JPH089334A - Noise reduction circuit - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to a noise reduction circuit using a 1H delay element, and an object thereof is to reduce the deterioration of vertical resolution while sufficiently reducing the noise in a low frequency band. . A decorrelation detection circuit 4 and a decorrelation control circuit 5 detect a decorrelation portion of a signal obtained by subtracting an input / output signal of a 1H delay element 2 by a subtraction circuit 3 to detect the decorrelation portion. By adjusting the level adjusting circuit 7 in accordance with the signal level, it is possible to increase the amount of attenuation of a small amplitude signal, which is a noise component, for attenuating the noise component including the uncorrelated signal from the input signal. Noise components are reduced in all bands from the frequency band to the high frequency band. On the other hand, in the low frequency band where the vertical resolution is conspicuous, it is possible to reduce the amount of attenuation that attenuates the noise component including the signal in the non-correlated portion from the input signal in the non-correlated portion of the large-amplitude signal. .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video tape recorder provided with a noise reduction circuit, and more particularly to a noise reduction circuit using a 1H delay element (H is a horizontal scanning period).

[0002]

2. Description of the Related Art In a two-head helical scan type video tape recorder (hereinafter referred to as VTR), an FM
A modulated luminance signal (hereinafter referred to as FM luminance signal) and F
The M luminance signal is mixed with the converted color signal converted into the low frequency band and recorded on the magnetic tape. Further, in order to prolong the recording time, high density recording is performed without providing a guard band between adjacent tracks. To avoid crosstalk interference from adjacent tracks during playback,
With respect to the luminance signal, inclined azimuth recording is performed, and with respect to the color signal, the rotation is switched every 1H and recording is performed, and then the adjacent crosstalk component is removed by a comb filter during reproduction. Especially for the luminance signal, the tilted azimuth recording alone is not sufficient.
As disclosed in Japanese Patent No. 5228, a noise reduction circuit is provided in the reproduction system to reduce adjacent crosstalk interference, reduce random noise, and improve the S / N ratio. The noise reduction circuit forms a comb filter by calculating signals around 1H using a 1H delay element, and reduces adjacent crosstalk components and noise components having no line correlation. However, since the noise reduction circuit calculates signals around 1H, it leads to deterioration of resolution in the vertical direction on the monitor screen. In this regard, Japanese Patent Publication No. 15228
In the gazette, a device for eliminating the comb filter characteristic in the low frequency band and achieving both vertical resolution deterioration and noise reduction is made.

[0003]

Since the above-mentioned conventional technique eliminates the comb filter characteristic in the low frequency band, the vertical resolution deterioration can be improved, but the noise removal capability in the low frequency band, which is visually noticeable, is reduced. There was a problem that it would decrease. Further, although the circuit processing is performed by dividing the band into a low frequency band and a high frequency band, the delay time of the calculation path is not necessarily the same, and the medium frequency band from the low frequency band to the high frequency band It is expected that the frequency shift of the zero point of the comb-shaped filter characteristic will occur, and the effect of reducing adjacent crosstalk and noise components will decrease.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a noise reduction circuit using a 1H delay element which constitutes a comb filter in a low frequency band and has little vertical resolution deterioration. It is in.

[0005]

As a first means, a signal obtained by subtracting signals around 1H is detected by a decorrelation detection circuit and a decorrelation control circuit to detect a decorrelation portion of the signal, and the decorrelation portion is detected. This is achieved by controlling the amount of attenuation for attenuating the noise component including the signal of the non-correlation part from the input signal according to the signal level of.

As a second means, a signal obtained by subtracting a signal around 1H is attenuated in a high frequency band by a low pass filter (hereinafter referred to as LPF), and then a limiter circuit limits the amplitude of a large amplitude signal. This is achieved by attenuating the low frequency band of the signal with a high-pass filter (hereinafter referred to as HPF) to make the band characteristic substantially flat and subtracting this signal from the input signal.

[0007]

In the first means, the small-amplitude signal, which is a noise component, is increased in attenuation amount by the decorrelation detection circuit and the decorrelation control circuit to attenuate the noise component including the uncorrelated signal from the input signal. The noise removal capability can be ensured because it acts on. On the other hand, in the non-correlation part of the large-amplitude signal where the vertical resolution is conspicuous, the decorrelation detection circuit and the decorrelation control circuit act to reduce the amount of attenuation that attenuates the noise component including the signal in the non-correlation part from the input signal. Therefore, deterioration of the vertical resolution can be reduced.

