Abstract:
A digital audio encoding method using an advanced psychoacoustic model is provided. The audio encoding method including determining the type of a window according to the characteristic of an input audio signal; generating a complex modified discrete cosine transform (CMDCT) spectrum from the input audio signal according to the determined window type; generating a fast Fourier transform (FFT) spectrum from the input audio signal, by using the determined window type; and performing a psychoacoustic model analysis by using the generated CMDCT spectrum and FFT spectrum.

Description:
[0001]    This application claims priorities from U.S. Provisional Patent Application No. 60/422,094 filed on Oct. 30, 2002, and Korean Patent Application No. 2002-75407 filed on Nov. 29, 2002, the contents of which are incorporated herein by reference in their entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an encoding method and apparatus for encoding digital audio data, and more particularly, to a method and apparatus in which an advanced psychoacoustic model is used so that the amount of computation and complexity needed in the encoding method and apparatus is reduced without degradation of sound quality.  
           [0004]    2. Description of the Related Art  
           [0005]    A moving picture experts group (MPEG) audio encoder allows a listener not to perceive quantization noise generated when data is encoded. At the same time, the MPEG audio encoder achieves a high compression rate. An MPEG-1 audio encoder standardized by the MPEG encodes an audio signal at a bit rate of 32 kbps˜448 kbps. The MPEG-1 audio standard has 3 different algorithms for encoding data.  
           [0006]    The MPEG-1 encoder has 3 modes, including layer  1 , layer  2 , and layer  3 . Layer  1  implements a basic algorithm, while layers  2  and  3  are enhanced modes. The layers at higher levels achieve a higher compression rate, but on the other hand, the size of the hardware becomes larger.  
           [0007]    The MPEG audio encoder uses a psychoacoustic model which closely mirrors a characteristic of human hearing, in order to reduce perceptual redundancy of a signal of an audio encoder. The MPEG1 and MPEG2, standardized by the MPEG, employ a perceptual coding method using a psychoacoustic model which reflects the characteristic of human perception and removes perceptual redundancy such that a good sound quality can be maintained after decoding data.  
           [0008]    The perceptual coding method, by which a human psychoacoustic model is analyzed and applied, uses a threshold in a quiet and a masking effect. The masking effect is a phenomenon in which a small sound less than a predetermined threshold is masked by a big sound, and this masking between signals existing in an identical time interval is also referred to as frequency masking. At this time, depending on the frequency band, the threshold of the masked sound varies.  
           [0009]    By using the psychoacoustic model, a maximum noise model that is inaudible in each subband of a filter band can be determined. With this noise level in each subband, that is, with the masking threshold, a signal to mask ratio (SMR) value of each subband can be obtained.  
           [0010]    The coding method using the psychoacoustic model is disclosed in the U.S. Pat. No. 6,092,041, “System and method of encoding and decoding a layered bitstream by re-applying psychoacoustic analysis in the decoder” assigned to Motorola, Inc.  
           [0011]    [0011]FIG. 1 is a block diagram showing an ordinary MPEG audio encoding apparatus. Here, among the MPEG audio encoders, the MPEG-1 layer  3  audio encoder, that is, the MP3 audio encoder, will now be explained as an example.  
           [0012]    The MP3 encoder comprises a filter bank  110 , a modified discrete cosine transform (MDCT) unit  120 , a fast Fourier transform (FFT) unit  130 , a psychoacoustic model unit  140 , a quantization and Huffman encoding unit  150 , and a bitstream formatting unit  160 .  
           [0013]    The filter bank  110  divides an input time domain audio signal into 32 frequency domain subbands in order to remove statistical redundancy of the audio signal.  
           [0014]    By using window switching information input from the psychoacoustic model unit  140 , the MDCT unit  120  divides the subbands, which are divided in the filter bank  110 , into finer frequency bands in order to increase frequency resolution. For example, if the window switching information, which is input from the psychoacoustic model unit  140 , indicates a long window, the 32 subbands are divided into finer frequency bands by using 36 point MDCT, and if the window switching information indicates short window, the 32 subbands are divided into finer frequency bands by using 12 point MDCT.  
