JPS6310907A - Noise elimination circuit - Google Patents

Abstract

PURPOSE:To eliminate residual noise and distortion of waveform by providing the 1st H.P.F whose cut-off frequency is variable and the 2nd H.P.F whose cut-off frequency is fixed so as to mix each output signal in opposite phase close to that of an original input signal. CONSTITUTION:An output of a H.P.F 18 is multiplied by a factor of B by an amplifier circuit 19 and the noise is also amplified by B. An output signal of a limiter circuit 13 and an output signal of the amplifier circuit 19 are inputted to a mixer 14 in opposite phase respectively and subtracted with each other. As a result, a signal (f) is obtained, a large signal level of the input video signal is eliminated completely and a small signal level such as noise is attenuated by a multiple of (1-B). Further, the noise extracting band of the output signal from the mixer 14 is limited toward higher frequencies by an L.P.F 15. In giving the signal to a mixer 16, where it is mixed with the original input video signal in opposite phase, a video signal (g) is obtained. Thus, the signal distortion having a comparatively large signal change is eliminated and a large noise rejection effect is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a noise removal circuit that reduces noise signal components in a specific band included in a reproduced video signal.

BACKGROUND OF THE INVENTION FIG. 3 shows a block circuit of a conventionally known noise removal circuit, and FIG. 4 is a waveform diagram for explaining the operation of this circuit.

In FIG. 3, high frequency components of a video signal supplied to an input terminal 31 are separated by a bypass filter (hereinafter referred to as H, P, , F) 32, amplitude limited by a limiter circuit 33, and a low-pass filter ( Hereinafter, the high range of the noise extraction band is limited by L, P, F) 34). The mixer 35 mixes the video signal inputted from the input terminal 31 and the output signal of the low-pass filter 34 at an appropriate ratio in opposite phases to each other.
A video signal with reduced noise is output to the output terminal 36.

FIG. 5 shows a specific circuit of the differentiating circuit constituting the bypass filter. If the capacitance value of the capacitor 52 is C1 and the resistance value of the resistor 53 is R, then the transfer characteristic from the input terminal 51 to the output terminal 54 is , contains the Laplace variable S, and becomes . Now, if a video signal containing noise as shown in FIG. 4(a) is input to the input terminal 51 of the bypass filter shown in FIG.
4, a signal shown in FIG. 4(b) is output. When the amplitude of this signal is limited by the limiter circuit 33, the waveform becomes as shown in FIG. 4(C). Now, if the level at which the input signal is amplitude limited is VL, and the amplification factor of the limiter circuit 33 is A, then during the period when the input signal exceeds VL, the output becomes a flat signal of AVL, and the period during which this output is flat is t. If LIM, tLllll is the potential difference from the pedestal region to the white beak region of the high-power video signal, for example, as shown in FIG. 4(a).
Then, it becomes . Also, if the cutoff frequency of the bypass filter shown in Fig. 5 is fc, then -1”-1 ・・・・・・(3
)2πCR, and it can be seen that the relationship between fc and tLIM is inversely proportional to each other.

In the conventional noise removal circuit shown in FIG. 3, when a composite video signal as shown in FIG. 4(a) is supplied to the input terminal 31, a signal as shown in FIG. 4(d) is output to the output terminal 36. is obtained.

Problems to be Solved by the Invention In such a conventional configuration, the cutoff frequency fc of the bypass filter for separating high frequency components is made small;
When trying to expand the noise extraction band to the lower frequency side, t L
Since IM becomes large and noise cannot be removed during the period tLIM, residual noise increases in the output signal and large waveform distortion occurs as shown in FIG. 4(d). Conversely, when the cutoff frequency 50 is large, tLIM becomes small, and the above drawbacks are alleviated, but the noise extraction band becomes narrow and a large noise removal effect cannot be expected.

Furthermore, in the conventional example shown in FIG. 3, the bypass filter (H, P, F) 32 has a high cutoff frequency fc for large signal levels, and a high cutoff frequency fc for small signal levels such as noise. By configuring the same cutoff frequency fc to be small, it is possible to reduce tLllll to almost zero and obtain a large noise removal effect. In the region where is small, Figure 4 (
As shown in C), a waveform with a large time constant is exhibited, and distortion in the output waveform cannot be removed.

