JPH04346510A - Filter device - Google Patents

Abstract

PURPOSE:To suppress the ringing caused by a low pass filter(LPF). CONSTITUTION:A filter device detects a part of an input signal 50 where a large level change is recognized and then shapes previously the waveform to the detected part of the signal 50 before application of an LPF operation. A control signal generating circuit 1 detects the level change of the signal 50 and produces a control signal 51 based on the level change value of the signal 50. A waveform shaping circuit 2 shapes the waveform of the signal 50 according to the value of the signal 51 and obtains 8 shaped signal 52. A filter circuit 3 applies an LPF operation to the signal 52 with the desired band pass characteristic.

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 device for removing or resampling high frequency components.

0002

2. Description of the Related Art Conventionally, low-pass filters have been used for the purpose of removing high frequency components or resampling sampled signals. In such cases, the amount of attenuation changes sharply at the cutoff point of the filter.In other words, by using a low-pass filter with a small rolloff rate, it is possible to extract more signal components below the cutoff point. Become. However, on the other hand, as the roll-off rate decreases, ringing disturbance increases. For example, Figure 8 (
When a low-pass filter with a small roll-off rate is applied to a step signal as shown in a), a signal as shown in FIG. 8(b) is obtained.

[0003] Conventionally, therefore, a method using a low-pass filter with a gentle attenuation characteristic, that is, a large roll-off rate, has been widely used, especially when applied to image signals.

0004

[Problem to be solved by the invention] In the above-mentioned prior art,
Since a low-pass filter with a large roll-off rate is used, the available signal band cannot be fully utilized. SUMMARY OF THE INVENTION An object of the present invention is to provide a filter device that eliminates or alleviates the drawbacks of the conventional system and suppresses the occurrence of ringing disturbance even when a low-pass filter with a small roll-off rate is used.

[0005]

[Means for Solving the Problems] The filter device of the first invention is a low-pass filter device that removes or resamples high-frequency components, and detects a level change in an input signal and changes the level. After waveform shaping is applied to a large portion of the signal, a low-pass filter having desired passband characteristics is applied.

Further, the filter device of the second invention is a low-pass filter device that performs resampling, which applies a low-pass filter to an input signal to produce a resampled signal, The sample value of the resampled signal is corrected using at least the sample value of the input signal located nearby, and the corrected resampled signal is output.

[0007]

[Operation] In the first invention, a pre-filter circuit detects a level change in an input signal, performs waveform shaping on a portion with a large level change, and then adjusts the roll-off rate of the waveform-shaped signal. Apply a small low-pass filter. That is, for example, when a step signal with a large level change as shown in FIG. The waveform is as shown in b). After that, if a low-pass filter with a small roll-off rate is applied, large ringing will no longer occur as shown in FIG. 6(c).

For example, by applying the filter device of the first invention to an image signal, it is possible to suppress the occurrence of noticeable ringing in areas where the level changes are large, and to prevent the resolution of areas where the level changes are small from lowering more than necessary. It becomes possible to obtain an output signal.

In the second invention, after applying a low-pass filter with a small roll-off rate to the sampled input signal to obtain a resampled signal, a correction circuit converts the sampled value of the resampled signal into a resampled signal. is corrected using the sample value of the input signal located in the vicinity. For example, consider the case where sampled signals S1, S2, . First, resampled signals F1, F2, . . . , F5 are obtained using a low-pass filter with a small roll-off rate. Next, a correction circuit applies correction to the resampled signal. For example, focusing on the resampling point F4, comparing the resampling signal F4 with the input signals S4 and S5 located in the vicinity, it is determined that the resampling signal F4 is not larger than the larger of S4 and S5, and the smaller of S4 and S5 is The signal level of the resampled signal F4 is corrected to a range that is not smaller. In the case of FIG. 7, since S4 and S5 are equal, F4 is corrected to the same value as S4 and S5. By adding the correction according to the second invention, it becomes possible to remove ringing that does not exist in the input signal from the resampled signal. Note that in the above example, the value of F4 does not necessarily need to completely match the values of S4 and S5, and may be reduced to a certain degree.

Furthermore, in the second aspect of the invention, when the correction circuit applies correction to the resampled signal, it may detect a change in level of the input signal and increase the degree of correction for a portion where the change is large. Generally, the greater the level change within a certain section, the greater the ringing and the more noticeable it becomes. Therefore, by increasing the degree of correction for parts where the level change is large, it is possible to prevent unnecessary correction from being applied to parts where the level change is small.

