JPS6035809A - Muting circuit - Google Patents

Abstract

PURPOSE:To prevent the generation of a transient sound and to switch stereoponic reproducing into monophonic reproducing automatically at the time of a weak input by controlling impedance of an impedance variable circuit in accordance with the intensity of an S curve detecting signal and a receiving signal. CONSTITUTION:The impedance variable circuit 28 of which impedance is adjusted by a control voltage applied by a signal intensity detecting circuit 14 and the S curve detecting circuit 16. An S curve signal and a signal intensity level signal are applied from an AND circuit 18 to the circuit 28. In said constitution, the output impedance of the circuit 28 is increased and a detecting signal is controlled at its muting. A transient sound, i.e. a pop sound, at the muting control can be previously prevented under said control. Since the output impedance is increased at the weak input time, stereophonic reproducing is converted into a monophonic reproducing automatically at the reception of an FM signal.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a muting circuit, and particularly to muting control of an FM detection stage.

FIG. 1 shows an FM detection system of a general FM radio receiver. That is, an FM reception signal changed to an intermediate frequency is applied to the input terminal 2, and the signal output amplified by the intermediate frequency amplifier 4 is subjected to FM detection by the FM detection circuit 6.
A frequency deviation is detected. This FM detection output is given to a stereo demodulation circuit 10 via an electronic volume circuit 8, and in the case of stereo broadcasting, left and right stereo signals are detected.
The output is taken out from the output terminal 12R112L.

The gain of the electronic volume circuit 8 is controlled by the received signal level and the S curve signal. That is, a detection signal is obtained from the received signal taken out from the intermediate frequency amplifier 4 by the signal strength detection circuit 14, and an S curve signal outputted from the FM detection circuit 6 is given to the S curve detection circuit 16, and each signal strength detection circuit 14 and the outputs of the S curve detection circuit 16 are applied to the electronic volume circuit 8 via an AND circuit 18.

FIG. 2 shows an equivalent circuit of the electronic volume circuit 8. The electronic volume circuit 8 includes a resistor 22, a variable voltage source 24, and a reference voltage source 26 between the output terminal 20 and a reference potential point.
The terminal voltage of the variable voltage source 24 is changed based on the control signal given from the AND circuit 18.

In this way, in the conventional muting circuit, the electronic volume circuit 8 is configured as a variable impedance (2), so as shown in FIG. are piled up,
Since the muting control lowers the reference voltage VREF of the reference voltage source 26, fluctuations in the DC potential cause transient noise (pumping noise). Furthermore, although Fig. 3A shows the case of strong input, as shown in Fig. 3B,
Similar fluctuations in DC input potential occur even during weak input,
This causes transient noise to occur.

In addition, when muting, the output impedance decreases, so in the case of stereo playback with weak input, the S/N ratio decreases, the stereo separation characteristics deteriorate, and it may become impossible to maintain stereo playback. .

Therefore, when there is a weak input, it is necessary to forcibly control the monaural reproduction state. Conventionally, this switching control has been performed by detecting the signal strength, and a special additional circuit has been required.

The purpose of this invention is to prevent the generation of such transient sounds, automatically switch stereo playback to monaural playback (3) when there is a weak input, and provide a muting circuit that does not require a special additional circuit. .

The present invention provides a variable output impedance circuit that varies the output impedance of the FM detection circuit at the output section thereof, and makes it possible to control the impedance of the variable output impedance circuit in accordance with the strength of the S-curve detection signal and the received signal. It is characterized by

Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings.

FIG. 4 shows an embodiment of the muting circuit of the present invention, in which the same parts as those of the muting circuit shown in FIG. 1 are given the same reference numerals. In the figure, the muting circuit of this embodiment is provided with a variable impedance circuit 28 whose impedance is adjusted by control voltages provided by the signal strength detection circuit 14 and the S-curve detection circuit 16. This variable impedance circuit 2B has an AND
An S-curve signal and a signal strength level signal are provided from circuit 18. A capacitor 32 and a stereo demodulation circuit 10 are connected between the output terminal 3o of the impedance variable path 28 and the reference potential point.