           [0015]    The FFT unit  130  converts the input audio signal into a frequency domain spectrum and outputs the spectrum to the psychoacoustic model unit  140 .  
           [0016]    In order to remove perceptual redundancy according to the characteristic of human hearing, the psychoacoustic model unit  140  uses the frequency spectrum output from the FFT unit  130  and determines a masking threshold that is a noise level inaudible in each subband, that is, an SMR. The SMR value determined in the psychoacoustic model unit  140  is input to the quantization and Huffman encoding unit  150 .  
           [0017]    In addition, the psychoacoustic model unit  140  calculates a perceptual energy level to determine whether or not to perform window switching, and outputs window switching information to the MDCT unit  120 .  
           [0018]    In order to process the frequency domain data which is input from the MDCT unit  120  after the MDCT is performed, the quantization and Huffman encoding unit  150  performs bit allocation to remove perceptual redundancy and quantization to encode the audio data, based on the SMR value input from the psychoacoustic model unit  140 .  
           [0019]    The bit stream formatting unit  160  formats the encoded audio signal, which is input from the quantization and Huffman encoding unit  150 , into bit streams specified by the MPEG and outputs the bit streams.  
           [0020]    As described above, the prior art psychoacoustic model shown in FIG. 1 uses the FFT spectrum obtained from the input audio signal in order to calculate the masking threshold. However, the filter bank causes aliasing and values obtained from components in which aliasing has occurred are used in the quantization step. In the psychoacoustic model, if an SMR is obtained based on the FFT spectrum and the SMR is used in the quantization step, an optimal result cannot be obtained.  
         SUMMARY OF THE INVENTION  
         [0021]    The present invention provides a digital audio encoding method and apparatus in which a modified psychoacoustic model is used so that the sound quality of an output audio stream can be improved and the amount of computation in the digital audio encoding step can be reduced, when compared to the prior art MPEG audio encoder.  
           [0022]    According to an aspect of the present invention, there is provided a digital audio encoding method comprising determining the type of a window according to the characteristic of an input audio signal; generating a complex modified discrete cosine transform (CMDCT) spectrum from the input audio signal according to the determined window type; generating a fast Fourier transform (FFT) spectrum from the input audio signal, by using the determined window type; and performing a psychoacoustic model analysis, by using the generated CMDCT spectrum and FFT spectrum.  
           [0023]    In the digital audio encoding method, when the determined window type is a long window, a long window is applied to generate a long CMDCT spectrum, a short window is applied to generate an FFT spectrum, and based on the generated long CMDCT spectrum and short FFT spectrum, a psychoacoustic model analysis is performed.  
           [0024]    According to another aspect of the present invention, there is provided a digital audio encoding apparatus comprising: a window switching unit which determines the type of a window according to the characteristic of an input audio signal; a CMDCT unit which generates a CMDCT spectrum from the input audio signal according to the window type determined in the window switching unit; an FFT unit which generates an FFT spectrum from the input audio signal, by using the window type determined in the window switching unit; and a psychoacoustic model unit which performs a psychoacoustic model analysis by using the CMDCT spectrum generated in the CMDCT unit and the FFT spectrum generated in the FFT unit.  
           [0025]    In the apparatus, if the window type determined in the window switching unit is a long window, the CMDCT unit generates a long CMDCT spectrum by applying a long window, the FFT unit generates a short FFT spectrum by applying a short window, and the psychoacoustic model unit performs a psychoacoustic model analysis based on the long CMDCT spectrum generated in the CMDCT unit and the short FFT spectrum generated in the FFT unit.  
           [0026]    According to still another aspect of the present invention, there is provided a digital audio encoding method comprising generating a CMDCT spectrum from an input audio signal; and performing a psychoacoustic model analysis by using the generated CMDCT spectrum.  
           [0027]    The method may further comprise generating a long CMDCT spectrum and a short CMDCT spectrum by performing CMDCT by applying a long window and a short window to an input audio signal.  
           [0028]    In the method, a psychoacoustic model analysis is performed by using the generated long CMDCT spectrum and short CMDCT spectrum.  
