CN111050250A

CN111050250A - Noise reduction method, device, equipment and storage medium

Info

Publication number: CN111050250A
Application number: CN202010043612.4A
Authority: CN
Inventors: 高均波; 陈孝良; 涂贤玲; 常乐
Original assignee: Beijing SoundAI Technology Co Ltd
Current assignee: Beijing SoundAI Technology Co Ltd
Priority date: 2020-01-15
Filing date: 2020-01-15
Publication date: 2020-04-21
Anticipated expiration: 2040-01-15
Also published as: CN111050250B

Abstract

The application provides a noise reduction method, a noise reduction device, noise reduction equipment and a storage medium, and belongs to the technical field of voice. The noise reduction method comprises the following steps: the method comprises the steps of obtaining a first signal to be played and a noise signal collected by environmental sound, synthesizing the first signal and an inverted signal to obtain and play a first target signal, collecting the played first target signal to obtain a second signal, adjusting the inverted signal according to the similarity of the second signal and the first signal, and then continuing to synthesize the first signal and the adjusted inverted signal to obtain and play the second target signal. Therefore, the reverse phase signal for offsetting the noise signal can be adjusted by comparing the audio signal actually heard by the user with the audio signal wanted to be heard by the user, so that the noise signal can be offset better by the reverse phase signal, and a better noise reduction effect is realized.

Description

Noise reduction method, device, equipment and storage medium

Technical Field

The present application relates to the field of speech technologies, and in particular, to a noise reduction method, apparatus, device, and storage medium.

Background

Nowadays, headphones increasingly receive user's liking, and headphones usually has the sponge cover that can wrap up people's ear, and this sponge cover makes wearing of headphones more comfortable, also can be to a certain extent the noise in the separation environment. In practice, however, it is difficult to completely isolate the noise in the environment only by means of the sponge cover.

In the related art, a technician usually sets a microphone outside the sponge cover, the microphone can collect environmental sound, the earphone can generate an inverse sound wave with the same frequency spectrum and the opposite phase as the noise sound wave according to the collected noise sound wave, and the inverse sound wave is transmitted into the ear through the loudspeaker, and the inverse sound wave can be offset from the noise sound wave transmitted into the ear, so that the noise reduction effect is achieved.

In the actual use process, the measurement of the microphone on the environmental sound may not be accurate enough, so that the noise sound wave cannot be completely offset by the reversed-phase sound wave generated by the earphone, and the noise reduction effect is poor.

Disclosure of Invention

In view of this, embodiments of the present application provide a noise reduction method, apparatus, device and storage medium, which can improve the noise reduction effect.

In one aspect, a method for noise reduction is provided, the method comprising:

acquiring a first signal to be played and a noise signal acquired from environmental sound;

synthesizing the first signal to be played and the inverted signal of the noise signal to obtain a first target signal, and playing the first target signal;

collecting the played first target signal to obtain a second signal;

adjusting the inverted signal of the noise signal according to the similarity between the second signal and the first signal to be played;

and continuously synthesizing the first signal to be played and the adjusted reversed-phase signal, and playing a second target signal obtained after synthesis.

Optionally, the synthesizing the first signal to be played and the inverted signal of the noise signal to obtain a first target signal, and playing the first target signal includes:

carrying out attenuation processing on the noise signal to obtain an attenuated noise signal;

carrying out phase reversal processing on the attenuation noise signal to obtain a phase reversal signal;

synthesizing the first signal to be played and the inverted signal to obtain a first target signal, and playing the first target signal;

the adjusting the inverted signal of the noise signal according to the similarity between the second signal and the first signal to be played includes:

adjusting the attenuation noise signal according to the similarity between the second signal and the first signal to be played to obtain an adjusted attenuation noise signal;

and carrying out phase inversion processing on the adjusted attenuation noise signal to obtain an adjusted phase inversion signal.

Optionally, the attenuating the noise signal to obtain an attenuated noise signal includes:

based on the attenuation coefficient, carrying out attenuation processing on the noise signal to obtain an attenuated noise signal;

the adjusting the attenuation noise signal according to the similarity between the second signal and the first signal to be played to obtain an adjusted attenuation noise signal includes:

and adjusting the attenuation coefficient according to the similarity between the second signal and the first signal to be played to obtain an adjusted attenuation noise signal.

Optionally, the adjusting the attenuation coefficient according to the similarity between the second signal and the first signal to be played to obtain an adjusted attenuated noise signal includes:

when the similarity between the second signal and the first signal to be played is smaller than a similarity threshold, increasing the attenuation coefficient to obtain an adjusted attenuation noise signal;

after the first signal to be played and the adjusted inverse signal are continuously synthesized and the synthesized second target signal is played, the method further includes:

collecting the played second target signal to obtain a third signal;

and when the similarity between the third signal and the first signal to be played is smaller than a similarity threshold value, continuing to execute the step of adjusting the attenuation coefficient until the adjustment is stopped when a target condition is met.

Optionally, the acquiring a first signal to be played and a noise signal collected for an ambient sound includes:

acquiring a first signal to be played;

acquiring environmental sound based on a first microphone facing the environment on sound acquisition equipment to obtain a noise signal;

the acquiring the played first target signal to obtain a second signal includes:

and acquiring the played first target signal based on a second microphone facing to the ear of the person on the sound acquisition equipment to obtain a second signal.

synthesizing the first signal to be played and the inverted signal of the noise signal to obtain a first target signal;

playing the first target signal according to the target time delay;

the continuously synthesizing the first signal to be played and the adjusted inverse signal, and playing the synthesized second target signal includes:

continuously synthesizing the first signal to be played and the adjusted reversed-phase signal to obtain a second target signal;

and playing the adjusted second target signal according to the target time delay.

Optionally, before the adjusting the inverted signal of the noise signal according to the similarity between the second signal and the first signal to be played, the method further includes:

and comparing the effective amplitude values of the second signal and the first signal to be played in a target time length to obtain the similarity of the second signal and the first signal to be played.

In one aspect, there is provided a noise reduction apparatus, the apparatus comprising:

the acquisition module is used for acquiring a first signal to be played and a noise signal acquired by environmental sound;

the synthesis module is used for synthesizing the first signal to be played and the inverted signal of the noise signal to obtain a first target signal and playing the first target signal;

the acquisition module is used for acquiring the played first target signal to obtain a second signal;

the adjusting module is used for adjusting the inverted signal of the noise signal according to the similarity between the second signal and the first signal to be played;

and the synthesis module is further configured to continue to synthesize the first signal to be played and the adjusted inverse signal, and play a second target signal obtained after synthesis.

