JPH03292072A - Channel selection device - Google Patents

Abstract

PURPOSE:To prevent a level of an output signal from being abnormally high by providing a control circuit whose output is fluctuated when a channel selection means selects an off-channel and providing a voltage level reduction means reducing a voltage level of a 3rd input terminal of a tuner with the output from the control circuit. CONSTITUTION:An anode of a diode 13 connects to a 3rd input terminal 2c of a tuner 2 and a cathode is connected to a connecting point between a transistor(TR) 10 and a relay 11 respectively. A cathode of the diode 13 connects to a collector of the TR 10 and an anode connects to the 3rd input terminal 2c of the tuner 2. Thus, when the TR 10 is turned off by selecting an off- channel, the 3rd input terminal 2c of the tuner 2 is forcibly nearly to 0V via the diode 13, resulting that the up-gain of the tuner 2 of up/down type is remarkably reduced, and an output signal level from a 1st output terminal 2d to a television receiver 7 is decreased. Thus, the output signal level is kept to a prescribed level or below.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a channel selection device for a cable television (hereinafter referred to as CATV) receiver.

2. Description of the Related Art A conventional channel selection device of this type is shown in FIG. That is, 1st. Second. The third input terminals 2a to 2C and the first. A tuner 2 having second output terminals 2d and 2e, and an input signal transmission line 1 connected to a first input terminal 2a of the tuner 2.
, a tuning means 16 connected to the second input terminal 2b of the tuner 2, and a third input terminal 2C of the tuner 2.
an automatic gain control circuit (hereinafter referred to as AGC circuit) 4 connected to the television receiver 7 connected to the first output terminal 2d of the tuner 2, and connected to the second output terminal 2e of the tuner 2; and a video detection circuit 3 whose output side is connected to the input side of the automatic gain control circuit.

Further, a micro combinator 14 is connected to the channel selection means 16 as an example of a control circuit.

And the output port 14 of this microcomputer 14
A transistor 10 is connected to a through a resistor 9, and a relay 1 is connected between the collector of this transistor 10 and ground.
1 and a diode 12 are connected in parallel. The relay 11 cuts off the power to the television receiver 7 when the channel selection means 16 selects an off channel.

Note that 5.6 in FIG. 2 is a resistor and a capacitor that constitute a low-pass filter.

Problems to be Solved by the Invention In the configuration described above, the AGC circuit 4 has the first output terminal 2
This controls the level of the output signal from d to the television receiver 7.

By the way, this AGC circuit 4 is a circuit in which the input RF multiplied signal is converted to a certain predetermined frequency by the tuner 2, and the video detection circuit 3 performs video detection on the signal of that frequency, and operates according to the level of the detected signal. It is. That is, if the level of the detected signal is high, the AGC circuit 4 outputs the first output terminal 2.
It lowers the level of the output signal from d, increases the level of this output signal if the level of the detected signal is low, and as a result works to keep the level of the output signal constant even if the gain of tuner 2 changes. This is what we are doing.

By the way, when you are not watching CATV, you would like to turn off the channel selection device shown in Figure 2, but in the case of CATV, addressable data is always sent from the CATV station to this second channel.
Since the signal is being sent to the channel selection device shown in the figure, its power cannot be shut off. Therefore, this channel selection device is operating, but at this time, the channel selection means 16 is set to an off channel and selects a frequency with no television transmission signal.

However, when an off-channel is selected in this way, there is a problem in that the signal level output from the first output terminal 2d to the television receiver 7 cannot be kept below a certain value for the following reason. That is, a plurality of CATV transmission signals (54 MHz to 550 MHz in the United States) configured with RF frequencies separated by 6 MHz) are input to the input signal transmission line 1 .

When a viewer is watching a CATV program, use tuner 2,
The frequency of the channel selected by the viewer is converted to, for example, 61.25 MHz (3CH in the United States) and output to the television receiver 7. On the other hand, the channel frequency converted to 61.25MHz is input to the video detection circuit 3,
Video is detected.

This detected signal is input to the AGC circuit 4, and a DC voltage corresponding to the level of the video detected signal is applied to the AGC circuit 4.
The signal is outputted from and supplied to the third input terminal 2C of the tuner 2 via a low-pass filter composed of a resistor 5 and a capacitor 6. That is, when the level of the signal input to the input signal transmission line 1 becomes high and the level of the video-detected signal becomes high, the DC voltage level of the output of the AGC circuit 4 becomes low and the level of the signal input to the third input terminal of the tuner 2 becomes high. The DC voltage level applied to 8 becomes lower. Therefore, the gain of the amplifier of the tuner 2 decreases, and the output level from the first output terminal 2d of the tuner 2 to the television receiver 7 decreases. Moreover, if the level of the signal input to the input signal transmission line 1 becomes low and the level of the video-detected signal becomes low, the opposite effect will occur, and the signal will be transmitted from the first output terminal 2d of the tuner 2 to the television receiver 7. Output level increases. In this way, when watching a CATV program, the output level of the tuner 2 is held constant even if the level of the input signal increases.

