JPS6142908B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise removal circuit for video signals such as television signals, and the present invention relates to a noise elimination circuit for video signals such as television signals,
The purpose of this invention is to propose a system that can faithfully remove noise from video signals, regardless of whether the system is a television system or a SECAM television system.

In particular, the present invention aims to propose a noise removal path that can reliably remove noise from both static and moving parts of a playback screen of a video signal, and which reduces the reduction in resolution.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a system diagram showing this embodiment. This will be explained below. Reference numeral 1 denotes an input terminal from which an input video signal to be removed from noise is supplied. The input video signal is quantized, and its sampling period is T
shall be. 6 is an output terminal from which an output video signal from which noise has been removed is obtained. The input video signal supplied to input terminal 1 is a black and white video signal, a color video signal,
Various types of television signals are possible, such as the NTSC television system, PAL television system, and SECAM television system, but in this example, we will use a case where a black and white video signal of the NTSC television system is supplied. I will explain. Reference numeral 18 indicates a prediction filter, which is a delay means to which the input video signal is supplied and delayed by a time of N frames (N=1, 2, 3, . . ., in this example, N=1). It is equipped with 2. Then, the input video signal supplied to the input terminal 1 is supplied to the delay means 2 and also to the synthesizer 3 as a subtracter, so that the output of the delay means 2 is subtracted from the input video signal in the synthesizer 3. and then
A prediction filter 18 is configured. 4 is a nonlinear circuit to which the output of the prediction filter 18 is supplied. Here, a nonlinear circuit is a circuit in which the input level vs. output level characteristic exhibits linearity when the absolute value of the input signal level is below a predetermined value or above a predetermined value, and otherwise exhibits nonlinearity. . In this embodiment, as shown in FIG. 2, the circuit has a limiter characteristic in which the input level vs. output level characteristic is a straight line when the absolute value of the input level is less than a predetermined value, and becomes a constant value when it exceeds a predetermined value. In this case, it is also possible to use a stripping circuit in which the output level becomes zero when the absolute value of the input level exceeds a predetermined value. A synthesizer 5 combines the output of the nonlinear circuit 4 and the input from the input terminal 1, and in this example, the output of the nonlinear circuit 4 is subtracted from the input video signal in the synthesizer 5. A feedback circuit 7 feeds back the output of the synthesizer 5 to the input side of the delay means 2, and in this example is an attenuator having an attenuation rate K (K≦1). 9 is the sum of the absolute values of the level differences of signals at a plurality of points in the vicinity of a certain point in the input video signal, and signals at corresponding points in time delayed by N frames, in this example, one frame each from the plural points. This is a detection circuit that detects. 22 is a noise remover to which an input video signal is supplied. 27 is synthesizer 5
This is a variable mixer to which the output of the noise remover 22 and the output of the noise remover 22 are supplied. Then, the feedback amount of the feedback circuit 7 (or the characteristics of the nonlinear circuit 4) and the mixing ratio of the variable mixer 27 are varied by the detection output of the detection circuit 9, and the noise is removed from the output of the variable mixer 27. This is to output a video signal.

Further explanation will be given with reference to FIG. The input video signal from input terminal 1 is sent to delay circuits 15-16 for phase compensation.
The output is supplied to an attenuator 13 having an attenuation ratio of 1-K/2, and the output is supplied to a combiner 8. Then, in the synthesizer 8, the output of the attenuator 13 and the output of the above-mentioned attenuator 7 are added, and the output is supplied to the delay means 2 through an amplifier 28 having an amplification ratio of 2/1+K. Further, the input video signal which is the output of the delay circuit 15 is supplied to the synthesizer 3, and the output of the delay means 2 is subtracted from the input video signal in the synthesizer 3. However,
In this example, the prediction filter 18 is constructed by adding an attenuator 13 and an amplifier 28 in addition to the delay means 2 and the synthesizer 3. The output of the synthesizer 3, that is, the output of the prediction filter 18, is supplied to the nonlinear circuit 4, and the output thereof is supplied to the synthesizer 5 through the attenuator 14 having an attenuation ratio of 1+K/2. In the synthesizer 5, the output of the nonlinear circuit 4, that is, the output of the attenuator 14, is subtracted from the input video signal obtained through the delay circuits 15-16. The output of the combiner 5 is supplied to the variable mixer 27 and also to the feedback circuit 7.

