JP2535826B2 - Noise cancellation circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a noise canceling circuit used in a video reproducing circuit of a VTR or the like.

[Conventional technology]

Since noise from adjacent tracks concentrates on the portion where the brightness level in the VTR playback waveform changes abruptly (hereinafter referred to as the edge portion), the noise at this edge portion is larger than the noise at the flat portion of the playback waveform. . Further, in the conventional VTR, in order to remove noise, a noise cancel circuit is used in which a signal obtained by passing a video signal through a high-pass filter is further passed through a limiter to subtract a signal obtained from the video signal.

[Problems to be solved by the invention]

The conventional noise canceling circuit described above can remove the noise in the flat portion of the waveform, but cannot sufficiently remove the noise in the edge portion. The noise in the edge part can be removed by lowering the response in the high frequency range. However, this causes problems such as loss of detail on the screen, lower limit resolution, and deterioration of reproduced waveform.

Even when the non-linear de-emphasis circuit is provided, the noise at the edge portion becomes conspicuous due to the reduction of the de-emphasis amount in the large amplitude portion.

[Means for solving problems]

In the present invention, a first high-pass filter for extracting a high-frequency component of an input video signal, an amplitude limiting circuit for limiting an amplitude of an output signal of the first high-pass filter, and a high-frequency component of the input video signal are extracted. A second high-pass filter, a subtraction circuit for obtaining a difference between the output signal of the amplitude limiting circuit and the output signal of the second high-pass filter, and a third high-pass filter to which the output signal of the subtraction circuit is supplied. , A combining means for combining the output signal of the third high-pass filter, the output signal of the amplitude limiting circuit, and the input video signal.

[Action]

With the high-pass filter, the vicinity of the edge of the waveform is not blunted, and the waveform is not distorted.

〔Example〕

 FIG. 1 shows a first embodiment.

In FIG. 1, the luminance signal reproduced from the magnetic tape 1 by the head 2 is applied to the FM demodulation circuit 4 through the reproduction amplifier 3 and demodulated. The demodulated luminance signal is added to the main de-emphasis circuit 5 to perform high-frequency correction, and then sent to the sub-de-emphasis circuit 6 for correction. The de-emphasis circuit 6 is constituted by an adder 7, a high pass filter 8 and a limiter 9 as shown in the figure.

In this embodiment, the edge noise canceling circuit 10 for removing the noise in the edge portion is provided. The noise canceling circuit 10 uses the de-emphasis circuit 6 as a part thereof.

The noise canceling circuit 10 comprises the de-emphasis circuit 6, a high pass filter 11, a frequency compensating circuit 12, an adder 13, a high pass filter 14, an attenuator 15 and an adder 16.

Next, the edge noise canceling circuit 10 having the above configuration
The operation will be described with reference to FIG. 2 a to f
Shows the signal waveforms at points a to f in FIG.

The output of the main de-emphasis circuit 5 is passed through an adder 7 to obtain a high-frequency corrected signal a
The output signal at the point a is added to the high-pass filter 8 to extract its high-frequency component, and after the level is limited by the limiter 9, it is added to the adder 7 to be added to the main demultiplexer. -Subtracted from the output signal of the emphasis circuit 5. This subtraction output becomes the output signal at the point a that has been corrected in the high frequency range. This output signal is applied to the high-pass filter 8 and the high-pass filter 11 having substantially the same characteristics as the high-pass filter 8.

When the level of the output signal suddenly changes and the edge portion 17 as shown in FIG. 2A is generated, the output waveform of the high-pass filter 8 is a differential waveform of the edge portion 17 as shown in FIG. 2B. Noise N is superimposed on the differential waveform and the waveform of the signal shown in FIG. By passing through the limiter 9, the differentiated waveform becomes a waveform cut at a predetermined limiter level as shown in FIG. 2c. In this waveform, noise N near the limiter level is removed as shown in the figure. The signal of this waveform is added to the adder 7 and the adder 13.

