal; A drive means is provided to reduce the Bedestal level fluctuation of the output signal due to the influence of color burst.
It's a round thing.

[Effect]

It has two sampling and holding circuits, a first and a second sampling and holding circuit, and these two sampling and holding circuits generate pulses based on a synchronizing signal supplied from the outside or a synchronizing signal obtained by synchronously separating the input video signal. When the delay time difference between the signals of the first and second sampling circuits is set to approximately 1,000 cycles, and the outputs of both are mixed, the component in which the pedestal level fluctuates between 1 horizontal cycle and 1 horizontal cycle is canceled out. Obtain a stabilized output signal.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention.

In the figure, 1 is an input terminal to which a humming video signal is supplied, 2 is a video amplification circuit, 3 is an output terminal, 4 is a low-pass filter that removes color burst components from the output composite power 2-signal, and 5 is this low-pass filter 4. A first sampling and holding circuit 6 receives the output of the first sampling and holding circuit 5 and obtains the voltage of the back porch of the video signal.
The sampling and holding circuit 5 which inputs the signal from
This is a sampling and hold circuit that outputs a signal delayed by about one horizontal period with respect to the output of .

T is the first and 20th sampling and holding circuits 5 and 6
8 is a loop filter that inputs the output of this adder T, 9 is a control signal amplification circuit that inputs the output of the loop filter 8, and 10 is from this control signal amplification circuit 9. A mixing circuit that mixes the output signal of the first and second sampling and holding circuits with the input composite color signal, and mixes the output signals of the first and second sampling and hold circuits with the input composite color signal via a loop filter to produce a clamped output. 011, which constitutes a means for obtaining a signal, is a synchronization separation circuit that separates a synchronization signal from an input video signal, and 12 is a first and second sampling and hold circuit 5 based on the synchronization signal obtained by this synchronization separation circuit 11. .

Next, the operation of the embodiment shown in FIG. 1 will be explained with reference to FIGS. 2 and 3 showing the waveforms of each part. The waveform of the video signal is the error signal +r from the first and second sampling and holding circuits 5 and 6 (see Fig. 3).
・) l (see f)) is added by the adding circuit T and the loop filter 8. The signal is mixed by the mixing circuit 10 with the inverted signal via the control signal amplification circuit 9, and then sent to the video amplification circuit 2.
is sent to output terminal 3. Furthermore, the output of the video amplification circuit 2 is passed through a low-pass filter 4 to remove the force 2-burst component, and then supplied to a second sampling and hold circuit 5, which generates a sampling pulse. Timing pulse c from circuit 12
and maintains the pedestal level for one horizontal cycle.

Further, the signals from the second sampling and holding circuit 6 and the first sampling and holding circuit 5 are sampled with a timing pulse d delayed by about one horizontal period, and the voltage is held.

The sampling pulse generation circuit 12 inputs the synchronization signal b (see FIG. 2(b)) separated by the synchronization separation circuit 11 from the input video signal a (see FIG. 2(&)). (1), (d) K timing pulse a,
d is generated and supplied to the first and second sampling and hold circuits 5 and 6, respectively. Here, the first sampling and holding circuit 5 is sampled with a pulse at a position where one color burst is added, as shown in FIG. Further, the second sampling and holding circuit 6 is sampled and output after a slightly shorter time than IH has passed since the first sampling and holding circuit 5 is sampled. Therefore, this first and second 2
The phases of the output signals of the two sampling and holding circuits 5 and 6 have an approximately IH time difference.

Like the color signal of the NTSC system, the phase of the color burst is inverted for each IH, and the components that remain without being removed by the low-pass filter 4 and that cause fluctuations in the pedestal level for each IH are the components of the first and second components described above. K can be reduced by adding the outputs of the two sampling and holding circuits 5 and 60.

In this manner, an output signal can be obtained that is not affected by pedestal level fluctuations for each IH due to distortion in the color burst portion that often occurs when color perspective is added.

It is also clear that the signal from the synchronous separation circuit 11 can be used as is as a pulse for driving the sampling and holding circuit 6. Furthermore, a differential amplifier circuit may be used as the control signal amplifier circuit 9, and a regulator may be provided to adjust the output clamp potential.

FIG. 3 is a waveform diagram of each part of the embodiment of FIG. 1, with time t plotted on the horizontal axis, and is also shown over a longer number H than in FIG.

In the clamp circuit according to the present invention, the component that fluctuates for each IH as shown in FIG.
It is also clear that the signal is significantly reduced as shown in f), and that it can be sufficiently attenuated by the loop filter 8.

〔Effect of the invention〕

As explained above, according to the present invention, by setting the delay time difference between the signals of the two sampling and holding circuits to about one horizontal period and mixing the outputs of both, the component in which the pedestal level fluctuates every horizontal period is canceled out. Since a stabilized output signal can be obtained, the practical effect is extremely large. Furthermore, it is extremely effective in that it is possible to obtain an output signal that is not affected by pedestal level fluctuations for each IH due to distortion in the color burst portion that occurs when color perspective is added.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIGS. 2 and 3 are waveform diagrams showing waveforms of various parts to explain the operation of FIG. 1. 4@... e low-pass filter, 5, 6... sampling hold circuit, T... adder circuit, 8... loop filter, 9... control signal amplification circuit, 10...
...Mixing circuit, 11...Synchronization separation circuit, 12...
・Sampling pulse generation circuit. 1II Figure 2 (d) Figure 3 □t

Claims (1)

[Claims] A first sampling and holding circuit which obtains the back porch voltage of the video signal after removing the color burst component from the output composite color signal with a filter, and a first sampling and holding circuit which receives the signal from the first sampling and holding circuit as an input. a second sampling and hold circuit that outputs a signal delayed by a predetermined horizontal period with respect to the output of the circuit;
means for adding the output signals of the first and second sampling and holding circuits and mixing them with the input composite color signal via a loop filter to obtain a clamped output signal; 1. A video signal clamp circuit, comprising means for driving a hold circuit, and capable of reducing pedestal level fluctuations of an output signal due to the influence of color burst.