Abstract:
A codec supporting switching between time-domain aliasing cancellation transform coding mode and time-domain coding mode is made less liable to frame loss by adding a further syntax portion to the frames, depending on which the parser of the decoder may select between a first action of expecting the current frame to have, and thus reading forward aliasing cancellation data from the current frame and a second action of not-expecting the current frame to have, and thus not reading forward aliasing cancellation data from the current frame. In other words, while a bit of coding efficiency is lost due to the provision of the new syntax portion, it is merely the new syntax portion which provides for the ability to use the codec in case of a communication channel with frame loss. Without the new syntax portion, the decoder would not be capable of decoding any data stream portion after a loss and will crash in trying to resume parsing. Thus, in an error prone environment, the coding efficiency is prevented from vanishing by the introduction of the new syntax portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of copending International Application No. PCT/EP2011/061521, filed Jul. 7, 2011, which is incorporated herein by reference in its entirety, and additionally claims priority from U.S. Patent Application No. 61/362,547, filed Jul. 8, 2010 and U.S. Patent Application No. 61/372,347, filed Aug. 10, 2010, all of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is concerned with a codec supporting a time-domain aliasing cancellation transform coding mode and a time-domain coding mode as well as forward aliasing cancellation for switching between both modes. 
     It is favorable to mix different coding modes in order to code general audio signals representing a mix of audio signals of different types such as speech, music or the like. The individual coding modes may be adapted for particular audio types, and thus, a multi-mode audio encoder may take advantage of changing the encoding mode over time corresponding to the change of the audio content type. In other words, the multi-mode audio encoder may decide, for example, to encode portions of the audio signal having speech content, using a coding mode especially dedicated for coding speech, and to use another coding mode in order encode different portions of the audio content representing non-speech content such as music. Time-domain coding modes such as codebook excitation linear prediction coding modes, tend to be more suitable for coding speech contents, whereas transform coding modes tend to outperform time-domain coding modes as far as the coding of music is concerned, for example. 
     There have already been solutions for addressing the problem of coping with the coexistence of different audio types within one audio signal. The currently emerging USAC, for example, suggests switching between a frequency domain coding mode largely complying with the AAC standard, and two further linear prediction modes similar to sub-frame modes of the AMR-WB plus standard, namely a MDCT (Modified Discrete Cosine Transformation) based variant of the TCX (TCX=transform coded excitation) mode and an ACELP (adaptive codebook excitation linear prediction) mode. To be more precise, in the AMR-WB+ standard, TCX is based on a DFT transform, but in USAC TCX has a MDCT transform base. A certain framing structure is used in order to switch between FD coding domain similar to AAC and the linear prediction domain similar to AMR-WB+. The AMR-WB+ standard itself uses an own framing structure forming a sub-framing structure relative to the USAC standard. The AMR-WB+ standard allows for a certain sub-division configuration sub-dividing the AMR-WB+ frames into smaller TCX and/or ACELP frames. Similarly, the AAC standard uses a basis framing structure, but allows for the use of different window lengths in order to transform code the frame content. For example, either a long window and an associated long transform length may be used, or eight short windows with associated transformations of shorter length. 
     MDCT causes aliasing. This is, thus, true, at TCX and FD frame boundaries. In other words, just as any frequency domain coder using MDCT, aliasing occurs at the window overlap regions, that is cancelled by the help of the neighbouring frames. That is, for any transitions between two FD frames or between two TCX (MDCT) frames or transition between either FD to TCX or TCX to FD, there is an implicit aliasing cancellation by the overlap/add procedure within the reconstruction at the decoding side. Then, there is no more aliasing after the overlap add. However, in case of transitions with ACELP, there is no inherent aliasing cancellation. Then, a new tool has to be introduced which may be called FAC (forward aliasing cancellation). FAC is to cancel the aliasing coming from the neighbouring frames if they are different from ACELP. 
     In other words, aliasing cancellation problems occur whenever transitions between transform coding mode and time domain coding mode, such as ACELP, occur. In order to perform the transformation from the time domain to the spectral domain as effective as possible. time-domain aliasing cancellation transform coding is used, such as MDCT, i.e. a coding mode using a overlapped transform where overlapping windowed portions of a signal are transformed using a transform according to which the number of transform coefficients per portion is less than the number of samples per portion so that aliasing occurs as far as the individual portions are concerned, with this aliasing being cancelled by time-domain aliasing cancellation, i.e. by adding the overlapping aliasing portions of neighboring re-transformed signal portions. MDCT is such a time-domain aliasing cancellation transform. Disadvantageously, the TDAC (time-domain aliasing cancellation) is not available at transitions between the transform coding (TC) coding mode and the time-domain coding mode. 
     In order to solve this problem, forward aliasing cancellation (FAC) may be used according to which the encoder signals within the data stream additional FAC data within a current frame whenever a change in the coding mode from transform coding to time-domain coding occurs. This, however, necessitates the decoder to compare the coding modes of consecutive frames in order to ascertain as to whether the currently decoded frame comprises FAC data within its syntax or not. This, in turn, means that there may be frames for which the decoder may not be sure as to whether the decoder has to read or parse FAC data from the current frame or not. In other words, in case that one or more frames were lost during transmission, the decoder does not know for the immediately succeeding (received) frames as to whether a coding mode change occurred or not, and as to whether the bit stream of the current frame encoded data contains FAC data or not. Accordingly, the decoder has to discard the current frame and wait for the next frame. Alternatively, the decoder may parse the current frame by performing two decoding trials, one assuming that FAC data is present, and another assuming that FAC data is not present, with subsequently deciding as to whether one of both alternatives fails. The decoding process would most likely make the decoder crash in one of the two conditions. That is, in reality, the latter possibility is not a feasible approach. The decoder should at any time know how to interpret the data and not rely on its own speculation on how to treat the data. 
     SUMMARY 
     According to an embodiment, a decoder for decoding a data stream having a sequence of frames into which time segments of an information signal are coded, respectively, may have a parser configured to parse the data stream, wherein the parser is configured to, in parsing the data stream, read a first syntax portion and a second syntax portion from a current frame; and a reconstructor configured to reconstruct a current time segment of the information signal associated with the current frame based on information acquired from the current frame by the parsing, using a first selected one of a Time-Domain Aliasing Cancellation transform decoding mode and a time-domain decoding mode, the first selection depending on the first syntax portion, wherein the parser is configured to, in parsing the data stream, perform a second selected one of a first action of expecting the current frame to have, and thus reading forward aliasing cancellation data from the current frame and a second action of not-expecting the current frame to have, and thus not reading forward aliasing cancellation data from the current frame, the second selection depending on the second syntax portion, wherein the reconstructor is configured to perform forward aliasing cancellation at a boundary between the current time segment and a previous time segment of a previous frame using the forward aliasing cancellation data. 
     According to another embodiment, an encoder for encoding an information signal into data stream such that the data stream has a sequence of frames into which time segments of the information signal are coded, respectively, may have a constructor configured to code a current time segment of the information signal into information of the current frame using a first selected one of a Time-Domain Aliasing Cancellation transform coding mode and a time-domain coding mode; and an inserter configured to insert the information into the current frame along with a first syntax portion and a second syntax portion, wherein the first syntax portion signals the first selection, wherein the constructor and inserter are configured to determine forward aliasing cancellation data for forward aliasing cancellation at a boundary between the current time segment and a previous time segment of a previous frame and insert the forward aliasing cancellation data into the current frame in case the current frame and the previous frame are encoded using different ones of the Time-Domain Aliasing Cancellation transform coding mode and the time-domain coding mode, and refraining from inserting any forward aliasing cancellation data into the current frame in case the current frame and the previous frame are encoded using equal ones of the Time-Domain Aliasing Cancellation transform coding mode and the time-domain coding mode, wherein the second syntax portion is set depending on as to whether the current frame and the previous frame are encoded using equal or different ones of the Time-Domain Aliasing Cancellation transform coding mode and the time-domain coding mode. 
     According to another embodiment, a method for decoding a data stream having a sequence of frames into which time segments of an information signal are coded, respectively, may have the steps of parsing the data stream, wherein parsing the data stream has reading a first syntax portion and a second syntax portion from a current frame; and reconstructing a current time segment of the information signal associated with the current frame based on information acquired from the current frame by the parsing, using a first selected one of a Time-Domain Aliasing Cancellation transform decoding mode and a time-domain decoding mode, the first selection depending on the first syntax portion, wherein, in parsing the data stream, a second selected one of a first action of expecting the current frame to have, and thus reading forward aliasing cancellation data from the current frame and a second action of not-expecting the current frame to have, and thus not reading forward aliasing cancellation data from the current frame is performed, the second selection depending on the second syntax portion, wherein the reconstructing includes performing forward aliasing cancellation at a boundary between the current time segment and a previous time segment of a previous frame using the forward aliasing cancellation data. 
     According to another embodiment, a method for encoding an information signal into data stream such that the data stream has a sequence of frames into which time segments of the information signal are coded, respectively, may have the steps of coding a current time segment of the information signal into information of the current frame using a first selected one of a Time-Domain Aliasing Cancellation transform encoding mode and a time-domain encoding mode; and inserting the information into the current frame along with a first syntax portion and a second syntax portion, wherein the first syntax portion signals the first selection, determining forward aliasing cancellation data for forward aliasing cancellation at a boundary between the current time segment and a previous time segment of a previous frame and inserting the forward aliasing cancellation data into the current frame in case the current frame and the previous frame are encoded using different ones of the Time-Domain Aliasing Cancellation transform encoding mode and the time-domain encoding mode, and refraining from inserting any forward aliasing cancellation data into the current frame in case the current frame and the previous frame are encoded using equal ones of the Time-Domain Aliasing Cancellation transform encoding mode and the time-domain encoding mode, wherein the second syntax portion is set depending on as to whether the current frame and the previous frame are encoded using equal or different ones of the Time-Domain Aliasing Cancellation transform encoding mode and the time-domain encoding mode. 
