JPH03163905A - Ringing removal system - Google Patents

Abstract

PURPOSE:To remove ringing regardless of the form of a filter by obtaining a ringing removing signal by inputting the differentiating signal of an input signal in a time block where the said input signal of the filter includes an edge, and subtracting the said ringing removing signal from the output of the filter. CONSTITUTION:A removing filter 4 is equipped with a waveform, for which a desired unit step response without ringing is subtracted from the unit step response of a filter 1 as a ringing removing object, as a unit step response. The input signal is inputted through a differentiating circuit 2 and a gate circuit 3 to the removing filter 4. The ringing removing signal is obtained as the output of the removing filter 4 and this signal is subtracted from the output of the filter 1 by an adder/subtractor 6. When the keen edge generating ringing is included in the input signal and the gate circuit 3 is turned ON, ringing can be removed. The control of the gate circuit can be executed while turning on the gate circuit by a control circuit 5 when an amplitude for the differentiating signal of the input signal is larger than a certain threshold value.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a ringing removal method, and particularly to a ringing removal method that is optimal for use in removing ringing in an image signal filter.

[Conventional technology]

BACKGROUND OF THE INVENTION There has been an active effort to improve the image quality of TV receivers.

In order to improve the image quality, it is necessary to filter the received TV signal without impairing its high frequency range. However, when the cutoff characteristic of the filter is made steep, ringing occurs near the edge portions of the input signal, which (3) significantly deteriorates the quality of the reproduced image. Therefore, various methods have been devised to eliminate this ringing, for example,
A method of adaptively switching filter amplitude characteristics and a nonlinear filter called a median filter are used. In addition, as a method for removing ringing without deteriorating the frequency characteristics of the filter, the above-mentioned TV Society technical report "Ringing compensation method applying an asymmetric coefficient filter" ITEJ vol. ↓3, no. 4
1, p31, BCS'89-6 (Sep.1989
There is a ringing compensation method described in ).

[Problem to be solved by the invention]

In the ringing compensation method using an asymmetric coefficient filter described in the above-mentioned prior art, edge portions in the input signal are detected and a filter switching signal is created from this. Switched to an asymmetric coefficient filter with zero post-ringing. This method eliminates ringing but does not attenuate high frequencies (4). This is a compensation method.

However, the above-mentioned conventional technique has a limitation in that it can only be applied to symmetric coefficient filters. Symmetric coefficient filters have linear phase characteristics and do not cause waveform distortion due to group delay, so they are often used in image signal processing. However, in reality, filters with asymmetric coefficients such as analog LC filters are often used, and in these cases, the above-mentioned conventional techniques can only be applied approximately, and the ringing removal effect is incomplete.

In addition, in the above filter switching method, it is necessary to switch the filter during the ringing duration of the impulse response, so there is a problem that the ringing removal effect is not sufficient for signals that include edges at shorter time intervals. Ta.

The main objective of the present invention is to provide a ringing removal method that can be applied regardless of the coefficients of the filter that causes ringing. Another object of the present invention is to provide a ringing removal method that is effective even for signals containing edges at short time intervals, such as short-period rectangular burst signals.

(5) [Means for solving the problem] In order to achieve the above object, a waveform that is the same as the ringing waveform of the filter whose ringing is to be removed is created, and this waveform is subtracted from the output of the filter to be removed. in particular,
A removal filter is used whose unit step response is a waveform obtained by subtracting a desired unit step response without ringing from the unit step response of the ringing removal target filter. A signal obtained by differentiating the input signal is input to a removal filter through a gate circuit to obtain a ringing removal signal. When the obtained removal signal is subtracted from the output signal of the ringing removal target filter, an output with ringing removed and a desired unit step response waveform is obtained.

If the transfer function of the filter to be removed is H(s), the transfer function of the filter with the desired step response is H*(s), the transfer function of the gate circuit is G(s), and the input signal is
The ringing removal signal is z Umbrella S *G(s)”(H(s
)−H*(s))/s=G(s) umbrella(H(s)H”(s
))*x. Subtracting this from the output H(s)*x of the ringing removal pair (6), the output signal y is y = {H(s)-G(s)*(H(s)-H(s)
))*x...(L) It is expressed as. In equation (1), when the gate circuit is on, G(
s) = 1, when the gate 1 circuit is off, G(s) = O, so y = H * (s) * x When the gate is on, y = H (s) umbrella X When the gate is off... ( 2), and if you turn on gate 1- when the input includes a steep edge signal that causes ringing, the desired response waveform H rate (s)
*Can be set to x.