Optionally, the synthesis module is to:

the adjustment module is configured to:

Optionally, the synthesis module is configured to perform attenuation processing on the noise signal based on an attenuation coefficient to obtain an attenuated noise signal;

the adjusting module is used for adjusting the attenuation coefficient according to the similarity between the second signal and the first signal to be played to obtain an adjusted attenuation noise signal.

Optionally, the adjusting module is configured to increase the attenuation coefficient when the similarity between the second signal and the first signal to be played is smaller than a similarity threshold, so as to obtain an adjusted attenuated noise signal;

the acquisition module is further used for acquiring the played second target signal to obtain a third signal;

the adjusting module is further configured to continue to execute the step of adjusting the attenuation coefficient when the similarity between the third signal and the first signal to be played is smaller than a similarity threshold value, and stop adjusting until a target condition is met.

Optionally, the obtaining module is configured to:

acquiring a first signal to be played;

the acquisition module is used for acquiring the played first target signal based on a second microphone facing to human ears on the sound acquisition equipment to obtain a second signal.

Optionally, the synthesis module is to:

playing the first target signal according to the target time delay;

Optionally, the apparatus further comprises:

and the comparison module is used for comparing the effective amplitude values of the second signal and the first signal to be played in a target time length to obtain the similarity of the second signal and the first signal to be played.

In one aspect, a computer device is provided that includes one or more processors and one or more memories having at least one instruction stored therein, the instruction being loaded and executed by the one or more processors to implement operations performed by the noise reduction method.

In one aspect, a computer-readable storage medium having at least one instruction stored therein is provided, which is loaded and executed by a processor to implement operations performed by the noise reduction method.

The beneficial effects brought by the technical scheme provided by the embodiment of the application at least can comprise:

the application provides a noise reduction method and a noise reduction device, wherein a first signal to be played and a noise signal collected from environmental sound are obtained, the first signal and an inverse signal are synthesized to obtain and play a first target signal, the played first target signal is collected to obtain a second signal, the inverse signal is adjusted according to the similarity between the second signal and the first signal, and then the first signal and the adjusted inverse signal are continuously synthesized to obtain and play a second target signal. Therefore, the reverse phase signal for offsetting the noise signal can be adjusted by comparing the audio signal actually heard by the user with the audio signal wanted to be heard by the user, so that the noise signal can be offset better by the reverse phase signal, and a better noise reduction effect is realized.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the application.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present application, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

Fig. 1 is a schematic implementation environment of a noise reduction method provided in an embodiment of the present application;

fig. 2 is a flowchart of a noise reduction method provided in an embodiment of the present application;

fig. 3 is a flowchart of a noise reduction method provided in an embodiment of the present application;

fig. 4 is a schematic structural diagram of an earphone according to an embodiment of the present application;

FIG. 5 is a flowchart of a noise reduction method provided by an embodiment of the present application;

fig. 6 is a schematic structural diagram of a noise reduction device according to an embodiment of the present application;

fig. 7 is a schematic structural diagram of a computer device according to an embodiment of the present application.

Detailed Description

To make the objects, technical solutions and advantages of the present application more clear, embodiments of the present application will be described in further detail below with reference to the accompanying drawings.

It should be noted that the embodiments described below are some embodiments of the present application, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.

In this application, the terms "first," "second," and the like are used for distinguishing between similar items and items having substantially the same function, and it should be understood that "first," "second," and "n" have no logical or temporal dependency, nor are they limited in number or order of execution.

The terms referred to in the present application are explained below.

Fig. 1 is a schematic diagram of an implementation environment of a noise reduction method provided by an embodiment of the present application, and referring to fig. 1, the implementation environment may include a computer device 101.

Where the computer device 101 may have two microphones and one speaker 1013, for example, the computer device may be a sound collection device, such as a headset, where the headset may be any one or more of an ear bud, an in-ear headphone, a headset, and an in-ear headphone. In this embodiment, fig. 1 is a schematic diagram of a headset implementation, two microphones may be disposed on two sides of an ear cup of the headset, when a user uses the headset, a first microphone 1011 disposed outside the ear cup of the headset may collect ambient noise to obtain a noise signal, a speaker 1013 may play an audio signal to the user, and a second microphone 1012 disposed inside the ear cup of the headset may collect sound actually heard by the user to obtain a second signal.

In this embodiment, the computer device 101 may acquire the environmental noise by using the first microphone 1011, further acquire an inverse signal capable of canceling the noise, and adjust the inverse signal according to the second signal acquired by the second microphone 1012, thereby improving the effect of eliminating the noise signal.

Fig. 2 is a flowchart of a noise reduction method according to an embodiment of the present application. Referring to fig. 2, the method is applied to a computer device, and includes:

201. the method comprises the steps of obtaining a first signal to be played and a noise signal collected for environmental sound.

202. And synthesizing the first signal to be played and the inverted signal of the noise signal to obtain a first target signal, and playing the first target signal.

203. And collecting the played first target signal to obtain a second signal.

204. And adjusting the inverted signal of the noise signal according to the similarity between the second signal and the first signal to be played.

205. And continuously synthesizing the first signal to be played and the adjusted reversed-phase signal, and playing a second target signal obtained after synthesis.

The application provides a noise reduction method, which comprises the steps of synthesizing a first signal to be played and a noise signal acquired by ambient sound, obtaining and playing a first target signal, acquiring the played first target signal, obtaining a second signal, adjusting the inverted signal according to the similarity between the second signal and the first signal, and then continuing to synthesize the first signal and the adjusted inverted signal, so as to obtain and play the second target signal. Therefore, the reverse phase signal for offsetting the noise signal can be adjusted by comparing the audio signal actually heard by the user with the audio signal wanted to be heard by the user, so that the noise signal can be offset better by the reverse phase signal, and a better noise reduction effect is realized.

should adjust this attenuation noise signal according to the similarity of this second signal and this first signal that waits to broadcast, obtain the attenuation noise signal after the adjustment, include:

after the first signal to be played and the adjusted inverted signal are continuously synthesized and the synthesized second target signal is played, the method further includes:

collecting the played second target signal to obtain a third signal;

and when the similarity between the third signal and the first signal to be played is smaller than the similarity threshold, continuing to execute the step of adjusting the attenuation coefficient until the third signal meets the target condition, and stopping adjusting.

