JPS6258193B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a malfunction prevention device for a television receiver, and is intended to prevent malfunctions in which a video intermediate frequency amplifier is fixed at the maximum gain when switching channels.

General television receivers have a system to keep the video output constant despite fluctuations in the antenna input level.
An AGC circuit and a noise removal circuit are provided to prevent the AGC voltage from fluctuating when noise enters the input. When a no-signal input suddenly changes to a strong input, such as when switching channels, the noise removal circuit treats this as noise and operates, changing the video intermediate frequency amplifier toward maximum gain. A so-called blocking condition occurs, which locks the AGC loop. Therefore, the present invention aims to provide a device in which such a situation does not occur, and embodiments of the present invention will be described below.

A video signal of approximately 2V _pp input to terminal A is detected by the reference voltage at the base of transistor 5 determined by resistors 2, 3, and 4, and the level at the tip of the synchronization signal is detected, and the signal portion higher than the reference voltage is transmitted to transistor 5. 6, further amplified by second stage transistors 7 and 8, and transistors 9,
Terminal 1 through the emitter follower circuit consisting of 10
It is taken out at 1. The voltage charged in the capacitor 12 externally connected to the terminal 11 is applied to the video intermediate frequency amplification circuit as an AGC voltage from the terminal B inside the integrated circuit.

At the base of the transistor 6, an integrating circuit consisting of a resistor 13 and a capacitor 14 removes high frequency components such as whiskers at the tip of the synchronizing signal, but the video signal itself is input to the base of the transistor 16. A reference voltage that is approximately 0.5V higher than the base of transistor 5 is applied to the base of transistor 15, and if there is noise of 0.5V or more from the tip of the synchronization signal, transistors 15, 16, 17, 1
7' amplifies only the noise component. Then, the transistor 18 is switched by a noise pulse, and the noise portion of the AGC voltage that enters the base of the transistor 7 is removed.

Transistor 1 is connected to the base of transistor 18.
9 is connected, but this circuit prevents the transistor 18 from remaining on and causing a blocking state when a strong input suddenly enters from a weak electric field or when noise with a sufficiently long pulse width enters. It is something to do. That is, the base of the transistor 19 has an integrating circuit consisting of a resistor 20 and an external capacitor 21, and relatively high frequency components such as noise are filtered out, but long pulses such as when switching channels are filtered out. At this time, the voltage at terminal 22 becomes sufficient to turn on transistor 19,
This turns off the noise removal transistor 18. In this way, the noise removal transistor 18 is turned off when there is a noise with a long pulse width, so that it is possible to prevent a blocking state caused by the noise removal transistor 18 remaining on at this time. In this way, the noise removal transistor 18 is turned off when there is no noise or when there is noise with a long pulse width, and is turned on to remove noise only when noise with a short pulse width is detected. So,
When the noise removal transistor 18 is turned off during noise with a long pulse width, the noise with a long pulse width from the transistor 5 is transmitted directly to the transistor 7,
8, the noise is not increased by turning off the noise canceling transistor 18.

However, resistors 20, 23 and capacitor 2
It is very difficult to perform frequency separation between the noise component and the pulse during channel switching using a time constant of 1. If the time constant is made large so as not to reduce the noise removal effect, the transistor 19 cannot be turned on sufficiently in response to sudden input fluctuations, resulting in blocking.

Therefore, in order to prevent such malfunctions, the present invention attempts to prevent blocking by using the AFT daily pulses generated when switching channels, and a pulse is applied to the terminal 24 in Fig. ing.

This pulse is generated, for example, as shown in FIG. FIG. 2 shows the case of a mechanical tuner, in which 12V is applied to one end of an AFT daylight switch 25 linked to a channel changeover switch, and pulses are extracted from the other end via a resistor 26 and a diode 27. In this way, a voltage with a long enough pulse width to reliably turn on the transistor 19 at the time of channel switching can be obtained.

FIG. 3 shows the case of an electronic tuner. When the channel selection button is pressed, an AFT defective pulse is generated from one terminal of the channel selection IC 28 linked thereto. This pulse is usually 12V,
It becomes OV only when selecting a channel. Since this pulse has a narrow pulse width, the transistor 29 and resistors 30, 31,
32, 33, capacitor 34, diode 35,
36, the polarity is reversed and the pulse width is approximately
Expanded to 50ms. That is, since the diode 35 is normally off at the collector of the transistor 29, the transistor 29 is also turned off, OV, and when selecting a channel, the diode 35 is conductive, and the transistor 29 is also turned on, resulting in a voltage of 12V. When conductive, the capacitor 34 is charged, so even if the channel selection IC 28 loses its output and the diode 35 is turned off, the transistor 29 remains on until the capacitor 34 is discharged. The pulse width can be widened.

As described above, according to the present invention, blocking can be completely prevented by applying a voltage from the outside only when switching channels. Furthermore, since a voltage with a wide pulse width is applied from the outside, a sufficiently large capacitor can be added so that the noise removal effect does not deteriorate. Moreover, since we used the switching pulse for AFC defect, which is always generated when switching channels in the tuner, as the pulse for canceling the operation of the noise removal circuit when switching channels, a special cancellation pulse generation means is required. It does not need to be provided separately and can be implemented with a simple circuit configuration.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a malfunction prevention device for a television receiver according to an embodiment of the present invention, and FIGS. 2 and 3 are specific circuit diagrams of a portion of the same device. 5, 6, 7, 8, 9, 10...amplification transistor, 18,19...switching transistor,
15, 16, 17, 17'... Noise extraction transistor, 25... Defeat switch, 24...
...Terminal, 28...IC for channel selection.

Claims (1)

[Claims] 1. An AGC circuit that generates an AGC voltage, a noise detection circuit that detects only noise, and a noise removal circuit that uses the noise detected by this noise detection circuit to remove noise from the AGC voltage obtained by the AGC circuit. A malfunction prevention device for a television receiver, comprising: a circuit that controls the noise removal circuit with a switch pulse for canceling AFT generated when switching channels, and cancels the operation of the noise removal circuit during the period of the switch pulse.