JPH0758881B2 - Through rate limiting filter - Google Patents

Description

TECHNICAL FIELD The present invention relates to a slew rate limiting filter for band limiting a signal having a large slew rate (time rate of voltage change).

The present invention relates to a slew rate limiting filter suitable for band limiting a video signal, for example.

[Prior Art] In order to limit the slew rate of an input signal, it is known to increase the value of a phase compensation capacitor of an operational amplifier, for example.

[Problems to be Solved by the Invention] The object is achieved by using the above-described method for a monochrome video signal, but in this case, the signal delay amount has a signal amplitude dependence. Therefore, with respect to the color video signal, the delay amount between the channels may be locally deviated. As a result, there is a drawback that phenomena such as color shift and color bleeding occur.

Further, if the band of the video signal is limited by a steep filter, there is a drawback that ringing occurs.

Further, in a method of band-compressing and transmitting by sub-sampling, a high frequency component of an input video signal is often superimposed on a high band of a transmission line band. In this case, an LPF (low pass filter) that defines the transmission line band is used. Requires roll-off characteristics. However, when the characteristic of the filter deviates from the roll-off characteristic, there is a drawback that ringing occurs in the received reproduced image.

When such ringing occurs, the image becomes unsightly, so
In the prior art, the trade-off between the steepness of the filter and the ringing was attempted.

In the sub-sampling method, efforts were made to make the transmission line filter as close to the roll-off characteristic as possible.

Therefore, in view of the above points, an object of the present invention is to provide a filter that can appropriately limit the slew rate of an input signal.

[Means for Solving the Problems] The present invention is a slew rate limiting filter for band limiting an input signal having a large slew rate, which is a high pass filter which introduces the input signal and passes only a predetermined high frequency component. A filter, an absolute value circuit for converting the output signal of the high-pass filter into a positive polarity component, and introducing the input signal,
A low-pass filter that passes only a predetermined low-pass component,
A delay circuit that introduces the input signal to give a delay time corresponding to the delay amount of the low-pass filter to the input signal, and an output of the low-pass filter according to the magnitude of the signal output from the absolute value circuit. And a selection circuit for switching and outputting a signal or an output signal of the delay circuit.

Another aspect of the present invention is a slew rate limiting filter that band-limits an input signal having a large slew rate, the high-pass filter introducing the input signal and passing only a predetermined high frequency component, and the output of the high-pass filter. An absolute value circuit that converts a signal into a positive component, a low-pass filter that introduces the input signal and passes only a predetermined low-pass component, and an input signal that is equivalent to the delay amount of the low-pass filter A delay circuit that gives a delay time to the input signal, and output signals of the low-pass filter and the delay circuit, respectively, and perform gain control according to the output signal of the absolute value circuit. And a complementary gain controller that outputs a summed signal whose gain is controlled for each signal.

Further, in the preferred embodiment of the present invention, the slew rate of the video signal is limited to a value smaller than the value determined by the required band so that the ringing caused by the limited band is less visible without deteriorating the resolution.

[Operation] The slew rate limiting filter according to the present invention is a filter that gently suppresses a high frequency component with respect to a large amplitude signal and does not perform any band limiting action with respect to a small amplitude signal.

In a preferred embodiment of the present invention, the ringing of an image reproduced from a band-limited or sub-sampled video signal is reduced without requiring strict filter characteristics in consideration of human visual characteristics. . That is,
The ringing is conspicuous in the vicinity of the portion where the video signal changes abruptly (the large amplitude contour portion), and the small amplitude contour portion is almost invisible. Therefore, if the slew rate of the large-amplitude contour portion is limited, this conspicuous ringing can be suppressed, and in this case, there is no effect on the small-amplitude contour portion, so the resolution does not change. .

[Examples] Hereinafter, the present invention will be described in detail based on Examples.

FIG. 1 is a basic configuration diagram of a slew rate limiting filter to which the present invention is applied. FIG. 2 is a waveform diagram of each part shown in FIG. First, the high-pass filter (HPF) 2 extracts the high frequency components of the video signal. The output signal 2a of HPF2 is introduced into the absolute value circuit 3, and the negative polarity component is converted into the positive polarity component. The output signal 3a of the absolute value circuit 3 becomes a large output signal mainly in the large amplitude contour portion of the image as shown in FIG. 2 (C).

On the other hand, the low-pass filter (LPF) 4 is a low-pass filter having a gentle cutoff characteristic such that ringing is not noticeable even if the band is limited to a required band. This output signal 4a,
The output signal 5a of the delay line 5 corresponding to the delay amount of the LPF 4 is switched according to the output signal of the absolute value circuit 3. That is, when the output signal 3a of the absolute value circuit 3 is large, the output of the LPF 4 is selected, and when the output signal 3a of the absolute value circuit 3 is small, the output signal 5a of the delay line 5 is selected.

Actually, a complementary gain controller (controls the gains of two input signals in conjunction with each other and adds the two gain-controlled signals. At this time, if the gains for the respective signals are A ₁ and A ₂ , ₁ A ₂
6), the selection control is continuously performed.
In this case, since the control signal mainly represents the large-amplitude contour portion of the image as described above, the LP signal is used in this portion.
Output 4a of F4 is selected. Since the control signal is small in the other portions, the delay line output 5a is selected. Therefore, a signal whose slew rate is limited by the LPF4 can be obtained only in the large amplitude contour portion of the image.

Since the ringing is inconspicuous in portions other than the large amplitude contour, it is preferable to faithfully output the same signal as the input signal as much as possible. Therefore, as shown in FIG. 2 (C), a threshold level may be set in the control signal to ignore a small control signal.