In the second means, the LPF is arranged in the preceding stage of the limiter circuit, so that the limiter gain in the low frequency band is equivalently higher than that in the high frequency band. Further, since the HPF arranged in the latter stage of the limiter circuit attenuates the low frequency band so that the frequency characteristic of the small amplitude signal becomes substantially flat, the noise component corresponding to the small amplitude signal is almost from the low frequency band to the high frequency band. It can be uniformly damped. On the other hand, since the limiter circuit limiter gain is higher than the high frequency band in the low frequency band of the large amplitude signal, the amount of attenuation in the low frequency band can be made smaller than the high frequency band, and the deterioration of the vertical resolution can be reduced.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIG.
FIG. 1 is a diagram showing an embodiment of a noise reduction circuit according to the present invention, in which 1 is an input terminal, 2 is a 1H delay element, 3 and 9 are subtraction circuits, 4 is a decorrelation detection circuit, and 5 is a decorrelation control circuit. , 6 is a limiter circuit, 7 is a level adjusting circuit, 8 is a delay circuit, 1
0 is an output terminal.

By subtracting the signal Y input from the terminal 1 and the signal Y _d delayed by 1H by the 1H delay element 2 in the subtraction circuit 3, nf _H (f _H is the horizontal synchronizing frequency, n = 0, 1,
2, ...) A comb filter with a zero point is constructed. This is (2n + 1) f _H / 2 (n = 0, 1, 2, ...
The adjacent crosstalk component including noise existing in *) and the non-correlated portion of the signal can be passed, and is called a C comb filter (hereinafter abbreviated as C comb). The output of the subtraction circuit 3 is sent to the decorrelation detection circuit 4 to detect decorrelation, and the detected signal is sent to the decorrelation control circuit 5. The decorrelation control circuit 5 controls the current flowing through the level adjusting circuit 7 to control the output level of the level adjusting circuit 7.

On the other hand, the output of the subtraction circuit 3 is limited by a limiter circuit 6 whose limiting level is set to a peak-to-peak value of noise. The level adjusting circuit 7 whose current is controlled by the decorrelation control circuit 5 controls the level of the output of the limiter circuit 6, and outputs only the noise component to the subtracting circuit 9. On the other hand, input terminal 1
The input signal is delayed by the delay circuit 8 by the delay time of the limiter circuit 6 and the level adjusting circuit 7, and the subtracting circuit 9
Send to. The subtraction circuit 9 subtracts the output of the level adjustment circuit 7 from the output of the delay circuit 8 and outputs it from the output terminal 10. This makes it possible to reduce the noise component from the input signal,
Further, the frequency deviation of the comb tooth characteristic can be reduced while being greatly limited.

Alternatively, the decorrelation detection circuit 4 detects the non-correlated portion of the large-amplitude signal, and the level adjustment circuit 7 is controlled via the decorrelation control circuit 5 to reduce the amount of attenuation in the non-correlated portion. Therefore, deterioration of the vertical resolution can be reduced. FIG. 2 shows the frequency characteristics at the output terminal 10. As shown by the curves 11 to 18, in the low frequency band, the large-amplitude comb that is the signal component of the uncorrelated portion can be made shallow (curves 11 and 12), and the small-amplitude comb that is the noise component can be made deep (curves 13 and 14). )it can. In the high frequency band, the depth gradually increases linearly from large amplitude to small amplitude (curves 15, 16, 17, 18). This makes it possible to suppress the deterioration of vertical resolution while enabling the removal of noise components in the low frequency band.

Further, by setting the delay time of the delay circuit 8 to be equal to the delay time generated in the subtraction circuit 3, the limiter circuit 6 and the level adjustment circuit 7, it is possible to eliminate the frequency deviation of the zero point of the comb characteristic. .