           [0029]    In the method, if the determined window type is a long window, quantization and encoding of a long MDCT spectrum are performed based on the result of the psychoacoustic model analysis, and if the determined window type is a short window, quantization and encoding of a short MDCT spectrum are performed based on the result of the psychoacoustic model analysis.  
           [0030]    According to yet still another aspect of the present invention, there is provided a digital audio encoding apparatus comprising a CMDCT unit which generates a CMDCT spectrum from an input audio signal; and a psychoacoustic model unit which performs a psychoacoustic analysis by using the CMDCT spectrum generated in the CMDCT unit.  
           [0031]    In the apparatus, the CMDCT unit generates a long CMDCT spectrum and a short CMDCT spectrum, by performing CMDCT by applying a long window and a short window to the input audio signal.  
           [0032]    In the apparatus, the psychoacoustic model unit performs a psychoacoustic analysis by using the long CMDCT spectrum and short CMDCT spectrum generated in the CMDCT unit.  
           [0033]    The apparatus further comprises a quantization and encoding unit and if the window type determined in the window type determining unit is a long window, the quantization and encoding unit performs quantization and encoding of a long MDCT spectrum, based on the result of the psychoacoustic model analysis and if the window type determined in the window type determining unit is a short window, performs quantization and encoding of a short MDCT spectrum, based on the result of the psychoacoustic model analysis.  
           [0034]    Since the MPEG audio encoder requires a very large amount of computation, it is difficult to apply the MPEG audio encoder to real-time processing. Though it is possible to simplify the encoding algorithm by degrading the sound quality of the output audio, it is very difficult to reduce the amount of computation without degrading the sound quality.  
           [0035]    In addition, the filter bank used in the prior art MPEG audio encoder causes aliasing. Since the values obtained from the components where the aliasing occurred are used in the quantization step, it is preferable that a psychoacoustic model is applied to a spectrum where the aliasing occurred.  
           [0036]    Also, as shown in Equation 2 which will be explained later, an MDCT spectrum provides the values of size and phase in a frequency 2π(k+0.5)/N, k=0, 1, . . . , N/2-1. Accordingly, it is preferable that a spectrum in the frequencies is calculated and a psychoacoustic model is applied.  
           [0037]    Also, CMDCT is applied to the output of the filter bank to calculate the spectrum of an input signal, and a psychoacoustic model is applied according to the spectrum such that the amount of computation needed in the FFT transform can be reduced compared to the prior art MPEG audio encoder, or the FFT transform process can be omitted.  
           [0038]    The present invention is based on the facts described above and an audio encoding method and apparatus according to the present invention can reduce the complexity of an MPEG audio encoding processor without degrading the sound quality of an MPEG audio stream. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:  
         [0040]    [0040]FIG. 1 is a block diagram showing a prior art MPEG audio encoding apparatus;  
         [0041]    [0041]FIG. 2 is a block diagram showing an MPEG audio encoding apparatus according to a preferred embodiment of the present invention;  
         [0042]    [0042]FIG. 3 is a diagram showing a method for detecting a transient signal used in a window switching algorithm according to the present invention;  
         [0043]    [0043]FIG. 4 is a flowchart of the steps performed by a window switching algorithm used in the present invention;  
         [0044]    [0044]FIG. 5 is a diagram showing a method for obtaining an entire spectrum from subband spectra according to the present invention;  
         [0045]    [0045]FIG. 6 is a flowchart of the steps performed by an MPEG audio encoding method according to another preferred embodiment of the present invention;  
         [0046]    [0046]FIG. 7 is a block diagram of an MPEG audio encoding apparatus according to another preferred embodiment of the present invention; and  
         [0047]    [0047]FIG. 8 is a flowchart of the steps performed by an MPEG audio encoding method according to still another preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0048]    Referring to Equations 1 through 4, algorithms used in the present invention will now be explained in detail.  
         [0049]    The filter bank divides an input signal to a resolution of π/32. As described below, it is possible to calculate the spectrum of an input signal by applying CMDCT to the output value of the filter bank. At this time, the transform length is much shorter than a transform length when CMDCT is directly applied to an input signal without using the output value of the filter bank. Using this short transform value for the filter bank output can reduce the amount of computation compared to using a long transform value.  
