JPH0139021Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a television receiver equipped with an electronic channel selection device using an automatic preset method, and more particularly to an automatic gain control device for stabilizing the automatic preset operation.

A television receiver equipped with an electronic channel selection device using an automatic channel search method is equipped with a channel preset switch and a one-touch channel selection switch, and when the channel preset switch is closed, When the auto preset button is operated, the tuning voltage is automatically swept to perform station searching. When a broadcast signal is caught, that tuning voltage is memorized, and while automatic station searching is performed again, the tuning voltage of the receiving channel is sequentially changed. Configured to preset. In the high frequency signal system where this electronic channel selection device is installed,
In order to prevent contrast fluctuations on the image receiving screen, for example, the output of a video intermediate frequency amplifier is taken in to determine the strength of the received signal, and a gain control signal with a value inversely proportional to the level of the input signal is sent out. A control circuit (AGC circuit) is provided, and the video signal level is controlled to be constant by supplying this gain control signal to the high frequency amplifier of the tuner section and performing gain control. In addition to preventing contrast fluctuations as described above, such an automatic gain control system also stabilizes the operation of the electronic tuning device in the auto preset mode by keeping the level of the video intermediate frequency signal constant. It has become

However, in the electronic tuning device having the above-mentioned automatic gain control system, the above-mentioned automatic gain control circuit (AGC circuit) is connected to the tuner and The gain of the video intermediate frequency amplification circuit is controlled to the maximum. When the gain control circuit receives a signal indicating that a broadcast signal has been caught, it operates to maintain the video signal level at a predetermined constant value. At this time, the gain control circuit operates with a delay time, so when searching for a broadcast signal with a strong received signal level (field strength), the video intermediate frequency amplification circuit and tuner are Nonlinear distortion occurs due to excessive input to the frequency mixing circuit, and for example, 1.5MHz interference waves are generated by the audio carrier of the adjacent channel. As a result, the problem arises that the electronic tuning device in the auto preset mode mistakes the 1.5 MHz interference wave as a received signal, stops the scanning operation, and stores the tuning voltage in memory.

In recent television receivers, video intermediate frequency amplifiers and video detectors have been integrated, and the maximum permissible input level has accordingly become lower, so such problems are likely to occur.

In addition, when the gain control signal output from the gain control circuit is matched to the UHF tuner section, when the VHF tuner section is selected to receive VHF band broadcasting,
The level of the video intermediate frequency signal output from this VHF tuner section becomes lower than the standard value, and as a result, auto preset cannot be performed for weak broadcast signals. have.

Therefore, an object of the present invention is to provide an automatic gain control device that can stably locate and automatically preset all broadcast signals from weak electric fields to strong electric fields in all reception bands of VHF and UHF. That's true.

In order to achieve this object, the present invention weights the gain control signal in accordance with the selection of the receiving band when the electronic tuning device is in the auto preset mode.

Hereinafter, the automatic gain control device according to the present invention will be described in detail with reference to the drawings.

FIG. 2 is a circuit diagram showing an embodiment of an automatic gain control device according to the present invention. In the figure, a receiving antenna 1 is connected to a tuner circuit 2. The tuner circuit 2 is composed of a VHF tuner section 3 and a UHF tuner section 4, and each VHF and UHF tuner section 3, 4 includes a high frequency amplifier 5a, 5b and a local oscillator, as is generally known. 6a, 6b and mixers 7a, 7b. 8 is a video intermediate frequency amplifier that amplifies the video intermediate frequency signal IF sent out from the tuner circuit 2, and its output is sent to the video detector 9.
A video signal VID is taken out, amplified by a video amplifier 10, and then supplied to a video reproduction system (not shown). 11 is a gain control circuit that receives the video intermediate frequency signal IF as an input and sends out a gain control circuit GC for making the signal level constant through feedback control; 12 is a gain control circuit that receives the output of the video detector 9 as an input for automatic fine adjustment; The discriminator circuit detects the normal tuning point of the video intermediate frequency signal using a window comparator and sends out an automatic fine adjustment signal AFT. 13 is a synchronization separation circuit that separates and outputs a synchronization signal from the video signal supplied from the video amplifier 10; 14 is a tuning device using an automatic station search method; The tuning voltage CSS supplied to each local oscillation circuit 6a, 6b is swept in one direction, and the discriminator circuit 1
When a broadcast signal finely adjusted by the automatic fine adjustment signal AFT supplied from 2 is received, the presence of a television broadcast signal is detected by the synchronization signal supplied from the synchronization separation circuit 13, and the tuning voltage CSS is detected. It is configured to memorize and sequentially move to the next tuning position. After the auto preset operation is completed, automatic channel selection is performed by selectively operating a channel selection switch (not shown) provided on the keyboard 15. In addition, the keyboard 15 has
A band select switch for specifying the VHF and UHF bands is provided, and the tuning device 14 sends band select signals VS and US for VHF and UHF selection to the tuner circuit 2 in response to the band specification by the band select switch. supply 16
is the gain control signal supplied from the gain control circuit 11
This is a weighting circuit that weights the GC according to the reception band and then supplies it to the high frequency amplifiers 5a and 5b to perform gain control.When the auto preset signal OPS is supplied from the keyboard 15, the band Weighting is performed in accordance with the select signals VS and US, and is canceled when the power is turned on or by the one-touch channel selection mode signal WS.

