JPH08116497A - Television signal reception circuit - Google Patents

Abstract

PURPOSE: To prevent deterioration in the image quality due to occurrence of intermodulation or cross modulation of two amplifiers even when two tuners receive radio waves of different electric field strength in the television receiver able to display two images simultaneously with the two tuners. CONSTITUTION: When the strength of a reception signal of a station selected at least by one of tuners 20, 21 reaches a 1st threshold level set in advance or over, the gain of the concerned tuner is controlled decreasingly. When the strength of the reception signal of the selected station is further strengthened and the strength reaches a 2nd threshold level or over at which amplifiers 5, 6 are on the verge of cross modulation or intermodulation, then a gain of an amplifier means 10 or 11 provided to a pre-stage of the concerned tuner is controlled decreasingly. Thus, even when the strength of the reception signal of the station selected by the tuner 20 or 21 is increased, since the gain of the amplifier 10 or 11 is decreased before cross modulation or intermodulation takes place in the amplifiers 10, 11 provided to the pre-stage of the tuners 20, 21, no cross modulation or intermodulation takes place in the amplifiers 10, 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television signal receiving circuit suitable for use in a television receiver capable of displaying two images at the same time.

[0002]

2. Description of the Related Art Conventionally, there is a television receiver capable of displaying two screens at the same time. Some TV receivers of this type have two tuners for displaying two screens (one screen is called a parent screen and the other screen is called a child screen) at the same time. The other is used for the child screen. In the method using these two tuners, in order to prevent the deterioration of the image quality due to the interference between the antenna terminal voltages of the tuners (that is, the leakage voltage of the local oscillator), the high frequency signal distribution arranged in front of these tuners is distributed. The amplifier is equipped with an amplifier to isolate the tuners from each other.

FIG. 3 is a block diagram showing a circuit configuration of a part of a television signal receiving circuit used in a conventional two-screen television receiver. In this figure, 1 is an antenna and 2 is a distributor, which divides a received signal from the antenna 1 into two. Each of 3 and 4 is an amplifier that amplifies a wide band signal. These amplifiers 3 and 4 are for isolating the tuners from each other as described above. Each of 5 and 6 is an attenuator for attenuating a signal amplification amount in the amplifiers 3 and 4 to increase input / output isolation. With the attenuators 5 and 6, the tuner 20
Since the strength of the antenna terminal voltage (or the leak voltage generated locally) of the tuner 21 (or the tuner 21) entering the tuner 21 (or the tuner 20) becomes small, the image quality does not deteriorate. Gain control voltages VA and VB are supplied to the tuners 20 and 21 from a voltage control unit (not shown), and the gains of the tuners 20 and 21 are set. In this case, the voltage control unit inputs the selection outputs of the tuners 20 and 21, that is, the reception signals obtained by the station selection, respectively, and when the intensity of at least one reception signal exceeds a preset first threshold value. Control to reduce the gain of the corresponding tuner.

[0004]

The conventional television signal receiving circuit described above has the following problems. (A) The signal amplification in the amplifiers 3 and 4 is attenuated in the attenuators 5 and 6 to increase the input / output isolation so that the antenna terminal voltage of the tuner 20 (or tuner 21) becomes the tuner 20 (or tuner 21). Although the input amount is reduced to prevent the deterioration of the image quality, there is a problem that the amplifiers 3 and 4 cause intermodulation or intermodulation when a radio wave having a strong electric field strength is received. (B) In order to improve this problem, an attenuator may be provided in front of each of the amplifiers 3 and 4, but in this case, switching of the attenuator becomes very complicated, which is not practical. (C) It is also possible to provide an attenuator in front of the distributor 2. In this case, however, it is effective only when the two tuners are receiving radio waves having substantially the same electric field strength, and therefore the amplifier is also used. Causes intermodulation and cross modulation.

Therefore, an object of the present invention is to provide a television signal receiving circuit in which two amplifiers do not cause intermodulation or intermodulation even when two tuners receive radio waves having different electric field strengths.

