JPS6245284A - Dc restoration circuit for television signal - Google Patents

Abstract

PURPOSE:To attain a DC restoration with high accuracy by providing an LPF that eliminates a color burst signal from a digital signal converted from an analog television signal. CONSTITUTION:A TV signal supplied from a terminal 1N is supplied to an A/D converter 1 and a synchronizing and separator circuit 6. The color burst signal is eliminated at an LPF2, and the signal is formed to be the one having a flat part over the whole of a back porch. The output of the LPF2 is subtracted from a corresponding digital value supplied from a restoring DC level holding circuit 3 by a subtractor 4 and supplied to a latching circuit. A circuit 7, at every receiving of a latching pulse from the circuit 6, holds a new subtraction result supplied from the subtractor 4, adding it one the digital signal outputted from the converter 1 at an adder 8. As a result, the level of the digital signal from the adder 8 is equivalent to the one that the level of the back porch is clamped to the content of the circuit 3.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a DC regeneration circuit for television signals used in television receivers.

BACKGROUND OF THE INVENTION In conventional television receivers, a DC regeneration circuit is installed in order to regenerate the DC component lost due to AC coupling in the video amplifier circuit.

Conventionally, such DC reproducing circuits include a pedestal clamp circuit that clamps the level in the back porch to a predetermined value, and a so-called soft clamp circuit that clamps the level by resonating with the color burst signal.

Problems to be Solved by the Invention The conventional pedestal clamp circuit described above requires a highly accurate synchronous separation circuit and a clamp pulse generation circuit.
There is a problem of high cost.

That is, as shown in the waveform diagram in Figure 6, the flat level that appears between the start point of the back porch and the start point of the color burst signal is fixed by a predetermined clamp pulse generated based on the horizontal synchronization signal. It is clamped to the regenerative DC level, but the period during which this flat level appears is (0,5
~1) Since the value is as short as μsec, high accuracy is required in the detection timing of the horizontal synchronization signal and the generation timing of the clamp pulse.

Furthermore, the above-mentioned soft clamp circuit has the problem of high cost because the capacitors and inductances constituting the resonant circuit are required to have highly accurate element constants and stability against aging.

Structure of the Invention Means for Solving the Problems The DC reproduction circuit of the present invention includes an A/D converter that converts an analog television signal into a digital signal, and a low voltage converter that removes a color burst signal from the converted digital signal. a pass-pass filter; an arithmetic unit that outputs a level difference between the level of the digital signal from which the color burst signal has been removed and a predetermined reproduction DC level; and a holding circuit that holds the level difference within the appearance period of the back porch. , and an arithmetic unit that adds the level difference held in the holding circuit to the output of the A/D converter.

That is, the DC reproducing circuit of the present invention provides a branch path for the television signal bus, removes the color burst signal in this branch bus, and expands the flat section inside the back porch, thereby reducing the signal to a considerable extent. It is configured to enable highly accurate DC regeneration while allowing timing errors.

Further, the DC regeneration circuit of the present invention enables highly accurate DC regeneration by performing DC regeneration based on digital processing.

Recently, there have been plans to perform various types of image quality improvement processing such as noise removal, scanning line interpolation, and edge enhancement on the television receiver side. There is a growing tendency for information to be converted to digital format. Therefore, if the A/D conversion circuit installed for the purpose of improving image quality as described above is used as is, the amount of hardware required for DC reproduction based on digital processing can be kept to a small amount.

Hereinafter, the operation of the present invention will be explained in detail together with examples.

Embodiment FIG. 1 is a block diagram showing the configuration of a DC regeneration circuit according to an embodiment of the present invention, and 1 is an A/D converter.

2.5 is a low-pass filter, 3 is a regenerative DC level holding circuit, 4 is a subtracter, 6 is a synchronous separation circuit, 7 is a launch circuit, 8
is an adder, and 9 is a D/A converter.

An analog television signal supplied from an input terminal IN is supplied to an A/D converter 1 and a sync separation circuit 6.

The A/D converter 1 samples the supplied analog television signal at a predetermined sampling period and converts it into a digital television signal. This digital television signal is applied to one input terminal of the low pass filter 2 and the subtractor 8.

As illustrated in FIG. 2, the low-pass filter 2 includes n delay units connected in series and each delaying a digital signal by one sampling period.
h . . . D, , +, k to (n+],) coefficient units that weight the inputs and outputs of these delay units in a predetermined manner.
z, *...k-+, and the weighting consists of a transversal filter with an adder ADR that adds the subsequent digital signals.

