Patent Publication Number: US-9847095-B2

Title: Method and apparatus for adaptively encoding and decoding high frequency band

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Ser. No. 13/686,015 filed Nov. 27, 2012, which is a continuation application of prior application Ser. No. 13/220,193, filed on Aug. 29, 2011, now U.S. Pat. No. 8,340,962, which is a continuation application of Ser. No. 11/766,331 filed Jun. 21, 2007, now U.S. Pat. No. 8,010,352, which claim the benefit of Korean Patent Application No. 10-2006-0056070, filed on Jun. 21, 2006 and Korean Patent Application No 10-2007-0060688, filed on Jun. 20, 2007, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in the entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for encoding and decoding an audio signal such as a speech signal or a music signal, and more particularly, to a method and apparatus for encoding and decoding a high frequency signal by using a signal or a spectrum of a low frequency band. 
     2. Description of the Related Art 
     In general, signals of high frequency bands are regarded as less important sound to be recognized by humans in comparison with low frequency signal. Accordingly, when an audio signal is coded, if coding efficiency has to be improved due to a restriction of available bits, a signal of a low frequency band is coded by allocating a great number of bits, while a high frequency signal is coded by allocating a small number of bits. 
     Thus, when the high frequency signal is coded, a method and apparatus for maximizing the quality of sound to be recognized by humans by using the small number of bits are demanded. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for adaptively encoding or decoding a high frequency signal above a preset frequency band in the time domain or in the temporal domain by using a signal of a low frequency band below the preset frequency band. 
     According to an aspect of the present invention, there is provided an apparatus for adaptively encoding a high frequency band, the apparatus including a domain conversion unit which converts a high frequency signal of the high frequency band above a preset frequency band to the time domain or to the frequency domain by frequency bands; a time domain encoding unit which encodes a frequency band converted to the time domain by using an excitation signal of a low frequency band below the preset frequency band; and a frequency domain encoding unit which encodes a frequency band converted to the frequency domain by using an excitation spectrum of the low frequency band. 
     According to another aspect of the present invention, there is provided an apparatus for adaptively encoding a high frequency band, the apparatus including a noise information encoding unit which selects a frequency band to be used to encode a high frequency spectrum of the high frequency band above a preset frequency band from an excitation spectrum of a low frequency band below the preset frequency band, and encodes information on the selected frequency band; and an envelope information encoding unit which extracts an envelope of the high frequency spectrum and encodes the envelope. 
     According to another aspect of the present invention, there is provided an apparatus for adaptively encoding a high frequency band, the apparatus including a domain selection unit which selects an encoding domain of a high frequency signal of the high frequency band above a preset frequency band from the time domain and the frequency domain; a time domain encoding unit which encodes the high frequency signal by using an excitation signal of a low frequency band below the preset frequency band, if the domain selection unit selects the time domain; and a frequency domain encoding unit which converts the high frequency signal to the frequency domain, generates a high frequency spectrum, and encodes the high frequency spectrum by using the excitation signal of the low frequency band, if the domain selection unit selects the frequency domain. 
     According to another aspect of the present invention, there is provided an apparatus for adaptively decoding a high frequency band, the apparatus including a domain determination unit which determines an encoding domain of each frequency band of the high frequency band above a preset frequency band; a time domain decoding unit which decodes a frequency band determined as having been encoded in the time domain by using an excitation signal of a low frequency band below the preset frequency band; and a frequency domain decoding unit which decodes a frequency band determined as having been encoded in the frequency domain by using an excitation spectrum of the low frequency band. 
     According to another aspect of the present invention, there is provided an apparatus for adaptively decoding a high frequency band, the apparatus including a noise generation unit which generates noise of the high frequency band above a preset frequency band by using information on a frequency band to be used to decode the high frequency band from an excitation spectrum of a low frequency band below the preset frequency band; and an envelope control unit which decodes an envelope of a high frequency spectrum of the high frequency band and controls an envelope of the noise. 
     According to another aspect of the present invention, there is provided an apparatus for adaptively decoding a high frequency band, the apparatus including a domain determination unit which determines an encoding domain of the high frequency band above a preset frequency band; a time domain decoding unit which decodes a high frequency signal of the high frequency band by using an excitation signal of a low frequency band below the preset frequency band, if the domain determination unit determines that the high frequency band has been encoded in the time domain; and a frequency domain decoding unit which decodes a high frequency spectrum of the high frequency band by using an excitation spectrum of the low frequency band, if the domain determination unit determines that the high frequency band has been encoded in the frequency domain. 
     According to another aspect of the present invention, there is provided a method of adaptively encoding a high frequency band, the method including converting a high frequency signal of the high frequency band above a preset frequency band to the time domain or to the frequency domain by frequency bands; encoding a frequency band converted to the time domain by using an excitation signal of a low frequency band below the preset frequency band; and encoding a frequency band converted to the frequency domain by using an excitation spectrum of the low frequency band. 
     According to another aspect of the present invention, there is provided a method of adaptively encoding a high frequency band, the method including selecting a frequency band to be used to encode a high frequency spectrum of the high frequency band above a preset frequency band from an excitation spectrum of a low frequency band below the preset frequency band, and encoding information on the selected frequency band; and extracting an envelope of the high frequency spectrum and encoding the envelope. 
     According to another aspect of the present invention, there is provided a method of adaptively encoding a high frequency band, the method including selecting an encoding domain of a high frequency signal of the high frequency band above a preset frequency band from the time domain and the frequency domain; encoding the high frequency signal by using an excitation signal of a low frequency band below the preset frequency band, if the domain selection unit selects the time domain; and converting the high frequency signal to the frequency domain, generates a high frequency spectrum, and encoding the high frequency spectrum by using the excitation signal of the low frequency band, if the domain selection unit selects the frequency domain. 
     According to another aspect of the present invention, there is provided a method of adaptively decoding a high frequency band, the method including determining an encoding domain of each frequency band of the high frequency band above a preset frequency band; decoding a frequency band determined as having been encoded in the time domain by using an excitation signal of a low frequency band below the preset frequency band; and decoding a frequency band determined as having been encoded in the frequency domain by using an excitation spectrum of the low frequency band. 
     According to another aspect of the present invention, there is provided a method of adaptively decoding a high frequency band, the method including generating noise of the high frequency band above a preset frequency band by using information on a frequency band to be used to decode the high frequency band from an excitation spectrum of a low frequency band below the preset frequency band; and decoding an envelope of a high frequency spectrum of the high frequency band and controlling an envelope of the noise. 
     According to another aspect of the present invention, there is provided a method of adaptively decoding a high frequency band, the method including determining an encoding domain of the high frequency band above a preset frequency band; decoding a high frequency signal of the high frequency band by using an excitation signal of a low frequency band below the preset frequency band, if the domain determination unit determines that the high frequency band has been encoded in the time domain; and decoding a high frequency spectrum of the high frequency band by using an excitation spectrum of the low frequency band, if the domain determination unit determines that the high frequency band has been encoded in the frequency domain. 
     According to another aspect of the present invention, there is provided a computer readable recording medium having recorded thereon a computer program for executing a method of adaptively encoding a high frequency band, the method including converting a high frequency signal of the high frequency band above a preset frequency band to the time domain or to the frequency domain by frequency bands; encoding a frequency band converted to the time domain by using an excitation signal of a low frequency band below the preset frequency band; and encoding a frequency band converted to the frequency domain by using an excitation spectrum of the low frequency band. 
     According to another aspect of the present invention, there is provided a computer readable recording medium having recorded thereon a computer program for executing a method of adaptively encoding a high frequency band, the method including selecting a frequency band to be used to encode a high frequency spectrum of the high frequency band above a preset frequency band from an excitation spectrum of a low frequency band below the preset frequency band, and encoding information on the selected frequency band; and extracting an envelope of the high frequency spectrum and encoding the envelope. 
     According to another aspect of the present invention, there is provided a computer readable recording medium having recorded thereon a computer program for executing a method of adaptively encoding a high frequency band, the method including selecting an encoding domain of a high frequency signal of the high frequency band above a preset frequency band from the time domain and the frequency domain; encoding the high frequency signal by using an excitation signal of a low frequency band below the preset frequency band, if the domain selection unit selects the time domain; and converting the high frequency signal to the frequency domain, generates a high frequency spectrum, and encoding the high frequency spectrum by using the excitation signal of the low frequency band, if the domain selection unit selects the frequency domain. 
     According to another aspect of the present invention, there is provided a computer readable recording medium having recorded thereon a computer program for executing a method of adaptively decoding a high frequency band, the method including determining an encoding domain of each frequency band of the high frequency band above a preset frequency band, decoding a frequency band determined as having been encoded in the time domain by using an excitation signal of a low frequency band below the preset frequency band, and decoding a frequency band determined as having been encoded in the frequency domain by using an excitation spectrum of the low frequency band. 
     According to another aspect of the present invention, there is provided a computer readable recording medium having recorded thereon a computer program for executing a method of adaptively decoding a high frequency band, the method including generating noise of the high frequency band above a preset frequency band by using information on a frequency band to be used to decode the high frequency band from an excitation spectrum of a low frequency band below the preset frequency band; and decoding an envelope of a high frequency spectrum of the high frequency band and controlling an envelope of the noise. 