     According to another embodiment, a data stream may have a sequence of frames into which time segments of an information signal are coded, respectively, each frame having a first syntax portion, a second syntax portion, and information into which a time segment associated with the respective frame is coded using a first selected one of a Time-Domain Aliasing Cancellation transform coding mode and a time-domain coding mode, the first selection depending on the first syntax portion of the respective frame, wherein each frame includes forward aliasing cancellation data or not depending on the second syntax portion of the respective frame, wherein the second syntax portion indicates that the respective frame has forward aliasing cancellation data of the respective frame and the previous frame are coded using different ones of the Time-Domain Aliasing Cancellation transform coding mode and the time-domain coding mode so that forward aliasing cancellation using the forward aliasing cancellation data is possible at the boundary between the respective time segment and a previous time segment associated with the previous frame. 
     According to another embodiment, a computer program may have a program code for performing, when running on a computer, a method for decoding a data stream having a sequence of frames into which time segments of an information signal are coded, respectively, which may have the steps of parsing the data stream, wherein parsing the data stream includes reading a first syntax portion and a second syntax portion from a current frame; and reconstructing a current time segment of the information signal associated with the current frame based on information acquired from the current frame by the parsing, using a first selected one of a Time-Domain Aliasing Cancellation transform decoding mode and a time-domain decoding mode, the first selection depending on the first syntax portion, wherein, in parsing the data stream, a second selected one of a first action of expecting the current frame to include, and thus reading forward aliasing cancellation data from the current frame and a second action of not-expecting the current frame to include, and thus not reading forward aliasing cancellation data from the current frame is performed, the second selection depending on the second syntax portion, wherein the reconstructing includes performing forward aliasing cancellation at a boundary between the current time segment and a previous time segment of a previous frame using the forward aliasing cancellation data. 
     According to another embodiment, a computer program may have a program code for performing, when running on a computer, a method for encoding an information signal into data stream such that the data stream has a sequence of frames into which time segments of the information signal are coded, respectively, which may have the steps of coding a current time segment of the information signal into information of the current frame using a first selected one of a Time-Domain Aliasing Cancellation transform encoding mode and a time-domain encoding mode; and inserting the information into the current frame along with a first syntax portion and a second syntax portion, wherein the first syntax portion signals the first selection, determining forward aliasing cancellation data for forward aliasing cancellation at a boundary between the current time segment and a previous time segment of a previous frame and inserting the forward aliasing cancellation data into the current frame in case the current frame and the previous frame are encoded using different ones of the Time-Domain Aliasing Cancellation transform encoding mode and the time-domain encoding mode, and refraining from inserting any forward aliasing cancellation data into the current frame in case the current frame and the previous frame are encoded using equal ones of the Time-Domain Aliasing Cancellation transform encoding mode and the time-domain encoding mode, wherein the second syntax portion is set depending on as to whether the current frame and the previous frame are encoded using equal or different ones of the Time-Domain Aliasing Cancellation transform encoding mode and the time-domain encoding mode. 
     The present invention is based on the finding that a more error robust or frame loss robust codec supporting switching between time-domain aliasing cancellation transform coding mode and time-domain coding mode is achievable if a further syntax portion is added to the frames depending on which the parser of the decoder may select between a first action of expecting the current frame to include, and thus reading forward aliasing cancellation data from the current frame and a second action of not-expecting the current frame to include, and thus not reading forward aliasing cancellation data from the current frame. In other words, while a bit of coding efficiency is lost due to the provision of the second syntax portion, it is merely the second syntax portion which provides for the ability to use the codec in case of a communication channel with frame loss. Without the second syntax portion, the decoder would not be capable of decoding any data stream portion after a loss and will crash in trying to resume parsing. Thus, in an error prone environment, the coding efficiency is prevented from vanishing by the introduction of the second syntax portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be detailed subsequently referring to the appended drawings, in which: 
         FIG. 1  is a schematic block diagram of a decoder according to an embodiment; 
         FIG. 2  is a schematic block diagram of an encoder according to an embodiment; 
         FIG. 3  is a block diagram of a possible implementation of the reconstructor of  FIG. 2 ; 
         FIG. 4  is a block diagram of a possible implementation of the FD decoding module of  FIG. 3 ; 
         FIG. 5  is a block diagram of possible implementation of the linear prediction domain (LPD) decoding modules of  FIG. 3 ; 
         FIG. 6  is schematic diagram illustrating the encoding procedure in order to generate FAC data in accordance with an embodiment; 
         FIG. 7  is a schematic diagram of the possible TDAC transform re-transform in accordance with an embodiment; 
         FIG. 8 ,  9  are block diagrams for illustrating a path lineation of the FAC data at the encoder of a further processing in the encoder in order to test the coding mode change in an optimization sense; 
         FIG. 10 ,  11  are block diagrams showing as to how the decoder handles the data stream in order to derive the FAC data of  FIGS. 8 and 9  from the data stream; 
         FIG. 12  is a schematic diagram of the FAC based reconstruction at the decoding side across from boundaries of frames of different coding mode; 
         FIGS. 13 ,  14  are schematically the processing performed at the transition handler of  FIG. 3  in order to perform the reconstruction of  FIG. 12 ; 
         FIGS. 15 to 19  are portions of a syntax structure in accordance with an embodiment; and 
         FIGS. 20 to 22  are portions of a syntax structure in accordance with another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a decoder  10  according to an embodiment of the present invention. Decoder  10  is for decoding a data stream comprising a sequence of frames  14   a ,  14   b  and  14   c  into which time segments  16   a - c  of an information signal  18  are coded, respectively. As is illustrated in  FIG. 1 , the time segments  16   a  to  16   c  are non-overlapping segments which directly abut each other in time and are sequentially ordered in time. As illustrated in  FIG. 1 , the time segments  16   a  to  16   c  may be of equal size but alternative embodiments are also feasible. Each of the time segments  16   a  to  16   c  is coded into a respective one of frames  14   a  to  14   c . In other words, each time segment  16   a  to  16   c  is uniquely associated with one of frames  14   a  to  14   c  which, in turn, have also an order defined among them, which follows the order of the segments  16   a  to  16   c  which are coded into the frames  14   a  to  14   c , respectively. Although  FIG. 1  suggests that each frame  14   a  to  14   c  is of equal length measured in, for example, coded bits, this is, of course, not mandatory. Rather, the length of frames  14   a  to  14   c  may vary according to the complexity of the time segment  16   a  to  16   c  the respective frame  14   a  to  14   c  is associated with. 
     For ease of explanation of the below-outlined embodiments, it is assumed that the information signal  18  is an audio signal. However, it should be noted that the information signal could also be any other signal, such as a signal output by a physical sensor or the like, such as an optical sensor or the like. In particular, signal  18  may be sampled at a certain sampling rate and the time segments  16   a  to  16   c  may cover immediately consecutive portions of this signal  18  equal in time and number of samples, respectively. A number of samples per time segment  16   a  to  16   c  may, for example, be 1024 samples. 
     The decoder  10  comprises a parser  20  and a reconstructor  22 . The parser  20  is configured to parse the data stream  12  and, in parsing the data stream  12 , read a first syntax portion  24  and a second syntax portion  26  from a current frame  14   b , i.e. a frame currently to be decoded. In  FIG. 1 , it is exemplarily assumed that frame  14   b  is the frame currently to be decoded whereas frame  14   a  is the frame which has been decoded immediately before. Each frame  14   a  to  14   c  has a first syntax portion and a second syntax portion incorporated therein with a significance or meaning thereof being outlined below. In  FIG. 1 , the first syntax portion within frames  14   a  to  14   c  is indicated with a box having a “1” in it and the second syntax portion indicated with a box entitled “2”. 
     Naturally, each frame  14   a  to  14   c  also has further information incorporated therein which is for representing the associated time segment  16   a  to  16   c  in a way outlined in more detail below. This information is indicated in  FIG. 1  by a hatched block wherein a reference sign  28  is used for the further information of the current frame  14   b . The parser  20  is configured to, in parsing the data stream  12 , also read the information  28  from the current frame  14   b.    
     The reconstructor  22  is configured to reconstruct the current time segment  16   b  of the information signal  18  associated with the current frame  14   b  based on the further information  28  using a selected one of the time-domain aliasing cancellation transform decoding mode and a time-domain decoding mode. The selection depends on the first syntax element  24 . Both decoding modes differ from each other by the presence or absence of any transition from spectral domain back to time-domain using a re-transform. The re-transform (along with its corresponding transform) introduces aliasing as far as the individual time segments are concerned which aliasing is, however, compensable by a time-domain aliasing cancellation as far as the transitions at boundaries between consecutive frames coded in the time-domain aliasing cancellation transform coding mode is concerned. The time-domain decoding mode does not necessitate any re-transform. Rather, the decoding remains in time-domain. Thus, generally speaking, the time-domain aliasing cancellation transform decoding mode of reconstructor  22  involves a re-transform being performed by reconstructor  22 . This retransform maps a first number of transform coefficients as obtained from information  28  of the current frame  14   b  (being of the TDAC transform decoding mode) onto a re-transformed signal segment having a sample length of a second number of samples which is greater than the first number thereby causing aliasing. The time-domain decoding mode, in turn, may involve a linear prediction decoding mode according to which the excitation and linear prediction coefficients are reconstructed from the information  28  of the current frame which, in that case, is of the time-domain coding mode. 
