JPH08321001A - Noise reduction circuit - Google Patents

Abstract

PURPOSE: To eliminate only noise even when a signal including consecutive pulsive voice signals is inputted by providing a noise discriminating circuit. CONSTITUTION: A discrimination signal NS indicating the presence of a pulsive signal is obtained by extracting the component B of the high frequency range signal including the pulsive signal from a voice signal A by a high frequency filter 2 and by passing it through a signal ratio comparing circuit 5. Next, the switch 12 of a single-shot noise discriminating circuit 10 closes a contact in accordance with the discrimination signal NS to transmit the reference voltage VR from a reference voltage source 11 to a release filter 13 only for the period of the pulse width of the signal NS. Moreover, a delay circuit 14 delays the release signal F by a prescribed fixed time to generate a delayed release signal G. When the level of the signal F is sufficiently smaller than that of the signal G, a discrimination circuit 15 discriminates the signal as a voice signal and transmits a discrimination signal MN to an amplitude judging circuit 6. Thus, the circuit 6 judges the signal as the voice signal when plural pulsive signals are generated in a fixed time and judges the signal as a pulsive noise signal when the pulsive signal is generated singly in that time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise eliminating circuit,
In particular, the present invention relates to a noise removal circuit that removes a pulsed noise signal generated when a scratched record is reproduced.

[0002]

2. Description of the Related Art Conventionally, a noise removing circuit of this type has been used for the purpose of reducing a pulse noise signal generated when a scratched record is reproduced. The pulse noise signal resulting from this flaw is generated by a sudden change in the waveform due to sound skipping or the like. Therefore, since the frequency of the noise signal component is generally higher than the frequency of the normal music signal component, it can be distinguished from the normal music signal to some extent by passing it through a high-pass filter.

Various methods have been proposed for detecting this pulse noise signal, and in the noise removing circuit disclosed in Japanese Patent Laid-Open No. 59-66230 (reference 1), high frequency components of an input audio signal are separated. It has a high-pass filter, a monostable multivibrator that is triggered by this high-pass filter output signal, and an analog switch for disconnecting the audio signal, and the rise time and amplitude of the high-pass filter output signal correspond to the trigger level. When it is larger than the set value of, it is determined that the input audio signal contains a pulse noise signal, the monostable multivibrator is triggered to generate a noise removal control pulse of a certain width, and the analog pulse is generated by this control pulse. Noise is removed by controlling the switch.

Further, in the noise removing circuit described in Japanese Utility Model Publication No. 60-7572 (reference 2), the envelope signal of the high-pass filter output signal is further extracted, and the level comparison result of the high-pass filter output signal and the envelope signal is obtained. When the output signal of the high-pass filter is significantly larger, it is determined that the input audio signal contains a pulse noise signal.

Referring to FIG. 5 which is a block diagram showing a general conventional noise removing circuit based on the noise removing circuits described in Documents 1 and 2, the conventional noise removing circuit shown in this figure shows that an input audio signal A is input. A delay circuit 1 that delays and outputs a delayed audio signal DA, a high-pass filter 2 that extracts the high-frequency component signal B of the audio signal A, and a full-wave rectifier circuit that rectifies the high-frequency component signal B to generate a detection signal C. 3, a low-pass filter 4 for extracting a signal HD which is a low-frequency component of the detection signal C, a signal C,
A signal ratio comparison circuit 5 that compares the levels of HD with each other and outputs a noise determination signal NS from the ratio of the two, and the amplitude of the delayed audio signal DA is suppressed in response to the supply of the signal NS to remove the noise component. And an amplitude suppression circuit 6 for outputting the output signal O.