FIG. 3 is a circuit diagram showing a specific example of the weighting circuit 16 shown in FIG.
The one-touch tuning mode signal WS is sent out, and when the mode select switch 17 is set to the opposite state as shown in the figure, the channel preset mode is entered.If the auto preset button 18 is momentarily closed in this state, the auto preset signal OPS is sent out. be done. 19 is an OR gate which receives as input the initialization signal IR and the one-touch tuning mode signal WS supplied from an initialization circuit constituted by a series body of a capacitor 20 and a resistor 21; 22 is set by an auto preset signal OPS;
A flip-flop is reset by the output of the OR gate 19, 33 is a NAND gate which receives the VHF band select signal VS and the set output of the flip-flop 22, and 34 receives the UHF band select signal US and the set output of the flip-flop 22. Nando Gate, 35,3
6 is a resistor that divides the gain control signal GC,
A weighted gain control signal GC' is sent from the voltage division point A to the high frequency amplifier circuits 5a and 5b. 37 is a diode connected through a resistor 38 between the voltage dividing point A and the output end of the NAND gate 33; 39 is a diode connected between the voltage dividing point A and the output end of the NAND gate 34 through a resistor 40; It is a diode. The resistors 35 and 36 are connected to a gain control signal that gives maximum gain to the high frequency amplifiers 5a and 5b.
GC′ (usually 8V) is set to the value sent out. Further, resistors 38 and 40 are resistors that control the gain control signal GC', and correspond to weighting for each reception band, and the resistor 40 in charge of the UHF band is smaller than the resistor 38 in charge of the VHF band. value (reverse AGC method).
Note that 41 and 42 are pull-down resistors.

In the automatic gain control device constructed as described above, the channel preset mode is entered by setting the mode switch 17 of the keyboard 15 shown in FIG. 3 to a state opposite to that shown. In this state, when the auto preset switch 18 is operated momentarily, the auto preset signal OPS is sent out and the flip-flop 22 is set. On the other hand, the tuning device 14 supplies the band selection signal VS or US to the tuner circuit 2 in response to the band designation and auto preset mode designation supplied from the keyboard 15 to turn on the tuner sections 3 and 4 for VHF or UHF. Along with selection, the tuning voltage CSS
sweep in one direction. Therefore, the VHF tuner unit 3, which is in charge of the band specified by the band select switch of the keyboard 15, receives the broadcast signal by sequentially moving the tuning point within the receiving band. In this case, the gain control circuit 11 determines the level of the video intermediate frequency signal IF and supplies the weighting circuit 16 with a gain control signal GC for making the signal level constant. The weighting circuit 16 has a flip-flop 22 set by an auto preset signal OPS, and receives VHF or
When a band select signal VS or US for selecting the UHF tuner sections 3 and 4 is supplied, the output of the NAND gate 33 or 34 becomes "L" in response to the band select signal. That is, in this case, since the band select switch provided on the keyboard 15 specifies the VHF band, the band select signal VS becomes "H" and the output of the NAND gate 33 becomes "L". When the output of the NAND gate 33 becomes "L", the point A set by the resistors 35 and 36 is connected to the diode 37, the resistor 38 and the NAND gate 33 so that the gain control signal GC' that provides the maximum gain is sent out. It will be pulled to earth via. As a result,
The gain control signal GC' output from point A is lowered from the value at which the maximum gain is obtained in accordance with the value of the resistor 38.
, that is, weighting corresponding to the VHF tuner section 3 is performed. Therefore, the video intermediate frequency signal IF output from the tuner circuit 2 is
The integrated video intermediate frequency amplifier 8, video detector 9, and gain control circuit 11 are kept constant at optimal values, and stable operation can be obtained even with weak broadcast signals. Become. and,
When the discrimination circuit 12 sends out the fine adjustment signal AFT to perform fine adjustment, and the synchronization separation circuit 13 separates the synchronization signal included in the video signal VID and supplies it to the tuning device, the tuning device 14 receives the broadcast signal. It determines that the channel has been received and stores the selected channel voltage CSS in memory. Then, once the memory operation is completed, the tuning voltage is reset again.
Continue the CSS sweep operation and repeat the operation to move to the next tuning position.

