KR900003078B1 - Noise reduction circuit for video signal - Google Patents

Abstract

The circuit for improving the S/N ratio and the resolution ratio of the vertical direction is characterised by a first (10), a second (9) , and a third adder (16). The first adder (10) adds the image signal extracted from a 1H delay circuit (7) and an input signal. The second adder (9) adds the input signal and an output of a counter (12) receiving the output signal of a high band filter (11). The third adder (16) performs the adding function according to the record or playback mode. When in the record mode, the signal from an equaliser (13) and the signal expanded by a circuit (17) are added, but the signal compressed by a circuit (15) is added when in the playback mode.

Description

Noise reduction circuit of video signal

1 and 2 are each a block diagram of one embodiment of a circuit according to the present invention and a characteristic diagram of a vertical component input level to a vertical output level / input level ratio.

3A to 3D are characteristic diagrams of output signal frequencies for explaining the operation of the circuit of the present invention.

4 is a characteristic diagram of an output signal frequency when a low pass filter is used during recording in the circuit of the present invention.

5 and 6A to 6D are block diagrams and signal waveform diagrams for explaining the operation of one embodiment of a conventional circuit, respectively.

7 and 8A and 8B are block diagrams and other characteristics of output signal frequencies of another example of a conventional circuit, respectively.

9 and 10A to 10C are block diagrams and other characteristics of output signal frequencies of another example in which a conventional circuit is changed.

* Explanation of symbols for main parts of the drawings

1: Video signal input terminal 6: Output terminal

7: 1H delay circuit 9, 10, 16: adder

11: high pass filter 12: counting circuit

13: equalizer 14: subtractor

15: slice circuit 17: small, medium level expansion circuit

S ₁ , S ₂ : switch

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise reduction circuit of a video signal, for example, used in a spiral scan type VTR. In particular, the input signal can smooth the characteristics of the output signal frequency at a low level and a medium level. It is characterized in that to provide a noise reduction circuit with small degradation of the vertical resolution.

5 shows an example block diagram in a conventional noise reduction circuit provided in a luminance signal reproducing system of a VTR. In Fig. 5, the signal a (Fig. 6a) entering the terminal 1 becomes a signal b (Fig. 6b) at a frequency of 1 MHz or more, for example, in the high pass filter 2, and the amplitude is limited in the limiter 3. After approximately the noise component c (FIG. 6C) is supplied to the subtractor 5 through the counting circuit 4, the noise component is subtracted from the signal a to become the signal d (FIG. 6D) to form the terminal 6 It is taken out from.

This had the problem that the noise component remained in the edge while the low-pass noise below 1MHz remained.

7 is a block diagram showing an example of a conventional H-correlation noise low circuit. In FIG. 7, the signal entered into the terminal 1 is delayed by 1H in the 1H delay circuit 7 and supplied to the subtractor 8, where it is subtracted from the input signal and the amplitude is limited in the limiter 3 so as to be approximately a noise component. do. In the subtractor 5, the output of the limiter 3 is subtracted from the input signal, and a reduced cytin signal is taken out to reduce the noise component.

This sufficiently reduces the noise component when there is a line correlation, leaving no noise component at the edges compared to the circuits of 6a to 6d, and sufficiently reducing the low pass noise. However, the degree of improvement of the SN ratio is logically about 3 dB, but is actually only 1.5 to 2.0 dB due to deterioration due to the characteristics of the 1H delay circuit 7, for example, as shown in FIG. 8A. When the input signal is at a small level, the output signal has a comb-shaped characteristic, and there is a problem such that the vertical resolution is deteriorated as compared with when the input signal is at a large level and the output signal level is large (Fig. 8B).

9 shows a circuit diagram of a circuit which the applicant first proposed as a patent application "Noise Reduction Circuit of Image Signal" of November 28, 58, 58. 9, a feedback comb-type filter comprising a return path into which an adder (9, 10), a 1H delay circuit (7), a high pass filter (11), and a counting circuit (12) are inserted into the signal entering the terminal (1); Passed through the equalizer 13, this signal is subtracted from the input signal by the subtractor 14, and the signal obtained through the slice circuit 15 and the output of the equalizer 13 are added by the adder 16 and taken out. It is configured to.

This greatly improves the SN ratio in the high frequency band by, for example, about 6 to 10 dB, and as shown in FIG. 10C, when the input signal is at a large level, the output signal level is increased, and the degradation in vertical resolution is small. However, when the input signal shown in FIG. 10B is at a medium level and when the input signal shown in FIG. 10C is at a low level, the output signal has comb-shaped characteristics, and the vertical resolution is deteriorated. There was a problem such that the vertical resolution deteriorated as compared with the circuit of FIG. 7 only by sharpening.

The present invention provides a first comb filter for adding and extracting a signal obtained by delaying an input video signal for 1H period and the input video signal, and a second comb filter for subtracting and outputting the output of the first comb filter from the input video signal. A filter, an extension circuit which expands and extracts a relatively small level of the output of the second comb-type filter at the time of recording and adds the output of the first comb-type filter, and outputs the second comb-type filter at the time of reproduction. Although the above-mentioned problem is solved by the structure which consists of a circuit which compresses and adds and takes out the output of the said 1st comb-type filter, this invention is demonstrated with reference to drawings and its Example below.