     Thus, as became clear from the above discussion, in the time-domain aliasing cancellation transform decoding mode, reconstructor  22  obtains from information  28  a signal segment for reconstructing the information signal at the respective time segment  16   b  by a re-transform. The re-transformed signal segment is longer than the current time segment  16   b  actually is and participates in the reconstruction of the information signal  18  within a time portion which includes and extends beyond time segment  16   b .  FIG. 1  illustrates a transform window  32  used in transforming the original signal or in both, transforming and re-transforming. As can be seen, window  32  may comprise the zero portion  32   1  at the beginning thereof and a zero-portion  32   2  at a trailing end thereof, and aliasing portions  32   3  and  32   4  at a leading and trailing edge of the current time segment  16   b  wherein a non-aliasing portion  32   5  where window  32  is one, may be positioned between both aliasing portions  32   3  and  32   4 . The zero-portions  32   1  and  32   2  are optional. It is also possible that merely one of the zero-portions  32   1  and  32   2  is present. As is shown in  FIG. 1 , the window function may be monotonically increasing/decreasing within the aliasing portions. Aliasing occurs within the aliasing portions  32   3  and  32   4  where window  32  continuously leads from zero to one or these versa. The aliasing is not critical as long as the previous and succeeding time segments are coded in the time-domain aliasing cancellation transform coding mode, too. This possibility is illustrated in  FIG. 1  with respect to the time segment  16   c . A dotted line illustrates a respective transform window  32 ′ for time segment  16   c  the aliasing portion of which coincides with the aliasing portion  32   4  of the current time segment  16   b . Adding the re-transformed segment signals of time segments  16   b  and  16   c  by reconstructor  22  cancels-out the aliasing of both re-transformed signal segments against each other. 
     However, in cases where the previous or succeeding frame  14   a  or  14   c  is coded in the time-domain coding mode, a transition between different coding modes results at the leading or trailing edge of the current time segment  16   b  and, in order to account for respective aliasing, the data stream  12  comprises forward aliasing cancellation data within the respective frame immediately following the transition for enabling the decoder  10  to compensate for the aliasing occurring at this respective transition. For example, it may happen that the current frame  14   b  is of the time-domain aliasing cancellation transform coding mode, but decoder  10  does not know as to whether the previous frame  14   a  was of the time-domain coding mode. For example, frame  14   a  may have got lost during transmission and decoder  10  has no access thereto, accordingly. However, depending on the coding mode of frame  14   a , the current frame  14   b  comprises forward aliasing cancellation data in order to compensate for the aliasing occurring at aliasing portion  32   3  or not. Similarly, if the current frame  14   b  was of the time-domain coding mode, and the previous frame  14   a  has not been received by decoder  10 , then the current frame  14   b  has forward aliasing cancellation data incorporated into it or not depending on the mode of the previous frame  14   a . In particular, if the previous frame  14   a  was of the other coding mode, i.e. time-domain aliasing cancellation transform coding mode, then forward aliasing cancellation data would be present in the current frame  14   b  in order to cancel the aliasing otherwise occurring at boundary between time segments  16   a  and  16   b . However, if the previous frame  14   a  was of the same coding mode, i.e. time-domain coding mode, then parser  20  would not have to expect forward aliasing cancellation data to be present in the current frame  14   b.    
     Accordingly, the parser  20  exploits a second syntax portion  26  in order to ascertain as to whether forward aliasing cancellation data  34  is present in the current frame  14   b  or not. In parsing the data stream  12 , parser  20  may selected one of a first action of expecting the current frame  14   b  to comprise, and thus reading forward aliasing cancellation data  34  from the current frame  14   b  and a second action of not-expecting the current frame  14   b  to comprise, and thus not reading forward aliasing cancellation data  34  from the current frame  14   b , the selection depending on the second syntax portion  26 . If present, the reconstructor  22  is configured to perform forward aliasing cancellation at the boundary between the current time segment  16   b  and the previous time segment  16   a  of the previous frame  14   a  using the forward aliasing cancellation data. 
     Thus, compared to the situation where the second syntax portion is not present, the decoder of  FIG. 1  does not have to discard, or unsuccessfully interrupt parsing, the current frame  14   b  even in case the coding mode of the previous frame  14   a  is unknown to the decoder  10  due to frame loss, for example. Rather, decoder  10  is able to exploit the second syntax portion  26  in order to ascertain as to whether the current frame  14   b  has forward aliasing cancellation data  34  or not. In other words, the second syntax portion provides for a clear criterion on as to whether one of the alternatives, i.e. FAC data for the boundary to the preceding frame being present or not, applies and ensures that any decoder may behave the same irrespective from their implementation, even in case of frame loss. Thus, the above-outlined embodiment introduces mechanisms to overcome the problem of frame loss. 
     Before describing more detailed embodiments further below, an encoder able to generate the data stream  12  of  FIG. 1  is described with the respective  FIG. 2 . The encoder of  FIG. 2  is generally indicated with reference sign  40  and is for encoding the information signal into the data stream  12  such that the data stream  12  comprises the sequence of frames into which the time segments  16   a  to  16   c  of the information signal are coded, respectively. The encoder  40  comprises a constructor  42  and an inserter  44 . The constructor is configured to code a current time segment  16   b  of the information signal into information of the current frame  14   b  using a first selected one of a time-domain aliasing cancellation transform coding mode and a time-domain coding mode. The inserter  44  is configured to insert the information  28  into the current frame  14   b  along with a first syntax portion  24  and a second syntax portion  26 , wherein the first syntax portion signals the first selection, i.e. the selection of the coding mode. The constructor  42 , in turn, is configured to determine forward aliasing cancellation data for forward aliasing cancellation at a boundary between the current time segment  16   b  and a previous time segment  16   a  of a previous frame  14   a  and inserts forward aliasing cancellation data  34  into the current frame  14   b  in case the current frame  14   b  and the previous frame  14   a  are encoded using different ones of a time-domain aliasing cancellation transform coding mode and a time-domain coding mode, and refraining from inserting any forward aliasing cancellation data into the current frame  14   b  in case the current frame  14   b  and the previous frame  14   a  are encoded using equal ones of the time-domain aliasing cancellation transform coding mode and the time-domain coding mode. That is, whenever constructor  42  of encoder  40  decides that it is advantageous, in some optimization sense, to switch from one of both coding modes to the other, constructor  42  and inserter  44  are configured to determine and insert forward aliasing cancellation data  34  into the current frame  14   b , while, if keeping the coding mode between frames  14   a  and  14   b , FAC data  34  is not inserted into the current frame  14   b . In order to enable the decoder to derive from the current frame  14   b , without knowledge of the content of the previous frame  14   a , as to whether FAC data  34  is present within the current frame  14   b  or not, the second syntax portion  26  is set depending on as to whether the current frame  14   b  and the previous frame  14   a  are encoded using equal or different ones of the time-domain aliasing cancellation transform coding mode and the time-domain coding mode. Specific examples for realizing the second syntax portion  26  will be outlined below. 
     In the following, an embodiment is described according to which a codec, a decoder and an encoder of the above described embodiments belong to, supports a special type of frame structure according to which the frames  14   a  to  14   c  themselves are the subject to sub-framing, and two distinct versions of the time-domain aliasing cancellation transform coding mode exist. In particular, according to these embodiments further described below, the first syntax portion  24  associates the respective frame from which same has been read, with a first frame type called FD (frequency domain) coding mode in the following, or a second frame type called LPD coding mode in the following, and, if the respective frame is of the second frame type, associates sub-frames of a sub-division of the respective frame, composed of a number of sub-frames, with a respective one of a first sub-frame type and a second sub-frame type. As will outlined in more detail below, the first sub-frame type may involve the corresponding sub-frames to be TCX coded while the second sub-frame type may involve this respective sub-frames to be coded using ACELP, i.e. Adaptive Codebook Excitation Linear Prediction. Either, any other codebook excitation linear prediction coding mode may be used as well. 
     The reconstructor  22  of  FIG. 1  is configured to handle these different coding mode possibilities. To this end, the reconstructor  22  may be constructed as depicted in  FIG. 3 . According to the embodiment of  FIG. 3 , the reconstructor  22  comprises two switches  50  and  52  and three decoding modules  54 ,  56  and  58  each of which is configured to decode frames and sub-frames of specific type as will be described in more detail below. 
     Switch  50  has an input at which the information  28  of the currently decoded frame  14   b  enters, and a control input via which switch  50  is controllable depending on the first syntax portion  24  of the current frame. Switch  50  has two outputs one of which is connected to the input of decoding module  54  responsible for FD decoding (FD=frequency domain), and the other one of which is connected to the input of sub-switch  52  which has also two outputs one of which is connected to an input decoding module  56  responsible for transform coded excitation linear prediction decoding, and the other one of which is connected to an input of module  58  responsible for codebook excitation linear prediction decoding. All coding modules  54  to  58  output signal segments reconstructing the respective time segments associated with the respective frames and sub-frames from which these signal segments have been derived by the respective decoding mode, and a transition handler  60  receives the signal segments at respective inputs thereof in order to perform the transition handling and aliasing cancellation described above and described in more detail below in order to output at its output of the reconstructed information signal. Transition handler  60  uses the forward aliasing cancellation data  34  as illustrated in  FIG. 3 . 
     According to the embodiment of  FIG. 3 , the reconstructor  22  operates as follows. If the first syntax portion  24  associates the current frame with a first frame type, FD coding mode, switch  50  forwards the information  28  to FD decoding module  54  for using frequency domain decoding as a first version of the time-domain aliasing cancellation transform decoding mode to reconstruct the time segment  16   b  associated with the current frame  14   b . Otherwise, i.e. if the first syntax portion  24  associates the current frame  14   b  with the second frame type, LPD coding mode, switch  50  forwards information  28  to sub-switch  52  which, in turn, operates on the sub-frame structure of the current frame  14 . To be more precise, in accordance with the LPD mode, a frame is divided into one or more sub-frames, the sub-division corresponding to a sub-division of the corresponding time segment  16   b  into un-overlapping sub-portions of the current time segment  16   b  as it will be outlined in more detail below with respect to the following figures. The syntax portion  24  signals for each of the one or more sub-portions as to whether same is associated with a first or a second sub-frame type, respectively. If a respective sub-frame is of the first sub-frame type sub-switch  52  forwards the respective information  28  belonging to that sub-frame to the TCX decoding module  56  in order to use transform coded excitation linear prediction decoding as a second version of the time-domain aliasing cancellation transform decoding mode to reconstruct the respective sub-portion of the current time segment  16   b . If, however, the respective sub-frame is of the second sub-frame type sub-switch  52  forwards the information  28  to module  58  in order to perform codebook excitation linear prediction coding as the time-domain decoding mode to reconstruct the respective sub-portion of the current time signal  16   b.    