Optionally, the acquiring the first signal to be played and the noise signal collected for the environmental sound includes:

acquiring a first signal to be played;

a noise signal collected for ambient sound based on a first microphone on the sound collection device facing the environment;

should gather this first target signal to the broadcast, obtain the second signal, include:

playing the first target signal according to the target time delay;

the continuously synthesizing the first signal to be played and the adjusted inverse signal, and playing the synthesized second target signal, includes:

and comparing the effective amplitude of the second signal with the effective amplitude of the first signal to be played in a target time length to obtain the similarity of the second signal and the first signal to be played.

Fig. 3 is a flowchart of a noise reduction method according to an embodiment of the present application. Referring to fig. 3, the method includes:

301. the computer device obtains a first signal to be played and a noise signal collected for ambient sound.

In an embodiment of the present application, a method for collecting and denoising sound through two microphones is provided. The first signal to be played may be a signal corresponding to an audio that the user wants to listen to, the first microphone may be used to collect ambient sound, and the second microphone may be used to collect a sound that the user actually hears. For example, the computer device may be a headset provided with a first microphone, a second microphone and a loudspeaker. When a user listens to a song through the earphone, the environment sound influences the user to listen to the song, and the environment sound is included besides the audio frequency the user wants to listen to in the sound actually heard by the user, so that the environment sound can be regarded as noise, the first microphone can collect the noise, the loudspeaker can play the audio frequency, the second microphone can collect the sound actually heard by the human ear, and then the collected sound is processed by the main control chip of the earphone.

In one possible implementation, the process of acquiring the noise signal by the computer device may be implemented based on the first microphone, and specifically, the computer device may acquire the noise signal of the ambient sound based on the first microphone facing the environment on the sound acquisition device.

In one possible implementation, the first microphone may be a microphone on the sound collection device facing the environment, and the second microphone may be a microphone on the sound collection device facing the human ear. In particular, the sound collection device may be a headset or a part thereof, the first microphone may be arranged on the ear cup of the headset towards the side of the environment, and the second microphone may be arranged on the ear cup of the headset towards the side of the human ear. Like this, through setting up first microphone and second microphone in the both sides of sound collection equipment for the user is when using sound collection equipment, and first microphone can just gather the ambient noise to external environment, and the second microphone can just to the people's ear, gathers the sound that the people's ear actually heard.

In another possible implementation manner, the computer device may also be a device in which the first microphone and the second microphone are connected, and the first microphone and the second microphone collect sound to obtain an audio signal, and send the audio signal to the connected computer device, and the computer device processes the collected audio signal. For example, the computer device may be a mobile phone of a user, the first microphone and the second microphone are disposed on an earphone, the earphone is connected to the mobile phone, the mobile phone is capable of sending a first signal to be played corresponding to an audio that the user wants to listen to the earphone, and the first microphone and the second microphone are capable of sending collected audio signals to the mobile phone for processing.

In a possible implementation manner, the computer device may be a headset, and after receiving an original audio signal corresponding to an audio that a user wants to listen to, the headset may perform a series of decoding on the original audio signal, and the computer device may perform extraction on the decoded signal to obtain a first signal to be played. The first signal to be played has a certain difference from the original audio signal, and the difference may be generated in the decoding process, and it is difficult for the human ear to hear the difference, so the first signal to be played may be regarded as a signal not including noise, and the first signal to be played may be regarded as a signal corresponding to the audio signal that the user wants to listen to.

302. The computer equipment synthesizes the first signal to be played and the inverted signal of the noise signal to obtain a first target signal, and plays the first target signal.

Wherein, the first signal to be played may be a signal corresponding to audio that the user wants to listen to. The noise signal may have the same frequency spectrum as the noise signal and an opposite phase, so that the noise signal can be cancelled. The synthesized first target signal comprises a first signal to be played and an inverted signal, the first target signal can be played under the control of computer equipment, and after the first target signal is played, the inverted signal in the first target signal and the noise signal can be mutually offset, so that the audio signal actually heard by a user is only the first signal.

For example, the computer device may be a headset provided with a first microphone, a second microphone and a loudspeaker, which is inevitably heard by the user when the user uses the headset. The earphone can synthesize a first signal to be played corresponding to the audio heard by the user and an inverse signal corresponding to the noise signal collected by the first microphone to obtain a first target signal, and controls the loudspeaker to play the first target signal to the user, the inverse signal in the first target signal can be offset with the noise signal, so that the audio signal actually heard by the user is only the first signal, and noise reduction is realized.

In a possible implementation manner, the inverse signal of the noise signal may also be an attenuated inverse signal of the attenuated noise signal, and in this implementation manner, the computer device may perform attenuation processing on the noise signal to obtain the attenuated noise signal, perform inverse processing on the attenuated noise signal to obtain an inverse signal, and synthesize the first signal to be played and the inverse signal to obtain and play the first target signal.

The computer equipment can be an earphone, the noise signal can be a signal corresponding to the environmental sound, when the user wears the earphone, the environmental sound can be attenuated to a certain degree in the process of transmitting the environmental sound into the ear of the user through the earphone cover, and therefore the attenuated noise signal obtained by carrying out attenuation processing on the noise signal can represent the noise signal corresponding to the part of the environmental sound transmitted into the ear of the user. The frequency spectrum of the inverted signal can be the same as that of the attenuated noise signal, and the phase of the inverted signal can be opposite to that of the attenuated noise signal, so that the inverted signal and the attenuated noise signal can be mutually offset when being superposed, the eliminated noise signal is the attenuated noise signal actually received by human ears, and the noise reduction effect is improved. The first target signal can be obtained by synthesizing the first signal to be played and the inverted signal, so that the inverted signal in the first target signal played by the computer device controlling the loudspeaker can be offset from the attenuated noise signal, and the sound finally heard by the user is only the audio corresponding to the first signal to be played, that is, the user can only hear the sound desired to be heard, thereby realizing noise reduction.

