Patent Publication Number: US-8996388-B2

Title: Method and an apparatus for processing an audio signal

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 13/391,992 of Feb. 23, 2012, which is National Phase of PCT/KR2010/006412 filed on Sep. 17, 2010, which claims priority under 35 U.S.C. 119 (e) to U.S. Provisional Application No. 61/243,514 filed on Sep. 17, 2009, the disclosure of which hereby incorporated by reference in its entirety into this application. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an apparatus for processing an audio signal and method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for encoding or decoding an audio signal. 
     BACKGROUND ART 
     Generally, an audio characteristic based coding scheme is applied to such an audio signal as a music signal, while a speech characteristic based coding scheme is applied to a speech signal. 
     DISCLOSURE OF THE INVENTION 
     Technical Problem 
     However, if a prescribed coding scheme is applied to a signal in which audio and speech characteristics are mixed with each other, audio coding efficiency is lowered or a sound quality is degraded. 
     Technical Solution 
     Accordingly, the present invention is directed to an apparatus for processing an audio signal and method thereof that substantially obviate one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an apparatus for processing an audio signal and method thereof, by which one of at least two kinds of coding schemes is applied to one frame or subframe. 
     Another object of the present invention is to provide an apparatus for processing an audio signal and method thereof, by which, in applying a different coding scheme to each frame or subframe of an audio signal including a series of frames, a mismatch generated from asymmetry of a window shape corresponding to each coding scheme can be solved. 
     Another object of the present invention is to provide an apparatus for processing an audio signal and method thereof, by which aliasing and the like can be cancelled when a rectangular window and a non-rectangular window come in contact with each other. 
     Another object of the present invention is to provide an apparatus for processing an audio signal and method thereof, by which, if a frequency domain scheme applied frame follows a linear prediction domain scheme applied frame, a window transmission for compensating a window length difference can be skipped. 
     Another object of the present invention is to provide an apparatus for processing an audio signal and method thereof, by which a mismatch attributed to asymmetry of a window shape, can be solved in a manner of switching a type of a window corresponding to a current frame according to a coding scheme of a following frame. 
     A further object of the present invention is to provide an apparatus for processing an audio signal and method thereof, by which bit efficiency in a frame of a linear prediction domain scheme can be raised in a manner of selectively applying a long-term prediction according to whether a previous frame is a frame of a frequency domain scheme. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     Advantageous Effects 
     Accordingly, the present invention provides the following effects or advantages. 
     First of all, the present invention compensates such a defect as aliasing due to inter-window asymmetry (e.g., asymmetry between a rectangular window and a non-rectangular window) and the like, thereby improving a sound quality of an audio signal considerably. 
     Secondly, as a scheme for compensating the aliasing and the like is applied, 100% overlapping between a rectangular window and a non-rectangular window become unnecessary. Therefore, the non-rectangular window can maintain a descending line or an ascending line with a gentle slope. 
     Thirdly, the present invention applies a non-rectangular window having a descending line or an ascending line with a gentle slope, whereby a crossing point between homogeneous windows (e.g., non-rectangular windows) is matched to a crossing point between heterogeneous windows (e.g., a non-rectangular window and a rectangular window). 
     Fourthly, as a crossing point of homogenous windows is matched to a crossing point of heterogeneous windows, a transition window for compensation of a window length difference becomes unnecessary and a direct transition between a first coding scheme (e.g., linear prediction domain scheme) and a second coding scheme (e.g., frequency domain scheme) becomes possible. 
     Fifthly, as the direct transition becomes possible, it is able to apply a window suitable for an audio signal characteristic of a corresponding block without using a window for solving a mismatch. Therefore, a sound quality can be considerably enhanced. 
     Sixthly, since a shape of a window corresponding to a non-rectangular window type is made to vary according to whether a short window is present at a previous or following block, TDAC condition is met. Therefore, a sound quality can be enhanced. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
       In the drawings: 
         FIG. 1  is a schematic block diagram of an audio signal processing apparatus according the present invention; 
         FIG. 2  is a block diagram of an encoder according to a first embodiment of the present invention; 
         FIG. 3  is a block diagram of a decoder according to a first embodiment of the present invention; 
         FIG. 4  is a diagram of an audio signal configured by a block unit, to which a different coding scheme is applied per frame (or subframe); 
         FIG. 5  is a diagram for transition to a heterogeneous coding scheme (i.e., rectangular coding scheme and non-rectangular coding scheme); 
         FIG. 6  is a diagram for characteristics when a rectangular window and a non-rectangular window are overlapped with each other; 
         FIG. 7  is a diagram for a correction part (CP), an aliasing part (AP) and an uncompensated signal; 
         FIG. 8  is a diagram for a characteristic of a non-rectangular window with symmetry (i.e., condition for TDAC); 
         FIG. 9  is a diagram for examples of a compensation signal for compensating a correction part and/or an aliasing part; 
         FIG. 10  is a diagram for examples of a non-rectangular window in combination of heterogeneous windows (i.e., rectangular window and non-rectangular window) shown in  FIG. 6 ; 
         FIG. 11  is a diagram for a case that a rectangular window following a rectangular window is overlapped 
         FIG. 12  is a block diagram of an encoder according to a second embodiment of the present invention; 
         FIG. 13  is a block diagram of a decoder according to a second embodiment of the present invention; 
         FIG. 14  is a diagram of a shape of a transition window according to whether a rectangular coding scheme is applied to a previous block; 
         FIG. 15  is a block diagram of an encoder according to a third embodiment of the present invention; 
         FIG. 16  is a block diagram of a decoder according to a third embodiment of the present invention; 
         FIG. 17A  is a diagram of a long_start window combined with a first coding scheme window or a second coding scheme window (short window); 
         FIG. 17B  is a diagram of a long_stop window combined with a first coding scheme window or a second coding scheme window (short window); 
         FIGS. 17C and 17D  are diagrams of a stop_start window combined with a first coding scheme window or a second coding scheme window (short window); 
         FIG. 18  is a diagram of a short window overlapped with a first coding scheme window or a second coding scheme window; 
         FIG. 19  is a block diagram of an encoder according to a fourth embodiment of the present invention; 
         FIG. 20  is a block diagram of a decoder according to a fourth embodiment of the present invention; 
         FIG. 21  is a table of inter-window paths or transitions; 
         FIG. 22  is a diagram for a case of transition to a long_stop window in a first coding scheme; 
         FIG. 23  is a diagram for a case of transition to a short window in a first coding scheme; 
         FIG. 24  is a diagram for a case that a first coding scheme window is overlapped with a short window in a new shape; 
         FIG. 25  is a block diagram of an encoder according to a fifth embodiment of the present invention; 
         FIG. 26  is a block diagram of a decoder according to a sixth embodiment of the present invention; 
         FIG. 27  is a diagram for a case that a window corresponding to a first coding scheme (e.g., TCX) is overlapped with a short window (or a long_stop window); 
         FIG. 28  is a table of a window corresponding to a non-rectangular scheme among first coding schemes varying within Shape  1  to Shape  4 ; 
         FIG. 29  is a block diagram of an encoder according to a sixth embodiment of the present invention; 
         FIG. 30  is a block diagram of a decoder according to a sixth embodiment of the present invention; 
         FIG. 31  is a diagram for examples of a coding scheme per block (frame or subframe); 
         FIG. 32  is a diagram for one examples of a signal waveform related to a long term prediction; 
         FIG. 33  is a diagram for an example of an audio signal encoding apparatus to which an encoder according to an embodiment of the present invention is applied; 
         FIG. 34  is a diagram for an example of an audio signal decoding apparatus to which a decoder according to an embodiment of the present invention is applied; 
         FIG. 35  is a schematic block diagram of a product in which an audio signal processing apparatus according to one embodiment of the present invention is implemented; and 
         FIG. 36  is a diagram for explaining relations between products in which an audio signal processing apparatus according to one embodiment of the present invention is implemented. 
     
    
    
