JPS628601Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to the improvement of a muting circuit,
In particular, it is an object of the present invention to provide a muting circuit that can perform a plurality of different types of muting using a single muting circuit.

When the power is turned on or off, a shock noise is generated due to the time constant of the power supply. Therefore, in order to prevent the above-mentioned shock noise from occurring, it is necessary to perform muting for a relatively long time until the power is sufficiently stabilized when the power is turned on, and a muting circuit that operates immediately when the power is turned off is required. be. Furthermore, when switching from AM broadcast reception to FM broadcast reception or other function switching, a shock noise will be generated due to the switching operation.
Since the above-mentioned shock noise is generated only during the switching operation, the muting circuit at the time of function switching must be configured to operate immediately upon switching and to be released after a predetermined period of time has elapsed. There are various other cases where mutating must be performed, such as during automatic search of the synthesizer tuner, but if the various mutating circuits mentioned above are specially provided, the circuits will become complicated and the cost will increase. This is not desirable because it occurs.

The present invention has been made in view of the above points, and aims to provide a muting circuit that can perform various types of muting using a single muting circuit. An embodiment of the present invention will be described below with reference to the drawings. In Figure 1, 1 is an antenna for receiving FM broadcasting, and 2 is an antenna for receiving FM broadcasting.
RF (radio frequency) amplification stage, 3 is a mixing circuit that mixes the RF signal and the local oscillation signal from the local oscillation circuit 4 to generate an IF (intermediate frequency) signal, 5 is an IF amplification stage that amplifies the IF signal, 6 is a detection stage for detecting an IF signal, 7 is a preamplification stage including a function changeover switch, 8 is a main amplification stage, 9 is a speaker, and the path from the antenna 1 to the detection stage 6 constitutes an FM tuner. . Further, numeral 10 indicates an AM tuner, and numeral 11 indicates a record player, which are switched by a function changeover switch included in the preamplification stage 7 and connected to the main amplification stage 8.

Furthermore, 12 is a relay consisting of a contact 13 and a relay coil 14 inserted between the main amplification stage 8 and the speaker 9, 15 is a drive transistor serving as a drive circuit for driving the relay 12, and 16 is the drive transistor 15. capacitor connected to the base of, 1
7 is a charging resistor for charging the capacitor 16;
18 is an NPN transistor that serves as a discharge circuit for discharging the capacitor 16; 19 is a PNP transistor that also serves as a discharge circuit for discharging the capacitor 16; 20 is composed of resistors 21 and 22, a capacitor 23, and a diode 24; , a first differentiating circuit for applying a mutating signal (first muting signal) corresponding to power-on to the base of the NPN transistor 18; 25 is a resistor 26, a capacitor 27, and a diode 28;
29 is a second differentiating circuit for applying a muting signal (second muting signal) corresponding to power off to the base of the PNP transistor 19, and 29 is an operation of a function changeover switch and an auto search operation of the local oscillation circuit 4. This is a pulse generating circuit that generates a pulse of a predetermined width that becomes a muting signal (third muting signal) to the NPN transistor 18 or the PNP transistor 19 in accordance with the above.

Next, the circuit operation will be explained. FM tuner and AM tuner are generally well known, so
The explanation of its operation will be omitted.

In order to operate the FM tuner, when the power switch is turned on at time to, the voltage of the power supply (+B) rises as shown in Figure 2 A, and stabilizes at time _t1 . Until the power supply voltage stabilizes, contact 13 of relay 12
If the contact 13 is closed, a loud shock noise will be generated due to the transient state of the circuit, so it is necessary to keep the contact 13 open. Therefore, a muting circuit that operates when the power is turned on is required, and this is accomplished by using the first differentiating circuit 20 and the NPN transistor 18. That is, when the power switch is turned on, the power supply voltage rises as shown in FIG. 2A, but when it is differentiated by the first differentiating circuit 20 , it becomes a waveform as shown in FIG. 2B. Therefore _, during the time T 1 from immediately after time t ₀ to time t ₂ when the output signal of the first differentiating circuit 20 exceeds the rising voltage (V _BE ), the NPN transistor 18 is turned on and the capacitor 16 is discharged. keep. Therefore, the drive transistor 15 is turned off during said time (T ₁ ), the relay 12 is not energized, and the contacts 13 remain open. At time _t2 , the output signal of the differentiating circuit 20 becomes less than or equal to V _BE , and the NPN transistor 18 is turned off. Therefore, charging of the capacitor 16 is started, and the capacitor 1
The voltage across the capacitor 6 increases according to a time constant determined by the charging resistor 17 and the capacitor 16, as shown in FIG. When the voltage across the capacitor 16 reaches the rising voltage of the drive transistor 15, the drive transistor 15 is turned on, the relay 12 is excited, the contact 13 is closed, and muting is canceled. The above is an explanation of the muting operation when the power is turned on. The muting time is (T ₁ +T ₂ ), which is a sufficient length, and the generation of shock noise when the power is turned on can be completely prevented.