The input video signal from input terminal 1 is sent to noise remover 2.
2, and its output is supplied to a variable mixer 27 through a delay circuit 17 for phase compensation. As the noise remover 22, for example, a two-dimensional low-pass wave filter (FIG. 3), which will be described later, can be used. In addition, ordinary low-pass waveformers may also be used. or,
This noise remover 22 can also be applied to a circuit that linearly combines a plurality of video signals at different times within the same field and removes noise by supplying the signal to a nonlinear circuit. An example of this type of circuit includes a prediction error detection circuit that includes a prediction filter for a video signal in the same field and detects a prediction error of the video signal, and a nonlinear circuit to which the detection output of the prediction error detection circuit is supplied. However, when the prediction error is relatively small, this noise removal circuit removes the prediction error from the video signal, thereby removing noise from the video signal.

Other examples of this type of circuit include a serial/parallel conversion circuit to which an input video signal is supplied, an orthogonal conversion circuit to which the output of the serial/parallel conversion circuit is supplied, and a nonlinear conversion circuit to which the output of the orthogonal conversion circuit is supplied. circuit, an inverse conversion circuit to which the output of the nonlinear circuit is supplied, and a parallel/serial conversion circuit to which the output of the inverse conversion circuit is supplied, and the output of the parallel/serial conversion circuit or its output is subtracted from the input video signal. This is a noise removal circuit that outputs a noise-free output as an output video signal.

The variable mixer 27 includes a variable attenuator 23 with an attenuation ratio n ₁ to which the output of the combiner 5 is supplied, and a variable attenuator 23 with an attenuation ratio n ₂ (where n ₁ + n ₂ = 1) to which the output of the delay circuit 17 is supplied. 24 and a combiner 26 to which the outputs of attenuators 23 and 24 are added.

In the synthesizer 25, the output of the delay means 2 is subtracted from the input video signal from the input terminal 1. The output of the synthesizer 25 is supplied to the detection circuit 9.

The detection circuit 9 includes two-dimensional low-pass waveforms 10 and 1.
2 and a double-wave rectifier circuit 11. That is, the output of the synthesizer 25 is supplied to the first two-dimensional low-pass wave generator 10, and the output is supplied to the double-wave rectifier circuit 1.
1, and its output is supplied to a second two-dimensional low-pass wave filter 12. The configurations of the first and second two-dimensional low-pass transducers 10 and 12 in this detection circuit 9 can be as shown in FIG. 3, for example.

That is, in FIG. 3, 28 is the input terminal of the two-dimensional low-pass wave generator, and 39 is its output terminal. The output from the input terminal 28 is supplied to a delay circuit 29 whose delay amount is 1 horizontal period, and the output from which is further delayed by 1 horizontal period.
horizontal period delay circuit 30 and synthesizer 34
The outputs of the delay circuits 29 and 30 are added together, and the output of the combiner 34 and the input signal from the input terminal 28 are added together in the combiner 35. Furthermore,
The output of the synthesizer 35 is supplied to a delay circuit 31 whose delay amount is T (sampling period), the output of the synthesizer 35 and the output of the delay circuit 31 are added in a synthesizer 36, and the output of the synthesizer 36 is further added. is supplied to a delay circuit 32 with a delay amount of 2T, the output of the synthesizer 36 and the output of the delay circuit 32 are added in a synthesizer 37, and the output of the synthesizer 37 is supplied to a delay circuit 33 with a delay amount of 3T. , the output of the combiner 37 and the output of the delay circuit 33 are added in the combiner 38, and the added output is obtained at the output terminal 39.