     BEST MODE 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for processing an audio signal, comprising: receiving, by an audio processing apparatus, an audio signal including a first data of a first block encoded with rectangular coding scheme and a second data of a second block encoded with non-rectangular coding scheme; receiving a compensation signal corresponding to the second block; estimating a prediction of an aliasing part using the first data; obtaining a reconstructed signal for the second block based on the second data, the compensation signal and the prediction of aliasing part is provided. 
     According to the present invention, the rectangular coding scheme is to encode or decode with rectangular window, the non-rectangular coding scheme is to encode or decode with non-rectangular window. 
     According to the present invention, the compensation signal is generated based on a correction part and an error of aliasing part, the correction part corresponds to a difference related to asymmetry between rectangular window and non-rectangular window, the error of aliasing part corresponds to a difference between the aliasing part and the prediction of aliasing part. 
     According to the present invention, the aliasing part corresponds to overlapping part between the first block and non-rectangular window used for the non-rectangular coding scheme. 
     According to the present invention, the estimating of the prediction comprises: generating an output signal for the first block using the first data of the first block based on the rectangular window scheme; obtaining the prediction of the aliasing part using the output signal for the first block and the non-rectangular window. 
     According to the present invention, the reconstructed signal is approximate to a signal processed with rectangular window that differs from non-rectangular window used for the non-rectangular coding scheme. 
     According to the present invention, the obtaining of the reconstructed signal comprises: inverse-frequency-transforming the second data to generate a time-domain second signal; inverse-frequency-transforming the compensation signal to generate a time-domain compensation signal; obtaining the reconstructed signal, by adding the time-domain compensation signal to the time-domain second signal and the prediction of the aliasing part; 
     According to the present invention, the first block corresponds to one of frame and subframe, and the second block corresponds to one of frame and subframe. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, an apparatus for processing an audio signal, comprising: a de-multiplexer receiving an audio signal including a first data of a first block encoded with rectangular coding scheme and a second data of a second block encoded with non-rectangular coding scheme, and receiving a compensation signal corresponding to the second block; a rectangular decoding unit estimating a prediction of an aliasing part using the first data; and, a non-rectangular decoding unit obtaining a reconstructed signal for the second block based on the second data, the compensation signal and the prediction of aliasing part is provided. 
     According to the present invention, the rectangular coding scheme is to encode or decode with rectangular window, the non-rectangular coding scheme is to encode or decode with non-rectangular window. 
     According to the present invention, the compensation signal is generated based on a correction part and an error of aliasing part, the correction part corresponds to a difference related to asymmetry between rectangular window and non-rectangular window, the error of aliasing part corresponds to a difference between the aliasing part and the prediction of aliasing part. 
     According to the present invention, the aliasing part corresponds to overlapping part between the first block and non-rectangular window used for the non-rectangular coding scheme. 
     According to the present invention, the rectangular decoding unit configured to: generate an output signal for the first block using the first data of the first block based on the rectangular window scheme, and obtain the prediction of the aliasing part using the output signal for the first block and the non-rectangular window. 
     According to the present invention, the reconstructed signal is approximate to a signal processed with rectangular window that differs from non-rectangular window used for the non-rectangular coding scheme. 
     According to the present invention, the non-rectangular decoding unit configured to: inverse-frequency-transform the second data to generate a time-domain second signal; inverse-frequency-transform the compensation signal to generate a time-domain compensation signal; and, obtain the reconstructed signal, by adding the time-domain compensation signal to the time-domain second signal and the prediction of the aliasing part. 
     According to the present invention, the first block corresponds to one of frame and subframe, and the second block corresponds to one of frame and subframe. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, a method for processing an audio signal, comprising: receiving, by an audio processing apparatus, coding identification information indicating whether to apply a first coding scheme or a second coding scheme to a current frame, when the coding identification information indicates that the second coding scheme to the current frame, receiving window type information indicating a particular window for the current frame, from among a plurality of windows; identifying that a current window is a long_start window based on the window type information, wherein the long_start window follows only_long window of a previous frame, wherein the long_start window includes a gentle long_start window and a steep long_start window; and, when the first coding scheme is applied to a following frame, applying the gentle long_start window to the current frame, wherein: the gentle long_start window comprise a descending line with first slope, the steep long_start window comprise a descending line with second slope, the first slope is gentler than the second slope is provided. 
     According to the present invention, a width of the first slope is equal to two-times a width of the second slope. 
     According to the present invention, a width of the first slope corresponds to N/4 (where N is frame length). 
     According to the present invention, a width of the first slope corresponds to 256 samples, and wherein a width of the first slope is equal to ⅛ of length of the long_start window. 
     According to the present invention, the only_long window is horizontal-symmetry, and the long_start window is horizontal-asymmetry, the long_start window has zero part in a right half. 
     According to the present invention, center point of the descending line with the first slope or the second slope is at 3N/2 distance from a start point of the long_start window (where N is frame length). 
     According to the present invention, the first coding scheme is based on frequency-domain, and the second coding scheme is based on linear-prediction domain. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, an apparatus for processing an audio signal, comprising: a de-multiplexer receiving, by an audio processing apparatus, coding identification information indicating whether to apply a first coding scheme or a second coding scheme to a current frame, and, when the coding identification information indicates that the second coding scheme to the current frame, receiving window type information indicating a particular window for the current frame, from among a plurality of windows; a second coding unit identifying that current window is a long_start window based on the window type information, wherein the long_start window follows only_long window of a previous frame, wherein the long_start window includes a gentle long_start window and a steep long_start window, and, when the first coding scheme is applied to a following frame, applying the gentle long_start window to the current frame, wherein: the gentle long_start window comprise a descending line with first slope, the steep long_start window comprise a descending line with second slope, the first slope is gentler than the second slope is provided. 
     According to the present invention, a width of the first slope is equal to two-times a width of the second slope. 
     According to the present invention, wherein a width of the first slope corresponds to N/4 (where N is length of the current frame). 
     According to the present invention, wherein a width of the first slope corresponds to 256 samples, and wherein a width of the first slope is equal to ⅛ of length of the long_start window. 
     According to the present invention, the only_long window is horizontal-symmetry, and the long_start window is horizontal-asymmetry, the long_start window has zero part in a right half. 
     According to the present invention, center point of the descending line with the first slope or the second slope is at 3N/2 distance from a start point of the long_start window (where N is frame length). 
     According to the present invention, the first coding scheme is based on frequency-domain, and the second coding scheme is based on linear-prediction domain. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, a method for processing an audio signal, comprising: receiving, by an audio processing apparatus, an audio signal including a first data of a first block and a second data of a second block; receiving a compensation signal corresponding to the second block; obtaining a reconstructed signal for the second block based on the second data, the compensation signal and a window of the second block, wherein, when the first data is encoded with a rectangular coding scheme and the window of the second block belongs to transition window class, the window of the second block has ascending line with a first slope, wherein the first slope is gentler than a second slope is provided. 
     According to the present invention, when the first data is encoded with a non-rectangular coding scheme and the window of the second block belongs to the transition window class, the window of the second block has ascending line with the second slope. 
     According to the present invention, when the transition window class comprises long_stop window and stop_start window, and the long_stop window and the stop_start window are horizontal-asymmetry, and have a zero part in a left half. 
     According to the present invention, the compensation signal is received, when the first data is encoded with the rectangular coding scheme. 
     According to the present invention, the compensation signal is generated based on at least one of a difference related to asymmetry between rectangular window and non-rectangular window, and a difference between the aliasing part and prediction of aliasing part. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, an apparatus for processing an audio signal, comprising: a de-multiplexer receiving an audio signal including a first data of a first block and a second data of a second block, and receiving a compensation signal corresponding to the second block; a non-rectangular decoding unit obtaining a reconstructed signal for the second block based on the second data, the compensation signal and a window of the second block, wherein, when the first data is encoded with a rectangular coding scheme and the window of the second block belongs to transition window class, the window of the second block has ascending line with a first slope, wherein the first slope is gentler than a second slope is provided. 
     According to the present invention, when the first data is encoded with a non-rectangular coding scheme and the window of the second block belongs to the transition window class, the window of the second block has ascending line with the second slope. 
     According to the present invention, when the transition window class comprises long_stop window and stop_start window, and the long_stop window and the stop_start window are horizontal-asymmetry, and have a zero part in a left half. 
     According to the present invention, the compensation signal is received, when the first data is encoded with the rectangular coding scheme. 
     According to the present invention, the compensation signal is generated based on at least one of a difference related to asymmetry between rectangular window and non-rectangular window, and a difference between the aliasing part and prediction of aliasing part. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, a method for processing an audio signal, comprising: receiving, by an audio processing apparatus, an audio signal including a first data of a first block and a second data of a second block; receiving a compensation signal corresponding to the second block; obtaining a reconstructed signal for the second block based on the second data, the compensation signal and a window of the second block, wherein, when the first data is encoded with a rectangular coding scheme and the window of the second block belongs to transition window class, the window of the second block has ascending line with a first slope, wherein the first slope is gentler than a second slope is provided. 
     According to the present invention, when the first data is encoded with a non-rectangular coding scheme and the window of the second block belongs to the transition window class, the window of the second block has ascending line with the second slope. 
     According to the present invention, when the transition window class comprises long_stop window and stop_start window, and the long_stop window and the stop_start window are horizontal-asymmetry, and have a zero part in a left half. 
     According to the present invention, the compensation signal is received, when the first data is encoded with the rectangular coding scheme. 
     According to the present invention, the compensation signal is generated based on at least one of a difference related to asymmetry between rectangular window and non-rectangular window, and a difference between the aliasing part and prediction of aliasing part. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, an apparatus for processing an audio signal, comprising: a de-multiplexer receiving an audio signal including a first data of a first block and a second data of a second block, and receiving a compensation signal corresponding to the second block; a non-rectangular decoding unit obtaining a reconstructed signal for the second block based on the second data, the compensation signal and a window of the second block, wherein, when the first data is encoded with a rectangular coding scheme and the window of the second block belongs to transition window class, the window of the second block has ascending line with a first slope, wherein the first slope is gentler than a second slope is provided. 
     According to the present invention, when the first data is encoded with a non-rectangular coding scheme and the window of the second block belongs to the transition window class, the window of the second block has ascending line with the second slope. 
     According to the present invention, when the transition window class comprises long_stop window and stop_start window, and the long_stop window and the stop_start window are horizontal-asymmetry, and have a zero part in a left half. 
     According to the present invention, the compensation signal is received, when the first data is encoded with the rectangular coding scheme. 
     According to the present invention, the compensation signal is generated based on at least one of a difference related to asymmetry between rectangular window and non-rectangular window, and a difference between the aliasing part and prediction of aliasing part. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, a method for processing an audio signal, comprising: when a second coding scheme is applied to a current frame, receiving, by an audio processing apparatus, window type information indicating a particular window for the current frame from among a plurality of windows; and, applying a current window to the current frame based on the window type information, wherein, when a first coding scheme is applied to a previous frame, the plurality of window consists of a short window, a first transition window, a second transition window, wherein the short window has at least one ascending line which width is N/8, and the first transition window and the second transition window have an ascending line which width is N/4 (where N is frame length) is provided. 
     According to the present invention, length of short window, the first transition window and the second transition window is 2N. 
     According to the present invention, left half of short window, the first transition window and the second transition window corresponds to 1024 samples. 
     According to the present invention, cross point between the current window and a previous window is at N/2 distance from start of the current window. 
     According to the present invention, the first transition window have no zero part in right half, the second transition window have zero part in right half, the short window has a plurality of short parts which are overlapped together, and the short part has the ascending line and a descending line. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, a method for processing an audio signal, comprising: receiving, by an audio processing apparatus, an audio signal including a current frame encoded with a first coding scheme and a following frame encoded with a second coding scheme; receiving sub-coding identification information indicating at least one block of the current frame is encoded with a rectangular coding scheme or a non-rectangular coding scheme; when the sub-coding identification information indicates that at least last block of the current frame is encoded with the non-rectangular coding scheme, deciding a window shape including a first shape and a second shape for a current window, according to whether a following window for the following frame is a short window or not; applying the current window of the decided window shape to the current frame, wherein: the first shape has a descending line with first slope, the second shape has a descending line with second slope, and, the first slope is gentler than the second slope is provided. 
     According to the present invention, a width of the first slope corresponds to 256 samples or N/4 and a width of the second slope corresponds to 128 samples or N/8 (N is frame length). 
     According to the present invention, cross point between the current window and a following window is at N/2 distance from start of the following window. 
     According to the present invention, the first slope is matched to a slope of an ascending slope in non-short window, and the second slope is matched to a slope of an ascending slope in the short window. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, an apparatus for processing an audio signal, comprising: a de-multiplexer, when a second coding scheme is applied to a current frame, receiving window type information indicating a particular window for the current frame from among a plurality of windows; a second coding unit applying a current window to the current frame based on the window type information, wherein, when a first coding scheme is applied to a previous frame, the plurality of window consists of a short window, a first transition window, a second transition window, wherein the short window has at least one ascending line which width is N/8, and the first transition window and the second transition window have an ascending line which width is N/4 (where N is frame length) is provided. 
     According to the present invention, length of short window, the first transition window and the second transition window is 2N. 
     According to the present invention, left half of short window, the first transition window and the second transition window corresponds to 1024 samples. 
     According to the present invention, cross point between the current window and a previous window is at N/2 distance from start of the current window. 
     According to the present invention, the first transition window have no zero part in right half, the second transition window have zero part in right half, the short window has a plurality of short parts which are overlapped together, and the short part has the ascending line and a descending line. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, an apparatus for processing an audio signal, comprising: a de-multiplexer receiving an audio signal including a current frame encoded with a first coding scheme and a following frame encoded with a second coding scheme, and receiving sub-coding identification information indicating at least one block of the current frame is encoded with a rectangular coding scheme or a non-rectangular coding scheme; a first coding unit, when the sub-coding identification information indicates that at least last block of the current frame is encoded with the non-rectangular coding scheme, deciding a window shape including a first shape and a second shape for a current window, according to whether a following window for the following frame is a short window or not; applying the current window of the decided window shape to the current frame, wherein: the first shape has a descending line with first slope, the second shape has a descending line with second slope, and, the first slope is gentler than the second slope is provided. 
     According to the present invention, a width of the first slope corresponds to 256 samples or N/4 and a width of the second slope corresponds to 128 samples or N/8 (N is frame length). 
     According to the present invention, cross point between the current window and a following window is at N/2 distance from start of the following window. 
     According to the present invention, the first slope is matched to a slope of an ascending slope in non-short window, and the second slope is matched to a slope of an ascending slope in the short window. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, a method for processing an audio signal, comprising: receiving, by an audio processing apparatus, coding identification information indicating whether to apply a first coding scheme or a second coding scheme to a current frame; when the coding identification information indicates that the second coding scheme is applied to the current frame, receiving window type information indicating a particular window for the current frame, from among a plurality of windows; identifying that a current window is a long_stop window based on the window type information, wherein the long_stop window is followed by only_long window of a following frame, wherein the long_stop window includes a gentle long_stop window and a steep long_stop window; and, when the first coding scheme is applied to a previous frame, applying the gentle long_stop window to the current frame, wherein: the gentle long_stop window comprise an ascending line with first slope, the steep long_stop window comprise an ascending line with second slope, and, the first slope is gentler than the second slope. 
     According to the present invention, a width of the first slope is equal to two-times a width of the second slope. 
     According to the present invention, a width of the first slope corresponds to N/4 (where N is frame length). 
     According to the present invention, a width of the first slope corresponds to 256 samples, and wherein a width of the first slope is equal to ⅛ of length of the long_stop window. 
     According to the present invention, the only_long window is horizontal-symmetry, and the long_stop window is horizontal-asymmetry, the long_stop window has zero part in a left half. 
     According to the present invention, center point of the ascending line with the first slope or the second slope is at N/2 distance from a start point of the long_stop window (where N is frame length). 
     According to the present invention, the first coding scheme is based on frequency-domain, and the second coding scheme is based on linear-prediction domain. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, an apparatus for processing an audio signal, comprising: a de-multiplexer receiving coding identification information indicating whether to apply a first coding scheme or a second coding scheme to a current frame, and when the coding identification information indicates that the second coding scheme is applied to the current frame, receiving window type information indicating a particular window for the current frame, from among a plurality of windows; and, a second coding unit and identifying that a current window is a long_stop window based on the window type information, wherein the long_stop window is followed by only_long window of a following frame, wherein the long_stop window includes a gentle long_stop window and a steep long_stop window; and, when the first coding scheme is applied to a previous frame, applying the gentle long_stop window to the current frame, wherein: the gentle long_stop window comprise an ascending line with first slope, the steep long_stop window comprise an ascending line with second slope, and, the first slope is gentler than the second slope. 
     According to the present invention, a width of the first slope is equal to two-times a width of the second slope. 
     According to the present invention, a width of the first slope corresponds to N/4 (where N is frame length). 
     According to the present invention, a width of the first slope corresponds to 256 samples, and wherein a width of the first slope is equal to ⅛ of length of the long_stop window. 
     According to the present invention, wherein the only_long window is horizontal-symmetry, and the long_stop window is horizontal-asymmetry, the long_stop window has zero part in a left half. 
     According to the present invention, center point of the ascending line with the first slope or the second slope is at N/2 distance from a start point of the long_stop window (where N is frame length). 
     According to the present invention, the first coding scheme is based on frequency-domain, and the second coding scheme is based on linear-prediction domain. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, a method for processing an audio signal, comprising: receiving, by an audio processing apparatus, coding identification information indicating whether to apply a first coding scheme or a second coding scheme to a current frame; when the coding identification information indicates that the second coding scheme is applied to the current frame, receiving window type information indicating a particular window for the current frame, from among a plurality of windows; identifying that a current window is a stop_start window based on the window type information, wherein the stop_start window follows one of a long_start window, a short window and a window of the first coding scheme for a previous frame, wherein the stop_start window is followed by one of a long_stop window, a short window and a window of the first coding scheme for a following frame, wherein the stop_start window includes a gentle-gentle stop_start window, a gentle-steep stop_start window, a steep-gentle stop_start window and a steep-steep stop_start window; when the first coding scheme is applied to a previous frame, applying one of the gentle-gentle stop_start window and the gentle-steep stop_start window to the current frame; and, when the first coding scheme is applied to a following frame, applying one of the gentle-gentle stop_start window and the steep-gentle stop_start window to the current frame, wherein: the gentle-gentle stop_start window comprise an ascending line with first slope and a descending line with the first slope, the gentle-steep stop_start window comprise an ascending line with the first slope and a descending line with second slope, the steep-gentle stop_start window comprise an ascending line with the second slope and a descending line with first slope, the steep-steep stop_start window comprise an ascending line with the second slope and a descending line with the second slope, and, the first slope is gentler than the second slope. 
     According to the present invention, a width of the first slope is equal to two-times a width of the second slope. 
     According to the present invention, a width of the first slope corresponds to N/4 (where N is frame length). 
     According to the present invention, a width of the first slope corresponds to 256 samples, and wherein a width of the first slope is equal to ⅛ of length of the stop_start window. 
     According to the present invention, the short window is horizontal-symmetry, the long_start window and the long_stop window are horizontal-asymmetry, the long_stop window has zero part in a left half, and the long_start window has zero part in a right half. 
     According to the present invention, wherein center point of the ascending line with the first slope or the second slope is at N/2 distance from a start point of the stop_start window (where N is frame length), center point of the descending line with the first slope or the second slope is at 3N/2 distance from a start point of the stop_start window (where N is frame length). 
     According to the present invention, the first coding scheme is based on frequency-domain, and the second coding scheme is based on linear-prediction domain. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, an apparatus for processing an audio signal, comprising: a de-multiplexer receiving coding identification information indicating whether to apply a first coding scheme or a second coding scheme to a current frame, and, when the coding identification information indicates that the second coding scheme is applied to the current frame, receiving window type information indicating a particular window for the current frame, from among a plurality of windows; and, a second coding unit identifying that a current window is a stop_start window based on the window type information, wherein the stop_start window follows one of a long_start window, a short window and a window of the first coding scheme for a previous frame, wherein the stop_start window is followed by one of a long_stop window, a short window and a window of the first coding scheme for a following frame, wherein the stop_start window includes a gentle-gentle stop_start window, a gentle-steep stop_start window, a steep-gentle stop_start window and a steep-steep stop_start window, and, when the first coding scheme is applied to a previous frame, applying one of the gentle-gentle stop_start window and the gentle-steep stop_start window to the current frame, when the first coding scheme is applied to a following frame, applying one of the gentle-gentle stop_start window and the steep-gentle stop_start window to the current frame, wherein: the gentle-gentle stop_start window comprise an ascending line with first slope and a descending line with the first slope, the gentle-steep stop_start window comprise an ascending line with the first slope and a descending line with second slope, the steep-gentle stop_start window comprise an ascending line with the second slope and a descending line with first slope, the steep-steep stop_start window comprise an ascending line with the second slope and a descending line with the second slope, and, the first slope is gentler than the second slope. 
     According to the present invention, a width of the first slope is equal to two-times a width of the second slope. 
     According to the present invention, a width of the first slope corresponds to N/4 (where N is frame length). 
     According to the present invention, a width of the first slope corresponds to 256 samples, and wherein a width of the first slope is equal to ⅛ of length of the stop_start window. 
     According to the present invention, the short window is horizontal-symmetry, the long_start window and the long_stop window are horizontal-asymmetry, the long_stop window has zero part in a left half, and the long_start window has zero part in a right half. 
     According to the present invention, center point of the ascending line with the first slope or the second slope is at N/2 distance from a start point of the stop_start window (where N is frame length), center point of the descending line with the first slope or the second slope is at 3N/2 distance from a start point of the stop_start window (where N is frame length). 
     According to the present invention, the first coding scheme is based on frequency-domain, and the second coding scheme is based on linear-prediction domain. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, a method for processing an audio signal, comprising: receiving, by an audio processing apparatus, coding identification information indicating whether to apply a first coding scheme or a second coding scheme to a current frame, when the coding identification information indicates that the second coding scheme is applied to the current frame, receiving window type information indicating a particular window for the current frame, from among a plurality of windows; identifying that a current window is a short window based on the window type information, wherein the short window has one fixed shape which comprises a plurality of short parts overlapped together, and, applying the short window of the fixed shape to the current frame, wherein the short window follows one of a long_start window, a stop_start window and a window of the first coding scheme for a previous frame, wherein the short window is followed by one of a long_stop window, the stop_start window and the window of the first coding scheme for a following frame. 
     According to the present invention, wherein the short part has an ascending line and a descending line, wherein a width of the ascending line is identical to a width of the descending line. 
     According to the present invention, the width of the ascending line and the width of the descending line correspond to N/8 (wherein N is frame length). 
     According to the present invention, cross point between the short window and a window for the previous frame is at N/2 distance from start of the short window, cross point between the short window and a window for the following frame is at 3N/2 distance from start of the short window (where N is frame length). 
     According to the present invention, the method further comprises receiving a frame data of the following frame; when a rectangular coding scheme from among the first coding scheme is applied to the following frame, receiving a compensation signal for the following frame; and, obtaining a reconstructed signal for the following frame based on the following data of the following frame, the compensation signal and the window of the first coding scheme. 
     According to the present invention, the compensation signal is generated based on at least one of a difference related to asymmetry between a rectangular window and a non-rectangular window, and a difference between the aliasing part and prediction of aliasing part. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, an apparatus for processing an audio signal, comprising: receiving, by an audio processing apparatus, coding identification information indicating whether to apply a first coding scheme or a second coding scheme to a current frame, when the coding identification information indicates that the second coding scheme is applied to the current frame, receiving window type information indicating a particular window for the current frame, from among a plurality of windows; identifying that a current window is a short window based on the window type information, wherein the short window has one fixed shape which comprises a plurality of short parts overlapped together, and, applying the short window of the fixed shape to the current frame, wherein the short window follows one of a long_start window, a stop_start window and a window of the first coding scheme for a previous frame, wherein the short window is followed by one of a long_stop window, the stop_start window and the window of the first coding scheme for a following frame. 
     According to the present invention, the short part has an ascending line and a descending line, wherein a width of the ascending line is identical to a width of the descending line. 
     According to the present invention, the width of the ascending line and the width of the descending line correspond to N/8 (wherein N is frame length). 
     According to the present invention, cross point between the short window and a window for the previous frame is at N/2 distance from start of the short window, cross point between the short window and a window for the following frame is at 3N/2 distance from start of the short window (where N is frame length). 
     According to the present invention, the apparatus further comprises receiving a frame data of the following frame; when a rectangular coding scheme from among the first coding scheme is applied to the following frame, receiving a compensation signal for the following frame; and, obtaining a reconstructed signal for the following frame based on the following data of the following frame, the compensation signal and the window of the first coding scheme. 
     According to the present invention, the compensation signal is generated based on at least one of a difference related to asymmetry between a rectangular window and a non-rectangular window, and a difference between the aliasing part and prediction of aliasing part. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, a method for processing an audio signal, comprising: receiving, by an audio processing apparatus, an audio signal including a current frame encoded with a first coding scheme and a following frame encoded with a second coding scheme; receiving sub-coding identification information indicating at least one block of the current frame is encoded with a rectangular coding scheme or a non-rectangular coding scheme; when the sub-coding identification information indicates that at least last block of the current frame is encoded with the non-rectangular coding scheme, deciding a window shape including a first shape and a second shape for a current window, according to whether a following window for the following frame is a short window or not; applying the current window of the decided window shape to the current frame, wherein: the first shape has a descending line with first slope, the second shape has a descending line with second slope, and, the first slope is gentler than the second slope. 
     According to the present invention, a width of the first slope corresponds to 256 samples or N/4 and a width of the second slope corresponds to 128 samples or N/8 (N is frame length). 
     According to the present invention, cross point between the current window and a following window is at N/2 distance from start of the following window. 
     According to the present invention, the first slope is matched to a slope of an ascending slope in non-short window, and the second slope is matched to a slope of an ascending slope in the short window. 
     To further achieve these and other advantages and in accordance with the purpose of the present invention, an apparatus for processing an audio signal, comprising: a de-multiplexer receiving an audio signal including a current frame encoded with a first coding scheme and a following frame encoded with a second coding scheme, and receiving sub-coding identification information indicating at least one block of the current frame is encoded with a rectangular coding scheme or a non-rectangular coding scheme; a first coding unit, when the sub-coding identification information indicates that at least last block of the current frame is encoded with the non-rectangular coding scheme, deciding a window shape including a first shape and a second shape for a current window, according to whether a following window for the following frame is a short window or not; applying the current window of the decided window shape to the current frame, wherein: the first shape has a descending line with first slope, the second shape has a descending line with second slope, and, the first slope is gentler than the second slope. 
     According to the present invention, a width of the first slope corresponds to 256 samples or N/4 and a width of the second slope corresponds to 128 samples or N/8 (N is frame length). 
     According to the present invention, cross point between the current window and a following window is at N/2 distance from start of the following window. 
     According to the present invention, the first slope is matched to a slope of an ascending slope in non-short window, and the second slope is matched to a slope of an ascending slope in the short window. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     MODE FOR INVENTION 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. First of all, terminologies or words used in this specification and claims are not construed as limited to the general or dictionary meanings and should be construed as the meanings and concepts matching the technical idea of the present invention based on the principle that an inventor is able to appropriately define the concepts of the terminologies to describe the inventor&#39;s invention in best way. The embodiment disclosed in this disclosure and configurations shown in the accompanying drawings are just one preferred embodiment and do not represent all technical idea of the present invention. Therefore, it is understood that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents at the timing point of filing this application. 
     According to the present invention, terminologies not disclosed in this specification can be construed as the following meanings and concepts matching the technical idea of the present invention. Specifically, ‘coding’ can be construed as ‘encoding’ or ‘decoding’ selectively and ‘information’ in this disclosure is the terminology that generally includes values, parameters, coefficients, elements and the like and its meaning can be construed as different occasionally, by which the present invention is non-limited. 
     In this disclosure, in a broad sense, an audio signal is conceptionally discriminated from a video signal and designates all kinds of signals that can be auditorily identified. In a narrow sense, the audio signal means a signal having none or small quantity of speech characteristics. Audio signal of the present invention should be construed in a broad sense. Yet, the audio signal of the present invention can be understood as an audio signal in a narrow sense in case of being used as discriminated from a speech signal. 
     Although coding is specified to encoding only, it can be construed as including both encoding and decoding. 
       FIG. 1  is a schematic block diagram of an audio signal processing apparatus according the present invention. 
     Referring to  FIG. 1 , an encoder  100  of an audio signal processing apparatus according the present invention includes a pair of coding units (i.e., a rectangular coding unit  120 R and a non-rectangular coding unit  120 N or a first coding unit  120 - 1  and a second coding unit  120 - 2 ) and is able to further include a signal classifier  110  and a multiplexer  130 . 
     In this case, the rectangular coding unit  120 R is a coding unit to which a rectangular coding scheme is applied. In particular, the rectangular coding scheme means a coding scheme of applying a window having a rectangular shape, while a non-rectangular coding scheme means a coding scheme of applying a window having a non-rectangular shape. 
     Moreover, the first and second coding units  120 - 1  and  120 - 2  are units for applying first and second coding schemes based on different domains, respectively. In this case, the domains can include a linear prediction domain, a frequency domain, a time domain and the like. For instance, the first coding scheme is a coding scheme based on the linear prediction domain and the second coding scheme is a coding scheme based on the frequency domain. And, definitions and properties according to domain types shall be descried in detail later. 
     The encoder  100  is able to include three specific coding units (i.e., A coding unit  120 A, B coding unit  120 B and C coding unit  120 C). For example shown in  FIG. 1 , A coding scheme applied to the A coding unit  120 A is a rectangular coding scheme and corresponds to a first coding scheme. B coding scheme applied to the B coding unit  120 B is a non-rectangular coding scheme and corresponds to a first coding scheme. C coding scheme applied to the C coding unit  120 C is a non-rectangular coding scheme and corresponds to a second coding scheme. As mentioned in the foregoing description, the drawing shown in  FIG. 1  is just exemplary, by which the present invention is non-limited. For clarity and convenience of the following description, the example shown in  FIG. 1  is taken as a reference. 
     Optionally, the A, B and C coding schemes can correspond to ACELP (algebraic code excited linear prediction), TCX (transform coded excitation) and MDCT (modified discrete Fourier transform), respectively, by which the present invention is non-limited. The A, B and C coding schemes shall be described in detail with reference to details of the rectangular coding scheme, the non-rectangular coding scheme, the first coding scheme and the second coding scheme later. 
     The signal classifier  110  analyzes characteristics of an input audio signal and then determines to apply which one of the above-mentioned at least two coding schemes to a current frame or subframe based on the analyzed characteristics. According to the determination, coding scheme information is generated. As mentioned in the foregoing description, the at least two coding schemes correspond to the rectangular and non-rectangular coding schemes, the first and second coding schemes or the A to C coding schemes, by which the present invention is non-limited. 
     For instance, in case of the examples shown in  FIG. 1 , the coding scheme information can include coding identification information and sub-coding identification information. In this case, the coding identification information indicates either the first coding scheme or the second coding scheme for a current frame. In case that a current frame corresponds to the first coding scheme, the sub-coding identification information is the information indicating whether the first coding scheme is the A coding scheme or the B coding scheme per frame or subframe. 
     Afterwards, the signal classifier  110  generates the coding scheme information and then delivers it to the multiplexer  130 . 
     Meanwhile, under the control of the signal classifier  110 , the input signal is classified per frame or subframe and is then inputted to the rectangular/non-rectangular coding unit  120 R/ 120 N or the first/second coding unit  120 - 1 / 120 - 2 . In case of the example shown in  FIG. 1 , the input signal is inputted one of the A to C coding units  102 A to  120 C. 
     In case of the example shown in  FIG. 1 , each of the A to C coding units  120 A to  120 C delivers data, which is a result from encoding the input signal by the corresponding coding scheme, to the multiplexer  120 . 
     The multiplexer  130  generates at least bitstream by multiplexing the coding scheme information and the data which is the result of the coding performed by the corresponding unit. 
     Meanwhile, a decoder  200  of the audio signal processing apparatus according to the present invention includes at least two decoding units  220 R and  220 N or  220 - 1  and  220 - 2  and is able to further include a demultiplexer  210 . In this case, the at least two decoding units are components in aspect of decoding to correspond to the former at least two coding units and include a rectangular decoding unit  220 R and a non-rectangular decoding unit  220 N (or a first decoding unit  220 - 1  and a second decoding unit  220 - 2 ), respectively. In a manner similar to that of the encoder  100 , the at least two decoding units can include A to C decoding units  220 A to  220 C, respectively.
         A rectangular coding scheme applied by the rectangular decoding unit  220 R and a non-rectangular coding scheme applied by the non-rectangular decoding unit  220 N are as good as those explained in the foregoing description. And, a first coding scheme applied by the first decoding unit  220 - 1  and a second coding scheme applied by the second decoding unit  220 - 2  are as god as those explained in the foregoing description. As mentioned in the foregoing description, in case that the A to C decoding units  220 A to  220 C are included as shown in  FIG. 1 , A to C coding schemes used by the respective coding units shall be described in detail later.       

     Afterwards, the demultiplexer  210  extracts the coding scheme information and the data per frame or subframe from the at least one bitstream. The extracted data is forwarded to the corresponding decoding unit  220 A,  220 B or  220 C according to the coding scheme information. Finally, each of the decoding units decodes the data by the corresponding decoding scheme to generate an output audio signal. 
     In the following description, embodiments of the audio signal processing apparatus according to the present invention shown in  FIG. 1  are described in order. 
       FIG. 2  is a block diagram of an encoder according to a first embodiment of the present invention, and  FIG. 3  is a block diagram of a decoder according to a first embodiment of the present invention. In particular, the first embodiments relates to an embodiment for compensating such a defect as aliasing and the like when a block encoded by a rectangular coding scheme come in contact with a block encoded by a non-rectangular coding scheme. 
     Referring to  FIG. 2 , like the former encoder  100  shown in  FIG. 1 , an encoder  100 A according to a first embodiment includes a rectangular coding unit  120 R and a non-rectangular coding unit  120 N and is able to further include a multiplexer  130 . In particular, the rectangular coding unit  120 R includes a rectangular scheme coding part  122  and a rectangular scheme synthesis part  124 . And, the non-rectangular coding unit  120 N includes a compensation information generating part  128  and is able to further include a non-rectangular scheme coding part  126 . 
     First of all, an input signal is divided by a unit of block and is then inputted to the rectangular coding unit  120 R or the non-rectangular coding unit  120 N per block. In this case, the block is a unit corresponding to a frame or a subframe. 
     In the following description, a coding scheme per frame (e.g., rectangular coding scheme, non-rectangular coding scheme) is examined with reference to  FIG. 4  and  FIG. 5  and various methods for compensating a defect (e.g., aliasing, etc.) generated from a transition to a heterogeneous coding scheme (e.g., rectangular coding scheme or non-rectangular coding scheme) are described with reference to  FIGS. 6 to 11 .  FIGS. 4 to 11  are preferentially described and the components shown in  FIG. 2  and  FIG. 3  shall be described again. 
       FIG. 4  shows a configuration unit of an audio signal and a coding scheme per configuration unit. 
     Referring to  FIG. 4 , it can be observed that an audio signal is configured with a series of frames including an i th  frame (frame i) and an (i+1) th  frame (frame i+1). In particular, it can be recognized that a single frame includes a plurality of subframes (e.g., 4 subframes). Moreover,  FIG. 4  shows that a different coding scheme is applicable to each frame or subframe. In particular,  FIG. 4  shows an example that there are three kinds of coding schemes [i.e., A coding scheme (ACELP), B coding scheme (TCX) and C coding scheme (FD)]. For instance, a frame can be configured with a plurality of subframes (e.g., 4 subframes). And, the A coding scheme (e.g., ACELP) is applicable per subframe, as shown in an i th  frame shown  FIG. 4  (A). The B coding scheme (e.g., TCX) is applicable to 1 subframe, 2 contiguous subframes and 4 contiguous subframes (i.e., one frame), as shown in an i th  frame of  FIG. 4  (B) and i th  and (i+1) th  frames shown in  FIG. 4  (D). The C coding scheme (e.g., FD) is applicable not by a subframe unit but by a frame unit, as shown in  FIG. 4  (A) and  FIG. 4  (B), by which the present invention is non-limited. 
       FIG. 5  is a diagram for transition to a heterogeneous coding scheme (i.e., rectangular coding scheme and non-rectangular coding scheme). 
     Referring to  FIG. 5  (A- 1 ), a transition in N th  block is made to a rectangular coding scheme and a transition in (N+1) th  block is made to a non-rectangular coding scheme. On the contrary, referring to  FIG. 5  (A- 2 ), a transition in N th  block is made to a non-rectangular coding scheme and a transition in (N+1) th  block is made to a rectangular coding scheme. In this case, a block can correspond to a frame or subframe explained in the foregoing description. Namely, the N th  or (N+1) th  frame or subframe can include a frame or subframe. In particular, total four kinds of combinations (e.g., frame-frame, frame-subframe, subframe-frame and subframe-frame) are possible. 
     The example of the transition from the rectangular coding scheme to the non-rectangular coding scheme, as shown in  FIG. 5  (A- 1 ), can be discovered from the former cases shown in  FIG. 4  (A) to  FIG. 4  (D). 
     As mentioned in the foregoing description with reference to  FIG. 1 , the A coding scheme (ACELP) corresponds to the rectangular coding scheme, while each of the B coding scheme (TCX) and the C coding scheme (FD) corresponds to the non-rectangular coding scheme. The case (i.e.,  FIG. 5  (A- 1 )) of the transition from the A coding scheme (ACELP) to the B coding scheme (TCX) or the C coding scheme (FD) corresponds to one of the parts indicated by dotted line shown in  FIG. 5  (B- 1 ) to  FIG. 5  (B- 4 ). 
     On the contrary, the case [i.e.,  FIG. 5  (A- 2 )] of the transition from the non-rectangular coding scheme to the rectangular coding scheme, i.e., the case of the transition from the B coding scheme (TCX) or the C coding scheme (FD) to the A coding scheme (ACELP) is not indicated in  FIG. 5  (B- 1 ) to  FIG. 5  (B- 4 ) but can be discovered from two or three locations (e.g., 1 st  and 2 nd  blocks in  FIG. 5  (B- 2 ), etc.). 
     Thus, such a defect as aliasing and the like can be generated due to asymmetry from a location at which a rectangular window and a non-rectangular window come in contact with each other. In the following description, a method of compensating this defect is described with reference to  FIGS. 6 to 9 . 
       FIG. 6  is a diagram for characteristics when a rectangular window and a non-rectangular window are overlapped with each other.  FIG. 7  is a diagram for a correction part (CP), an aliasing part (AP) and an uncompensated signal. In particular,  FIG. 6  corresponds to a case that a rectangular window is followed by a non-rectangular window. Yet, a case that a non-rectangular window is followed by a rectangular window in a manner of being overlapped with the following rectangular window shall be explained later in this disclosure. 
     Referring to  FIG. 6 , it can be observed that a rectangular window and a non-rectangular window are overlapped with each other in part. Regarding an audio signal including blocks A to F, a rectangular window is applied to both of the block B and the block C and a non-rectangular window is applied to the blocks C to F. In particular, the rectangular window and the non-rectangular window are overlapped with each other at the block C.  FIG. 6  ( a ) to  FIG. 6  ( d ) show that results from applying windowing, folding, unfolding and windowing to the blocks A to F in order. In this case, each of the windowing, folding, unfolding and windowing is applied to a corresponding block in order for the application of time domain aliasing cancellation (TDAC) in association with a non-rectangular window. 
     Referring to  FIG. 6  ( a ), a rectangular window is applied to each of the block B and the block C (i.e., dotted blocks) and a non-rectangular window is applied to each of the blocks C to F. C(L 1 ) indicates a result from applying a part L 1  of the non-rectangular window to the block C. And, D(L 2 ) indicates a result from applying a part L 2  of the non-rectangular window to the block D. subsequently, if the folding is performed on the non-rectangular window applied result, it results in the blocks shown in  FIG. 6  ( b ). In this case, Er, Dr or the like means that the folding is performed on the corresponding blocks and that the folded blocks are then reversed with reference to a block boundary. Afterwards, the unfolding is performed to result in the diagram shown in  FIG. 6  ( c ). Finally, if a non-rectangular window is applied to the unfolded blocks, the same result as shown in  FIG. 6  ( d ) is generated. 
     In particular, an uncompensated signal corresponding to the block D of the original signal, i.e., a signal acquired as the transmitted data only can be represented as follows.
 
Uncompensated signal=(− Cr ( L   1 ) r+D ( L   2 ))( L   2 )  [Formula 1]
 
     In Formula 1, ‘C’ indicates data corresponding to the block C, ‘D’ indicates data corresponding to the block D, ‘r’ indicates reversion, indicates a result from applying the part L 1  of the non-rectangular window, and indicates a result from applying the part L 2  of the non-rectangular window. 
     In the following description, a method of compensating an uncompensated signal to become identical or similar to an original signal is described with reference to  FIGS. 7 to 9 . First of all, referring to  FIG. 7 , an uncompensated signal corresponding to Formula 1 is shown. 
     Meanwhile, a non-rectangular window has symmetry. Characteristics of the non-rectangular window, as shown in  FIG. 8 , are explained as follows.  FIG. 8  is a diagram for a characteristic of a non-rectangular window with symmetry (i.e., condition for TDAC).
 
 L   i   2   +R   i   2 =1, where  i= 1 or 2
 
 L   1r   =R   2  
 
 L   2r   =R   1   [Formula, 2]
 
     In Formula 2, ‘L 1 ’ indicates a left first part, ‘L 2 ’ indicates a left second part, ‘R 1 ’ indicates a right first part, and ‘R 2 ’ indicates a right second part. 
     Hence, if the above characteristics of the non-rectangular window are applied, Formula 1 can be summarized in the following.
 
Uncompensated signal=(− Cr ( L   1 ) r+D ( L   2 ))( L   2 )= D ( L   2 ) 2   −Cr ( R   2   L   2 ) (because  L   1r   =R   2 )  [Formula 3]
 
     Hence, in order for the uncompensated signal to become equal to the original signal D, i.e., in order to perform a perfect compensation, a needed signal is shown in  FIG. 7  and can be represented as follows. 
     
       
         
           
             
               
                 
                   
                     Needed 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     signal 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     for 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     perfect 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     compensation 
                   
                   = 
                   
                     
                       
                         original 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         signal 
                       
                       - 
                       
                         uncompensated 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         signal 
                       
                     
                     = 
                     
                       D 
                       - 
                       
                         ( 
                         
                           
                             
                               D 
                               ⁡ 
                               
                                 ( 
                                 
                                   L 
                                   2 
                                 
                                 ) 
                               
                             
                             2 
                           
                           - 
                           
                             Cr 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   R 
                                   2 
                                 
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                                   L 
                                   2 
                                 
                               
                               ) 
                             
                           
                         
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                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                     ⁢ 
                     
                       - 
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     Meanwhile, using the characteristics shown in Formula 2, Formula 4-1 can be summarized into the following.
 
Needed signal for perfect compensation= D ( R   2 ) 2+   C ( R   2   L   2 ) (because 1 −L   2   2   =R   2   2 )  [Formula 4-2]
 
     In Formula 4-2, a first term (D(R 2 ) 2 ) corresponds to a correction part and a second term (Cr(R 2 L 2 )) can be named an aliasing part. 
     If homogeneous windows (e.g., non-rectangular window and non-rectangular window) are overlapped with each other, the correction part CP and the aliasing part AP correspond parts to be deleted in a manner of being added by performing time domain aliasing cancellation (TDAC). In other words, since heterogeneous windows (i.e., rectangular window and non-rectangular window) are overlapped with each other, the correction part CP and the aliasing part AP are remaining errors instead of being cancelled. 
     Specifically, the correction part CP corresponds to a part of a current block (e.g., block D) (i.e., a block behind a window crossing point) to which a non-rectangular window (particularly, R 2 ) is applied. And, the aliasing part AP corresponds to a part of a previous block (e.g., block C) (i.e., a block behind a window crossing point) (e.g., a block at which a rectangular window and a non-rectangular block are overlapped with each other) to which a non-rectangular window (particularly, R 2  and L 2 ) is applied. 
     Meanwhile, since a decoder is able to reconstruct a previous block (e.g., block C) using data of the previous block, it is able to generate a prediction of an aliasing part using the reconstructed previous block. This is represented as Formula 5.
 
Prediction of aliasing part= qCr ( R   2   L   2 )  [Formula 5]
 
     Meanwhile, an error of an aliasing part, which is a difference (or a quantization error) between a prediction of the aliasing part and an original aliasing part can be represented as Formula 6.
 
Error of aliasing part= er ( R   2   L   2 )= Cr ( R   2   L   2 )− qCr ( R   2   L   2 )  [Formula 6]
 
     Using Formula 5 and Formula 6, Formula 4-2 is summarized into Formula 7.
 
Needed signal for perfect compensation= D ( R   2 ) 2+   Cr ( R   2   L   2 )= D ( R   2 ) 2 +( qCr+er )( R   2   L   2 )  [Formula 7]
 
     In Formula 7, D(R 2 ) 2  indicates a correction part CP, qCr(R 2 L 2 ) indicates a prediction of an aliasing part AP, and er(R 2 L 2 ) indicates an error of the aliasing part. 
     Hence, the signal needed for perfect compensation is a sum of the correction part CP and the aliasing part AP, as shown in Formula 7. 
     In the following description, three kinds of methods for compensating a correction part CP and an aliasing part AP are explained with reference to  FIG. 9 . 
       FIG. 9  is a diagram for embodiments of a compensation signal for compensating a correction part and/or an aliasing part. 
     Referring to  FIG. 9 , a compensation signal of a first embodiment shown in  FIG. 9  (A) includes a correction part CP and an error of an aliasing part, while a compensation signal of a second embodiment shown in  FIG. 9  (B) includes a correction part CP only. According to a third embodiment shown in  FIG. 9  (B), a compensation signal is not sent to a decoder but a correction part CP and an aliasing part AP are estimated by the decoder.
 
Method  A : Compensation signal= D ( R   2 ) 2   +er ( R   2   L   2 ), where ‘ D ’ is a reconstructed signal.  [Formula 8-1]
 
     In case of a compensation signal according to the first embodiment, as mentioned in the foregoing description with reference to Formula 5, a prediction of an aliasing part AP can be obtained by a decoder based on data of a previous block (i.e., a block corresponding to an overlapped part between a rectangular window and a non-rectangular window) without transmission from an encoder to a decoder. Even if a compensation signal includes a correction part CP and an error of an aliasing part, the decoder is able to generate a prediction of the aliasing part. Therefore, it is able to obtain a signal for perfect compensation (cf. Formula 7). According to the first embodiment, it is able to save the number of bits by transmitting an error instead of the aliasing part AP itself. Moreover, it is able to obtain a perfectly compensated signal by compensating the error of the aliasing part AP. 
     According to the second embodiment, a compensation signal includes a signal corresponding to a correction part CP only.
 
Method  B : Compensation signal= D ( R   2 ) 2 , where a reconstructed signal is  D−er ( R   2   L   2 ).  [Formula 8-2]
 
     As mentioned in the foregoing description (or like the first embodiment), a decoder generates a prediction of an aliasing part AP and then obtains a compensated signal using a compensation signal corresponding to a correction part CP together with the prediction. According to the second embodiment, since an error of the aliasing part AP may remain in the compensated signal, a reconstruction rate or a sound quality may be degraded. Yet, a compression ratio of the compensation signal can be raised higher than that of the first embodiment. 
     According to the third embodiment, a compensation signal is not transmitted but a decoder estimates a correction part CP and an aliasing part AP.
 
Method  C : Compensation signal=Not transmitted, generated compensation signal in the decoder= qCr ( L   2   R   2 )+ D ( R   2 ) 2 , where a reconstructed signal is  D−er ( L   2 )/( R   2 ).  [Formula 8-3]
 
     As mentioned in the foregoing description (or like the first embodiment and the second embodiment), a prediction of an aliasing part AP can be generated by a decoder. 
     Meanwhile, a correction part CP can be generated in a manner of compensating a window shape for a signal corresponding to a current block (e.g., block D). In particular, qCr((L 2 R 2 ) generated using data of the previous block (qC) is added to un-compensated signal like the formula 1. Then D(L 2 ) 2 −er(L 2 R 2 ) is generated, by dividing D(L 2 ) 2 −er(L 2 R 2 ) by (L 2 ) 2  (which may correspond to adding D(R 2 ) 2  to D(L 2 ) 2 −er(L 2 R 2 )), D−er(R 2 )/(L 2 ) is obtained. In formula 8-3, quantized error of current block (block D) is not represented. 
     A reconstruction rate of the third embodiment may be lower than that of the first or second embodiment. Yet, since the third embodiment does not need bits for transmitting a compensation signal at all, a compression ratio of the third embodiment is considerably high. 
       FIG. 10  is a diagram for examples of a non-rectangular window in combination of heterogeneous windows (i.e., rectangular window and non-rectangular window) shown in  FIG. 6 . In the examples of a non-rectangular window, as shown in  FIG. 10  (A) to  FIG. 10  (C), each corner is not rectangular but has an ascending line with a slope. Shapes of non-rectangular windows corresponding to  FIG. 10  (A) to  FIG. 10  (C) can be represented as Table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Total length 
                 Left zero part 
                 Ascending line 
                 Top line 
                 Descending line 
                 Right zero part 
               
               
                   
               
             
            
               
                 (A) 
                 N/4 or 256 
                 0 
                 N/4 or 256 
                 0 
                 N/4 or 256 
                 0 
               
               
                 (B) 
                 N/2 or 512 
                 N/8 or 128 
                 N/4 or 256 
                 N/4 or 256 
                 N/4 or 256 
                 N/8 or 128 
               
               
                 (C) 
                 N or 1024 
                 N3/8 or 384 
                 N/4 or 256 
                 3N/4 or 768 
                 N/4 or 256 
                 N/8 or 128 
               
               
                   
               
            
           
         
       
     
     In Table 1, ‘N’ indicates a frame length and a numeral indicates the number of samples (e.g., ‘256’ indicates 256 samples.). 
     Referring to Table 1 and  FIG. 10 , each of the windows of the three kinds of types can have ascending and descending lines of which widths are set to N/4 and N/4, respectively. In this case, ‘N’ indicates a frame length. 
     Non-rectangular windows shown in  FIG. 10  (A) to  FIG. 10  (C) can respectively correspond to windows in mode  1 , mode  2  and mode  3  of the B coding scheme (e.g., TCX), by which the present invention is non-limited. As mentioned in the foregoing description with reference to  FIG. 4 , the mode  1  corresponds to the window when the B coding scheme is applied to one subframe. The mode  2  corresponds to the window when the B coding scheme is applied to two contiguous subframes. And, the mode  3  corresponds to the window when the B coding scheme is applied to four contiguous subframes, i.e., one frame. 
     In the above description, the examples of the non-rectangular window corresponding to the B coding scheme are explained. Examples of a non-rectangular window corresponding to the C coding scheme (e.g., MDCT) shall be described later together with an audio signal processing apparatus according to a second embodiment. 
       FIG. 11  is a diagram for a case that a rectangular window following a rectangular window is overlapped. In particular,  FIG. 11  shows a case that a rectangular window is overlapped after a non-rectangular window, whereas  FIG. 6  shows a case that a rectangular window is followed by a non-rectangular window. 
     Referring to  FIG. 11  (A), like the case shown in  FIG. 6 , it can be observed that a correction part CP and an aliasing part AP are generated from a block corresponding to a non-rectangular window. Since the block, at which non-rectangular and rectangular windows are overlapped, is not a previous block but a following block unlike  FIG. 6 , it is able to generate a prediction of the aliasing part AP using data of the following block. Moreover, by transmitting one of the examples of the compensation signal described with reference to  FIG. 9 , it is able to solve a defect (i.e., the correction part CP and the aliasing part AP) generated due to the overlapping between the non-rectangular and rectangular windows. 
     Referring to  FIG. 11  (B), an embedding part EP of a rectangular window is embedded as an aliasing part AP in data coded according to a coding scheme corresponding to a non-rectangular window. Assuming that a whole signal corresponding to a rectangular window is set to D and that an embedding part EP is set to C rw , the embedding part EP can be represented as Formula 9.
 
 C   rw   =Cr ( L   1 ) r+D ( R   2 )  [Formula 9]
 
     For reference, the signal is a signal before a decoder applies a window. 
     The embedding part EP (C rw ) can be calculated by a decoder. Instead of coding the whole signal D according to a rectangular coding scheme, transmission can be performed by encoding ‘D−C rw ’ (i.e., a transmission part TP shown in the drawing) only. And, the transmission part TP is represented as Formula 10.
 
 TP=D−Crw=−Cr ( L   1 ) r−D (1 −R   2 )  [Formula 10]
 
     The decoder is able to reconstruct an original signal in a manner of overlapping unfolded data corresponding to a non-rectangular coding scheme with data corresponding to a rectangular coding scheme. 
     In the above description so far, contents for compensating the defect in case of the overlapping of the heterogeneous coding schemes and the heterogeneous windows (i.e., rectangular window and non-rectangular window) are explained in detail with reference to  FIGS. 4 to 11 . In the following description, an audio signal processing apparatus and method according to a first embodiment are explained with reference to  FIG. 2  and  FIG. 3  again. 
     Referring now to  FIG. 2 , explained in the following description is a case that N th  block and (N+1) th  block correspond to a rectangular coding scheme and a non-rectangular coding scheme, respectively. Of course, a reverse case that N th  block and (N+1) th  block correspond to a non-rectangular coding scheme and a rectangular coding scheme, respectively is applicable as mentioned in the foregoing description with reference to  FIG. 10  (A). 
     The rectangular scheme coding part  122  encodes N th  block of an input signal according to a rectangular coding scheme and then delivers the encoded data (for clarity, this data is named a first data) to the rectangular scheme synthesis part  124  an the multiplexer  130 . In this case, as mentioned in the foregoing description, the rectangular coding scheme is the coding scheme for applying a rectangular window. ACELP belongs to the rectangular coding scheme, by which the present invention is non-limited. The rectangular scheme coding part  122  is able to output a result encoded by applying a rectangular window to be block B and the block C by the A coding scheme in  FIG. 6 . 
     The rectangular scheme synthesis part  124  generates a prediction of an aliasing part AP using the encoded data, i.e., the first data. In particular, the rectangular scheme synthesis part  124  generates an output signal by performing decoding with the rectangular coding scheme. For instance, the block C (and the block B) is reconstructed into its original form by the A coding scheme. Using the output signal and the non-rectangular window, the prediction of the aliasing part AP is obtained, In this case, the prediction of the aliasing part AP can be represented as Formula 5. In Formula 5, ‘qC’ indicates the output signal and ‘R 2 L 2 ’ indicates the non-rectangular window. And, the prediction of the aliasing part AP is inputted to the compensation information generating part  128 . 
     The non-rectangular scheme coding part  126  generates an encoded data (for clarity, named a second data) by encoding the (N+1) th  block by the non-rectangular coding scheme. For instance, the second data can correspond to a result from applying the non-rectangular window to the blocks C to F and then folding the blocks. As mentioned in the foregoing description, the non-rectangular coding scheme can correspond to the B coding scheme (e.g., TCX) or the C coding scheme (e.g., MDCT), by which the present invention is non-limited. And, the second data is delivered to the multiplexer  130 . 
     The compensation information generating part  124  generates a compensation signal using the prediction of the aliasing part and an original input signal. In this case, the compensation signal can be generated according to one of the three kinds of the methods shown in  FIG. 9 . In case of using the method A, both of the prediction of the aliasing part and the original input signal are used. In case of using the method B, the original input signal is used only. In case of the method C, the compensation signal is not generated. Each of the three kinds of the methods is applicable to a whole frame or subframes in the same manner. Alternatively, in consideration of a bit efficiency of each frame, a different method is applicable to each frame. Definition and generation process of the compensation signal are explained in the foregoing description with reference to  FIGS. 6 to 9  and shall not be redundantly explained in the following description. Meanwhile, the compensation signal generated by the compensation information generating part  124  is delivered to the multiplexer  130 . 
     The multiplexer  130  generates at least one bitstream by multiplexing the first data (e.g., data of the N th  block), the second data (e.g., data of the (N+1) th  block) and the compensation signal together and then transmits the generated at least one bitstream to an encoder. Of course, like the former multiplexer  130  shown in  FIG. 1 , the latter multiplexer  130  enables coding scheme information and the like to be contained in the corresponding bitstream. 
     Referring to  FIG. 3 , like the former decoder  200  shown in  FIG. 1 , a decoder  200 A according to a first embodiment of the present invention includes a rectangular decoding unit  220 R and a non-rectangular decoding unit  220 N and is able to further include a demultiplexer  210 . In this case, the non-rectangular decoding unit  220 N includes a compensation part  228 . In particular, the rectangular decoding unit  220 R is able to further include a rectangular scheme decoding part  222  and an aliasing prediction part  224 . And, the non-rectangular decoding unit  220 N is able to further include a non-rectangular scheme decoding part  226 . 
     The demultiplexer  210  extracts the first data (e.g., data of the N th  block), the second data (e.g., data of the (N+1) th  block) and the compensation signal from the at least one bitstream. In this case, the compensation signal can correspond to one of the three types described with reference to  FIG. 9 . 
     The rectangular scheme decoding part  222  generates an output signal by decoding the first data by the rectangular coding scheme. This is as good as obtaining the block C (and the block B) shown in  FIG. 6 . 
     Like the rectangular scheme synthesis part  124  shown in  FIG. 2 , the aliasing prediction part  224  generates a prediction of the aliasing part using the output signal and a non-rectangular window. In this case, the prediction of the aliasing part may correspond to Formula 5. 
     The non-rectangular scheme decoding part  226  generates a signal by decoding the second data by the non-rectangular coding scheme. Since the generated signal is the signal before the compensation of aliasing and the like, it corresponds to the uncompensated signal mentioned in the foregoing description. Hence, this signal can be equal to the former signal represented as Formula 1. 
     The compensation part  228  generates a signal reconstructed using the compensation signal delivered from the demultiplexer  210 , the prediction of the aliasing, part obtained by the aliasing prediction part  224  and the uncompensated signal generated by the non-rectangular scheme decoding part  226 . In this case, the reconstructed signal is the same as described with reference to  FIG. 9  and Formulas 8-1 to 8-3. 
     In the following description, an audio signal processing apparatus according to a second embodiment is explained with reference to  FIG. 12  and  FIG. 13 . 
     First of all, regarding the first embodiment, the N th  block corresponds to the rectangular coding scheme (e.g., A coding scheme) and the (N+1) th  block corresponds to the non-rectangular coding scheme (e.g., B coding scheme or C coding scheme), and vice versa. On the contrary, regarding the second embodiment, when (N+1) th  block corresponds to the C coding scheme, a window type of the C coding scheme is changed according to whether N th  block corresponds to a rectangular coding scheme (e.g., A coding scheme). In this case, it is a mater of course that the N th  block and the (N+1) th  block can be switched to each other in order. 
       FIG. 12  is a block diagram of an encoder according to a second embodiment of the present invention. 
     Referring to  FIG. 12 , like the first embodiment, an encoder  100 B according to a second embodiment includes a rectangular coding unit  120 R and a non-rectangular coding unit  120 N. Yet, the non-rectangular coding unit  120 N further includes a window type determining part  127 . The rest of components (i.e., a rectangular scheme coding part  122  and a rectangular scheme synthesis part  124 , a non-rectangular scheme coding part  126  and a compensation information generating part  128 ) have the same functionality of the former components of the same names according to the first embodiments. And, the same parts shall not be described in the following description. 
     In case that a second block (i.e., a current block) is encoded by a non-rectangular coding scheme, the window type determining part  127  determines a type of a window of the second block according to whether a first block (e.g., a previous block, a following block, etc.) is encoded by a rectangular coding scheme. In particular, if the second block is encoded by the C coding scheme belonging to the non-rectangular coding schemes and a window applied to the second block belongs to a transition window class, the window type determining part  127  determines the type (and a shape) of the window of the second block according to whether the first block is encoded by the rectangular coding scheme. Examples of the window type are shown in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Examples of window type in non-rectangular coding scheme (particularly, C coding scheme) 
               
            
           
           
               
               
            
               
                   
                 Window shape 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Previous/ 
                   
                 Width of 
                   
                 Width of 
                   
               
               
                   
                 Window 
                   
                 Name per 
                 following 
                 Left zero 
                 ascending 
                 Top 
                 descending 
                 Right zero 
               
               
                   
                 type 
                 Classification 
                 shape 
                 block 
                 interval 
                 line 
                 line 
                 line 
                 interval 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 Only_long 
                 Non- 
                   
                 Irrespective 
                 0 
                 N 
                 0 
                 N 
                 0 
               
               
                   
                 window 
                 transition 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 window 
                   
                   
                   
                   
                   
                   
                   
               
               
                 2 
                 Long_start 
                 Transition 
                 Steep 
                 C coding 
                 0 
                 N 
                 7N/16 
                 N/8 
                 7N/16 
               
               
                   
                 window 
                 window 
                 long_start 
                 scheme 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 window 
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 Gentle 
                 Rectangular 
                   
                   
                 3N/8 
                 N/4 
                 3N/8 
               
               
                   
                   
                   
                 long_start 
                 window 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 window 
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 3 
                 Shirt 
                 Non- 
                   
                 Irrespective 
                 0 
                 Overlapping of 8 short parts, each 
               
               
                   
                 window 
                 transitional 
                   
                   
                   
                 having ascending and descending line 
               
               
                   
                   
                 window 
                   
                   
                   
                 width set to N/8 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 4 
                 Long_stop 
                 Transition 
                 Steep 
                 C coding 
                 7N/16 
                 N/8 
                 7N/16 
                 N 
                 0 
               
               
                   
                 window 
                 window 
                 long_stop 
                 scheme 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 window 
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 Gentle 
                 Rectangular 
                 3N/8 
                 N/4 
                 3N/8 
                   
                   
               
               
                   
                   
                   
                 long_stop 
                 window 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 window 
                   
                   
                   
                   
                   
                   
               
               
                 5 
                 Stop_start 
                 Transition 
                 Steep- 
                 C coding 
                 7N/16 
                 N/8 
                 7N/8 
                 N/8 
                 7N/16 
               
               
                   
                 window 
                 window 
                 steep 
                 scheme window for 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 stop_start 
                 both previous 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 window 
                 and following 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                   
                 blocks 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 Gentle- 
                 C coding 
                 3N/8 
                 N/4 
                 3N/16 
                 N/8 
                 7N/16 
               
               
                   
                   
                   
                 steep 
                 scheme window 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 stop_start 
                 only for 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 window 
                 following block 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 Steep- 
                 C coding 
                 7N/16 
                 N/8 
                 13N/16 
                 N/4 
                 3N/16 
               
               
                   
                   
                   
                 gentle 
                 scheme window 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 stop_start 
                 only for 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 window 
                 previous block 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 Gentle- 
                 Rectangular 
                 3N/8 
                 N/4 
                 3N/4 
                 N/4 
                 3N/8 
               
               
                   
                   
                   
                 gentle 
                 window for 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 stop_start 
                 both previous 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                 window 
                 and following 
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                   
                 blocks 
               
               
                   
               
               
                 In Table 1, ‘N’ indicates a frame length, 1,024 or 960 samples or the like. 
               
            
           
         
       