     The reconstructed signal segments output by modules  54  to  58  are put together by transition handler  60  in the correct (presentation) time order with performing the respective transition handling and overlap-add and time-domain aliasing cancellation processing as described above and described in more detail below. 
     In particular, the FD decoding module  54  may be constructed as shown in  FIG. 4  and operate as describe below. According to  FIG. 4 , the FD decoding module  54  comprises a de-quantizer  70  and a re-transformer  72  serially connected to each other. As described above, if the current frame  14   b  is an FD frame, same is forwarded to module  54  and the de-quantizer  70  performs a spectral varying de-quantization of transform coefficient information  74  within information  28  of the current frame  14   b  using scale factor information  76  also comprised by information  28 . The scale factors have been determined at encoder side using, for example, psycho acoustic principles so as to keep the quantization noise below the human masking threshold. 
     Re-transformer  72  then performs a re-transform on the de-quantized transform coefficient information to obtain a re-transformed signal segment  78  extending, in time, over and beyond the time segment  16   b  associated with the current frame  14   b . As will be outlined in more detail below, the re-transform performed by re-transformer  72  may be an IMDCT (Inverse Modified Discrete Cosine Transform) involving a DCT IV followed by an unfolding operation wherein after a windowing is performed using a re-transform window which might be equal to, or deviate from, the transform window used in generating the transform coefficient information  74  by performing the afore-mentioned steps in the inverse order, namely windowing followed by a folding operation followed by a DCT IV followed by the quantization which may be steered by psycho acoustic principles in order to keep the quantization noise below the masking threshold. 
     It is worthwhile to note that the amount of transform coefficient information  28  is due to the TDAC nature of the re-transform of re-transformer  72 , lower than the number of samples which the reconstructed signal segment  78  is long. In case of IMDCT, the number of transform coefficients within information  74  is rather equal to the number of samples of time segment  16   b . That is, the underlying transform may be called a critically sampling transform necessitating time-domain aliasing cancellation in order to cancel the aliasing occurring due to the transform at the boundaries, i.e. the leading and trailing edges of the current time segment  16   b.    
     As a minor note it should be noted that similar to the sub-frame structure of LPD frames, the FD frames could be the subject of a sub-framing structure, too. For example, FD frames could be of long window mode in which a single window is used to window a signal portion extending beyond the leading and trailing edge of the current time segment in order to code the respective time segment, or of a short window mode in which the respective signal portion extending beyond the borders of the current time segment of the FD frame is sub-divided into smaller sub-portions each of which is subject to a respective windowing and transform individually. In that case, FD coding module  54  would output a re-transformed signal segment for sub-portion of the current time segment  16   b.    
     After having described a possible implementation of the FD coding module  54 , a possible implementation of the TCX LP decoding module and the codebook excitation LP decoding module  56  and  58 , respectively, is described with respect to  FIG. 5 . In other words,  FIG. 5  deals with the case where the current frame is an LPD frame. In that case, the current frame  14   b  is structured into one or more sub-frames. In the present case a structuring into three sub-frames  90   a ,  90   b  and  90   c  is illustrated. It might be that a structuring is, by default, restricted to certain sub-structuring possibilities. Each of the sub-portions is associated with a respective one of sub-portions  92   a ,  92   b  and  92   c  of the current time segment  16   b . That is, the one or more sub-portions  92   a  to  92   c  gap-less cover, without overlap, the whole time segment  16   b . According to the order of the sub-portions  92   a  to  92   c  within the time segment  16   b , a sequential order is defined among the sub-frames  92   a  to  92   c . As is illustrated in  FIG. 5 , the current frame  14   b  is not completely sub-divided into the sub-frames  90   a  to  90   c . In even other words, some portions of the current frame  14   b  belong to all sub-frames commonly such as the first and second syntax portions  24  and  26 , the FAC data  34  and potentially further data as the LPC information as will be described below in further detail although the LPC information may also be sub-structured into the individual sub-frames. 
     In order to deal with the TCX sub-frames the TCX LP decoding module  56  comprises a spectral weighting derivator  94 , a spectral weighter  96  and a re-transformer  98 . For illustration of purposes, the first sub-frame  90   a  is shown to be a TCX sub-frame, whereas the second sub-frame  90   b  is assumed to be ACELP sub-frame. 
     In order to process the TCX sub-frame  90   a , derivator  94  derives a spectral weighting filter from LPC information  104  within information  28  of the current frame  14   b , and spectral weighter  96  spectrally weights transform coefficient information within the respect of sub-frame  90   a  using the spectral weighting filter received from derivator  94  as shown by arrow  106 . 
     Re-transformer  98 , in turn, re-transforms the spectrally weighted transform coefficient information to obtain a re-transformed signal segment  108  extending, in time t, over and beyond the sub-portion  92   a  of the current time segment. The re-transform performed by re-transformer  98  may be the same as performed by re-transformer  72 . In effect, re-transformer  72  and  98  may have hardware, a software-routine or a programmable hardware portion in common. 
     The LPC information  104  comprised by the information  28  of the current LPD frame  14   b  may represent LPC coefficients of one-time instant within time segment  16   b  or for several time instances within time segment  16   b  such as one set of LPC coefficients for each sub-portion  92   a  to  92   c . The spectral weighting filter derivator  94  converts the LPC coefficients into spectral weighting factors spectrally weighting the transform coefficients within information  90   a  according to a transfer function which is derived from the LPC coefficients by derivator  94  such that same substantially approximates the LPC synthesis filter or some modified version thereof. Any de-quantization performed beyond the spectral weighting by weighter  96 , may be spectrally invariant. Thus, differing from FD decoding mode, the quantization noise according to the TCX coding mode is spectrally formed using LPC analysis. 
     Due to the use of the re-transform, however, the re-transformed signal segment  108  suffers from aliasing. By using the same re-transform, however, re-transform signal segments  78  and  108  of consecutive frames and sub-frames, respectively, may have their aliasing cancelled out by transition handler  60  merely by adding the overlapping portions thereof. 
     In processing the (A)CELP sub-frames  90   b , the excitation signal derivator  100  derives an excitation signal from excitation update information within the respective sub-frame  90   b  and the LPC synthesis filter  102  performs LPC synthesis filtering on the excitation signal using the LPC information  104  in order to obtain an LP synthesized signal segment  110  for the sub-portion  92   b  of the current time segment  16   b.    
     Derivators  94  and  100  may be configured to perform some interpolation in order to adapt the LPC information  104  within the current frame  14   b  to the varying position of the current sub-frame corresponding to the current sub-portion within the current time segment  16   b.    
     Commonly describing  FIGS. 3 to 5 , the various signal segments  108 ,  110  and  78  enter transition handler  60  which, in turn, puts together all signal segments in the correct time order. In particular, the transition handler  60  performs time-domain aliasing cancellation within temporarily overlapping window portions at boundaries between time segments of immediately consecutive ones of FD frames and TCX sub-frames to reconstruct the information signal across these boundaries. Thus, there is no need for forward aliasing cancellation data for boundaries between consecutive FD frames, boundaries between FD frames followed by TCX frames and TCX sub-frames followed by FD frames, respectively. 
     However, the situation changes whenever an FD frame or TCX sub-frame (both representing a transform coding mode variant) precedes an ACELP sub-frame (representing a form of time domain coding mode). In that case, transition handler  60  derives a forward aliasing cancellation synthesis signal from the forward aliasing cancellation data from the current frame and adds the first forward aliasing cancellation synthesis signal to the re-transformed signal segment  100  or  78  of the immediately preceding time segment to re-construct the information signal across respective the boundary. If the boundary falls into the inner of the current time segment  16   b  because a TCX sub-frame and an ACELP sub-frame within the current frame define the boundary between the associated time segment sub-portions, transition handler may ascertain the existence of the respective forward aliasing cancellation data for these transitions from first syntax portion  24  and the sub-framing structure defined therein. The syntax portion  26  is not needed. The previous frame  14   a  may have got lost or not. 
     However, in case of the boundary coinciding with the boundary between consecutive time segments  16   a  and  16   b , parser  20  has to inspect the second syntax portion  26  within the current frame in order to determine as to whether the current frame  14   b  has forward aliasing cancellation data  34 , the FAC data  34  being for cancelling aliasing occurring at the leading end of the current time segment  16   b , because either the previous frame is an FD frame or the last sub-frame of the preceding LPD frame is a TCX sub-frame. At least, parser  20  needs to know syntax portion  26  in case, the content of the previous frame got lost. 
     Similar statements apply for transitions into the other direction, i.e. from ACELP sub-frames to FD frames or TCX frames. As long as the respective boundaries between the respective segments and segment sub-portions fall within the inner of the current time segment, the parser  20  has no problem in determining the existence of the forward aliasing cancellation data  34  for these transitions from the current frame  14   b  itself, namely from the first syntax portion  24 . The second syntax portion is not needed and is even irrelevant. However, if the boundary occurs at, or coincides with, a boundary between the previous time segment  16   a  and the current time segment  16   b , parser  20  needs to inspect the second syntax portion  26  in order to determine as to whether forward aliasing cancellation data  34  is present for the transition at the leading end of the current time segment  16   b  or not—at least in case of having no access to the previous frame. 
     In case of transitions from ACELP to FD or TCX, the transition handler  60  derives a second forward aliasing cancellation synthesis signal from the forward aliasing cancellation data  34  and adds the second forward aliasing cancellation synthesis signal to the re-transformed signal segment within the current time segment in order to reconstruct the information signal across the boundary. 
     After having described embodiments with regard to  FIGS. 3 to 5  which generally referred to an embodiment according to which frames and sub-frames of different coding modes existed, a specific implementation of these embodiments will be outlined in more detail below. The description of these embodiments concurrently includes possible measures in generating the respective data stream comprising such frames and sub-frames, respectively. In the following, this specific embodiment is described as an unified speech and audio codec (USAC) although the principles outlined therein would also be transferrable to other signals. 
