JPH07321905A - Simple noise reduction circuit - Google Patents

Abstract

PURPOSE:To reduce a noise signal mixed into a speech signal by connecting in parallel a pair of diodes provided with mutually equal characteristic with the polarities reverse to each other and serially connecting this parallel circuit to the signal line of the speech signal. CONSTITUTION:A pair of the diodes 3A and 3B provided with mutually equal characteristic are connected in parallel with the polarities reverse to each other and this parallel circuit is serially connected to the signal line of the speech signal SS so as to take out the speech signal SS from a stage in the rear of the parallel circuit of this signal line. Then, the gain, etc., of an amplifier 2 is set lest the level of the noise signal SN outputted from the amplifier 2 is over the threshold levels of the diodes 3A and 3B. Thereby, the diodes 3A and 3B show high resistance with regard to the noise signal SN and the noise signal SN is not provided to n amplifier 4. Consequently, even when the noise signal SN is incorporated to a sound signal system, the noise signal SN is blocked by the diodes 3A and 3B so as to remove or reduce the noise signal SN.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a simple noise reduction circuit suitable for use in telephones and the like.

[0002]

2. Description of the Related Art Carbon microphones are used as transmitters in standard telephones, so-called black telephones.
Since this carbon microphone hardly responds to sounds below a certain volume, it is effective for noise noise around the transmitter.

A high-pass filter and a low-pass filter are effective for reducing a noise signal mixed in the audio signal system. Moreover, the configuration for that is simple,
It is low cost.

Further, if the audio signal is level-compressed and transmitted on the transmitting side and the level is expanded on the receiving side, the noise signal mixed in the audio signal transmission line can be reduced.

[0005]

However, since the carbon microphone is larger and heavier than the electret condenser microphone and the ceramic microphone, it is unsuitable for a recent telephone such as a cordless telephone handset.

Further, neither the high-pass filter nor the low-pass filter can remove the noise signal within the band of the audio signal.

In this respect, when the audio signal is compressed / expanded, the noise signal can be reduced even within the band of the audio signal, but the target is the noise signal mixed in the transmission line. is there. Moreover, the response characteristic (time constant) of the processing system causes a breathing phenomenon, and the variation in gain of the transmission line causes unnaturalness in the level and frequency characteristic of the audio signal obtained at the receiving side. Furthermore, since a large number of elements are required, the circuit scale becomes complicated and large, and the cost increases.

The present invention is intended to solve the above problems.

[0009]

By the way, in a general telephone, high fidelity to a voice signal is not required. That is, even if the quality of the audio signal is somewhat deteriorated, there is no problem. This is also clear from the fact that a carbon microphone, which does not have very good sound quality, is used as the transmitter of the telephone.

The present invention focuses on such a point and reduces the noise signal mixed in the audio signal.

That is, in the present invention, when the reference numerals of the respective parts correspond to the embodiments described later, a pair of diodes 3A, 3B having the same characteristics are connected in parallel in opposite polarities, and this parallel circuit is The audio signal SS is connected in series to the signal line of the audio signal SS, and the audio signal SS is taken out from a stage subsequent to the parallel circuit of the signal line.

[0012]

When the noise signal SN is mixed in the audio signal system, the noise signal SN is blocked by the diodes 3A and 3B, and the noise signal SN is removed or reduced.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a transmitter system of a telephone, in which 1 is a transmitter of which a voice signal SS from the transmitter 1 is supplied to a microphone amplifier 2. In this case, in this example, the transmitter 1 is composed of an electret condenser microphone, and therefore the amplifier 2
Supplies the transmitter 1 with the voltage required to operate the electret condenser microphone.

Further, 3A and 3B are diodes having the same characteristics, 4 is a line drive amplifier, and 5 is a telephone line. The diodes 3A and 3B are connected in parallel to each other in opposite polarities, and this parallel circuit is connected in series to the audio signal line between the output end of the amplifier 2 and the input end of the amplifier 4. In this example, the diodes 3A and 3B are Schottky diodes having a forward characteristic as shown in FIG. 2A. The output impedance of the amplifier 2 is set to a sufficiently small value, and the input impedance of the amplifier 4 is set to a predetermined small value.

According to such a configuration, the diode 3
The forward voltage-resistance characteristics of A and 3B are as shown in FIG. 2B, for example, and the diodes 3A and 3B have forward voltage V
About 150 mV of F is the threshold level VTH, and V
When F ≧ VTH, it is turned on, and when VF <VTH, it is in a high resistance state (off).

Further, when there is a noise sound source around the transmitter 2 and the noise sound from the noise source is collected by the transmitter 1, as shown in FIG. 3A, the noise signal SN is output from the amplifier 2. Although outputted, the level of the noise signal SN is generally smaller than the threshold level VTH of the diodes 3A and 3B. Also, when the noise signal SN is generated inside the transmitter 1 or the amplifier 2 or when the noise signal SN jumps in from the outside, the noise signal SN is output from the amplifier 2 as well. The level is generally less than the threshold level VTH.

Therefore, since the diodes 3A and 3B have a high resistance to the noise signal SN from the amplifier 2, the noise signal SN is blocked by the diodes 3A and 3B, and as shown in FIG. Noise signal S
N will not be supplied.