Furthermore, in the second invention, when the correction circuit applies correction to the resampled signal, it detects a difference between the sampled value of the resampled signal and the average value of the sampled values of the input signal located in the vicinity thereof. Then, the degree of correction may be increased for a portion with a larger difference. In other words, the larger the deviation of the resampled signal from the input signal, the larger the correction is applied, thereby making it possible to prevent unnecessary correction in areas where ringing is not noticeable.

0012

Embodiments Next, embodiments of the present invention will be described with reference to FIGS. 1 to 5. It is assumed that each filter circuit used in these embodiments is, for example, a one-dimensional low-pass filter, and that the input signal is sampled.

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

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

In the embodiment of FIG. 1, an input signal 50 is input to a control signal generation circuit 1 and a waveform shaping circuit 2 which constitute a prefilter circuit 100. Control signal generation circuit 1
, detects a level change in an input signal 50 and generates a control signal 51 for adjusting the degree of waveform shaping. In the waveform shaping circuit 2, the input signal 50 is processed by applying a reduced pass filter with a large roll-off rate to the input signal 50.
The waveform-shaped signal and the input signal 50 are weighted and added according to the value of the control signal 51, thereby outputting a shaped signal 52. In the filter circuit 3, the shaped signal 52
A low-pass filter with a small roll-off rate is applied to the signal using a known technique, and a signal 53 is output. A desired signal band is removed from the signal 53 by the filter circuit 3, and the signal 53 is made into a signal with less noticeable ringing by the waveform shaping circuit 2.

FIG. 2 shows a specific example of the configuration of the prefilter circuit 100.

In prefilter circuit 100, input signal 50 is first input to delay circuit D1, and output signal 60 of delay circuit D1 is input to delay circuit D2. Here, the delay circuits D1 and D2 are circuits that delay the signal by a period T corresponding to one sampling interval. Next, the difference 62 between the input signal 50 and the signal 60 delayed by one sampling period T, and 1
Signal 60 delayed by sampling interval T and two sampling intervals 2T
A difference 63 between the signal 61 and the delayed signal 61 is obtained. The differences 62 and 63 are converted to absolute values 64 and 64 by absolute value circuits 8 and 9, respectively.
65, and furthermore, in the maximum value circuit 10, the larger one of the absolute values 64 and 65 is selected to produce the maximum value signal 66.
is required. This maximum value signal 66 is taken as the amount of change in signal level before and after the sample point of interest. The maximum value signal 66 is converted by the multiplier generating circuit 11 into weighted signals 67 and 68 representing the degree of waveform shaping. Weighting signals 67 and 6
The sum with 8 is always 1, and in the part where the maximum value signal 66 is large, the weighting signal 68 is 1 and the weighting signal 67 is 0, and in the part where the maximum value signal 66 is small, the weighting signal 68 is 0 and the weighting signal 67 is 1. do it like this. The weighting signal is made to change continuously depending on the value of the maximum value signal 66. The above is the part corresponding to the control signal generation circuit 1 in FIG. 1, and the weighting signals 67 and 68 are the control signal 5 in FIG.
Corresponds to 1.

Next, a portion corresponding to the waveform shaping circuit 2 will be explained. Input signal 50 is input to filter circuit 12 and delay circuit 13. In the filter circuit 12, a reduced pass type filter with a large roll-off rate is applied to the input signal 50 using a known technique to obtain a signal 69. The delay circuit 13 delays the input signal 50 by the time necessary for filtering in the filter circuit 12 and outputs it as a signal 70. The multiplier M1 multiplies the signal 69 and the weighted signal 68, the multiplier M2 multiplies the signal 70 and the weighted signal 67, and the multiplication results are added to output the shaped signal 52.

In the above embodiment, the degree of waveform shaping is achieved by weighted addition in the prefilter circuit 100, but the same effect can also be obtained by varying the filter coefficients in the filter circuit 12. . In that case, the delay circuit 13, multipliers M1, M2,
The multiplier generating circuit 11 is no longer necessary, and a circuit for generating the filter coefficient of the filter circuit 12 in accordance with the maximum value signal 66 is provided instead.

Furthermore, the control signal generation 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 of the previous and subsequent sample points, the control signal 51 may be changed depending on the way the signal level changes.

[0021] When displaying the image signal band-limited by the filter device of the first invention on a display device, the level change is enhanced by emphasizing the high-frequency component in the part where the signal level change is large. It is possible to reduce image blur in large parts of the image.