     According to another aspect of the present invention, there is provided a computer readable recording medium having recorded thereon a computer program for executing a method of adaptively decoding a high frequency band, the method including determining an encoding domain of the high frequency band above a preset frequency band; decoding a high frequency signal of the high frequency band by using an excitation signal of a low frequency band below the preset frequency band, if the domain determination unit determines that the high frequency band has been encoded in the time domain; and decoding a high frequency spectrum of the high frequency band by using an excitation spectrum of the low frequency band, if the domain determination unit determines that the high frequency band has been encoded in the frequency domain. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1A  is a block diagram of an apparatus for adaptively encoding a high frequency band, according to an embodiment of the present invention; 
         FIG. 1B  is a block diagram of a high frequency band encoding unit  160  included in the apparatus illustrated in  FIG. 1A , according to an embodiment of the present invention; 
         FIG. 2A  is a block diagram of an apparatus for adaptively encoding a high frequency band, according to another embodiment of the present invention; 
         FIG. 2B  is a block diagram of a high frequency band encoding unit  250  included in the apparatus illustrated in  FIG. 2A , according to an embodiment of the present invention; 
         FIG. 3A  is a block diagram of an apparatus for adaptively encoding a high frequency band, according to another embodiment of the present invention; 
         FIG. 3B  is a block diagram of a high frequency band encoding unit  360  included in the apparatus illustrated in  FIG. 3A , according to an embodiment of the present invention; 
         FIG. 4A  is a block diagram of an apparatus for adaptively decoding a high frequency band, according to an embodiment of the present invention; 
         FIG. 4B  is a block diagram of a high frequency band decoding unit  440  included in the apparatus illustrated in  FIG. 4A , according to an embodiment of the present invention; 
         FIG. 5A  is a block diagram of an apparatus for adaptively decoding a high frequency band, according to another embodiment of the present invention; 
         FIG. 5B  is a block diagram of a high frequency band decoding unit  525  included in the apparatus illustrated in  FIG. 5A , according to an embodiment of the present invention; 
         FIG. 6A  is a block diagram of an apparatus for adaptively decoding a high frequency band, according to another embodiment of the present invention; 
         FIG. 6B  is a block diagram of a high frequency band decoding unit  635  included in the apparatus illustrated in  FIG. 6A , according to an embodiment of the present invention; 
         FIG. 7A  is a graph of an envelope restored by linear predictive coding (LPC) coefficients, according to an embodiment of the present invention; 
         FIG. 7B  is a graph of a result obtained by multiplying an excitation signal by an envelope restored by a low frequency signal and LPC coefficients, according to an embodiment of the present invention; 
         FIG. 7C  is a graph of a result obtained by compensating for a mismatch between a low frequency signal and a high frequency signal, according to an embodiment of the present invention; 
         FIG. 8A  is a graph of an excitation spectrum of a low frequency band, according to an embodiment of the present invention; 
         FIG. 8B  is a graph of an excitation spectrum of a low frequency band when the excitation spectrum is patched to a high frequency band, according to an embodiment of the present invention; 
         FIG. 8C  is a graph of a controlled envelope of a high frequency spectrum, according to an embodiment of the present invention; 
         FIG. 9A  is a flowchart of a method of adaptively encoding a high frequency band, according to an embodiment of the present invention; 
         FIG. 9B  is a flowchart of operation  960  included in the method of  FIG. 9A , according to an embodiment of the present invention; 
         FIG. 10A  is a flowchart of a method of adaptively encoding a high frequency band, according to another embodiment of the present invention; 
         FIG. 10B  is a flowchart of operation  1050  included in the method of  FIG. 10A , according to an embodiment of the present invention; 
         FIG. 11A  is a flowchart of a method of adaptively encoding a high frequency band, according to another embodiment of the present invention; 
         FIG. 11B  is a flowchart of operation  1160  included in the method of  FIG. 11A , according to an embodiment of the present invention; 
         FIG. 12A  is a flowchart of a method of adaptively decoding a high frequency band, according to an embodiment of the present invention; 
         FIG. 12B  is a flowchart of operation  1240  included in the method of  FIG. 12A , according to an embodiment of the present invention; 
         FIG. 13A  is a flowchart of a method of adaptively decoding a high frequency band, according to another embodiment of the present invention; 
         FIG. 13B  is a flowchart of operation  1325  included in the method of  FIG. 13A , according to an embodiment of the present invention; 
         FIG. 14A  is a flowchart of a method of adaptively decoding a high frequency band, according to another embodiment of the present invention; and 
         FIG. 14B  is a flowchart of operation  1435  included in the method of  FIG. 14A , according to an embodiment of the present invention; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be described in detail by explaining embodiments of the invention with reference to the attached drawings. 
       FIG. 1A  is a block diagram of an apparatus for adaptively encoding a high frequency band, according to an embodiment of the present invention. 
     Referring to  FIG. 1A , the apparatus includes a first conversion unit  100 , a domain selection unit  105 , a linear prediction unit  110 , a long term prediction unit  115 , an excitation signal encoding unit  120 , a second conversion unit  125 , a quantization unit  130 , an inverse quantization unit  135 , a second inverse conversion unit  140 , a storage unit  145 , an excitation signal decoding unit  150 , an excitation spectrum generation unit  155 , a high frequency band encoding unit  160 , and a multiplexing unit  165 . 
     The first conversion unit  100  converts a signal input through an input terminal IN into a signal of the time domain by frequency bands. The first conversion unit  100  may convert the signal by using a quadrature mirror filterbank (QMF) method or a lapped orthogonal transformation (LOT) method. 
     However, the first conversion unit  100  may convert the signal into a signal of the time domain and a signal of the frequency domain signal by using, for example, a frequency varying-modulated lapped transformation (FV-MLT) method. In this case, the apparatus may not include the second conversion unit  125  so that the first conversion unit  100  may converts the signal into a signal of a domain selected by the domain selection unit  105 . 
     The domain selection unit  105  determines whether to encode each signal of a low frequency band below a preset frequency band from the signal of a frequency band converted by the first conversion unit  100  in the time domain or in the frequency domain in accordance with a preset standard. Also, the domain selection unit  105  encodes information on an encoding domain of each frequency band and outputs the information to the multiplexing unit  165 . 
     Here, the preset standard may be a gain of linear predictive coding (LPC), spectral variations between linear prediction filters of neighboring frames, a pitch delay gain, a long term prediction gain, etc. 
     The linear prediction unit  110  extracts and encodes LPC coefficients by performing an LPC analysis on a signal of a frequency band determined to be encoded in the time domain by the domain selection unit  105 , and extracts a first excitation signal by removing short term correlations from a signal of a frequency band determined to be encoded in the time domain. 
     The long term prediction unit  115  extracts a second excitation signal by performing long term prediction on the first excitation signal extracted by the linear prediction unit  110 . Also, the long term prediction unit  115  encodes the result obtained by performing the long term prediction and output the result to the multiplexing unit  165 . 
     The long term prediction unit  115  may perform the long term prediction, for example, by measuring continuity of periodicity, frequency spectral tilt, or frame energies. Here, the continuity of periodicity may be a degree of continuity of frames which have low variations of pitch lags and high pitch correlations over a certain section. Also, the continuity of periodicity may be a degree of continuity of frames which have very low first formant frequencies and high pitch correlations over a certain section. 
     The excitation signal encoding unit  120  encodes the second excitation signal extracted by the long term prediction unit  115 . 
     The second conversion unit  125  generates a spectrum by converting a signal of a frequency band determined to be encoded in the frequency domain by the domain selection unit  105  from the time domain to the frequency domain. 
     The quantization unit  130  quantizes the spectrum generated by the second conversion unit  125 . The spectrum quantized by the quantization unit  130  is output to the multiplexing unit  165 . 
     The inverse quantization unit  135  inverse quantizes the spectrum quantized by the quantization unit  130 . 
     The second inverse conversion unit  140  performs inverse operation of the conversion performed by the second conversion unit  125  by inverse converting the spectrum inverse quantized by the inverse quantization unit  135  from the frequency domain to the time domain. 
     The storage unit  145  stores the signal inverse converted by the second inverse conversion unit  140 . The storage unit  145  stores the inverse converted signal in order to use the inverse converted signal when the long term prediction unit  115  performs the long term prediction on a signal of a frequency band to be encoded in the time domain from a next frame. 
     The excitation signal decoding unit  150  decodes the second excitation signal encoded by the excitation signal encoding unit  120 . 
     The excitation spectrum generation unit  155  generates an excitation spectrum by whitening the spectrum inverse quantized by the inverse quantization unit  135 . 
     The high frequency band encoding unit  160  adaptively encodes a signal of a high frequency band above the preset frequency band in the time domain or in the frequency domain by using a signal of a low frequency band below the preset frequency band. If the high frequency band encoding unit  160  encodes the signal in the time domain, the second excitation signal decoded by the excitation signal decoding unit  150  is used, and if the high frequency band encoding unit  160  encodes the signal in the frequency domain, the excitation spectrum generated by the excitation spectrum generation unit  155  is used. 
     The multiplexing unit  165  generates a bitstream by multiplexing the information on the encoding domain of each frequency band, the information encoded by the domain selection unit  105 , the LPC coefficients encoded by the linear prediction unit  110 , the result of the long term prediction performed by the long term prediction unit  115 , the second excitation signal encoded by the excitation signal encoding unit  120 , the spectrum quantized by the quantization unit  130 , the result encoded by the high frequency band encoding unit  160 , etc. The bitstream is output through an output terminal OUT. 
       FIG. 1B  is a block diagram of the high frequency band encoding unit  160  included in the apparatus illustrated in  FIG. 1A , according to an embodiment of the present invention. 
       FIG. 7A  is a graph of an envelope restored by LPC coefficients, according to an embodiment of the present invention. 
       FIG. 7B  is a graph of a result obtained by multiplying an excitation signal by an envelope restored by a low frequency signal and LPC coefficients, according to an embodiment of the present invention. 
       FIG. 7C  is a graph of a result obtained by compensating for a mismatch between a low frequency signal and a high frequency signal, according to an embodiment of the present invention. 
     Referring to  FIG. 1B , the high frequency band encoding unit  160  includes a domain selection unit  170 , a linear prediction unit  175 , a multiplier  180 , a gain encoding unit  185 , a noise information encoding unit  190 , and an envelope information encoding unit  195 . 
     The domain selection unit  170  determines whether to encode a signal of a high frequency band above a preset frequency band in the time domain or in the frequency domain. 
     The domain selection unit  170  may determine whether to encode the high frequency band in the time domain or in the frequency domain in accordance with whether a low frequency band below the preset frequency band, which is used when the high frequency band is encoded, is encoded in the time domain or in the frequency domain. If a low frequency band, which is used when the high frequency band is encoded, is encoded in the time domain, the high frequency band is determined to be encoded in the time domain, and if the low frequency band, which is used when the high frequency band is encoded, is encoded in the frequency domain, the high frequency band is determined to be encoded in the frequency domain. 
