JP2979712B2 - Filter device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-pass filter for removing or resampling high frequency components.

[0002]

2. Description of the Related Art Conventionally, a low-pass filter has been used for the purpose of removing high-frequency components or re-sampling a sampled signal. In such a case, the amount of attenuation changes abruptly at the cutoff point of the filter. In other words, if a low-pass filter having a small roll-off rate is used, it is possible to extract more signal components below the cutoff point. Become. However, on the other hand, when the roll-off rate is reduced, ringing disturbance increases. For example, FIG.
When a low-pass filter having a small roll-off ratio is applied to the step signal shown in FIG. 8A, a signal as shown in FIG.

For this reason, a system using a low-pass filter having a gradual attenuation characteristic, that is, a large roll-off rate, has been frequently used, particularly when applied to an image signal.

[0004]

In the above-mentioned prior art,
The use of a low-pass filter with a large roll-off rate has a disadvantage that the available signal band cannot be fully utilized. SUMMARY OF THE INVENTION An object of the present invention is to eliminate or alleviate the drawbacks of the conventional system and to provide a filter device in which occurrence of ringing disturbance is suppressed even when a low-pass filter having a small roll-off ratio is used.

[0005]

SUMMARY OF THE INVENTION The filter device of the first invention, the low-pass filter device for removing or re-sampling of high frequency components, the desired pass band characteristic
A low-pass filter with a small roll-off rate having
Connected to the input side of this low-pass filter
A waveform shaping circuit that can control the degree of waveform shaping
Detects the level change of the force signal and detects the large part of the level change
Control the waveform shaping circuit to apply a large waveform shaping to the
And a control signal generating circuit for outputting a control signal.
Have been.

A filter device according to a second aspect of the present invention is a low-pass filter device for performing resampling.
Have the desired passband characteristics to pass through the signal and perform resampling.
Low-pass filter with low roll-off rate
Two input signals before and after the resampling point, delaying the signal
A delay circuit that outputs a sample value of the low-pass filter;
The sampled value of the resampled signal output from the
Between the sample values of the two input signals obtained from
And a correction circuit for correcting the error.

[0007]

According to the first aspect of the present invention, the level change of the input signal is detected by the pre-filter circuit, and the waveform shaping is performed on the portion where the level change is large. Apply a small low-pass filter. That is, for example, when a step signal having a large level change as shown in FIG. 6A is input, a portion where the level is rapidly changing is first subjected to waveform shaping by a pre-filter circuit, and the waveform is changed as shown in FIG. The waveform is as shown in b). Thereafter, if a low-pass filter having a small roll-off rate is applied, large ringing does not occur as shown in FIG.

For example, by applying the filter device of the first invention to an image signal, occurrence of conspicuous ringing in a portion having a large level change is suppressed, and the resolution of a portion having a small level change is not reduced more than necessary. An output signal can be obtained.

In the second aspect of the invention, the sampled input signal is subjected to a low-pass filter having a small roll-off rate to obtain a resampled signal. Is corrected using the sample value of the input signal located in the vicinity thereof. For example, consider a case where when sampled signals S1, S2,..., S6 as shown in FIG. 7 are input, resampling is performed at the center position between adjacent sample points. First, resampled signals F1, F2,..., F5 are obtained by a low-pass filter having a small roll-off rate. next,
The resampling signal is corrected by a correction circuit. For example, focusing on the resampling point F4, comparing the resampling signal F4 with the input signals S4 and S5 located near the resampling signal F4,
The signal level of the resampled signal F4 is corrected to a range not larger than the larger one of S4 and S5 and smaller than the smaller one of S4 and S5. In the case of FIG. 7, since S4 and S5 are equal, F4 is corrected to the same value as S4 and S5. By applying the correction according to the second aspect, it is possible to remove ringing that does not exist in the input signal from the resampled signal. In the above example, the value of F4 is completely changed to S4, S5
It is not always necessary to make the value coincide with the value of.