From the above explanation, since equation (2) holds regardless of the functional form of H (s), it can be seen that the filter for removing ringing may be any type of filter, not limited to symmetric coefficients.

Next, as a problem with the conventional method described in the [Problem] section, when the edge portion repeats at a short period, such as a short period rectangular wave burst, ringing removal is incomplete. This is because the filter switching in conventional technique (7) is performed over the impulse response duration, so in the case of edges that repeat in a short period, the ringing of adjacent edges overlaps, resulting in a ringing waveform that is different from that of a single edge. It's for a reason. To avoid this problem, ringing generated from each edge can be removed independently. For sledding (2)
It can be seen from the equation that it is sufficient to shorten the gate-on time and switch only the time when the edge appears. That is, this can be achieved by differentiating the input signal and inputting this differential signal to the removal filter only when the amplitude of the differential signal exceeds a certain threshold (the time when the slope of the edge exceeds a certain value).

[Effect]

It is desirable that the ringing removal has the characteristic of removing only the ringing without changing the cutoff characteristics of the filter to be removed. The ringing compensation method using an asymmetric coefficient filter mentioned in the prior art utilizes the property that the asymmetric coefficient filter has the same frequency characteristics as the original filter, and that the ringing is O before (after) the impulse response. This ensures that the frequency characteristics do not change. Since the method of the present invention does not utilize the symmetry of filter coefficients, it is not possible to make the frequency characteristics completely the same, but as shown in equation (2), the filter switching time is limited to only the vicinity of the edge. By doing this, it is possible to obtain characteristics that are almost the same as those of the original filter. At this time, for a large amplitude signal such as an edge signal, the filter behaves in the same way as a filter with desired characteristics from which ringing has been removed. In other words, from a large-amplitude signal that causes ringing, only the edge portion that causes the ringing is extracted by differentiation (time-compressed, so to speak), passed through the circuit to Gate 1, integrated by a removal filter, and returned to the original signal. . Therefore, equation (2) behaves as if it were a different transfer function for large-amplitude signals and small-amplitude signals, and ringing (leakage from large-amplitude signals to small-amplitude signals) is removed, and For amplitude signals, the filter operates almost the same as the original filter characteristics.

〔Example〕

(9) Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of a ringing removal method according to the present invention. In the figure, 1 is a filter that generates ringing, 2 is a differential circuit, 3 is a gate circuit, and 4 is a filter that generates ringing.
5 is a ringing removal filter, 5 is a control circuit that determines the threshold value of the gate circuit, and 6 is an adder/subtractor. The transfer function of filter 1 is H(s), and the transfer function of removal filter 4 is (H(s)
) H*(s))/S. Here H*(s)
is the transfer function of a filter with desired characteristics that has an impulse response that does not cause ringing. That is, the removal filter 4 is a filter that has a waveform obtained by subtracting a desired unit I step response without ringing from the unit 1 step response of the filter 1 from which ringing is to be removed, as a unit step response. An input signal is input to the removal filter 4 through the differentiating circuit 2 and the gate circuit 3. A ringing removal signal is obtained as the output of the removal filter 4, and this is subtracted from the output of the filter 1 by an adder/subtractor 6.

The output signal is as shown in equation (1), and the output signal is as shown in equation (1).
10) If path 3 is on, He(s). If it is off, press H (
s). Therefore, when the input signal contains a sharp edge that causes ringing, the ringing can be removed by turning on the gate circuit 3.

The gate circuit can be controlled by turning on the control circuit 5 when the amplitude of the differential signal of the input signal is larger than a certain threshold.

The operation of the embodiment shown in FIG. 1 will be explained using the operation waveform diagram shown in FIG. 2. Assume that a step-like signal as shown by the solid line in 2-a is input. The output signal of filter 1 is 2-b
The result is a signal containing ringing (not symmetrical before and after the edge), as in A signal obtained by differentiating the input signal as shown in 2-c is obtained from the differentiating circuit 2. Since the differential signal at the edge has an amplitude larger than the threshold value, it passes through the gate 1 circuit 3 and is input to the removal filter 4, where a ringing compensation signal like 2-d is obtained. Subtracting this from 2-b gives us 2-
A ringing-free filter output as shown in e is obtained. From the above explanation, it can be seen that ringing can be removed by the present invention regardless of the filter type (11).

When the input signal is superimposed with a small amplitude sine wave signal as shown by the dotted line in 2-a, a sine wave signal as shown in the dotted line is added to the differential signal 2-c, but this signal does not exceed the threshold. Therefore, it does not appear in the ringing compensation signal 2d. The sinusoidal signal at the filter output therefore appears at the output without any modification. That is, although the ringing removal method of the present invention removes ringing, the Takagi signal component is not attenuated.