     Window switching in USAC has several purposes. It mixes FD frames, i.e. frames encoded with frequency coding, and LPD frames which are, in turn, structured into ACELP (sub-) frames and TCX (sub-)frames. ACELP frames (time-domain coding) apply a rectangular, non-overlapping windowing to the input samples while TCX frames (frequency-domain coding) apply a non-rectangular, overlapping windowing to the input samples and then encode the signal using a time-domain aliasing cancellation (TDAC) transform, namely the MDCT, for example. To harmonize the overall windows, TCX frames may use centered windows with homogeneous shapes and to manage the transitions at ACELP frame boundaries, explicit information for cancelling the time-domain aliasing and windowing effects of the harmonized TCX windows are transmitted. This additional information can be seen as forward aliasing cancellation (FAC). FAC data is quantized in the following embodiment in the LPC weighted domain so that quantization noises of FAC and decoded MDCT are of the same nature. 
       FIG. 6  shows the processing at the encoder in a frame  120  encoded with transform coding (TC) which is preceded and followed by a frame  122 ,  124  encoded with ACELP. In line with the above discussion, the notion of TC includes MDCT over long and short blocks using AAC, as well as MDCT based TCX. That is, frame  120  may either be an FD frame or an TCX (sub-)frame as the sub-frame  90   a ,  92   a  in  FIG. 5 , for example.  FIG. 6  shows time-domain markers and frame boundaries. Frame or time segment boundaries are indicated by dotted lines while the time-domain markers are the short vertical lines along the horizontal axes. It should be mentioned that in the following description the terms “time segment” and “frame” are sometimes used synonymously due to the unique association there between. 
     Thus, the vertical dotted lines in  FIG. 6  show the beginning and end of the frame  120  which may be a sub-frame/time segment subpart or a frame/time segment. LPC 1  and LPC 2  shall indicate the center of an analysis window corresponding to LPC filter coefficients or LPC filters which are used in the following in order to perform the aliasing cancellation. These filter coefficients are derived at the decoder by, for example, the reconstructor  22  or the derivators  94  and  100  by use of interpolation using the LPC information  104  (see  FIG. 5 ). The LPC filters comprise: LPC 1  corresponding to a calculation thereof at the beginning of the frame  120 , and LPC 2  corresponding to a calculation thereof at the end of frame  120 . Frame  122  is assumed to have been encoded with ACELP. The same applies to frame  124 . 
       FIG. 6  is structured into four lines numbered at the right hand side of  FIG. 6 . Each line represents a step in the processing at the encoder. It is to be understood that each line is time alined with the line above. 
     Line  1  of  FIG. 6  represents the original audio signal, segmented in frames  122 ,  120  and  124  as stated above. Hence, at the left of marker “LPC 1 ”, the original signal is encoded with ACELP. Between markers “LPC 1 ” and “LPC 2 ”, the original signal is encoded using TC. As described above, in TC the noise shaping is applied directly in the transform domain rather than in the time domain. To the right of marker LPC 2 , the original signal is again encoded with ACELP, i.e. a time domain coding mode. This sequence of coding modes (ACELP then TC then ACELP) is chosen so as to illustrate the processing in FAC since FAC is concerned with both transitions (ACELP to TC and TC to ACELP). 
     Note, however, that the transitions at LPC 1  and LPC 2  in  FIG. 6  may occur within the inner of a current time segment or may coincide with the leading end thereof. In the first case, the determination of the existence of the associated FAC data may be performed by parser  20  merely based on the first syntax portion  24 , whereas in case of frame loss, parser  20  may need the syntax portion  26  to do so in the latter case. 
     Line  2  of  FIG. 6  corresponds to the decoded (synthesis) signals in each of frames  122 ,  120  and  124 . Accordingly, the reference sign  110  of  FIG. 5  is used within frame  122  corresponding to the possibility that the last sub-portion of frame  122  is an ACELP encoded sub-portion like  92   b  in  FIG. 5 , while a reference sign combination  108 / 78  is used in order to indicated the signal contribution for frame  120 , analogously to  FIGS. 5 and 4 . Again, at the left of marker LPC 1 , the synthesis of that frame  122  is assumed to have been encoded with ACELP. Hence, the synthesis signal  110  at the left of marker LPC 1  is identified as an ACELP synthesis signal. There is, in principle, a high similarity between the ACELP synthesis and the original signal in that frame  122  since ACELP attempts to encode the wave form as accurately as possible. Then, the segment between markers LPC 1  and LPC 2  on line  2  of  FIG. 6  represents the output of the inverse MDCT of that segment  120  as seen at the decoder. Again, segment  120  may be the time segment  16   b  of an FD frame or a sub-portion of a TCX coded sub-frame, such as  90   a  in  FIG. 5 , for example. In the figure, this segment  108 / 78  is named “TC frame output”. In  FIGS. 4 and 5 , this segment was called re-transformed signal segment. In case of frame/segment  120  being a TCX segment sub-part, the TC frame output represents a re-windowed TLP synthesis signal, where TLP stands for “Transform-coding with Linear Prediction” to indicate that in case of TCX, noise shaping of the respective segment is accomplished in the transform domain by filtering the MDCT coefficients using spectral information from the LPC filters LPC 1  and LPC 2 , respectively, what has also been described above with respect to  FIG. 5  with regard to spectral weighter  96 . Note also, that the synthesis signal, i.e. the preliminarily re-constructed signal including the aliasing, between markers “LPC 1 ” and “LPC 2 ” on line  2  of  FIG. 6 , i.e. signal  108 / 78 , contains windowing effects and time-domain aliasing at its beginning and end. In case of MDCT as the TDAC transform, the time-domain aliasing may be symbolized as unfoldings  126   a  and  126   b , respectively. In other words, the upper curve in line  2  of  FIG. 6  which extends from the beginning to the end of that segment  120  and is indicated with reference signs  108 / 78 , shows the windowing effect due to the transform windowing being flat in the middle in order to leave the transformed signal unchanged, but not at the beginning and end. The folding effect is shown by the lower curves  126   a  and  126   b  at the beginning and end of the segment  120  with the minus sign at the beginning of the segment and the plus sign at the end of the segment. This windowing and time-domain aliasing (or folding) effect is inherent to the MDCT which serves as an explicit example for TDAC transforms. The aliasing can be cancelled when two consecutive frames are encoded using the MDCT as it has been described above. However, in case where the “MDCT coded” frame  120  is not preceded and/or followed by other MDCT frames, its windowing and time-domain aliasing is not cancelled and remains in the time-domain signal after the inverse MDCT. Forward aliasing cancellation (FAC) can then be used to correct these effects as has been described above. Finally, the segment  124  after marker LPC 2  in  FIG. 6  is also assumed to be encoded using ACELP. Note that to obtain the synthesis signal in that frame, the filter states of the LPC filter  102  (see  FIG. 5 ), i.e. the memory of long-term and short-term predictors, at the beginning of the frame  124  are to be set properly which implies that the time-aliasing and windowing effects at the end of the previous frame  120  between markers LPC 1  and LPC 2  is to be cancelled by the application of FAC in a specific way which will be explained below. To summarize, line  2  in  FIG. 6  contains the synthesis of preliminary reconstructed signals from the consecutive frames  122 ,  120  and  124 , including the effect of windowing in time-domain aliasing at the output of the inverse MDCT for the frame between markers LPC 1  and LPC 2 . 
     To obtain line  3  of  FIG. 6 , the difference between line  1  of  FIG. 6 , i.e. in the original audio signal  118 , and line  2  of  FIG. 6 , i.e. the synthesis signals  110  and  108 / 78 , respectively, as described above, is computed. This yields a first difference signal  128 . 
     The further processing at the encoder side regarding frame  120  is explained in the following with respect to line  3  of  FIG. 6 . At the beginning of frame  120 , firstly, two contributions taken from the ACELP synthesis  110  at the left of marker LPC 1  on line  2  of  FIG. 6 , are added to each other as follows: 
     The first contribution  130  is a windowed and time-reversed (of folded) version of the last ACELP synthesis samples, i.e. the last samples of signal segment  110  shown in  FIG. 5 . The window length and shape for this time-reversed signal is the same as the aliasing part of the transform window to the left of frame  120 . This contribution  130  can be seen as a good approximation of the time-domain aliasing present in the MDCT frame  120  of line  2  in  FIG. 6 . 
     The second contribution  132  is a windowed zero-input response (ZIR) of the LPC 1  synthesis filter with the initial state taken as the final states of this filter at the end of the ACELP synthesis  110 , i.e. at the end of frame  122 . The window length and shape of this second contribution may be the same as for the first contribution  130 . 
     With new line  3  in  FIG. 6 , i.e. after adding the two contributions  130  and  132  above, a new difference is taken by the encoder to obtain line  4  in  FIG. 6 . Note that the difference signal  134  stops at marker LPC 2 . An approximate view of the expected envelope of the error signal in the time-domain is shown on line  4  in  FIG. 6 . The error in the ACELP frame  122  is expected to be approximately flat in amplitude in the time-domain. Then, the error in the TC frame  120  is expected to exhibit the general shape, i.e. time-domain envelope, as shown in this segment  120  of line  4  in  FIG. 6 . This expected shape of the error amplitude is only shown here for illustration purposes. 
     Note that if the decoder were to use only the synthesis signals of line  3  in  FIG. 6  to produce or reconstruct the decoded audio signal, then the quantization noise would be typically as the expected envelope of the error signal  136  on line  4  of  FIG. 6 . It is thus to be understood that a correction should be sent to the decoder to compensate for this error at the beginning and end of the TC frame  120 . This error comes from the windowing and time-domain aliasing effects inherent to the MDCT/inverse MDCT pair. The windowing and time-domain aliasing have been reduced at the beginning of the TC frame  120  by adding the two contributions  132  and  130  from the previous ACELP frame  122  as stated above, but cannot be completely cancelled as in the actual TDAC operation of consecutive MDCT frames. At the right of the TC frame  120  on line  4  in  FIG. 6  just before marker LPC 2 , all the windowing and time-domain aliasing remains from the MDCT/inverse MDCT pair and has to be, thus, completely cancelled by forward aliasing cancellation. 
     Before proceeding to describe the encoding process in order to obtain the forward aliasing cancellation data, reference is made to  FIG. 7  in order to briefly explain the MDCT as one example of TDAC transform processing. Both transform directions are depicted and described with respect to  FIG. 7 . The transition from time-domain to transform-domain is illustrated in the upper half of  FIG. 7 , whereas the re-transform is depicted in the lower part of  FIG. 7 . 
     In transitioning from the time-domain to transform-domain, the TDAC transform involves a windowing  150  applied to an interval  152  of the signal to be transformed which extends beyond the time segment  154  for which the later resulting transform coefficients are actually be transmitted within the data stream. The window applied in the windowing  150  is shown in  FIG. 7  as comprising an aliasing part L k  crossing the leading end of time segment  154  and an aliasing part R k  at a rear end of time segment  154  with a non-aliasing part M k  extending therebetween. An MDCT  156  is applied to the windowed signal. That is, a folding  158  is performed so as to fold a first quarter of interval  152  extending between the leading end of interval  152  and the leading end of time segment  154  back along the left hand (leading) boundary of time segment  154 . The same is done with regard to aliasing portion R k . Subsequently, a DCT IV  160  is performed on the resulting windowed and folded signal having as much samples as time signal  154  so as to obtain transform coefficients of the same number. A conversion is performed then at  162 . Naturally, the quantization  162  may be seen as being not comprised by the TDAC transform. 