The computer device obtains the attenuated noise signal by attenuating the noise signal and performs an inverse phase process on the attenuated noise signal, so that the obtained inverse phase signal can better correspond to the noise signal transmitted into the ear. The inverse signal and the first signal to be played are synthesized to obtain a first target signal, the computer equipment controls and plays the first target signal, the inverse signal in the first target signal and an attenuated noise signal obtained by attenuating a noise signal can be mutually offset, the sound finally heard by human ears is only the audio frequency corresponding to the first signal to be played, and noise reduction is achieved.

303. And the computer equipment collects the played first target signal to obtain a second signal.

The computer equipment can also collect the played first target signal, and analyze whether the signal actually heard by the human ear still includes a noise signal or not through the collected second signal, and whether further noise reduction is needed or not according to the current noise reduction effect.

In a possible implementation manner, the computer device may collect the played first target signal based on a second microphone facing the human ear on the sound collection device, so as to obtain a second signal.

The second microphone may be a microphone facing the human ear on the sound collection device, for example, the second microphone may be disposed on an ear cup of the earphone on a side facing the human ear, so that the second microphone may be configured to collect the sound actually heard by the human ear to obtain a second signal, where the second signal may be a signal corresponding to the sound actually heard by the human ear after the opposite-phase signal in the first target signal is offset by the attenuated noise signal.

In a possible implementation, since the actual attenuated noise signal cannot be measured separately, the frequency spectrum of the inverted signal obtained by the computer device is difficult to be identical to the actual attenuated noise signal, so that the inverted signal is difficult to completely cancel the attenuated noise signal, and the second signal may include a residual noise signal. The computer device may acquire the sound actually heard by the human ear using the second microphone to obtain a second signal including a residual noise signal, which is used as a data basis for adjusting the noise reduction process.

304. And the computer equipment adjusts the inverted signal according to the similarity between the second signal and the first signal to be played.

The similarity is used for representing the similarity between the second signal and the first signal to be played, and the higher the similarity is, the less the noise signals in the second signal are, and the better the elimination effect of the inverted noise signal on the attenuation noise signal is. The computer equipment can adjust the inverted signal according to the similarity of the second signal and the first signal to be played, so that the frequency spectrum of the adjusted inverted signal is closer to the frequency spectrum of the actual attenuated noise signal, the attenuated noise signal can be better filtered, the sound actually heard by human ears is closer to the sound wanted to be heard by a user, and a better noise reduction effect is achieved.

In a possible implementation manner, the computer device may perform attenuation processing on the noise signal to obtain an attenuated noise signal, and further perform inverse processing on the attenuated noise signal to obtain an inverse signal. Correspondingly, the computer device can adjust the attenuation noise signal according to the similarity between the second signal and the first signal to be played, and perform phase inversion processing on the adjusted attenuation noise signal to obtain an adjusted phase inversion signal.

The attenuation noise signal can represent a signal corresponding to a part of the environment sound transmitted into the ear of the user, the opposite-phase signal can be obtained by performing opposite-phase processing on the attenuation noise signal, and further, the computer equipment can adjust the attenuation noise signal and perform opposite-phase processing on the adjusted attenuation noise signal to realize adjustment of the opposite-phase signal.

The computer equipment adjusts the attenuation noise signal according to the similarity of the second signal and the first signal to be played, so that the inverted signal is adjusted, and the residual noise signal in the second signal can be better filtered by the inverted signal.

In a possible implementation manner, the computer device may perform attenuation processing on the noise signal based on the attenuation coefficient to obtain an attenuated noise signal, and correspondingly, the computer device may also adjust the attenuation coefficient according to the similarity between the second signal and the first signal to be played to obtain an adjusted attenuated noise signal.

The process that the environmental sound passes through the earphone cover and is transmitted into the ear can generate attenuation to a certain degree, and therefore the noise signal can be processed based on the attenuation coefficient to obtain the attenuated noise signal. For example, the attenuated noise signal can be obtained by the following equation.

S₂＝*S₁(0＜K≤1)

Where K is the attenuation coefficient, S₁Is a noise signal, S₂Is attenuating the noise signal.

The noise signal is attenuated through the attenuation coefficient, and the attenuation noise signal is adjusted through adjusting the attenuation coefficient, so that the noise signal is processed and adjusted more simply and accurately by a computer.

In a possible implementation manner, when the similarity between the second signal and the first signal to be played is smaller than the similarity threshold, the computer device may increase the attenuation coefficient to obtain the adjusted attenuated noise signal. Correspondingly, the computer equipment synthesizes the first signal and the adjusted reversed-phase signal to obtain and play a second target signal, then the played second target signal can be collected to obtain a third signal, and when the similarity between the third signal and the first signal to be played is smaller than the similarity threshold, the step of adjusting the attenuation coefficient is continuously executed until the target condition is met, and the adjustment is stopped.

The target condition may be that the similarity is smaller than a similarity threshold, or that the similarity is converged, or that the adjustment number reaches a target number, and the like, which is not limited in the embodiment of the present application.

The second signal may be a signal corresponding to a sound actually heard by the user, the first signal to be played may be a signal corresponding to an audio that the user wants to hear, and in a case where it is difficult for the inverted signal to completely cancel the actual attenuated noise signal, the second signal is hardly identical to the first signal to be played. Similarly, even if the second target signal comprises an adjusted inverse signal, it is difficult to ensure that the adjusted inverse signal can completely cancel the actual in-ear noise, and therefore the similarity threshold can be set to the desired result, and the computer device can continuously adjust the inverse signal by adjusting the attenuation coefficient before the desired result is achieved. The smaller the similarity is, the larger the residual noise signal in the second signal is, that is, the cancellation effect of the inverted signal on the actual attenuated noise signal is not good, so that the attenuation coefficient can be increased, the adjusted inverted signal can be closer to the inverted signal corresponding to the actual attenuated noise signal, and the noise reduction effect is further improved.

For example, the similarity threshold may be 0.9, if the obtained similarity is less than 0.9, the attenuation coefficient may be increased by 0.1, and then the similarity may be obtained again, and in the case that the similarity is less than 0.9, the attenuation coefficient is continuously increased until the similarity is not less than 0.9. The similarity threshold and the adjustment step size of the attenuation coefficient may also be preset by a related technician according to actual needs, own experience, or experimental results, which is not limited in this application.