     
     Referring to Table 1, 2 nd , 4 th  and 5 th  windows (i.e., a long_start window, a long_stop window and a stop_start window) among total 5 windows belong to a transition window class. The window belonging to the transition window class, as shown in the table, differs in shape according to a previous or following block corresponds to a rectangular window. In case corresponding to a rectangular coding scheme, a width of an ascending or descending line is N/4. Yet, it can be observed that a class of a transition window has a width of an ascending or descending line becomes N/8 in case corresponding to a non-rectangular coding scheme (e.g., C coding scheme). 
       FIG. 13  is a block diagram of a decoder according to a second embodiment of the present invention. 
       FIG. 14  is a diagram of a shape of a transition window according to whether a rectangular coding scheme is applied to a previous block. Although a right non-rectangular shown in  FIG. 14  (A) or  FIG. 14  (B) corresponds to the long_stop window shown in Table 1, it can be replaced by a long_start window or a stop_start window. 
     Referring to  FIG. 14  (A), in case that a previous block corresponds to a rectangular window, an ascending line of a transition window of a current block has a first slope. Referring to  FIG. 14  (B), in case that a previous block does not correspond to a rectangular window (particularly, in case that a previous block corresponds to a window of the C coding scheme), an ascending line of a transition window of a current block has a second slope. In this case, the first slope is gentler than the second slope. And, a width of the first slope can correspond to twice that of the second slope. In particular, the width of the first lope is N/4, while the width, of the second slope is N/8. 
     In other words, the window type determining part  127  preferentially determines a type of a window corresponding to a current block, generates window type information for specifying a specific window applied to the current block (e.g., a frame or subframe) among a plurality of windows (i.e., for indicating a window type), and then delivers the generated window type information to the multiplexer  130 . In case that the type of the window corresponding to the current block is classified into a transition window, the window type determining part  127  determines a shape of a window, and more particularly, a width (and a corresponding top line and a length of a left or right zero part) of an ascending or descending line according to whether a previous or following block corresponds to a rectangular coding scheme and then applies the determined window shape to the current block. 
     Meanwhile, like the former compensation information generating part  128  of the first embodiment, the compensation information generating part  128  generates a compensation signal when heterogeneous windows (e.g., a non-rectangular window and a rectangular window) are overlapped with each other (e.g., the case corresponding to (A) in  FIG. 14 ). 
     As mentioned in the foregoing description, since a defect generated from the heterogeneous windows overlapped with each other can be corrected using the compensation signal, 50% of the heterogeneous windows can be overlapped instead of 100%. Since the heterogeneous windows need not to be overlapped with each other by 100%, it is not necessary to narrow a width of an ascending or descending line of each window classified into a transition window. Therefore, a window can have a slope relatively gentler than that of the case of the 100% overlapping. 
     Referring to  FIG. 13 , in a decoder  200 B according to a second embodiment, a non-rectangular decoding unit  220 N further includes a window shape determining part  127  rather than that of the first embodiment. In the following description, components having the same names of the former components of the first embodiment shall not be explained in detail. 
     In case that a current block or a second block corresponds to a non-rectangular coding scheme (particularly, the C coding scheme), the window shape determining part  127  determines a specific window (i.e., a window type) applied to the current block among a plurality of windows based on the window type information delivered from the demultiplexer  210 . In case that a window of a current block belongs to a transition window class, the window shape determining part  127  determines a shape of a window of the determined window type according to whether a previous/following block (i.e., a first block) is coded by a rectangular coding scheme. In particular, if the previous/following block is encoded by the rectangular coding scheme and a window of the current block belongs to the transition window class, as mentioned in the foregoing description, the window shape is determined to have an ascending or descending line with a first slope gentler than a second slope. For instance, in case of a long_start window, the window shape is determined as a gentle long_start window (having a descending line with a first slope (e.g., N/4) in Table 1. In case of a long_stop window, the window shape is determined as a gentle long_stop window (e.g., an ascending line with a first slope (N/4)). And, in case of a stop_start window, the window shape is determined in the same manner. In this case, as mentioned in the foregoing description, the first slope (e.g., N/4) is gentler than the second slope. In particular, the second slope is a slope of an ascending or descending line of a steep transition window (e.g., a steep long_stop window, etc.). 
     The window type and shape determined in the above manner are delivered to the non-rectangular scheme decoding part  226 . Subsequently, the non-rectangular scheme decoding part  226  generates an uncompensated signal by decoding a current block by the non-rectangular scheme according to the determined window type and shape. 
     Like the first embodiment, in case that the overlapping of heterogeneous windows (e.g., a rectangular window and a non-rectangular window) occurs, the compensation part  228  generates a reconstructed signal using the uncompensated signal and the compensation signal (and the prediction of the aliasing part). 
     In the following description, an audio signal processing apparatus according to a third embodiment is explained with reference to  FIG. 15  and  FIG. 16 . The third embodiment includes the first coding unit  120 - 1 , the second coding unit  120 - 2 , the first decoding unit  220 - 1  and the second decoding unit  220 - 2  in the former audio signal processing apparatus shown in  FIG. 1 . In particular, when a current block (e.g., N th  block) is encoded by a second coding scheme (i.e., C coding scheme), according to whether a following block (e.g., the (N+1) th  block) or a previous block (e.g., the (N-1) th  block) is encoded by a first coding scheme (i.e., A coding scheme or B coding scheme), a shape of a current window corresponding to the current block is determined by the third embodiment. 
       FIG. 15  is a block diagram of an encoder according to a third embodiment of the present invention. 
     Referring to  FIG. 15 , in an encoder  100 C according to a third embodiment, a first coding unit  120 - 1  includes a first scheme coding part  122 - 1  and a second coding unit  120 - 2  includes a second scheme coding part  126 - 2  and a window type determining part  127 - 2 . And, the encoder  100  can further include a multiplexer  130 . In this case, an input signal is inputted to the first coding unit  120 - 1  or the second coding unit  120 - 2  by a unit of block (e.g., a frame, a subframe, etc.). 
     The first scheme coding part  122 - 1  encodes the input signal by a first coding scheme and the second scheme coding part  126 - 2  encodes the input signal by a second coding scheme. In this case, the first and second coding schemes are as good as those described with reference to  FIG. 1 . In particular, the first coding scheme is a linear prediction domain based coding scheme and the second coding scheme can correspond to a frequency domain based scheme. Meanwhile, as mentioned in the foregoing description with reference to  FIG. 1 , the first coding scheme can include the A coding scheme (e.g., ACELP) corresponding to the rectangular window scheme and the B coding scheme (e.g., TCX) corresponding to the non-rectangular window scheme and the second coding scheme can include the C coding scheme (e.g., MDCT) corresponding to the non-rectangular window scheme. 
     In case that the input signal corresponds to the second coding scheme, the window type determining part  127 - 2  determines a window type and shape of a current block with reference to a characteristic (and a window type) of a previous or following block, generates window type information indicating the window type corresponding to the current block (frame or subframe), and then delivers the generated window type information to the multiplexer  130 . 
     In the following description, a window type is explained in detail with reference to Table 1, a window type and shape of a current block according to a coding scheme of a previous/following block are explained with reference to  FIG. 17A  to  FIG. 18 , and the components shown in  FIG. 15  and  FIG. 16  are then explained again. 
     First of all, one example of a window type corresponding to a second coding scheme can be identical to Table 1. Referring to Table 1, windows (e.g., only_long, long_start, short, long_stop and stop_start) of total five types exist. In this case, the only_long window is a window applied to a signal suitable for a long window due to a stationary characteristic of the signal and the short window is a window applied to a signal suitable for a short window due to a transient characteristic of the signal. The long_start window, the long_stop window and the stop_start window, which are classified as transition windows, are necessary for a process of transition to the short window (or a window with a first coding scheme) from the only_long window or a process for transition to the only_long window (or a window with a first coding scheme) from the short window. The stop_start window is the window used if a previous/following frame corresponds to the short window (or a window with a first coding scheme) despite that a long window is suitable for a current block or frame. 
     Shapes of the windows of the five types shown in Table 1 are examined in detail as follows. First of all, each of the only_long, short, and stop_start windows has horizontal symmetry, while the rest of the windows have horizontal asymmetry. The long_start window includes a zero part in a right half only, whereas the long_stop window includes a zero part in a left half only. 
     In the following description, three cases where a window shape of a current frame is determined according to a previous frame or a following frame are explained in detail. 
     The first case, in which a window shape of a current frame is determined according to a following frame when the current frame is a long_start window, is described below with reference to  FIG. 17A  and the second case, in which a window shape of a current frame is determined according to a previous frame when the current frame is a long_stop window, is described below with reference to  FIG. 17B . In addition, the third case, in which a window shape of a current frame is determined according to a previous frame or a following frame when the current frame is a stop_start window, is described below with reference to  FIGS. 17C and 17D . 
     In the first case, when a previous frame is an only_long window and a current frame is a long_start window, a shape of a current long_start window is determined according to whether a following frame corresponds to a short window or a window with a first coding scheme. In particular, a slope of a descending line of the long_start window can vary. A long_start window having a gentle slope of a descending line shall be named a gentle long_start window (cf. a name per shape in Table 1) and a long_start window having a steep slope of a descending line shall be named a steep long_start window. This shall be described in detail with reference to  FIG. 17A  as follows. 
       FIG. 17A  is a diagram of a long_start window combined with a first coding scheme window or a second coding scheme window (short window).  FIG. 17A  (A- 1 )/(A- 2 ) shows a combination between a long_start window and a window of a first coding scheme.  FIG. 17A  (B) shows a combination between a long_start window and a short window. 
     In particular, a window of a first coding scheme shown in  FIG. 17A  (A- 1 ) is a window corresponding to ‘A scheme’ (i.e., rectangular window scheme). And,  FIG. 17A  (A- 2 ) shows a window corresponding to ‘B coding scheme’ (non-rectangular window scheme) in the first coding scheme window. Referring to  FIG. 17A  (A- 1 ) and  FIG. 17A  (A- 2 ), in case that a following frame corresponds to a first coding scheme, a current long_start window includes a descending line having a first slope. Referring to  FIG. 17A  (B), in case that a following frame corresponds to a second coding scheme (i.e., a short window), a current long_start window includes a descending line having a second slope. A width of the first slope can be twice that of the second slope and can correspond to N/4, where ‘N’ is a length of a frame. Besides, the width of the first slope amounts to 256 samples and can correspond to ⅛ of a total length of the long_start window. 
     Like the case shown in  FIG. 17A  (A- 1 ), in case that a rectangular window is overlapped with a long_start window followed by the rectangular window, as mentioned in the foregoing descriptions of the first and second embodiments, it is able to compensate a correction part (CP) and an aliasing part (AP) using a received compensation signal. If this compensation is not performed, the long_start window should be 100% overlapped with the rectangular window. Therefore, in order not to waste bits, a slope of a descending line overlapped with the rectangular window should have been set steep. Yet, as the above-mentioned compensation is enabled, a sound quality avoids being distorted with 50% of the overlapping with the rectangular window. Hence, a slope of the descending line can be maintained as the first slope shown in  FIG. 17A  (A- 1 ). Thus, as the descending line is gently maintained with the first slope, a crossing point between the two windows becomes a point at 3N/2. If 100% of the overlapping is achieved, a crossing point between the two windows should become 3N/2-N/16. In particular, the corresponding crossing point is ahead of that of the case shown in  FIG. 17A  (A- 1 ) by N/16. 
     In other words, in case that a following window is a window corresponding to a first coding scheme, 50% of the overlapping is acceptable. Hence, a descending line of a long_start window is maintained gentle with a first slope. As a result, a location of a crossing point becomes the same location (e.g., a point of 3N/2 from a window start point) if the following window follows the first or second coding scheme or is irrespective of the first or second coding scheme. Thus, as the crossing points become equal to each other, inter-window transition becomes different. This shall be described together with a fourth embodiment later in this disclosure. 
     The case of  FIG. 17A  (A- 2 ) meets a condition of TDAC since a first slope is symmetric with an ascending line of a non-rectangular window of a following frame (more correctly, the first slope is horizontally symmetric with the ascending line or the absolute value of the first slope is equal to the absolute value of the ascending line) when a descending line of a current frame has the first slope. 
     Referring to  FIG. 17A  (B), as a second slope is matched to a slope of an ascending line of a window (short window) corresponding to a following frame (i.e., a second coding scheme), a condition of TDAC is met. In this case, the meaning of ‘being matched’ may indicate that an absolute value of a slope is identical. In particular, a width of a slope of a descending line is N/4 and a width of a slope of an ascending line of a following frame is N/4 as well. 
     The second case is described below with reference to  FIG. 17B . As described above, in the second case, when a following frame is an only_long window and a current frame is a long_stop window, a shape of a current long_stop window is determined according to whether a previous frame corresponds to a second coding scheme (short window) or a window with a first coding scheme. In particular, a slope of an ascending line of the long_stop window can vary. A long_stop window having a gentle slope of an ascending line shall be named a gentle long_stop window (cf. a name per shape in Table 1) and a long_stop window having a steep slope of an ascending line shall be named a steep long_stop window. 
     Similar to  FIG. 17A ,  FIG. 17B  (A- 1 ) illustrates an example in which a rectangular coding scheme among the first coding scheme is applied to a previous frame, (A- 2 ) illustrates an example in which a non-rectangular coding scheme among the first coding scheme is applied to a previous frame, and (B) illustrates an example in which the second coding scheme is applied to a previous frame. An ascending line of a long_stop window of a current frame has a first slope (i.e., the long_stop window of the current frame is a gentle long_stop window) when a previous frame corresponds to the first coding scheme for the same reason as described above with reference to  FIG. 17A . An ascending line of a long_stop window of a current frame has a second slope (i.e., the long_stop window of the current frame is a steep long_stop window) when a previous frame corresponds to the second coding scheme. 
     Here, the first slope is gentler than the second slope. A width of the first slope can be twice that of the second slope and can correspond to N/4, where ‘N’ is a length of a frame. Additionally, the width of the first slope amounts to 256 samples and can correspond to ⅛ of a total length of the long_stop window. 
     The third case is described below with reference to  FIGS. 17C and 17D . As described above, in the third case, when a current frame is a stop_start window, a shape of a current stop_start window is determined according to whether a previous frame or a following frame corresponds to a second coding scheme (short window) or a window with a first coding scheme. In particular, a slope of an ascending line or a descending line of the stop_start window can vary. A stop_start window, which has a gentle slope in both ascending and descending lines, shall be named a gentle-gentle stop_start window (cf. a name per shape in Table 1) and a stop_start window, which has a gentle slope (second slope) in both ascending and descending lines, shall be named a steep-steep stop_start window. A stop_start window, which has a gentle slope (first slope) only in an ascending line, shall be named a gentle-steep stop_start window and a stop_start window, which has a gentle slope (first slope) only in a descending line, shall be named a steep-gentle stop_start window. 
       FIG. 17C  illustrates the case where a slope of a descending line of a stop_start window is determined to be the first slope or the second slope according to a following frame and  FIG. 17D  illustrates the case where a slope of an ascending line of a stop_start window is determined to be the first slope or the second slope according to a previous frame. 
     The stop_start window may follow one of a long_start window, a short window, and a window corresponding to the first coding scheme for a previous frame and may be followed by one of a long_stop window, a short window, and a window corresponding to the first coding scheme for a following frame. 
     Referring to  FIG. 17C , when the first coding scheme is applied to a following frame, a descending line of a stop_start window is determined to be the first slope as shown in (A- 1 ) and (A- 2 ) of  FIG. 17C . That is, one of a gentle-gentle stop_start window or a steep-gentle stop_start window is applied to a current frame for the same reason as described above with reference to  FIGS. 17A and 17B . 
     On the other hand, when a following frame does not correspond to the first coding scheme but corresponds to a short window or a long_stop window (i.e., in the case of (B) of  FIG. 17C ), a descending line is determined to be the second slope. That is, a gentle-steep stop_start window or a steep-steep stop_start window is applied to a current frame. 
     Referring to  FIG. 17D , when the first coding scheme is applied to a following frame, an ascending line of a stop_start window is determined to be the second slope as shown in (A- 1 ) and (A- 2 ) of  FIG. 17C . That is, one of a gentle-gentle stop_start window or a gentel-steep stop_start window is applied to a current frame. 
     On the other hand, when a following frame does not correspond to the first coding scheme but corresponds to a short window or a long_start window (i.e., in the case of (B) of  FIG. 17D ), a descending line is determined to be the second slope. That is, a gentle-steep stop_start window or a steep-steep stop_start window is applied to a current frame. Referring now to Table 1, a short window has a single shape irrespective of a coding scheme of a previous or following block. This is explained with reference to  FIG. 18  as follows.  FIG. 18  is a diagram of a short window overlapped with a first coding scheme window (A) or a second coding scheme window (B). Referring to  FIG. 18  (A- 1 ), a first coding scheme, and more particularly, a rectangular coding scheme (e.g., A coding scheme) appears behind a short window. Referring to  FIG. 18  (A- 2 ), a first coding scheme, and more particularly, a non-rectangular coding scheme (e.g., B coding scheme) appears behind a short window. Here, a short window may follow one of a long_start window or a stop_start window when a previous frame corresponds to the second coding scheme, and a short window may be followed by one of a long_stop window or a stop_start window when a following frame corresponds to the second coding scheme. 
     Irrespective of a case that a short window is overlapped with a window of a first coding scheme following the short window, as shown in  FIG. 18  (A- 1 ) or  FIG. 18  (A- 2 ), or a case that a short window is overlapped with a window (particularly, a long_stop window or a stop_start window) of a second coding scheme following the short window, as shown in  FIG. 18  (B), a slope (cf. ‘slope A’ in the drawing) of a descending line of the short window is identical. Thus, the reason why the short window in the same fixed shape is possible is explained as follows. First of all, as mentioned in the foregoing descriptions of the first and second embodiments, even if a rectangular coding scheme appears behind a short window, it is able to compensate a correction part (CP) and an aliasing part (AP) using a compensation signal [ FIG. 18  (A- 1 )]. A detailed description of compensation using a correction part (CP) and an aliasing part (AP) is omitted herein since it is already described above in the first and second embodiments. Such compensation is possible if overlapping of 50% or more is achieved. And, a descending line of a last one of 8 short parts (i.e., triangular shapes) included in a short window needs not to have a steep slope as well. Therefore, it is able to maintain a relatively gentle slope (i.e., ‘slope A’) (e.g., width of N/8, where N is a frame length) at the same level of an ascending line, as shown in  FIG. 18  (A- 1 ) like the case shown in  FIG. 17A  (A- 1 ). Accordingly, it is able to use a short window of the same fixed shape irrespective of whether a following block corresponds to a first or second coding scheme. 
     Meanwhile, if a current frame is a long_stop window and a following frame is an only_long window, a shape of a current long_stop window can be determined according to a previous frame corresponds to a window of a first coding scheme. This shall be explained in detail with reference to a fourth embodiment. 
     Referring now to  FIG. 15 , the window type determining part  127 - 2 , as mentioned in the foregoing description with reference to Table 1, determines a specific window to apply to a current block among of a plurality of windows, generates window type information indicating the determined specific window, and then delivers the generated window type information to the multiplexer. 
     Afterwards, the multiplexer  130  generates at least one stream by multiplexing data (e.g., data of the (N+1) th  or (N−1) th  block) encoded by a first coding scheme, data (e.g., data of N th  block) encoded by a second coding scheme and the window type information together. 
     Referring to  FIG. 16 , a decoder  200 C according to a third embodiment includes a first decoding unit  220 - 1  and a second decoding unit  220 - 2  and is able to further include a demultiplexer  210 . The first decoding unit  220 - 1  includes a first scheme decoding part  222 - 1  and the second decoding unit  20 - 2  includes a second scheme decoding part  226 - 2  and a window shape determining part  227 - 2 . 
     The demultiplexer  210  receives the coding scheme information (e.g., coding identification information and sub-coding identification information) described with reference to  FIG. 1  and then delivers data to the first decoding unit  220 - 1  or the second decoding unit  220 - 2  per block based on the received coding scheme information. Moreover, the demultiplexer  210  extracts the window type information and then delivers it to the second decoding unit  220 - 2 . In this case, the window type information can include information indicating one of the five kinds of window types corresponding to Table 1. Yet, as mentioned in the foregoing description, a window type of a current block can be limited due to a coding scheme or window type of a previous or following block instead of the availability o all of the five kinds of window types. Hence, the window type information may include the information indicating one of two or three kinds of types except unavailable window types instead of indicating one of total five kinds. This transition limitation shall be additionally explained together with a fourth embodiment later. 
     The first scheme decoding part  222 - 1  is a component configured to perform a process reverse to that of the first scheme encoding part  122 - 1 . The first scheme decoding part  222 - 1  generates an output signal [e.g., an output signal of the (N+1) th  block or (N-1) th ] by decoding data by a first coding scheme (e.g., ACELP, TCX, etc.). And, the second scheme decoding part  226 - 2  generates an output signal (e.g., an output signal of N th  block) by decoding data by a second coding scheme (e.g., MDCT, etc.). 
     The window shape determining part  227 - 2  identifies a window type of a current block based on the window type information and then determines a window type among the window types according to a coding scheme of a previous or following block. As mentioned in the foregoing description with reference to  FIG. 17A , in the first case, when a current window is a long_start window and a previous window is an only_long window, a window shape is determined by selecting either a steep long_start window or a gentle long_start window according to whether a following window corresponds to a first coding scheme or a second coding scheme. Also, as mentioned in the foregoing description with reference to  FIGS. 17B to 17D , a current window shape is determined according to whether a previous or a following window corresponds to the first coding scheme in both the second case (i.e., when a current window is a long_stop window) and the third case (i.e., when a current window is a stop_start window). 
     In the example described with reference to  FIG. 18 , if a current block is a short window, a short window of the same shape is determined irrespective of a window type of a following block. 
     Subsequently, the second scheme decoding part  226 - 2  applies the window in the shape determined by the window shape determining part  227 - 2  to the current block. 
     In the following description, a fourth embodiment of the present invention is explained with reference to  FIGS. 19 to 23 . A fourth embodiment of the present invention determines a window shape of a current block according to a coding scheme o a previous block, whereas the third embodiment determines a window shape of a current block according to a coding scheme of a following block. Thus, the fourth embodiment of the present invention is almost identical to the third embodiment of the present invention but just differs from the third embodiment in determining a window shape. And, the redundant description of the same parts shall be omitted from the following description. 
       FIG. 19  is a block diagram of an encoder according to a fourth embodiment of the present invention, and  FIG. 20  is a block diagram of a decoder according to a fourth embodiment of the present invention. 
     Referring to  FIG. 19  and  FIG. 20 , components of an encoder  100 D and a decoder  200 D according to a fourth embodiment of the present invention are almost identical to the respective components of the former encoder and decoder  100 C and  200 C according to the third embodiment of the present invention shown in  FIG. 15  and  FIG. 16  but the fourth embodiment of the present invention differs from the third embodiment of the present invention in that N th  block and (N+1) th  block are encoded by a first coding scheme and a second coding scheme, respectively. Therefore, the former description of the same parts explained with reference to  FIG. 15  and  FIG. 16  shall be substituted for the description of the fourth embodiment of the present invention. 
     A window type determining part  127 - 2  determines a window of a current block in consideration of inter-block window transition. In particular, the window type determining part  127 - 2  determines a window type and shape of a current block [e.g., (N+1) th  block] according to whether a previous block (e.g., N th  block) is coded by a first coding scheme. In particular, in case that a previous block is coded by a first coding scheme, one (e.g., a short window, a long_stop window and a stop_start window) of three types except an only-log window and a long_start window among 5 kinds of types shown in Table 1 is determined as a window type. Thus, without going through a transition window necessary for inter-coding scheme transition in the first coding scheme, it is able to directly move to a short window used in the second coding scheme or a transition window (i.e., a long_stop window or a stop_start window) used for transition between a short window and a long window. 
     Such an inter-window path is shown in  FIG. 21 .  FIG. 21  is a table of inter-window paths or transitions. Referring to  FIG. 21 , a row direction indicates a window corresponding to a previous block, while a column direction indicates a window corresponding to a current block. A part having a mark of circle or star indicates an available window transition path. For instance, in case that a previous block corresponds to an only_long window, an only_long window o a long_start window is available for a current block only. 
     Referring to the star marks, in case that a previous block is a block corresponding to a first coding scheme (e.g., ACELP or TCX), as mentioned in the foregoing description, one of a short window, a long_stop window and a stop_start window can become a window corresponding to a second coding scheme. In particular, it is unnecessary to go through a window (e.g., a window corresponding to 1,152 samples) separately provided for a transition to a second coding scheme from a first coding scheme. This is because a crossing point coincides irrespective of a coding scheme, as mentioned in the foregoing description of the third embodiment. The following description is made with reference to  FIG. 22  and  FIG. 23 . 
       FIG. 22  is a diagram for a case of transition to a long_stop window in a first coding scheme, which corresponds to the star mark ★(1) shown in  FIG. 21 .  FIG. 23  is a diagram for a case of transition to a short window in a first coding scheme, which corresponds to the star mark ★(2) shown in  FIG. 21 . 
     First of all,  FIG. 22  (A) shows a crossing between a window corresponding to a rectangular coding scheme (e.g., ACELP) belonging to a first coding scheme and a long_stop window.  FIG. 22  (B) shows a crossing between a window corresponding to a non-rectangular coding scheme (e.g., TCX) belonging to a first coding scheme and a long_stop window. In both  FIG. 22  (A) and  FIG. 22  (B), it can be observed that a transition to a long_stop window from a block corresponding to a first coding scheme is possible. 
     Since a rectangular window is shown in  FIG. 22  (A), as mentioned in the foregoing description of the first or second embodiment, it is able to compensate a correction part (CP) and an aliasing part (AP), which are errors caused by the overlapping between a rectangular window and a non-rectangular window. Hence, 50% of the overlapping is enough and an ascending line of a long_stop window, as mentioned in the foregoing description with reference to  FIG. 14  (A), can have a gentle slope (e.g., N/4 width). Accordingly, since an inter-window crossing point is located in a distance of N/2, a long-sop window corresponding to 1.024 samples or a length of 2N (where N indicates a frame) can be directly connected unlike the case that 100% of the overlapping is required. 
     A third case (i.e., a transition to a stop_start window) is not shown in  FIG. 21 . Like the case of the long_stop window or the short window, a stop_start window corresponds to 1,024 samples or has a length of 2N. In this case, it is ale to make a direct transition to a stop_start window from a window corresponding to a first coding scheme. 
     In case of  FIG. 22  (A), a slope of an ascending line of a long_stop window shall be described in addition to the second embodiment. In case that a current frame and a following frame are a long_stop window and an only_long window, respectively, a shape of a current long_stop window can be determined according to whether a previous frame corresponds to a window of a first coding scheme. This is as good as the former description with reference to  FIG. 14 . In particular, like the case shown in  FIG. 14  (A), in case that a previous frame corresponds to a first coding scheme [e.g., A coding scheme (i.e., a rectangular coding scheme) in  FIG. 14  (A)], an ascending line of a current long_stop window has a first slope. Like the case shown in  FIG. 14  (B), in case that a previous frame corresponds to a second coding scheme [e.g., C coding scheme (i.e., a non-rectangular coding scheme) in  FIG. 14  (B)], an ascending line of a current long_stop window has a second slope. In this case, the first slope is gentler than the second slope. 
     Referring now to the fourth embodiment, as mentioned in the above description with reference to  FIG. 21 , in case that a previous block and a current block correspond to a first coding scheme and a second coding scheme, respectively, one of a short window, a long_stop window and a stop_start window is determined. 
     The window type determining part  127 - 2  shown in  FIG. 19  determines a window type of a current block by referring to coding schemes and window types of previous and following blocks. In doing so, the window type determining part  127 - 2  determines the window type of the current block according to the above-explained path limitation. Occasionally, the window type determining part  12702  determines a shape of a window of the current block as well. Afterwards, the window type determining part  127 - 2  delivers window type information indicating the determined window type to the multiplexer  130 . 
     The second scheme coding part  126 - 2  encodes the current block according to the second coding scheme using the determined window type and shape. And, the multiplexer  130  generates at least one bitstream by multiplexing the data of the previous block, the data of the current block and the window type information of the current block together. 
     Referring to  FIG. 20 , components except the window shape determining part  227 - 2  have functions or roles similar to the former components shown in  FIG. 16  and shall not described in detail in the following description. 
     The window shape determining part  227 - 2  determines a specific window for a current block among a plurality of windows based on window type information. In doing so, it is able to determine one of a plurality of the windows in consideration of the transition limitation shown in  FIG. 21 . This is explained in detail as follows. 
     Referring to  FIG. 21 , if a current block corresponds to a second coding scheme, the total number of kinds of available window types does not exceed 3 according to a window type of a previous block [e.g., 2, 3, 3, 2, 3 and 3 kinds from the top in order]. Hence, the window type information can be encoded with 2 bits. One example of the window type information is shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Window type information 
               