That is, the gain of the amplifier 2 is set so that the level of the noise signal SN output from the amplifier 2 does not exceed the threshold level VTH of the diodes 3A and 3B. The diodes 3A and 3B exhibit high resistance with respect to SN, and the amplifier 4
The noise signal SN is no longer supplied to.

When the voice signal SS is output from the transmitter 1, the voice signal SS is amplified to a predetermined large level by the amplifier 2 and supplied to the diodes 3A and 3B. Then, the diodes 3A and 3B are turned on for the audio signal SS having such a level.
The S is supplied to the amplifier 4 through the diodes 3A and 3B and sent to the telephone line 5.

At this time, the small level portion of the audio signal SS (the portion smaller than the threshold level VTH)
On the other hand, the diodes 3A and 3B exhibit high resistance, and the low level portions thereof are not supplied to the amplifiers 3A and 3B. Therefore, as shown in FIG. 4B, the audio signal SS supplied to the amplifier 4 is
Produces distortion similar to crossover distortion, but the phone does not originally demand high fidelity, so there is no problem.

In this way, even if the noise signal SN is output from the amplifier 2, the noise signal SN is removed or reduced, so that the S / N in a silent state can be improved.
Further, in the presence of sound, the noise signal SN is not removed or reduced, but since it is masked by the voice signal SS, the S / N is less deteriorated in terms of hearing.

Therefore, an electret condenser microphone or a ceramic microphone can be used as the transmitter 1, and it can be applied to recent telephones.

Moreover, in that case, since the circuit has no time constant, the breathing phenomenon does not occur. Further, even if the gain of the transmission line varies, the level of the voice obtained on the receiving side, the frequency characteristic, and the like are not unnatural.
Further, as is clear from FIG. 1, the structure is simple and the cost is low.

FIG. 4 shows the invention as an OG of an answering machine.
An example of a transmission system in the case of being applied to a reproduction circuit of M (a response message that informs the other party of the telephone that there is no answer at the time of an incoming call) is shown. That is, 11 is a ROM or RAM in which OGM audio data is written, that is, a memory, and 12 is a memory control circuit.
Reference numeral 2 controls the reading of the memory 11 and its read address. Then, O
When a GM reproduction request is issued, the control circuit 12 reads the OGM audio data from the memory 11, and the read audio data is supplied to the D / A converter 13 and converted into an analog audio signal SS. D / A converted.

Further, diodes 14A and 14B similar to the diodes 3A and 3B are connected in parallel in mutually opposite polarities, and this parallel circuit is connected to the D / A converter 13.
Is connected in series to the audio signal line between the output end of the line drive amplifier and the input end of the line drive amplifier 15. Further, the output signal of the amplifier 15 is supplied to the telephone line 16.

According to such a configuration, the circuits 11 to 13
Even if the noise signal SN due to the clock signal in FIG.
For the same reason as above, the diodes 14A and 14B are
And is not supplied to the amplifier 15.

Therefore, also in this example, the S / N in the silent state can be improved. In addition, when there is sound, the noise signal SN is masked by the voice signal SS, and the S / N on the auditory sense is not significantly reduced.

[0028]

According to the present invention, even if the noise signal SN is generated in the audio signal system, the noise signal SN is removed or reduced, so that the S / N in a silent state can be improved. Further, when there is sound, the noise signal SN is not removed or reduced, but since it is masked by the audio signal SS, the S / N on the auditory sense is not significantly reduced.

Therefore, in the telephone, the transmitter 1
An electret condenser microphone or a ceramic microphone can be used as, and it can correspond to a recent telephone.

Moreover, in that case, since the circuit has no time constant, no breathing phenomenon occurs. Further, even if the gain of the transmission line varies, the level of the voice obtained on the receiving side, the frequency characteristic, and the like are not unnatural.
Further, as is clear from FIG. 1, the structure is simple and the cost is low.

[Brief description of drawings]

FIG. 1 is a connection diagram showing an example of the present invention.

FIG. 2 is a diagram showing an example of characteristics of a diode.

FIG. 3 is a waveform diagram for explaining the present invention.

FIG. 4 is a waveform diagram for explaining the present invention.

FIG. 5 is a connection diagram showing another example of the present invention.

[Explanation of symbols]

 1 microphone 2 microphone amplifier 3A, 3B diode 4 line drive amplifier 5 telephone line 11 memory 12 memory control circuit 13 D / A converter 14A, 14B diode 15 line drive amplifier 16 telephone line

Claims (4)

[Claims] 1. A pair of diodes having the same characteristics are connected in parallel to each other in opposite polarities, the parallel circuit is connected in series to a signal line of an audio signal, and the parallel circuit of the signal line is connected to a stage subsequent to the parallel circuit. A simple noise reduction circuit that extracts audio signals. 2. The simple noise reduction circuit according to claim 1, wherein the simple noise reduction circuit is provided in a telephone and supplies a voice signal output from a transmitter to a signal line of the voice signal. 3. The simple noise reduction circuit according to claim 2, wherein the transmitter is an electret condenser microphone. 4. The simple noise reduction circuit according to claim 1, wherein a memory in which audio data is written, a control circuit for controlling reading of the memory, and a D / A converter for D / A converting the audio data. And a simple noise reduction circuit for supplying the audio signal output from the D / A converter to the signal line of the audio signal.