The active low-pass filter 2, as shown in FIG.
It has gain-frequency characteristics that eliminate color burst signals.

That is, if the analog-equivalent waveform of the digital signal supplied to the low-pass filter 2 is as shown in FIG. 5(A), the analog-equivalent waveform of the digital signal output from the low-pass filter supply 2 As shown in FIG. 3B, the color burst signal is removed and the flat portion spreads over the entire back porch.

The digital signal output from the low-pass filter 2 is subtracted by the corresponding digital value supplied from the regenerative DC level holding circuit 3 in the subtracter 4 . The result of this subtraction is supplied to the launch circuit 7 via the low-pass filter 5.

The low-pass filter 5 is composed of a transversal filter similar to the low-pass filter 2, and has a gain-frequency characteristic as illustrated in FIG. Reduces low-frequency noise caused by fluctuations on the time axis, which is a particular problem when the signal is a reproduced signal.

The synchronization separation circuit 6 detects a horizontal synchronization signal from the analog television signal supplied from the input terminal IN, and sends a latch pulse delayed by a predetermined time from the horizontal synchronization signal to the launch circuit 7.
supply to. This latch pulse is timed so that it appears within the back porch where the color burst has been removed and is flattened over most of its area, as illustrated in FIG. 4B. This timing adjustment may be fairly rough because the entire back porch is flattened.

Each time the launch circuit 7 receives a latch pulse from the sync separator circuit 6, it holds a new subtraction result supplied from the subtractor 4 via the low pass filter supply 5. Lunch circuit 7
The subtraction result held in is added to the digital signal output from the A/D converter 1 in the adder 8 .

As a result, the level of the digital signal output from the adder 8 is such that the level of the back porch is clamped to the content of the reproduction DC level holding circuit 3.

The digital signal output from the adder 8 is converted into an analog signal by the D/A converter 9 if various digital processing for image quality improvement is not performed at the subsequent stage, and the digital signal is sent from the output terminal OUT to the analog signal at the subsequent stage. It is supplied to the processing section.

Above, we have exemplified a configuration in which a low-pass filter 5 is installed between the subtracter 4 and the launch circuit 7 in order to deal with cases where there is large jitter such as in the case of a reproduced signal from a VTR. Since the level is flattened over the entire back porch and a considerable timing shift can be tolerated, the low-pass filter 5 can be omitted.

Alternatively, the subtracter 4 may be configured to operate only immediately before the appearance of the latch pulse.

Effects of the Invention As explained in detail above, the DC regeneration circuit of the present invention has the following effects:
A configuration in which a branch path is provided for the television signal path, a color burst signal is removed within this branch bus, a flat section within the back porch is expanded, and the level of this flat section is clamped to a predetermined reproduction DC level. Therefore, highly accurate DC reproduction can be performed while allowing a considerable timing error.

Further, since the DC regeneration circuit of the present invention is configured to perform DC regeneration based on digital processing, highly accurate DC regeneration is possible.

Furthermore, if the present invention is applied to a television receiver that performs various digital processing to improve image quality, the amount of hardware required for DC reproduction can be reduced by using the A/D conversion circuit for digital processing as is. It is particularly suitable in that it can be used.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a television signal DC regeneration circuit according to an embodiment of the present invention. 2 is a circuit diagram showing an example of the configuration of the low-pass filter 2 shown in FIG. 1, and FIG. 3 is a characteristic diagram showing an example of the gain-frequency characteristics of the low-pass filter 2 shown in FIG. Fig. 4 is a waveform diagram for explaining the operation of Fig. 1, Fig. 5 is a characteristic diagram showing an example of the gain-frequency characteristic of the low-pass filter 5 of Fig. 1, and Fig. 6 is a problem with the conventional circuit. FIG. 3 is a waveform diagram for explaining the points. 1...A/D converter, 2...Low pass filter for removing color burst, 3...Regenerating DC level holding circuit, 4...Subtractor, 5...Low pass filter, 6 ...Synchronization separation circuit, 7..Launch circuit. 8...Adder. Patent applicant: NEC Home Electronics Co., Ltd.

Claims (1)

[Claims] An A/D converter for converting an analog television signal into a digital signal; a low pass filter for removing a color burst signal from the converted digital signal; and a low pass filter for removing a color burst signal from the converted digital signal. an arithmetic unit that outputs a level difference between the level of the digital signal and a predetermined reproduction DC level; a holding circuit that holds the level difference within the appearance period of the back porch; 1. A DC reproducing circuit for a television signal, comprising: an arithmetic unit that adds to the output of a /D converter.