Furthermore, when the power switch is turned off at time _t4 , the voltage of the power supply (+B) gradually decreases as shown in FIG.
A negative differential signal as shown in FIG. 2E is applied to the base of the PNP transistor 19. Therefore, the above
PNP transistor 19 turns on immediately after time _t4 ,
The charge stored in the capacitor 16 is discharged through the emitter-collector path of the PNP transistor 19. Therefore, the drive transistor 15
At _t4 , it is immediately turned off, relay 12 is opened, and muting is achieved when the power is turned off. At time _t5 , the signal applied to the base of the PNP transistor 19 approaches zero and the PNP transistor 19 is turned off.
Since the base voltage of transistor 5 has sufficiently decreased as shown in FIG. 2D, drive transistor 15 will not turn on after time _t4 .

Furthermore, mutating at the time of function switching will be explained. In the circuit shown in Figure 1, FM
If the tuner is operating normally, the voltage of the power supply (+B) is applied normally, the NPN transistor 18 and the PNP transistor 19 are off, and the capacitor 16 is fully charged.
Drive transistor 15 is on and relay 12 is energized. If the function selector switch of the preamplifier stage 7 is switched from the FM position to the AM position in this state, a shock sound may occur.
Therefore, in the present invention, the start of operation of the function changeover switch is transmitted to the pulse generation circuit 29, and the pulse generation circuit 29 generates a pulse of a predetermined width to perform muting. When the output pulse of the pulse generating circuit 29 is a positive pulse, the positive pulse is applied to the base of the NPN transistor 18 via a resistor 30 and a diode 31. Therefore, the NPN transistor 18 is turned on, the charge in the capacitor 16 is discharged, the drive transistor 15 is turned off, the relay 12 is released, and muting is performed. When the output pulse of the pulse generating circuit 29 is negative, the negative pulse is applied to the base of the PNP transistor 19 via the resistor 32 and the diode 33, turning on the PNP transistor 19, and the same pulse as in the case of a positive pulse is generated. Muting is achieved.

The pulse width of the output pulse of the pulse generating circuit 29 is not very wide. Therefore, as shown in FIG. 2, if a positive pulse of width W is applied at time _t6 , the NPN transistor 18 is turned on from time _t6 to time _t7 , and then turned off. Thus, the capacitor 16 is completely discharged while the NPN transistor 18 is on, and is charged immediately after the NPN transistor 18 is turned off. The state is shown in FIG. At time _t8 , the voltage across the capacitor 16 reaches the on level of the drive transistor 15, so the drive transistor 1
5 is turned on, relay 12 is excited and muting is canceled.

Therefore, muting at the time of function switching is carried out for a time _T3 , as shown in FIG.

Muting, etc. during automatic search of the synthesizer tuner can be considered in exactly the same way as the muting during function switching.
For example, auto search is performed by changing the oscillation frequency of the local oscillation circuit 4, and when the auto search button is operated, the auto search control section (included in the local oscillation circuit 4 in Fig. 1) generates a pulse. A signal is applied to the circuit 29, and the pulse generation circuit 29 generates a pulse, thereby performing muting until the local oscillation circuit 4 becomes stable.

As described above, the present invention is a practical device that can perform various types of muting with a single muting circuit, and can prevent the occurrence of shock noise.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 A, B, C, D, H, H, and T are characteristic diagrams for explaining the present invention. Explanation of main symbols, 15...drive transistor,
16...Capacitor, 18, 19...Transistor, 20 , 25 ...Differentiating circuit, 29...Pulse generating circuit.

Claims (1)

[Scope of utility model registration request] A drive circuit that drives a switching element for performing muting, a single capacitor that determines the operating time of the drive circuit, a charging circuit that charges the capacitor, and a discharge circuit that discharges the capacitor. , a circuit that generates a first muting signal based on power on, a circuit that generates a second muting signal based on power off, and a third muting signal based on operation switching other than power on/off. a circuit that generates a signal, and when the power is turned off, the driving circuit is controlled during the operating time of the discharge circuit based on the second mutating signal, thereby performing mutating, and when the power is turned on or the third mutating signal is When the mutating signal is generated, the drive circuit is controlled for a time equal to the sum of the operating time of the discharge circuit based on the first mutating signal or the third mutating signal and the charging time of the capacitor, thereby performing mutating. A mutating circuit characterized by the following.