FIG. 5 shows an equivalent circuit of the variable impedance diagram FI&28. That is, this variable impedance circuit 28 has a variable impedance element 34, a signal source 36, and a reference voltage source 38 connected between the output terminal 3o and a reference potential point.

According to such a configuration, the impedance of the variable impedance element 34 is variably adjusted in response to the control voltage generated during weak input and side peak. That is, in this case, the output impedance of the variable impedance circuit 28 becomes high, and the detection signal is subjected to muting control.

According to such muting control, there is no fluctuation in the DC potential as in conventional circuits, so it is possible to prevent the occurrence of transient pop sounds during muting control.

Moreover, during muting control, the output impedance of the variable impedance circuit 28 becomes high (5).As a result, a low-pass filter is formed by the impedance of the variable impedance circuit 28 and the capacitor 32, making it possible to reduce high-frequency noise and control separation. become. That is,
Since the output impedance becomes high when there is a weak input, stereo playback is automatically changed to monaural playback while receiving an FM signal. Therefore, there is no need to add a special switching circuit to change the reproduction method at the time of weak input as in the past, and the circuit configuration can be simplified.

FIG. 6 shows a specific example of the circuit configuration of the variable impedance circuit 28. That is, this variable impedance circuit 28 is provided with a pair of transistors 40 and 42, and a detected output is applied to the base of the transistor 40 from an input terminal 44.

The collector of each transistor 40.42 is connected to the power supply terminal 46.
A resistor 48 is connected to the power supply line to which the drive voltage Vcc is applied from
．． 50 and is also connected to an output amplifier 52 that amplifies its signal output. and,
The variable impedance output of the output amplifier 52 is taken out from the output terminal 54 (6), and a resistor 55 is connected to the base of the transistor 42.
．． 57 and a bias voltage source 59 via a feedback circuit.

Further, the emitters of the transistors 40 and 42 are connected in common, and a transistor 56 is connected between the emitters and the reference potential point via a resistor 58 with the emitters facing the reference potential point. The base and collector of a transistor 60 are commonly connected to the base of this transistor 56, and a resistor 62 is inserted between the emitter of this transistor 60 and a reference potential point. Transistors 56, 60 and resistors 58, 62 constitute a current transistor circuit, and a constant current flowing through transistor 60 is caused by a control voltage applied to a control input terminal 66 formed between the base and collector of transistor 60 via resistor 64. The operating currents of transistors 40 and 42 constituting the controlled Z differential amplifier are controlled.

According to such a configuration, it is easy to implement a semiconductor integrated circuit,
By varying the inbee (7) dance according to the control voltage applied to the control input terminal 66 during weak input and side peak times, it is possible to obtain good muting control without transient amounts and separation control.

As described above, according to the present invention, it is possible to suppress the occurrence of a transient amount due to muting control, and it is also possible to automatically change from stereo playback to monaural playback by separation control at the time of weak input.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing a conventional muting circuit.
Fig. 2 is a circuit diagram showing its equivalent circuit, Fig. 3 is an explanatory diagram showing its operating characteristics, Fig. 4 is a block diagram showing an embodiment of the muting circuit of the present invention, and Fig. 5 is its equivalent circuit. FIG. 6 is a circuit diagram showing a specific example of the circuit configuration of the variable impedance circuit. 6...FM detection circuit, 14...signal strength detection circuit,
16...S curve detection circuit, 2nd...impedance variable circuit. (8) Figure 4 Figure 5 IQLj〒■YoteEach set of values] ``22f1-17■←e≠
"Figure 6 6 52-

Claims (1)

[Claims] Muting characterized in that a variable impedance circuit for adjusting the output impedance is installed in the output section of the FM detection circuit, and the impedance of the variable impedance circuit can be controlled according to the strength of the S curve detection signal and the received signal. circuit.