JPH0640418B2 - Noise eliminator - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal noise eliminator which is applied to a VTR, a video disc device or the like to reduce a noise component in a minute specific area contained in a video signal. is there.

2. Description of the Related Art As a conventional noise eliminator, for example, JP-A-58-96 is used.
No. 473 publication.

FIG. 4 is a block diagram of this conventional noise eliminator, in which 1 is an input terminal of a video signal and 2 is an input terminal 1.
Differentiating circuit for separating high frequency components from the video signal input from (which may have a filter characteristic that cuts off the low frequency band and passes the high frequency band of a specific band), 3 is an output signal of the differential circuit 2 An amplifier for amplifying, 4 is an amplitude limiter, 5 is an attenuator for amplifying the signal amplified by the amplifier 3 and limited to a constant amplitude by the amplitude limiter 4 to an appropriate level, and 6 is input from the input terminal 1. A mixer for mixing the video signal and the output signal of the attenuator 5 in opposite phases, 7 is an output terminal for outputting the output signal of the mixer 6, that is, a video signal with reduced noise, 8 is an operation of the amplitude limiter 4. Is a charging / discharging circuit for increasing the charging / discharging current of the differentiating circuit 2.

FIG. 5 is a specific circuit diagram of the differentiating circuit 2, the amplitude limiter 4, and the charging / discharging circuit 8. The capacitor 9, the resistor 10 constitutes the differentiating circuit 2, and the resistor 11, the transistors 12, 13 and the voltage source 14 are The amplitude limiter 4 is formed, the resistor 15 and the transistor 16 form a charging circuit, and the resistor 17 and the transistor 18 form a discharging circuit. The voltage source 19 is for properly biasing the output of the charging / discharging circuit 8, and can also serve as the input bias power source of the amplifier 3.

Regarding the conventional noise eliminator configured as described above,
This will be described below with reference to FIG. Here, for the sake of simplicity, it is assumed that a step signal that changes to the level V _A at t = 0 as shown in FIG. 6A is used, and a minute level high-frequency noise N is superposed on the step signal.
Now, assuming that the capacitance value of the capacitor 9 is C and the resistance value of the resistor 10 is R, the transfer characteristic of the differentiating circuit 2 is SRC / (SRC + 1) (S is a Laplace transform variable). This differentiating circuit is shown in FIG.
When the step signal shown in FIG. 6 is input, the signal shown by the solid line in FIG. 6b is output when the charge / discharge circuit does not operate. That is, when the input signal changes stepwise by a constant level, the output of the differentiating circuit also changes by the same level, and then the discharge or charging current flows according to the positive or negative of the change in the input signal, and the output potential of the differentiating circuit changes. The resistance of the resistor 10 changes toward the potential of the reference point to which one end of the resistor 10 is coupled. Note that, here, the case where a current flows from the capacitance 9 to the resistor 10 is expressed as discharge, and the case where a current flows to the capacitance 9 on the contrary is expressed as charge. The charging / discharging time constant in this case is RC
Is. Then, the waveform when the differential circuit output signal is amplified and the amplitude is limited is as shown by the solid line in FIG. 6c. If the amplitude-limited level of the input signal is V _B and the amplification degree of the amplifier 3 is K, the output waveform of the amplitude limiter 4 is amplitude-limited at the portion where the output of the differentiating circuit 2 exceeds V _B. The level becomes a KV _B flat signal. The time width of this flat portion is determined by the change level of the input signal and the time constant of the differentiating circuit. For example, at time t = 0, level V
Assuming a step signal that changes by _A , the output voltage of the differentiation circuit Is And the time t at which this reaches the amplitude limit level V _B
1 becomes t ₁ = RC ln (V _B / V _A ) ... (2). That is, the amplitude of the differential signal is limited by this time interval. The solid line in FIG. 6d is the output waveform obtained by attenuating the output whose amplitude has been limited to a predetermined level by the attenuator 5 and mixing by the mixer 6 in a phase opposite to the input signal.

Next, a high-frequency noise N is added to the input signal as shown in FIG. 6a.
Considering the case where is superimposed, noise N is also superimposed on the output of the differentiating circuit according to its pass band. This noise N
The part of the amplitude limiter 4 which is not limited is shown in FIG. 6c.
, And is canceled in the mixer 6.