     The linear prediction unit  175  extracts LPC coefficients by performing an LPC analysis on the frequency band determined to be encoded in the time domain by the domain selection unit  170 . The LPC coefficients extracted by the linear prediction unit  175  are encoded and output to the multiplexing unit  165  illustrated in  FIG. 1A  through a first output terminal OUT  1 , and are used to restore an envelope as illustrated in  FIG. 7A  by a decoder. 
     The multiplier  180  multiplies the second excitation signal which is decoded by the excitation signal decoding unit  150  illustrated in  FIG. 1A , and is input through a first input terminal IN  1  by an envelope generated by the LPC coefficients extracted by the linear prediction unit  175 . An example of the signal multiplied by the multiplier  180  may be a signal  710  illustrated in  FIG. 7B . 
     The gain encoding unit  185  calculates a gain which compensates for a mismatch between the signal multiplied by the multiplier  180  and a low frequency signal of a low frequency band below the preset frequency band, and encodes the gain. By the gain calculated by the gain encoding unit  185 , the mismatch between a low frequency signal  720  and the multiplied signal  710  which are illustrated in  FIG. 7B  may be compensated for as illustrated in  FIG. 7C  by the decoder. Also, the gain encoded by the gain encoding unit  185  is output to the multiplexing unit  165  illustrated in  FIG. 1A  through a second output terminal OUT  2 . 
     The noise information encoding unit  190  selects a frequency band of the excitation spectrum generated by the excitation spectrum generation unit  155 , which is to be used to generate noise of the frequency band determined to be encoded in the frequency domain by the domain selection unit  170 , and encodes information on the selected frequency band. The information encoded by the noise information encoding unit  190  is output to the multiplexing unit  165  illustrated in  FIG. 1A  through a third output terminal OUT  3 . 
     The envelope information encoding unit  195  extracts envelope information of a spectrum of the frequency band determined to be encoded in the frequency domain by the domain selection unit  170  from a high frequency band above the preset frequency band, and encodes the envelope information. The envelope information encoded by the envelope information encoding unit  195  is output to the multiplexing unit  165  illustrated in  FIG. 1A  through a fourth output terminal OUT  4 . 
     The present invention is not limited to an open-loop method in which an encoding domain is firstly selected and then encoding is performed in accordance with the selected domain as described above with reference to  FIGS. 1A and 1B . Alternatively, a close-loop method in which encoding is performed both in the time domain and in the frequency domain and then more appropriate domain is selected later by comparing encoding results may be used. 
       FIG. 2A  is a block diagram of an apparatus for adaptively encoding a high frequency band, according to another embodiment of the present invention. 
     Referring to  FIG. 2A , the apparatus includes a frequency band division unit  200 , a linear prediction unit  205 , a conversion unit  210 , a quantization unit  215 , an inverse quantization unit  220 , an inverse conversion unit  225 , a storage unit  230 , a signal analyzation unit  235 , a long term prediction unit  240 , a switching unit  245 , a high frequency band encoding unit  250 , and a multiplexing unit  255 . 
     The frequency band division unit  200  divides a signal input through an input terminal IN into a low frequency signal of a low frequency band below a preset frequency band and a high frequency signal of a high frequency band above the preset frequency band. 
     The linear prediction unit  205  extracts LPC coefficients by performing an LPC analysis on the low frequency signal divided by the frequency band division unit  200 , and extracts a first excitation signal by removing short term correlations from the low frequency signal. Also, the linear prediction unit  205  encodes the LPC coefficients and outputs the encoded LPC coefficients to the multiplexing unit  255 . 
     The conversion unit  210  generates an excitation spectrum by converting the first excitation signal extracted by the linear prediction unit  205  from the time domain to the frequency domain. 
     The quantization unit  215  quantizes the excitation spectrum generated by the conversion unit  210 . The excitation spectrum quantized by the quantization unit  215  is output to the multiplexing unit  255 . 
     The inverse quantization unit  220  inverse quantizes the excitation spectrum quantized by the quantization unit  215 . 
     The inverse conversion unit  225  performs inverse operation of the conversion performed by the conversion unit  210  by inverse converting the excitation spectrum inverse quantized by the inverse quantization unit  220  from the frequency domain to the time domain, thereby generating a second excitation signal. 
     The storage unit  230  stores the second excitation signal inverse converted by the inverse conversion unit  225 . The storage unit  230  stores the second excitation signal in order to use the second excitation signal when the long term prediction unit  240  performs long term prediction on a signal of a frequency band to be encoded in the time domain from a next frame. 
     The signal analyzation unit  235  analyzes the first excitation signal extracted by the linear prediction unit  205  and determines whether to perform long term prediction by the long term prediction unit  240  or not in accordance with characteristics of the low frequency signal. Here, the characteristics of the low frequency signal may be an LPC gain, spectral variations between linear prediction filters of neighboring frames, a pitch delay gain, a long term prediction gain, etc. 
     If the signal analyzation unit  235  determines to perform the long term prediction on the first excitation signal, the long term prediction unit  240  extracts a third excitation signal by performing the long term prediction on the first excitation signal extracted by the linear prediction unit  205 . The long term prediction unit  240  may perform the long term prediction, for example, by measuring continuity of periodicity, a frequency spectral tilt, or a frame energy. Here, the continuity of periodicity may be a degree of continuity of frames which have low variations of pitch lags and high pitch correlations over a certain section. Also, the continuity of periodicity may be a degree of continuity of frames which have very low first formant frequencies and high pitch correlations over a certain section. 
     The switching unit  245  switches the third excitation signal extracted by the long term prediction unit  240  in accordance with the determination of the signal analyzation unit  235 . 
     The high frequency band encoding unit  250  encodes the high frequency signal in the frequency domain by using the excitation spectrum of the low frequency band below the preset frequency band, which is inverse quantized by the inverse quantization unit  220 . 
     The multiplexing unit  255  generates a bitstream by multiplexing the LPC coefficients encoded by the linear prediction unit  205 , the excitation spectrum quantized by the quantization unit  215 , the result of the long term prediction performed by the long term prediction unit  240 , the result encoded by the high frequency band encoding unit  250 , etc. The bitstream is output through an output terminal OUT. 
       FIG. 2B  is a block diagram of the high frequency band encoding unit  250  included in the apparatus illustrated in  FIG. 2A , according to an embodiment of the present invention. 
     Referring to  FIG. 2B , the high frequency band encoding unit  250  includes a noise information encoding unit  260  and an envelope information encoding unit  265 . 
     The noise information encoding unit  260  encodes information on a frequency band to be used to encode a high frequency spectrum of a high frequency band above a preset frequency band from an excitation spectrum which is inverse quantized by the inverse quantization unit  220  illustrated in  FIG. 2A , and are input through a first input terminal IN  1 . The information encoded by the noise information encoding unit  260  is output to the multiplexing unit  255  illustrated in  FIG. 2A  through a first output terminal OUT  1 . 
     The envelope information encoding unit  265  receives a high frequency spectrum through a second input terminal IN  2 , extracts an envelope of the high frequency spectrum, and encodes information on the extracted envelope. The envelope information may be energy values calculated by frequency bands. The envelope information encoding unit  265  output the envelope information to the multiplexing unit  255  illustrated in  FIG. 2A  through a second output terminal OUT  2 . 
       FIG. 3A  is a block diagram of an apparatus for adaptively encoding a high frequency band, according to another embodiment of the present invention. 
     Referring to  FIG. 3A , the apparatus includes a frequency band division unit  300 , a linear prediction unit  305 , a domain selection unit  310 , a long term prediction unit  315 , an excitation signal encoding unit  320 , a conversion unit  325 , a quantization unit  330 , an inverse quantization unit  335 , an inverse conversion unit  340 , a storage unit  345 , an excitation signal decoding unit  350 , a high frequency band encoding unit  360 , and a multiplexing unit  365 . 
     The frequency band division unit  300  divides a signal input through an input terminal IN into a low frequency signal of a low frequency band below a preset frequency band and a high frequency signal of a high frequency band above the preset frequency band. 
     The linear prediction unit  305  extracts LPC coefficients by performing an LPC analysis on the low frequency signal divided by the frequency band division unit  300 , and extracts a first excitation signal by removing short term correlations from the low frequency signal. The LPC coefficients extracted by the linear prediction unit  305  are encoded and output to the multiplexing unit  365 . 
     The domain selection unit  310  determines whether to encode the first excitation signal extracted by the linear prediction unit  305  in the time domain or in the frequency domain in accordance with a preset standard. Here, the preset standard may be an LPC gain, spectral variations between linear prediction filters of neighboring frames, a pitch delay gain, a long term prediction gain, etc. 
     If the domain selection unit  310  determines to encode the first excitation signal in the time domain, the long term prediction unit  315  performs the long term prediction on the first excitation signal extracted by the linear prediction unit  305  and extracts a second excitation signal. 
     The long term prediction unit  315  may perform the long term prediction, for example, by measuring continuity of periodicity, frequency spectral tilt, or frame energies. Here, the continuity of periodicity may be a degree of continuity of frames which have low variations of pitch lags and high pitch correlations over a certain section. Also, the continuity of periodicity may be a degree of continuity of frames which have very low first formant frequencies and high pitch correlations over a certain section. 
     The excitation signal encoding unit  320  encodes the second excitation signal extracted by the long term prediction unit  315 . 
     If the domain selection unit  310  determines to encode the first excitation signal in the frequency domain, the conversion unit  325  generates a spectrum by converting the first excitation signal extracted by the linear prediction unit  305  from the time domain to the frequency domain. 
     The quantization unit  330  quantizes the excitation spectrum generated by the conversion unit  325 . The excitation spectrum quantized by the quantization unit  330  is output to the multiplexing unit  365 . 
     The inverse quantization unit  335  inverse quantizes the excitation spectrum quantized by the quantization unit  330 . 
     The inverse conversion unit  340  performs inverse operation of the conversion performed by the conversion unit  325  by inverse converting the excitation spectrum inverse quantized by the inverse quantization unit  335  from the frequency domain to the time domain. 
     The storage unit  345  stores the third excitation signal inverse converted by the inverse conversion unit  340 . The storage unit  345  stores the third excitation signal in order to use the third excitation signal when the long term prediction unit  315  performs the long term prediction on a signal of a frequency band to be encoded in the time domain from a next frame. 