         [0050]    CMDCT can be obtained by the following Equation 1:  
           {tilde over (X)} ( k )= X   C ( k )+ jX   S ( k )   EQN.(1)  
         [0051]    wherein, k=0, 1, 2, . . . , N/2-1.  
         [0052]    In this case, X C (k) denotes MDCT and X S (k) denotes modified discrete sine transform (MDST). The following derivative Equations 2 through 4 explain the relationships between CMDCT and FFT.  
                       X   C          (   k   )       =       ∑     n   =   0       N   -   1              x        (   n   )          Cos        {     2        π        (     k   +   0.5     )              (     n   +   0.5   +     N   /   4       )     /   N       }                     =       ∑     n   =   0       N   -   1              x        (   n   )          Cos        {       2      π                     n        (     k   +   0.5     )       /   N       +     Φ   k       }                       EQN   .                (   2   )                                 
 
         [0053]    wherein, Φ k =2π(k+0.5)(N/4+0.5)/N, and k=0, 1, . . . , N/2-1.  
         [0054]    Also, MDST can be expressed as the MDCT in the following Equation 3:  
                       X   S          (   k   )       =       ∑     n   =   0       N   -   1              x        (   n   )          Sin        {     2        π        (     k   +   0.5     )              (     n   +   0.5   +     N   /   4       )     /   N       }                     =       ∑     n   =   0       N   -   1              x        (   n   )          Sin        {       2      π                     n        (     k   +   0.5     )       /   N       +     Φ   k       }                       EQN   .                (   3   )                                 
 
         [0055]    wherein, k=0, 1, . . . , N/2-1.  
         [0056]    Also, assuming that {overscore (x)}(k) denotes the complex conjugate of CMDCT, {overscore (x)}(k) can be obtained as the following Equation 4:  
                       x   _          (   k   )       =         X   C          (   k   )       -       jX   S          (   k   )                     =       ∑     n   =   0       N   -   1                 j        {       2      π                     n        (     k   +   0.5     )       /   N       +     Φ   k       }                       =              -   j                     Φ   k                X   ′          (   k   )                       EQN   .                (   4   )                                 
 
         [0057]    wherein,  
             X   ′          (   k   )       =       ∑     n   =   0       N   -   1                 j        {     2      π                     n        (     k   +   0.5     )       /   N       }             ,       and                 k     =   0     ,   1   ,   …              ,       N   /   2     -   1.                           
 
         [0058]    As shown in Equation 4, the complex conjugate of CMDCT is obtained by calculating a spectrum between frequencies of DFT spectrum, that is, frequencies of 27π(k+0.5)/N, k=0, 1, . . . , N/2-1.  
         [0059]    The phase of CMDCT is obtained by shifting the phase of X′(k), and this phase shift does not affect the calculation of an unpredictability measure in a psychoacoustic model of the MPEG-1 layer  3 .  
         [0060]    Considering this, the psychoacoustic model according to the present invention uses a CMDCT spectrum instead of an FFT spectrum, or a long CMDCT spectrum or a short CMDCT spectrum instead of a long FFT spectrum or a short FFT spectrum when a psychoacoustic model is analyzed. Accordingly, the amount of computation needed in FFT transform can be reduced.  
         [0061]    The present invention will now be explained in detail referring to preferred embodiments.  
         [0062]    [0062]FIG. 2 is a block diagram showing an audio encoding apparatus according to a preferred embodiment of the present invention.  
         [0063]    A filter bank  210  divides an input time domain audio signal into a plurality of frequency domain subbands in order to remove the statistical redundancy of the input audio signal. In the present embodiment, the audio signal is divided into 32 subbands each having a bandwidth of π/32. Though a 32 poly-phase filter bank is used in the present embodiment, other filters capable of subband encoding can be used selectively.  
         [0064]    The window switching unit  220  determines a window type to be used in a CMDCT unit  230  and an FFT unit  240 , based on the characteristic of an input audio signal, and inputs the determined window type information to the CMDCT unit  230  and the FFT unit  240 .  
         [0065]    The window type is broken down into a short window and a long window. In the MPEG-1 layer  3 , a long window, a start window, a short window, and a stop window are specified. At this time, the start window or the stop window is used to switch the long window to the short window. Although in the present embodiment, the window types specified in the MPEG-1 are explained as examples, the window switching algorithm can be performed according to other window types selectively. The window switching algorithm according to the present invention will be explained later in detail by referring to FIGS. 3 and 4.  
         [0066]    The CMDCT unit  230  performs CMDCT by applying the long window or short window to the output data of the filter bank  210 , based on the window type information input from the window switching unit  220 .  
         [0067]    The real part of the CMDCT value that is calculated in the CMDCT unit  230 , that is, the MDCT value, is input to a quantization and encoding unit  260 .  
         [0068]    Also, the CMDCT unit  230  calculates a full spectrum by adding calculated subband spectra and sends the calculated full spectrum to the psychoacoustic model unit  250 . The process of obtaining a full spectrum from subband spectra will be explained later referring to FIG. 5.  
         [0069]    Selectively, a LAME algorithm may be used for fast execution of MDCT. In the LAME algorithm, MDCT is optimized by unrolling the Equation 1. By using the symmetry of trigonometric coefficients related to calculation, contiguous multiplications by identical coefficients are replaced by addition operations. For example, the number of multiplications is reduced by replacing 224 multiplications with 324 additions, and for 36 point MDCT, the MDCT time decreases by about 70%. This algorithm can also be applied to the MDST.  
         [0070]    Based on the window type information from the window switching unit  220 , the FFT unit  240  uses a long window or a short window for the input audio signal to perform FFT, and outputs the calculated long FFT spectrum or short FFT spectrum to the psychoacoustic model unit  250 . At this time, if the window type used in the CMDCT unit  230  is a long window, the FFT unit  240  uses a short window. That is, if the output of the CMDCT unit  230  is a long CMDCT spectrum, the output of the FFT unit  240  becomes a short FFT spectrum. Likewise, if the output of the CMDCT unit  230  is a short CMDCT spectrum, the output of the FFT unit  240  becomes a long FFT spectrum.  
         [0071]    The psychoacoustic model unit  250  combines the CMDCT spectrum from the CMDCT unit  230  and the FFT spectrum from the FFT unit  240 , and calculates the unpredictability used in a psychoacoustic model.  
         [0072]    For example, when a long window is used in CMDCT, the long spectrum is calculated by using the resultant values of long MDCT and long MDST, and the short spectrum is calculated by using the FFT. Here, the reason why the CMDCT spectrum calculated in the CMDCT unit  230  is used for the long spectrum is based on the fact that the sizes of FFT and MDCT are similar to each other, which can be shown in the Equations 3 and 4.  
         [0073]    Also, when a short window is used in CMDCT, the short spectrum is calculated by using the resultant values of short MDCT and short MDST, and the long spectrum is calculated by using the FFT.  
         [0074]    Meanwhile, the CMDCT spectrum calculated in the CMDCT unit  230  has the length of 1152 (32 subbands×36 sub-subbands) when the long window is applied, and has the length of 384 (32 subbands×12 sub-subbands) when the short window is applied. On the other hand, the psychoacoustic model unit  250  needs a spectrum having a length of 1024 or 256.  
         [0075]    Accordingly, the CMDCT spectrum is re-sampled from the length of 1152 (or 384) into the length of 1024 (or 256) by linear mapping before the psychoacoustic model analysis is performed.  
         [0076]    Also, the psychoacoustic model unit  250  obtains an SMR value, by using the calculated unpredictability, and outputs the SMR value to the quantization and encoding unit  260 .  
         [0077]    The quantization and encoding unit  260  determines a scale factor, and determines quantization coefficients based on the SMR value calculated in the psychoacoustic model unit  250 . Based on the determined quantization coefficients, the quantization and encoding unit  260  performs quantization, and with the quantized data, performs Huffman encoding.  
         [0078]    A bitstream formatting unit  270  converts the data input from the quantization and encoding unit  260 , into a signal having a predetermined format. If the audio encoding apparatus is an MPEG audio encoding apparatus, the bitstream formatting unit  270  converts the data into a signal having a format specified by the MPEG standards, and outputs the signal.  
         [0079]    [0079]FIG. 3 is a diagram showing a method for detecting a transient signal used in a window switching algorithm based on the output of the filter bank  210  used in the window switching unit  220  of FIG. 2.  
         [0080]    According to the MPEG audio standards specified by the MPEG, an actual window type is determined based on the window type of a current frame and the window-switching flag of the next frame. The psychoacoustic model determines a window switching flag based on perceptual entropy. Accordingly, the psychoacoustic modeling needs to be performed on at least one frame that precedes a frame that is being processed in a filter bank and MDCT unit.  
         [0081]    On the other hand, the psychoacoustic model according to the present invention uses a CMDCT spectrum as described above. Therefore, the window type should be determined before CMDCT is applied. Also, with this reason, a window-switching flag is determined from the output of the filter bank, and the filter bank unit and window switching unit process a frame that precedes one frame before a frame being processed for quantization and psychoacoustic modeling.  
         [0082]    As shown in FIG. 3, the input signal from the filter bank is divided into 3 time bands and 2 frequency bands, that is, 6 bands in total. In FIG. 3, on the horizontal axis, a frame is divided into 36 samples, that is, 3 time bands each having 12 samples. On the vertical axis, a frame is divided into 32 subbands, that is, 2 frequency bands each having 16 subbands. Here, 36 samples and 32 subbands correspond to 1152 sample inputs.  
         [0083]    The parts marked by slanted lines indicate parts used for detecting a transient signal, and for convenience of explanation, the parts marked by slanted lines will be referred to as ( 1 ), ( 2 ), ( 3 ), and ( 4 ) as shown in FIG. 3. Assuming that energies in regions ( 1 ) through ( 4 ) are E 1 , E 2 , E 3 , and E 4 , respectively, energy ratio E 1 /E 2  between regions ( 1 ) and ( 2 ), and energy ratio E 3 /E 4  between regions ( 3 ) and ( 4 ) are transient indicators that indicate whether or not there is a transient signal.  
         [0084]    When a signal is a non-transient signal, the value of the transient indicator is within a predetermined range. Accordingly, if a transient indicator exceeds the predetermined range, the window switching algorithm indicates that a short window is needed.  
         [0085]    [0085]FIG. 4 is a flowchart of the steps performed by a window switching algorithm used in the window switching unit  220  shown in FIG. 2.  
         [0086]    In step  410 , a filter bank output of one frame having 32 subbands, each of which has 36 output samples, is input.  
         [0087]    In step  420 , as shown in FIG. 3, the input signal is divided into 3 time bands, each having 12 sample values, and 2 frequency bands, each having 16 subbands.  
         [0088]    In step  430 , energies E 1 , E 2 , E 3 , and E 4  of bands, which are used to detect a transient signal, are calculated.  
         [0089]    In step  430 , in order to determine whether or not there is transient in the input signal, the calculated energies are compared. That is, E 1 /E 2  and E 3 /E 4  are calculated.  
         [0090]    In step  440 , based on the calculated energy ratios of neighboring bands, it is determined whether or not there is transient in the input signal. When there is transient in the input signal, a window flag to indicate a short window is generated, and when there is no transient, a window switching flag to indicate a long window is generated.  
         [0091]    In step  450 , based on the window switching flag generated in the step  440  and the window used in the previous frame, a window type that is actually applied is determined. The applied window type may be one of ‘short’, ‘long stop’, ‘long start’, and ‘long’ used in the MPEG-1 standards.  
         [0092]    [0092]FIG. 5 is a diagram showing a method for obtaining an entire spectrum from subband spectra according to the present invention.  
         [0093]    Referring to FIG. 5, a method for approximately calculating a signal spectrum from a spectrum calculated from the output of a subband filter bank will now be explained.  
         [0094]    As shown in FIG. 5, an input signal is filtered by analysis filters, H 0 (Z), H 1 (Z), H 2 (Z), . . . , H M-1 (Z), and downsampled. Then, the downsampled signals, y 0 (n), y 1 (n), y 2 (n), . . . , y M-1 (n), are upsampled, filtered by synthesis filters, G 0 (Z), G 1 (Z), G 2 (Z), . . . , G M-1 (Z), and combined in order to reconstruct a signal.  