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

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

In the embodiment of FIG. 3, an input signal 50 is input to a filter circuit 3 and a 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 with a small roll-off rate. The delay circuit 4 receives the input signal 5 for the time necessary to perform filter processing in the filter circuit 3.
0 is delayed and output as a signal 54. The signal 54 and the resampled signal 55 which is the output of the filter circuit 3 are input to the correction circuit 5, and the value of the signal 54 is used to convert the resampled signal 5
5 will be corrected.

A more specific example of the basic configuration of FIG. 3 is shown in FIG. In the configuration of FIG. 4, 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 in 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, delay circuits D3, D4, and D5 convert the input signal 50 into one sampling period T, two sampling periods 2T, and
Signals 71, 72, and 73 are generated delayed by 3 sampling periods 3T. In multipliers M3, M4, M5, M6, each signal 5
A resampled signal 55 is obtained by multiplying 0, 71, 72, and 73 by a filter coefficient and calculating the sum. In this configuration, the number of filter taps is four, but a similar configuration can also be used to increase the number of taps. Figure 7
To explain using , the input signal 50 is the signal S1, S2,...
, S6, and the resampled signal 55 is the signal F1, F2,...
, F5. However, the resampled signal 55 shown in FIG.
The waveform is an example of a waveform that appears when the number of filter taps is greater 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 determination circuit 14 outputs the larger one of the signals 71 and 72 as a signal 75 and the smaller one as a signal 76. signal 75
, 76 and the resampled signal 55 are input to the limit circuit 15, and the resampled signal 55 is corrected to have a signal level within the range between the signals 75 and 76, and becomes a corrected signal 56. In FIG. 7, for example, signals 50, 71, 7
2 and 73 are respectively S3, S4, S5, and S6, the resampled signal 55 becomes F4. At this time, signal 75
becomes S4, and the signal 76 becomes S5. In Figure 7, S4 happens to be
Since F4 and S5 are equal, F4 is corrected to the same value as S4 and S5. Through the above processing, ringing included in the resampled signal 55 is removed. Note that, as a correction in the limit circuit 15, it is not necessarily necessary to make the value of F4 completely match the values of S4 and S5, and it is sufficient to reduce the value to some extent.

The filter device of the second 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 using FIG. 3, and the correction circuit 7 is controlled by a control signal 57.

In the embodiment of 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 in the embodiment described using FIG. 3, and output a resampled signal 55 and a signal 54, respectively. In the control signal generation circuit 6, the amount of change in the signal level before and after resampling is detected in the same way as the control signal generation circuit 1 in FIG. 1, and two weighting values according to the amount of change are generated as the control signal 57. do. signal 54, resampled signal 55
and the control signal 57 are input to the correction circuit 7, which generates a corrected resampled signal with the same configuration as the correction circuit 5 in FIG.
A signal obtained by weighting and adding the resampled signal 55 from the control signal 57 based on the control signal 57 is output as a correction signal 58.

Note that the correction signal 58 is not obtained by weighted addition based on the control signal 57, but when the resampled signal 55 is corrected by the limit circuit 15 (see FIG. 4), the degree of correction is determined by the control signal 57. It is also possible to obtain a configuration by changing it according to.

Furthermore, when generating the control signal 57, it may be generated using not only the amount of change in the signal level but also the manner in which the signal level changes, and the control signal 57 may be generated by using not only the amount of change in the signal level but also the manner in which the signal level changes. It may be generated using the difference between the sample value of the input signal 50 and the average value.

Although the filter circuits used in each of the above-described embodiments are one-dimensional filters, the same effect can be obtained even if the filter circuits are two-dimensional or more.

[0033]

[Effects of the Invention] As explained above, according to the present invention, it is possible to provide a filter device that can suppress the occurrence of ringing interference even when a low-pass filter with a small roll-off rate is used. becomes.

[Brief explanation of drawings]

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

FIG. 2 shows a prefilter circuit 100 in the embodiment of FIG.
FIG. 2 is a block diagram showing the configuration of FIG.

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

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

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

FIG. 8 is a diagram showing signal waveforms 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 circuits 8, 9 Absolute value circuit 10 Maximum value circuit 11 Multiplier generation circuit 14 Size determination circuit 15 Limit circuit 100 Prefilter circuits M1 to M6 Multiplier

Claims (2)

[Claims] Claim 1: In a low-pass filter device that removes or resamples high frequency components, after detecting level changes in an input signal and applying waveform shaping to portions with large level changes in advance. A filter device characterized by applying a low-pass filter having desired passband characteristics. 2. A low-pass filter device that performs resampling, which applies a low-pass filter to an input signal to obtain a resampled signal, and sets sample values of the resampled signal to at least a nearby one. A filter device that corrects the input signal using a sample value of the input signal located therein, and outputs a corrected resampled signal.