Next, the decorrelation detection circuit 4 and the decorrelation control circuit 5 will be described in detail. FIG. 3 is a block diagram of the decorrelation detection circuit 4. 19 is an input terminal, 20 is an LPF, 2
1 is a limiter circuit, 22 is a full-wave rectifier circuit, and 23 is an output terminal. From the output of the subtraction circuit 3 input from the input terminal 19, only the low frequency component is extracted by the LPF 20 and sent to the limiter circuit 21. If the maximum value of the signal level in the non-correlation part is 0 dB, the limiter circuit 21 assumes -10
The limiting level is set so that the amount of attenuation of about dB or more is reduced and the circuit forming the voltage-current conversion circuit 27 in the subsequent stage is not turned off. The output of the LPF 20 is limited by the limiter circuit 21 to a signal of about -10 dB or more. The input / output characteristics of the limiter circuit 21 are shown in FIG. If the input level is a signal less than about -10 dB as shown by the curve 24, the limiter shows a linear output characteristic, and the output level of the signal above -10 dB is limited. After that, the signal that is full-wave rectified by the full-wave rectifier circuit 22 is a non-correlated signal, and is output from the output terminal 23.

FIG. 5 is a block diagram of the decorrelation control circuit 5. 25 is an input terminal, 26 is a slicer circuit, 27
Is a voltage-current conversion circuit, and 28 is an output terminal. The decorrelation signal input from the input terminal 25 is sent to the slicer circuit 26. The input / output characteristics of the slicer circuit 26 are shown in FIG. In the slicer circuit 26, as shown by a curve 29, in order to prevent the attenuation amount of a level of about -20 dB or less from being switched between the low frequency band and the high frequency band, and to prevent the level adjusting circuit 7 from malfunctioning due to noise components. , Signal components of about −20 dB or less, so-called noise components are removed. The signal from which the noise component has been removed by the slicer circuit 26 is then converted into a current change by the voltage-current conversion circuit 27, and the current is sent to the level adjustment circuit 7 to control the amount of attenuation.

FIG. 7 shows a second embodiment of the present invention. Figure 7
The same components as those of FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. Reference numerals 30 and 31 are addition circuits. By adding the signal Y input from the terminal 1 and the signal Y _d delayed by 1H by the 1H delay element 2 in the adding circuit 30, (2n +
1) A comb filter having a zero point at f _H / 2 (n = 0, 1, 2, ...) Is constructed. nf _H (n = 0, 1, 2, ...
・ ・) Luminous comb filter (hereinafter abbreviated as Y comb) because only the fundamental and higher harmonics of the luminance signal present in
Call. The subtractor circuit 3 generates a C comb for subtracting the output of the adder circuit 30, that is, the Y comb from the input signal Y.
Further, the adder circuit 31 adds the output of the adder circuit 30, that is, the Y comb and the output of the subtractor circuit 3, that is, the C comb, and returns the input signal Y to the subtractor circuit 9. The delay time generated in the limiter circuit 6 and the level adjusting circuit 7 is calculated by the adding circuit 31.
The delay time in the subtraction circuit 9 can be adjusted by performing the addition again with. Therefore, the delay times of the two input to the subtraction circuit 9 can be matched, and the frequency deviation of the zero point of the comb tooth characteristic can be reduced.

FIG. 8 shows a third embodiment of the present invention. Figure 8
The same components as those of FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. 32 is an LPF, 33 is an HPF, and 34 is a level setting circuit. The output of the subtraction circuit 3 has a characteristic in which the high frequency band is attenuated by the LPF 32 from the low frequency band. Then, after passing through the limiter circuit 6, LPF3
The low-frequency band is attenuated by the HPF 32 having the same cut-off frequency and order as 2, so that the low-frequency band and the high-frequency band become substantially flat. By making the cutoff frequencies and the orders of the LPF 32 and HPF 33 equal, the delay characteristics in the low frequency band and the high frequency band can be made substantially the same. afterwards,
The level adjustment setting circuit 34 sets the attenuation amount and sends it to the subtraction circuit 9. By subtracting the output of the level setting circuit 34 from the input signal in the subtraction circuit 9, the noise component in the low frequency band can be removed. Moreover, since the amount of attenuation in the non-correlated portion of the low frequency band can be reduced, it is possible to reduce the deterioration of vertical resolution. Furthermore, LPF32,
Since the delay times generated in the limiter circuit 6, HPF 33, and level setting circuit 34 can be matched by the delay circuit 8, it is possible to reduce the frequency deviation of the zero point of the comb tooth characteristic.

Next, a more detailed description will be given with reference to FIG.
FIG. 9 shows the characteristics of each part in the block diagram shown in FIG.