In FIG. 3, the HPF2 and LPF4 shown in FIG.
This is an example configured using 9 etc. Here, the output signal of the subtractor 14 corresponds to the output of the HPF. Further, the circuit 15 for performing the square operation is used instead of the absolute value circuit 3 shown in FIG. That is, when the square calculation is performed and the output signal of the subtractor 14 is small, the output signal of the square calculation circuit 15 becomes smaller. Therefore, the threshold function of the control signal is included in a pseudo manner.

FIG. 4 is a block diagram showing an embodiment different from FIG. Therefore, the output signal 13a of the adder 13 corresponds to the LPF output for limiting the slew rate. The output signal 14a of the subtractor 14 corresponds to the HPF output. That is, this is the output of the adder 13 subtracted from the output of the delay line 18.
Therefore, the sum of the output of the adder 13 and the output of the subtractor 14 is
The output of the delay line 8 is always output regardless of the LPF configuration (ie,
The input signal is simply delayed).

The nonlinear amplifier 16 shown in FIG. 4 has the following characteristics. That is, when the input signal of the amplifier 16 is small (close to zero), the gain is 1, and as the absolute value of the input signal increases, the differential gain decreases and saturates at a certain input signal level. If the absolute value of the input signal is further increased, the differential gain becomes negative, and if it exceeds a certain value, the gain becomes zero.

In the small portion of the output signal of the subtractor 14, that is, in the portion that is not the large amplitude contour portion, the input / output characteristic of the non-linear amplifier 16 is a linear characteristic with a gain of approximately 1. Therefore, the outputs of the adder 13 and the subtractor 1 are It is almost equal to the sum. That is, the output signal 17a of the adder 17 is almost the restored input video signal. Also, the output signal 14a of the subtractor 14
Since the output of the non-linear amplifier 16 is not so large even when is increased, the input signal is not restored even if this is added to the output of the adder 13, and the slew rate is reduced. Further, when the output signal 14a of the subtractor 14 becomes large, the output signal 16a of the nonlinear amplifier 16 becomes zero, so the output signal 13a of the adder 13 is output as it is. This is shown in FIG.

In the configuration shown in FIG. 4, if the characteristic of the non-linear amplifier 16 is set to "gain is 1 or more in a small amplitude region", a contour compensation function can be provided. In this case, a small-amplitude contour in which ringing is not noticeable is emphasized, and ringing occurs at a portion where ringing is noticeable due to the slew rate limitation, so that introduction to a band compression transmission system often gives good results.

Although the above description has been made in one dimension, it is easy to extend it in two dimensions. That is, it is possible to deal with each filter only by making it a two-dimensional filter configuration.

Further, it goes without saying that the present invention can be applied to analog signals other than video signals.

[Advantages of the Invention] As described above, according to the present invention, ringing due to band limitation can be suppressed.

In particular, in the technical field dealing with video signals, conventionally, this suppression has made a trade-off with deterioration of resolution by making the steepness of the band limiting filter gentle, but by implementing the present invention, a small amplitude contour part However, since the same signal as the input signal is obtained, the resolution is not deteriorated.

In addition, a contour compensation device by the applicant (Japanese Patent Application No. 61-64852).
No.) and the present invention in combination, a further effect can be obtained. That is, in the contour compensator, without adding overshoot or undershoot,
The slew rate of the signal can be increased to enhance sharpness. Therefore, the signal of which the slew rate is limited according to the present invention can be band-limited and transmitted, and the limited slew rate can be restored at the receiving end by using the contour compensating device. This makes it possible to obtain an image having no ringing at the receiving end and a high slew rate, that is, an image almost equal to the input signal at the transmitting end, regardless of the band limitation of the transmission path.

[Brief description of drawings]

FIG. 1 is a basic configuration diagram showing an embodiment of the present invention, FIGS. 2 (A) to (F) are diagrams showing waveforms of respective parts of the basic configuration diagram, and FIG. 3 is a block showing a specific example based on the basic configuration. FIG. 4 is a block diagram showing another embodiment, and FIGS. 5 (A) to 5 (F) are waveform charts showing the waveform of each part of FIG. 4 and the input / output characteristic example of the nonlinear amplifier. 1 ... Input terminal, 2 ... High-pass filter (HPF), 3 ... Absolute value circuit, 4 ... Low-pass filter (LPF), 5 ... Delay line, 6 ... Complementary gain controller, 7 ... Output Terminals, 8,9 …… Delay line, 10,11,12 …… Coefficient multiplier, 13 …… Converter, 14 …… Subtractor, 15 …… Square arithmetic circuit, 16 …… Non-linear amplifier, 17 …… Adder.

Claims (2)

[Claims] 1. A slew rate limiting filter for band limiting an input signal having a large slew rate, the high pass filter introducing the input signal and passing only a predetermined high frequency component, and an output signal of the high pass filter. To a positive component, a low-pass filter that introduces the input signal and passes only a predetermined low-pass component, and introduces the input signal that corresponds to the delay amount of the low-pass filter. A delay circuit for giving a delay time to the input signal, and according to the magnitude of the signal output from the absolute value circuit,
A slew rate limiting filter, comprising: a selection circuit for switching and outputting the output signal of the low-pass filter or the output signal of the delay circuit. 2. A slew rate limiting filter for band limiting an input signal having a large slew rate, the high pass filter introducing the input signal and passing only a predetermined high frequency component, and an output signal of the high pass filter. To a positive component, a low-pass filter that introduces the input signal and passes only a predetermined low-pass component, and introduces the input signal that corresponds to the delay amount of the low-pass filter. A delay circuit for giving a delay time to the input signal, and introducing the output signals of the low-pass filter and the delay circuit, respectively, by performing gain control according to the output signal of the absolute value circuit, each signal introduced And a complementary gain controller that outputs a summed signal whose gain is controlled for each slew rate limiting filter.