However, since the limited part of this noise component is clipped and noise cannot be extracted, the noise is not canceled at the output of the mixer 6, and the noise becomes as shown by N ′ in FIG. 6D. Moreover, since the limited portion has a constant DC level, the DC component in the output of the mixer 6 decreases, in other words, the signal component decreases. In short, this noise eliminator removes noise in the flat portion of the input signal, but has no noise elimination effect in the portion where the DC level changes and the portion having a large amplitude AC component, and conversely the signal component. Has the drawback of being reduced. Furthermore, the differentiating circuit 2
The time width t _{1 of the} portion where the output signal of the signal is limited and becomes flat is proportional to the time constant RC of the differentiating circuit 2, and RC must be increased in order to pass low-frequency noise. Therefore, the time width t ₁ gets bigger.

On the other hand, when the charge / discharge circuit 8 operates, the absolute value of the output level of the differentiating circuit 2 becomes equal to the limiter level V of the amplitude limiter 4.
_When it exceeds _B , the charging / discharging circuit 8 operates and the differentiation circuit 2
When the output of is positive, the discharge circuit is operated, and when it is negative, the charging circuit is operated to increase the charge / discharge speed as shown by the broken line in FIG. 6d. As a result, the level V _B is reached at time t ₂ which is shorter than time t ₁ . When the output level of the differentiating circuit 2 reaches V _B , the discharge has a time constant RC determined by the differentiating circuit 2, and the output of the amplitude limiter 4 is limited to the time t ₂ as shown by the broken line in FIG. 6c. Will be taken out.

The limiter characteristic of the amplitude limiter 4 shown in FIG.
It is determined by the conduction and interruption characteristics of 1 and the transistors 12 and 13, and when the transistor 12 or 13 is conducting, the output of the amplitude limiter is limited to a constant level, and at the same time, the resistor 15, the transistor 16 or the discharging circuit which constitutes the charging circuit is constituted. Either the resistor 17 or the transistor 18 operates.

Problems to be Solved by the Invention However, in the above configuration, even if the charging / discharging current of the charging / discharging circuit 8 is increased and t ₂ is sufficiently reduced, the sixth
There remains a component that attenuates from the level of KV _B toward zero as shown in FIG.
That is, in the VTR, the S / N at the edge portion is originally bad, and although there is a demand to improve the SNR as much as possible, the conventional noise eliminator adversely deteriorates the S / N at this portion. Further, if the charging / discharging current is increased to reduce t ₂ , or the time constant of the differentiating circuit 2 is increased to remove noise in a lower frequency band,
Since the charging / discharging current flowing to the input terminal 1 via the differentiating circuit 2 becomes large, the influence on the input signal becomes large, and the output signal easily becomes distorted, which makes the design very difficult. It was

In view of such a point, the present invention does not cause signal loss in a high level signal such as the step signal of FIG. 6a, has a large noise removal effect at the edge portion, and removes even low frequency band noise. Even so, it is an object of the present invention to provide a noise elimination device for video signals with extremely little waveform distortion.

Means for Solving the Problems The present invention relates to a difference circuit for extracting a change amount of an input signal per predetermined time, and when the input level is small, the output is approximately proportional to the input or non-linearly compressed according to the input level. A non-linear circuit whose output is sufficiently small or zero when the level is large, a delay circuit which delays the output of this non-linear circuit by the predetermined time, and the non-linear circuit which adds the output of this delay circuit and the output of the difference circuit. It is a noise eliminator comprising an adder input to a circuit, a multiplier that multiplies an output of the nonlinear circuit by a predetermined value, and a subtracter that subtracts an output of the multiplier from a signal input to the difference circuit.

Operation According to the present invention, when the input signal level is small, the output signal at the output terminal of the non-linear circuit exhibits a high-pass characteristic having a time constant determined by the gain of the non-linear circuit and compresses when the input signal level increases. It exhibits amplitude limiting characteristics, and when the input level becomes higher, the output level becomes lower, and when the input signal has a large amplitude, the signal is eliminated, that is, the S / N is not degraded and the output is obtained. It has a noise removal effect, and has a low S / N ratio, and even if the noise is removed even in the low frequency region, the noise is removed with less waveform distortion.