Next, FIG. 6A to FIG. 6A, which are signal waveform diagrams showing the operation signal waveforms of each block on the same time axis.
The operation of the conventional noise removal circuit will be described with reference to (E). First, the audio signal A including the pulse noise signal 21 shown in FIG. 6A is input from the input terminal TI, and the input audio signal is input. A is a delay circuit 1 and a high-pass filter 2
Is supplied to each. The cut-off frequency of the high-pass filter 2 is set to 7 kHz, and the high-pass filter 2 responds to the supply of the audio signal A as shown in FIG.
The high frequency component signal B shown in (B) is output. As is clear from this figure, in the high frequency component signal B, the signal level of the normal audio signal is attenuated, and only the high frequency noise signal 21A corresponding to the noise signal 21 is projected. The full-wave rectifier circuit 4 full-wave rectifies the high frequency component signal B, and the detection signal C shown in FIG.
Is supplied to the low-pass filter 4 and the signal ratio comparison circuit 5. The cut-off frequency of the low-pass filter 4 is extremely low, and in this case, it is set to 10 Hz, and the waveform of the output signal HD obtained by smoothing the signal C by the low-pass filter 4 is shown in FIG. In this way, by setting the cutoff frequency of the low-pass filter 4 to be extremely lower than the cutoff frequency of the high-pass filter 2, it does not react very much to the pulse signal such as the noise signal 21A which is instantaneously generated, and the level fluctuation is small. It is possible to obtain the low-pass filter output signal HD showing the envelope corresponding to the signal component. Next, the detection signal C and the low-pass filter output signal HD are compared by the signal ratio comparison circuit 5, and the detection noise signal 21B in which the level of the detection signal C is significantly larger than the low-pass filter output signal HD is generated as a pulse signal. The period is determined and the noise determination signal NS is output. The delay circuit 1 is
A delay time corresponding to the time from the input to the high-pass filter 2 to the generation of the noise determination signal NS is given to the audio signal A, and the delay signal DA is output. Amplitude suppression circuit 6
Is the delay signal DA in response to the supply of the noise determination signal NS.
Suppress the level of to 0. In this case, the delayed audio signal DA
When the level of is suddenly set to 0, the signal waveform becomes discontinuous and causes another noise. Therefore, the output signal level is gradually changed to 0 so that the signal waveform does not become discontinuous. To do. Referring to FIG. 6E showing the waveform of the output audio signal O output to the output terminal TO, the amplitude suppression circuit 6 suppresses the amplitude of only the pulse noise signal 21.

However, in this conventional noise suppressing circuit, as shown in FIG. 2A, when the audio signal contains a pulsed audio signal, for example, an audio signal 22 of a brass instrument such as a trumpet. In the detection signal C shown in FIG. 2C, the pulse signal 22B has a period of about 3 ms and a pulse width of about 0.5 ms. Even if compared with the low-pass filter output signal HD corresponding to this signal C, the audio signal 22B is Like the noise signal 21B, it is determined to be a pulse noise signal, and this signal 22 is also suppressed.

In this case, if the cutoff frequency of the low-pass filter 4 is increased, the signal HD can follow the fluctuation of the level of the audio signal 22B, but it is difficult to detect the pulse noise signal 21B.

[0009]

SUMMARY OF THE INVENTION In the above-mentioned conventional noise removing circuit, all the signals in which the level of the high frequency component signal of the audio signal after passing through the high frequency filter is significantly higher than the envelope level of the high frequency component signal are treated as noise. Since it was determined to be a signal, when an audio signal of a brass instrument such as a trumpet containing a pulsed audio signal continuous to the original sound is input,
There is a drawback in that this voice signal is suppressed because it cannot be distinguished from the noise signal.

[0010]

A noise removing circuit of the present invention comprises a high-pass filter for extracting a high-frequency component signal from an input audio signal, and a pulse-shaped signal protruding from the high-frequency component signal from the envelope of the high-frequency component signal. A pulse determination circuit that determines the presence or absence of a pulsed signal and outputs a pulse determination signal having a predetermined pulse width; a delay circuit that delays the input audio signal by a predetermined time and outputs a delayed audio signal; and the pulse determination In a noise removal circuit comprising an amplitude suppression circuit that suppresses the amplitude of the delayed audio signal during the pulse width of the pulse determination signal in response to the supply of a signal to remove pulsed noise, the pulse determination circuit comprises: , A pulsed voice signal composed of continuous pulses in which the pulsed signal is repeated within a predetermined first period, and a single-shot pulse other than that Determine the noise is configured to include a noise discrimination circuit for generating the pulse determination signal.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Next, referring to FIG. 1, which is also a block diagram in which common characters / symbols are used for common constituent elements in common with FIG. 5, an embodiment of the present invention shown in FIG. The noise removing circuit includes a delay circuit 1 and a high-pass filter 2 which are common to the conventional one.
In addition to the full-wave rectification circuit 3, the low-pass filter 4, the signal ratio comparison circuit 5, and the amplitude suppression circuit 6, only a single pulsed noise signal is extracted from the pulsed signal detected from the audio signal A. A noise discrimination circuit 10 is provided.