The computer equipment sets the similarity threshold value, and continuously increases the attenuation coefficient before the similarity is not less than the similarity threshold value, so that the adjusted reversed phase signal can be more and more close to the reversed phase signal corresponding to the actual attenuated noise signal, the similarity between the second signal and the first signal to be played is continuously increased until the similarity is not less than the expected similarity threshold value, and the noise reduction effect can be improved.

In a possible implementation manner, the computer device may compare the effective amplitudes of the second signal and the first signal to be played within the target time duration to obtain the similarity between the second signal and the first signal to be played.

The second signal and the first signal to be played may be waveform signals, the second signal may include the first signal to be played and residual noise signals, and the computer device may intercept signal waveforms of the second signal and the first signal to be played in the same time period, and compare effective amplitudes of the signal waveforms of the intercepted second signal and the first signal to be played to obtain a similarity between the second signal and the first signal to be played. The length of the time period is a target time period, and the target time period may be preset by a related technician according to actual needs, own experience, or experimental results, which is not limited in this application.

The amplitudes of the signal waveforms of the second signal and the first signal to be played may be positive values or negative values. The effective amplitude may be greater than a positive effective threshold when the amplitude of the signal waveform is a positive value, and may be less than a negative effective threshold when the amplitude of the signal waveform is a negative value. For example, the signal waveforms of the second signal and the first signal to be played may be regarded as sine functions, the positive effective threshold of the signal waveform may be + a, the negative effective threshold may be-B, when the amplitude of the signal waveform is a positive value, the effective amplitude may be greater than + a, and when the amplitude of the signal waveform is a negative value, the effective amplitude may be less than-B. Wherein, A and B can be positive numbers, and the values of A and B can be the same or different. When the values of a and B are the same, the absolute values of a and B may be the effective threshold, and the absolute value of the effective amplitude may be greater than the effective threshold.

With respect to the positive and negative valid threshold settings described above, in some embodiments, the valid threshold may be set to 0.005 volts (V). In an exemplary case, the effective amplitude of the first signal may be 0.01V, the effective amplitude of the second signal may be 0.005V, and at this time, the effective amplitudes of the first signal and the second signal are larger and differ more, so that the effective amplitudes of the first signal and the second signal are compared, and the comparison is performed, so that the reference value of the obtained similarity is high.

If the effective amplitude is not set to be greater than the effective threshold, the comparison result may be meaningless, in an exemplary case, the amplitude of the first signal may be 0.001V, the amplitude of the second signal may be 0.002V, and at this time, the amplitudes of the first signal and the second signal are smaller and have a smaller difference, and the reference value of the similarity obtained by comparing the effective amplitudes of the first signal and the second signal is not high. Therefore, the effective amplitudes of the first signal and the second signal can be made to be comparable by setting the effective threshold, the obtained similarity is guaranteed to be meaningful, and the reference value is high.

By comparing the effective amplitudes of the second signal and the first signal to be played in the target duration, the computer device can more simply and accurately acquire the similarity between the second signal and the first signal to be played, and the effective amplitude can be larger than a positive effective threshold or smaller than a negative effective threshold, so that the effective amplitudes of the first signal and the second signal have comparability, and the obtained similarity is significant. Of course, the computer device may also obtain the similarity between the second signal and the first signal to be played by other methods, which is not limited in this application.

305. And the computer equipment continues to synthesize the first signal to be played and the adjusted reversed-phase signal and plays the synthesized second target signal.

Wherein, the first signal to be played may be a signal corresponding to audio that the user wants to listen to. The adjusted reverse phase signal can be obtained according to the similarity between the second signal and the first signal to be played, the frequency spectrum of the adjusted reverse phase signal is closer to the frequency spectrum of the actual attenuated noise signal, and the attenuated noise signal can be better offset. The computer device may synthesize the first signal to be played and the adjusted inverted signal to obtain a second target signal and play the second target signal. The second target signal may include the first signal to be played and the adjusted inverted signal. Therefore, in the second target signal played under the control of the computer equipment, the adjusted reversed phase signal can better offset the attenuated noise signal, and a better noise reduction effect is achieved

In a possible implementation manner, the computer device synthesizes a first signal to be played and an inverted signal of a noise signal to obtain a first target signal, and then can play the first target signal according to a target time delay; and continuously synthesizing the first signal to be played and the adjusted reversed-phase signal to obtain a second target signal, and playing the adjusted second target signal according to the target time delay.

Taking the first target signal as an example, the first target signal may include a first signal to be played and an inverse signal, and the inverse signal may be used to cancel the attenuated noise signal, so that the inverse signal in the first target signal controlled to be played by the computer device needs to be aligned in inverse phase with the attenuated noise signal, so that the inverse signal may be cancelled out with the attenuated noise signal. Accordingly, the computer device may control the timing of playing the first target signal based on the target time delay, so as to achieve the inverse alignment of the inverse signal and the attenuated noise signal. Similarly, the computer device may also control the timing of playing the second target signal based on the target time delay to achieve the inverse alignment of the inverse signal and the attenuated noise signal. The target time delay may be preset by a relevant technician according to actual needs, experience of the relevant technician, or experimental results, which is not limited in the present application.

The computer equipment plays the first target signal and the adjusted first target signal based on the target time delay, so that the reversed phase alignment of the reversed phase signal and the adjusted reversed phase signal with the attenuation noise signal is realized, and the reversed phase signal and the adjusted reversed phase signal can be ensured to offset the attenuation noise signal.

The application provides a noise reduction method, which comprises the steps of synthesizing a first signal to be played and a noise signal acquired by ambient sound, obtaining and playing a first target signal, acquiring the played first target signal, obtaining a second signal, adjusting the inverted signal according to the similarity between the second signal and the first signal, and then continuing to synthesize the first signal and the adjusted inverted signal, so as to obtain and play the second target signal. Therefore, the computer equipment can adjust the inverted signal for offsetting the noise signal by comparing the audio signal actually heard by the user with the audio signal wanted to be heard by the user, so that the inverted signal can better offset the noise signal, and a better noise reduction effect is realized.

The embodiment shown in fig. 3 above describes a specific flow of the noise reduction method from the perspective of the computer device. A specific embodiment is provided below to exemplarily explain a specific flow of the noise reduction method.