In the second invention, when the correction circuit corrects the resampled signal, the level change of the input signal may be detected and the degree of the correction may be increased as the level of the change increases. In general, the ringing becomes larger and more conspicuous in a portion where the level change within a certain section is large. Therefore, by increasing the degree of correction in a portion having a large level change, it is possible to prevent unnecessary correction in a portion having a small level change.

Further, in the second invention, when the correction circuit corrects the resampled signal, a difference between a sample value of the resampled signal and an average value of sample values of an input signal located near the resampled signal is detected. Then, the degree of correction may be increased as the difference becomes larger. That is, by performing a larger correction as the resampled signal deviates greatly from the input signal, it is possible to prevent unnecessary correction from being performed in a portion where ringing is not conspicuous.

[0012]

Next, an embodiment of the present invention will be described with reference to FIGS. Each filter circuit used in these embodiments is, for example, a one-dimensional low-pass filter, and the input signal is sampled.

First, an embodiment of the first invention will be described.

FIG. 1 is a block diagram showing a basic configuration of an embodiment of the first invention.

In the embodiment shown in FIG. 1, an input signal 50 is input to a control signal generating circuit 1 and a waveform shaping circuit 2 which constitute a pre-filter circuit 100. Control signal generation circuit 1
, A level change of the input signal 50 is detected, and a control signal 51 for adjusting the degree of waveform shaping is generated.
In the waveform shaping circuit 2, the input signal 50 is subjected to a low pass filter having a large roll-off rate, so that the input signal 50
Is shaped, and the shaped signal and the input signal 50 are weighted and added by the value of the control signal 51 to output a shaped signal 52. In the filter circuit 3, the shaped signal 52
Is subjected to a low-pass filter with a small roll-off rate using a known technique, and a signal 53 is output. The signal 53 is a signal in which a desired signal band is removed by the filter circuit 3 and the ringing is inconspicuous by the waveform shaping circuit 2.

FIG. 2 shows a specific configuration example of the pre-filter circuit 100.

In the pre-filter circuit 100, the input signal 50 is first input to the delay circuit D1, and the output signal 60 of the delay circuit D1 is input to the delay circuit D2. Here, the delay circuits D1 and D2 are circuits for delaying a signal by a period T for one sampling interval. Next, the difference 62 between the input signal 50 and the signal 60 delayed by one sampling period T, and 1
The signal 60 delayed by the sampling interval T and two sampling intervals 2T
A difference 63 from the signal 61 delayed by only this is obtained. The differences 62 and 63 are calculated by the absolute value circuits 8 and 9,
65, and the maximum value circuit 10 selects the larger one of the absolute values 64 and 65 to obtain the maximum value signal 66.
Is required. The maximum value signal 66 is defined as the amount of change in the signal level before and after the sample point of interest. The maximum value signal 66 is converted by the multiplier generation circuit 11 into weighting signals 67 and 68 indicating the degree of waveform shaping. Weighting signals 67 and 6
The sum with 8 is always 1, the weighting signal 68 is 1 and the weighting signal 67 is 0 in the portion where the maximum value signal 66 is large, and the weighting signal 68 is 0 and the weighting signal 67 is 1 in the portion where the maximum value signal 66 is small. To do. The weighting signal is made to continuously change according to the value of the maximum value signal 66. The above is the portion corresponding to the control signal generation circuit 1 of FIG. 1, and the weighting signals 67, 68
Equivalent to 1.

Next, a portion corresponding to the waveform shaping circuit 2 will be described. The input signal 50 is input to the filter circuit 12 and the delay circuit 13. In the filter circuit 12, a reduction pass filter having a large roll-off ratio is applied to the input signal 50 by using a known technique, and a signal 69 is obtained. In the delay circuit 13, the input signal 50 is delayed by a time necessary for performing filter processing in the filter circuit 12 and output as a signal 70. The multiplier M1 multiplies the signal 69 by the weighting signal 68, the multiplier M2 multiplies the signal 70 by the weighting signal 67, and adds the respective multiplication results to output the shaped signal 52.

