JPS6251878A - Comb line filter - Google Patents

Abstract

PURPOSE:To set freely the value of feedback coefficient to suppress the occurrence of a flase signal by taking out a circular delay signal input from the coefficient output and providing a low pass filter and a killer circuit in the circulating loop. CONSTITUTION:When a video signal having line correlations is inputted, a killer circuit 9 is operated as a buffer and has not the influence upon the signal. Relations of a formula are true between an input signal Vi and an output signal V0. In this formula, G1 and G2 are transfer functions of an LPF 3b and a delay part 3a respectively and K is the feedback coefficient. When taking notice of components having integer-fold line frequency, the signal amplitude is not changed though the feedback coefficient K is changed, and the value K is changed to set freely the degree of S/N improvement. When a video signal having no line correlations is inputted, the circuit is cut off by the killer circuit 9 not to output the signal to an output terminal 11, thereby preventing the occurrence of the flase signal.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a comb filter, and particularly relates to the configuration of a circulating comb filter used for improving the signal-to-noise (S/N) ratio of video signals. .

[Conventional technology]

Circulating comb filters are used to improve the S/N ratio of video signals. -As an example, Figure 2 shows NHK in April 1920.
A circulating comb filter described in Giken Monthly Report ``1 Video signal processing and image quality using a comb filter'' is shown.
The 8/N ratio improvement operation will be explained. In Figure 2,
(1) is a video signal input terminal, (2) is an addition circuit, (3)
is one horizontal time (abbreviated as 1H in the following explanation) delay circuit, (3a) is a delay section, (3b) is a low-pass filter, (4) is an adder circuit, (5) is a multiplier, and (6) is a The signal output terminal (7) is the signal amount normalization circuit, and (8) is the normalized signal output terminal.Next, the operation will be explained.0Now, the video signal input terminal (1)
) is V, and the input signal of IH delay circuit (3) is VH.
9 The signal of the signal output terminal (6) is set to vo, and the multiplier (5)
If the feedback system is , and the transfer function of the IH delay circuit (3) is G, then equation (1)?(2) holds true.

V=V+KV　・・・[1] ilI O The relationship with the force signal vi is expressed by equation [3].

1+G V = -V, ... [3] 0l-KO' 1+c) □ in the formula [3] is the transmission amount of the filter in Figure 2, 1 = KG, and is the transmission of the 5.1H delay circuit (3). When the function G handles a television signal with a large line correlation, fL = 1.
(fL is the line frequency), G== e−j
O) r (,=coaωτ, -jθ1nωτ,) Therefore, the transfer function of equation [3] can be expressed as equation [4].

If n is a positive integer, then formula [4] becomes 2 when f=nfL.
The filter shown in the figure is a comb-shaped filter that has abdomens at integral multiples of the line frequency, and by changing the feedback rate K, the sharpness of the amplitude frequency characteristic of the filter changes.
This indicates that the degree of improvement in the SIN ratio can be set freely.

At this time, the 1H delay circuit (3) is composed of a delay section (3a) and a low-pass filter (3b) that removes unnecessary signals (sampling pulse leakage, etc.) from the delay section (3a). and the low-pass filter (3t+) are set so that the sum of the delay times is 1H.

Further, the signal amount normalization circuit (7) receives the signal v0. of the signal output terminal (6). is a function of K, so the signal V. (1-K)
By multiplying, the normalized signal output terminal (8) is provided to output a signal that does not cause an output amplitude change due to a change in X.

[Problem that the invention seeks to solve]

In the case of conventional circuits, in order to change the SZN ratio improvement degree, when the coefficient K of the multiplier is changed, the multiplier of the signal amount normalization circuit must also be changed in proportion to KIC, which makes it difficult to maintain the consistency of both circuits. It was difficult to obtain, and it was difficult to substantially vary X. Additionally, if a video signal with no line correlation is input, a false signal may be generated, and in order to limit the horizontal frequency band, another low-pass filter is required in addition to the low-pass filter for the delay circuit. C,
There was a problem that it was uneconomical.

This invention was made in order to solve the above-mentioned problems, and it is possible to easily change the coefficient K of the coefficient generator, and it does not generate false signals, and it can be used as a low-pass filter for horizontal frequency band limitation. , low-pass filter for delay circuit and 1
The present invention aims to provide a circulating comb filter circuit that can be used in combination with two low-pass filters.

[Means for solving problems]

The cyclic filter according to the present invention takes the cyclic delay signal input from the system output, provides a low-pass filter in the cyclic loop, and further provides a killer circuit in the cyclic loop.