In an embodiment of the present application, a method for collecting sound by two microphones and reducing noise is provided, and the noise reduction method can be applied to an earphone provided with two microphones and a loudspeaker. Fig. 4 is a schematic structural diagram of an earphone according to an embodiment of the present disclosure, and as shown in fig. 4, the earphone may include a first microphone, a second microphone, a main control chip and a speaker, where the first microphone, the second microphone and the speaker may be respectively electrically connected to the main control chip, and the main control chip may receive and process audio signals collected by the first microphone and the second microphone, and may also control the speaker to play the audio signals. The first microphone may be directed towards the environment for picking up ambient sounds and the second microphone may be directed towards the human ear for picking up sounds actually heard by the user.

The specific flow of the noise reduction method may be as shown in fig. 5, where the first microphone may collect ambient sound to obtain a corresponding noise signal S₁And sending to the main control chip. The master control chip may be based on the attenuation coefficient K,to S₁Carrying out attenuation adjustment to obtain an attenuation noise signal S₂I.e. the signal corresponding to the ambient noise transmitted into the human ear.

S₂＝K*S₁(0＜K≤1)

The main control chip can attenuate the noise signal S₂Performing an inversion process to obtain and attenuate a noise signal S₂Are identical in frequency spectrum and are opposite in phase₂。

The earphone can also comprise an audio frequency back-sampling module which can acquire the audio frequency signal S to be played₀And sending to a main control chip, the audio signal S₀I.e. the audio signal corresponding to the audio the user wants to listen to.

The main control chip can be used for inverting the phase of the signal S₂And an audio signal S₀Synthesizing to obtain audio signal S₄And controls the loudspeaker to play the audio signal S₄. Audio signal S₄Including and attenuating noise signals S₂Are identical in frequency spectrum and are opposite in phase₃Thus inverting the signal S₃Capable of attenuating noise signal S₂Mutually cancel each other, so that only the audio signal S which the user wants to hear is included in the audio signals actually heard by the user₀。

Inverse signal S₂The noise signal transmitted into the ear of the user cannot be completely eliminated because of the attenuated noise signal S based on the attenuation coefficient K₂May not be exactly the same as the noise signal that is transmitted into the user's ear. The second microphone can collect the sound in the ear of the user to obtain a corresponding audio signal S₅. The main control chip can convert the audio signal S₅And the audio signal S that the user wants to hear₀Comparing to obtain similarity C, and comparing the similarity C with a similarity threshold C₀A comparison is made. When the similarity C is less than the similarity C₀When the master control chip is used, the attenuation coefficient K can be increased to attenuate the noise signal S₂Performing adjustment and reverse phase treatmentThe adjusted inverse signal is obtained, so that the frequency spectrum of the adjusted inverse signal is closer to the frequency spectrum of the noise signal transmitted into the ear of the user, and thus, the adjusted inverse signal can better counteract the noise signal transmitted into the ear of the user, and better noise reduction effect is realized. When the similarity C is larger than C₀In the meantime, it may be considered that the human ear cannot recognize that the actually heard audio includes noise, and the main control chip may stop adjusting the attenuation coefficient K.

It should be noted that the above embodiments are only optional embodiments of the present application, and are not intended to limit the present application.

Fig. 6 is a schematic structural diagram of a noise reduction device provided in an embodiment of the present application, and referring to fig. 6, the device includes:

an obtaining module 601, configured to obtain a first signal to be played and a noise signal acquired from an environmental sound;

a synthesizing module 602, configured to synthesize the first signal to be played and the inverted signal of the noise signal to obtain a first target signal, and play the first target signal;

an acquisition module 603, configured to acquire the played first target signal to obtain a second signal;

an adjusting module 604, configured to adjust an inverted signal of the noise signal according to a similarity between the second signal and the first signal to be played;

the synthesizing module 602 is further configured to continue to synthesize the first signal to be played and the adjusted inverse signal, and play a second target signal obtained after synthesis.

Optionally, the synthesis module 602 is configured to:

the adjustment module 604 is configured to:

Optionally, the synthesizing module 602 is configured to perform attenuation processing on the noise signal based on an attenuation coefficient to obtain an attenuated noise signal;

the adjusting module 604 is configured to adjust the attenuation coefficient according to the similarity between the second signal and the first signal to be played, so as to obtain an adjusted attenuated noise signal.

Optionally, the adjusting module 604 is configured to increase the attenuation coefficient when the similarity between the second signal and the first signal to be played is smaller than a similarity threshold, so as to obtain an adjusted attenuated noise signal;

the collecting module 603 is further configured to collect the played second target signal to obtain a third signal;

the adjusting module 604 is further configured to continue to perform the step of adjusting the attenuation coefficient when the similarity between the third signal and the first signal to be played is smaller than the similarity threshold, and stop adjusting until a target condition is met.

Optionally, the obtaining module 601 is configured to:

acquiring a first signal to be played;

the collecting module 603 is configured to collect the played first target signal based on a second microphone facing towards the ear of the person on the sound collecting device, so as to obtain a second signal.

Optionally, the synthesis module 602 is configured to:

playing the first target signal according to the target time delay;

Optionally, the apparatus further comprises:

The application provides a noise reduction device, through the noise signal who acquires the first signal of treating the broadcast and gather environmental sound, synthesize first signal and opposition signal, obtain and broadcast first target signal, gather the first target signal of broadcast again, obtain the second signal, adjust the opposition signal according to the similarity of second signal and first signal, later continue to synthesize first signal and the opposition signal after the adjustment, obtain and broadcast the second target signal. Therefore, the noise reduction device can adjust the inverted signal for offsetting the noise signal by comparing the audio signal actually heard by the user with the audio signal wanted to be heard by the user, so that the inverted signal can better offset the noise signal, and a better noise reduction effect is realized.

It should be noted that: in the noise reduction device provided in the above embodiment, only the division of the above functional modules is used for illustration when noise is reduced, and in practical applications, the above functions may be distributed by different functional modules according to needs, that is, the internal structure of the device is divided into different functional modules to complete all or part of the above described functions. In addition, the noise reduction device and the noise reduction method provided by the above embodiments belong to the same concept, and specific implementation processes thereof are described in the method embodiments in detail and are not described herein again.

The computer device in the noise reduction method may be the computer device shown in fig. 7, and fig. 7 is a schematic structural diagram of a computer device provided in an embodiment of the present application. Referring to fig. 7, the computer device 700 may be: a smart phone, a tablet computer, an MP3(Moving Picture Experts Group Audio Layer III, motion video Experts compression standard Audio Layer 3) player, an MP4(Moving Picture Experts Group Audio Layer IV, motion video Experts compression standard Audio Layer 4) player, a notebook computer or a desktop computer. Computer device 700 may also be referred to by other names such as user equipment, portable terminals, laptop terminals, desktop terminals, and the like. The computer device 700 may also be: in-ear headphones, head phones, or ear-hook headphones.