[0006]

In order to achieve the above-mentioned object, a television signal receiving circuit according to the invention of claim 1 divides a received signal into two parts, and one receiving part distributed by this distributing part. First amplification means for amplifying the signal,
The second amplifying means for amplifying the other received signal distributed by the distributing means, the first attenuating means for attenuating the received signal amplified by the first amplifying means, and the second amplifying means. Second attenuating means for attenuating the amplified received signal, first tuner for receiving the received signal attenuated by the first attenuating means, and input for the received signal attenuated by the second attenuating means A second tuner to be output, and outputs of the first and second tuners respectively, and when at least one output exceeds a preset first threshold value, the corresponding first or second In a television signal receiving circuit including a gain control means for controlling the gain of the tuner to be lowered, each of the first amplifying means and the second amplifying means is capable of gain control from the outside, and the gain is The control means is the first , When the output of at least one of the second tuners exceeds the first threshold value and the first and second amplifying means exceed a second threshold value set before intermodulation or intermodulation occurs. , The control is performed so as to reduce the gain of the first or second amplifying means that outputs the reception signal to the corresponding tuner.

[0007]

According to the present invention, when the strength of the received signal of the station selected by at least one of the first and second tuners exceeds the preset first threshold value, the gain of the corresponding tuner is lowered. Then, the strength of the received signal of the selected station becomes stronger, and the first and second amplifying means provided before the first and second tuners have the second threshold value or more before the intermodulation or the intermodulation. Then, control is performed to reduce the gain of the first amplifying means or the second amplifying means provided in the preceding stage of the corresponding tuner. In this case, the first and second
In the case where the same station selects the same station, the first and second amplifying means do not cause intermodulation or intermodulation even if the intensity of the received signal becomes strong. When the tuners are receiving stations different from each other, if the strength of one received signal becomes strong, the first and second amplifying means cause intermodulation or intermodulation. In such a case, it works effectively. Therefore, even if the strength of the received signal of the station selected by one of the first and second tuners becomes strong, the tuner for one of the first and second tuners is tuned before the intermodulation or the intermodulation occurs by the first and second amplification means. Since the gain of the first or second amplifying means provided in the preceding stage is reduced, the first and second amplifying means do not cause intermodulation or intermodulation.

[0008]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a circuit configuration of an embodiment of a television signal receiving circuit according to the present invention. In addition,
In this figure, the same parts as those in FIG. 3 described above are designated by the same reference numerals and the description thereof will be omitted. In FIG. 1, 1
Each of 0 and 11 is an amplifier that amplifies a wide band signal, and amplifies and outputs the received signal distributed by the distributor 2. These amplifiers 10 and 11 are capable of gain control from the outside, and are controlled by a gain control unit 12. The gain controller 12 includes a CPU, ROM, and
RAM, an A / D converter, and a variable resistor 12A for setting two threshold values (first and second threshold values shown below),
And 12B. In this case, ROM has C
A program for controlling the PU is written,
RAM is used in the operation of the CPU. Also, A /
The D converter digitally converts the channel selection output of the tuners 20 and 21 (output of the reception signal of the selected station).

The gain control section 12 controls the gains of the tuners 20 and 21 as well as the gains of the amplifiers 10 and 11 based on the selected output from each of the tuners 20 and 21. That is, when the selected output exceeds the preset first threshold value, the gain of the tuner 20 or the tuner 21 is controlled to be decreased, and when the selected output becomes larger and exceeds the preset second threshold value, the amplifier 10 is controlled. Alternatively, the gain of the amplifier 11 is controlled to decrease. In this case, one of the tuners 20 and 21 is not always controlled,
Both may be controlled at the same time. In short, control is performed when the selected output exceeds the first threshold value. Similarly, one of the amplifiers 10 and 11 is not necessarily controlled, and both of them may be controlled simultaneously. In short, control is performed when the selected output exceeds the second threshold value.

Here, FIG. 2 is a graph showing how the gain control section 12 performs control. When the selected output (input level) becomes large and reaches the first threshold level L1,
Gain control of the tuner 20 or the tuner 21 is started. Then, when the selected output further increases and reaches the second threshold level L2, the gain control of the amplifier 10 or the amplifier 11 is started. In this case, the first and second threshold values can be arbitrarily set by the variable resistors 12A and 12B as described above. However, the tuners 20 and 21 are set to start control first. The reason for this will be described below.

