JPH0358232B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a noise reduction circuit for video signals, and is used, for example, in a helical scanning VTR.
In particular, when the input signal is at a small or medium level, the output signal frequency characteristics can be flattened, and the SN ratio can be improved.
An object of the present invention is to provide a noise reduction circuit that causes less deterioration in vertical resolution.

Prior Art and Its Problems FIG. 5 shows a block system diagram of an example of a conventional noise reduction circuit provided in a luminance signal reproduction system of a VTR, for example. In the figure, a signal a (FIG. 6A) input to terminal 1 is passed through high-pass filter 2, for example.
The signal b (Figure B) with a frequency of 1 MHz or higher is amplitude limited by the limiter 3 and becomes approximately a noise component e (Figure C).
After being determined as , the signal is supplied to the subtracter 5 via the coefficient circuit 4, where the noise component is subtracted from the signal a, and the signal d (D in the figure) is outputted from the terminal 6.

This had the problem that while noise components remained in the edge portion, reduced noise of 1MHz or less remained.

FIG. 7 shows a block system diagram of an example of a conventional H-correlated noise reduction circuit. In the figure, the signal input to terminal 1 is delayed by 1H in 1H delay circuit 7 and then supplied to subtracter 8, where it is subtracted from the input signal and then limited in amplitude by limiter 3 to become a substantially noise component. be done. A subtracter 5 subtracts the output of the limiter 3 from the input signal to obtain a signal with reduced noise components.

This circuit can sufficiently subtract noise components when there is line correlation, does not leave any noise components at the edge parts compared to the circuit shown in Figure 6, and can sufficiently reduce low-frequency noise. obtain. However, although theoretically the degree of improvement in the SN ratio can be improved by about 3 dB, in reality it can only be improved by about 1.5 to 2.0 dB due to deterioration due to the characteristics of the 1H delay circuit 7, for example.
As shown in Figure A, when the input signal is at a low level, the output signal has a pronounced comb-shaped characteristic, and the vertical resolution is degraded compared to when the input signal is at a high level and the output signal level is high (Figure B). There were some problems, such as:

FIG. 9 shows a circuit diagram of a circuit previously proposed by the present applicant in the patent application ``Video Signal Noise Reduction Circuit'' dated November 28, 1981. In the figure, a signal input to terminal 1 is passed through a feedback comb filter and equalizer 13 consisting of adders 9, 10, a 1H delay circuit 7, and a feedback path in which a high-pass filter 11 and a coefficient circuit 12 are inserted. , this signal is subtracted from the input signal by a subtracter 14, and the signal obtained through the slice circuit 15 and the output of the equalizer 13 are added together by an adder 16 and extracted.

This device can significantly improve the SN ratio, especially in the high frequency band, to about 6 to 10 dB, and also
As shown in FIG. 0C, when the input signal is at a high level, the output signal level can be high, and there is little deterioration in vertical resolution. However, when the input signal shown in FIG. 10B is at a medium level, and when the input signal shown in FIG. There were problems such as the vertical resolution being worse than the circuit shown in FIG. 7 due to the sharpness of the shape characteristics.

Means for Solving the Problems and Their Effects The present invention provides a first method that adds and extracts a signal obtained by delaying an input video signal for 1H period and the input video signal.
a comb filter, a second comb filter that subtracts the output of the first comb filter from the input video signal, and expands a relatively low level signal during recording from the second comb filter. an expansion circuit that compresses the output of the second comb filter during playback and adds the output to the output of the first comb filter and extracts the output; The above-mentioned problems are solved as a configuration consisting of the following, and one embodiment thereof will be described below with reference to the drawings.

Embodiment FIG. 1 shows a block system diagram of an embodiment of the circuit of the present invention. In the figure, the same components as in FIG. 9 are given the same numbers and their explanations will be omitted. This device has interlocking switches S ₁ and S ₂ between the subtracter 14 and the adder 16, a slice circuit 15 and a small to medium level expansion circuit 17 (which expands small to medium level signals below the slice level in the slice circuit 15). The other configuration is the same as the circuit shown in FIG. 9.

When recording, connect switches S ₁ and S ₂ to the Rec side.
Among the outputs of the feedback comb filter, small to medium level signals below the slice level of the slice circuit 15 are expanded by a small to medium level expansion circuit 17 and sent to the adder 1.
6, it is added to the output of the equalizer 13 and taken out. In this way, small to medium level signals are also extracted beyond the slice level, and as shown in the characteristics in FIG. 2, the vertical component signal from terminal 6 is extracted without loss above the slice level.
Compared to the characteristics of the illustrated circuit, it becomes flat above the slice level.