Generally, the computer device 700 includes: one or more processors 701 and one or more memories 702.

The processor 701 may include one or more processing cores, such as a 4-core processor, an 8-core processor, and so on. The processor 701 may be implemented in at least one hardware form of a DSP (Digital Signal Processing), an FPGA (Field-Programmable Gate Array), and a PLA (Programmable Logic Array). The processor 701 may also include a main processor and a coprocessor, where the main processor is a processor for processing data in an awake state, and is also called a Central Processing Unit (CPU); a coprocessor is a low power processor for processing data in a standby state. In some embodiments, the processor 701 may be integrated with a GPU (Graphics Processing Unit), which is responsible for rendering and drawing the content required to be displayed on the display screen. In some embodiments, the processor 701 may further include an AI (Artificial Intelligence) processor for processing computing operations related to machine learning.

Memory 702 may include one or more computer-readable storage media, which may be non-transitory. Memory 702 may also include high-speed random access memory, as well as non-volatile memory, such as one or more magnetic disk storage devices, flash memory storage devices. In some embodiments, a non-transitory computer readable storage medium in memory 702 is used to store at least one instruction for execution by processor 701 to implement a noise reduction method provided by method embodiments herein.

In some embodiments, the computer device 700 may also optionally include: a peripheral interface 703 and at least one peripheral. The processor 701, the memory 702, and the peripheral interface 703 may be connected by buses or signal lines. Various peripheral devices may be connected to peripheral interface 703 via a bus, signal line, or circuit board. Specifically, the peripheral device includes: at least one of a radio frequency circuit 704, a display screen 705, a camera assembly 706, an audio circuit 707, a positioning component 708, and a power source 709.

The peripheral interface 703 may be used to connect at least one peripheral related to I/O (Input/Output) to the processor 701 and the memory 702. In some embodiments, processor 701, memory 702, and peripheral interface 703 are integrated on the same chip or circuit board; in some other embodiments, any one or two of the processor 701, the memory 702, and the peripheral interface 703 may be implemented on a separate chip or circuit board, which is not limited in this embodiment.

The Radio Frequency circuit 704 is used for receiving and transmitting RF (Radio Frequency) signals, also called electromagnetic signals. The radio frequency circuitry 704 communicates with communication networks and other communication devices via electromagnetic signals. The rf circuit 704 converts an electrical signal into an electromagnetic signal to transmit, or converts a received electromagnetic signal into an electrical signal. Optionally, the radio frequency circuit 704 includes: an antenna system, an RF transceiver, one or more amplifiers, a tuner, an oscillator, a digital signal processor, a codec chipset, a subscriber identity module card, and so forth. The radio frequency circuitry 704 may communicate with other computer devices via at least one wireless communication protocol. The wireless communication protocols include, but are not limited to: metropolitan area networks, various generation mobile communication networks (2G, 3G, 4G, and 5G), Wireless local area networks, and/or WiFi (Wireless Fidelity) networks. In some embodiments, the radio frequency circuit 704 may also include NFC (Near Field Communication) related circuits, which are not limited in this application.

The display screen 705 is used to display a UI (User Interface). The UI may include graphics, text, icons, video, and any combination thereof. When the display screen 705 is a touch display screen, the display screen 705 also has the ability to capture touch signals on or over the surface of the display screen 705. The touch signal may be input to the processor 701 as a control signal for processing. At this point, the display 705 may also be used to provide virtual buttons and/or a virtual keyboard, also referred to as soft buttons and/or a soft keyboard. In some embodiments, the display screen 705 may be one, providing the front panel of the computer device 700; in other embodiments, the display 705 can be at least two, respectively disposed on different surfaces of the computer device 700 or in a folded design; in some embodiments, the display 705 may be a flexible display disposed on a curved surface or on a folded surface of the computer device 700. Even more, the display 705 may be arranged in a non-rectangular irregular pattern, i.e. a shaped screen. The Display 705 may be made of LCD (Liquid Crystal Display), OLED (Organic Light-emitting diode), or the like.

The camera assembly 706 is used to capture images or video. Optionally, camera assembly 706 includes a front camera and a rear camera. Typically, the front camera is disposed on the front panel of the computer device 700 and the rear camera is disposed on the back of the computer device 700. In some embodiments, the number of the rear cameras is at least two, and each rear camera is any one of a main camera, a depth-of-field camera, a wide-angle camera and a telephoto camera, so that the main camera and the depth-of-field camera are fused to realize a background blurring function, and the main camera and the wide-angle camera are fused to realize panoramic shooting and VR (Virtual Reality) shooting functions or other fusion shooting functions. In some embodiments, camera assembly 706 may also include a flash. The flash lamp can be a monochrome temperature flash lamp or a bicolor temperature flash lamp. The double-color-temperature flash lamp is a combination of a warm-light flash lamp and a cold-light flash lamp, and can be used for light compensation at different color temperatures.

The audio circuitry 707 may include a microphone and a speaker. The microphone is used for collecting sound waves of a user and the environment, converting the sound waves into electric signals, and inputting the electric signals to the processor 701 for processing or inputting the electric signals to the radio frequency circuit 704 to realize voice communication. For stereo sound acquisition or noise reduction purposes, the microphones may be multiple and located at different locations on the computer device 700. The microphone may also be an array microphone or an omni-directional pick-up microphone. The speaker is used to convert electrical signals from the processor 701 or the radio frequency circuit 704 into sound waves. The loudspeaker can be a traditional film loudspeaker or a piezoelectric ceramic loudspeaker. When the speaker is a piezoelectric ceramic speaker, the speaker can be used for purposes such as converting an electric signal into a sound wave audible to a human being, or converting an electric signal into a sound wave inaudible to a human being to measure a distance. In some embodiments, the audio circuitry 707 may also include a headphone jack.

The Location component 708 is used to locate the current geographic Location of the computer device 700 for navigation or LBS (Location Based Service). The positioning component 708 may be a positioning component based on the GPS (global positioning System) in the united states, the beidou System in china, the graves System in russia, or the galileo System in the european union.