[Effect]

In this invention, since the cyclic delay signal input is taken from the coefficient output, a signal amount normalization circuit is not required.
By simply changing the coefficient K of the coefficient multiplier, the desired S/N ratio improvement can be obtained without changing the signal amount. Further, since a low-pass filter is provided in the circulation loop, it can be used both as a low-pass filter for horizontal frequency band limitation and as a low-pass filter for a delay circuit. Furthermore, if there is no line correlation, the signal circulation loop can be cut off by the killer circuit to suppress the generation of false signals.

[Embodiments of the invention]

FIG. 1 is a block diagram showing an embodiment of the present invention. In Figure 1, (1) is a video signal input terminal, (2
) is an adder circuit, (3a) is a delay section, (3b) is a low-pass filter, (5) is a multiplier, (9) is a killer circuit, (1
1 is a subtraction circuit, and (b) is a signal output terminal. Note that the first
In the figure, the same reference numerals are used for parts equivalent to those of the conventional example shown in FIG.

Next, the operation will be explained. If a video signal with line correlation is being input now, the killer circuit (9) acts as a buffer and does not affect the signal in any way. At this time, the signal of the video signal input terminal (1) is set to vI, and the output signal of the adder circuit (2) is set to 1. The output signal of the system multiplier (5) is converted to v2. The output of the signal output terminal (b) is set to V. , low-pass filter (3b)
The transfer function of 01. If the transfer function of the delay section (3a) is 02, the following equations (5) and (6) hold true.

■o= C)1v1-v2= GlVl-KG1V1=
(1-K) Glvl... [5] V = V + () V = V +KGGV ・-(6
)(”5), (6':] If you delete ■1 from the expression,
Formula 7a holds true for the input/output relationship.

Now, the delay time of the low-pass filter (3b) and the delay unit (3a) is τ1. If τ2 is set so that the sum of τ1 and τ2 is IH, 01 in the denominator of formula [7]
G2 is expressed as follows.

()()=. −jωτ1. . −Jωτ2=o−jω(τ
1+τ2) = , -jωτ1+==() Therefore, formula [Ma] becomes formula [8a].

-K. l-K. 1,...[8] has 1+ and -G1, and this is a comb that has an abdomen at an integral multiple of the line frequency and a comb section at an odd multiple, just like the filter in Figure 2. This indicates that it is a shaped filter. Now, if we focus on components that are integral multiples of the line frequency, the signal amplitude will not change even if the feedback coefficient K is changed, so a normalization circuit is not necessary. can be set freely. [01 in formula 8a is the transfer function of the low-pass filter (3b), which removes unnecessary signals from the low-pass filter (3'b) ti% delay section (3a) and limits the horizontal frequency band of the entire filter. I understand what you are doing.

Next, consider the case where a video signal without line correlation is input. At this time, the above formulas [5] to [8a] do not hold, and a false signal is generated. Therefore, if there is no line correlation, the killer circuit (9) can be used to shut off the circuit and prevent the signal input from the signal input terminal (1) from being output to the signal output terminal (b), thereby generating a false signal. do not.

In the above embodiment, the killer circuit (9) was provided on the output side of the low-pass filter (3b), but it may also be provided on the input side of the low-pass filter (31), and the same effect as in the above embodiment can be obtained. play.

〔Effect of the invention〕

As described above, in the comb filter according to the present invention, S
The value of the feedback system that determines the degree of improvement in the /N ratio can be freely set without the need for a signal amount normalization circuit, and one low-pass filter, which conventionally required two, can be used.
This has the effect of suppressing the generation of false signals.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a comb filter according to an embodiment of the present invention, and FIG. 2 is a block diagram showing a conventional comb filter. In the figure, (1) is the video signal input terminal, (2) is the addition circuit, (3a) is the delay section, (3b) is the low-pass filter, (5) is the multiplier, (9) is the killer circuit, and the wire is the subtraction circuit. In the circuit, (b) is a signal output terminal. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (4)

[Claims] (1) an adder circuit to which a video input signal is input to a first input terminal; a series connection body of a low-pass filter and a killer circuit that are connected in series with each other and to which the output of the adder circuit is input;
A series unit to which the output of the series connection unit is input; a delay unit to which the output of the series unit is input and the output is supplied to the second input terminal of the adding circuit; and the output of the series connection unit is the signal to be subtracted. The input terminal is provided with a subtraction circuit in which the output of the multiplier is input to the subtraction signal input terminal, and the sum of the delay times of the low-pass filter and the delay section is one horizontal time, and the output of the subtraction circuit is A comb filter that obtains the output signal from the terminal. (2) The comb filter according to claim 1, wherein the series connection body has a killer circuit connected after the low-pass filter. (3) The comb filter according to claim 1, wherein the series connection body includes a low-pass filter connected after the killer circuit. (4) Claims 1 to 3, characterized in that the coefficient of the coefficient generator can be arbitrarily set from the outside.
A comb filter according to any of paragraphs.