JPH0342022B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a muting device for an FM receiver that mutes a detected signal according to the electric field strength.

[Conventional technology]

FM receivers generally have an automatic gain control (AGC) function, which ensures that the level of the detected signal remains constant regardless of the received field strength. Therefore, if the detected signal is applied directly to the audio amplifier, the received electric field strength will drop and the S/N of the detected signal will decrease.
Every time the ratio deteriorates, harsh noise will be output from the speaker.

Therefore, conventionally, a field strength detection circuit that outputs a signal corresponding to the received field strength (hereinafter referred to as an S level signal) and a mute circuit that mutes the detected signal are provided, and the detected signal is muted according to the S level signal. This prevents harsh noise from being output when the electric field is weak.

[Problem that the invention seeks to solve]

However, the above-mentioned conventional example has the following problems. That is, in a weak electric field region where miute is required, the electric field strength is not constant, and the S level signal output from the electric field strength detection circuit has noise superimposed thereon as shown in FIG. 3A.
If the mute circuit is operated directly by an S-level signal, the mute circuit responds to whiskers of noise and repeats its operation in a very short period of time, so the sound output from the speaker also repeats its amplitude in a short period of time. As a result, there was a problem in not being able to apply appropriate Miyuto.

Therefore, it has been proposed to remove the noise component superimposed on the S level signal using a low-pass filter, etc., and operate the mute circuit using the signal from which the noise component has been removed. There was a problem. That is, when the S level signal shown in Figure 3A is applied to the low pass filter, the output signal has a delay with respect to the S level signal as shown in Figure 3B, so there is a sudden change in the electric field strength. In this case, there was a problem that the mute operation was delayed and a harsh noise was output.

The present invention solves the above-mentioned problems, and its purpose is to be able to apply mute in response to steep fluctuations in electric field strength, and to apply mute in a very short time in response to whiskers of noise. The goal is to avoid repeating the miute action.

[Means for solving problems]

In order to solve the above-mentioned problems, the present invention provides a field strength detection circuit that outputs a signal corresponding to the received electric field strength in a muting device of an FM receiver that mutes the detected signal of the FM detection circuit according to the received electric field strength. an integrator for smoothing the output signal of the field strength detection circuit; clipping means for clipping the output signal of the field strength detection circuit at a level that is a predetermined amount higher and lower than the output signal level of the integrator; A muting circuit for muting the detected signal based on the output signal of the clipping means is provided.

[Effect]

The output signal of the field strength detection circuit is smoothed by an integrator. The clipping means clips the output signal of the field strength detection circuit at a level that is a predetermined amount higher and lower than the output signal of the integrator. Therefore, the signal output from the clipping means has the amplitude of the noise component superimposed on the output signal of the field strength detection circuit limited, so the detected signal is muted based on the output signal of the clipping means. By doing so, since muting is basically performed in response to the output signal of the electric field strength detection circuit, it is possible to apply muting in response to sudden fluctuations in the electric field strength, and it is also possible to mute in response to sudden fluctuations in the electric field strength. Since the steep part has been removed, there is no need to repeat the miute movement in a very short period of time in response to this whisker-like part.

〔Example〕

FIG. 1 is a block diagram of an embodiment of the present invention.
1 is an antenna, 2 is an RF (high frequency) amplifier, 3 is a mixer, 4 is a local oscillator, 5 is an IF (intermediate frequency) amplifier, 6 is an FM detection circuit, 7 is a field strength detection circuit,
8 is a mute signal generation circuit, 9 is a mute circuit,
10 is an audio amplifier, 11 is a speaker, Q1
Q5 is a transistor, D1 and D2 are diodes, C1 to C6 are capacitors, R1 to R10 are resistors, and IN is an input terminal.

The signal received by antenna 1 is sent to RF amplifier 2,
FM detection circuit 6 via mixer 3 and IF amplifier 5
and is added to the electric field strength detection circuit 7. The FM detection circuit 6 applies a detection signal to the input terminal IN of the mute circuit 9, and the electric field strength detection circuit 7 generates an S level signal corresponding to the received electric field strength, for example, as shown in FIG. 2A, to generate a mute signal generating circuit. Add to 8.
In other words, as can be seen from A in the same figure, the S level signal has a strong received electric field strength, and when it is constant, it is almost flat, but when the received electric field strength changes rapidly, its DC component changes to the received electric field strength. It changes accordingly and becomes a shape in which noise is superimposed.

