JPH0744996A - Noise reduction circuit - Google Patents

Abstract

PURPOSE:To provide a noise reduction circuit capable of perfectly removing noise at the time of non-signal without a sound signal, or sufficiently reducing it. CONSTITUTION:A regenerative signal of a head 2 is supplied to an equalizer amplifier 3, and the output of the amplifier 3 is supplied to a fixed terminal of (a) side of a switch 5 directly, and further, is supplied to the fixed terminal of (b) side of the switch 5 through a resistor 6, and further, is supplied to the movable terminal side of the switch 5 through an FET 16. When the sound signal exists as the output of the amplifier 3, the FET 16 is turned on, and when no sound signal exists, the FET 16 is turned off. When the switch 5 is connected to the (b) side, when no sound signal exists as the output of the amplifier 3, the output of the amplifier 3 is supplied to the movable terminal side of the switch 5 through the resistor 6, and the noise at the time of nonsignal is reduced sufficiently. When the switch 5 is connected to a (c) side, when no sound signal exists as the output of the amplifier 3, a signal path is interrupted, and no output of the amplifier 3 is supplied to the movable terminal of the switch 5, and the noise at the time of non-signal is removed perfectly.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise reduction circuit for reducing noise in an output signal of a tape recorder, a radio receiver or the like.

[0002]

2. Description of the Related Art As is well known in tape recorders and radio receivers, a weak audio signal is amplified by an amplifier and finally supplied to a speaker or headphones. in this case,
White noise generated by circuit elements, pulsed external noise, electrostatic noise, inductive noise, motor noise, etc. jump into the amplifier system and deteriorate the S / N (signal-to-noise ratio). Therefore, when there is no audio signal, a noise sound such as a rustle or a beep is output from the speaker or the headphones, and it becomes harsh.

[0003]

Conventionally, various kinds of noise described above have been reduced to some extent by devising circuit design and measures such as shielding, and compromised within a range where there is no problem in practical use.

Therefore, the present invention provides a noise reduction circuit capable of completely eliminating or sufficiently reducing noise when there is no voice signal and no signal.

[0005]

According to a first aspect of the present invention, there is provided a noise reduction circuit which inserts a switching element into a signal path of an audio signal and detects whether or not there is an audio signal in the signal path. A means is provided, and the signal path is disconnected by the switching element when the absence of an audio signal is detected by the audio signal detection means.

A noise reduction circuit according to a second aspect of the invention is
A level reducing means and an audio signal detecting means for detecting whether or not there is an audio signal in the signal path are provided, and when the absence of the audio signal is detected by the audio signal detecting means, the level reducing means is inserted in the signal path. It is a thing.

[0007]

According to the first aspect of the invention, since the signal path is disconnected by the switching element when there is no audio signal and no signal is present, noise can be completely removed.

In the second aspect of the invention, since the level reducing means is inserted in the signal path when there is no audio signal and no signal is present, it is possible to sufficiently reduce noise.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. This example is an example applied to the signal system of a tape recorder.

In FIG. 1, the audio signal reproduced by the reproducing magnetic head 2 from the magnetic tape 1 is supplied to the noise reduction circuit 4 after the frequency characteristic is corrected by the equalizer amplifier 3. That is, the output side of the amplifier 3 is directly connected to the fixed terminal on the a side of the changeover switch 5, and is also connected to the fixed terminal on the b side of the changeover switch 5 via the resistor 6 constituting the level reducing means. .

The output side of the amplifier 3 is grounded via a series circuit of a resistor 7 and a capacitor 8 which form a low pass filter. The low-pass filter composed of the resistor 7 and the capacitor 8 is for reducing or eliminating pulse noise. The output side of this low-pass filter, that is, the connection point of the resistor 7 and the capacitor 8 is connected to the input side of the amplifier 9.

The output side of the amplifier 9 is grounded via the cathode / anode of the diode 10 and is connected to the anode of the diode 11. And diode 1
The cathode of No. 1 is grounded via a parallel circuit of a capacitor 12 and a resistor 13 which form a smoothing circuit. here,
The diodes 10 and 11 form a full-wave rectifier circuit.

The connection point P of the parallel circuit of the diode 11, the capacitor 12 and the resistor 13 is a resistor 14 for voltage division.
And 15 are grounded via a series circuit, and the connection point of these resistors 14 and 15 is connected to the gate of a field effect transistor (FET) 16 which constitutes a switching element. The drain of the FET 16 is connected to the output side of the amplifier 3, and the source thereof is connected to the movable terminal side of the changeover switch 5.

In the above structure, when the level of the output signal of the amplifier 3 is high, the level of the smoothed output obtained at the connection point P is high and the FET 16 is turned on. On the contrary, when the level of the output signal of the amplifier 3 is low, the level of the smoothed output obtained at the connection point P is low and the FET
16 turns off.

Here, the level of the output signal of the amplifier 3 at which the FET 16 is turned off, that is, the judgment level of no signal with no audio signal is determined by the gain of the amplifier 9 and the resistance values of the resistors 14 and 15. Will be set.

The output signal of the changeover switch 5 becomes the output signal of the noise reduction circuit 4. That is, the output signal of the changeover switch 5 is supplied to the speaker 19 via the variable resistor 17 for volume adjustment and the audio amplifier 18.

Next, the operation of this example will be described.