The single noise discrimination circuit 10 is closed so as to supply the reference voltage VR in response to the reference voltage source 11 which supplies the reference voltage VR of a constant level and the noise judgment signal NS which is the output of the signal ratio comparison circuit 5. A switch 12 for generating a released signal E, a release filter 13 for generating a release signal F from the signal E, a delay circuit 14 for delaying the release signal F and outputting a delayed release signal G, and signals F, G
Discriminating circuit 15 which outputs the single noise discriminating signal MN.

Next, FIG. 2A to FIG. 2A, which are signal waveform diagrams showing the operation signal waveforms of FIG. 1 and each block on the same time axis.
The operation of the present embodiment will be described with reference to (D) and FIGS. 3 (A) to 3 (C). First, the high-pass filter 2 from the supplied audio signal A includes high-pass signals including pulse signals 21 and 22. The signal component B is extracted, and the full-wave rectifier circuit 3, low-pass filter 4,
And the operation until the signal ratio comparison circuit 5 outputs the determination signal NS indicating the presence of the pulse signal is the same as the conventional one. 2 (A)-(D) are the conventional ones of FIGS. 6 (A)-(D).
Corresponding to the input audio signal A, the high-frequency component signal B, the detection signal C, and the low-pass filter output signal HD.
Indicates.

For convenience of explanation, the input audio signal A has a period of about 3 m in the detected signal C as described in the prior art.
It is assumed that a trumpet audio signal 22 that generates a pulse signal 22B having a pulse width of about 0.5 ms and a pulse noise signal 21 is included. In addition, the cutoff frequency of each of the high-pass filter 2 and the low-pass filter 4 is 7 KH.
z and 10 Hz are also the same as the conventional one.

Referring to FIG. 2B, the high frequency component signal B
So, although other low frequency signal component levels are attenuated,
The levels of the signals 21A and 22A, which are pulse signals, remain high. Therefore, the signal ratio comparison circuit 5 determines that the signals 21 and 22 corresponding to these signals 21A and 22A are pulse signals, outputs the determination signal NS, and supplies it to the one-shot noise determination circuit 10.

Next, the switch 12 of the single noise determination circuit 10 closes the contact in response to the supply of the determination signal NS and releases the reference voltage VR from the reference voltage source 11 only during the period of the pulse width of the determination signal NS. Supply to. That is, the release filter 13 is supplied with the voltage VR only during the period in which the pulse signal is present, and has the level 0 during the other periods. Therefore, the release signal F output from the release filter 13 has a saw-tooth wave signal waveform that peaks when the pulse signal is generated and then gradually decreases in level as shown in FIG. The delay circuit 14 delays the release signal F for a certain time to generate a delayed release signal G. The discriminating circuit 15 compares the levels of the signals F and G and discriminates the pulse noise signal 21 if the level of the signal F at the time of comparison is sufficiently lower than the signal G and the audio signal 22 if the level is not sufficiently low. Then, the determination signal MN is output to the amplitude determination circuit 6.

That is, while the pulsed voice signal is generated at a constant interval, in this example, about 3 ms,
Since the pulse noise signal is one shot, as described above,
If a plurality of pulse signals are generated during a certain period of time, it can be regarded as an audio signal, and if they are generated individually, it can be regarded as a pulse noise signal.

The above operation of the present embodiment will be described in detail using numerical values. The time t1 from the state of the peak level L1 of the release signal F to the level 0 is set to 8.5 ms, and the delay of the delay circuit 14 is delayed. The time t2 is 8 ms. The level when the release signal F has passed 0.5 ms from the state of the peak level L1 is L2, and the level when 6 ms has passed also is L3. The determination circuit 15 determines the presence or absence of a pulse noise signal when the level of the delayed release signal G is L2 or higher. When the level of the delayed release signal G is L2 or more and the level of the release signal F is L3 or less,
Since the next pulsed signal is not detected for at least 6 ms after the first pulsed signal is detected, the detected pulsed signal at this time is determined to be a single noise signal. However, 2m from the detection of the first pulsed signal
The pulsed signal detected within s is regarded as the same signal as the first pulsed signal. The pulse noise signal 21 generated at the beginning of the audio signal A can be detected in this way. However, when another pulse signal is detected within 2 ms or more and 8 ms from the detection of the first pulse signal, the level of the release signal F becomes L3 or more, so this detected pulse signal is regarded as continuous Judge as a signal. At this time, the presence or absence of the pulse noise signal is not determined until the levels of the release signal F and the delayed release signal G become 0. In this way, the trumpet audio signal 22 included in the latter half of the audio signal A can be determined as an audio signal without malfunctioning.

The amplitude suppression circuit 6 suppresses the level of the delay signal DA to 0 in the same manner as in the conventional case in response to the supply of the discrimination signal MN.

