Apparatus, method and medium for coding an audio signal using correlation between frequency bands

0001The present invention relates to audio signal processing using, for example, a moving picture expert group (MPEG)-4 audio signal encoding and decoding, and more particularly, to an apparatus, method, computer program and medium for processing an audio signal using a correlation between bands.

0002In a conventional method of processing an audio signal, such as perceptual noise substitution (PNS) which is used as an MPEG-4 audio coding tool, an audio signal can be effectively processed at a low bit rate such as 64 kbps/stereo, but sound quality is degraded. In the conventional method, in particular, when a transient audio signal is processed, sound quality is more degraded. In addition, in the conventional method, the audio signal is encoded by reducing an audio frequency bandwidth since the number of available bits is small. In this case, since the audio frequency bandwidth is reduced, sound quality is more degraded.

0003<patcit id="pcit0001" dnum="EP1441330A2"><text>European patent application number EP 1441330A2</text></patcit> discloses an advanced digital audio encoding and/or decoding method and apparatus.

0004WO-A1-2005/076260 falls under Article 54(3) EPC. It discloses an extended band coder that determines similarity using a least-means-square comparison.

0005According to an aspect of the present invention, there is provided an apparatus for processing an audio signal using a correlation between bands according to claim 1.

0006According to another aspect of the present invention, there is provided a method of processing an audio signal using a correlation between bands according to claim 9.

0007The invention also relates to a computer program as claimed in claim 17 and to a computer readable medium as claimed in claim 18.

0008Thus, there is provided an apparatus for processing an audio signal using a correlation between bands in which an audio signal is effectively processed without reducing a bandwidth even at a low bit rate.

0009There is also provided a method of for processing an audio signal using a correlation between bands in which an audio signal is effectively processed without reducing a bandwidth even at a low bit rate.

BRIEF DESCRIPTION OF THE DRAWINGS

0010These and/or other aspects, features, and advantages of the invention will become apparent and more readily appreciated from the following description of exemplary embodiments, taken in conjunction with the accompanying drawings of which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is a block diagram of an apparatus for processing an audio signal according to an exemplary embodiment of the present invention;</li><li><figref idref="f0002">FIG. 2</figref> is a flowchart illustrating a method of processing an audio signal by which an input audio signal is encoded, according to an exemplary embodiment of the present invention;</li><li><figref idref="f0002">FIG. 3</figref> is a flowchart illustrating a method of processing an audio signal by which an encoded audio signal is decoded, according to another exemplary embodiment of the present invention;</li><li><figref idref="f0003">FIG. 4</figref> is a block diagram of a correlation analyzer shown in <figref idref="f0001">FIG 1</figref> according to another exemplary embodiment of the present invention;</li><li><figref idref="f0004">FIG. 5</figref> is a flowchart illustrating operation 72 shown in <figref idref="f0002">FIG. 2</figref> according to another exemplary embodiment of the present invention;</li><li><figref idref="f0005">FIG. 6</figref> is a block diagram of the correlation analyzer shown in <figref idref="f0001">FIG 1</figref> according to another exemplary embodiment of the present invention;</li><li><figref idref="f0006">FIG 7</figref> is a flowchart illustrating operation 72 shown in <figref idref="f0002">FIG 2</figref> according to another exemplary embodiment of the present invention;</li><li><figref idref="f0007">FIG. 8</figref> is a block diagram of a high frequency component restoring portion according to another exemplary embodiment of the present invention;</li><li><figref idref="f0007">FIG. 9</figref> is a flowchart illustrating operation 94 shown in <figref idref="f0002">FIG. 3</figref> according to another exemplary embodiment of the present invention; and</li><li><figref idref="f0008 f0009 f0010">FIGS. 10A through 10E</figref> are illustrative waveforms of subbands for explaining a correlation between a low frequency band and a high frequency band.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0011Reference will now be made in detail to exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. Exemplary embodiments are described below to explain the present invention by referring to the figures.

0012<figref idref="f0001">FIG. 1</figref> is a block diagram of an apparatus for processing an audio signal according to an exemplary embodiment of the present invention. The apparatus of <figref idref="f0001">FIG. 1</figref> comprises an encoding unit 10 and a decoding unit 12.

0013The encoding unit 10 encodes an input audio signal input through an input terminal IN1 and transmits the result of encoding to the decoding unit 12. In this case, the decoding unit 12 decodes the input audio signal encoded by the encoding unit 10 and outputs the result of decoding through an output terminal OUT1.

0014In exemplary embodiments, subbands having a high frequency are referred to as first subbands, and subbands having a low frequency are referred to as second subbands.

0015When encoding, the encoding unit 10 searches the second subbands to obtain the most similar subband having a correlation, of more than a predetermined value, between the first subband and the most similar subband. Encoding unit 10 generates information about the second searched subband, for example, information about an index of the second searched subband, where the second searched subband is the most similar subband. The encoding unit 10 performs the operation on each of the first subbands.

0016In this case, the encoding unit 10 encodes an input audio signal using a general audio encoding method in first subband(s) having no similar subband(s) and second subbands. Hereinafter, similar subband refers to a second subband having a correlation of more than a predetermined value between the first subband and the similar subband. In this case, the general audio encoding method may be random noise substitution (RNS), which will be described later.

0017According to an exemplary embodiment of the present invention, the encoding unit 10 may comprise a subband filter analyzer 30, a correlation analyzer 32, a quantizer 34, an outputting portion 36, and a quantization controller 38, as shown in <figref idref="f0001">FIG. 1</figref>.

0018Hereinafter, the configuration and operation of the encoding unit 10 shown in <figref idref="f0001">FIG. 1</figref> and a method of processing an audio signal performed in the encoding unit 10 will be described.

0019<figref idref="f0002">FIG. 2</figref> is a flowchart illustrating a method of processing an audio signal by which an input audio signal is encoded, according to an exemplary embodiment of the present invention. The method of <figref idref="f0002">FIG. 2</figref> includes subband-filtering an input audio signal (operation 70), searching for the most similar subband for each of first subbands included in the result of subband-filtering and generating information about the searched most similar subband (operation 72), performing quantization using the result of analyzing hearing sensitivity (operations 74 and 76), and lossless encoding and bit packing the result of quantization (operation 78).

0020In operation 70, the subband filter analyzer 30 of the encoding unit 10 inputs an input audio signal through an input terminal IN1, subband-filters the inputted input audio signal, and outputs the result of subband-filtering to each of the correlation analyzer 32 and the quantization controller 38. In this case, the subband filter analyzer 30 may also output the result of subband-filtering to the quantizer 34, which is also referred to as quantization portion 34.

0021After operation 70, in operation 72, the correlation analyzer 32 searches for the most similar subband, having a correlation of more than a predetermined value between the first subband and the most similar subband, from second subbands, generates information about the second searched subband, and outputs generated information to the quantizer 34. For example, the correlation analyzer 32 searches for the most similar subband from the second subbands and matches each first subband having a most similar subband with information about the most similar subband to generate information about the second searched subband.

0022After operation 72, in operation 74, the quantization controller 38 analyzes hearing sensitivity from the result of subband-filtering inputted by the subband filter analyzer 30, generates a step size control signal according to the result of analyzing, and outputs the generated step size control signal to the quantizer 34. To this end, the quantization controller 38 may be implemented as an address generator (not shown) and a lookup table (not shown). Here, the address generator (not shown) generates an address by reflecting hearing sensitivity from the result of subband filtering inputted by the subband filter analyzer 30 and outputs the generated address to the lookup table (not shown). The lookup table selects a corresponding step size from step sizes stored as data, in response to the address generated by the address generator and outputs the selected step size as a step size control signal to the quantizer 34. Here, the step size stored in the lookup table may be generated based on information used to properly perform quantization, for example, a psychological sound model.

0023According to the present invention, operations 72 and 74 shown in <figref idref="f0002">FIG. 2</figref> may be performed simultaneously, and operation 74 may be performed earlier than operation 71.

0024After operation 74, in operation 76, the quantizer 34 quantizes information about the second generated subband inputted by the correlation analyzer 32 and the result of subband-filtering and outputs the result of quantization to the outputting portion 36. To this end, the quantizer 34 may directly input the result of subband-filtering from the subband filter analyzer 30 or through the correlation analyzer 32. In this case, the quantizer 34 controls a quantization step size in response to the step size control signal inputted by the quantization controller 38.

0025After operation 76, in operation 78, the outputting portion 36 lossless encodes and bit packs the result of quantization performed by the quantizer 34, converts the result of lossless-encoding and bit-packing into a bit stream format, stores the converted bit stream, and transmits the stored bit stream to the decoding unit 12. Here, Huffman encoding may be used for lossless encoding.

0026According to the present invention, the encoding unit 10 may not comprise the quantization controller 38. In this case, the encoding unit 10 comprises a subband filter analyzer 30, a correlation analyzer 32, a quantizer 34, and an outputting portion 36.

0027When decoding, the decoding unit 12 receives information about the second generated subband in a bit stream format transmitted from the encoding unit 10 and copies data about the second searched subband as data about a first subband using received information.

0028In this case, an input audio signal having no matched most similar subband between a first subband(s) and second subbands, is decoded using a general audio decoding method. To this end, according to an exemplary embodiment of the present invention, the decoding unit 12 comprises an inputting portion 50, an inverse quantizer 52, a high frequency component restoring portion 54, and a_subband filter synthesizer 56, as shown in <figref idref="f0001">FIG. 1</figref>.

0029Hereinafter, the configuration and operation of the decoding unit 12 shown in <figref idref="f0001">FIG. 1</figref> and a method of processing an audio signal performed in the decoding unit 12 will be described.

0030<figref idref="f0002">FIG. 3</figref> is a flowchart illustrating a method of processing an audio signal by which an encoded audio signal is decoded, according to another exemplary embodiment of the present invention. The method of <figref idref="f0002">FIG. 3</figref> includes bit unpacking, lossless decoding, and extracting various information (operation 90), performing inverse quantization (operation 92), copying data (operation 94), and performing subband filtering and restoring an input audio signal (operation 96).

0031In operation 90, the inputting portion 50 receives a bit stream transmitted from the outputting portion 36 of the encoding unit 10, bit unpacks and lossless decodes the received bit stream, outputs the bit-unpacked and lossless-decoded bit stream to the inverse quantizer 52, extracts various information and outputs extracted information to the high frequency component restoring portion 54. Here, Huffman decoding is an example of lossless decoding.

0032After operation 90, in operation 92, the inverse quantizer 52 inputs and inverse quantizes the result of lossless decoding performed by the inputting portion 50 and outputs the result of inverse quantization to the high frequency component restoring portion 54.

0033After operation 92, in operation 94, the high frequency component restoring portion 54 copies data corresponding to information about the second generated subband included in various information extracted by the inputting portion 50 among data about second subbands included in the result of inverse quantization as data about the first subband and outputs the result of copying to the subband filter synthesizer 56.

0034After operation 94, in operation 96, the subband filter synthesizer 56 subband filters the first subband having copied data inputted by the high frequency component restoring portion 54 and the result of inverse quantization and outputs the result of subband-filtering as an audio signal in which the input audio signal is restored, through an output terminal OUT1. The result of inverse quantization subband-filtered in operation 96 refers to data about the first subband having no copied data and the second subband among data included in the result of inverse quantization.

0035To this end, the subband filter synthesizer 56 may input the result of inverse quantization through the high frequency component restoring portion 54 or directly from the inverse quantizer 52.

0036Hereinafter, the configuration and operation of the correlation analyzer 32 shown in <figref idref="f0001">FIG. 1</figref> according to exemplary embodiments of the present invention and a method of processing an audio signal performed in exemplary embodiments will be described with reference to the attached drawings.

0037<figref idref="f0003">FIG. 4</figref> is a block diagram of the correlation analyzer 32 shown in <figref idref="f0001">FIG. 1</figref> according to another exemplary embodiment 32A of the present invention. The correlation analyzer 32A comprises a correlation calculator 110, a subband comparator and selector 113, and an information generator 116.

0038<figref idref="f0004">FIG. 5</figref> is a flowchart illustrating operation 72 shown in <figref idref="f0002">FIG. 2</figref> according to another exemplary embodiment of the present invention. Operation 72 includes selecting second subbands used in obtaining the largest correlation among correlations between respective first subbands and the second subbands (operations 130 and 132), generating information according to similarity of correlations (operations 134 and 138), and generating information about a noise power (operation 140).

0039In operation 130, the correlation calculator 110 of <figref idref="f0003">FIG. 4</figref> calculates correlations between second subbands that belong to a low frequency band, and each of the first subbands that belongs to a high frequency band and outputs the calculated correlations in each of the first subbands to the subband comparator and selector 113. To this end, the correlation calculator 110 discriminates a high frequency band and a low frequency band based on a reference frequency in a band of the result of subband-filtering inputted through an input terminal IN2. According to the present invention, the reference frequency which is a basis for discriminating a high frequency band and a low frequency, may be changed by a user or may be set in advance.

0040According to the present invention, a correlation can be obtained using Equation 1 <maths id="math0001" num="(1),"><math display="block"><mi>cor</mi><mo>=</mo><mfrac><mrow><mi>abs</mi><mo>⁢</mo><mfenced><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi mathvariant="normal">i</mi><mo>=</mo><mn>0</mn></mrow><mrow><mi mathvariant="normal">I</mi><mo>−</mo><mn>1</mn></mrow></munderover><mfenced><mi>samp</mi><mfenced open="[" close="]"><mi mathvariant="normal">s</mi><mo>⁢</mo><msub><mi mathvariant="normal">b</mi><mn>1</mn></msub></mfenced><mo>⁢</mo><mfenced open="[" close="]"><mi mathvariant="normal">i</mi></mfenced><mo>⋅</mo><mi>samp</mi><mfenced open="[" close="]"><mi mathvariant="normal">s</mi><mo>⁢</mo><msub><mi mathvariant="normal">b</mi><mn>2</mn></msub></mfenced><mo>⁢</mo><mfenced open="[" close="]"><mi mathvariant="normal">i</mi></mfenced></mfenced></mstyle></mfenced></mrow><msqrt><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi mathvariant="normal">i</mi><mo>=</mo><mn>0</mn></mrow><mrow><mi mathvariant="normal">I</mi><mo>−</mo><mn>1</mn></mrow></munderover><mfenced><mi>samp</mi><mfenced open="[" close="]"><mi mathvariant="normal">s</mi><mo>⁢</mo><msub><mi mathvariant="normal">b</mi><mn>1</mn></msub></mfenced><mo>⁢</mo><mfenced open="[" close="]"><mi mathvariant="normal">i</mi></mfenced><mo>⋅</mo><mi>samp</mi><mfenced open="[" close="]"><mi mathvariant="normal">s</mi><mo>⁢</mo><msub><mi mathvariant="normal">b</mi><mn>1</mn></msub></mfenced><mo>⁢</mo><mfenced open="[" close="]"><mi mathvariant="normal">i</mi></mfenced></mfenced></mstyle><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi mathvariant="normal">i</mi><mo>=</mo><mn>0</mn></mrow><mrow><mi mathvariant="normal">I</mi><mo>−</mo><mn>1</mn></mrow></munderover><mfenced><mi>samp</mi><mfenced open="[" close="]"><mi mathvariant="normal">s</mi><mo>⁢</mo><msub><mi mathvariant="normal">b</mi><mn>2</mn></msub></mfenced><mo>⁢</mo><mfenced open="[" close="]"><mi mathvariant="normal">i</mi></mfenced><mo>⋅</mo><mi>samp</mi><mfenced open="[" close="]"><mi mathvariant="normal">s</mi><mo>⁢</mo><msub><mi mathvariant="normal">b</mi><mn>2</mn></msub></mfenced><mo>⁢</mo><mfenced open="[" close="]"><mi mathvariant="normal">i</mi></mfenced></mfenced></mstyle></msqrt></mfrac></math><img file="EP1667112B1_D0001.tif" /></maths> wherein abs() is an absolute value of (), sb₁ is an index of a second subband that belongs to a low frequency band and is one selected from 0 to k-1. In addition, k is the number of second subbands that belong to the low frequency band, and sb₂ is an index of a first subband. I is the number of time domain samples which belong to the first subband. In this case, it is assumed that the number of time domain samples that belong to the first subbands is equal to that of the second subbands. In addition, samp[sb₁][i] is an i-th time domain sample placed in an sb₁ -th second subband, and samp[sb₂][i] is an i-th time domain sample placed in an sb₂ -th first subband.

0041After operation 130, in operations 132 and 134, a subband selector 112 selects second subbands used in calculating the largest correlation of more than a predetermined value among correlations calculated in each of first subbands and inputted by the correlation calculator 110 and outputs the second selected subbands to the information generator 116. Here, 'the second subbands used in calculating correlations' refers to second subbands compared with first subbands to calculate correlations.

0042To this end, in operation 132, the subband selector 112 selects second subbands used in calculating the largest correlation of more than a predetermined value among correlations calculated by the correlation calculator 110 in each of first subbands, outputs the second selected subbands to the information generator 116, and outputs the largest correlation to a comparator 114. After operation 132, in operation 134, the comparator 114 compares a correlation calculated using the second subbands selected in each of first subbands, that is, the largest correlation in each of first subbands, with a predetermined value and outputs the result of comparing to the information generator 116. In other words, the comparator 114 determines whether the largest correlation of each of the first subbands is more than or equal to the predetermined value.

0043In operations 136 to 140, the information generator 116 generates information about the second selected subband inputted from the subband selector 112, information about whether first subbands have similar subbands, and information about a noise power of the first subbands and outputs the generated information through an output terminal OUT2 in response to the result compared by the comparator 114.

0044For example, if it is recognized from the result of comparing inputted by the comparator 114 that the largest correlation of the first subbands is more than or equal to the predetermined value, in operation 136, the information generator 116 generates information about the second selected subbands inputted from the subband selector 112, that is, information about an index of the second selected subbands and information indicating that the first subbands have similar subbands, for example, in a mode bit format, and outputs the generated information through an output terminal OUT2. However, if it is recognized from the result of comparing inputted from the comparator 114 that the largest correlation of the first subband is not more than the predetermined value, in operation 138, the information generator 116 generates information indicating that the first subband has no similar subbands, in a mode bit format. Here, the mode bit is a bit indicating whether the first subband has similar subband. For example, if the first subbands have the similar subbands, in operation 136, the mode bit may be set to '1' (or '0') to indicate a correlation noise substitution (CNS) mode. If the first subbands have no similar subbands, in operation 138, the mode bit may be set to '0' (or '1') to indicate a random noise substitution (RNS) mode. Operations 136 and 138 are performed on each first subblock.

0045<figref idref="f0005">FIG. 6</figref> is a block diagram of the correlation analyzer 32 shown in <figref idref="f0001">FIG. 1</figref> according to another exemplary embodiment 32B of the present invention. The correlation analyzer 32B comprises a correlation calculator 110, a subband comparator and selector 150, and an information generator 156.

0046<figref idref="f0006">FIG. 7</figref> is a flowchart illustrating operation 72 shown in <figref idref="f0002">FIG. 2</figref> according to another exemplary embodiment of the present invention. Operation 72 includes determining whether there are correlations of more than a predetermined value among correlations of respective first subbands (operations 130 and 162), selecting second subbands used in obtaining the largest correlation from the existing correlations (operation 164), and generating information (operations 136 to 140).

0047Since the correlation calculator 110 shown in <figref idref="f0003">FIGS. 4</figref> and <figref idref="f0005">6</figref> performs the same operation, the same reference numeral is used therefor, and a detailed description thereof will be omitted. Further, since operations 130 and 140 shown in <figref idref="f0004">FIGS. 5</figref> and <figref idref="f0006">7</figref> are performed in the same manner, the same reference numeral is used therefor, and a detailed description thereof will be omitted.

0048After operation 130, in operations 162 and 164, the subband comparator and selector 150 selects second subbands used in calculating the largest correlation of more than a predetermined value among correlations calculated in each of first subbands and inputted from the correlation calculator 110 and outputs the second selected subbands to the information generator 156.

0049To this end, in operation 162, a comparator 152 compares the correlations calculated in each of first subbands with the predetermined value and outputs the result of comparing to each of a subband selector 154 and an information generator 156. In other words, the comparator 152 determines whether there is correlation of more than the predetermined value among correlations calculated in each of subbands. If it is recognized from the result compared by the comparator 152 that there is correlation of more than the predetermined value, in operation 164, the subband selector 154 selects second subbands used in calculating the largest correlation among the correlations of more than the predetermined value and outputs the second selected subbands to the information generator 156.

0050In operations 166 and 168, the information generator 156 generates information about the second subbands selected by the subband selector 154, generates information about whether the first subband has similar subband, using the result of comparing inputted from the comparator 152, and outputs the generated information through an output terminal OUT2. The information generator 156 also generates information about a noise power of the first subband, like the information generator 116 shown in <figref idref="f0003">FIG. 4</figref>.

0051For example, if it is recognized from the result of comparing inputted from the comparator 152 that there is correlation of more than the predetermined value, in operation 166, the information generator 156 generates information about the second selected subband inputted from the subband selector 154, that is, information about an index of the second selected subband and information indicating that the first subband has similar subband, for example, in a mode bit format, and outputs the generated information through an output terminal OUT2. However, if it is recognized from the result of comparing inputted from the comparator 152 that there is no correlation of more than the predetermined value, in operation 168, the information generator 156 generates information indicating that the first subband has no similar subband, in the mode bit format. Operations 166 and 168 are performed on each first subblock.

0052Hereinafter, the configuration and operation of the high frequency component restoring portion 54 shown in <figref idref="f0001">FIG. 1</figref> according to an exemplary embodiment of the present invention and a method of processing an audio signal performed in an exemplary embodiment will be described with reference to the attached drawings.

0053<figref idref="f0007">FIG. 8</figref> is a block diagram of the high frequency component restoring portion 54 according to another exemplary embodiment 54A of the present invention. The high frequency component restoring portion 54A includes a correlation checking portion 180, a data copying portion 182, a random noise generator 184, and a normalizing portion 186.

0054<figref idref="f0007">FIG. 9</figref> is a flowchart illustrating operation 94 shown in <figref idref="f0002">FIG. 3</figref> according to another exemplary embodiment of the present invention. Operation 94 includes decoding first subbands differently depending on whether the first subband has similar subband (operations 190 to 194) and normalizing copied data (operation 196).

0055In operation 190, the correlation checking portion 180 checks whether each of first subbands of the result of quantization performed by the inverse quantization portion 52 has similar subband. To this end, the correlation checking portion 180 inputs additional information extracted from the inputting portion 50 through an input terminal IN3 and determines from the inputted additional information whether each of the first subbands has similar subbands. For example, the extracted additional information may include the above-described mode bit. In this case, the correlation checking portion 180 checks whether the mode bit is '1' or '0' and can determine through the result of checking whether the first subband has the similar subband.

0056If it is recognized through the result of checking performed by the correlation checking portion 180 that the first subbands has the similar subband, in operation 192, the data copying portion 182 extracts data included in information about the second selected subbands from the result of inverse quantization inputted from the inverse quantization portion 52 through an input terminal IN4 and copies the extracted data as data about the first subbands. However, if it is recognized through the result of checking performed by the correlation checking portion 180 that the first subbands have no similar subbands, in operation 194, the random noise generator 184 randomly generates noise about the first subbands and outputs the randomly-generated noise to the normalizing portion 186. Here, the above-described RNS method includes a general encoding method by which operation 138 or 168 of setting the mode bit to a bit value indicating an RNS mode is performed and a general decoding method by which operation 194 is performed according to the mode bit set to the bit value indicating the RNS mode.

0057Operations 192 and 194 shown in <figref idref="f0007">FIG. 9</figref> are performed on each of first subbands. In this case, decoding on the second subbands is performed using a general decoding method. In other words, noise of the second subbands is randomly generated in operation 194.

0058After operation 192 or 194, the normalizing portion 186 normalizes the copied data and the randomly-generated noise so that a total noise power about first subbands, that is, a total energy is maintained at the same level as that of the first subbands calculated from the encoding unit 10, and outputs the result of normalization to the subband filter synthesizer 56 through an output terminal OUT3. To this end, the normalizing portion 186 inputs additional information including information about the noise power generated by the encoding unit 10 from the inputting portion 50 through an input terminal IN5, so as to see a total noise power of the first subbands calculated from the encoding unit 10.

0059Here, when data included in the information about the second selected subband is copied as data about the first subbands, the level of the first original subband may be changed. Thus, in order to restore the level of the first original subbands before encoding, the normalizing portion 186 normalizes the copied data and the randomly-generated noise.

0060In the apparatus and method for processing an audio signal according to the present invention, when a correlation between a low frequency band and a high frequency band is high, a more improved performance can be provided to the user.

0061In general, the correlation between the low frequency band and the high frequency band increases when a sudden attack occurs on a time region and even when a harmonic component is strong and identical with a subband boundary.

0062<figref idref="f0008 f0009 f0010">FIGS. 10A through 10E</figref> are illustrative waveforms of subbands for explaining a correlation between a low frequency band and a high frequency band. Specifically, <figref idref="f0008">FIG. 10A</figref> illustrates a sample size about 6th to 9th subbands, <figref idref="f0008">FIG. 10B</figref> illustrates a sample size about 10th to 13th subbands, <figref idref="f0009">FIG. 10C</figref> illustrates a sample size about 14th to 17th subbands, <figref idref="f0009">FIG. 10D</figref> illustrates a sample size about 18th to 21 st subbands, and <figref idref="f0010">FIG. 10E</figref> illustrates a sample size about 22nd to 25th subbands. In each drawing, a horizontal axis represents time, and a vertical axis represents the size of a sample. 1 to 16 shown in each of <figref idref="f0008 f0009 f0010">FIGS. 10A through 10E</figref> represent indices on a time region.

0063If a reference frequency is the 10th subband of <figref idref="f0008">FIG. 10B</figref>, the size of a sample of an index 2 on a time region about the 14th subband of <figref idref="f0009">FIG. 10C</figref> in a high frequency band is very similar to the size of a sample of an index 2 on a time region about the 7th subband of <figref idref="f0008">FIG. 10A</figref> in a low frequency band, that is, correlation is very high.

0064As described above, in the apparatus and method for processing an audio signal using a correlation between bands according to the present invention, when the audio signal is encoded and decoded, a noise component is effectively substituted such that sound quality is improved, in particular, noise of a transient audio signal can be effectively substituted. Furthermore, without reducing a bandwidth even at a low bit rate, a high frequency signal can be effectively encoded and decoded, with respect to a signal having a strong harmonic component, more stable sound quality than in a conventional RNS method can be provided to the user, and when an audio signal with a large change according to time is processed, natural sound quality can be provided to the user.

0065In addition to the above-described exemplary embodiments, exemplary embodiments of the present invention can also be implemented by executing computer readable code/instructions in/on a medium, e.g., a computer readable medium. The medium can correspond to any medium/media permitting the storing and/or transmission of the computer readable code. The code/instructions may form a computer program.

0066The computer readable code/instructions can be recorded/transferred on a medium in a variety of ways, with examples of the medium including magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.), optical recording media (e.g., CD-ROMs, or DVDs), and storage/transmission media such as carrier waves, as well as through the Internet, for example. The medium may also be a distributed network, so that the computer readable code/instructions is stored/transferred and executed in a distributed fashion. The computer readable code/instructions may be executed by one or more processors.

0067Although a few exemplary embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in exemplary embodiments without departing from the scope of the invention which is defined in the claims.