First, the changeover switch 5 of the noise reduction circuit 4
The case where is connected to the a side will be described. In this case, the output signal of the amplifier 3 is supplied to the speaker 19 via the changeover switch 5, the variable resistor 17 and the audio amplifier 18. That is, the noise reduction circuit 4 has the same configuration as the conventional circuit to which the noise reduction circuit 4 is not connected, and operates in the same manner.

Next, the changeover switch 5 of the noise reduction circuit 4
The case where is connected to the b side will be described. When there is an audio signal as the output signal of the amplifier 3, the level of the smoothed output obtained at the connection point P increases and the FET 16 is turned on, so that the output side of the amplifier 3 is connected to the movable terminal of the changeover switch 5. . Therefore, the output signal of the amplifier 3 is directly supplied to the movable terminal side of the changeover switch 5, and further supplied to the speaker 19 via the variable resistor 17 and the audio amplifier 18.

On the other hand, when there is no audio signal as the output signal of the amplifier 3, the level of the smoothed output obtained at the connection point P becomes small and the FET 16 is turned off. Not connected to terminals. Therefore, the output signal of the amplifier 3 is supplied to the movable terminal side via the resistor 6, and further supplied to the speaker 19 via the variable resistor 17 and the audio amplifier 18.

Therefore, when the change-over switch 5 of the noise reduction circuit 4 is connected to the side b and there is no audio signal as the output signal of the amplifier 3, the level of the output signal of the amplifier 3 is sufficiently reduced by the resistor 6. Then, the noise is supplied to the movable terminal side of the changeover switch 5 and the noise when there is no audio signal and no signal can be sufficiently reduced. For example, amplifier 3
2A, the signal obtained at the movable terminal side of the changeover switch 5 is as shown in FIG. 2B, and the level of the audio signal period is not reduced,
Only the noise level during the no-signal period is sufficiently reduced.

Next, the changeover switch 5 of the noise reduction circuit 4
The case where is connected to the c side will be described. When there is an audio signal as the output signal of the amplifier 3, the level of the smoothed output obtained at the connection point P increases and the FET 16 turns on, so that the output side of the amplifier 3 is connected to the movable terminal of the changeover switch 5. . Therefore, the output signal of the amplifier 3 is directly supplied to the movable terminal side of the changeover switch 5, and further supplied to the speaker 19 via the variable resistor 17 and the audio amplifier 18.

On the other hand, when there is no audio signal as the output signal of the amplifier 3, the level of the smoothed output obtained at the connection point P becomes small and the FET 16 is turned off. Not connected to terminals. That is, the signal path is cut off, and the output signal of the amplifier 3 is not supplied to the movable terminal side of the changeover switch 5 at all, and thus is not supplied to the speaker 19 via the variable resistor 17 and the audio amplifier 18.

Therefore, when the changeover switch 5 of the noise reduction circuit 4 is connected to the c side, and when there is no audio signal as the output signal of the amplifier 3, the output signal of the amplifier 3 is completely on the movable terminal side of the changeover switch 5. It is possible to completely eliminate the noise when no signal is supplied and no signal is supplied. For example, when the output signal of the amplifier 3 is as shown in FIG. 2A, the signal obtained at the movable terminal side of the changeover switch 5 is as shown in FIG. 2C, and the level of the audio signal period is not reduced, The noise level during the no signal period is drastically reduced.

In this case, when the level of the output signal of the amplifier 3 is equal to or lower than the predetermined level, that is, equal to or lower than the noise level, the signal path is cut because it is in the non-signal period. So as not to be removed
In this example, although not described above, the time constant of the capacitor 12 and the resistor 13 is a value suitable for the purpose of use, for example, 0.3.
It is set to about 0.5 sec.

In the above embodiment, the example applied to the signal system of the tape recorder has been shown, but it goes without saying that the present invention can be applied to the signal system of a radio receiver, a CD player and the like.

[0027]

According to the first aspect of the present invention, since the signal path is cut by the switching element when there is no audio signal and no signal is present, noise can be completely removed.

According to the second aspect of the invention, since the level reducing means is inserted in the signal path when there is no voice signal and no signal is present, noise can be sufficiently reduced.

[Brief description of drawings]

FIG. 1 is a circuit connection diagram showing an embodiment of a noise reduction circuit according to the present invention.

FIG. 2 is a waveform diagram for explaining the operation of the embodiment.

[Explanation of symbols]

1 Magnetic Tape 2 Playback Magnetic Head 3 Equalizer Amplifier 4 Noise Reduction Circuit 5 Changeover Switch 6 Resistor Constituting Level Reducing Unit 16 Field Effect Transistor Constituting Switching Element 17 Variable Resistor for Volume Adjustment 18 Audio Amplifier 19 Speaker

Claims (2)

[Claims] 1. An audio signal detecting means for detecting whether or not there is an audio signal in the signal path while inserting a switching element in the signal path of the audio signal, wherein there is no audio signal in the audio signal detecting means. A noise reduction circuit characterized by disconnecting the signal path with the switching element when is detected. 2. A level reducing means and an audio signal detecting means for detecting whether or not there is an audio signal in the signal path, and when the absence of the audio signal is detected by the audio signal detecting means, the signal path is provided. A noise reduction circuit characterized in that the level reduction means is inserted into the noise reduction circuit.