As a result, the frequency characteristics of the output signal when the input signal is at a noise level, a medium level, and a large level do not have a comb-shaped characteristic, but become flat as shown in Figure 3 B, C, and D, respectively, and the vertical resolution deteriorates. does not occur. Note that A in the figure shows the output signal level when the input signal is at an extremely low level below the noise level.
Has comb-shaped characteristics.

Furthermore, as shown in the characteristics of FIG. 2, when the vertical component input level is below the slice level, the rate at which that component is lost is small compared to the characteristics. Generally, the slice level is selected to be sufficiently small compared to the video signal level, so the slice circuit 15
The loss in the vertical component due to is hardly noticeable.

On the other hand, playback noise occurs after recording, so
During reproduction, switches S ₁ and S ₂ are connected to the PB side to reduce noise through the same circuit as the circuit shown in FIG. 9. By performing such signal processing, the deterioration of the vertical resolution can be reduced compared to the one shown in Figure 9, and the large degree of improvement in the S/N ratio of the feedback comb filter can be utilized, resulting in a significant S/N ratio improvement. obtain.

Note that a small-medium level compression circuit may be used instead of the slice circuit 15, and if the characteristics of the small-medium level expansion circuit 15 and the characteristics of the small-medium level compression circuit are appropriately selected, the loss of the video signal can be reduced to zero.

Also, between the small/medium level expansion circuit 17 and the switch _S1 , or between the small/medium level expansion circuit 17 and the switch S2 _.
If a reduction filter is connected between the subtractor 14
As shown in FIG. 4, the frequency characteristic of the output signal can be made flat in the reduction and comb-shaped in the high frequency range.

In addition, by appropriately selecting the cutoff frequency of the reduction filter, a high-pass comb filter can be realized, and when such a high-pass filter is used during recording, the band of the luminance signal can be widened, as is well known.
It is possible to obtain a reproduced signal with a wide band and a high SN ratio.

Effects As described above, the video signal noise reduction circuit according to the present invention includes a first comb filter that adds and extracts a signal obtained by delaying an input video signal by 1H period and the input video signal, and a second comb filter that subtracts the output of the first comb filter from the signal and extracts the signal; and a second comb filter that extracts the signal by subtracting the output of the first comb filter; The output signal is Vertical component loss does not occur above the slice level during compression, and as a result, the frequency response becomes flat without becoming a comb-like characteristic above the slice level, especially when the input signal is at low or medium levels. It has features such as flat output signal frequency characteristics, significantly improved S/N ratio, and little deterioration of vertical resolution.

[Brief explanation of drawings]

1 and 2 are a block system diagram and a vertical component input level versus vertical output level/input level ratio characteristic diagram of an embodiment of the circuit of the present invention, respectively, and FIG. 3 is for explaining the operation of the circuit of the present invention. Output signal frequency characteristic diagram. Figure 4 is an output signal frequency characteristic diagram when a low-pass filter is used during recording in the circuit of the present invention.
5 and 6 are a block system diagram and a signal waveform diagram for explaining operation of an example of a conventional circuit, respectively, and FIGS. 7 and 8 are a block system diagram and an output signal frequency characteristic diagram of another example of a conventional circuit, respectively. , 9 and 10 are a block system diagram and an output signal frequency characteristic diagram of still other examples of the conventional circuit, respectively. 1...Video signal input terminal, 6...Output terminal, 7
...1H delay circuit, 9, 10, 16...adder,
11...High-pass filter, 12...Coefficient circuit, 13
... Equalizer, 14 ... Subtractor, 15 ... Slice circuit, 17 ... Small and medium level expansion circuit, S ₁ , S ₂
...Switch.

Claims (1)

[Scope of Claims] 1. A first comb filter that adds and extracts a signal obtained by delaying an input video signal by 1H period and the input video signal, and a first comb filter that extracts a signal obtained by delaying the input video signal by a period of 1H; A second comb filter subtracts and extracts the output of the second comb filter, and a relatively low level signal during recording is expanded and added to the output of the first comb filter. Noise in a video signal, comprising: an expansion circuit for extracting the noise; and a circuit for compressing the output of the second comb filter during playback, adding it to the output of the first comb filter, and extracting the noise. reduction circuit. 2. The video signal noise reduction circuit according to claim 1, wherein the first comb filter is a feedback comb filter. 3. The video signal noise reduction circuit according to claim 1 or 2, wherein the expansion circuit is provided with a low-pass filter on its input side or output side.