The mute signal generation circuit 8 is the electric field strength detection circuit 7
A mute signal is created by limiting the amplitude of the noise component superimposed on the S level signal from the signal, and is added to the mute circuit 9. For example, if the S level signal shown by the solid line in Figure 2B is applied from the field strength detection circuit 7, the output signal a of the integrator consisting of the resistor R2 and capacitor C3 will be as shown by the dotted line in Figure 2B. . Note that the S level signal shown by a solid line in FIG. 1B is an enlarged version of the portion (A) in FIG. Then, the output signal a of the integrator is applied to the bases of the transistors Q1 and Q2, and the transistor Q1 receives the level of its emitter, that is, the electric field strength detection circuit 7 via the resistor R3.
When the level of the S level signal supplied to the emitters of transistors Q1 and Q2 becomes lower than the level of signal a by more than Vd, which is the voltage drop between the base and emitter, the transistor Q2 becomes on state, and the transistor Q2 is connected to the resistor by the field strength detection circuit 7. The level of the S level signal supplied to the emitter of transistor Q2 via R3 is lower than the level of signal a by the voltage drop between the base and emitter.
When it becomes larger than Vd, it becomes on state. The operation of the transistors Q1 and Q2 is stabilized when the output level of the mute signal generating circuit 8 is Vd lower and Vd higher than the level of the signal a, respectively. Therefore, the mute signal output from the mute signal generation circuit 8 is as shown in FIG. 2C.
The S level signal is ± centered on the level of signal a.
It is clipped within the range of Vd. In addition, S
The level difference between the level signal and the integrator output signal a is
If the voltage is within Vd, both transistors Q1 and Q2 are off, so the S level signal is output as is as a mute signal.

The mute signal generated by the mute signal generating circuit 8 is applied to the base of the transistor Q3 in the mute circuit 9. Here, as the level of the mute signal applied to the base of the transistor Q3 becomes smaller, the collector current of the output transistor Q4 decreases, so as the received electric field strength becomes lower and the level of the mute signal becomes smaller, The amount of mute increases, and speaker 11
The sound output from the will be reduced.

That is, in the mute circuit 9, the current flowing from the emitter to the collector of the transistor Q5 changes depending on the level of the detection signal applied to the input terminal IN, and becomes small when the level of the detection signal is high and large when the level of the detection signal is low. Further, a predetermined bias voltage is applied to the base of the transistor Q4, and when the base of the transistor Q3 is at a high potential, almost all of the current from the transistor Q5 flows to the transistor Q4 side, and does not flow to the transistor Q3.
Therefore, the detected signal applied to the input terminal IN is output from the collector of the transistor Q4 without being attenuated, and is reproduced by the speaker 11.

However, as the base potential of transistor Q3 decreases, the current flowing to the transistor Q3 side increases, and therefore the current flowing to the transistor Q4 side decreases. Therefore, as the potential applied to the base of transistor Q3, that is, the output voltage of mute signal generation circuit 8, decreases, the amount of mute increases, and the sound output from speaker 11 decreases.

〔Effect of the invention〕

As explained above, the present invention includes clipping means consisting of transistors Q1, Q2, etc., which clips the output signal of the field strength detection circuit at a level higher and lower by a predetermined amount than the level of the output signal of the integrator. Since the detected signal is muted based on the output signal of the means, the muting can be applied in response to sudden fluctuations in the received electric field strength, and the whisker-like steep parts of the noise are removed. , the miute motion is not repeated in a very short time in response to this whisker-like portion. Furthermore, since the present invention does not use a low-pass filter with a large delay time as in the conventional example, there is an advantage that appropriate muting can be applied even when the electric field strength changes rapidly.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG.
The figures are an explanatory diagram of the operation of FIG. 1, and FIG. 3 is an explanatory diagram of the drawbacks of the conventional example. In the figure, 1 is an antenna, 2 is an RF amplifier,
3 is a mixer, 4 is a local oscillator, 5 is an IF amplifier,
6 is an FM detection circuit, 7 is an electric field strength detection circuit, 8 is a mute signal generation circuit, 9 is a mute circuit, 10
is the audio amplifier, 11 is the speaker, Q1~Q
5 is a transistor, D1 and D2 are diodes, C
1 to C6 are capacitors, R1 to R10 are resistors,
IN is an input terminal.

Claims (1)

[Claims] 1. A muting device for an FM receiver that mutes a detected signal of an FM detection circuit according to a received electric field strength, comprising: a field strength detection circuit that outputs a signal corresponding to the received electric field strength; and a field strength detection circuit that outputs a signal corresponding to the received electric field strength. an integrator for smoothing the output signal of the circuit; clipping means for clipping the output signal of the field strength detection circuit at a level higher and lower than the level of the output signal of the integrator by a predetermined amount; and an output of the clipping means. 1. A mute device for an FM receiver, comprising a mute circuit that mutes the detected signal based on a signal.