In this way, two-dimensional low-pass wave transmitters 10 and 12 are constructed, and their transfer functions are as follows.

G(Z)=(1+Z ^-h +Z ^-2h ) ・(1+Z ^-1 )・(1+Z ^-2 )・(1+Z ^-3 ) However, Z is expressed as Z=e ^j 〓 ^T , where ω
is the angular frequency. Further, h is a value obtained by dividing the horizontal period by T.

Then, the output of the detection circuit 9 is transmitted to the control circuits 19 and 2.
0 and 51, respectively. The output of the control circuit 19 controls the K of the attenuator 7, which is a feedback circuit, as well as the K of the attenuators 13, 14 and 28. The output of the control circuit 20 controls the characteristics of the nonlinear circuit 4, in this example, the input level value at which the characteristic of input level versus output level changes from linear to nonlinear, that is, the threshold level of the input. Control circuit 2
The attenuation ratios n ₁ and n ₂ of the variable attenuators 23 and 24 of the variable mixer 27 are controlled by the output of the variable mixer 27 .

Next, the operation of the detection circuit 9 will be explained. 1st
The output of the prediction filter 18 in the figure is a signal of the difference between the input video signal and a signal delayed by a time corresponding to one frame. signal
If X' is assumed to change as time t changes as shown in Figure 5A, then X-X' changes as shown by the curve S _1. In this case, at the point where this curve S ₁ crosses the horizontal axis t,
This means that the image is stationary between frames, and it can be seen that the image moves between frames as the distance increases above and below the horizontal axis. In FIG. 5A, S ₂ indicates irregular noise superimposed on this video signal. FIG. 5B shows the output when the signal X-X' shown in FIG _.
Let it be (X−X′). According to this, the noise curve from which the high-frequency components of the above-mentioned noise S ₂ of the video signal have been removed
It can be seen that S ₂ ′ is shown. FIG. 5C shows a curve when the output of the first two-dimensional low-pass wave generator 10 is supplied to the double-wave rectifier circuit 11 for double-wave rectification, and this curve is expressed as |L ₁ (X−X′) Indicated as |.
According to this, the noise S ₂ ″ in Fig. 5C is the noise S 2 ″ in Fig. 5B
It can be seen that the noise S ₂ ' is double-wave rectified and the portion below the horizontal axis t is folded upward. FIG. 5D shows the output obtained by further supplying the signal in FIG. 5C to the second two-dimensional low-pass wave generator 12, and this is expressed as L _{2 |} do. According to this, it can be seen that even if pulse-like noise is mixed in the output side of the double-wave rectifier circuit 11, the noise _S2 is removed. Thus, on the output side of the second two-dimensional low-pass wave filter 12, a detection output indicating whether the screen is stationary or moving during one frame of the input video signal is obtained. That is, the output of the prediction filter 18 was the output obtained by combining the curves S ₁ and S ₂ shown in FIG.
As shown in FIG. D, it can be seen that even if these curves _S1 and S are combined and added, a curve that is approximately similar to the video signal _S1 will be obtained.

That is, the detection circuit 9 detects the sum of the absolute values of the level differences of signals at a plurality of time points near a certain time point of the input video signal and signals at a plurality of corresponding time points each delayed by N frames from the plurality of time points. The reason for this is that in the case of a still image, the absolute value of each level difference is very small, and the sum of the absolute values is also small, indicating that this is a still image. This is because even if the absolute value of the level difference is small to some extent, most of the level differences are large and can be regarded as a moving image.