On the other hand, the signal having the edge portion 17 added to the high pass filter 11 is added to the frequency compensation circuit 12. Even if the two high-pass filters 8 and 11 have the same characteristic, the frequency compensating circuit 12 actually causes the characteristic that the high frequency is lowered by the limiter 9, so that the output of the high-pass filter 11 is amplified, the phase, It has a function of performing correction for matching the frequency characteristics and the like with the output of the limiter 9.
Therefore, the output waveform of the frequency compensation circuit 12 is substantially the same as the output waveform of the high pass filter 8 of FIG. 2b.
The signal of this output waveform is added to the adder 13 and the limiter 9
Is subtracted from the output waveform of. Since this subtraction is performed by a system that adds the waveform of FIG. 2b and the waveform of FIG. 2c inverted, the waveform of the subtraction output is as shown in FIG.
The noise N remains in the portion corresponding to the vicinity of the limiter level of the waveform in FIG. 7C, and the noise is removed in the other portions. Further, this waveform is similar to the waveform shown in FIG. 11B and smaller than the waveform shown in FIG.

The signal having the waveform of FIG. 2d is a high pass filter.
By passing through 14, the waveform of FIG. This waveform is a differentiated form of the waveform shown in FIG. 7D, appears on both the positive and negative sides, and the noise N remains at the portion corresponding to the remaining portion of the noise N shown in FIG. The signal of this waveform is adjusted in the amount of cancellation by the attenuator 15 and then added to the adder 16 to be subtracted from the waveform of FIG.

This subtraction is performed by adding the waveform shown in FIG. 2a and the waveform obtained by inverting the waveform shown in FIG. 2e. Therefore, the waveform of the subtraction output is the same as the waveform shown in FIG. The noise is removed from the portion corresponding to the vicinity of the limiter level, and the original noise in FIG.

The dotted line in FIG. 2f represents a waveform equal to the noise-removed waveform in FIG. 2a. As is clear from this figure, the waveform near the edge portion 17 'and the peak of the waveform shown by the solid line is corrected by the waveform shown in FIG. That is, although the response in the high frequency range is reduced to remove the noise N, the loss of detail, the reduction of the limit resolution, the waveform deterioration, etc. do not occur. The signal having the waveform of FIG.
By being added to 18, it is possible to obtain the output signal S _{0 from} which the residual noise N is almost completely removed.

In the embodiment described above, the sub
I am using the de-emphasis circuit 6.
When the de-emphasis circuit 6 is not provided, the adder 7 and the transmission line connecting the adder 7 and the limiter 9 are omitted, and the output signal of the main de-emphasis circuit 5 is used as an input signal. It will appear directly at point a.

FIG. 2 shows a second embodiment, in which parts corresponding to those in FIG. 1 are designated by the same reference numerals.

In the embodiment shown in FIG. 1, the output signal of the adder 13 is a
Although it is obtained based on the input signal of the point, this embodiment is an adder.
This is a case where the output signal of 13 is obtained on the basis of the output signal S ₀ and is configured as a feed hack type. That is, the adder 13
Is added to the output signal of the limiter 9 in the adder 19 through the high pass filter 14, and the added output signal is added to the adder 7 and subtracted from the input signal.

Also in this embodiment, if the sub-de-emphasis circuit 6 is not provided, the adder 19 and the transmission line connecting the adder 19 and the limiter 9 are omitted.

〔The invention's effect〕

According to the present invention, the amplitude-limited first high-frequency component extracted from the video signal by the high-pass filter, and the first high-frequency component and the high-frequency component extracted from the video signal by the high-pass filter are provided. By synthesizing the second high-frequency component obtained by the difference of the input video signal and the input video signal, it is possible to effectively remove the noise remaining in the edge portion of the conventional input video signal.

[Brief description of drawings]

FIG. 1 is a block diagram showing the first embodiment of the present invention, and FIG.
FIG. 3 is a signal waveform diagram of points a to f in FIG. 1, and FIG. 3 is a block diagram showing a second embodiment. In the reference numerals used in the drawings, 10 ... edge noise canceling circuit 8, 11, 14 ... high-pass filter 9 ... limiter 13, 16 ... adder.

Claims (1)

(57) [Claims] 1. A first high-pass filter for extracting a high-frequency component of an input video signal, an amplitude limiting circuit for limiting an amplitude of an output signal of the first high-pass filter, and a high-frequency component of the input video signal. A second high-pass filter, a subtraction circuit for extracting a difference between the output signal of the amplitude limiting circuit and the output signal of the second high-pass filter, and a third high-pass filter to which the output signal of the subtraction circuit is supplied, A noise canceling circuit comprising a combining means for combining the output signal of the third high pass filter, the output signal of the amplitude limiting circuit and the input video signal.