         [0095]    This process corresponds to the process in the frequency domain in which spectra of all bands are added. Accordingly, if these filters are idealistic, the result will be the same as a spectrum obtained by adding Y m (k) for each band, and, as a result, an input FFT spectrum can be obtained. Also, if these filters approximate an idealistic filter, an approximate spectrum can be obtained, which a psychoacoustic model according to the present invention uses.  
         [0096]    As the results of experiments, even when filters used are not ideal band-pass filters, if the filters are a filter bank used in the MPEG-1 layer  3 , the spectrum obtained by the method described above was similar to the actual spectrum.  
         [0097]    Thus, the spectrum of an input signal can be obtained by adding CMDCT spectra in all bands. While the spectrum obtained by using CMDCT is 1152 points, the spectrum needed in the psychoacoustic model is 1024 points. Accordingly, the CMDCT spectrum is re-sampled by using simple linear mapping, and then can be used in the psychoacoustic model.  
         [0098]    [0098]FIG. 6 is a flowchart of the steps performed by an MPEG audio encoding method according to another preferred embodiment of the present invention.  
         [0099]    In step  610 , an audio signal is input to the filter bank, and the input time domain audio signal is divided into frequency domain subbands in order to remove the statistical redundancy of the input audio signal.  
         [0100]    In step  620 , based on the characteristic of the input audio signal, the window type is determined. If the input signal is a transient signal, step  630  is performed, and if the input signal is not a transient signal, step  640  is performed.  
         [0101]    In step  630 , by applying a short window to the audio data processed in the step  610 , short CMDCT is performed, and at the same time, by applying a long window, long FFT is performed. As a result, a short CMDCT spectrum and a long FFT spectrum are obtained.  
         [0102]    In step  640 , by applying a long window to the audio data processed in the step  610 , long CMDCT is performed, and at the same time, by applying a short window, short FFT is performed. As a result, a long CMDCT spectrum and a short FFT spectrum are obtained.  
         [0103]    In step  650 , if the window type determined in the step  620  is a short window, by using the short CMDCT spectrum and long FFT spectrum obtained in the step  630 , unpredictability used in the psychoacoustic model is calculated.  
         [0104]    If the window type determined in the step  620  is a long window, by using the long CMDCT spectrum and short FFT spectrum obtained in the step  640 , unpredictability is calculated. Also, based on the calculated unpredictability, the SMR value is calculated.  
         [0105]    In step  660 , quantization of the audio data obtained in the step  610  is performed according to the SMR value calculated in the step  650 , and Huffman encoding of the quantized data is performed.  
         [0106]    In step  670 , the data encoded in the step  660  is converted into a signal having a predetermined format and then the signal is output. If the audio encoding method is an MPEG audio encoding method, the data is converted into a signal having a format specified by the MPEG standards.  
         [0107]    [0107]FIG. 7 is a block diagram explaining an audio encoding apparatus according to another preferred embodiment of the present invention.  
         [0108]    The audio encoding apparatus shown in FIG. 7 comprises a filter bank unit  710 , a window switching unit  720 , a CMDCT unit  730 , a psychoacoustic model unit  740  a quantization and encoding unit  750 , and a bitstream formatting unit  760 .  
         [0109]    Here, for simplification of explanation, explanations of the filter bank unit  710 , the quantization and encoding unit  750 , and the bitstream formatting unit  760  will be omitted because these units perform functions similar to that of the filter bank unit  210 , the quantization and encoding unit  260 , and the bitstream formatting unit  270 , respectively, of FIG. 2.  
         [0110]    The window switching unit  720 , based on the characteristic of the input audio signal, determines the type of a window to be used in the CMDCT unit  730 , and sends the determined window type information to the CMDCT unit  730 .  
         [0111]    The CMDCT unit  730  calculates a long CMDCT spectrum and a short CMDCT spectrum together. In the present embodiment, the long CMDCT spectrum used in the psychoacoustic model unit  740  is obtained by performing 36 point CMDCT, adding all the results, and then re-sampling the spectrum having a length of 1152 into a spectrum having a length of 1024. Also, the short CMDCT spectrum used in the psychoacoustic model unit  740  is obtained by performing 12 point CMDCT, adding all the results, and then re-sampling the resulting spectrum having a length of 384 into a spectrum having a length of 256.  