     The excitation signal decoding unit  350  decodes the second excitation signal encoded by the excitation signal encoding unit  320 . 
     The high frequency band encoding unit  360  adaptively encodes a high frequency signal of a high frequency band above the preset frequency band in the time domain or in the frequency domain by using a signal or spectrum of the low frequency band below the preset frequency band. If the high frequency band encoding unit  360  encodes the high frequency signal in the time domain, the second excitation signal decoded by the excitation signal decoding unit  350  is used, and if the high frequency band encoding unit  360  encodes the high frequency signal in the frequency domain, the excitation spectrum inverse quantized by the inverse quantization unit  335  is used. 
     The multiplexing unit  365  generates a bitstream by multiplexing the LPC coefficients extracted by the linear prediction unit  305 , the result of the long term prediction performed by the long term prediction unit  315 , the information on the encoding domain of the low frequency signal selected by the domain selection unit  305 , the second excitation signal encoded by the excitation signal encoding unit  320 , the excitation spectrum quantized by the quantization unit  330 , the result encoded by the high frequency band encoding unit  360 , etc. The bitstream is output through an output terminal OUT. 
       FIG. 3B  is a block diagram of the high frequency band encoding unit  360  included in the apparatus illustrated in  FIG. 3A , according to an embodiment of the present invention. 
     Referring to  FIG. 3B , the high frequency band encoding unit  360  includes a domain selection unit  370 , a linear prediction unit  375 , a multiplier  380 , a gain encoding unit  385 , a noise information encoding unit  390 , and an envelope information encoding unit  395 . 
     The domain selection unit  370  determines whether to encode a high frequency signal of a high frequency band above a preset frequency band in the time domain or in the frequency domain in accordance with an encoding domain of a low frequency signal of a low frequency band below the preset frequency band, the low frequency signal input through a first input terminal IN  1 , the encoding domain selected by the domain selection unit  310  illustrated in  FIG. 3A . If the low frequency signal is determined to be encoded in the frequency domain by the domain selection unit  310  illustrated in  FIG. 3A , the domain selection unit  370  determines to encode the high frequency signal in the frequency domain, and if the low frequency signal is determined to be encoded in the time domain by the domain selection unit  310  illustrated in FIG.  3 A, the domain selection unit  370  determines to encode the high frequency signal in the time domain. 
     If the high frequency signal is determined to be encoded in the time domain by the domain selection unit  370 , the linear prediction unit  375  extracts LPC coefficients by performing an LPC analysis on the high frequency signal input through a second input terminal IN  2 . The LPC coefficients extracted by the linear prediction unit  375  are encoded and output to the multiplexing unit  365  illustrated in  FIG. 3A  through a first output terminal OUT  1 , and are used to restore an envelope as illustrated in  FIG. 7A  by a decoder. 
     The multiplier  380  multiplies the second excitation signal which is decoded by the excitation signal decoding unit  350  illustrated in  FIG. 3A , and is input through a third input terminal IN  3  by an envelope of the high frequency signal generated by the LPC coefficients extracted by the linear prediction unit  375 . An example of the signal multiplied by the multiplier  380  may be the signal  710  illustrated in  FIG. 7B . 
     The gain encoding unit  385  calculates a gain which compensates for a mismatch between the signal multiplied by the multiplier  380  and a low frequency signal, and encodes the gain. The mismatch existing at the boundary between the low frequency signal  720  and the multiplied signal  710  which are illustrated in  FIG. 7B  is compensated for as illustrated in  FIG. 7C . Also, the gain encoded by the gain encoding unit  385  is output to the multiplexing unit  365  illustrated in  FIG. 3A  through a second output terminal OUT  2 . 
     The noise information encoding unit  390  selects a frequency band to be used to decode a high frequency spectrum from the excitation spectrum inverse quantized by the inverse quantization unit  335  illustrated in  FIG. 3A  by the decoder, and encodes information on the selected frequency band. The information encoded by the noise information encoding unit  390  is output through a third output terminal OUT  3 . 
     The envelope information encoding unit  395  extracts envelope information of the high frequency spectrum, and encodes the envelope information. The envelope information may be energy values calculated by frequency bands. The envelope information encoded by the envelope information encoding unit  395  is output to the multiplexing unit  365  illustrated in  FIG. 3A  through a fourth output terminal OUT  4 . 
     The present invention is not limited to an open-loop method in which an encoding domain is firstly selected and then encoding is performed in accordance with the selected domain as described above with reference to  FIGS. 3A and 3B . Alternatively, a close-loop method in which encoding is performed both in the time domain and in the frequency domain and then more appropriate domain is selected later by comparing encoding results may be used. 
       FIG. 4A  is a block diagram of an apparatus for adaptively decoding a high frequency band, according to an embodiment of the present invention. 
     Referring to  FIG. 4A , the apparatus includes an inverse multiplexing unit  400 , a domain determination unit  405 , an excitation signal decoding unit  410 , a long term combination unit  415 , a linear combination unit  420 , an inverse quantization unit  430 , a second inverse conversion unit  433 , an excitation spectrum generation unit  435 , a high frequency band decoding unit  440 , and a first inverse conversion unit  445 . 
     The inverse multiplexing unit  400  inverse multiplexes a bitstream input from an encoder through an input terminal IN. The inverse multiplexing unit  400  inverse multiplexes information on an encoding domain of a frequency band encoded by the encoder, LPC coefficients encoded by the encoder, a result of long term prediction performed by the encoder, an excitation signal encoded by the encoder, a spectrum quantized by the encoder, information required for decoding a high frequency signal by using a low frequency signal or a low frequency spectrum, etc. 
     The domain determination unit  405  receives the information on the encoding domain of a low frequency band below a preset frequency band, which is encoded by the encoder, and determines the encoding domain of each frequency band. 
     The excitation signal decoding unit  410  receives the excitation signal of a frequency band determined as having been encoded in the time domain by the domain determination unit  405 , the excitation signal encoded by the encoder, from the inverse multiplexing unit  400  and decodes the excitation signal. 
     The long term combination unit  415  receives the result of the long term prediction performed by the encoder on the frequency band determined as having been encoded in the time domain by the domain determination unit  405  from the inverse multiplexing unit  400 , decodes the result, and combines the excitation signal decoded by the excitation signal decoding unit  410  and the result of the long term prediction. 
     The linear combination unit  420  receives the LPC coefficients of the frequency band determined as having been encoded in the time domain by the domain determination unit  405  from the inverse multiplexing unit  400 , decodes the LPC coefficients, and combines the LPC coefficients and the signal combined by the long term combination unit  415 . 
     The inverse quantization unit  430  receives the spectrum of the frequency band determined as having been encoded in the frequency domain by the domain determination unit  405  from the inverse multiplexing unit  400 , and inverse quantizes the spectrum. 
     The second inverse conversion unit  433  performs inverse operation of the conversion performed by the second conversion unit  125  illustrated in  FIG. 1A  by inverse converting the spectrum inverse quantized by the inverse quantization unit  430  from the frequency domain to the time domain. 
     The excitation spectrum generation unit  435  generates an excitation spectrum by whitening the spectrum inverse quantized by the inverse quantization unit  430 . 
     The high frequency band decoding unit  440  decodes a high frequency signal of a high frequency band above the preset frequency band by using the excitation signal decoded by the excitation signal decoding unit  410  or the excitation spectrum generated by the excitation spectrum generation unit  435 . 
     The first inverse conversion unit  445  performs inverse operation of the conversion performed by the first conversion unit  100  illustrated in  FIG. 1A . The first inverse conversion unit  445  performs inverse conversion by combining the signal combined by the linear combination unit  420  or the spectrum inverse converted by the second inverse conversion unit  433  and the high frequency signal decoded by the high frequency band decoding unit  440  into a time domain signal, and outputs the combined time domain signal through an output terminal OUT. The first inverse conversion unit  445  may perform the inverse conversion by using a QMF method or an LOT method. 
     However, the first inverse conversion unit  445  may combine a time domain signal and a frequency domain signal by frequency bands into a time domain signal by using, for example, a FV-MLT method. In this case, the high frequency band decoding unit  440  may not include an additional inverse conversion unit in order to convert a frequency domain signal into a time domain signal. 
       FIG. 4B  is a block diagram of the high frequency band decoding unit  440  included in the apparatus illustrated in  FIG. 4A , according to an embodiment of the present invention. 
       FIG. 8A  is a graph of an excitation spectrum of a low frequency band, according to an embodiment of the present invention. 
       FIG. 8B  is a graph of an excitation spectrum of a low frequency band when the excitation spectrum is patched to a high frequency band, according to an embodiment of the present invention. 
       FIG. 8C  is a graph of a controlled envelope of a high frequency spectrum, according to an embodiment of the present invention. 
     Referring of  FIG. 4B , the high frequency band decoding unit  440  includes a domain determination unit  450 , a linear combination unit  455 , a multiplier  460 , a gain application unit  465 , a noise information decoding unit  470 , an envelope control unit  475 , and an inverse conversion unit  480 . 
     The domain determination unit  450  determines whether a signal of a high frequency band above a preset frequency band has been encoded in the time domain or in the frequency domain. An encoding domain of each frequency band may be determined by using information on an encoding domain, which is transmitted from an encoder and is received through the inverse multiplexing unit  400  illustrated in  FIG. 4A  or by using information on a decoded domain of a low frequency band below the preset frequency band, which is used when the high frequency band is decoded and is received from the domain determination unit  405  illustrated in  FIG. 4A . 
     The linear combination unit  455  receives LPC coefficients of a frequency band determined as having been encoded in the time domain from the inverse multiplexing unit  400  through a first input terminal IN  1 , and decodes the LPC coefficients. By the LPC coefficients decoded by the linear combination unit  455 , an envelope may be restored as illustrated in  FIG. 7A . 
     The multiplier  460  multiplies the excitation signal which is decoded by the excitation signal decoding unit  410  illustrated in  FIG. 4A , and are input through a second input terminal IN  2  by an envelope generated by the LPC coefficients decoded by the linear combination unit  455 . An example of the signal multiplied by the multiplier  460  may be the signal  710  illustrated in  FIG. 7B . 
     The gain application unit  465  decodes the gain received through a third input terminal IN  3  and applies the gain to the signal multiplied by the multiplier  460 . By applying the gain, a mismatch between a decoded low frequency signal and a decoded high frequency signal may be compensated for. For example, the high frequency signal multiplied by the multiplier  460  has the mismatch at the boundary to the low frequency signal as illustrated in  FIG. 7B . However, when the gain application unit  465  applies the gain, the mismatch does not exist between the low frequency signal and the high frequency signal as illustrated in  FIG. 7C . The signal to which the gain is applied to by the gain application unit  465  is output to the first inverse conversion unit  445  illustrated in  FIG. 4A  through a first output terminal OUT  1 . 
     The noise information decoding unit  470  receives information on a frequency band to be used to decode a high frequency spectrum from the excitation spectrum generated by the excitation spectrum generation unit  435  illustrated in  FIG. 4A  from the inverse multiplexing unit  400  illustrated in  FIG. 4A  through a fourth input terminal IN  4 , and decodes the information. The noise information decoding unit  470  generates noise by patching or symmetrically folding the excitation spectrum of the corresponding frequency band to the frequency band determined to be encoded in the frequency domain by the domain determination unit  450 . For example, an excitation spectrum illustrated in  FIG. 8A  is patched to the high frequency band as illustrated in  FIG. 8B . 
     The envelope control unit  475  receives envelope information of a high frequency spectrum encoded by the encoder from the inverse multiplexing unit  400  illustrated in  FIG. 4A  through a fifth input terminal IN  5 , and decodes the envelope information. An envelope of the noise generated by the noise information decoding unit  470  is controlled by using the envelope information of the high frequency spectrum decoded by the envelope control unit  475 . For example, the envelope control unit  475  controls the noise generated by the noise information decoding unit  470  as illustrated in  FIG. 8B  into an envelope illustrated in  FIG. 8C  by using the envelope information of the high frequency spectrum. 
     The inverse conversion unit  480  performs inverse operation of the conversion performed by the second conversion unit  125  illustrated in  FIG. 1A  by inverse converting the noise of which envelope is controlled by the envelope control unit  475  from the frequency domain to the time domain, thereby generating a high frequency signal. 
       FIG. 5A  is a block diagram of an apparatus for adaptively decoding a high frequency band, according to another embodiment of the present invention. 
     Referring to  FIG. 5A , the apparatus includes an inverse multiplexing unit  500 , an inverse quantization unit  505 , an inverse conversion unit  510 , a long term combination unit  515 , a linear combination unit  520 , a high frequency band decoding unit  525 , and a frequency band combination unit  530 . 
     The inverse multiplexing unit  500  inverse multiplexes a bitstream input from an encoder through an input terminal IN. The inverse multiplexing unit  500  inverse multiplexes LPC coefficients encoded by the encoder, an excitation spectrum encoded by the encoder, a result of long term prediction performed by the encoder, information required for decoding a high frequency signal of a high frequency band above a preset frequency band by using an excitation spectrum of a low frequency band below the preset frequency band, etc. 
     The inverse quantization unit  505  receives the low frequency excitation spectrum quantized by the encoder from the inverse multiplexing unit  500  and inverse quantizes the low frequency excitation spectrum. 
     The inverse conversion unit  510  performs inverse operation of the conversion performed by the conversion unit  210  illustrated in  FIG. 2A  by inverse converting the excitation spectrum inverse quantized by the inverse quantization unit  505  from the frequency domain to the time domain, thereby generating an excitation signal. 
     The long term combination unit  515  receives the result of the long term prediction performed by the encoder on the low frequency excitation signal from the inverse multiplexing unit  500 , decodes the result, and selectively combines the excitation signal generated by the inverse conversion unit  510  and the result of the long term prediction. 
     The linear combination unit  520  receives the LPC coefficients from the inverse multiplexing unit  500 , and decodes the LPC coefficients. After the LPC coefficients are decoded, if the long term combination unit  515  did not combine the result of the long term prediction, the linear combination unit  520  combines the excitation signal generated by the inverse conversion unit  510  and the LPC coefficients, and if the long term combination unit  515  combined the result of the long term prediction, the linear combination unit  520  combines the signal combined by the long term combination unit  515  and the LPC coefficients. The signal combined by the linear combination unit  520  is a restored low frequency signal of a low frequency band. 
     The high frequency band decoding unit  525  decodes a high frequency signal by using the excitation spectrum of the low frequency signal inverse quantized by the inverse quantization unit  505 . 
     The frequency band combination unit  530  combines the low frequency signal restored by the linear combination unit  520  and the high frequency signal decoded by the high frequency band decoding unit  525 , and outputs the combined signal through an output terminal OUT. 
       FIG. 5B  is a block diagram of a high frequency band decoding unit  525  included in the apparatus illustrated in  FIG. 5A , according to an embodiment of the present invention. 
     Referring of  FIG. 5B , the high frequency band decoding unit  525  includes a noise information decoding unit  535 , an envelope control unit  540 , an inverse conversion unit  545 . 
     The noise information decoding unit  535  receives information on a frequency band to be used to decode a high frequency spectrum from an excitation spectrum of a low frequency band below a preset frequency band from the inverse multiplexing unit  500  illustrated in  FIG. 5A  through a first input terminal IN  1 , and decodes the information. The noise information decoding unit  535  selects an excitation spectrum to be used from excitation spectrums inverse quantized by the inverse quantization unit  505  through a first′ input terminal IN  1 ′ in accordance with the decoded information, and generates noise by patching or symmetrically folding the corresponding excitation spectrum to a high frequency band above the preset frequency band. For example, the excitation spectrum illustrated in  FIG. 8A  is patched to the high frequency band as illustrated in  FIG. 8B . 
     The envelope control unit  540  receives envelope information of a high frequency spectrum encoded by the encoder from the inverse multiplexing unit  500  illustrated in  FIG. 5A  through a second input terminal IN  2 , and decodes the envelope information. The envelope control unit  540  controls an envelope of the noise generated by the noise information decoding unit  535  by using the envelope information of the high frequency spectrum. For example, the envelope control unit  540  controls the noise generated by the noise information decoding unit  535  as illustrated in  FIG. 8B  into an envelope illustrated in  FIG. 8C  by using the envelope information of the high frequency spectrum. 
     The inverse conversion unit  545  performs inverse operation of the conversion performed by the conversion unit  210  illustrated in  FIG. 2A  by inverse converting the noise of which envelope is controlled by the envelope control unit  540  from the frequency domain to the time domain, thereby generating a high frequency signal. The high frequency signal generated by the inverse conversion unit  545  is output to the frequency band combination unit  530  illustrated in  FIG. 5A  through a first output terminal OUT  1 . 
       FIG. 6A  is a block diagram of an apparatus for adaptively decoding a high frequency band, according to another embodiment of the present invention. 
     Referring to  FIG. 6A , the apparatus includes an inverse multiplexing unit  600 , a domain determination unit  605 , an excitation signal decoding unit  610 , a long term combination unit  615 , an inverse quantization unit  620 , an inverse conversion unit  625 , a linear combination unit  630 , a high frequency band decoding unit  635 , and a frequency band combination unit  640 . 
     The inverse multiplexing unit  600  inverse multiplexes a bitstream input from an encoder through an input terminal IN. The inverse multiplexing unit  600  inverse multiplexes information on an encoding domain of a low frequency signal selected by the encoder, LPC coefficients encoded by the encoder, a result of long term prediction performed by the encoder, an excitation spectrum quantized by the encoder, information required for decoding a high frequency signal by using a low frequency signal or a low frequency spectrum of a low frequency band below a preset frequency band, etc. 
     The domain determination unit  605  receives the information on the encoding domain of the low frequency band encoded by the encoder from the inverse multiplexing unit  600 , decodes the information on the encoding domain, and determines whether the low frequency band has been encoded in the time domain or in the frequency domain. 
     If the domain determination unit  605  determines that the low frequency band has been encoded in the time domain, the excitation signal decoding unit  610  receives an excitation signal of the low frequency band encoded by the encoder from the inverse multiplexing unit  600  and decodes the excitation signal. 
     The long term combination unit  615  receives the result of the long term prediction performed by the encoder on the low frequency band signal from the inverse multiplexing unit  600 , decodes the result, and combines the excitation signal decoded by the excitation signal decoding unit  610  and the result of the long term prediction. 
     If the domain determination unit  605  determines that the low frequency band has been encoded in the frequency domain, the inverse quantization unit  620  receives an excitation spectrum quantized by the encoder from the inverse multiplexing unit  600 , and inverse quantizes the excitation spectrum. 
     The inverse conversion unit  625  performs inverse operation of the conversion performed by the conversion unit  325  illustrated in  FIG. 3A  by inverse converting the excitation spectrum inverse quantized by the inverse quantization unit  620  from the frequency domain to the time domain, thereby generating an excitation signal. 
     The linear combination unit  630  receives the LPC coefficients of the low frequency signal from the inverse multiplexing unit  600 , decodes the LPC coefficients, and combines the decoded LPC coefficients and the excitation signal combined by the long term combination unit  615  or the excitation signal generated by the inverse conversion unit  625 . The signal combined by the linear combination unit  630  is a restored low frequency signal of a low frequency band. 
     The excitation spectrum generation unit  635  decodes the high frequency signal by using the excitation spectrum inverse quantized by the inverse quantization unit  620  or the excitation signal decoded by the excitation signal decoding unit  610 . If the low frequency band has been encoded in the time domain, the high frequency band decoding unit  635  decodes the high frequency signal by using the excitation spectrum inverse quantized by the inverse quantization unit  620 , and if the low frequency band has been encoded in the frequency domain, the high frequency band decoding unit  635  decodes the high frequency signal by using the excitation spectrum decoded by the excitation signal decoding unit  610 . 
     The frequency band combination unit  640  combines the low frequency signal restored by the linear combination unit  630  and the high frequency signal decoded by the high frequency band decoding unit  525 , and outputs the combined signal through a first output terminal OUT. 
       FIG. 6B  is a block diagram of a high frequency band decoding unit  635  included in the apparatus illustrated in  FIG. 6A , according to an embodiment of the present invention. 
     Referring of  FIG. 6B , the high frequency band decoding unit  635  includes a domain determination unit  645 , a linear combination unit  650 , a multiplier  655 , a gain application unit  660 , a noise information decoding unit  665 , an envelope control unit  670 , and an inverse conversion unit  675 . 
     The domain determination unit  645  determines whether to decode a high frequency band above a preset frequency band in the time domain or in the frequency domain by determining an encoding domain of a low frequency band below the preset frequency band. 
     If the domain determination unit  645  determines to decode the high frequency band in the time domain, the linear combination unit  650  receives LPC coefficients of a high frequency signal from the inverse multiplexing unit  600  illustrated in  FIG. 6A  through a first input terminal IN  1 , and decodes the LPC coefficients. By the LPC coefficients decoded by the linear combination unit  650 , an envelope may be restored as illustrated in  FIG. 7A . 
     The multiplier  655  multiplies the excitation signal which is decoded by the excitation signal decoding unit  610  illustrated in  FIG. 6A  and are input through a second input terminal IN  2  by the envelope generated by the LPC coefficients decoded by the linear combination unit  650 . An example of the signal multiplied by the multiplier  655  may be the signal  710  illustrated in  FIG. 7B . 
     The gain application unit  660  decodes a gain received through a third input terminal IN  3  from the inverse multiplexing unit  600  illustrated in  FIG. 6A , decodes the gain, and applies the gain to the signal multiplied by the multiplier  655 . By applying the gain, a mismatch between a low frequency signal and a high frequency signal, which are restored by the linear combination unit  630  illustrated in  FIG. 6A , may be compensated for. For example, the high frequency signal multiplied by the multiplier  655  has the mismatch at the boundary to the low frequency signal as illustrated in  FIG. 7B . However, when the gain application unit  660  applies the gain, the mismatch does not exist between the low frequency signal and the high frequency signal as illustrated in  FIG. 7C . The signal to which the gain is applied to by the gain application unit  660  is output to the frequency band combination unit  640  illustrated in  FIG. 6A  through a first output terminal OUT  1 . 
     If the domain determination unit  645  determines to decode the high frequency band in the frequency domain, the noise information decoding unit  665  receives an excitation spectrum inverse quantized by the inverse quantization unit  620  illustrated in  FIG. 6A  through a fourth input terminal IN  4 , and generates a spectrum by patching or symmetrically folding the excitation spectrum to the high frequency band. For example, the excitation spectrum illustrated in  FIG. 8A  is patched to the high frequency band as illustrated in  FIG. 8B . 
     The envelope control unit  670  receives envelope information of a high frequency spectrum encoded by the encoder from the inverse multiplexing unit  600  illustrated in  FIG. 6A  through a fifth input terminal IN  5 , and decodes the envelope information. The envelope control unit  670  controls an envelope of the noise generated by the noise information decoding unit  665  by using the decoded envelope information of the high frequency spectrum. For example, the envelope control unit  670  controls the noise generated by the noise information decoding unit  665  as illustrated in  FIG. 8B  into the envelope illustrated in  FIG. 8C  by using the envelope information of the high frequency spectrum. 
     The inverse conversion unit  675  performs inverse operation of the conversion performed by the conversion unit  325  illustrated in  FIG. 3A  by inverse converting the noise of which envelope is controlled by the envelope control unit  670  from the frequency domain to the time domain, thereby generating a high frequency signal. 
       FIG. 9A  is a flowchart of a method of adaptively encoding a high frequency band, according to an embodiment of the present invention. 
     In operation  900 , an input signal is converted into a signal of the time domain by frequency bands. The conversion of operation  900  may be performed by using a QMF method or an LOT method. 
     However, the input signal may be converted into a signal of the time domain and a signal of the frequency domain signal by using, for example, a FV-MLT method in operation  900 . In this case, operation  925  may not be performed and the conversion may be performed in operation  900  in a domain selected in operation  905 . 
     In operation  905 , whether to encode each signal of a low frequency band below a preset frequency band in the time domain or in the frequency domain is determined from the signal converted in operation  900  in accordance with a preset standard. Here, the preset standard may be an LPC gain, spectral variations between linear prediction filters of neighboring frames, a pitch delay gain, a long term prediction gain, etc. 
     In operation  910 , LPC coefficients are extracted and encoded by performing an LPC analysis on a signal of a frequency band determined to be encoded in the time domain in operation  905 , and a first excitation signal is extracted by removing short term correlations from a signal of a frequency band determined to be encoded in the time domain in operation  905 . 
     In operation  915 , long term prediction is performed on the extracted first excitation signal and a second excitation signal is extracted. 
     The long term prediction of operation  915  may be performed by measuring continuity of periodicity, frequency spectral tilt, or frame energies. Here, the continuity of periodicity may be a degree of continuity of frames which have low variations of pitch lags and high pitch correlations over a certain section. Here, the continuity of periodicity may be a degree of continuity of frames which have very low first formant frequencies and high pitch correlations over a certain section. 
     In operation  920 , the second excitation signal extracted in operation  915  is encoded. 
     In operation  925 , a spectrum is generated by converting a signal of a frequency band determined to be encoded in the frequency domain from the time domain to the frequency domain. 
     In operation  930 , the spectrum generated in operation  925  is quantized. 
     In operation  935 , the spectrum quantized in operation  930  is inverse quantized. 
     In operation  940 , inverse operation of the conversion of operation  925  is performed by inverse converting the spectrum inverse quantized in operation  935  from the frequency domain to the time domain. 
     In operation  945 , the signal inverse converted in operation  940  is stored; The inverse converted signal is stored in order to use the inverse converted signal when the long term prediction is performed in operation  915  on a signal of a frequency band to be encoded in the time domain from a next frame. 
     In operation  950 , the second excitation signal encoded in operation  920  is decoded. 
     In operation  955 , an excitation spectrum is generated by whitening the spectrum inverse quantized in operation  935 . 
     In operation  960 , a signal of a high frequency band above the preset frequency band is adaptively encoded in the time domain or in the frequency domain by using a signal of a low frequency band below the preset frequency band. If the signal is encoded in the time domain, the second excitation signal decoded in operation  950  is used, and if the signal is encoded in the frequency domain, the excitation spectrum generated in operation  955  is used. 
     In operation  965 , a bitstream is generated by multiplexing the information on the encoding domain of each frequency band which is encoded in operation  905 , the LPC coefficients encoded in operation  910 , the result of the long term prediction performed in operation  915 , the second excitation signal encoded in operation  920 , the spectrum quantized in operation  930 , and the result encoded in operation  960 . 
       FIG. 9B  is a flowchart of operation  960  included in the method of  FIG. 9A , according to an embodiment of the present invention. 
     In operation  970 , whether to encode a signal of a high frequency band above a preset frequency band in the time domain or in the frequency domain is determined. 
     The determination of operation  970  may be performed in accordance with whether a low frequency band below the preset frequency band, which is used when the high frequency band is encoded, is encoded in the time domain or in the frequency domain. If a low frequency band, which is used when the high frequency band is encoded, is encoded in the time domain, the high frequency band is determined to be encoded in the time domain, and if the low frequency band, which is used when the high frequency band is encoded, is encoded in the frequency domain, the high frequency band is determined to be encoded in the frequency domain. 
     In operation  975 , LPC coefficients are extracted by performing an LPC analysis on the frequency band determined to be encoded in the time domain in operation  970 . The LPC coefficients extracted in operation  975  are used to restore an envelope as illustrated in  FIG. 7A  by a decoder. 
     In operation  980 , the second excitation signal decoded in operation  950  of  FIG. 9A  is multiplied by an envelope generated by the LPC coefficients extracted in operation  975 . An example of the signal multiplied in operation  980  may be a signal  710  illustrated in  FIG. 7B . 
     In operation  985 , a gain which compensates for a mismatch between the signal multiplied in operation  980  and a low frequency signal of a low frequency band below the preset frequency band is calculated and encoded. By the gain calculated in operation  985 , the mismatch between a low frequency signal  720  and the multiplied signal  710  which are illustrated in  FIG. 7B  may be compensated for as illustrated in  FIG. 7C  by the decoder. 
     In operation  990 , a frequency band of the excitation spectrum generated in operation  955 , which is to be used to generate noise of the frequency band determined to be encoded in the frequency domain in operation  970  is selected and information on the selected frequency band is encoded. 
     In operation  995 , envelope information of a spectrum of the frequency band determined to be encoded in the frequency domain in operation  970  from a high frequency band above the preset frequency band is extracted and encoded. 
     The present invention is not limited to an open-loop method in which an encoding domain is firstly selected and then encoding is performed in accordance with the selected domain as described above with reference to  FIGS. 9A and 9B . Alternatively, a close-loop method in which encoding is performed both in the time domain and in the frequency domain and then more appropriate domain is selected later by comparing encoding results may be used. 
       FIG. 10A  is a flowchart of a method of adaptively encoding a high frequency band, according to another embodiment of the present invention. 
     In operation  1000 , an input signal is divided into a low frequency signal of a low frequency band below a preset frequency band and a high frequency signal of a high frequency band above the preset frequency band. 
     In operation  1005 , LPC coefficients are extracted by performing an LPC analysis on the low frequency signal divided in operation  1000 , and a first excitation signal is extracted by removing short term correlations from the low frequency signal divided in operation  1000 . 
     In operation  1010 , an excitation spectrum is generated by converting the first excitation signal extracted in operation  1005  from the time domain to the frequency domain. 
     In operation  1015 , the excitation spectrum generated in operation  1010  is quantized. 
     In operation  1020 , the excitation spectrum quantized in operation  1015  is inverse quantized. 
     In operation  1025 , inverse operation of the conversion performed in operation  1010  is performed by inverse converting the excitation spectrum inverse quantized in operation  1020  from the frequency domain to the time domain, thereby generating a second excitation signal. 
     In operation  1030 , the second excitation signal inverse converted in operation  1025  is stored. The second excitation signal is stored in order to use the second excitation signal when long term prediction is performed in operation  1040  on a signal of a frequency band to be encoded in the time domain from a next frame. 
     In operation  1035 , the first excitation signal extracted in operation  1005  is analyzed and whether to perform the long tem prediction in operation  1040  or not is determined in accordance with characteristics of the low frequency signal. Here, the characteristics of the low frequency signal may be an LPC gain, spectral variations between linear prediction filters of neighboring frames, a pitch delay gain, a long term prediction gain, etc. 
     In operation  1040 , if the long term prediction is determined to be performed in operation  1035 , a third excitation signal is extracted by performing the long term prediction on the first excitation signal extracted in operation  1005 . 
     The long term prediction of operation  1040  may be performed by measuring continuity of periodicity, frequency spectral tilt, or frame energies. Here, the continuity of periodicity may be a degree of continuity of frames which have low variations of pitch lags and high pitch correlations over a certain section. Here, the continuity of periodicity may be a degree of continuity of frames which have very low first formant frequencies and high pitch correlations over a certain section. 
     In operation  1050 , the high frequency signal is encoded in the frequency domain by using the excitation spectrum of the low frequency band below the preset frequency band, which is inverse quantized in operation  1020 . 
     In operation  1055 , a bitstream is generated by multiplexing the LPC coefficients encoded in operation  1005 , the excitation spectrum quantized in operation  1015 , the result of the long term prediction performed in operation  1040 , and the result encoded in operation  1050 . 
       FIG. 10B  is a flowchart of operation  1050  included in the method of  FIG. 10A , according to an embodiment of the present invention. 
     In operation  1060 , information on a frequency band to be used to encode a high frequency spectrum of a high frequency band above a preset frequency band from an excitation spectrum which is inverse quantized in operation  1020  of  FIG. 10A  is encoded. The information encoded by the noise information encoding unit  1060  is output to the multiplexing unit  1055  illustrated in  FIG. 10A  through a first output terminal OUT  1 . 
     In operation  1065 , a high frequency spectrum is received, and an envelope of the high frequency spectrum is extracted, and information on the extracted envelope is encoded. The envelope information may be energy values calculated by frequency bands. 
       FIG. 11A  is a flowchart of a method of adaptively encoding a high frequency band, according to another embodiment of the present invention. 
     In operation  1100 , an input signal is divided into a low frequency signal of a low frequency band below a preset frequency band and a high frequency signal of a high frequency band above the preset frequency band. 
     In operation  1105 , LPC coefficients is extracted by performing an LPC analysis on the low frequency signal divided in operation  1100 , and a first excitation signal is extracted by removing short term correlations from the low frequency signal. 
     In operation  1110 , whether to encode the first excitation signal extracted in operation  1105  in the time domain or in the frequency domain is determined in accordance with a preset standard. Here, the preset standard may be an LPC gain, spectral variations between linear prediction filters of neighboring frames, a pitch delay gain, a long term prediction gain, etc. 
     In operation  1115 , if the first excitation signal is determined to be encoded in the time domain in operation  1110 , the long term prediction is performed on the first excitation signal extracted in operation  1105  and a second excitation signal is extracted. 
     The long term prediction of operation  1115  may be performed by measuring continuity of periodicity, frequency spectral tilt, or frame energies. Here, the continuity of periodicity may be a degree of continuity of frames which have low variations of pitch lags and high pitch correlations over a certain section. Here, the continuity of periodicity may be a degree of continuity of frames which have very low first formant frequencies and high pitch correlations over a certain section. 
     In operation  1120 , the second excitation signal extracted in operation  1115  is encoded. 
     In operation  1125 , if the first excitation signal is determined to be encoded in the time domain in operation  1110 , a spectrum is generated by converting the first excitation signal extracted in operation  1105  from the time domain to the frequency domain. 
     In operation  1130 , the excitation spectrum generated in operation  1125  is quantized. 
     In operation  1135 , the excitation spectrum quantized in operation  1130  is inverse quantized. 
     In operation  1140 , inverse operation of the conversion performed in operation  1125  is performed by inverse converting the excitation spectrum inverse quantized in operation  1135  from the frequency domain to the time domain. 
     In operation  1145 , the third excitation signal inverse converted in operation  1140  is stored. The third excitation signal is stored in order to use the third excitation signal when the long term prediction is performed in operation  1115  on a signal of a frequency band to be encoded in the time domain from a next frame. 
     In operation  1150 , the second excitation signal encoded in operation  1120  is decoded. 
     In operation  1160 , a high frequency signal of a high frequency band above the preset frequency band is adaptively encoded in the time domain or in the frequency domain by using a signal or spectrum of the low frequency band below the preset frequency band. If the signal is encoded in the time domain, the second excitation signal decoded in operation  1150  is used, and if the signal is encoded in the frequency domain, the excitation spectrum generated in operation  1135  is used. 
     In operation  1165 , a bitstream is generated by multiplexing the LPC coefficients extracted in operation  1105 , the result of the long term prediction performed in operation  1115 , the information on the encoding domain of the low frequency signal selected in operation  1105 , the second excitation signal encoded in operation  1120 , the excitation spectrum quantized in operation  1130 , and the result encoded in operation  1160 . 
       FIG. 11B  is a flowchart of operation  1160  included in the method of  FIG. 11A , according to an embodiment of the present invention. 
     In operation  1170 , whether to encode a high frequency signal of a high frequency band above a preset frequency band in the time domain or in the frequency domain is determined in accordance with an encoding domain of a low frequency signal of a low frequency band below the preset frequency band, the encoding domain selected in operation  1110  of  FIG. 11A . If the low frequency signal is determined to be encoded in the frequency domain in operation  1110  of  FIG. 11A , the high frequency signal is determined to be encoded in the frequency domain, and if the low frequency signal is determined to be encoded in the time domain in operation  1110  of  FIG. 11A , the high frequency signal is determined to be encoded in the time domain. 
     In operation  1175 , if the high frequency signal is determined to be encoded in the time domain in operation  1170 , LPC coefficients are extracted by performing an LPC analysis on the high frequency signal. The LPC coefficients extracted in operation  1175  are used to restore an envelope as illustrated in  FIG. 7A  by a decoder. 
     In operation  1180 , the second excitation signal decoded in operation  1150  of  FIG. 11A  is multiplied by an envelope of the high frequency signal generated by the LPC coefficients extracted in operation  1175 . An example of the signal multiplied in operation  1180  may be the signal  710  illustrated in  FIG. 7B . 
     In operation  1185 , a gain which compensates for a mismatch between the signal multiplied in operation  1180  and a low frequency signal is calculated and encoded. The mismatch existing at the boundary between the low frequency signal  720  and the multiplied signal  710  which are illustrated in  FIG. 7B  is compensated for as illustrated in  FIG. 7C . 
     In operation  1190 , a frequency band to be used to decode a high frequency spectrum is selected from the excitation spectrum inverse quantized in operation  1135  of  FIG. 11A  by the decoder, and information on the selected frequency band is encoded. 
     In operation  1195 , envelope information of the high frequency spectrum is extracted and encoded. The envelope information may be energy values calculated by frequency bands. 
     The present invention is not limited to an open-loop method in which an encoding domain is firstly selected and then encoding is performed in accordance with the selected domain as described above with reference to  FIGS. 11A and 11B . Alternatively, a close-loop method in which encoding is performed both in the time domain and in the frequency domain and then more appropriate domain is selected later by comparing encoding results may be used. 
       FIG. 12A  is a flowchart of a method of adaptively decoding a high frequency band, according to an embodiment of the present invention. 
     In operation  1200 , a bitstream input from an encoder is inverse multiplexed. The inverse multiplexing is performed on information on an encoding domain of a frequency band encoded by the encoder, LPC coefficients encoded by the encoder, a result of long term prediction performed by the encoder, an excitation signal encoded by the encoder, a spectrum quantized by the encoder, and information required for decoding a high frequency signal by using a low frequency signal or a low frequency spectrum. 
     In operation  1205 , the information on the encoding domain of a low frequency band below a preset frequency band, which is encoded by the encoder, is received and the encoding domain of each frequency band is determined. 
     In operation  1210 , the excitation signal of a frequency band determined as having been encoded in the time domain in operation  1205 , the excitation signal encoded by the encoder, is decoded. 
     In operation  1215 , the result of the long term prediction performed by the encoder on the frequency band determined as having been encoded in the time domain in operation  1205  is decoded, and the excitation signal decoded in operation  1210  and the result of the long term prediction are combined. 
     In operation  1220 , the LPC coefficients of the frequency band determined as having been encoded in the time domain in operation  1205  are decoded, and the LPC coefficients and the signal combined in operation  1215  are combined. 
     In operation  1230 , the spectrum of the frequency band determined as having been encoded in the frequency domain in operation  1205  is inverse quantized. 
     In operation  1233 , inverse operation of the conversion performed in operation  1225  of  FIG. 9A  is performed by inverse converting the spectrum inverse quantized in operation  1230  from the frequency domain to the time domain. 
     In operation  1235 , an excitation spectrum is generated by whitening the spectrum inverse quantized in operation  1230 . 
     In operation  1240 , a high frequency signal of a high frequency band above the preset frequency band is decoded by using the excitation signal decoded in operation  1210  or the excitation spectrum generated in operation  1235 . 
     In operation  1245 , inverse operation of the conversion performed in operation  900  illustrated in  FIG. 9A  is performed. The inverse conversion is performed by combining the signal combined in operation  1220  or the spectrum inverse converted in operation  1233  and the high frequency signal decoded in operation  1240  into a time domain signal. The inverse conversion may be performed by using a QMF method or an LOT method. 
     However, a time domain signal and a frequency domain signal by frequency bands may be combined into a time domain signal by using, for example, a FV-MLT method. In this case, an additional operation for converting a frequency domain signal into a time domain signal may not be performed. 
       FIG. 12B  is a flowchart of operation  1240  included in the method of  FIG. 12A , according to an embodiment of the present invention. 
     In operation  1250 , whether a signal of a high frequency band above a preset frequency band has been encoded in the time domain or in the frequency domain is determined. An encoding domain of each frequency band may be determined by using information on an encoding domain, which is transmitted from an encoder or by using information on a decoded domain of a low frequency band below the preset frequency band, which is used when the high frequency band is decoded in operation  1205  of  FIG. 12A . 