Next, when the band select switch provided on the keyboard 15 is set to the UHF band,
The band selection signal US is sent from the channel selection device 14. As a result, the output of the NAND gate 34 shown in FIG. in this case,
The resistance value of the resistor 40 is set lower than that of the resistor 38 which is in charge of the VHF band. Therefore, when the UHF band is selected, the level of the gain control signal GC' output from point A is VHF.
However, since the tuner section 4 in charge of the UHF band has high gain reduction characteristics as shown in FIG. The level of the frequency signal IF is constant whether the VHF band or UHF band is selected, and this allows
Fluctuations due to differences in characteristics between the VHF tuner section and the UHF tuner section are prevented. In other words, the weighting circuit 16 performs weighting to correct the gain reduction characteristics of the tuner sections 3 and 4 responsible for VHF and UHF when selecting the VHF band or UHF band. This weighting value is set by the resistors 38 and 40. Therefore, even if the UHF band is selected, input signals exceeding the permissible values are prevented from being supplied to the integrated video intermediate frequency amplifier 8, video detector 9, etc. as in the past, and interference waves are prevented due to nonlinearity. This solves problems such as erroneous presets occurring during auto preset.

Next, when the mode select switch 17 provided on the keyboard 15 is set to the one-touch tuning mode as shown in FIG. is reset. Therefore, in this one-touch tuning mode, the outputs of the NAND gates 33 and 34 are always fixed at the "H" level, so the diode 3
7 and 39 are reverse biased and cut off.
Then, the potential at point A, that is, the gain control signal GC' is completely unaffected by the resistors 38 and 40, and the resistor 3
A gain control signal GC' for maximum gain control determined by the values of 5 and 36 will be sent out. Further, the reset operation of the flip-flop 22, that is, the weighting operation of the gain control signal, is also canceled by the initialization signal IR generated by the initialization circuit composed of the capacitor 20 and the resistor 21 when the power is turned on.

In addition, in the above-mentioned embodiment, VHF and UHF
We have explained the case where the weighting is changed according to the band selection, but the VHF band tuner is divided into low band and high band,
It goes without saying that weighting may be given to these band selections as well.

As explained above, the automatic gain control device according to the present invention is equipped with a weighting circuit that weights the gain control signal supplied to the high frequency amplifier of the tuner circuit in accordance with the selection of the reception band. , when changing the reception band, deviations from the optimum value of the video intermediate frequency signal due to differences in gain reduction characteristics of the tuner section are prevented, and along with this, deviations in contrast are also prevented.
It has various excellent effects such as stabilizing the station searching operation in the auto preset mode of the station selection device.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing gain reduction characteristics in the tuner section of each receiving band, and FIG. 2 is a circuit diagram showing an automatic gain control device according to an embodiment of the present invention.
FIG. 3 is a circuit diagram showing a specific example of the weighting circuit shown in FIG. 2. 2... tuner circuit, 3, 4... tuner section,
5a, 5b...High frequency amplifier, 6a, 6b...Local oscillator, 7a, 7b...Mixer, 8...Video intermediate frequency amplifier, 9...Video detector, 10...Video amplifier, 11...Gain control circuit , 12... Discrimination circuit, 13... Synchronization separation circuit, 14... Tuning device,
15...Keyboard, 16...Weighting circuit.

Claims (1)

[Scope of utility model registration request] With the automatic gain control circuit controlling the gain of the tuner and video intermediate frequency amplifier to the maximum state,
A television receiver having an electronic tuning circuit equipped with a tuning voltage preset function that automatically detects a broadcast signal by sweeping the tuning voltage of a tuner and stores a tuning voltage corresponding to the detected broadcast signal. A weighting circuit is provided that stabilizes the video signal level by weighting the gain control of the tuner and video intermediate frequency amplifier by the automatic gain control circuit in accordance with the reception band of the tuner when presetting the voltage. An automatic gain control device characterized by: