Apparatus and method for noise estimation, and noise reduction apparatus employing the same

0001This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. <patcit id="pcit0001" dnum="KR1020080099699"><text>10-2008-0099699, filed on October 10, 2008</text></patcit> in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference for all purposes.

0002The following description relates to audio signal processing, and more particularly, to an apparatus and method for estimating noise, and a noise reduction apparatus employing the same.

0003Voice telephony using communication terminals such as mobile phones may not ensure high voice quality in a noisy environment. In order to enhance voice quality in noisy environments, technology to estimate background noise components to extract only the actual voice signals is desired.

0004As technology develops, voice-based applications for various terminals such as camcorders, notebook PCs, navigation systems, game machines, and the like, which operate in response to voice or store audio data are emerging. Accordingly, technology for reducing or eliminating background noise to extract high-quality voice is increasingly needed.

0005Various methods for estimating or reducing background noise have been proposed. However, it has been difficult to obtain a desired noise reduction or elimination performance where the statistical characteristics of noise change with time or where unexpected sporadic noise is generated upon initial operation for updating the statistical characteristics of noise.

0006According to one general aspect, there is provided a noise estimation apparatus including an audio input unit to receive audio signals from a plurality of directions and transform the audio signals into frequency-domain signals, a target sound blocker to block audio signals coming from a direction of a target sound source, and a compensator to compensate for distortions from directivity gains of the target sound blocker.

0007The audio input unit may include two microphones adjacent to each other from 1cm to 8 cm in distance, and transform audio signals received through the two microphones into frequency-domain signals.

0008The target sound blocker may block the audio signals from the target sound source by calculating differences between the audio signals received through the two microphones.

0009The compensator may calculate weights of the audio signals in which the audio signals from the target sound source are blocked, based on an average value of the audio signals in which the audio signals from the target sound source are blocked, and multiply the audio signals in which the audio signals from the target sound source are blocked by the corresponding weights.

0010The noise estimation apparatus may further include a target sound detector to detect the audio signals from the target sound source, and in a section where the audio signals from the target sound source are not detected, calculate a scaling coefficient which corresponds to a ratio of a magnitude of an audio signal received in the section relative to noise components estimated by the compensator, wherein the compensator may multiply the estimated noise components by the scaling coefficient.

0011The scaling coefficient may be calculated and updated in the section where the audio signals from the target sound source are not detected, and in a section where the audio signals from the target sound source are detected, a scaling coefficient that is previously calculated may be used.

0012The noise estimation apparatus may further include a gain calibrator to calibrate the two microphones to equalize gains of the two microphones.

0013The target sound blocker may output audio signal in which the audio signals from the target sound source are blocked.

0014According to another aspect, there is provided a noise reduction apparatus including a noise estimator configured to receive audio signals from a plurality of directions, transform the audio signals into frequency-domain signals, block audio signals coming from a direction of a target sound source from the frequency-domain signals, and compensate for gain distortions of the audio signals in which the audio signals from the target sound source are blocked, so as to estimate noise components, and a noise reduction filter to remove the noise components estimated by the noise estimator using a filter coefficient calculated based on the estimated noise components.

0015The noise estimator may include two microphones adjacent to each other from 1cm to 8 cm in distance, and the noise estimator may transform audio signals received through the two adjacent microphones into frequency-domain signals, calculate differences between the frequency-domain signals to block the audio signals from the target sound source, calculate weights of the audio signals in which the audio signals from the target sound source are blocked, using an average value of the audio signals in which the audio signals from the target sound source are blocked, and multiply the audio signals in which the audio signals from the target sound source are blocked by the corresponding weights.

0016According to still another aspect, there is provided a noise estimation method of a noise estimation apparatus, the method including receiving audio signals from a plurality of directions and transforming the audio signals into frequency-domain signals, blocking audio signals from a direction of a target sound source from the frequency-domain signals, compensating for gain distortions of the audio signals in which the audio signals from the target sound source are blocked.

0017The receiving of the audio signals may include receiving audio signals using two microphones adjacent to each other from 1cm to 8 cm in distance, and the blocking of the audio signals may include blocking the audio signals from the target sound source by calculating differences between the audio signals received through the two microphones.

0018The compensating may include calculating weights of the audio signals in which the audio signals from the target signal source are blocked, using an average value of the audio signals in which the audio signals from the target sound source are blocked, and multiplying the audio signals in which the audio signals from the target sound source are blocked by the corresponding weights.

0019The compensating may include detecting the presence of the audio signals from the target sound source, and in a section where the audio signals from the target sound source are not detected, calculating a scaling coefficient which corresponds to a ratio of a magnitude of an audio signal received in the section relative to previously calculated noise components.

0020The scaling coefficient may be calculated and updated in the section where the audio signals from the target sound source are not detected, and in a section where the audio signals from the target sound source are detected, a scaling coefficient that is previously calculated may be used.

0021The noise estimation apparatus may include two microphones, the method may further include calibrating the two microphones to equalize gains of the two microphones, and the receiving of the audio signals may include receiving audio signals using the calibrated two microphones.

0022According to yet another aspect, there is provided an apparatus for reducing noise, including an audio input unit having a plurality of microphones, which receives audio signals from a plurality of directions and transforms the audio signals into frequency-domain signals, a target sound blocker which blocks an audio signal coming from a direction of a target sound source from the frequency-domain signals, by calculating differences between audio signals received by the plurality of microphones, and outputs audio signals in which the audio signal from the target sound source is blocked, and a noise reduction unit which removes the audio signals in which the audio signal from the target sound source is blocked, to output the audio signal from the target sound source.

0023The noise reduction unit may be a filter which removes the audio signals in which the audio signal from the target sound source is blocked, using a filter coefficient determined based on the audio signals in which the audio signal from the target sound source is blocked.

0024The apparatus may further include a compensator which compensates for distortions from directivity gains of the target sound blocker.

0025The compensator may calculate weights of the audio signals in which the audio signal from the target sound source is blocked, based on an average value of the audio signals in which the audio signal from the target sound source is blocked, and multiply the audio signals in which the audio signal from the target sound source is blocked by the corresponding weights.

0026The apparatus may further include a target sound detector which detects the audio signal from the target sound source, and in a section where the audio signal from the target sound source is not detected, calculates a scaling coefficient which corresponds to a ratio of a magnitude of an audio signal received in the section relative to noise components estimated by the compensator, wherein the compensator multiplies the estimated noise components by the scaling coefficient.

0027The scaling coefficient may be calculated and updated in the section where the audio signal from the target sound source is not detected, and in a section where the audio signals from the target sound source is detected, a scaling coefficient that is previously calculated may be used.

0028The apparatus may further include a gain calibrator which calibrates the plurality of microphones to equalize gains of the microphones.

0029Other features and aspects will be apparent from the following detailed description, the drawings, and the claims.

<u>BRIEF DESCRIPTION OF THE DRAWINGS</u>

0030<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is a block diagram illustrating an exemplary noise estimation apparatus.</li><li><figref idref="f0002">FIG. 2</figref> is a diagram illustrating a location relationship between sound sources and an arrangement of a microphone array of the noise estimation apparatus of <figref idref="f0001">FIG. 1</figref>.</li><li><figref idref="f0003">FIG. 3</figref> is a graph illustrating a directivity pattern obtained by a target sound blocker of the noise estimation apparatus of <figref idref="f0001">FIG. 1</figref>.</li><li><figref idref="f0004">FIG. 4</figref> is a block diagram illustrating another exemplary noise estimation apparatus having a target sound detector.</li><li><figref idref="f0005">FIG. 5</figref> is a block diagram illustrating another exemplary noise estimation apparatus having a gain calibrator.</li><li><figref idref="f0006">FIG. 6</figref> is a block diagram illustrating an exemplary noise reduction apparatus having a noise estimator.</li><li><figref idref="f0006">FIG. 7</figref> is a flowchart illustrating an exemplary noise estimation method.</li></ul>

0031Throughout the drawings and the detailed description, unless otherwise described, the same drawing reference numerals will be understood to refer to the same elements, features, and structures. The relative size and depiction of these elements may be exaggerated for clarity, illustration, and convenience.

<u>DETAILED DESCRIPTION</u>

0032The following detailed description is provided to assist the reader in gaining a comprehensive understanding of the methods, apparatuses, and/or systems described herein. Accordingly, various changes, modifications, and equivalents of the systems, apparatuses and/or methods described herein will be suggested to those of ordinary skill in the art. Also, descriptions of well-known functions and constructions may be omitted for increased clarity and conciseness.

0033<figref idref="f0001">FIG. 1</figref> shows an exemplary noise estimation apparatus 100.

0034As shown in <figref idref="f0001">FIG. 1</figref>, the noise estimation apparatus 100 includes an audio input unit 110, a target sound blocker 120, and a compensator 130.

0035The audio input unit 110 receives audio signals from a plurality of directions and transforms them into frequency-domain signals. The target sound blocker 120 blocks audio signals coming from the direction of a target sound source. The compensator 130 compensates for gain distortions from the target sound blocker 120.

0036As one example, the audio input unit 110 includes two microphones (not shown) which are adjacent to each other, and transforms audio signals received by the microphones into frequency-domain signals. The transformation may be, for example, a Fourier transformation. Further exemplary details including the arrangement and number of microphones, the location of a target-sound source, and the locations of noise sources will be described with reference to <figref idref="f0002">FIG. 2</figref>.

0037In the example of audio input unit 110 having two microphones, the target sound blocker 120 blocks the target sound by calculating the differences between the audio signals received by the two microphones. For example, two omni-directional microphones for receiving audio signals from a plurality of directions are spaced apart by a predetermined distance (for example, 1 cm), so that audio signals coming from, for example, a front direction in which the target sound is generated are blocked and audio signals coming from different directions are received.

0038For example, a distance between two microphones may be from 1cm to 8cm. If a distance between two microphones is under 1cm, overall audio signals coming from a plurality of directions may be reduced. And if a distance between two microphones is over 8cm, audio signals coming from directions except a direction of target source may be blocked.

0039As an illustration, where frequency-transformed values of audio signals received by the microphones are S₁(f) and S₂(f), a frequency-transformed value B(f) of an audio signal in which target sound is blocked may be calculated by Equation 1: <maths id="math0001" num="[equation 1]"><math display="block"><mi mathvariant="normal">B</mi><mfenced><mi mathvariant="normal">f</mi></mfenced><mo>=</mo><msub><mi mathvariant="normal">w</mi><mi mathvariant="normal">i</mi></msub><mfenced><mi mathvariant="normal">f</mi></mfenced><mo>⋅</mo><msub><mi mathvariant="normal">S</mi><mn>1</mn></msub><mfenced><mi mathvariant="normal">f</mi></mfenced><mo>+</mo><msub><mi mathvariant="normal">w</mi><mn>2</mn></msub><mfenced><mi mathvariant="normal">f</mi></mfenced><mo>⋅</mo><msub><mi mathvariant="normal">S</mi><mn>2</mn></msub><mfenced><mi mathvariant="normal">f</mi></mfenced><mo>,</mo></math><img file="EP2175446A2_D0001.tif" /></maths> where w₁(f) and w₂(f) are coefficients for blocking target sound and may be set appropriately through an undue experiment. For example, where w₁(f) and w₂(f) are set to +1 and -1, respectively, the frequency-transformed value B(f) of the audio signal in which target sound is blocked becomes the difference between the frequency-transformed values S₁(f) and S₂(f) of the audio signals received by the microphones.

0040Where w₁(f) and w₂(f) are set to +1 and -1, respectively, since audio signals received from the front direction of the two microphones, that is, from the direction of a target-sound source, are ideally the same, and audio signals received from other directions are different from each other, only the audio signals received from the front direction of the two microphones ideally become zero. Accordingly, the target sound received from the front direction may be blocked.

0041The audio signal in which target sound is blocked may be noise components. However, the frequency characteristics of an audio signal output from the target sound blocker 120 may vary significantly depending on, for example, the microphone array aperture size, number of microphones, and so on. Accordingly, to reduce errors in noise estimation, the compensator 130 may be used to calculate weights based on an average value of audio signals in which target sound is blocked, and multiply the audio signals by the corresponding weights, respectively.

0042A directivity pattern D(f, ϕ) of the audio signals in which target sound is blocked, which is obtained by the target sound blocker 120, may be calculated by Equation 2: <maths id="math0002" num="[equation 2]"><math display="block"><mi>D</mi><mfenced><mi>f</mi><mo mathvariant="italic">□</mo></mfenced><mo mathvariant="italic">□</mo><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>n</mi><mo mathvariant="italic">□</mo><mo mathvariant="italic">□</mo><mfrac><mrow><mi>N</mi><mo mathvariant="italic">□</mo><mn>1</mn></mrow><mn>2</mn></mfrac></mrow><mfrac><mrow><mi>N</mi><mo mathvariant="italic">□</mo><mn>1</mn></mrow><mn>2</mn></mfrac></munderover></mstyle><msub><mi>w</mi><mi>n</mi></msub><mfenced><mi>f</mi></mfenced><mo>⁢</mo><msup><mi>e</mi><mrow><mi>j</mi><mo>⁢</mo><mfrac><mrow><mn>2</mn><mo mathvariant="italic">□</mo></mrow><mo mathvariant="italic">□</mo></mfrac><mo>⁢</mo><msub><mi mathvariant="italic">nd</mi><mspace width="1em" /></msub><mo>⁢</mo><mi>cos</mi><mo mathvariant="italic">□</mo></mrow></msup><mo>,</mo></math><img file="EP2175446A2_D0002.tif" /></maths> where N represents the number of microphones, d represents distance between the microphones, ϕ represents direction, f represents frequency, and w_n(f) represents weight relative to a microphone located at coordinate n, wherein the weights are related to the coefficients for blocking target in Equation 1. For example, if the number of the microphones are two, the w_-0.5(f) and w_0.5(f) are +1 and -1, respectively.

0043The compensator 130 receives the audio signal B(f) in which target sound is blocked, calculated by Equation 1, and multiplies the audio signal B(f) by the corresponding weight, so as to estimate noise components in real time. The weight may be calculated by Equation 3: <maths id="math0003" num="[equation 3]"><math display="block"><mi>W</mi><mfenced><mi>f</mi></mfenced><mo mathvariant="italic">□</mo><mfrac><mo mathvariant="italic">□</mo><mrow><mfrac><mn>1</mn><mo mathvariant="italic">□</mo></mfrac><munderover><mo>∫</mo><mn>0</mn><mo mathvariant="italic">□</mo></munderover><mfenced open="|" close="|"><mi>D</mi><mfenced><mi>f</mi><mo mathvariant="italic">□</mo></mfenced></mfenced><mo>⁢</mo><mi>d</mi><mo mathvariant="italic">□</mo></mrow></mfrac><mo>,</mo></math><img file="EP2175446A2_D0003.tif" /></maths> where α is a constant which is a global scaling coefficient, and is applied to all frequency components to adjust weights. The α value may be obtained through an undue experiment.

0044As a result, the noise components estimated by the compensator 130 may be written by Equation 4: <maths id="math0004" num="[equation 4]"><math display="block"><msub><mover><mi>N</mi><mo>˜</mo></mover><mi>a</mi></msub><mfenced><mi>f</mi></mfenced><mo>=</mo><mfenced open="|" close="|"><mi>B</mi><mfenced><mi>f</mi></mfenced><mo>⋅</mo><mi>W</mi><mfenced><mi>f</mi></mfenced></mfenced><mo>,</mo></math><img file="EP2175446A2_D0004.tif" /></maths>

0045As shown in Equation 4, noise of a current frame may be estimated without using noise information of the previous frame, and the existence and amount of directional noise may be estimated in real time regardless of detection of target sound.

0046An exemplary embodiment has been described with two microphones for an illustrative purpose. Accordingly, it is understood that the number of microphones can be other than two. For example, an audio input unit of a noise estimation apparatus may have three or more microphones. Based on the number of microphones, an appropriate combination of coefficients w may be selected to block audio signals received from a direction of a target-sound source.

0047<figref idref="f0002">FIG. 2</figref> shows a location relationship between sound sources 220 and 230-1 through 230-n, and an arrangement of a microphone array 210 of the noise estimation apparatus 100 of <figref idref="f0001">FIG. 1</figref>.

0048As shown, the microphones comprising the microphone array 210 are, for example, adjacent to each other, and the target-sound source 220 is located, for example, in front of (vertically above/below) the microphone array 210 so that audio signals are input to the microphone array 210. The audio signals input to the microphone array 210 are transferred to a noise reduction apparatus 240 to perform noise estimation and noise reduction.

0049The noise reduction apparatus 240 blocks audio signals received from the target-sound source 220 by, for example, the target sound blocking method described above with reference to <figref idref="f0001">FIG. 1</figref>, and extracts noise signals received from noise sources 230-1, 230-2, ..., 230-n located in directions other than the direction in which the target-sound source 220 is located.

0050<figref idref="f0003">FIG. 3</figref> shows an exemplary directivity pattern obtained by the target sound blocker 120 of the noise estimation apparatus 120 of <figref idref="f0001">FIG. 1</figref>.

0051Referring to <figref idref="f0002">FIG. 2</figref>, in the view shown, the angle between the microphone array 210 and the target-sound source 220 is 90°. Referring to <figref idref="f0003">FIG. 3</figref>, all frequency bands received at an angle of 90° at which target sound is received have a gain of about zero. That is, target sound received at the angle of 90° is blocked, and the more the angle of the sound sources deviates from 90° , the larger the gain becomes. The gain depends on frequency band. For example, gains of high-frequency components are larger and gains of low-frequency components are smaller.

0052Meanwhile, the directivity pattern may depend on the target sound blocker 120.

0053As shown in <figref idref="f0003">FIG. 3</figref>, the gain differences of the directivity pattern according to direction of noise become greater at higher frequencies. Accordingly, weights w(f) calculated by the compensator 130 (see <figref idref="f0001">FIG. 1</figref>) may be used to average the gains of the directivity pattern.

0054<figref idref="f0004">FIG. 4</figref> shows another exemplary noise estimation apparatus 400 having a target sound detector 410.

0055The target sound detector 410 detects the presence or absence of target sound, and in a section where target sound is not detected, that is, in a noise section, calculates a scaling coefficient which corresponds to a ratio of the magnitude of an audio signal received in the noise section relative to noise components calculated by the compensator 420, and provides the scaling coefficient to the compensator 420. Then to estimate the noise components, the compensator 420 multiplies the previously calculated noise components by the scaling coefficient calculated by the target sound detector 410.

0056Although the compensator 420 compensates for the gains of the directivity pattern using the average value as described above, the compensator 420 may not compensate for directivities of noise signals correctly at all frequencies. Accordingly, the exemplary noise estimation apparatus 400 compensates for variation of gain according to direction of noise, in a mute section where target sound is not detected, under the assumption that the direction of noise does not sharply change as the characteristics of noise change with time. That is, where the target sound detector 410 detects a noise section where target sound does not exist, the previously estimated noise is adjusted by calculating a ratio of the magnitude of a noise signal received in the noise section relative to a noise signal calculated by Equation 4.

0057The ratio, that is, a local scaling coefficient β(f) may be calculated by Equation 5: <maths id="math0005" num="[equation 5]"><math display="block"><mi>β</mi><mfenced><mi>f</mi></mfenced><mo>=</mo><mfrac><mfenced open="|" close="|"><mi>S</mi><mfenced><mi>f</mi></mfenced></mfenced><mrow><msub><mover><mi>N</mi><mo>˜</mo></mover><mi>a</mi></msub><mfenced><mi>f</mi></mfenced></mrow></mfrac></math><img file="EP2175446A2_D0005.tif" /></maths>

0058Since calculation of an estimated noise value in a frequency domain may be performed in units of frames, Equation 5 may be rewritten as Equation 6 including frame information: <maths id="math0006" num="[equation 6]"><math display="block"><mi>β</mi><mfenced><mi>n</mi><mo>⁢</mo><mi>f</mi></mfenced><mo>=</mo><mfenced open="{" close="}"><mtable><mtr><mtd><mi>γ</mi><mo>⋅</mo><mfrac><mfenced open="|" close="|"><mi>S</mi><mfenced><mi>n</mi><mo>⁢</mo><mi>f</mi></mfenced></mfenced><mrow><msub><mover><mi>N</mi><mo>˜</mo></mover><mi>a</mi></msub><mfenced><mi>n</mi><mo>⁢</mo><mi>f</mi></mfenced></mrow></mfrac><mo>+</mo><mfenced><mn>1</mn><mo>-</mo><mi>γ</mi></mfenced><mo>⋅</mo><mi>β</mi><mo>⁢</mo><mfenced><mi>n</mi><mo>-</mo><mn>1</mn><mo>,</mo><mi>f</mi></mfenced></mtd><mtd><mo>,</mo><mi mathvariant="italic">if</mi><mspace width="1em" /><msup><mi mathvariant="normal">n</mi><mi>th</mi></msup><mspace width="1em" /><mi>frame has no target signal</mi></mtd></mtr><mtr><mtd><mi>β</mi><mo>⁢</mo><mfenced><mi>n</mi><mo>-</mo><mn>1</mn><mo>,</mo><mi>f</mi></mfenced></mtd><mtd><mi>otherwise</mi></mtd></mtr></mtable></mfenced></math><img file="EP2175446A2_D0006.tif" /></maths>

0059That is, the local scaling coefficient β(f) is recalculated and updated in sections where target sound is not detected, and in sections where target sound is detected, the previous local scaling coefficient is used as is. In Equation 6, γ is an update rate, and as γ approaches 1, the target sound detector 410 responds more quickly to changes in input noise, while as γ approaches 0, it responds with less sensitivity to sudden errors. Accordingly, an estimated noise value reflecting the local scaling coefficient β(f) output from the compensator 420 may be calculated by Equation 7: <maths id="math0007" num="[equation 7]"><math display="block"><msub><mover><mi>N</mi><mo>˜</mo></mover><mi>b</mi></msub><mfenced><mi>f</mi></mfenced><mo>=</mo><mi>B</mi><mfenced><mi>f</mi></mfenced><mo>⋅</mo><mi>W</mi><mfenced><mi>f</mi></mfenced><mo>⋅</mo><mi>β</mi><mfenced><mi>f</mi></mfenced></math><img file="EP2175446A2_D0007.tif" /></maths>

0060It is understood that general voice activity detection methods may be used for the target sound detector 410, and accordingly, further description is omitted for conciseness. It is also understood that various known or to be known methods may be used to detect target sound.

0061<figref idref="f0005">FIG. 5</figref> shows another exemplary noise estimation apparatus 500 having a gain calibrator 510.

0062The gain calibrator 510 calibrates, for example, two microphones to which target sound is input, to equalize gains of the microphones. Generally, different microphones manufactured according to a standard may have different gains due to errors in manufacturing processes. If two microphones have a gain difference, the target sound blocker 120 may not block target sound correctly. Accordingly, gain calibration may be performed before receiving audio signals through microphones.

0063The gain calibration may be performed once. However, since the gain may depend on environmental factors such as temperature or humidity, gain calibration may also be performed at regular time intervals. It is understood that general gain calibration methods may be used, and accordingly, further description is omitted for conciseness.

0064<figref idref="f0006">FIG. 6</figref> shows an exemplary noise reduction apparatus 600 having a noise estimator.

0065Referring to <figref idref="f0006">FIG. 6</figref>, the noise reduction apparatus 600 includes a noise estimator 610 and a noise reduction filter 620.

0066The noise estimator 610 may perform noise estimation described above with reference to <figref idref="f0001 f0002 f0003 f0004 f0005">FIGS. 1 through 5</figref>. For example, to estimate noise, the noise estimator 610 receives audio signals from a plurality of directions, transforms them into frequency-domain signals, blocks audio signals coming from a direction of a target sound source to be detected from the frequency-domain signals, and compensates for gain distortions of the resultant audio signals in which target sound is blocked.

0067The noise estimator 610 transforms audio signals received through, for example, two adjacent microphones into frequency-domain signals, calculates differences between the frequency-domain signals to block target sound, calculates weights of the audio signals in which target sound is blocked using an average value of the audio signals, and multiplies the audio signals in which the target sound is blocked by the corresponding weights, so as to estimate noise components.

0068The noise reduction filter 620 may be designed based on filter coefficients that are calculated using the estimated noise components. The noise reduction filter 620 may be one of various filters, such as spectral subtraction, a Wiener filter, an amplitude estimator, and the like.

0069<figref idref="f0006">FIG. 7</figref> is a flowchart illustrating an exemplary noise estimation method. It is understood that an exemplary noise estimation apparatus described above may perform the method.

0070In operation 710, audio signals are received from a plurality of directions and transformed into frequency-domain signals.

0071In operation 720, audio signals coming from a direction of a target sound source to be detected are blocked from among the frequency-domain signals. For example, by calculating differences between audio signals received through, for example, two adjacent microphones, only target sound may be blocked.

0072In operation 730, the distortions from the directivity gains of a target sound blocker are compensated for. For example, weights of the audio signals in which target sound is blocked are calculated based on an average value of the audio signals, and the audio signals are multiplied by the corresponding weights, so as to estimate noise components. To estimate the noise components, the presence or absence of target sound may be detected, in sections where no target sound is detected, a ratio (a scaling coefficient) of the magnitude of an input audio signal relative to the previously estimated noise components may be calculated, and the previously estimated noise components may be multiplied by the scaling coefficient.

0073The scaling coefficient may be a local scaling coefficient described above. The local scaling coefficient may be recalculated and updated in sections where target sound is not detected, and in sections where target sound is detected, the previous scaling coefficient may be used as is.

0074In the operation 730, the spectral distortions originated from the directivity gains of the target sound blocker may be compensated for.

0075To equalize gains of the microphones, the microphones may be calibrated before the operation 710 of receiving audio signals.

0076According to examples described above, since estimation of non-stationary noise which changes with time is possible, audio or voice quality as well as audio or voice recognition performance may be improved in various apparatuses which receive audio or voice.

0077As one example, exemplary noise estimation method described above may be applied to communication terminals such as mobile phones to improve audio or voice quality. Because noise estimation may be carried out uniformly over all frequency domains, and also in sections where audio or voice exists, effective or improved noise estimation may be possible.

0078According examples described above, there is provided an apparatus and method for estimating non-stationary noise by blocking target sound, and a noise reduction apparatus employing the same.

0079It is understood that the terminology used herein may be different in other applications or when described by another person of ordinary skill in the art. For example, a noise "reduction" filter or a noise "reduction" apparatus may also be referred to as a noise "elimination" filter or a noise "elimination" apparatus, respectively. Moreover, with respect to target sound described as being blocked, it is understood that a target sound blocker may not "completely" block target sound due to, for example, gain mismatch of microphones.

0080The methods described above may be recorded, stored, or fixed in one or more computer-readable media that includes program instructions to be implemented by a computer to cause a processor to execute or perform the program instructions. The media may also include, alone or in combination with the program instructions, data files, data structures, and the like. Examples of computer-readable media include magnetic media, such as hard disks, floppy disks, and magnetic tape; optical media such as CD ROM disks and DVDs; magneto-optical media, such as optical disks; and hardware devices that are specially configured to store and perform program instructions, such as read-only memory (ROM), random access memory (RAM), flash memory, and the like. Examples of program instructions include machine code, such as produced by a compiler, and files containing higher level code that may be executed by the computer using an interpreter. The described hardware devices may be configured to act as one or more software modules in order to perform the operations and methods described above, or vice versa.

0081A number of exemplary embodiments have been described above. Nevertheless, it will be understood that various modifications may be made. For example, suitable results may be achieved if the described techniques are performed in a different order and/or if components in a described system, architecture, device, or circuit are combined in a different manner and/or replaced or supplemented by other components or their equivalents. Accordingly, other implementations are within the scope of the following claims.