Then, the constant K (or the characteristic of the nonlinear circuit 4) and the variable mixer 27 are determined based on the detection output of the detection circuit 9.
First, the case where K is changed will be explained. The characteristics of the output video signal of the synthesizer 5 with respect to the frequency f are as shown in FIG. This is similar to the characteristics of a comb waveform (C type) (the amount of delay is different). Note that f ₀ is the frame frequency (30Hz). In this case, the solid line shows the case when K=0, and the broken line shows the case when K=0.5. Then, this output video signal becomes a periodic function curve whose frequency becomes zero at f ₀ , 3f ₀ , 5f ₀ , . . . . As K approaches 0→1, the shape of the curve changes to become thinner. In this noise removal circuit, the closer K is to 1, the better the S/N becomes. Therefore, in the detection circuit 9, when the screen movement between one frame is small, this K is set to a value as large as possible, that is, a value close to 1, and the output of the detection circuit 9 is used to detect the image between one frame of the input video signal. It can be seen that when the movement of K is large, it is sufficient to control K to be small. If K is made small, a decrease in resolution on a moving screen can be avoided.

Also, as shown in FIG. 6, the threshold level of the nonlinear circuit 4 is set to the maximum when the output level of the detection circuit 9 is zero (still image), and as the output level increases (as the movement of the screen increases) ) Decrease the threshold level. Incidentally, in the case of a still image, the slope of the straight line portion of the nonlinear circuit 4 may be increased, and in the case of a moving screen, the slope may be decreased.

Furthermore, the attenuation ratios n ₁ and n ₂ of the variable attenuators 23 and 24 of the variable mixer 27 change the output level of the detection circuit 9 from small to large (quiet), as shown by the solid line and broken line curves in FIG. (image → moving screen), the size changes from large to small and from small to large, respectively, and n ₁ +n ₂ =1. In this case, switching of n ₁ =1, n ₂ =0; n ₁ =0, n ₂ =1 is also possible.

According to the present invention described above, it is possible to obtain a noise removal circuit that can reliably remove noise from both static and moving parts of a playback screen of a video signal, and further reduces resolution.

Although the case where N is 1 has been described above, cases where N is 2, 3, . . . are also possible. However, if the movement of the screen is rapid, it is better for N to be small, and if the movement of the entire screen is relatively slow, N may be slightly larger. however,
In the case of completely still images, N can be set to a fairly large value, but in the case of normal moving images, N may not be too large.
cannot be made larger.

[Brief explanation of the drawing]

Figure 1 is a system diagram showing one embodiment of the present invention, Figure 2 is a system diagram showing an embodiment of the present invention.
The figure is a characteristic curve diagram, Figure 3 is a system diagram showing a specific configuration of a part of Figure 1, Figure 4 is a characteristic curve diagram, Figure 5 is a waveform diagram, and Figures 6 and 7 are characteristic curve diagrams. It is. 1 is an input terminal, 2 is a delay means, 3 is a synthesizer as a subtracter, 4 is a nonlinear circuit, 5 is a synthesizer, 6 is an output terminal, 7 is an attenuator as a feedback circuit, 8 is a synthesizer, 9 is a synthesizer Detection circuit, 10 is a first two-dimensional low-pass wave generator, 11 is a double-wave rectifier circuit, and 12 is a second two-dimensional low-pass wave generator.
dimensional low-pass wave filter, 18 is a prediction filter, 22
is a noise remover, and 27 is a variable mixer.

Claims (1)

[Claims] 1. A prediction filter provided with a delay means to which an input video signal is supplied and delayed by a time of N frames (N = 1, 2, 3, ...), and a nonlinear circuit to which the output of the prediction filter is supplied. a synthesizer in which the output of the nonlinear circuit and the input video signal are combined; a feedback circuit in which the output of the synthesizer is fed back to the input side of the delay means; and a synthesizer in the vicinity of a certain point in time of the input video signal a detection circuit that detects the sum of the absolute values of each level difference of signals at a plurality of time points and signals at a plurality of corresponding time points each delayed by N frames from the plurality of time points; and a noise to which the input video signal is supplied. a remover; and a variable mixer to which the output of the combiner and the output of the noise remover are supplied,
The feedback amount of the feedback circuit (or the characteristics of the nonlinear circuit) and the mixing ratio of the variable mixer are varied by the detection output of the detection circuit, and the noise is removed from the output video signal from the variable mixer. A noise removal circuit characterized in that it outputs.