First Embodiment FIG. 1 shows a block diagram of a noise removing device according to a first embodiment of the present invention. In FIG. 1, reference numeral 20 is an input terminal of a video signal digitized at a sampling period T, and 21 is a change per unit time n times (n is a positive integer) the sampling period T of the digitized video signal. A delay circuit 2 for extracting a minute and having a delay time nT
2 and a subtractor 23 that subtracts the output signal from the input signal of the delay circuit 22. Reference numeral 24 denotes a non-linear circuit, which exhibits a substantially proportional or monotonically increasing characteristic when the input level is small, and exhibits a negative characteristic in which the output level conversely decreases and becomes zero when the input level increases. 25 is a difference circuit 21
, A delay circuit having the same delay time nT as that of the delay circuit 22 constituting the above, 26 is an adder for inputting to the nonlinear circuit 24 for adding the output signal of the difference circuit 21 and the output signal of the delay circuit 25,
Is a multiplier that multiplies the output signal of the non-linear circuit 24 by a predetermined multiplier, 28 is a subtractor that subtracts the output signal of the multiplier 27 from the input signal of the difference circuit 21, 29 is an output terminal, and the output signal of the subtractor 28, that is, noise is reduced. And outputs the digitized video signal.

The operation of the noise elimination characteristic of the present embodiment configured as described above will be described below by using the Z-transform equation representing a discrete system.

First, for simplification, it is assumed that the difference circuit 21 has a delay time T (n = 1) of the delay circuit 22 and extracts a change per unit time T of the input signal. As a result, the delay circuit 2
5 also delays the signal by one unit time T. The coefficient by which the signal is compressed by the non-linear circuit 24 is R
Let K be the multiplier multiplied by the multiplier 27.
Here, using the transfer function H (Z) of the noise eliminator shown in FIG. 1 and the above R, K and Z ⁻¹ (unit delay operator in Z conversion), R is regarded as a constant value and the Z conversion formula is obtained. When expressed by Becomes Since the second term on the right side of the above equation is a high-pass filter, H (Z) has a characteristic of attenuating only the high band. The time constant at this time is determined by R.

FIG. 2 shows an example of the non-linear characteristic R with a solid line, where the input signal level of the non-linear circuit 24 is V _i and the output signal level is V ₀ . Here, V ₀ = RV _i . In digital signal processing, ROM (read-only memory) can be used to obtain an arbitrary R characteristic. The broken line is another example of R and is also a polygonal line approximation characteristic of the characteristic shown by the solid line. The broken line characteristic can be easily obtained by a multiplier or an adder / subtractor and a switching circuit. The operation will be described below by using this polygonal line characteristic. The broken line characteristic is V _i <V _B and R = a, V
_{When B} V _i <V _C , V ₀ = aV _B , and when V _i V _C , V ₀ = 0
Then, when t = nT <0, 0 at the input terminal 20, t = nT
When a step signal of V _S is input at 0, the difference circuit 2
At 1 output, a signal V _S appears when t = 0, and a signal 0 appears when t ≠ 0. Here, when V _S <V _B , the output of the nonlinear circuit 24 is 0 when t <0 and a ^{n + 1} V _S when t = nT0. This is a signal that attenuates a times at every time T when a <1, and the time constant is −T / l in comparison with the equation (1).
na. On the other _hand, when _{V B <V S <V C} , the non-linear circuit 24 outputs become aV _B at t = 0, hereinafter, the signal decays at a times every time T. Further, when V _S > V _C, the output of the nonlinear circuit 24 is always 0, and no output is generated.

The characteristic feature of the nonlinear circuit 24 of the present invention is that it is proportional to a minute input signal that should be regarded as noise, exhibits compression characteristics when it exceeds a predetermined value, and indicates a large input signal that should be regarded as a signal. Is when the output becomes 0. here,
The compression characteristic is not particularly indispensable, and it may be a characteristic in which the compression ratio is proportional to a constant input level, and the standing-out is zero for signals higher than that. However, in this case, since the case of removing noise and the case of not removing noise are rapidly changed with respect to the change of the input level, there is a possibility that unnaturalness may occur due to the input signal. Therefore, as shown by the solid line in FIG. Characteristics are desirable,
The characteristics shown by the broken line alleviate the drawbacks. Incidentally, in the characteristic of Figure 2 the solid line R, the input signal level V _i output is maximum and V _D, the output signal of the nonlinear circuit 24 is V _D
The following minute input signals are attenuated with a time constant of −T / lnR, and input signals of V _{D and} above are instantaneously compressed to a level determined by the nonlinear characteristic R, and then, when −T / lnR. Decays with a constant. This compressed level decreases as the input level increases.