The power supply 709 is used to supply power to the various components of the computer device 700. The power source 709 may be alternating current, direct current, disposable batteries, or rechargeable batteries. When power source 709 includes a rechargeable battery, the rechargeable battery may support wired or wireless charging. The rechargeable battery may also be used to support fast charge technology.

In some embodiments, the computer device 700 also includes one or more sensors 710. The one or more sensors 710 include, but are not limited to: acceleration sensor 711, gyro sensor 712, pressure sensor 713, fingerprint sensor 714, optical sensor 715, and proximity sensor 716.

The acceleration sensor 711 may detect the magnitude of acceleration in three coordinate axes of a coordinate system established with the computer apparatus 700. For example, the acceleration sensor 711 may be used to detect components of the gravitational acceleration in three coordinate axes. The processor 701 may control the display screen 705 to display the user interface in a landscape view or a portrait view according to the gravitational acceleration signal collected by the acceleration sensor 711. The acceleration sensor 711 may also be used for acquisition of motion data of a game or a user.

The gyro sensor 712 may detect a body direction and a rotation angle of the computer device 700, and the gyro sensor 712 may cooperate with the acceleration sensor 711 to acquire a 3D motion of the user with respect to the computer device 700. From the data collected by the gyro sensor 712, the processor 701 may implement the following functions: motion sensing (such as changing the UI according to a user's tilting operation), image stabilization at the time of photographing, game control, and inertial navigation.

Pressure sensors 713 may be disposed on a side bezel of computer device 700 and/or underneath display screen 705. When the pressure sensor 713 is disposed on a side frame of the computer device 700, a user's holding signal to the computer device 700 may be detected, and the processor 701 performs left-right hand recognition or shortcut operation according to the holding signal collected by the pressure sensor 713. When the pressure sensor 713 is disposed at a lower layer of the display screen 705, the processor 701 controls the operability control on the UI interface according to the pressure operation of the user on the display screen 705. The operability control comprises at least one of a button control, a scroll bar control, an icon control and a menu control.

The fingerprint sensor 714 is used for collecting a fingerprint of a user, and the processor 701 identifies the identity of the user according to the fingerprint collected by the fingerprint sensor 714, or the fingerprint sensor 714 identifies the identity of the user according to the collected fingerprint. When the user identity is identified as a trusted identity, the processor 701 authorizes the user to perform relevant sensitive operations, including unlocking a screen, viewing encrypted information, downloading software, paying, changing settings, and the like. The fingerprint sensor 714 may be disposed on the front, back, or side of the computer device 700. When a physical key or vendor Logo is provided on the computer device 700, the fingerprint sensor 714 may be integrated with the physical key or vendor Logo.

The optical sensor 715 is used to collect the ambient light intensity. In one embodiment, the processor 701 may control the display brightness of the display screen 705 based on the ambient light intensity collected by the optical sensor 715. Specifically, when the ambient light intensity is high, the display brightness of the display screen 705 is increased; when the ambient light intensity is low, the display brightness of the display screen 705 is adjusted down. In another embodiment, processor 701 may also dynamically adjust the shooting parameters of camera assembly 706 based on the ambient light intensity collected by optical sensor 715.

A proximity sensor 716, also known as a distance sensor, is typically disposed on a front panel of the computer device 700. The proximity sensor 716 is used to capture the distance between the user and the front of the computer device 700. In one embodiment, the processor 701 controls the display screen 705 to switch from the bright screen state to the dark screen state when the proximity sensor 716 detects that the distance between the user and the front face of the computer device 700 is gradually decreased; when the proximity sensor 716 detects that the distance between the user and the front of the computer device 700 is gradually increased, the processor 701 controls the display screen 705 to switch from the breath-screen state to the bright-screen state.

Those skilled in the art will appreciate that the configuration illustrated in FIG. 7 is not intended to be limiting of the computer device 700 and may include more or fewer components than those illustrated, or some components may be combined, or a different arrangement of components may be employed.

In an exemplary embodiment, a computer-readable storage medium, such as a memory, including instructions executable by a processor to perform the noise reduction method in the above-described embodiments is also provided. For example, the computer-readable storage medium may be a Read-Only Memory (ROM), a Random Access Memory (RAM), a Compact Disc Read-Only Memory (CD-ROM), a magnetic tape, a floppy disk, an optical data storage device, and the like.

It will be understood by those skilled in the art that all or part of the steps for implementing the above embodiments may be implemented by hardware, or may be implemented by a program instructing relevant hardware, and the program may be stored in a computer-readable storage medium, and the above-mentioned storage medium may be a read-only memory, a magnetic disk or an optical disk, etc.

The above description is intended to be exemplary only, and not to limit the present application, and any modifications, equivalents, improvements, etc. made within the spirit and scope of the present application are intended to be included therein.

Claims

1. A method of noise reduction, the method comprising:

collecting the played first target signal to obtain a second signal;

2. The method according to claim 1, wherein the synthesizing the first signal to be played and the inverse signal of the noise signal to obtain a first target signal, and playing the first target signal comprises:

3. The method of claim 2, wherein the attenuating the noise signal to obtain an attenuated noise signal comprises:

4. The method according to claim 3, wherein the adjusting the attenuation coefficient according to the similarity between the second signal and the first signal to be played to obtain an adjusted attenuated noise signal comprises:

collecting the played second target signal to obtain a third signal;

5. The method of claim 1, wherein the obtaining the first signal to be played and the noise signal collected for the environmental sound comprises:

acquiring a first signal to be played;

6. The method according to claim 1, wherein the synthesizing the first signal to be played and the inverse signal of the noise signal to obtain a first target signal, and playing the first target signal comprises:

playing the first target signal according to the target time delay;

7. The method according to claim 1, wherein before the adjusting the inverse signal of the noise signal according to the similarity between the second signal and the first signal to be played, the method further comprises:

8. A noise reduction apparatus, characterized in that the apparatus comprises a plurality of functional modules for performing the noise reduction method of any one of claims 1 to 7.

9. A computer device comprising one or more processors and one or more memories having stored therein at least one instruction that is loaded and executed by the one or more processors to implement operations performed by the noise reduction method of any of claims 1-7.

10. A computer-readable storage medium having stored therein at least one instruction, which is loaded and executed by a processor to perform operations performed by the noise reduction method of any one of claims 1 to 7.