JPH04235491A - Filter circuit - Google Patents

Abstract

PURPOSE:To provide a filter circuit capable of selectively using an internal LPF circuit and an externally attached trap circuit corresponding to required specifications. CONSTITUTION:A switch (8) is provided between an LPF circuit (3) and an output terminal (2), and internal capacity (6), a coil terminal (5) and a coil (7) one end of which is grounded are connected to the side of a contact (B). Throwing the switch (8) to the side of contact (A) activates an internal LPF circuit mode; while throwing the switch to the side of contact (B) activates a trap circuit mode. Thus, desired output characteristics can be easily obtained as necessary.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Y/C (luminance signal/color signal) separation filter.

0002

2. Description of the Related Art FIG. 6 shows an example of the configuration of a conventional Y/C separation filter circuit. A primary or secondary LPF (low pass filter) circuit (3) connects the input terminal (1) and output terminal (2).
There are multiple LPF circuits (3) connected between
A monitor terminal (5) is connected between the output terminal (2) and the output terminal (2).

FIG. 7 is a graph showing the input/output characteristics of the filter circuit according to FIG. 6, where fc is the cutoff frequency of the LPF circuit (3), fo is the subcarrier frequency of the color signal, and in the NTSC system, 3. 58 MHz, and 4.43 MHz in the PAL system. This filter circuit is constructed by combining the characteristics of each LPF.
It is designed to make the fc of the PF output as high as possible and the amount of attenuation of the color signal as large as possible. Note that the monitor terminal (5) monitors the output characteristics of the LPF.

0004

[Problems to be Solved by the Invention] In the conventional filter circuit configured as described above, in order to obtain a high fc and a large attenuation amount at fo, it is necessary to prepare a plurality of LPFs with different filter characteristics and We have no choice but to adopt a method of combining various methods.

[0005] Therefore, when such a system is used as a Y/C separation filter, the user side
Although it has the advantage of being built-in and does not require any external connections, the circuit becomes larger because it is necessary to find the optimal combination for each LPF in order to achieve the desired filter characteristics. There were drawbacks. The present invention has been devised in view of the above-mentioned conventional problems, and its purpose is to selectively change the LPF characteristics built into an IC and the trap by an external coil, without increasing the size of the circuit, according to the required specifications. The objective is to provide a filter circuit that can be used for different purposes.

[0006]

[Means for Solving the Problems] A filter circuit according to the present invention is provided with a switch that enables selective switching control between an external coil internal LPF circuit and an external trap circuit by connecting to an external coil. It is something.

[0007]

[Operation] By connecting the changeover switch in the present invention to an external coil, it is possible to control the changeover and operate as a trap circuit.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. In the figure, the same components as those in the conventional circuit are denoted by the same reference numerals, and the explanation thereof will be omitted.

FIG. 1 shows a filter circuit according to an embodiment of the present invention, FIG. 2 shows an equivalent circuit when the switch (8) in FIG. 1 is connected to the A side, and FIG. 4 shows the circuit in the state shown in FIG. Output frequency characteristics, Figure 3 shows the switch (8) in Figure 1.
) is connected to the B side, and FIG. 5 shows the output frequency characteristics of the circuit in the state of FIG. 3.

In FIG. 1, there are contacts (A) and (B) between the primary or secondary LPF circuit (3) and the output terminal (2).
) is connected to a changeover switch (8) having a
This changeover switch (8) is connected to the switch control circuit (4)
controlled by An internal capacitor (6) consisting of a capacitor, a coil connection terminal (5), and an external coil (7) are connected in series between the LPF circuit (3) and the contact (B). One end is grounded. Furthermore, a connection between the internal capacitor (6) and the coil connection terminal (5) is connected to the input of the switch control circuit (4). A trap circuit is configured by the internal capacitor (6) and the coil (7).

Next, the operation will be explained.

[0012] In the case where the cutoff frequency as a characteristic of the LPF circuit (3) when Y/C separating the composite video signal inputted from the input terminal (1) does not have to be very high as shown in FIG. opens the coil connection terminal (5). If the switch is set so that the control circuit (4) is tilted toward the contact (A) at this time, the circuit of FIG. 1 equivalently becomes the circuit configuration of FIG. 2. This allows the LPF inside the IC to function as a Y/C separation filter having the characteristics shown in FIG. 4 without requiring any external attachment.

On the other hand, if the cutoff frequency fc inside the IC is insufficient for filter characteristics for Y/C separation, the coil connection terminal (5) is connected to an external coil (7) and grounded. As a result, a trap circuit consisting of the internal capacitor (6) and the external coil (7) is constructed, and functions as a filter that attenuates the chroma component. The output of the terminal (5) when not connected to the coil (7) has filter characteristics with a high cutoff frequency fc. Therefore, as shown in Fig. 5, the output frequency characteristics of the contact (B) when grounded via the external coil (7) are higher in fc and attenuation at fo (chroma frequency) than in Fig. 4. A large filter output will be obtained. The changeover switch (8) is turned to the contact (B) side by lowering the coil connection terminal (5) to the L level, that is, by grounding it via the coil (7). Attaching the coil (7) externally for this purpose also serves to configure a trap circuit in combination with the internal capacitance (6), so the external attachment of the coil can be done efficiently and with a simplified circuit configuration. It becomes possible.

[0014]

[Effects of the Invention] As explained above, according to the present invention, the switch for switching between the internal LPF circuit and the trap circuit using the external coil is controlled by grounding the external coil. It is possible to provide a flexible Y/C separation filter that can meet output characteristic standards.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of a filter circuit according to an embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram for explaining the operation of the circuit in FIG. 1;

FIG. 3 is an equivalent circuit diagram for explaining the operation of the circuit in FIG. 1;

FIG. 4 is a diagram showing circuit characteristics in the state of FIG. 2;

FIG. 5 is a diagram showing circuit characteristics in the state of FIG. 3;

FIG. 6 is a configuration diagram of a conventional filter circuit.

FIG. 7 is a diagram showing the circuit characteristics of FIG. 6;

[Explanation of symbols]

(1) Input terminal (2) Output terminal (3) LPF circuit (4) Switch control circuit (5) Coil connection terminal (6) Internal capacity (7) External coil (8) Selector switch

Claims (1)

[Claims] 1. An internal low-pass filter composed of an n-th order filter, a trap circuit constructed by connecting an internal capacitor and an external inductance to an output taken out from the midpoint of the filter, and both outputs. 1. A filter circuit comprising: a switch for switching a switch, wherein the switch can be switched by connecting the external inductance.