In the above-described embodiment, the degree of waveform shaping is realized by weighted addition in the pre-filter circuit 100, but the same effect can be obtained by changing the filter coefficient in the filter circuit 12. . In that case, the delay circuit 13, the multipliers M1, M2,
The multiplier generation circuit 11 becomes unnecessary, and a circuit for generating the filter coefficient of the filter circuit 12 according to the maximum value signal 66 is provided instead.

Further, the control signal generating circuit 1 of the above-described embodiment
Although the control signal 51 is generated simply based on the amount of change in the signal level at the preceding and following sample points, the control signal 51 may be changed according to the manner in which the signal level changes.

When the image signal band-limited by the filter device of the first invention is displayed on the display device, the higher the frequency change, the more the higher frequency components are emphasized. It is possible to reduce blurring of an image in a large portion of the image.

Next, an embodiment of the second invention will be described.

FIG. 3 is a block diagram showing a basic configuration of an embodiment of the second invention.

In the embodiment shown in FIG. 3, an input signal 50 is input to the filter circuit 3 and the delay circuit 4. The filter circuit 3 is realized with exactly the same configuration as the filter circuit 3 of FIG. 1, and is a low-pass filter circuit having a small roll-off rate. In the delay circuit 4, the input signal 5 is provided only for a time necessary for performing the filter
0 is delayed and output as a signal 54. The signal 54 and the resampled signal 55 output from the filter circuit 3 are input to the correction circuit 5, and the resampled signal 5
5 is corrected.

FIG. 4 shows a more specific configuration example of the basic configuration shown in FIG. 4, the delay circuits D3, D4,
A portion constituted by D5, multipliers M3, M4, M5, M6 and an adder corresponds to the filter circuit 3 in FIG. The delay circuit 4 of FIG. 3 is shared with the delay circuits D3 and D4 of the filter circuit. The magnitude determination circuit 14 and the limit circuit 15 correspond to the correction circuit 5 in FIG.

First, the input signals 50 are firstly sampled for one sampling period T, two sampling periods 2T,
Signals 71, 72 and 73 delayed by 3 sampling periods and 3T are generated. Signals 5 are output from multipliers M3, M4, M5 and M6, respectively.
The resampled signal 55 is obtained by multiplying 0, 71, 72, 73 by a filter coefficient and calculating the sum thereof. In this configuration, the number of filter taps is four, but the same configuration can be used when a large number of taps are used. Referring to FIG. 7, if the input signal 50 is the signal S1, S2,
, S6, and the resampled signal 55 is a signal F1, F2,
..., F5. However, the resampled signal 5 shown in FIG.
The waveform 5 is an example of a waveform that appears when the number of filter taps is larger than 4.

The signals 71 and 72 delayed by the delay circuits D3 and D4 are input to the magnitude determination circuit 14. The magnitude judgment circuit 14 outputs the larger one of the signals 71 and 72 as the signal 75 and the smaller one as the signal 76. Signal 7
The resampling signal 55 is input to the limit circuit 15, and the resampling signal 55 is corrected so as to have a signal level in a range between the signals 75 and 76, and becomes a correction signal 56. In FIG. 7, for example, signals 50, 71,
When 72 and 73 are S3, S4, S5 and S6, respectively, the resampled signal 55 is F4. At this time, signal 7
5 is S4, and the signal 76 is S5. It happens to be S in FIG.
Since F4 and S5 are equal, F4 is equal to S4, S5
Will be corrected to the same value as. The ringing included in the resampled signal 55 is removed by the above processing. It is not always necessary to completely match the value of F4 with the values of S4 and S5 as a correction in the limit circuit 15,
It is sufficient to make the size somewhat smaller.