As described above, according to the present embodiment, when the input level is small, the output is substantially proportional to or compressed by the input, and when the input level is large, the output is sufficiently small or zero. A noise removing device is constructed by providing a noise component from the output of the non-linear circuit, which is provided between the output of the adder for adding the outputs of the differential circuit and the delay circuit for extracting the variation of the hit and the input of the delay circuit. As a result, no output is generated in the non-linear circuit for an input signal that changes at a large level, and the signal is eliminated at the edge portion of the signal, that is, S
/ N is not deteriorated, S / N is improved up to just before the edge, and its operation is not affected by the time constant of the circuit. Even if noise in the low frequency band is removed, waveform distortion is reduced. You can Moreover, since it is digital signal processing,
The response is instantaneous and the analog-like transient characteristics and signal distortion that accompany signal compression do not occur.

FIG. 3 shows the difference circuit 21 used in the embodiment of the present invention shown in FIG. 1 provided with an amplitude limiter 30. In the figure, the amplitude limiter 30 limits the output level of the difference circuit composed of the extension circuit 22 and the subtractor 23. The input / output dynamic range of the adder 26 in the subsequent stage and the input dynamic range of the nonlinear circuit 24 are shown. Can be made smaller. That is, the output level of the delay circuit 25 is limited to a constant value, for example, aV _B by the non-linear circuit 24, and the output becomes a constant value of zero when the input level of the non-linear circuit 24 exceeds a certain value (V _E ). As a result, when the input from the difference circuit 21 to the adder 26 exceeds a constant value aV _B + V _E , the output of the non-linear circuit 24 becomes zero, and the difference circuit 2
This is because the output of 1 does not need a dynamic range larger than this.

As described above, according to the present embodiment, by providing the amplitude limiter 30 in the difference circuit 21, it is possible to reduce the scale when the adder 26 in the subsequent stage, particularly the nonlinear circuit 24, is composed of a ROM.

EFFECTS OF THE INVENTION As described above, according to the present invention, when a high level signal is input, the signal lacking at the edge portion of the signal, that is, the S / N inferior does not occur, and even between the edges It is possible to provide a noise elimination device for video signals that has a noise elimination effect and has extremely little waveform distortion even if noise in the low frequency band is eliminated, and its practical effect is great.

[Brief description of drawings]

FIG. 1 is a block diagram of a noise eliminator of one embodiment of the present invention, FIG. 2 is an input / output characteristic diagram of a non-linear circuit constituting the noise eliminator of the present invention, and FIG. 3 is another embodiment of the present invention. Block diagram of the difference circuit that constitutes the noise elimination circuit of
FIG. 4, FIG. 5, and FIG. 6 are a block diagram, a concrete circuit diagram, and a waveform diagram of a main part of a conventional noise eliminator, respectively. 20 ... Input terminal, 21 ... Difference circuit, 22 ... Delay circuit, 23 ... Subtractor, 24 ... Non-linear circuit, 25 ... Delay circuit, 26 ... Adder, 27 ... Multiplier, 28 ... … Subtractor, 29… Output terminal.

Claims (2)

[Claims] Claim: What is claimed is: 1. A difference means for extracting a variation of an input signal per predetermined time, and when the input level is small, the output is approximately proportional to the input or nonlinearly compressed according to the input level, and when the input level is large, the output is A non-linear means that is sufficiently small or zero, a delay means that delays the output of the non-linear means by the predetermined time, and an addition signal of the differential means output and the non-linear means output delayed by the predetermined time is input to the non-linear means. Means for multiplying the output of the non-linear means by a predetermined value, and subtraction means for subtracting the output of the multiplying means from the signal input to the difference means. 2. A noise eliminating device according to claim 1, further comprising an amplitude limiting means for limiting the output level of the difference means when the output level exceeds a predetermined value.