FIG. 1 is a block diagram showing an embodiment of a circuit according to the present invention. In FIG. This is because the switch S ₁ , S ₂ and the slice circuit 15 and the small and medium level expansion circuit 17 (slicing circuit 15) are smaller than the slice level between the subtractor 14 and the adder 16. Is configured to extend a medium level signal), and the rest of the configuration is the same as that of the circuit of FIG.

During recording, connect the switches S ₁ and S ₂ to the Rec side. During the output of the feedback comb filter, the small to medium level signals below the slice level of the slice circuit 15 are extended in the small and medium level expansion circuit 17 and together with the output of the equalizer 13 in the adder 16. It is added and taken out. In this way, small to medium level signals are also taken out beyond the slice level, and as shown in characteristic I of FIG. 2, signals of the vertical component from the terminal 6 are taken out without loss of the slice level above. Compared to the characteristic II of the circuit shown in Fig. 7, the flatness becomes higher than the slice level.

From this, when the input signal is noise level, medium level, and large level, the frequency characteristic of the output signal does not become a comb-like characteristic, but becomes flat as shown in FIGS. 3b, 3c, and 3d, respectively, Deterioration does not occur. 3A is an output signal level when the input signal is at an extremely small level that is less than or equal to the noise level, and has a comb-tooth shaped characteristic.

As shown in the characteristic I of FIG. 2, when the vertical direction component input level is less than or equal to the slice level, there is less ratio of the component losing than the characteristic II. In general, since the slice level is selected sufficiently small with respect to the video signal level, there is almost no loss of vertical components due to the slice arc 15.

On the other hand, since the reproduction noise is generated after recording, at the time of reproduction, the switches S ₁ and S ₂ are connected to the PB side and the noise is reduced by intervening in the same path as in the circuit of FIG. By performing such signal processing, degradation in the vertical resolution can be reduced as compared with that in FIG. 9, and the SN ratio of the feedback comb filter can be greatly improved, and the SN ratio can be greatly improved.

Instead of the slice circuit 15, a small and medium level compression circuit may be used. If the characteristics of the small and medium level expansion circuit 17 and the characteristics of the small and medium level compression circuit are properly selected, the loss of the video signal can be eliminated.

When the low pass filter is connected between the small and medium level expansion circuit 17 and the switch S ₁ , the high pass component of the output of the subtractor 14 can be reduced, and the frequency characteristic of the output signal is shown in FIG. As shown in the figure, it is flat in the low range and comb-like in the high range.

In addition, by appropriately selecting the cutoff frequency of the low pass filter, a high pass comb filter can be realized, and when such a high pass filter is used for recording, as is well known, the band of the luminance signal can be widened so that the band is wide and the SN ratio is A high reproduction signal can be obtained.

As the effect of the present invention, as described above, the noise low-pass circuit of the video signal according to the present invention comprises a first comb filter for adding and extracting a signal obtained by delaying the input video signal for 1H period and an input video signal, and an input video signal. A second comb filter for subtracting the output of the first comb-type filter from the second comb filter, and a relatively small level of the output of the second comb-type filter at the time of recording is extended and added to the output of the first comb-type filter And a circuit for compressing the output of the second comb-type filter and adding it to the output of the first comb-type filter during regeneration, so that the loss of the vertical component of the output signal is compressed. It does not occur above the slice level of, and from this the frequency characteristic becomes flat and does not become a comb-like characteristic above the slice level. In particular, the input signal has the advantages of flattening the output signal frequency characteristics at a low level and a medium level, greatly improving the SN ratio, and less deterioration in the vertical resolution.

Claims (3)

A delay circuit for delaying an input video signal for 1H period, a first addition circuit 10 for adding the video signal extracted from the delay circuit 7 and the input video signal, and the first addition circuit 10; Obtained by adding the high-pass filter 11 to which the output signal of?, The counting circuit 12 to which the output signal of the high-pass filter 11 is supplied, and the signal extracted from the counting circuit 12 and the input video signal A second adder circuit 9 for supplying a signal to the input side of the delay circuit 7, an equalizer 13 to which an output signal of the first adder circuit 10 is supplied, and the input image signal A subtraction circuit 14 for subtracting the output signal taken out from the equalizer 13, an extension circuit 17 for extending a relatively small level of the output signal of the subtraction circuit 14 at the time of recording, and at the time of reproduction, Compression (or slice) circuit 1 for compressing the output signal of subtraction circuit 14 5) and a signal obtained by adding the output signal of the expansion circuit 17 and the output signal of the equalizer 13 at the time of recording, and outputting the output signal of the compression circuit 15 and the equalizer (at the time of reproduction). And a third addition circuit (16) for outputting a signal obtained by adding the output signal of step (13). 2. The delay circuit (7), the first addition circuit (10), the second addition circuit (9), the high pass filter (11) and the counting circuit (12) constitute a feedback comb filter. Noise reduction circuit of video signal. 3. The noise reduction circuit according to claim 1 or 2, wherein the expansion circuit (17) is formed by providing a low pass filter on an input side or an output side thereof.

Images