The filter device according to the second aspect of the present invention can also be realized by the configuration shown in FIG. The embodiment of FIG. 5 has a configuration in which a control signal generation circuit 6 is added to the embodiment described with reference to FIG. 3 and the correction circuit 7 is controlled by a control signal 57.

In the embodiment shown in FIG. 5, an input signal 50 is input to a filter circuit 3, a delay circuit 4, and a control signal generation circuit 6. The configurations of the filter circuit 3 and the delay circuit 4 are exactly the same as those of the embodiment described with reference to FIG. 3, and output a resampled signal 55 and a signal 54, respectively. The control signal generation circuit 6 detects the amount of change in the signal level before and after the re-sampling point in the same manner as the control signal generation circuit 1 of FIG. 1, and uses two weights according to the change as the control signal 57. Occur. The signal 54, the resampling signal 55, and the control signal 57 are input to the correction circuit 7,
A resampled signal corrected by the same configuration as that of the correction circuit 5, and a signal obtained by weighting and adding the corrected resampled signal and the resampled signal 55 from the filter circuit 3 based on the control signal 57. Is output as the correction signal 58.

It should be noted that the correction signal 58 is not obtained by weighted addition based on the control signal 57, but the degree of the correction is corrected when the resampling signal 55 is corrected by the limit circuit 15 (see FIG. 4). May be obtained by changing according to the following.

When generating the control signal 57, the control signal 57 may be generated not only by using the amount of change in the signal level but also by using the manner of change in the signal level. It may be generated using a difference between the average value of the sample values of the input signal 50 and the average value.

Although the filter circuit used in each of the above embodiments is a one-dimensional filter, the same effect can be obtained with a two-dimensional or more filter circuit.

[0033]

As described above, according to the present invention, it is possible to provide a filter device capable of suppressing occurrence of ringing disturbance even when a low-pass filter having a small roll-off rate is used. Becomes

[Brief description of the drawings]

FIG. 1 is a block diagram showing a basic configuration of an embodiment of the first invention.

FIG. 2 shows a pre-filter circuit 100 in the embodiment of FIG.
FIG. 3 is a block diagram showing the configuration of FIG.

FIG. 3 is a block diagram showing a basic configuration of an embodiment of the second invention.

FIG. 4 is a block diagram showing a specific configuration of the embodiment of FIG. 3;

FIG. 5 is a block diagram showing a basic configuration of another embodiment of the second invention.

FIG. 6 is a diagram showing a signal waveform in the filter device of the first invention.

FIG. 7 is a diagram showing a signal waveform in the filter device of the second invention.

FIG. 8 is a diagram showing a signal waveform in a conventional filter device.

[Explanation of symbols]

 1, 6 control signal generation circuit 2 waveform shaping circuit 3, 12 filter circuit 4, 13, D1 to D5 delay circuit 5, 7 correction circuit 8, 9 absolute value circuit 10 maximum value circuit 11 multiplier generation circuit 14 size determination circuit 15 limit Circuit 100 Pre-filter circuit M1-M6 Multiplier

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H03H 17/00-17/08

Claims (2)

(57) [Claims] In a low-pass filter device for removing or resampling a high frequency component, a desired pass
Low-pass filter with band characteristic and low roll-off rate
Connected to the input side of this low-pass filter.
Waveform shaping circuit that can control the degree of waveform shaping applied to force signals
Circuit and the level change of the input signal
The waveform shaping circuit so as to apply a large waveform shaping to the part
A control signal generating circuit for outputting a control signal for controlling the
Filter and wherein the was e. 2. A low-pass filter apparatus for performing resampling, wherein the input signal is transmitted and resampling is performed.
Low pass with low roll-off rate with pass band characteristics of
Type filter, delays the input signal and moves around the resampling point
A delay circuit that outputs sample values of two neighboring input signals,
Of the resampled signal output from the low-pass filter.
This value is a sample of two input signals obtained from the delay circuit.
And a correction circuit for correcting the value to be between the values .