     A re-transform does the reverse. That is, following a de-quantization  164 , an IMDCT  166  is performed involving, firstly, a DCT −1  IV  167  so as to obtain time samples the number of which equals the number of samples of the time segment  154  to be re-constructed. Thereafter, an unfolding process  168  is performed on the inversely transformed signal portion received from module  167  thereby expanding the time interval or the number of time samples of the IMDCT result by doubling the length of the aliasing portions. Then, a windowing is performed at  170 , using a re-transform window  172  which may be same as the one used by windowing  150 , but may also be different. The remaining blocks in  FIG. 7  illustrate the TDAC or overlap/add processing performed at the overlapping portions of consecutive segments  154 , i.e. the adding of the unfolded aliasing portions thereof, as performed by the transition handler in  FIG. 3 . As illustrated in  FIG. 7 , the TDAC by blocks  172  and  174  results in aliasing cancellation. 
     The description of  FIG. 6  is now proceeded further. To efficiently compensate windowing and time-domain aliasing effects at the beginning and end of the TC frame  120  on line  4  of  FIG. 6 , and assuming that the TC frame  120  uses frequency-domain noise shaping (FDNS), forward aliasing correction (FAC) is applied following the processing described in  FIG. 8 . First, it should be noted that  FIG. 8  describes this processing for both, the left part of the TC frame  120  around marker LPC 1 , and for the right part of the TC frame  120  around marker LPC 2 . Recall that the TC frame  120  in  FIG. 6  are assumed to be preceded by an ACELP frame  122  at the LPC 1  marker boundary and followed by an ACELP frame  124  at the LPC 2  marker boundary. 
     To compensate for the windowing and time-domain aliasing effects around marker LPC 1 , the processing is described in  FIG. 8 . First, a weighting filter W(z) is computed from the LPC 1  filter. The weighting filter W(z) might be a modified analysis or whitening filter A(z) of LPC 1 . For example W(z)=A(z/λ) with λ being a predetermined weighting factor. The error signal at the beginning of the TC frame is indicated with reference sign  138  just as it is the case on line  4  of  FIG. 6 . This error is called the FAC target in  FIG. 8 . The error signal  138  is filtered by filter W (z) at  140 , with an initial state of this filter, i.e. with an initial state if its filter memory, being the ACELP error  141  in the ACELP frame  122  on line  4  in  FIG. 6 . The output of filter W(z) then forms the input of a transform  142  in  FIG. 6 . The transform is exemplarily shown to be an MDCT. The transform coefficients output by the MDCT are then quantized and encoded in processing module  143 . These encoded coefficients might form at least a part of the afore-mentioned FAC data  34 . These encoded coefficients may be transmitted to the coding side. The output of process Q, namely the quantized MDCT coefficients, is then the input of an inverse transform such as an IMDCT  144  to form a time-domain signal which is then filtered by the inverse filter 1/W(z) at  145  which has zero-memory (zero initial state). Filtering through 1/W(z) is extended to past the length of the FAC target using zero-input for the samples that extend after the FAC target. The output of filter 1/W(z) is a FAC synthesis signal  146 , which is a correction signal that may now be applied at the beginning of the TC frame  120  to compensate for the windowing and time-domain aliasing effect occurring there. 
     Now, the processing for the windowing and time-domain aliasing correction at the end of the TC frame  120  (before marker LPC 2 ) is described. To this end, reference is made to  FIG. 9 . 
     The error signal at the end of the TC frame  120  on line  4  in  FIG. 6  is provided with reference sign  147  and represents the FAC target in  FIG. 9 . The FAC target  147  is subject to the same process sequence as FAC target  138  of  FIG. 8  with the processing merely differing in the initial state of the weighting filter W(z)  140 . The initial state of filter  140  in order to filter FAC target  147  is the error in the TC frame  120  on line  4  of  FIG. 6 , indicated by reference sign  148  in  FIG. 6 . Then, the further processing steps  142  to  145  are the same as in  FIG. 8  which dealt with the processing of the FAC target at the beginning of the TC frame  120 . 
     The processing in  FIGS. 8 and 9  is performed completely from left to right when applied at the encoder to obtain the local FAC synthesis and to compute the resulting reconstruction in order to ascertain as to whether the change of the coding mode involved by choosing the TC coding mode of frame  120  is the optimum choice or not. At the decoder, the processing in  FIGS. 8 and 9  is only applied from the middle to the right. That is, the encoded and quantized transform coefficients transmitted by processor Q  143  are decoded to form the input of the IMDCT. Look, for example to  FIGS. 10 and 11 .  FIG. 10  equals the right hand side of  FIG. 8  whereas  FIG. 11  equals the right hand side of  FIG. 9 . Transition handler  60  of  FIG. 3  may, in accordance with the specific embodiment outlined now, be implemented in accordance with  FIGS. 10 and 11 . That is, transition handler  60  may subject transform coefficient information within the FAC data  34  present within the current frame  14   b  to a re-transform in order to yield a first FAC synthesis signal  146  in case of transition from an ACELP time segment sub-part to an FD time segment or TCX sup-part, or a second FAC synthesis signal  149  when transitioning from an FD time segment or TCX sub-part of an time segment to an ACELP time segment sub-part. 
     Note again, the FAC data  34  may relate to such a transition occurring inside the current time segment in which case the existence of the FAC data  34  is derivable for parser  20  from solely from syntax portion  24 , whereas parser  20  needs to, in case of the previous frame having got lost, exploit the syntax portion  26  in order to determine as to whether FAC data  34  exists for such transitions at the leading edge of the current time segment  16   b.    
       FIG. 12  shows how to the complete synthesis or reconstructed signal for the current frame  120  can be obtained by using the FAC synthesis signals in  FIGS. 8 to 11  and applying the inverse steps of  FIG. 6 . Note again, that even the steps which are shown now in  FIG. 12 , are also performed by the encoder in order to ascertain as to whether the coding mode for the current frame leads to the best optimization in, for example, rate/distortion sense or the like. In  FIG. 12 , it is assumed that the ACELP frame  122  at the left of marker LPC 1  is already synthesized or reconstructed such as by module  58  of  FIG. 3 , up to marker LPC 1  thereby leading to the ACELP synthesis signal on line  2  of  FIG. 12  with reference sign  110 . Since a FAC correction is also used at the end of the TC frame, it is also assumed that the frame  124  after marker LPC 2  will be an ACELP frame. Then, to produce a synthesis or reconstructed signal in the TC frame  120  between markers LPC 1  and LPC 2  in  FIG. 12 , the following steps are performed. These steps are also illustrated in  FIGS. 13 and 14 , with  FIG. 13  illustrating the steps performed by transition handler  60  in order to cope with transitions from a TC coded segment or segment sub-part to an ACELP coded segment sub-part, whereas  FIG. 14  describes the operation of transition handler for the reverse transitions. 
     1. One step is to decode the MDCT-encoded TC frame and position the thus obtained time-domain signal between markers LPC 1  and LPC 2  as shown in line  2  of  FIG. 12 . Decoding is performed by module  54  or module  56  and includes the inverse MDCT as an example for a TDAC re-transform so that the decoded TC frame contains windowing and time-domain aliasing effects. In other words, the segment or time segment sub-part currently to be decoded and indicated by index k in  FIGS. 13 and 14 , may be an ACELP coded time segment sub-part  92   a  as illustrated in  FIG. 13  or a time segment  16   b  which is FD coded or a TCX coded sub-part  92   a  as illustrated in  FIG. 14 . In case of  FIG. 13 , the previously processed frame is thus a TC coded segment or time segment sub-part, and in case of  FIG. 14 , the previously processed time segment is ACELP coded sub-part. The reconstructions or synthesis signal as output by modules  54  to  58  partially suffer from the aliasing effects. This is also true for the signal segments  78 / 108 . 
     2. Another step in the processing of the transition handler  60  is the generation of the FAC synthesis signal according to  FIG. 10  in case of  FIG. 14 , and in accordance with FIG.  11  in case of  FIG. 13 . That is, transition handler  60  may perform a re-transform  191  onto transform coefficients within the FAC data  34 , in order to obtain the FAC synthesis signals  146  and  149 , respectively. The FAC synthesis signals  146  and  149  are positioned at the beginning and end of the TC coded segment which, in turn, suffers from the aliasing effects and is registered to the time segment  78 / 108 . In case of  FIG. 13 , for example, transition handler  60  positions FAC synthesis signal  149  at the end of the TC coded frame k−1 as also shown in line  1  of  FIG. 12 . In case of  FIG. 14 , transition handler  60  positions the FAC synthesis signal  146  at the beginning of the TC coded frame k as is also shown in line  1  of  FIG. 12 . Note again that frame k is the frame currently to be decoded, and that frame k−1 is the previously decoded frame. 
     3. As far as the situation of  FIG. 14  is concerned where the coding mode change occurs at the beginning of the current TC frame k, the windowed and folded (inverted) ACELP synthesis signal  130  from the ACELP frame k−1 preceding the TC frame k, and the windowed zero-input response, or ZIR, of the LPC 1  synthesis filter, i.e. signal  132 , are positioned so as to be registered to the re-transformed signal segment  78 / 108  suffering from aliasing. This contribution is shown in line  3  of  FIG. 12 . As shown in  FIG. 14  and as already being described above, transition handler  60  obtains aliasing cancellation signal  132  by continuing the LPC synthesis filtering of the preceding CELP sub-frame beyond the leading boundary of the current time segment k and windowing the continuation of signal  110  within the current signal k with both steps being indicated with reference signs  190  and  192  in  FIG. 14 . In order to obtain aliasing cancellation signal  130 , the transition handler  60  also windows in step  194  the reconstructed signal segment  110  of the preceding CELP frame and uses this windowed and time-reversed signal as the signal  130 . 
     4. The contributions of lines  1 ,  2  and  3  of  FIG. 12  and the contributions  78 / 108 ,  132 ,  130  and  146  in  FIG. 14  and contributions  78 / 108 ,  149  and  196  in  FIG. 13 , are added by transition handler  60  in the registered positions explained above, to form the synthesis or reconstructed audio signal for the current frame k in the original domain as shown in line  4  of  FIG. 12 . Note that the processing of  FIGS. 13 and 14  produces a synthesis or reconstructed signal  198  in a TC frame where time-domain aliasing and windowing effects are cancelled at the beginning and end of the frame, and where the potential discontinuity of the frame boundary around marker LPC 1  has been smoothed and perceptually masked by the filter 1/W(z) in  FIG. 12 . 
     Thus,  FIG. 13  pertains the current processing of the CELP coded frame k and leads to forward aliasing cancellation at the end of the preceding TC coded segment. As illustrated at  196 , the finally reconstructed audio signal is aliasing less reconstructed across the boundary between segments k−1 and k. Processing of  FIG. 14  leads to forward aliasing cancellation at the beginning of the current TC coded segment k as illustrated at reference sign  198  showing the reconstructed signal across the boundary between segments k and k−1. The remaining aliasing at the rear end of the current segment k is either cancelled by TDAC in case the following segment is a TC coded segment, or FAC according to  FIG. 13  in case the subsequent segment is ACELP coded segment.  FIG. 13  mentions this latter possibility by assigning reference sign  198  to signal segment of time segment k−1. 
     In the following, specific possibilities will be mentioned as to how the second syntax portion  26  may be implemented. 
     For example, in order to handle the occurrence of lost frames, the syntax portion  26  may be embodied as a 2-bit field prev_mode that signals within the current frame  14   b  explicitly the coding mode that was applied in the previous frame  14   a  according to the following table: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 prev_mode 
                   
