JPH0535628Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an audio mute circuit suitable for a receiving circuit of a television receiver (hereinafter referred to as a TV receiver).

(Prior Art) An audio mute circuit provided in a conventional TV receiver will be described with reference to FIG. 4, which is a block diagram showing an example thereof. This audio mute circuit is a circuit that mutes harsh noise when receiving no signal.

The composite color video signal and audio signal received by the antenna 1 are high-frequency amplified by the tuner 2, and then converted into an intermediate frequency signal (in the NTSC system, the video signal is
58.75MHz, audio 54.25MHz) and is supplied to an intermediate frequency amplification circuit (hereinafter referred to as IF circuit) 3.
The intermediate frequency signal amplified by the IF circuit 3 is supplied to the video detection circuit 4. Here, from the intermediate frequency signal, a composite color video signal and a 4.5MHz (NTSC
The audio intermediate frequency signal (the audio carrier frequency of the system) is extracted.

4.5MHz audio intermediate frequency signal has a tuning point of 4.5MHz
The signal is supplied to the audio FM detection circuit 5c and becomes an audio low frequency signal. The audio low frequency signal is further amplified by an output amplifier 6 and reproduced as sound from a speaker 7.

On the other hand, the composite color video signal is transmitted to the video detection circuit 4.
is supplied to the video signal processing circuit 11 from
The signal is supplied to the synchronization signal separation circuit 12. The synchronization signal separated from the composite color video signal by the synchronization signal separation circuit 12 and the flyback pulse output from the deflection circuit (not shown) are supplied to an AND circuit 13.

The AND circuit 13 determines the presence or absence of a video signal based on the presence or absence of this synchronization signal. Only when the synchronization signal is "present", the output of the AND circuit 13 becomes High,
NPN transistor _Q2 turns on. Then,
The NPN transistor _Q3 is turned off, and a mute operation release signal is supplied from the transistor _Q3 to the mute terminal 6d of the output amplifier 6c. As a result, the output amplifier 6c performs normal amplification operation,
The audio signal is reproduced from the speaker 7.

On the other hand, if there is no synchronization signal, AND circuit 1
The output of transistor Q 3 becomes Low, transistor Q ₂ is turned off, and transistor Q ₃ is turned on. A mute operation start signal is then supplied to the mute terminal 6d of the output amplifier 6c. Therefore, the output amplifier 6c enters a mute operating state, and harsh noise is no longer reproduced from the speaker 7.

(Problem to be solved by the invention) However, in the conventional audio mute circuit described above, in order to use the synchronization signal separated from the video signal,
There were problems such as a complicated circuit configuration and high cost. This will be explained in detail below.

Many external noise signals are similar to synchronization signals. Therefore, even when receiving no signal,
There are cases where the AND circuit determines that the synchronization signal is present and the mute is not applied. To prevent this malfunction, it is necessary to adjust the sensitivity of the AND circuit. That is, it is necessary to set the lowest signal level for determining that the synchronization signal is "present." (in general,
The level of the pseudo synchronization signal due to the external noise signal is lower than the level of the regular synchronization signal. ) However, this sensitivity adjustment was difficult, making the circuit configuration of the AND circuit complicated.

(Means for Solving the Problems) Therefore, in order to solve the above problems, the present invention provides an audio intermediate frequency signal of an audio carrier frequency that is supplied, the tuning point is deviated from the audio carrier frequency by a predetermined amount, and the feedback signal is an audio FM detector to which a terminal and an output terminal are connected; an audio output amplifier to which an audio low frequency signal is supplied from the output terminal of the audio FM detector; and an audio output amplifier to which the audio low frequency signal is supplied from the feedback terminal of the audio FM detector. an integrator to which a low frequency signal is supplied; a comparator to which a DC voltage is supplied from the integrator to a negative input terminal and a reference voltage to a positive input terminal; the DC voltage is the reference voltage; The present invention provides an audio mute circuit comprising: a switching element that is turned on by the voltage supplied from the comparator and supplies a mute operation start signal to the audio output amplifier when be.

(Example) The audio mute circuit according to the present invention will be described below with reference to FIGS. 1 to 3. FIG. 1 is a block diagram showing one embodiment of this invention, and FIGS. 2 and 3 are diagrams for explaining the operation of the audio FM detection circuit. Note that the same parts as in the conventional example are given the same reference numerals, and a detailed explanation of the parts will be omitted.

In the audio mute circuit of the present invention, the tuning point of the audio FM detection circuit is slightly shifted from audio carrier frequency ₃ . Then, the DC level of the audio signal output from the audio FM detection circuit changes depending on the field strength of the received radio waves. Using this change in DC level, the audio mute circuit of the present invention determines a no-signal reception state and puts the output amplifier into a mute operating state.

As shown in FIG. 1, the antenna 1 includes a tuner 2,
Audio FM via IF circuit 3 and video detection circuit 4
It is connected to the detection circuit 5. The audio FM detection circuit 5 has a feedback terminal 5a and a detection output terminal 5b. The feedback terminal 5a and the detection output terminal 5b are coupled within the audio FM detection circuit 5.

The detection output terminal 5b is connected to a speaker 7 via an output amplifier 6. On the other hand, the feedback terminal 5a
is grounded through a series connected resistor R ₁ and capacitor C ₁ . This resistor R ₁ and capacitor C ₁ are
It constitutes an integrator 8.