         [0112]    The CMDCT unit  730  outputs the calculated long CMDCT spectrum and short CMDCT spectrum to the psychoacoustic model unit  740 . Also, if the window type input from the window switching unit  720  is a long window, the CMDCT unit  730  inputs the long MDCT spectrum to the quantization and encoding unit  750 , and if the input window type is a short window, inputs the short MDCT spectrum to the quantization and encoding unit  750 .  
         [0113]    The psychoacoustic model unit  740  calculates unpredictability according to the long spectrum and short spectrum sent from the CMDCT unit  730  and, based on the calculated unpredictability, calculates the SMR value. The calculated SMR value is sent to the quantization and encoding unit  750 .  
         [0114]    The quantization and encoding unit  750 , based on the long MDCT spectrum and short MDCT spectrum sent from the CMDCT unit  730  and the SMR information input from the psychoacoustic model unit  740  determines scale factors and quantization coefficients. Based on the determined quantization coefficients, quantization is performed and Huffman encoding of the quantized data is performed.  
         [0115]    The bitstream formatting unit  760  converts the data input from the quantization and encoding unit  750  into a signal having a predetermined format and outputs the signal. If the audio encoding apparatus is an MPEG audio encoding apparatus, the data is converted into a signal having a format specified by the MPEG standards and output.  
         [0116]    [0116]FIG. 8 is a flowchart of the steps performed by an MPEG audio encoding method according to still another preferred embodiment of the present invention.  
         [0117]    In step  810 , the filter bank receives an audio signal, and in order to remove the statistical redundancy of the input audio signal, the input time domain audio signal is divided into frequency domain subbands.  
         [0118]    In step  820 , based on the characteristic of the input audio signal, the window type is determined.  
         [0119]    In step  830 , by applying a short window to the audio data processed in the step  810 , short CMDCT is performed, and at the same time, by applying a long window, long FFT is performed. As a result, a short CMDCT spectrum and a long FFT spectrum are obtained.  
         [0120]    In step  840 , by using the short CMDCT spectrum and long CMDCT spectrum obtained in the step  830 , unpredictability to be used in the psychoacoustic model is calculated. Also, based on the calculated unpredictability, the SMR value is calculated.  
         [0121]    In step  850 , if the window type determined in the step  820  is a long window, the long MDCT value in the spectrum obtained in the step  830  is input, quantization of the long MDCT value is performed according to the SMR value calculated in the step  840 , and Huffman encoding of the quantized data is performed.  
         [0122]    In step  860 , the data encoded in the step  850  is converted into a signal having a predetermined format and then the signal is output. If the audio encoding method is an MPEG audio encoding method, the data is converted into a signal having a format specified by the MPEG standards.  
         [0123]    The present invention is not limited to the preferred embodiment described above, and it is apparent that variations and modifications by those skilled in the art can be effected within the spirit and scope of the present invention. In particular, in addition to the MPEG-1 layer  3 , the present invention can be applied to all audio encoding apparatuses and methods that use MDCT and the psychoacoustic model, such as MPEG-2 advanced audio coding (AAC), MPEG-4, and windows media audio (WMA).  
         [0124]    The present invention may be embodied in a code, which can be read by a computer, on a computer readable recording medium. The computer readable recording medium includes all kinds of recording apparatuses on which computer readable data are stored.  
         [0125]    The computer readable recording media includes storage media such as magnetic storage media (e.g., ROM&#39;s, floppy disks, hard disks, etc.), optically readable media (e.g., CD-ROMs, DVDs, etc.) and carrier waves (e.g., transmissions over the Internet). Also, the computer readable recording media can be scattered on computer systems connected through a network and can store and execute a computer readable code in a distributed mode.  
         [0126]    As described above, by applying the advanced psychoacoustic model according to the present invention, the CMDCT spectrum is used instead of the FFT spectrum such that the amount of computation needed in FFT transform and the complexity of an MPEG audio encoder can be decreased without degrading the sound quality of an output audio stream compared to the input audio signal.