                   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 ACELP 
                 0 
                 0 
               
               
                   
                 TCX 
                 0 
                 1 
               
               
                   
                 FD_long 
                 1 
                 0 
               
               
                   
                 FD_short 
                 1 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     With other words, this 2-bit field may be called prev_mode and may thus indicate a coding mode of the previous frame  14   a . In case of the just-mentioned example, four different states are differentiated, namely: 
     1) The previous frame  14   a  is an LPD frame, the last sub-frame of which is an ACELP sub-frame; 
     2) the previous frame  14   a  is an LPD frame, the last sub-frame of which is a TCX coded sub-frame; 
     3) the previous frame is an FD frame using a long transform window and 
     4) the previous frame is an FD frame using short transform windows. 
     The possibility of potentially using different window lengths of FD coding mode has already been mentioned above with respect to the description of  FIG. 3 . Naturally, the syntax portion  26  may have merely three different states and the FD coding mode may merely be operated with a constant window length thereby summarizing the two last ones of the above-listed options 3 and 4. 
     In any case, based on the above-outlined 2-bit field, the parser  20  is able to decide as to whether FAC data for the transition between the current time segment and the previous time segment  16   a  is present within the current frame  14   b  or not. As will be outlined in more detail below, parser  20  and reconstructor  22  are even able to determine based on prev_mode as to whether the previous frame  14   a  has been an FD frame using a long window (FD_long) or as to whether the previous frame has been an FD frame using short windows (FD_short) and as to whether the current frame  14   b  (if the current frame is an LPD frame) succeeds an FD frame or an LPD frame which differentiation is needed according to the following embodiment in order to correctly parse the data stream and reconstruct the information signal, respectively. 
     Thus, in accordance with the just-mentioned possibility of using a 2-bit identifier as the syntax portion  26 , each frame  14   a  to  14   c  would be provided with an additional 2-bit identifier in addition to the syntax portion  24  which defines the coding mode of the current frame to be a FD or LPD coding mode and the sub-framing structure in case of LPD coding mode. 
     For all of the above embodiments, it should be mentioned that other inter-frame dependencies should be avoided as well. For example, the decoder of  FIG. 1  could be capable of SBR. In that case, a crossover frequency could be parsed by parser  20  from every frame  14   a  to  14   c  within the respective SBR extension data instead of parsing such a crossover frequency with an SBR header which could be transmitted within the data stream  12  less frequently. Other inter-frame dependencies could be removed in a similar sense. 
     It is worthwhile to note for all the above-described embodiments, that the parser  20  could be configured to buffer at least the currently decoded frame  14   b  within a buffer with passing all the frames  14   a  to  14   c  through this buffer in a FIFO (first in first out) manner. In buffering, parser  20  could perform the removal of frames from this buffer in units of frames  14   a  to  14   c . That is, the filling and removal of the buffer of parser  20  could be performed in units of frames  14   a  to  14   c  so as to obey the constraints imposed by the maximally available buffer space which, for example, accommodates merely one, or more than one, frames of maximum size at a time. 
     An alternative signaling possibility for syntax portion  26  with reduced bit consumption will be described next. According to this alternative, a different construction structure of the syntax portion  26  is used. In the embodiment described before, the syntax portion  26  was a 2-bit field which is transmitted in every frame  14   a  to  14   c  of the encoded USAC data stream. Since for the FD part it is only important for the decoder to know whether it has to read FAC data from the bit stream in case the previous frame  14   a  was lost, these 2-bits can be divided into two 1-bit flags where one of them is signaled within every frame  14   a  to  14   c  as fac_data_present. This bit may be introduced in the single_channel_element and channel_pair_element structure accordingly as shown in the tables of  FIGS. 15 and 16 .  FIGS. 15 and 16  may be seen as a high level structure definition of the syntax of the frames  14  in accordance with the present embodiment, where functions “function_name( . . . )” call subroutines, and bold written syntax element names indicate the reading of the respective syntax element from the data stream. In other words, the marked portions or hatched portions in  FIGS. 15 and 16  show that each frame  14   a  to  14   c  is, in accordance with this embodiment, provided with a flag fac_data_present. Reference signs  199  show these portions. 
     The other 1-bit flag prev_frame_was_lpd is then only transmitted in the current frame if same was encoded using the LPD part of USAC, and signals whether the previous frame was encoded using the LPD path of the USAC as well. This is shown in the table of  FIG. 17 . 
     The table of  FIG. 17  shows a part of the information  28  in  FIG. 1  in case of the current fame  14   b  being an LPD frame. As shown at  200 , each LPD frame is provided with a flag prev_frame_was_lpd. This information is used to parse the syntax of the current LPD frame. That the content and the position of the FAC data  34  in LPD frames depends on the transition at the leading end of the current LPD frame being a transition between TCX coding mode and CELP coding mode or a transition from FD coding mode to CELP coding mode is derivable from  FIG. 18 . In particular, if the currently decoded frame  14   b  is an LPD frame just preceded by an FD frame  14   a , and fac_data_present signals that FAC data is present in the current LPD frame (because the leading sub-frame is an ACELP sub-frame) then FAC data is read at the end of the LPD frame syntax at  202  with the FAC data  34  including, in that case, a gain factor fac_gain as shown at  204  in  FIG. 18 . With this gain factor, the contribution  149  of  FIG. 13  is gain-adjusted. 
     If, however, the current frame is an LPD frame with the preceding frame being also an LPD frame, i.e. if a transition between TCX and CELP sub-frames occurs between the current frame and the previous frame, FAC data is read at  206  without the gain adjustability option, i.e. without the FAC data  34  including the FAC gain syntax element fac_gain. Further, the position of the FAC data read at  206  differs from the position at which FAC data is read at  202  in case of the current frame being an LPD frame and the previous frame being an FD frame. While the position of reading  202  occurs at the end of the current LPD frame, the reading of the FAC data at  206  occurs before the reading of the sub-frame specific data, i.e. the ACELP or TCX data depending on the modes of the sub-frames of the sub-frames structure, at  208  and  210 , respectively. 
     In the example of  FIGS. 15 to 18 , the LPC information  104  ( FIG. 5 ) is read after the sub-frames specific data such as  90   a  and  90   b  (compare  FIG. 5 ) at  212 . 
     For completeness only, the syntax structure of the LPD frame according to  FIG. 17  is further explained with regard to FAC data potentially additionally contained within the LPD frame in order to provide FAC information with regard to transitions between TCX and ACELP sub-frames in the inner of the current LPD coded time segment. In particular, in accordance with the embodiment of  FIGS. 15 to 18 , the LPD sub-frame structure is restricted to sub-divide the current LPD coded time segment merely in units of quarters with assigning these quarters to either TCX or ACELP. The exact LPD structure is defined by the syntax element lpd_mode read at  214 . The first and the second and the third and the fourth quarter may form together a TCX sub-frame whereas ACELP frames are restricted to the length of a quarter only. A TCX sub-frame may also extend over the whole LPD encoded time segment in which case the number sub-frames is merely one. The while loop in  FIG. 17  steps through the quarters of the currently LPD coded time segment and transmits, whenever the current quarter k is the beginning of a new sub-frame within the inner of the currently LPD coded time segment, FAC data at  216  provided the immediately preceding sub-frame of the currently beginning/decoded LPD frame is of the other mode, i.e. TCX mode if the current sub-frame is of ACELP mode and these versa. 
     For sake of completeness only,  FIG. 19  shows a possible syntax structure of an FD frame in accordance with the embodiment of  FIGS. 15 to 18 . It can be seen that FAC data is read at the end of the FD frame with the decision as to whether FAC data  34  is present or not, merely involving the fac_data_present flag. Compared thereto, parsing of the fac_data  34  in case of LPD frames as shown in  FIG. 17  necessitates, for a correct parsing, the knowledge of the flag prev_frame_was_lpd. 
     Thus, the 1-bit flag prev_frame_was_lpd is only transmitted if the current frame is encoded using the LPD part of USAC and signals whether the previous frame was encoded using the LPD path of the USAC codec (see Syntax of lpd_channel_stream( ) in  FIG. 17 ) 
     Regarding the embodiment of  FIGS. 15 to 19 , it should be further noted, that a further syntax element could be transmitted at  220 , i.e. in the case the current frame is an LPD frame and the previous frame is an FD frame (with a first frame of the current LPD frame being an ACELP frame) so that FAC data is to be read at  202  for addressing the transition from FD frame to ACELP sub-frame at the leading end of the current LPD frame. This additional syntax element read at  220  could indicate as to whether the previous FD frame  14   a  is of FD_long or FD_short. Depending on this syntax element, the FAC data  202  could be influenced. For example, the length of the synthesis signal  149  could be influenced depending on the length of the window used for transforming the previous LPD frame. Summarizing the embodiment of  FIGS. 15 and 19  and transferring features mentioned therein onto the embodiment described with respect to  FIGS. 1 to 14 , the following could be applied onto the latter embodiments either individually or in combination: 
     1) The FAC data  34  mentioned in the previous figures was meant to primarily note the FAC data present in the current frame  14   b  in order to enable forward aliasing cancellation occurring at the transition between the previous frame  14   a  and the current frame  14   b , i.e. between the corresponding time segments  16   a  and  16   b . However, further FAC data may be present. This additional FAC data, however, deals with the transitions between TCX coded sub-frames and CELP coded sub-frames positioned internally to the current frame  14   b  in case the same is of the LPD mode. The presence or absence of this additional FAC data is independent from the syntax portion  26 . In  FIG. 17 , this additional FAC data was read at  216 . The presence or existence thereof merely depends on lpd_mode read at  214 . The latter syntax element, in turn, is part of the syntax portion  24  revealing the coding mode of the current frame. lpd_mode along with core_mode read at  230  and  232  shown in  FIGS. 15 and 16  corresponds to syntax portion  24 . 
     2) Further, the syntax portion  26  may be composed of more than one syntax element as described above. The flag FAC_data_present indicates as to whether fac_data for the boundary between the previous frame and the current frame is present or not. This flag is present at an LPD frame as well as FD frames. A further flag, in the above embodiment called prev_frame_was_lpd, is transmitted in LPD frames only in order to denote as to whether the previous frame  14   a  was of the LPD mode or not. In other words, this second flag included in the syntax portion  26  indicates as to whether the previous fame  14   a  was an FD frame. The parser  20  expects and reads this flag merely in case of the current frame being an LPD frame. In  FIG. 17 , this flag is read at  200 . Depending on this flag, parser  20  may expect the FAC data to comprise, and thus read from the current frame, a gain value fac_gain. The gain value is used by the reconstructor to set a gain of the FAC synthesis signal for FAC at the transition between the current and the previous time segments. In the embodiment of  FIGS. 15 to 19 , this syntax element is read at  204  with the dependency on the second flag being clear from comparing the conditions leading to reading  206  and  202 , respectively. Alternatively or additionally, prev_frame_was_lpd may control a position where parser  20  expects and reads the FAC data. In the embodiment of  FIGS. 15 to 19  these positions were  206  or  202 . Further, the second syntax portion  26  may further comprise a further flag in case of the current frame being an LPD frame with the leading sub-frame of which being an ACELP frame and a previous frame being an FD frame in order indicate as to whether the previous FD frame is encoded using a long transform window or a short transform window. The latter flag could be read at  220  in case of the previous embodiment of  FIGS. 15 to 19 . The knowledge about this FD transform length may be used in order to determine the length of the FAC synthesis signals and the size of the FAC data  34 , respectively. By this measure, the FAC data may be adapted in size to the overlap length of the window of the previous FD frame so that a better compromise between coding quality and coding rate may be achieved. 
     3) By dividing-up the second syntax portion  26  into the just-mentioned three flags, it is possible to transmit merely one flag or bit to signal the second syntax portion  26  in case of the current frame being an FD frame, merely two flags or bits in case of the current frame being an LPD frame and the previous frame being an LPD frame, too. Merely in case of a transition from an FD frame to a current LPD frame, a third flag has to be transmitted in the current frame. Alternatively, as stated above, the second syntax portion  26  may be a 2-bit indicator transmitted for every frame and indicating the mode the frame preceding this frame to the extent needed for the parser to decide as to whether FAC data  34  has to be read from the current frame or not, and if so, from where and how long the FAC synthesis signal is. That is, the specific embodiment of  FIGS. 15 to 19  could be easily transferred to the embodiment of using the above 2-bit identifier for implementing the second syntax portion  26 . Instead of FAC_data_present in  FIGS. 15 and 16 , the 2-bit identifier would be transmitted. Flags at  200  and  220  would not have to be transmitted. Instead, the content of fac_data_present in the if-clause leading to  206  and  218 , could be derived by the parser  20  from the 2-bit identifier. The following table could be accessed at the decoder to exploit the 2-bit indicator. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 core_mode 
                   
