JPS5924225Y2 - Muting circuit of television receiver - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a muting circuit that prevents unnecessary noise from being generated from speakers of a television receiver when changing channels or cutting off power.

Generally, in television receivers, the local oscillation frequency of the tuner deviates from the normal value when switching channels or shutting off the power, so the tuner derives the audio signal of the unnecessary channel, which is detected and amplified by the audio reproduction circuit and output to the speaker. may be played as noise.

However, in the case of conventional mechanical rotary tuners, the mechanical noise generated when the tuner shaft rotates is large, so the above-mentioned noise generated from the speaker is camouflaged by this mechanical sound. It didn't really make me feel that uncomfortable.

However, television receivers that use electronic tuners do not have the above-mentioned camouflage effect, so the above-mentioned noise from the speakers can be heard directly.
It was quite jarring.

However, the present invention proposes a muting circuit that eliminates these drawbacks, and the details thereof will be explained below with reference to the drawings.

FIG. 1 is a block diagram showing a schematic configuration of the muting circuit of the present invention, and FIG. 2 is a circuit diagram showing an embodiment thereof.
Corresponding parts in both figures are given the same figure number.

In Figures 1 and 2, 1 is a so-called AF that temporarily shorts the local oscillation frequency control voltage application terminal of the tuner to ground when switching channels to prevent malfunction of the automatic fine tuning circuit.
The T-defeat circuit is basically configured as a switching circuit that is switched by pulses generated in connection with channel switching operations.

2 is a circuit newly added to the receiver for the muting circuit of the present invention, and this circuit 2 includes a switching circuit 3 consisting of a transistor TR, resistors R1 and R2, and the AF
The pulses derived from the switching transistor TR2 in the T-type feed circuit 1 are transferred to the transistor TR,
A voltage drop sensing circuit 4, which is composed of a diode D1 leading to the base of the receiver, a differentiator circuit consisting of a capacitor C1 and a resistor R3 connected to the DC power supply V1 in the receiver, and generates a pulse at a certain time after the power is cut off, and the receiver Other DC voltages on board +
(2) A voltage holding circuit 5 consisting of two voltage dividing resistors R4 and R5, a diode D2, and a capacitor C2, and a waveform shaping circuit 6 configured as an integrating circuit of a resistor R6 and a capacitor C3 and shaping the output of the switching circuit 3. The circuit 7 is an audio reproduction circuit for a television receiver including an audio detection circuit and an audio amplification circuit, and the IC (AN
340), the DC voltage derived from the 6th pin is divided by the resistors R7, R and the variable resistor Rv, and the volume is adjusted by applying a voltage according to the voltage division ratio to the 4th pin via the resistor R9. The output terminal of the waveform shaping circuit 6 is connected to the connection point between the resistor R9 and the variable resistor Rv.

According to the muting circuit of the present invention having such a configuration, when the power is turned on, the voltage holding circuit 5 acts as a voltage stabilizing circuit and the switching transistor TR.

Since a constant voltage is supplied to the emitter of
When the pulse shown in FIG. 3A appears at the base of the switching transistor TR1, the pulse shown in FIG. A rectangular wave voltage similar to that shown in the figure appears at the output end.

Therefore, the amplitude to this square wave voltage is determined by IC (AN 340
) Voltage applied to pin 4 at maximum and minimum volume■,■
If the setting is made as shown in the figure for the max.

In this case, it goes without saying that the width of pulse a is appropriately selected so that this muting function continues until the end of the channel switching operation.

Also, when the power is cut off, the power supply voltage V1. (2) decreases as shown in FIG. 4A, but if the capacitance of the capacitor C2 is selected to be relatively large, the voltage (2) is sufficient to operate the switching transistor TR1 by the voltage holding circuit 5.

can be supplied to its emitter for a certain period of time T2 after the power is cut off, and at that time the voltage drop sensing circuit 4 applies the pulse shown in FIG. 4 to the base of the transistor TR1. In this case, a rectangular wave voltage similar to that shown in FIG. 3 (FIG. 4 C) appears at the output terminal of the waveform shaping circuit 6, and the generation of noise from the speaker can be prevented.

The reason why the waveform shaping circuit 6 was provided in the above embodiment is to prevent the sound from suddenly occurring on the receiver after the muting operation is completed, especially when switching channels, and this point has been taken into account. If not, there is no need to provide it.

As described above, according to the muting circuit of the present invention, it is possible to reliably and completely prevent noise generated from the speakers when changing channels or cutting off the power, so it is especially suitable for television receivers that use electronic tuners and have little mechanical noise. This circuit is particularly effective, and it also has the advantage that it can be implemented at a very low cost since it only requires adding a few circuits to a conventional receiver.

[Brief explanation of the drawing]

The drawings all relate to the muting circuit of the present invention, and the first
FIG. 2 is a block diagram showing a schematic configuration thereof, FIG. 2 is a circuit diagram showing one embodiment thereof, and FIGS. 3 and 4 are waveform diagrams illustrating its operation. 1: AFT defeat circuit (first pulse generation circuit), 4: Voltage drop sensing circuit (second pulse generation circuit),
3: Niswitching circuit, 5: Voltage holding circuit, 6: Waveform shaping circuit, 7: Audio reproduction circuit.

Claims (1)

[Scope of utility model registration request] A circuit that generates a first pulse within the switching time in connection with a channel switching operation, a circuit that senses a voltage drop of the power supply when the power is cut off and generates a second pulse of a constant width, and a circuit that generates a second pulse of a constant width when the power is turned on. a circuit that maintains a voltage equal to or higher than a predetermined value until the second pulse disappears;
A muting circuit for a television receiver, comprising a circuit that switches in response to a pulse, and a circuit that de-energizes an audio reproduction circuit in response to the output.