Referring now to FIG. 4, which is also a block diagram of a second embodiment of the present invention, in which common characters / symbols are used for common constituent elements in common with FIG. 1, reference is made to FIG. The difference between the example circuit and the first embodiment described above is that the low-pass filter for smoothing the detection signal C is omitted and the signal ratio comparison circuit 5 is replaced by the detection signal C and a noise determination level signal supplied from the outside. The level comparison circuit 16 is provided to compare the level with LN and generate the noise determination signal NS when the signal C is larger.

Therefore, the switch 6 is closed not only when the pulse signal is detected, but also when the audio signal A contains a high-frequency component having a large level. But,
Since the circuit of the subsequent stage detects only a single pulsed signal, as a result, an output audio signal similar to that of the first embodiment can be obtained.

[0022]

As described above, in the noise elimination circuit of the present invention, the pulse judgment circuit discriminates the pulsed signal from the pulsed audio signal consisting of continuous pulses and the noise of the other single-shot pulse to pulse the signal. Since a noise determination circuit that generates a determination signal is provided, even if an audio signal of a brass instrument such as a trumpet including continuous pulsed audio signals is input, only the noise can be removed without affecting the audio signal. There is.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of a noise removing circuit of the present invention.

FIG. 2 is a signal waveform diagram showing a signal waveform of each part of the circuit of FIG.

FIG. 3 is a signal waveform diagram showing a signal waveform of each part of FIG.

FIG. 4 is a block diagram showing a second embodiment of the noise removing circuit of the present invention.

FIG. 5 is a block diagram showing an example of a conventional noise removal circuit.

6 is a signal waveform diagram showing signal waveforms of respective parts of the circuit of FIG.

[Explanation of symbols]

 1,14 Delay circuit 2 High-pass filter 3 Full-wave rectifier circuit 4 Low-pass filter 5 Signal ratio comparison circuit 6 Amplitude suppression circuit 10 Single noise discrimination circuit 11 Reference voltage source 12 Switch 13 Release filter 15 Discrimination circuit 16 Signal level comparison circuit

Claims (3)

[Claims] 1. A high-pass filter for extracting a high-frequency component signal from an input audio signal, and the presence or absence of a pulse-like signal which is a pulse-like signal protruding from the high-frequency component signal from the envelope of the high-frequency component signal is determined in advance. A pulse determination circuit that outputs a pulse determination signal having a predetermined pulse width, a delay circuit that delays the input audio signal by a predetermined time and outputs a delayed audio signal, and a pulse determination signal of the pulse determination signal in response to the supply of the pulse determination signal. A noise removal circuit comprising: an amplitude suppression circuit that suppresses the amplitude of the delayed audio signal to remove pulse-like noise during the pulse width period, wherein the pulse determination circuit has a first predetermined pulse signal. The pulse determination signal is obtained by discriminating the pulsed voice signal consisting of continuous pulses repeated within a period and the noise that is a single-shot pulse other than that. Noise removing circuit comprising: a noise determination circuit formed. 2. A rectifier circuit in which the noise discrimination circuit rectifies the high-frequency component signal and outputs a detection signal, and a low-pass filter which smooths the detection signal to generate a smoothed detection signal corresponding to the envelope. A first comparison circuit for comparing the amplitudes of the detection signal and the smoothed detection signal to detect the pulse signal and outputting a pulse detection signal; and a first reference voltage for a predetermined period in response to the supply of the pulse detection signal. A release circuit that discharges the first reference voltage with a predetermined voltage drop curve to generate a release signal, and delays the release signal for a predetermined second period to generate a delayed release signal. And a release signal delay circuit for comparing the amplitudes of the release signal and the delayed release signal. The noise elimination circuit according to claim 1, further comprising a second comparison circuit that generates a constant signal. 3. A rectifying circuit for rectifying the high frequency component signal to output a detection signal, and a third circuit for outputting a pulse detection signal by comparing the detection signal with a second reference voltage. Comparator circuit, a switch circuit that supplies a first reference voltage for a predetermined period in response to the supply of the pulse detection signal, and a release circuit that discharges the reference voltage with a predetermined voltage drop curve to generate a release signal. And a release signal delay circuit that delays the release signal for a predetermined second period to generate a delayed release signal, and compares the amplitudes of the release signal and the delayed release signal, and based on the comparison result, the pulsed voice signal and noise. The noise removal circuit according to claim 1, further comprising a second comparison circuit that determines whether or not to generate the pulse determination signal.