The connection point between the resistor R ₁ and the capacitor C ₁ is connected to the negative input terminal of the comparator 9. A positive input terminal of the comparator 9 is connected to a variable resistor _R2 . A resistor _R3 is connected to one end of the variable resistor _R2 , and a resistor _R4 is connected to the other end. Variable resistance R ₂
is a resistor for setting the reference voltage applied to the positive input terminal of the comparator 9, and the resistors R ₃ and R ₄
is the voltage dividing resistance.

The output terminal of the comparator 9 is connected to the NPN transistor Q ₁
This transistor is connected to the base of
The collector of _Q1 is connected to the mute terminal 6a of the output amplifier 6, and the emitter is grounded.

Next, circuit operation will be explained. 4.5M separated from composite color video signal by video detection circuit 4
The Hz audio intermediate frequency signal is input to the audio FM detection circuit 5. The audio FM detection circuit 5 has a tuning point of
It deviates from the voice carrier frequency _S (4.5MHz in Japan) within the receivable range. In this audio FM detection circuit 5, the audio signal that has been FM detected and becomes a low frequency signal is sent from the detection output terminal 5b to the output amplifier 6.
is supplied to

Since the feedback terminal 5a of the audio FM detection circuit 5 is coupled to the detection output terminal 5b, the audio low frequency signal is also output from this terminal 5a.

The DC level of the audio low frequency signal output from the terminals 5a and 5b changes depending on the electric field strength of the received radio waves. This is because the tuning point of the audio FM detection circuit 5 is shifted from the audio carrier frequency _S. This change in DC level will be explained using FIGS. 2 and 3.

First, as shown in FIG. 2, when the tuning point of the audio FM detection circuit 5 matches the audio carrier frequency _S , the signals S ₂ and S ₃ output from the feedback terminal 5a
The DC level of does not change with electric field strength.
(In the figure, _S1 is the input signal to the audio FM detection circuit 5,
S ₂ and S ₃ are output signals from the feedback terminal 5a during normal electric field and weak electric field, respectively. ) On the other hand, as shown in FIG. 3, when the tuning point of the audio FM detection circuit 5 deviates from the audio carrier frequency _S , the signals S ₄ and S ₅ output from the feedback terminal 5a
The DC level varies depending on the electric field strength. (S ₄ and S ₅ in the figure are output signals for normal electric field and weak electric field, respectively.) Therefore, this DC level fluctuation can be used to determine the no-signal reception state (weak electric field).

The audio low frequency signal output from the feedback terminal 5a of the audio FM detection circuit 5 is supplied to the integrator 8 and converted into a DC voltage _V1 according to the signal. this
The DC voltage V ₁ is supplied to the negative input terminal of the comparator 9, and the reference voltage V ₂ is supplied to the positive input terminal.
compared to Then, the DC voltage V ₁ is the reference voltage
When the voltage drops below V ₂ , the comparator 9 supplies sufficient voltage to the base of the transistor Q ₁ to turn on the transistor Q ₁ . Transistor Q ₁ turned on
supplies a mute operation start signal to the mute terminal 6a of the output amplifier 6 from its collector. As a result, the output amplifier 6 enters a mute operating state, and the harsh noise when receiving no signal is eliminated from the speaker 7.
It will no longer be played.

Note that when the DC voltage V ₁ becomes higher than the reference voltage V ₂ , the transistor Q ₁ is turned off and the mute operation state of the output amplifier 6 is released.

Further, the reference voltage V ₂ may be set as follows depending on the reception state:
It can be adjusted by variable resistor _R2 .

In this way, in the audio mute circuit of the present invention, the circuit for determining the no-signal reception state can be configured with an integrator, a comparator, and one switching element.
The configuration of the entire circuit can be simplified.

(Effects of the invention) As described above, the audio mute circuit of the invention can be realized with a simple circuit configuration and cost can be reduced.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing an embodiment of the audio mute circuit according to the present invention, and Figures 2 and 3 are audio mute circuits.
FIG. 4, which is a diagram for explaining the operation of the FM detection circuit, is a block diagram of a conventional audio mute circuit. 1...Antenna, 2...Chyuna, 3...IF
Circuit, 4...Video detection circuit, 5...Audio FM detection circuit, 5a...Feedback terminal, 5b...Detection output terminal, 6...Output amplifier, 6a...Mute terminal,
7...Speaker, 8...Integrator, 9...Comparator,
C ₁ ... Capacitor, Q ₁ ... NPN transistor,
R ₁ , R ₃ , R ₄ ...Resistance, R ₂ ... Variable resistance, _S ...
Audio carrier frequency.

Claims (1)

[Claim for Utility Model Registration] An audio FM detector to which an audio intermediate frequency signal at an audio carrier frequency is supplied, a tuning point deviates from the audio carrier frequency by a predetermined amount, and a feedback terminal and an output terminal are connected. an audio output amplifier to which the audio low frequency signal is supplied from the output terminal of the audio FM detector; an integrator to which the audio low frequency signal is supplied from the feedback terminal of the audio FM detector; and this integrator. a comparator to which a DC voltage is supplied to the negative input terminal and a reference voltage to the positive input terminal; 1. An audio mute circuit comprising a switching element that is turned on by a voltage and supplies a mute operation start signal to the audio output amplifier.