However, when the viewer is not watching the CATV program, the CAT
The V receiver is set to an off-channel tuning state in which it tunes to an RF frequency (for example, 49.25 Mtlz in the United States) where there is no signal to be transmitted, and at this time the power supply to the television receiver 7 is cut off by the relay 11. It turns off because it is connected to an AC outlet. However, at this time, there is no output signal at the second output terminal of the tuner 2 at 61.25 MHz, so the AGC does not work, and therefore, depending on the input signal level from the input signal transmission line 1, the first output terminal 2d
The output signal level from the TV receiver 1!7 became extremely high. Therefore, an object of the present invention is to prevent the level of the output signal from the first output terminal to the television receiver from becoming abnormally high during off-channel selection.

Means for Solving the Problems and a distant relative of the above object, the present invention provides a control circuit whose output changes when an off channel is selected by the tuning means, and the output from the control circuit changes the third channel of the tuner. A voltage level lowering means is provided for lowering the voltage level of the input terminal of the input terminal.

According to the present invention, during normal CATV viewing, the AGC circuit operates and the output signal level can be kept below a certain value, and when an off-channel with no TV transmission signal is selected, the output from the control circuit is activated. By lowering the voltage level at the third input terminal of the tuner, the output signal level from the first output terminal to the television receiver is kept below a certain value.

EXAMPLE An example of the present invention will be described below with reference to FIG. Note that the same parts as those in the conventional FIG. 2 are given the same numbers to simplify the explanation.

In FIG. 1, 13 is a diode whose anode side is connected to the third input terminal 2C of the tuner 2, and its cathode side is connected to the connection point between the transistor 10 and the relay 11.

In the above configuration, in this embodiment as well, the contacts of the relay 11 control the power supply to the AC outlet to which the television receiver 7 is connected, as in the conventional example, and when the off-channel is selected by the channel selection means 16, the The output port 14a of the microcomputer 14 that controls the transistors 10.10 becomes high level, and the transistors 10. Relay 11 is turned off, power supply to the AC outlet is cut off, and television receiver 7 is turned off. In addition, when the channel is not in the off-channel selection state, the output of the microcomputer 14 that controls the transistor 10 becomes low level, the relay 11 is turned on, and power is supplied to the AC outlet. Machine 7 turns on. In this embodiment, as described above, the cathode of the diode 13 is connected to the collector of the transistor 10, and the anode is connected to the third input terminal 2C of the tuner 2. Therefore, by selecting the off-channel as described above, transistor 1
0 is turned off, the third input terminal 2C of the tuner 2 is forced to become almost OV via the diode 13, and as a result, the up gain of the up-down tuner 2 is significantly reduced, and the first output From terminal 2d to TV receiver 7
The output signal level to is also reduced. Therefore, even at this time, the output signal level can be kept below a certain value. As another embodiment, a method of directly using the output port 14a of the microcomputer 14,
output port 14a and relay driving transistor 10
A similar effect can be obtained by using a voltage change at the connection between the two.

Effects of the Invention As described above, the present invention can maintain the signal level of the output of a CATV receiver below a certain value, for example, whether or not a CATV program is being viewed.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a channel selection device according to an embodiment of the present invention, and FIG. 2 is a circuit diagram of a conventional example. DESCRIPTION OF SYMBOLS 1... Input signal transmission path, 2... Tuner, 2a to 2C... First to third input terminals, 2
d, 2e... 1st. Second output terminal, 3...
...Video detection circuit, 4...AGC circuit, 5.
9...Resistor, 6...Capacitor, 7.
...TV receiver, 10...transistor, 11...relay 12.13...
Diode, 14...Microcomputer.

Claims (2)

[Claims] (1) A tuner having first, second, and third input terminals and first and second output terminals, an input signal transmission path connected to the first input terminal of this tuner, and a first input terminal of this tuner. 2
an automatic gain control circuit connected to a third input terminal of the tuner; a television receiver connected to a first output terminal of the tuner; and a video detection circuit whose output side is connected to the input side of the automatic gain control circuit, and whose output changes when an off channel is selected by the channel selection means. A channel selection device further comprising a voltage level lowering means for lowering the voltage level of the third input terminal of the tuner by an output from the control circuit. (2) The voltage level lowering means includes a diode whose anode side is connected to the third input terminal, and a relay connected between the cathode of this diode and the ground, and which turns on and off the power of the television receiver; 2. The channel selection device according to claim 1, comprising a switching element connected between the relay, a connection point between the cathode of the diode, and an output terminal of the control circuit.