            
           
           
               
               
               
            
               
                   
                   
                 window type info 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 only_long window 
                 0 
               
               
                   
                 long_start window 
                 1 
               
               
                   
                 short window 
                 2 
               
               
                   
                 long_stop window 
                 3 
               
               
                   
                 stop_start window 
                 1 
               
               
                   
                   
               
            
           
         
       
     
     If window type information is set to 1, it indicates a long_start window and a stop_start window, i.e., two cases. Meanwhile, according to the transition limitation disclosed in  FIG. 21 , in case that a previous block corresponds to a first coding scheme, a short window, a long_stop window and a stop_start window are available for a current block only. Hence, in the above two cases, the stop_start window is determined as a window of the current block except one case violating the limitation (i.e., a long_start window). 
     The window shape determining part  227 - 2  determines a window shape such as a slope of an ascending line of the current block, a slope of a descending line of the current block and the like based on the coding scheme of the previous or following block, according to the above-determined window type. Thus, the fourth embodiment has been described so far. In the following description, another method for solving a problem of a window transition between a first coding scheme and a second coding scheme is explained with reference to  FIG. 24 . 
       FIG. 24  is a diagram for a case that a first coding scheme window is overlapped with a short window in a new shape. As mentioned in the foregoing description, when a block of a first coding scheme and a block of a second coding scheme are adjacent to each other, it is not possible for the two blocks to be overlapped with each other by 50%. Instead, since the two blocks should be overlapped with each other by 10%, a crossing point is located ahead of a point N/2. In order to solve this problem of mismatch, a transition block having a length of 1,152 should be provided between the block of the first coding scheme and the block of the second coding scheme. In particular, although it is necessary to go over into a short window belonging to the second coding scheme behind the block of the first coding scheme, a long window having a length of 1,152 should be gone through. Therefore, in this case, a long window is applied to a current block that should be processed with a short window and a short window is applied to a following block. Thus, since a current block supposed to be processed with a short window is processed with a long window due to a transition problem, a sound quality becomes distorted. 
     In addition to the long window having the length of 1,152, in case that a short window, which includes total 9 short parts including a short part, having a length of 1,152 is used, as shown in  FIG. 24 , the problem of the sound quality distortion is reduced. Yet, as mentioned in the foregoing description, the short window having the length of 1,152 shown in  FIG. 24  is applicable only if a crossing point variation due to the 50% overlapping and a corresponding direct transition (cf. Third or fourth embodiment) are impossible. 
     In the following description, a fifth embodiment of the present invention is explained with reference to  FIG. 25  and  FIG. 26 . According to the fifth embodiment of the present invention, in case that a current block (e.g., N th  block) corresponds to a non-rectangular coding scheme (e.g., TCX) belonging to a first coding scheme, a window shape of a current block is determined according to whether a previous or following block [e.g., (N−1) th  or (N+1) th  block] corresponds to a short window of a second coding scheme.  FIG. 25  is a block diagram of an encoder according to a fifth embodiment of the present invention. Referring to  FIG. 25 , since an encoder  100 E according to a fifth embodiment of the present invention is almost identical to the former encoder  100 C/ 100 D of the third/fourth embodiment except a mode determining part  123 - 2 , the redundant description shall be omitted from the following description. 
     First of all, when a current block corresponds to a first coding scheme, the mode determining part  123 - 1  identifies whether the current block corresponds to a rectangular coding scheme (e.g., ACELP) or a non-rectangular coding scheme (e.g., TCX). If the current block corresponds to the non-rectangular coding scheme, the mode determining part  123  determines one of modes 1 to 3. As each of the modes 1 to 3 can correspond to a length for applying the non-rectangular scheme thereto, one of a single subframe, two contiguous subframes and four contiguous subframes (i.e., a single frame) can be determined. Moreover, the length can be determined into one of 256 samples, 512 samples and 1,024 samples, as shown in  FIG. 28 . 
     Thus, in case of a non-rectangular coding scheme, after a mode has been determined, a shape of a window of a current block is determined according to whether a window of a previous or following block is a short window. This process is explained in detail with reference to  FIG. 27  and  FIG. 28  as follows. 
       FIG. 27  (A) is a diagram for a case that a window corresponding to a first coding scheme (e.g., TCX) is overlapped with a short window.  FIG. 27  (A) is a diagram for a case that a window corresponding to a first coding scheme (e.g., TCX) is overlapped with or a long_stop window. In particular,  FIG. 27  (A) shows a window corresponding to the mode  1  (cf. Shape  1  and Shape  2  in  FIG. 28 ) among windows of a first coding scheme and  FIG. 27  (B) also shows a window corresponding to the mode  1  (cf. Shape  1  and Shape  2  in  FIG. 28 ) among windows of a first coding scheme. In more particular,  FIG. 27  (A) is identical to  FIG. 23  (B), while  FIG. 27  (B) is identical to  FIG. 22  (B). 
     In case that a window corresponding to a first coding scheme is overlapped with a long_stop window, as shown in  FIG. 27  (B), the window corresponds to Shape  1  and has a descending line of which width is equal to a width (e.g., N/4) of an ascending line of the long_stop window. In particular, a first slope of a descending line of Shape  1  is matched to a slope of an ascending line of a non-short window (e.g., long_stop window) of a next frame. In this case, the meaning of ‘match’ can indicate that an absolute value of a slope is equal. 
     On the contrary, in case that a window corresponding to a first coding scheme is overlapped with a short window, as shown in  FIG. 27  (A), the window corresponds to Shape  2  and has a descending line of which width is equal to a width (e.g., N/5) of an ascending line of the short window. In particular, a second slope of a descending line of Shape  2  is matched to a slope of an ascending line of a short window of a next frame. 
     Thus, a width of a descending or ascending line can vary according to a previous or following block is a short window. By equalizing the width, it is able to met the TDAC condition described with reference to  FIG. 8 , Therefore, the sound quality distortion can be considerable reduced if the TDAC condition is met. 
       FIG. 28  is a table of a window corresponding to a non-rectangular scheme among first coding schemes varying within Shape  1  to Shape  4 . 
     Referring to  FIG. 28 , according to whether a previous block and/or a following block corresponds to a short window, it can be observed that a shape of a window by a non-rectangular scheme belonging to a first coding scheme varies from Shape  1  to Shape  4 . In case that each of the previous block and the following block does not correspond to the short window, Shape  1  indicates a case that a width of an ascending line L and a width of a descending line R correspond to 256 samples (i.e., N/4) and 256 samples (i.e., N/4), respectively. In Shape  2 , since the following block corresponds to the short window only, a width of a descending line R is reduced into 128, a top line M is increased by 64, and a right zero part ZR is increased by 64. In shape  3 , since the previous block corresponds to the short window only, a width of an ascending line L is reduced into 128 only, a length of a left zero part ZL is increased by 64 greater than that of Shape  1 , and a length of a top line M is increased by 64 greater than that of Shape  1 . 
     Shape  4  indicates a case that each of the previous block and the following block corresponds to the short window. In Shape  4 , an ascending line L corresponds to 128 and a descending line R corresponds to 128, irrespective of a mode (e.g., mode  1 , mode  2  and mode  3 ). 
     For reference, windows corresponding to modes 1 to 3 in Shape  1  can be equal to  FIG. 10  (A),  FIG. 10  (B) and  FIG. 10  (C), respectively. 
     Moreover, the previous block corresponds to a last subframe of a previous frame at least and the following block can correspond to a first subframe of a following frame at least. 
     Referring now to  FIG. 25 , when a first coding scheme (particularly, a non-rectangular scheme) is applied, the mode determining part  123 - 1  determines one of a plurality of modes including the modes 1 to 3 shown in  FIG. 28 . Information corresponding to the determined mode can be encoded together with the above-mentioned sub-coding identification information. For instance, if the sub-coding identification information is set to 0, it is able to indicate A coding scheme (i.e., a rectangular coding scheme as a first coding scheme). If the sub-coding identification information is set to 1 to 3, it is able to indicate the modes 1 to 3 of B coding scheme (i.e., a non-rectangular coding scheme as a first coding scheme), respectively. 
     Once the mode is determined, the mode determining part  123 - 1  determines a shape of a window among Shapes 1 to 4 according to whether a previous block and/or a following block corresponds to a short window. 
     And, the multiplexer  123 - 1  generates at least one bitstream by multiplexing the sub-coding identification information, data of the current block and data of the previous or following block together. 
     Referring to  FIG. 26 , the window shape determining part  223 - 2  determines whether a current block is encoded by A coding scheme (i.e., a rectangular coding scheme) or B coding scheme (i.e., a non-rectangular coding scheme) belonging to a first coding scheme using the sub-coding identification information. Moreover, in case of the B coding scheme, using the sub-coding identification information, the window shape determining part  223 - 2  identifies one of the modes 1 to 3. 
     The window shape determining part  223 - 2  determines a shape of a window for the determined mode in a manner of identifying one of the Shapes 1 to 4 by determining whether a previous block and/or a following block corresponds to a short window. 
     The rest of components shall not be described from the following description. 
     An encoder  100 F and a decoder  200 F according to a sixth embodiment of the present invention are described with reference to  FIGS. 29 to 32  as follows. According to the sixth embodiment of the present invention, it is determined whether to perform a long-term prediction (LTP) according to a coding scheme of a previous block. 
       FIG. 29  is a block diagram of an encoder according to a sixth embodiment of the present invention and  FIG. 30  is a block diagram of a decoder according to a sixth embodiment of the present invention. 
     Referring to  FIG. 29  and  FIG. 30 , an encoder  100 F and a decoder  200 F according to a sixth embodiment of the present invention are similar to the former encoder  100 E and the decoder  200 E of the fifth embodiment of the present invention but differ in including a long prediction determining part  121 - 1  and a long prediction control part  221 - 2 . The long prediction determining part  121 - 2  determines whether to perform a long term prediction on a current block according to whether a first coding scheme (e.g., ACELP, TCX) or a second coding scheme (e.g., MDCT) is applied to a previous block. This is explained in detail with reference to  FIG. 31  and  FIG. 32  as follows. 
       FIG. 31  shows examples of a coding scheme per block (frame or subframe).  FIG. 31  (A) to  FIG. 31  (B- 3 ) show examples that a block having a first coding scheme (e.g., ACELP) applied to thereto appears behind a block having a second coding scheme (e.g., MDCT) applied thereto, respectively. Thus, in case that there is a change of a coding scheme [mode switching], efficiency of a long term prediction in the first coding scheme (e.g., ACELP) may be considerably lowered.  FIG. 32  is a diagram for one examples of a signal waveform related to a long term prediction.  FIG. 32  (A) shows an example that a second coding scheme (e.g., MDCT) and a rectangular coding scheme (e.g., ACELP) of a first coding scheme are applied to a previous block and a following block, respectively according to a characteristic of a signal.  FIG. 32  (B) shows one example of a signal of a block corresponding to a first coding scheme and a waveform of a signal as a result of performing a long term prediction (LTP). For a block after a second coding scheme, an original signal exists in a previous memory instead of a residual signal as a result of performing a linear prediction. Since a long term prediction is based on waveform correlation, if the long term prediction is applied to the above case, it is inevitable that coding efficiency is considerably lowered. Referring to  FIG. 32  (B), it can be observed that there is no big difference in waveform between a long term prediction result and an original signal. Therefore, in this case, it is able to save bits allocated to the long term prediction without applying the long term prediction that lowers coding efficiency considerably. 
     Referring to  FIG. 31  (B- 1 ), a long term prediction (LTP) may not be unconditionally applied to a first appearing block (i.e., a first frame) after applying a second coding scheme (e.g., MDCT). Occasionally, referring to  FIG. 31  (B- 2 ), it is able to adaptively apply a long term prediction (LTP). For instance, only if coding efficiency is good in applying a long term prediction (LTP), the long term prediction (LTP) is performed. Thus, in case that the long term prediction is conditionally performed, it is able to set a long term flag (LTP flag) indicating whether a long term prediction (LTP) has been performed. Moreover, referring to  FIG. 31  (B- 3 ), a long term prediction is not performed on blocks (e.g., 2 nd  to fourth blocks) unconditionally as well as a first appearing block or may not be performed thereon conditionally. Thus, in case that a long term prediction is not used conditionally, it is able to set a long term flag for a random block having a small effect of the long term prediction instead of setting a long term flag on a boundary with a block corresponding to a second coding scheme only. For instance, a long term prediction may not be performed in a voiceless part, a mute part or other music parts, in which a pitch does not exist, despite coding by a first coding scheme. 
     Referring now to  FIG. 29 , as mentioned in the foregoing description, the long prediction determining part  121 - 1  determines by a block unit whether to perform a long term prediction, based on a coding scheme of a previous block. If the long term prediction is not performed conditionally, the long term prediction determining part  121 - 1  delivers the long term flag (LTP flag) to the multiplexer  130 . 
     In case of a block corresponding to a first coding scheme, if a long term prediction (LTP) is not performed, the first scheme coding part  122 - 1  generates new information amounting to bits that are saved in case of not performing the long term prediction. Examples of the new information are described as follows. 
     1) It is able to utilize an excitation codebook. In particular, more code books are designed rather than previous codebooks or a dedicated codebook in a size of surplus bits. In case of using the dedicated codebook, an excitation signal is generated by a combination of an excitation by an original codebook and an excitation by an additional codebook. In case of the dedicated codebook, it is possible to use a codebook configured to encode a pitch component well like the functionality of a long term prediction. 
     2) It is able to enhance quantization performance of LPC coefficient by allocating additional bits to a linear prediction coding [LPC]. 
     3) It is able to allocate bits to code a compensation signal (i.e., a signal for compensating correction and aliasing parts generated from the overlapping between a non-rectangular window of a second coding scheme and a rectangular window of a first coding scheme) of the first or second embodiment. 
     4) Transmission amounting to saved bits is not performed. In particular, since a used bit amount is variable as many as a frame in case of audio coding, the saved bits are utilized in other frames. 
     Meanwhile, the first scheme coding part  122 - 1  delivers additional bits to the multiplexer  130  by encoding the new information for a block on which the long term prediction is not performed. 
     Finally, the multiplexer  130  generates at least one bitstream by multiplexing the long term flag (LTP flag), the additional bits corresponding to the new information and data corresponding to each block together. 
     Referring to  FIG. 30 , in case that a long term prediction is not performed conditionally, the demultiplexer  210  extracts the long term flag (LTP flag) and then delivers it to the long term prediction control part  221 - 2 . If the long term prediction is not performed unconditionally in consideration of a coding scheme of a previous block, the long term prediction control part  221 - 2  determines whether the previous block corresponds to a second coding scheme. If the long term prediction is not performed conditionally despite that the coding scheme of the previous block corresponds to the second coding scheme, the long term prediction control part  221 - 2  determines whether to perform the long term prediction based on the long term flag (LTP flag) delivered from the multiplexer  130 . 
     If so, the first scheme decoding part  222 - 1  performs the long term prediction on a block becoming a target of the long term prediction according to the determination made by the long term prediction control part  222 - 1 . In case that additional bits are transmitted, the first scheme decoding part  222 - 1  extracts the new information corresponding to the additional bits and then performs decoding of the corresponding block based on the extracted new information. 
     In the following description, applications of the encoder and decoder according to the present invention described with reference to  FIG. 1  and  FIG. 2  are explained. 
       FIG. 33  is a diagram for an example of an audio signal encoding apparatus to which an encoder according to an embodiment of the present invention is applied, and  FIG. 34  is a diagram for an example of an audio signal decoding apparatus to which a decoder according to an embodiment of the present invention is applied. 
     Referring to  FIG. 33 , an audio signal encoding apparatus  300  includes an encoder  100  according to the present invention and further includes a plural channel encoder  310 , a band extension coding unit  320  and a multiplexer  330 . In this case, the multiplexer  300  can include the former multiplexer  130  described with reference to  FIG. 1 . 
     The plural channel encoder  310  receives a plurality of channel signal (e.g., at least two channel signals) (hereinafter named a multi-channel signal) and then downmixes a plurality of the received channel signal to generate a mono or stereo downmix signal. And, the plural channel encoder  310  generates spatial information required for upmixing the downmix signal into a multi-channel signal. In this case, the spatial information can include channel level difference information, inter-channel correlation information, a channel prediction coefficient, downmix gain information and the like. Optionally, in case that the audio signal encoding apparatus  300  receives a mono signal, the plural channel encoder  310  does not downmix the received mono signal but the mono signal bypasses the plural channel encoder  310 . 
     The band extension encoder  320  is able to generate spectral data corresponding to a low frequency band and extension information for high frequency band extension by applying a band extension scheme to the downmix signal outputted from the plural channel encoder  310 . In particular, spectral data of a partial band of the downmix signal is excluded and the band extension information for reconstructing the excluded data can be generated. 
     The signal generated by the band extension coding unit  320  is inputted to an A coding unit  120 A, a B coding unit  120 B or a C coding unit  120 C according to coding scheme information generated by a signal classifier (not shown in the drawing) (e.g., the former signal classifier  110  shown in  FIG. 1 ). 
     The A to C coding units  10 A to  120 C are identical to the former coding units described with reference to  FIG. 1  and the redundant description shall be omitted from the following description. Additional contents are described as follows. 
     First of all, in case that a specific frame or segment of the downmix signal has a dominant speech characteristic, the A coding unit  120 A encodes the downmix signal by the A coding scheme (i.e., a rectangular coding scheme belonging to a first coding scheme). In this case, the A coding scheme can follow AMR-WB (adaptive multi-rate wideband) standard, by which the present invention is non-limited. Meanwhile, the A coding unit  120 A is able to further use a linear prediction coding (LPC) scheme. In case that a harmonic signal has high redundancy on a time axis, it can be modeled by linear prediction for predicting a current signal from a past signal. In this case, if the linear prediction coding scheme is adopted, coding efficiency can be raised. Meanwhile, the A coding unit  120 A can include a time domain encoder. 
     Secondly, in case that audio and speech characteristics coexist in a specific frame or segment of the downmix signal, the B coding unit  120 B encodes the downmix signal by the B coding scheme (i.e., a non-rectangular coding scheme belonging to the first coding scheme). In this case, the B coding scheme may correspond to TCX (transform coded excitation), by which the present invention is non-limited. In this case, the TCX can include a scheme for performing frequency transform on an excitation signal obtained from performing linear prediction (LPC). In this case, the frequency transform can include MDCT (modified discrete cosine transform). 
     Thirdly, in case that a specific frame or segment of the downmix signal has a dominant audio characteristic, the C coding unit  120 C encodes the downmix signal by the C coding scheme (i.e., a non-rectangular coding scheme belonging to a second coding scheme). In this case, the C coding scheme can follow AAC (advanced audio coding) standard or HE-AAC (high efficiency advanced audio coding) standard, by which the present invention is non-limited. Meanwhile, the C coding unit  120 C can include an MDCT (modified discrete transform) encoder. 
     And, the multiplexer  330  generates at least one bitstream by multiplexing spatial information, band extension information and the signal encoded by each of the A to C coding units  120 A to  120 C together. 
     Referring to  FIG. 34 , an audio signal decoding apparatus  400  includes a demultiplexer  410 , A to C decoding units  220 A to  220 C, a band extension decoding unit  420  and a plural channel decoder  430 . 
     The demultiplexer  410  extracts the data encoded by the A to C coding schemes, the band extension information, the spatial information and the like from an audio signal bitstream. 
     The A to C decoding units  220 A to  220 C correspond to the former A to C encoding units  120 A to  120 C to perform reverse processes thereof, respectively and their details shall be omitted from the following description. 
     The band extension decoding unit  420  reconstructs a high frequency band signal based on the band extension information by performing a band extension decoding scheme on an output signal of each of the A to C decoding units  220 A to  220 C. 
     In case that the decoded audio signal is a downmix signal, the plural channel decoder  430  generates an output channel signal of a multichannel signal stereo signal included) using the spatial information. 
     The audio signal processing apparatus according to the present invention is available for various products to use. Theses products can be mainly grouped into a stand alone group and a portable group. A TV, a monitor, a settop box and the like can be included in the stand alone group. And, a PMP, a mobile phone, a navigation system and the like can be included in the portable group. 
       FIG. 35  shows relations between products, in which an audio signal processing apparatus according to an embodiment of the present invention is implemented. 
     Referring to  FIG. 35 , a wire/wireless communication unit  510  receives a bitstream via wire/wireless communication system. In particular, the wire/wireless communication unit  510  can include at least one of a wire communication unit  510 A, an infrared unit  510 B, a Bluetooth unit  510 C and a wireless LAN unit  510 D. 
     A user authenticating unit  520  receives an input of user information and then performs user authentication. The user authenticating unit  520  can include at least one of a fingerprint recognizing unit  520 A, an iris recognizing unit  520 B, a face recognizing unit  520 C and a voice recognizing unit  520 D. The fingerprint recognizing unit  520 A, the iris recognizing unit  520 B, the face recognizing unit  520 C and the speech recognizing unit  520 D receive fingerprint information, iris information, face contour information and voice information and then convert them into user informations, respectively. Whether each of the user informations matches pre-registered user data is determined to perform the user authentication. 
     An input unit  530  is an input device enabling a user to input various kinds of commands and can include at least one of a keypad unit  530 A, a touchpad unit  530 B and a remote controller unit  530 C, by which the present invention is non-limited. 
     A signal coding unit  540  performs encoding or decoding on an audio signal and/or a video signal, which is received via the wire/wireless communication unit  510 , and then outputs an audio signal in time domain. The signal coding unit  540  includes an audio signal processing apparatus  545 . As mentioned in the foregoing description, the audio signal processing apparatus  545  corresponds to the above-described encoder  100  (first to sixth embodiments included) or the decoder  200  (first to sixth embodiments included). Thus, the audio signal processing apparatus  545  and the signal coding unit including the same can be implemented by at least one or more processors. 
     A control unit  550  receives input signals from input devices and controls all processes of the signal decoding unit  540  and an output unit  560 . In particular, the output unit  560  is an element configured to output an output signal generated by the signal decoding unit  540  and the like and can include a speaker unit  560 A and a display unit  560 B. If the output signal is an audio signal, it is outputted to a speaker. If the output signal is a video signal, it is outputted via a display. 
       FIG. 36  is a diagram for relations of products provided with an audio signal processing apparatus according to an embodiment of the present invention.  FIG. 36  shows the relation between a terminal and server corresponding to the products shown in  FIG. 35 . 
     Referring to  FIG. 36  (A), it can be observed that a first terminal  500 . 1  and a second terminal  500 . 2  can exchange data or bitstreams bi-directionally with each other via the wire/wireless communication units. Referring to  FIG. 36  (B), it can be observed that a server  600  and a first terminal  500 . 1  can perform wire/wireless communication with each other. 
     An audio signal processing method according to the present invention can be implemented into a computer-executable program and can be stored in a computer-readable recording medium. And, multimedia data having a data structure of the present invention can be stored in the computer-readable recording medium. The computer-readable media include all kinds of recording devices in which data readable by a computer system are stored. The computer-readable media include ROM, RAM, CD-ROM, magnetic tapes, floppy discs, optical data storage devices, and the like for example and also include carrier-wave type implementations (e.g., transmission via Internet). And, a bitstream generated by the above mentioned encoding method can be stored in the computer-readable recording medium or can be transmitted via wire/wireless communication network. 
     INDUSTRIAL APPLICABILITY 
     Accordingly, the present invention is applicable to processing and outputting an audio signal. 
     While the present invention has been described and illustrated herein with reference to the preferred embodiments thereof, it will be apparent to those skilled in the art that various modifications and variations can be made therein without departing from the spirit and scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention that come within the scope of the appended claims and their equivalents.