     In operation  1255  LPC coefficients of a frequency band determined as having been encoded in the time domain are decoded. By the LPC coefficients decoded in operation  1255 , an envelope may be restored as illustrated in  FIG. 7A . 
     In operation  1260 , the excitation signal decoded in operation  1210  of  FIG. 12A  is multiplied by an envelope generated by the LPC coefficients decoded in operation  1255 . An example of the signal multiplied in operation  1260  may be the signal  710  illustrated in  FIG. 7B . 
     In operation  1265 , the gain is decoded and applied to the signal multiplied in operation  1260 . By applying the gain, a mismatch between a decoded low frequency signal and a decoded high frequency signal may be compensated for. For example, the high frequency signal multiplied in operation  1260  has the mismatch at the boundary to the low frequency signal as illustrated in  FIG. 7B . However, when the gain is applied to, the mismatch does not exist between the low frequency signal and the high frequency signal as illustrated in  FIG. 7C . 
     In operation  1270 , information on a frequency band to be used to decode a high frequency spectrum from the excitation spectrum generated in operation  1235  of  FIG. 12A  is decoded. Noise is generated by patching or symmetrically folding the excitation spectrum of the corresponding frequency band to the frequency band determined to be encoded in the frequency domain in operation  1250 . For example, an excitation spectrum illustrated in  FIG. 8A  is patched to the high frequency band as illustrated in  FIG. 8B . 
     In operation  1275 , envelope information of a high frequency spectrum encoded by the encoder is decoded. An envelope of the noise generated in operation  1270  is controlled by using the envelope information of the high frequency spectrum decoded in operation  1275 . For example, the noise generated in operation  1270  of in  FIG. 8B  is controlled to an envelope illustrated in  FIG. 8C  by using the envelope information of the high frequency spectrum. 
     In operation  1280 , inverse operation of the conversion performed in operation  925  illustrated in  FIG. 9A  is performed by inverse converting the noise of which envelope is controlled in operation  1275  from the frequency domain to the time domain, thereby generating a high frequency signal. 
       FIG. 13A  is a flowchart of a method of adaptively decoding a high frequency band, according to another embodiment of the present invention. 
     In operation  1300  a bitstream input from an encoder is inverse multiplexed. The inverse multiplexing is performed on LPC coefficients encoded by the encoder, an excitation spectrum encoded by the encoder, a result of long term prediction performed by the encoder, and information required for decoding a high frequency signal of a high frequency band above a preset frequency band by using an excitation spectrum of a low frequency band below the preset frequency band. 
     In operation  1305 , the low frequency excitation spectrum quantized by the encoder is inverse quantized. 
     In operation  1310 , inverse operation of the conversion performed in operation  1010  of  FIG. 10A  is performed by inverse converting the excitation spectrum inverse quantized in operation  1305  from the frequency domain to the time domain, thereby generating an excitation signal. 
     In operation  1315 , the result of the long term prediction performed by the encoder on the low frequency excitation signal is decoded, and the excitation signal generated in operation  1310  and the result of the long term prediction are selectively combined. The combining of the result of the long term prediction is performed when the result of the long term prediction performed by the encoder on the excitation signal is transmitted from the encoder. 
     In operation  1320 , the LPC coefficients are decoded. After the LPC coefficients are decoded in operation  1320 , if the result of the long term prediction is not combined, the excitation signal generated in operation  1310  is combined with the LPC coefficients, and if the result of the long term prediction is combined, the signal combined in operation  1315  is combined with the LPC coefficients. The signal combined in operation  1320  is a restored low frequency signal of a low frequency band. 
     In operation  1325 , a high frequency signal is decoded by using the excitation spectrum of the low frequency signal inverse quantized in operation  1305 . 
     In operation  1330 , the low frequency signal restored in operation  1320  and the high frequency signal decoded in operation  1325  are combined. 
       FIG. 13B  is a flowchart of operation  1325  included in the method of  FIG. 13A , according to an embodiment of the present invention. 
     In operation  1335 , information on a frequency band to be used to decode a high frequency spectrum from an excitation spectrum of a low frequency band below a preset frequency band is decoded. An excitation spectrum to be used is selected from excitation spectrums inverse quantized in operation  1305  in accordance with the decoded information, and noise is generated by patching or symmetrically folding the corresponding excitation spectrum to a high frequency band above the preset frequency band. For example, the excitation spectrum illustrated in  FIG. 8A  is patched to the high frequency band as illustrated in  FIG. 8B . 
     In operation  1340 , envelope information of a high frequency spectrum encoded by the encoder is decoded. An envelope of the noise generated in operation  1335  is controlled by using the envelope information of the high frequency spectrum. For example, the noise generated in operation  1335  as illustrated in  FIG. 8B  is controlled to an envelope illustrated in  FIG. 8C  by using the envelope information of the high frequency spectrum. 
     In operation  1345 , inverse operation of the conversion performed in operation  1010  illustrated in  FIG. 10A  is performed by inverse converting the noise of which envelope is controlled in operation  1340  from the frequency domain to the time domain, thereby generating a high frequency signal. 
       FIG. 14A  is a flowchart of a method of adaptively decoding a high frequency band, according to another embodiment of the present invention. 
     In operation  1400 , a bitstream input from an encoder is inverse multiplexed. The inverse multiplexing is performed on information on an encoding domain of a low frequency signal selected by the encoder, LPC coefficients encoded by the encoder, a result of long term prediction performed by the encoder, an excitation spectrum quantized by the encoder, and information required for decoding a high frequency signal by using a low frequency signal or a low frequency spectrum of a low frequency band below a preset frequency band. 
     In operation  1405 , the information on the encoding domain of the low frequency band encoded by the encoder is decoded, and whether the low frequency band has been encoded in the time domain or in the frequency domain is determined. 
     In operation  1410 , if the low frequency band is determined as having been encoded in the time domain in operation  1405 , an excitation signal of the low frequency band encoded by the encoder is decoded. 
     In operation  1415 , the result of the long term prediction performed by the encoder on the low frequency band signal is decoded, and the excitation signal decoded in operation  1410  and the result of the long term prediction are combined. 
     In operation  1420 , if the low frequency band is determined as having been encoded in the frequency domain in operation  1405 , an excitation spectrum quantized by the encoder is inverse quantized. 
     In operation  1425 , inverse operation of the conversion performed in operation  1125  of  FIG. 11A  is performed by inverse converting the excitation spectrum inverse quantized in operation  1420  from the frequency domain to the time domain, thereby generating an excitation signal. 
     In operation  1430 , the LPC coefficients of the low frequency signal are decoded, and the decoded LPC coefficients are combined with the excitation signal combined in operation  1415  or the excitation signal generated in operation  1425 . The signal combined in operation  1430  is a restored low frequency signal of a low frequency band. 
     In operation  1435 , the high frequency signal is decoded by using the excitation spectrum inverse quantized in operation  1420  or the excitation signal decoded in operation  1410 . If the low frequency band has been encoded in the time domain, the high frequency signal is decoded by using the excitation spectrum inverse quantized in operation  1420 , and if the low frequency band has been encoded in the frequency domain, the high frequency signal is decoded by using the excitation spectrum decoded in operation  1410 . 
     In operation  1440 , the low frequency signal restored in operation  1430  and the high frequency signal decoded in operation  1325  are combined. 
       FIG. 14B  is a flowchart of operation  1435  included in the method of  FIG. 14A , according to an embodiment of the present invention. 
     In operation  1445 , whether to decode a high frequency band above a preset frequency band in the time domain or in the frequency domain is determined by determining an encoding domain of a low frequency band below the preset frequency band. 
     In operation  1450 , if the high frequency band is determined to be decoded in the time domain, LPC coefficients of a high frequency signal are decoded. By the LPC coefficients decoded in operation  1450 , an envelope may be restored as illustrated in  FIG. 7A . 
     In operation  1455 , the excitation signal which is decoded in operation  1410  of  FIG. 14A  is multiplied by the envelope generated by the LPC coefficients decoded in operation  1450 . An example of the signal multiplied in operation  1455  may be the signal  710  illustrated in  FIG. 7B . 
     In operation  1460 , a gain encoded by the encoder is decoded, and the gain is applied to the signal multiplied in operation  1455 . By applying the gain, a mismatch between a low frequency signal and a high frequency signal, which are restored in operation  1430  of  FIG. 14A , may be compensated for. For example, the high frequency signal multiplied in operation  1455  has the mismatch at the boundary to the low frequency signal as illustrated in  FIG. 7B . However, when the gain is applied to, the mismatch does not exist between the low frequency signal and the high frequency signal as illustrated in  FIG. 7C . 
     In operation  1465 , if the high frequency band is determined to be decoded in the frequency domain in operation  1445 , a spectrum is generated by patching or symmetrically folding an excitation spectrum inverse quantized in operation  1420  of  FIG. 14A  to the high frequency band. For example, the excitation spectrum illustrated in  FIG. 8A  is patched to the high frequency band as illustrated in  FIG. 8B . 
     In operation  1470 , envelope information of a high frequency spectrum encoded by the encoder is received and decoded. An envelope of the noise generated in operation  1465  is controlled by using the decoded envelope information of the high frequency spectrum. For example, the noise generated in operation  1465  as illustrated in  FIG. 8B  is controlled to the envelope illustrated in  FIG. 8C  by using the envelope information of the high frequency spectrum. 
     In operation  1475 , inverse operation of the conversion performed in operation  1125  of  FIG. 11A  is performed by inverse converting the noise of which envelope is controlled in operation  1470  from the frequency domain to the time domain, thereby generating a high frequency signal. 
     The present invention can also be embodied as computer readable code on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves. 
     As described above, according to the present invention, a signal of a high frequency band above a preset frequency band is adaptively encoded or decoded in the time domain or in the frequency domain by using a signal of a low frequency band below the preset frequency band. 
     As such, the sound quality of a high frequency signal is not deteriorate even when an audio signal is encoded or decoded by using a small number of bits and thus coding efficiency may be maximized. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The exemplary embodiments should be considered in a descriptive sense only and not for purposes of limitation. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims, and all differences within the scope will be construed as being included in the present invention.