JPS5936071Y2 - Input filter switching circuit - Google Patents

Description

[Detailed description of the invention] This invention relates to a color television receiver that can selectively receive television broadcast signals of both the PAL system and the SECAM system, and particularly relates to an input filter used in the front stage of the color signal processing circuit. This invention relates to a switching circuit.

In a color television receiver that can selectively receive television broadcast signals of PA system and SECAM system, when the system of the broadcast signal to be received is switched and selected, an input filter circuit provided before the color signal processing circuit is used. The frequency characteristics of can also be changed.

In the IPALPAL method, the linear slope near the center frequency of the color signal output from the video detection circuit is approximately 5
It is necessary to perform reverse correction using the slope on the low frequency side of the single tuned circuit having a resonance point above MH2 so that the chroma frequency characteristics between the antenna and the color signal processing circuit become symmetrical frequency characteristics with respect to the center frequency.

Second, in the SECAM system, in addition to the same correction as in the PAL system described above, inverse Since the carrier wave is suppressed in advance using a Bell correction, that is, a filter having characteristics as expressed in equation (1) before being sent, it is necessary to correct this.

When receiving this signal, it is necessary to perform frequency correction using a bell-shaped filter opposite to the inverse bell filter on the transmitting side in order to remove the AM signal component.

As mentioned above, in the input filter circuit of the color signal processing circuit when receiving the SECAM system, in addition to the same correction as in the PAL system, frequency correction based on the characteristics of the bell-shaped filter is required.

Conventionally, the characteristic switching of the input fill circuit as described above was performed using the PA
This is done by a passive diode switch or the like using L or SECAM killer voltage.

However, when receiving data using the SECAM method, since the FM detection method is employed for chroma processing, gain correction is also required as characteristic correction, and a circuit that can be used in combination with such a manual filter circuit is desired.

This idea was made in view of the above circumstances, and PAL
, the manual frequency characteristics for the SECAM system can be switched using PAL or SECAM killer voltage, and the chroma signal amplification degree when receiving the SECAM system can be changed to PAL.
It is an object of the present invention to provide an input filter switching circuit that can obtain a sufficiently large input filter compared to that when receiving a system.

An embodiment of this invention will be described below in detail with reference to the drawings.

In FIG. 1, 11 is a chroma signal input terminal, and this input terminal is connected to one electrode of a capacitor 13 of a passive filter circuit 12.

The other electrode of this capacitor 13 is connected to an inductor 14.
It is connected to the reference potential end via the terminal and also to one electrode of the capacitor 15.

The other electrode of this capacitor 15 is connected to a power supply terminal 17 via a resistor 16, and also to the base of a transistor 18 that constitutes an active filter circuit using a differential amplifier circuit, and is further connected to a transistor 20 via a resistor 19.
connected to the base of

The emitter of the transistor 18 is connected to the emitter of the transistor 20 through a series circuit of resistors 21 and 22, and the emitter of the transistor 18 is connected to the emitter of the transistor 20 through a series circuit of a capacitor 23 and a coil 24. connected to the connection midpoint of

The midpoint of this connection is the transistor 2 which becomes the current source.
5 collector.

The emitter of this transistor 25 is connected to a reference potential terminal, and the base is connected to the reference potential terminal via a diode 26 and to a power supply terminal 28 via a resistor 27.

The base of the transistor 20 is connected to a reference potential terminal via a resistor 30 and a capacitor 31, and is also connected to a bias power supply terminal 32.

Furthermore, the collector of the transistor 20 is connected to a resistor 33.
The collector of the transistor 18 is connected to the collector of the transistor 18 via a resistor 35 and to the chroma output terminal 36.

The input filter switching circuit of this invention is constructed as described above, and the passive filter circuit is PAL.

Commonly used when receiving both SECAM methods.

As shown in FIG. 2, the characteristics of this passive filter circuit are frequency characteristics having a resonance point around approximately 5.2 MH7.

Next, on the active filter circuit side, a PAL or SECAM killer voltage is applied to the killer voltage input terminal 32.

That is, during PAL reception, a high voltage is applied, turning on transistor 20 and turning off transistor 18.

Further, when receiving SECAM, a low voltage is applied, turning on transistor 18 and turning off transistor 20.

Therefore, the active filter circuit is switched to the equivalent circuit shown in FIG. 3 when receiving the SECAM system, and is switched to the equivalent circuit shown in FIG. 4 when receiving the PAL system.

When receiving the SECAM method, the passive filter circuit 12 corrects the chroma signal in the 4.43 MHz band, and
It is added to the equivalent circuit shown in the figure and is input to the base of transistor 18.

Amplification degree A of the equivalent circuit formed by this transistor 18
v is.

Therefore, the value of C and L is f=4.286MH. (color subcarrier), this equivalent circuit operates as a complete common emitter amplifier.

(Obtaining gain compensation) That is, by selecting the values of C and L so as to meet the above conditions, a filter amplifier having a steep single-peak characteristic at 4.286 MHz can be obtained during SECAM reception.

The frequency characteristic in this case is as shown in characteristic A in FIG.

When receiving the PAL system, the input signal is first RB, , R
The signal is attenuated by the output voltage, and then amplified depending on the settings of the emitter and collector resistors RE and Rc1 of the transistor 20.

(Equivalent circuit of FIG. 4) The amplification degree of the equivalent circuit of FIG. 4 is as follows.That is, this amplification degree can be set to any gain by using any constant.

The frequency characteristic in this case is as shown in characteristic B in FIG.

According to the input filter switching circuit of this invention described above,
Due to its simple configuration, there is a large degree of freedom in setting the amplification degree of the equivalent circuit of the PAL system and the SECAM system.
High gain correction characteristics required by the CAM system can be obtained.

As explained above, this idea is PAL.

To provide an input filter switching circuit capable of switching the input frequency characteristics for the SECAM method using PAL or SECAM killer voltage, and obtaining a sufficiently larger chroma signal amplification degree when receiving the SECAM method than when receiving the PAL method. I can do it.

[Brief Description of the Drawings] Figure 1 is a circuit diagram showing an embodiment of the input filter switching circuit of this invention, Figure 2 is a characteristic diagram of the passive filter circuit in the circuit of Figure 1, Figures 3 and 4. FIG. 5 is a diagram showing the characteristics that the active filter circuit of the circuit in FIG. 1 switches when receiving SECAM and PAL systems, respectively, and when receiving each system. 12...Passive filter, 18.20.25...Transistor, 23...Capacitor, 24...Inductor, 32...Killer voltage input terminal.

Claims (1)

[Scope of utility model registration request] A PAL or SECAM chroma signal is selectively added to the input terminal, and the PAL chroma signal 4.43
a differential amplifier circuit constituted by a passive filter circuit whose characteristics are set to obtain frequency correction in the MH2 band; and first and second transistors having an output terminal of the passive filter circuit connected to each control terminal; A killer voltage input terminal connected to the control terminal of the second transistor and to which a PAL or SECAM killer voltage is applied for switching one transistor on and the other off; An input filter comprising: a resonant circuit section set in the current path of the first transistor so that an equivalent circuit of the amplifier circuit forms a circuit that resonates with the chroma signal frequency of the SECAM system in addition to the amplification action. switching circuit.