Further, the ringing removed signal is processed by filter switching for a very short period of time when the edge differential signal exceeds the threshold value. Therefore, it can be seen that even when edges appear frequently, such as in a short-period rectangular wave burst l signal, filter switching can be performed accurately and ringing can be compensated for.

FIG. 3 shows an embodiment of the control circuit 5 of the embodiment shown in FIG. In the figure, 2 is the differential circuit of Fig. 1, and 31 is the first differential circuit.
A changeover switch constituting the gate circuit 3 shown in the figure, 32.
33 is an absolute value circuit, 34 and 35 are filters, 36 is a reference value circuit that provides a threshold value, and (12) 37 is a comparator. The threshold value needs to be determined based on the amplitude level of the input signal or the amplitude level of the differential signal of the input signal. That is, even if there is an edge, if there is a nearby signal with a large amplitude, the ringing will not cause much interference. Further, even if the amplitude of the differential signal is large, not all of the differential signals are edges. Therefore, these amplitudes are detected, a reference value is determined from these, and when an edge signal with a large amplitude appears, a ringing removal operation is performed. Various configurations of the control circuit 5 in FIG. 3 can be considered. The reference value given as a threshold value can be given as a function of the signal level or the signal level of the differential signal. In some cases, it may be a constant value.

FIG. 4 shows another embodiment of the invention. FIG. 4 shows an embodiment in which ringing removal according to the present invention is performed after a filter whose transfer function is known. In the figure, 4l is a ringing removal target filter, 42 is a ringing removal device, and 43 is a delay circuit. Components 2-6 of ringing removal device 42
are the same as the structural elements in the embodiment shown in FIG. 1 (■3), and are given the same numbers. The transfer function of the ringing removal filter 4 is the filter 41
For the transfer function H(s) of (H(s)−H*(s)
)/s. Here, H*(s) is a transfer function of a filter having desired characteristics and having an impulse response that does not cause ringing. The operation is almost the same as the embodiment shown in FIG. 1, except that the input signal from which the ringing removed signal is obtained is the output signal of the filter 41. Filter 41
The edge portion of the output signal has a slightly gentler slope than that of the input signal. Therefore, the amplitude and waveform of the differential signal obtained are different from those obtained from the input signal of the filter 41, but the difference is small and can be dealt with by slightly changing the threshold of the control circuit 5. The ringing removal signal obtained from the removal filter 4 is subtracted from the signal obtained by delaying the output signal of the filter 41 by the delay circuit 43, and ringing is removed. In the embodiment shown in FIG. 4, ringing removal is performed after the filter to be removed, so even if a plurality of filters generate ringing, only one removal device can handle the ringing.

Ringing generally occurs at a filter with the steepest cutoff characteristic in a transmission path. In the case of TV signals, it can be said that the output filter of the transmitter and the filter of the transmission path (such as in the case of broadcast multiplex relay) are more likely to cause ringing because of their steepness than the circuit inside the receiver. Even in such a case, if the transfer function of the transmission path can be estimated, the ringing generated in the transmission path can be removed in the receiver according to the present invention.

FIG. 5 shows an example in which the present invention is implemented when reringing generated by a transmitter and a transmission path filter is removed on the receiver side. In the figure, 5 is a signal adder, 52 is a transmitter output filter, 53 is a transmitter, 54 is a signal transmission line,
55 is a receiver, 42 is the same ringing removal device as in FIG. 4, and 56 is a transmission path characteristic calculation circuit. At the transmitter 53, the transmitted signal and the reference signal are added by a signal adder 51 before passing through the output filter 52. The reference signal is a signal whose waveform is known, and is made so that it does not overlap in time with the transmission signal (15). As long as the reference signal is a signal that can be reproduced on the receiver side, there is no need to use a special signal. In the case of TV signals, horizontal synchronization signals are used, and various test signals and GCR (goose l to
It is also possible to use these since the removal reference signal) etc. are inserted. The signal to which the reference signal has been added passes through an output filter 52 and a signal transmission line 54, and is input to a receiver 55. In the case of TV broadcasting, the transmission path 54 can take various forms such as a broadcast repeater or direct radio wave transmission. The received reference signal is sent to the transmission output filter 52 and the signal transmission line 5.
The received signal is subjected to waveform distortion due to the transfer characteristics of No. 4. Since the time position at which the reference signal is inserted is known on the receiving side, the reference signal is extracted from the received signal, inputted to the transmission path characteristic calculation circuit 56, and deconvolved with the reference signal generated on the receiver side. By doing this, the impulse response of the transmission line can be determined. From the transfer function of the transmission path obtained in this way, find the transfer function of the ringing removal filter,
It is input as the transfer function (16) of the ringing removal circuit 42. The structure of the ringing removal circuit 42 or the fourth
Although it is almost the same as the figure, the difference is that the transfer function of the ringing removal filter 4 is given by the transmission path characteristic calculation circuit 56.

