JPH07105928B2 - Filter circuit - Google Patents

Description

The present invention relates to a filter circuit suitable for application to a noise canceller arranged in a reproduction luminance signal system of a video tape recorder (VTR), for example.

[Outline of Invention]

According to the present invention, the input signal is supplied to the adder, and the input signal is phase-inverted after being passed through the series circuit of the first high-pass filter and the limiter and then supplied to the adder, and the output signal from the adder is output. In the filter circuit thus obtained, the input signal is further supplied to the adder through the second high-pass filter to obtain a flat characteristic at a large amplitude level and improve the waveform characteristic, frequency characteristic, and phase characteristic. This is what was planned.

[Conventional technology]

Conventionally, a noise canceller arranged in the reproduction luminance signal system of the VTR has been proposed as shown in FIG. That is, the low amplitude level component in the high frequency band is regarded as a noise component and the noise component is removed.

In the figure, (1) is an input terminal, and this input terminal (1) is supplied to the subtractor (2). The input terminal (1) is connected to a subtractor (2) via a high pass filter (3), a limiter (4), a low pass filter (5) for phase correction, and a variable attenuator (6). The output terminal (7) is derived from 2).

In the above structure, when the reproduction luminance signal is supplied to the input terminal (1), the high frequency component of the reproduction luminance signal is taken out from the high pass filter (3), and the amplitude is low in the high frequency range from the limiter (4). A level component, that is, a noise component is taken out, and this noise component is supplied to the subtractor (2) via the low-pass filter (5) and the variable attenuator (6). After all, in the subtractor (2), the input terminal (1)
The noise component is subtracted from the luminance signal supplied to the output terminal, and the luminance signal from which the noise component is removed is obtained at the output terminal (7) derived from the subtractor (2).

[Problems to be solved by the invention]

By the way, FIG. 8A shows the frequency characteristic when the attenuation amount in the variable attenuator (6) is set to a predetermined value in the example of FIG. As is clear from this figure, the high-frequency attenuation characteristic, that is, the low-pass characteristic is obtained even at a large amplitude level. This is due to the incomplete operation of the limiter (4). According to the example in FIG. 7, since the low-pass characteristic is obtained even at the large amplitude level, the waveform characteristic, the frequency characteristic, and the phase characteristic are deteriorated at the large amplitude level.
When the attenuation amount of the variable attenuator (6) is reduced to increase the amount of noise components, for example, as shown in FIG. 8B, the low-pass characteristic becomes stronger at a large amplitude level, and the waveform characteristic, frequency characteristic, Phase characteristics deteriorate.

In view of this point, the present invention aims to improve waveform characteristics, frequency characteristics, and phase characteristics at a large amplitude level.

[Means for solving problems]

According to the present invention, the input terminal is connected to the adder, and the input terminal is the first high-pass filter (3) and the limiter (4).
And a filter circuit in which an output terminal is derived from the adder connected to the adder through a series circuit of the phase inversion circuit, the input signal of the input terminal is supplied, and the cut is approximately equal to that of the first high-pass filter (3). A second high-pass filter (12) having an off frequency and a level setting circuit (13) connected to the second high-pass filter (12) and outputting a correction signal for subtracting from the output signal of the limiter (4) And the level of the correction signal is set so that the amplitude-frequency characteristic at the output terminal derived from the adder (11) becomes substantially flat when the input signal has a large amplitude level.

[Action]

In the above configuration, the frequency characteristic of the portion excluding the path of the second high pass filter (12) has a low pass characteristic even at a large amplitude level (see FIGS. 8A and 8B). However, by providing the path of the second high pass filter (12),
The low-pass characteristic at the large amplitude level described above is canceled out, and a flat characteristic is obtained.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. In FIG. 1, parts corresponding to those in FIG. 7 are designated by the same reference numerals, and detailed description thereof will be omitted.

In the figure, a subtractor (11) is provided between the limiter (4) and the low-pass filter (5). The input terminal (1) is connected to the subtractor (11) via the high pass filter (12) and the level setting circuit (13). And
In the subtractor (11), the output of the level setting circuit (13) is subtracted from the output of the limiter (4), and the subtracted output is supplied to the low pass filter (5). Here, the cut-off frequency of the high-pass filter (12) is made substantially equal to the cut-off frequency of the high-pass filter (3).

Others are the same as those in the example of FIG.

In the above configuration, the input / output frequency characteristics, excluding the paths of the high pass filter (12) and the level setting circuit (13), have low pass characteristics even at a large amplitude level, as shown in FIG. 2A. This frequency characteristic is when the attenuation amount in the variable attenuator (6) is a predetermined value.

In addition, the high-pass filter (12) and the level setting circuit (1
The frequency characteristics of input and output through the path of 3) are as shown in FIG. 2B. In this case, although not described above, the attenuation amount of the level setting circuit (13) is determined so that the high pass characteristic cancels the low pass characteristic at the large amplitude level as shown in FIG. 2A. .

Eventually, the input / output frequency characteristic becomes as shown in FIG. 2C, and a flat characteristic can be obtained at a large amplitude level.

When the attenuation amount of the variable attenuator (6) is reduced, the high pass filter (12) and the level setting circuit (13)
The frequency characteristic of input and output when the path of is removed is shown in Fig. 3A.
As shown in, the low-pass characteristic at a large amplitude level becomes stronger. However, in response to this, the high pass filter (1
2) and the frequency characteristic of the input / output passing through the path of the level setting circuit (13), the high pass characteristic becomes strong as shown in FIG. 3B. Therefore, the frequency characteristic of the entire input / output is also as shown in FIG. 3C at this time, and the flat characteristic can be obtained at the large amplitude level.