First, the case where a small amplitude signal is input will be described. As shown in FIG. 9A1, the output of the subtraction circuit 3 is a C comb having a zero point at nf _H. LP
F32 has a characteristic as shown in 35 of (B1), and the high frequency band is attenuated. In the case of a small amplitude signal, the limiter circuit functions as a linear amplifier, so the signal (B1) is input to the HPF 33. HPF33 is LPF3
Since the cutoff frequency and the order are the same as those of 2, the output of the HPF 33 has the same level in the low frequency band and the high frequency band as shown in (C1). Since this signal is subtracted from the input signal, it is possible to have comb-teeth characteristics in the entire band as shown in (D1), and noise components can be removed even in the low frequency band.

Next, the case where a large amplitude signal is input will be described. Similarly to the small amplitude signal, the output of the subtraction circuit 3 is
As shown in FIG. 9 (A2), a C comb having a zero point at nf _H is formed, and the output of the LPF 32 has a characteristic in which the high frequency band is attenuated as shown in (B2). When a large-amplitude signal is input, the limiter circuit 6 operates as a limiter, so that the low-frequency band and the high-frequency band have the same level as shown in (C2). By passing this signal through the HPF 33, the level of the low frequency band becomes smaller than that of the high frequency band as shown in (D2). Therefore, by subtracting this signal from the input signal, as compared with the high frequency band as shown in (E2), The depth of the Y comb in the low frequency band can be made shallow, and vertical resolution deterioration can be reduced.

[0021]

As described above, according to the present invention, 1
The decorrelation part of the signal obtained by subtracting the signals before and after H is detected, and the amount of attenuation for attenuating the noise component including the decorrelation part signal from the input signal is controlled according to the signal level of this decorrelation part. As a result, in the case of a small-amplitude signal that is a noise component, it is possible to increase the amount of attenuation that attenuates the noise component including the uncorrelated signal from the input signal, and the noise component in all bands from the low frequency band to the high frequency band. Can be reduced. On the other hand, in the low frequency band where the vertical resolution is conspicuous, it is possible to reduce the amount of attenuation that attenuates the noise component including the signal in the non-correlated portion from the input signal in the non-correlated portion of the large-amplitude signal. be able to. Further, since the delay time generated in the circuit for extracting the noise component is corrected, the frequency shift of the zero point of the comb-teeth characteristic can be eliminated, so that the noise component can be sufficiently reduced.

Further, the signal obtained by subtracting the signal around 1H is supplied to the limiter circuit via the LPF, and then the HPF is supplied.
Even in the case of a small-amplitude signal, which is a noise component, the amount of attenuation for attenuating the noise component including the uncorrelated signal from the input signal can be increased even in the low-frequency band where vertical resolution deterioration is noticeable. Since the limiter gain is high, the amount of attenuation for attenuating the noise component including the signal of the non-correlated portion from the input signal can be reduced, and the deterioration of the vertical resolution can be reduced. Further, since the delay time generated in the circuit for extracting the noise component is corrected, the frequency shift of the zero point of the comb-teeth characteristic can be eliminated, so that the noise component can be sufficiently reduced.

[Brief description of drawings]

FIG. 1 is a block diagram of a noise reduction circuit according to an exemplary embodiment of the present invention.

FIG. 2 is a frequency characteristic diagram at an output terminal.

FIG. 3 is a block diagram of a decorrelation detection circuit.

FIG. 4 is an input / output characteristic diagram of a limiter.

FIG. 5 is a block diagram of a decorrelation control circuit.

FIG. 6 is an input / output characteristic diagram of a slicer circuit.

FIG. 7 is a block diagram of a noise reduction circuit according to another embodiment of the present invention.

FIG. 8 is a block diagram of a noise reduction circuit according to another embodiment of the present invention.

FIG. 9 is a characteristic diagram of each part for explaining the operation of FIG.

[Explanation of symbols]

 2 ... 1H delay element, 4 ... decorrelation detection circuit, 5 ... decorrelation control circuit, 6, 21 ... Limiter circuit, 7 ... Level adjustment circuit, 20, 32 ... Low pass filter, 22 ... Full wave rectification circuit, 26 ... Slicer Circuit, 27 ... Voltage-current conversion circuit, 33 ... High-pass filter, 34 ... Level setting circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuyuki Watanabe 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Ltd.