The present invention is intended to solve these problems, and aims to eliminate residual noise and waveform distortion that occur when the above-described noise removal circuit is used.

Means for Solving the Problem In order to solve this problem, the present invention provides first H, P, and F whose cutoff frequency changes depending on the input level, and second H, P, and F whose cutoff frequency does not change. F',
The respective output signals are mixed in mutually opposite phases, and the output is further mixed with the original human power signal in mutually opposite phases.

Effect: With this configuration, the cutoff frequency fc is increased to reduce tLIM for large-level input signals, and the same cutoff frequency fc is decreased for small-level input signals, resulting in large noise removal. effect, and it is possible to remove waveform distortion for large signal levels.

Embodiment FIG. 1 shows an embodiment of the noise removal circuit of the present invention. In FIG. 1, 11 is an input terminal, 12 is a nonlinear type H, P, and F whose time constant changes depending on the input signal level, 13 is an amplitude limiting circuit, so-called a limiter circuit, 14.16 is a mixer, 15 is a beam, and P ,F.

17 is the output terminal, 18 is the constant time constant H, P, F, 19
is an amplifier circuit with a gain of 1 or less. FIG. 2 is a waveform diagram showing the operation of one embodiment of the present invention.

In FIG. 1, the video signal input to the input terminal 11 is input to the mixer 16, the nonlinear H, P, F 12, and the H, P
, F 18. First, nonlinear H, P, F12
As shown in Figure 2(b), a differential signal whose time constant changes depending on the input level is output, and H, P,
A differential signal having a constant time constant for all input levels is outputted from F18. Nonlinear H, P, F
The output signal of 12 is converted into non-linear H by the limiter circuit 13.
, P, F 12 time constant switching level BV+(B<1
) are removed, resulting in a signal as shown in FIG. 2(d).

On the other hand, the outputs of H, P, and F 18 are multiplied by eight by an amplifier circuit 19 to obtain a signal as shown in FIG. 2(e).

At this time, the noise is also multiplied by eight.

The output signal of the limiter circuit 13 and the output signal of the amplifier circuit 19 are input to the mixer 14 in opposite phases to each other and are subtracted. As a result, a signal as shown in Figure 2) is obtained, in which the large signal level of the input video signal is completely removed, and small signal levels such as noise are removed (
1-B) It is only attenuated by a factor of 1-B). Further, the noise extraction band of the output signal of the mixer 14 is limited on the high frequency side by L, P, and F 15. Then, this signal is sent to the mixer 16
When mixed with the original input video signal in opposite phases to each other, a video signal as shown in FIG. It is possible to eliminate signal distortion in some parts and obtain a large noise removal effect.

Effects of the Invention According to the present invention, residual noise and waveform distortion in parts where signal changes are large can be reduced by using nonlinear H, P, and F whose time constants change depending on the input level, and nonlinear H, P, and F whose time constants do not change depending on the input level. By subtracting the respective outputs of H, P, and F at an appropriate ratio, it is possible to suppress the output, and it is possible to output a video signal that is visually more natural and has a high S/N ratio. .

[Brief explanation of the drawing]

A timing chart diagram for explaining the operation, and FIG. 5 is a specific circuit diagram of H, P, and F in FIG. 3. 11...Input terminal, 12...Nonlinear H
, P, F, 13...Limiter circuit, 14.16
...Mixer, 15...L, P, Fl,
17...Output terminal, 18.Old...H, P, F,
19...Amplifier circuit. Name of agent: Patent attorney Toshio Nakao and 1 other person, 2nd Ward, 3rd District

Claims (1)

[Claims] A first differentiating circuit that separates high frequency components in the input signal and changes the time constant depending on the magnitude of the signal level, a second differentiating circuit whose time constant remains unchanged, and the first differentiating circuit separate the high frequency components in the input signal. a limiter circuit that limits the amplitude of high-frequency components of the input signal, and a first mixer that combines the signal obtained by the limiter circuit and the signal obtained by the second differentiation circuit in opposite phases to each other. , a second mixer that combines the signal obtained by the first mixer and the input signal in opposite phases to each other.