As described above, according to this example, since the flat characteristic is obtained at the large amplitude level, the waveform characteristic, the frequency characteristic, and the phase characteristic at the large amplitude level are improved.

Further, according to this example, even if the attenuation amount of the variable attenuator (6) is changed, for example, the noise cancel amount is changed, the flat characteristic at the large amplitude level is maintained and the characteristic is not deteriorated. Also, according to this example, the signal of the path of the primary high-pass filter (12) and the level setting circuit (13) is subtracted from the outputs of the primary high-pass filter (3) and the limiter (4) to correct it. Therefore, the phase characteristic is not deteriorated by the correction. Further, by varying the gain of the level setting circuit (13), the level of the amplitude to be flattened can be arbitrarily set. Further, according to the present example, the path from the input terminal (1) to the subtractor (2) is left as it is, so even if an active filter is used for the high pass filter (12), the S / N is not deteriorated.

Incidentally, FIG. 4 shows a part of a concrete circuit example of the example of FIG. 1, and corresponding parts are designated by the same reference numerals. In FIG. 4, (31) is a power supply terminal, (32) is a terminal to which a reference voltage V _REF corresponding to the central potential of the signal supplied to the terminal (1) is supplied, and (33) is a buffer. Circuit.

Next, FIG. 5 shows another embodiment of the present invention.
In FIG. 5, parts corresponding to those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted.

In the figure, the output side of the level setting circuit (13) is connected to the subtractor (2). Then, in the subtractor (2),
The outputs of this level setting circuit (13) are added. Others are the same as those in the example of FIG.

In the above configuration, the input / output frequency characteristics, excluding the paths of the high pass filter (12) and the level setting circuit (13), have low pass characteristics even at a large amplitude level, as shown in FIG. 2A. The input / output characteristics of the high-pass filter (12) and the level setting circuit (13) passing through the path are as shown in FIG. 2B. In this case, although not mentioned above. The attenuation amount of the level setting circuit (13) is determined so that the high pass characteristic cancels the low pass characteristic at the large amplitude level as shown in FIG. 2A. Therefore,
The overall input / output frequency characteristic is as shown in FIG. 2C, and a flat characteristic can be obtained at a large amplitude level.

In the example of FIG. 5, the variable attenuator (6)
As shown in FIG. 3A, the low-pass characteristic at a large amplitude level shows the input / output frequency characteristics when the attenuation amount of is reduced and the paths of the high-pass filter (12) and the level setting circuit (13) are excluded. Become stronger. In such a case, unlike the example of FIG. 1, the gain of the level setting circuit (13) is adjusted to adjust the high pass filter (12) and the level setting circuit (13).
The frequency characteristic of the input and output through the path of must be set as shown in FIG. 3B. As a result, the frequency characteristic of the entire input / output can be made flat with a large amplitude level as shown in FIG. 3C.

As described above, according to the example of FIG. 5, the flat characteristic is obtained at the large amplitude level, so that the waveform characteristic, the frequency characteristic and the phase characteristic are improved at the large amplitude level.

Next, FIG. 6 shows an example in which the filter circuit (22) as shown in FIG. 1 is inserted in the negative feedback path of the amplifier (21), which is suitable for use as a pre-emphasis circuit for VTR recording, for example. It is something.

In the figure, an input terminal (23) is connected to an input side of an amplifier (21) via a subtractor (24), an output terminal (25) is derived from an output side of this amplifier (21), and this amplifier ( 2
The output side of 1) is connected to the subtractor (2
4) connected to.

As described above, the filter circuit (22) has a non-linear characteristic in which the higher frequency range is attenuated as the amplitude level becomes smaller, and is a flat characteristic at the large amplitude level. The smaller the amplitude level is, the higher the high frequency region is emphasized, and the higher the amplitude is, the more flat the characteristic is. Therefore, the waveform characteristics, frequency characteristics, and phase characteristics at a large amplitude level are good.

In FIG. 7, a high-pass filter may be arranged after the subtractor (2) to correct a large amplitude level, but a low-pass filter for phase correction is required and the S / N ratio is increased. There is an inconvenience that causes deterioration of

〔The invention's effect〕

According to the present invention described above, since the flat characteristic is obtained at the large amplitude level, the waveform characteristic, the frequency characteristic and the phase characteristic at the large amplitude level are improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIGS. 2 and 3 are diagrams for explaining the same, and FIG. 4 is a partial circuit diagram of a part of the example of FIG. 1, FIG. FIG. 6 is a block diagram showing another embodiment of the present invention, FIG. 7 is a block diagram of a conventional example, and FIG. 8 is a diagram for explaining it. (1) is an input terminal, (2) and (11) are subtractors, (3)
And (12) is a high pass filter, (4) is a limiter,
(5) is a low-pass filter, (6) is a variable attenuator, (7) is an output terminal, and (13) is a level setting circuit.

Claims (1)

[Claims] 1. An input terminal is connected to an adder, and the input terminal is connected to the adder via a series circuit of a first high pass filter, a limiter and a phase inversion circuit,
In a filter circuit whose output terminal is derived from the adder, an input signal of the input terminal is supplied, a second high-pass filter having a cutoff frequency substantially equal to that of the first high-pass filter, and a second high-pass filter. An amplitude-frequency at the output terminal derived from the adder when the input signal has a large amplitude level, the level setting circuit being connected to a filter and outputting a correction signal for subtracting from the output signal of the limiter. A filter circuit characterized in that the level of the correction signal is set so that the characteristics become substantially flat.