               
               
                   
                   
                 of current frame 
                   
               
               
                   
                 prev_mode 
                 (superframe) 
                 first_lpd_flag 
               
               
                   
                   
               
             
             
               
                   
                 ACELP 
                 1 
                 0 
               
               
                   
                 TCX 
                 1 
                 0 
               
               
                   
                 FD_long 
                 1 
                 1 
               
               
                   
                 FD_short 
                 1 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     A syntax portion  26  could also merely have three different possible values in case FD frames will use only one possible length. 
     A slightly differing, but very similar syntax structure to that described above with respect to  15  to  19  is shown in  FIGS. 20 to 22  using the same reference signs as used with respect to  FIGS. 15 to 19 , so that reference is made to that embodiment for explanation of the embodiment of  FIGS. 20 to 22 . 
     With regard to the embodiments described with respect to  FIG. 3  et seq., it is noted that any transform coding scheme with aliasing propriety may be used in connection with the TCX frames, other than MDCT. Furthermore, a transform coding scheme such as FFT could also be used, then without aliasing in the LPD mode, i.e. without FAC for subframe transitions within LPD frames, and thus, without the need for transmitting FAC data for sub-frame boundaries in between LPD boundaries. FAC data would then merely be included for every transition from FD to LPD and vice versa. 
     With regard to the embodiments described with respect to  FIG. 1  et seq., it is noted that same were directed to the case where the additional syntax portion  26  was set in line, i.e. uniquely depending on a comparison between the coding mode of the current frame and the coding mode of the previous frame as defined in the first syntax portion of that previous frame, so that in all of the above embodiments the decoder or parser was able to uniquely anticipate the content of the second syntax portion of the current frame by use of, or comparing, the first syntax portion of these frames, namely the previous and the current frame. That is, in case of no frame loss, it was possible for the decoder or parser to derive from the transitions between frames whether FAC data is present or not in the current frame. If a frame is lost, the second syntax portion such as the flag fac_data_present bit explicitly gives that information. However, in accordance with another embodiment, the encoder could exploit this explicit signalisation possibility offered by the second syntax portion  26  so as to apply a converse coding according which the syntax portion  26  is adaptively, i.e. with the decision there upon being performed on a frame by frame basis, for example—set such that although the transition between the current frame and the previous frame is of the type which usually comes along with FAC data (such as FD/TCX, i.e. any TC coding mode, to ACELP, i.e. any time domain coding mode, or vice versa) the current frame&#39;s syntax portion indicates the absence of FAC. The decoder could then be implemented to strictly act according to the syntax portion  26 , thereby effectively disabling, or suppressing, the FAC data transmission at the encoder which signals this suppression merely by setting, for example, fac_data_present=0. The scenario where this might be a favourable option is when coding at very low bit rates where the additional FAC data might cost too much bits whereas the resulting aliasing artefact might be tolerable compared to the overall sound quality. 
     Although some aspects have been described in the context of an apparatus, it is clear that these aspects also represent a description of the corresponding method, where a block or device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus. Some or all of the method steps may be executed by (or using) a hardware apparatus, like for example, a microprocessor, a programmable computer or an electronic circuit. In some embodiments, some one or more of the most important method steps may be executed by such an apparatus. 
     The inventive encoded audio signal can be stored on a digital storage medium or can be transmitted on a transmission medium such as a wireless transmission medium or a wired transmission medium such as the Internet. 
     Depending on certain implementation requirements, embodiments of the invention can be implemented in hardware or in software. The implementation can be performed using a digital storage medium, for example a floppy disk, a DVD, a Blue-Ray, a CD, a ROM, a PROM, an EPROM, an EEPROM or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer readable. 
     Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed. 
     Generally, embodiments of the present invention can be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. The program code may for example be stored on a machine readable carrier. 
     Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier. 
     In other words, an embodiment of the inventive method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer. 
     A further embodiment of the inventive methods is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. The data carrier, the digital storage medium or the recorded medium are typically tangible and/or non-transitory. 
     A further embodiment of the inventive method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. The data stream or the sequence of signals may for example be configured to be transferred via a data communication connection, for example via the Internet. 
     A further embodiment comprises a processing means, for example a computer, or a programmable logic device, configured to or adapted to perform one of the methods described herein. 
     A further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein. 
     A further embodiment according to the invention comprises an apparatus or a system configured to transfer (for example, electronically or optically) a computer program for performing one of the methods described herein to a receiver. The receiver may, for example, be a computer, a mobile device, a memory device or the like. The apparatus or system may, for example, comprise a file server for transferring the computer program to the receiver. 
     In some embodiments, a programmable logic device (for example a field programmable gate array) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, the methods are advantageously performed by any hardware apparatus. 
     The above described embodiments are merely illustrative for the principles of the present invention. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to others skilled in the art. It is the intent, therefore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein. 
     While this invention has been described in terms of several embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations and equivalents as fall within the true spirit and scope of the present invention.