As described above, when the intensity of the received signal of the station selected by at least one of the tuners 20 and 21 becomes equal to or higher than the first threshold value set in advance, the gain of the corresponding tuner is controlled to be reduced and selected. When the strength of the received signal of the station becomes stronger and becomes equal to or higher than the second threshold value before the amplifiers 10 and 11 cause the intermodulation or the intermodulation, the gains of the amplifiers 10 and 11 provided in the preceding stage of the corresponding tuner are lowered. Take control. Therefore, the tuners 20, 21
Even if the received signal strength of the station selected by one of the tuners becomes strong, before the amplifiers 10 and 11 cause the intermodulation and the intermodulation, the amplifiers 10 and 1 provided at the front stage of the one tuner
Since the gain of 1 is lowered, the amplifiers 10 and 11 do not cause intermodulation or intermodulation. On the other hand, the gain control range of the tuners 20 and 21 is about 50 dB, but the amplifiers 10 and 11
By setting the gain control range of 10 dB to 20 dB, the gain control range on the tuner side can be 30 to 40 dB. As a result, an inexpensive transistor or FET for high frequency amplification can be used for the tuner.

In the above embodiment, the tuners 20 and 21
The reason why control must be started first is as follows. That is, normally, the gain control of the tuner begins to work when the electric field strength is about -50 dBm, and therefore the gain controllable range is about 50 dB, so that the electric field strength can be controlled to about 0 dBm. On the other hand, the gain control range of the amplifier is narrow, and depends on the semiconductor used, it is about 20 to 30 dB. If the gain is controlled first from the amplifier, it is necessary to control the gain up to about 0 dBm, so the gain control is started at -20 to -30 dBm at the input end of the amplifier. For this reason, it is not possible to control the gain of the amplifier first because of the characteristics of the amplifier.

In the above embodiment, the first and second threshold values are set by the variable resistors 12A and 12B. However, these first and second threshold values are set in advance in the gain control section 12 by the R value.
It may be stored in the OM. Further, although the television signal receiving circuit of the television is used in the above embodiment, it goes without saying that the present invention can be applied to a video deck having a function of receiving television broadcasting.

[0014]

According to the present invention, even if the strength of the received signal of the station selected by one of the first and second tuners becomes strong, the first and second amplifiers perform intermodulation or intermodulation. Since the gain of the first or second amplifier provided in the preceding stage of the one tuner is lowered before the occurrence of the above, the first and second amplifiers do not cause intermodulation or cross modulation modulation. ,
Image quality does not deteriorate due to these causes. Further, since the gain control of the amplifying means is performed, the gain control range on the tuner side can be reduced, so that an inexpensive transistor or FET for high frequency amplification can be used for the tuner.
There is also an effect that the cost can be reduced as a whole.

[Brief description of drawings]

FIG. 1 is a block diagram showing a partial circuit configuration of an embodiment of a television signal receiving circuit according to the present invention.

FIG. 2 is a graph showing a gain control operation of the television signal receiving circuit of the embodiment.

FIG. 3 is a block diagram showing a circuit configuration of a part of a conventional television signal receiving circuit.

[Explanation of symbols]

 1 Distributor 2 Antenna 5 and 6 Attenuator 10 and 11 Amplifier 12 Gain control part 20 and 21 Tuner 110 High frequency signal distributor

Claims (1)

[Claims] 1. A dividing means for dividing a received signal into two parts, a first amplifying means for amplifying one received signal distributed by the distributing means, and another receiving signal distributed by the distributing means. Second amplifying means for amplifying, first attenuating means for attenuating the received signal amplified by the first amplifying means, and second attenuating for attenuating the received signal amplified by the second amplifying means Means, a first tuner for inputting the reception signal attenuated by the first attenuating means, a second tuner for inputting a reception signal attenuated by the second attenuating means, the first and the first tuners Gain control means for inputting the outputs of the two tuners, respectively, and performing control so as to reduce the gain of the corresponding first or second tuner when at least one of the outputs exceeds a preset first threshold value. And a TV with In the signal reception circuit, each of the first amplification means and the second amplification means is capable of gain control from the outside, and the gain control means outputs at least one of the first and second tuners. When exceeding the first threshold value and exceeding the second threshold value set before the intermodulation and the intermodulation by the first and second amplifying means, the reception signal is output to the corresponding tuner. A television signal receiving circuit, wherein control is performed so as to reduce the gain of the first or second amplifying means.