Claims (8)

[Claims] 1. A noise reduction circuit for reducing a non-correlated noise component by utilizing the correlation between video signals before and after 1H (1H is an abbreviation for horizontal scanning period). An arithmetic circuit, a decorrelation detection circuit that detects a decorrelation portion from the output of the first arithmetic circuit, and a decorrelation control that outputs a decorrelation signal according to the decorrelation detection signal output from the decorrelation detection circuit. A circuit, a limiter circuit that limits the signal amplitude of the output of the first arithmetic circuit, and receives a decorrelation control signal output from the decorrelation control circuit and adjusts the level of the output signal of the limiter circuit accordingly. A noise including a level adjusting circuit, a delay circuit that delays an input signal by a certain time, and a second arithmetic circuit that calculates an output signal of the level adjusting circuit and an output signal of the delay circuit. Low Circuit. 2. The first arithmetic circuit is a first subtraction circuit that subtracts an output signal Y _d from an input signal Y of a 1H delay element to obtain Y−Y _d , and the second arithmetic circuit is 2. The noise reduction circuit according to claim 1, comprising a second subtraction circuit for subtracting the output of the level adjustment circuit from the output of the delay circuit. 3. A noise reduction circuit for reducing a non-correlated noise component by utilizing the correlation between video signals before and after 1H, wherein Y + Y _d and Y−Y _d are calculated by using signals around 1H. One arithmetic circuit, a second arithmetic circuit for arithmetically operating two outputs of the first arithmetic circuit, a non-correlation detection circuit for detecting non-correlation of output signals of the first arithmetic circuit, and the non-correlation detection A decorrelation control circuit that outputs a decorrelation control signal according to a decorrelation detection signal output from the circuit, a limiter circuit that limits the signal amplitude of the output of the first arithmetic circuit, and an output from the decorrelation control circuit A level adjusting circuit that receives the generated decorrelation control signal and adjusts the level of the output signal of the limiter circuit in accordance therewith; and a third operation that calculates the output signal of the level adjusting circuit and the output of the second arithmetic circuit. Characterized by having a circuit Noise reduction circuit. Wherein said first arithmetic circuit includes a first adder circuit for obtaining a Y + Y _d by adding the input signal Y and the output signal Y _d of 1H delay elements, the addition of the input signal Y of the first is composed from the first subtraction circuit subtracts the output signal Y + Y _d of the circuit to obtain a Y-Y _d, the second operation circuit outputs Y + Y _d and the first subtraction circuit of the first adder circuit a second adder circuit for adding the output Y-Y _d, the third arithmetic circuit constituting from the second subtraction circuit for subtracting an output of said level adjusting circuit from the output of the second adder circuit The noise reduction circuit according to claim 3, wherein: 5. The decorrelation detection circuit and the decorrelation control circuit at least include a low-pass filter for limiting a signal pass band to a low frequency band only, a limiter circuit for limiting a signal amplitude, and a predetermined signal level or less. 2. A slicer circuit for removing the signal of FIG.
Alternatively, the noise reduction circuit according to claim 3. 6. A noise reduction circuit for reducing uncorrelated noise components by utilizing the correlation between video signals before and after 1H, and a first arithmetic circuit for arithmetically operating signals around 1H and the first arithmetic operation. A low-pass filter for attenuating the high frequency band of the output signal of the circuit, a limiter circuit for limiting the amplitude of the output signal of the low-pass filter, a high-pass filter for attenuating the low frequency band of the output of the limiter circuit, and an output of the high-pass filter A level setting circuit that adjusts the signal level to a specified level, a delay circuit that delays an input signal by a certain fixed time, and a second arithmetic circuit that calculates the output signal of the level adjustment circuit and the output signal of the delay circuit. A noise reduction circuit characterized by having. 7. The first arithmetic circuit is a first subtraction circuit that subtracts an output signal Y _d from an input signal Y of a 1H delay element to obtain Y−Y _d , and the second arithmetic circuit is 7. The noise reduction circuit according to claim 6, comprising a second subtraction circuit for subtracting the output of the level adjustment circuit from the input signal Y. 8. The noise reduction circuit according to claim 6, wherein the cut-off frequency and the order of the low-pass filter and the high-pass filter are set to be the same.