〔Effect of the invention〕

According to the present invention, ringing can be removed regardless of the type of filter that generates the ringing. Furthermore, ringing can be removed from short-period rectangular wave burst signals, which has been difficult to remove in the past. Furthermore, since the filter switching time is extremely short, the frequency characteristics are almost the same as the original filter. Therefore, even if it is attached after a Takagi emphasis filter such as an edge enhancer, ringing can be removed without reducing the effect. Although the above explanation has been made using an analog transfer function H (s), it goes without saying that the present invention can be implemented using a digital circuit.

Furthermore, instead of making the filter itself a ringing-free filter, only the ringing of a signal that already contains ringing is removed (I7). Therefore, ringing can be removed by connecting it after a filter that generates ringing.

Furthermore, if the transfer function is known by some means, it is also possible to remove ringing generated in the transmission path or by a filter in the transmitter on the receiver side. In the field of image signal processing, ringing may occur not only in the horizontal direction but also in the vertical direction due to filter processing. The present invention can be applied not only to horizontal ringing but also to vertical ringing.

Furthermore, with this method, the characteristics of the removal filter can be set to any desired characteristics. Therefore, the edge portion of the signal can be modified arbitrarily, and it is possible to perform contour extraction, contour enhancement, etc. of the image signal.

Although ringing is often a problem in the field of image signals, the present invention can also be applied to fields other than image signal processing.

[Brief explanation of the drawing]

Fig.] is a configuration diagram of one embodiment of the present invention, Fig. 2 is a waveform diagram explaining the operation of Fig. 1, and Fig. 3 is a configuration diagram of an embodiment of the control circuit of Fig. 1 (18). FIG. 4 is a diagram showing the structure of another embodiment of the invention, and FIG. 5 is a diagram showing the structure of still another embodiment of the invention. 1, 34, 35. 41... Filter, 2... Differentiator circuit, 3... Gate circuit, 4... Ringing removal filter, 5... Control circuit, 6... Adder/subtractor, 3l.・
・Selector switch. 32.33... Absolute value circuit, 3
6... Reference value circuit, 37... Comparator, 42... Ringing removal device, 43... Delay circuit, 51... Signal adder, 52... Transmitter output filter, 53...・
Transmitter, 54...signal transmission (l9)

Claims (1)

[Claims] 1. In a ringing removal method in which a filter is connected in parallel to remove ringing contained in the output signal of the filter, a waveform obtained by subtracting a desired unit step response from the unit step response of the filter is a unit. A removal filter having a step response is provided, and a differential signal of the input signal is input to the removal filter in a time interval in which the input signal of the filter includes an edge to obtain a ringing removal signal, and the ringing removal signal is used as the ringing removal signal of the filter. A ringing removal method characterized by subtraction from the output. 2. In the ringing removal method described in claim 1, a threshold value is determined as a signal level of an input signal, a signal level of a differential signal of the input signal, or a function of these signal levels, and the absolute value of the differential signal is determined. 1. A ringing removal method, characterized in that the ringing removal signal is subtracted when the ringing removal signal exceeds the threshold value. 3. In a ringing removal method in which a ringing waveform included in the output signal of the filter is removed by cascade-connecting the latter stage of a filter with a known transfer function, a waveform obtained by subtracting a desired unit step response from the unit step response of the filter is used. A removal filter having a unit step response is provided, a differential signal of the output signal is input to the removal filter at an edge portion of the output signal of the filter to obtain a ringing removal signal, and the ringing removal signal is extracted from the output of the filter. A ringing removal method characterized by subtraction. 4. In a ringing removal method in which the ringing removal method according to claim 3 is applied in cascade after a filter whose transfer function is unknown, an output signal when a known reference signal is input to the filter. and the reference signal,
Ringing removal characterized by performing Laplace transform or Z transform in the case of sample value signals, determining a transfer function of the compensated filter from the ratio of each transform value, and determining a unit step response of the filter from the transfer function. method. 5. In the ringing removal method according to claim 4, the compensated filter is a transmission system for a transmission signal, and the reference signal is a known signal inserted into the transmission signal on the transmitter side, Furthermore, the ringing removal method is characterized in that the ringing removal operation is performed on the receiver side.