Abstract:
An improved encoder of the type which generates long windows and short windows, and in which the short windows are grouped. The improvement lies in adaptively grouping the short windows, rather than in statically grouping them all together or all individually. In one embodiment, a new group is begun when a perceptual entropy value of a window crosses a predetermined threshold value with respect to its predecessor. In another embodiment, each group whose perceptual entropy value exceeds the threshold is its own group. The invention can be embodied as a digital audio encoder, for example.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field of the Invention  
           [0002]    This invention relates generally to digital audio encoding, and more particularly to an improved audio encoder with adaptive grouping of short windows.  
           [0003]    2. Background Art  
           [0004]    A digital audio encoder creates a bitstream, typically including both auditory data and header data. It is desirable for the encoder to achieve high compression to reduce the transmission bandwidth and filesize of the bitstream output. It is also desirable that when a decoder plays the bitstream, the analog audio output faithfully reproduces the original with as little noise, corruption, distortion, and artifacting as possible.  
           [0005]    Modern encoders rely upon psychoacoustic perceptual models to determine, for example, what aspects of the original audio data need not be represented in the output bitstream. In short, if the listener cannot hear something, there is no sense encoding it in the bitstream.  
           [0006]    One audio characteristic which the human ear is especially sensitive to, and which is somewhat difficult to handle in conventional digital audio encoders, is the presence of sharp transients in the audio signal, such as occur often with percussion instruments such as drums and castanets, and with some other non-percussive “pitched signals” including some digitized speech. Due to the way that many encoders process and compress the audio signal, sharp transients often produce so-called “pre-echo distortion” in which the portion of the signal immediately preceding the transient becomes distorted due to the sudden and greater amplitude of the signal at the transient. Pre-echo occurs when there is a sharp transient near the end of a block, and the earlier part of the block includes a low-energy signal. In block-based algorithms, block average spectral estimation and time-frequency uncertainty cause the inverse transform function to spread quantization distortion even over the whole block. When there is a low-energy segment in the same block with a sharp transient near the end of the block, this quantization distortion can be of significant magnitude with respect to the low-energy segment&#39;s actual signal content. Other distortions may also occur, but pre-echo is a useful representative for them.  
           [0007]    Some recent encoders, such as the MPEG 2, 4 Advanced Audio Coder (AAC), attempt to reduce pre-echo distortion and other problems caused by sharp transients and by performing quantization and encoding upon shorter sections of audio data when sharp transients are present, and longer sections in their absence.  
           [0008]    [0008]FIG. 1 illustrates a high-level abstraction of an encoder  10  such as is known in the prior art. The encoder includes a filterbank analyzer  12  and a psychoacoustic perceptual model  14 , both of which receive the audio input data, typically in the form of a .WAV or other pulse coding modulation (PCM) file. The psychoacoustic perceptual model determines, among other things, where transients are found and how they should be handled. The perceptual model determines the existence of transients, and decides whether to use short windows for time-to-frequency domain mapping. The filterbank analyzer uses this information to perform the time-to-frequency domain mapping. The filterbank analyzer outputs one set of spectral coefficients if the perceptual model indicated a long window, or multiple sets if the perceptual model indicated short windows. Both provide input to a quantization and encoding module  16 , which performs the encoding of audio data from the filterbank analyzer in response to transient windowing controls from the psychoacoustic perceptual model. The quantization and encoding module quantizes and encodes spectral data according to a set of allowed noise threshold values provided by the perceptual model. A bitstream encoder  18  collects quantized spectral values, scale factors, and some additional information necessary for a decoder (not shown) to reconstruct the encoded data, and generates the output bitstream. Some encoders use entropy coding, such as Huffman coding, to further reduce the number of bits to be placed in the bitstream. The decoder can decode the bitstream and reproduce the original audio signal, within the limits imposed by the quality of the bitstream, of course.  
           [0009]    [0009]FIG. 2 illustrates a high-level abstraction of portions of the psychoacoustic perceptual model  14  such as is suggested by the MPEG AAC encoder standard. The audio input data is received by a perceptual entropy detector  22 , which provides input to a window length selector  24 . If the current audio segment does not contain sufficiently sharp transients, the window length selector will indicate that a long window should be used to encode the audio segment. If the audio segment contains sufficiently sharp transients, the window length selector will indicate that short windows should be used. In the case of the MPEG AAC encoder, short windows exist in sets of eight consecutive short windows. A perceptual entropy threshold value  26  is used to determine what constitutes a sufficiently sharp transient to warrant using short windows.  
           [0010]    [0010]FIG. 3 illustrates an audio signal having a sharp transient, as shown.  
           [0011]    [0011]FIG. 4 illustrates the pre-echo distortion that results from encoding the audio signal of FIG. 3 with too long of a window. The longer the amount of audio signal (or time) that precedes the transient in the window, the longer will be the duration of the pre-echo distortion. An excellent analysis of the state of the prior art is found in “Perceptual Coding of Digital Audio”, by Ted Painter and Andreas Spanias, Dept. of Electrical Engineering, Telecommunications Research Center, Arizona State University.  
           [0012]    What is needed is an improved audio encoder which gives advantages such as improved sound quality, such as one which has improved ability to encode audio which has sharp transients. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only.  
         [0014]    [0014]FIG. 1 shows an audio encoder according to the prior art.  
         [0015]    [0015]FIG. 2 shows a psychoacoustic perceptual model according to prior art.  
         [0016]    [0016]FIG. 3 shows an audio signal having a sharp transient, as is known in the prior art.  
         [0017]    [0017]FIG. 4 shows pre-echo distortion resulting from encoding the audio signal of FIG. 3, as is known in the prior art.  
         [0018]    [0018]FIG. 5 shows one embodiment of an audio encoder according to this invention.  
         [0019]    [0019]FIG. 6 shows another embodiment of an audio encoder according to this invention.  
         [0020]    FIGS.  7 - 10  show various groupings of short windows according to this invention.  
         [0021]    [0021]FIG. 11 shows one embodiment of a method of operation of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]    [0022]FIG. 5 illustrates one embodiment of an encoder  50  including this invention. The filterbank analyzer  12 , quantization and coding module  16 , and bitstream encoder  18  are not necessarily different than in the prior art. The perceptual model of the prior art is improved, and may be termed an adaptive grouping psychoacoustic perceptual model  54 .  
         [0023]    The adaptive grouping psychoacoustic perceptual model includes a perceptual entropy detector  22 , and a window length selector  24 , as before, for determining whether to use long windows or short windows. The window length selector operates according to a first perceptual entropy threshold value  26 , as before. Once a determination has been made that short windows should be used, a short window grouper  56  determines the value of the parameter (scale_factor_grouping) which defines group boundaries of the short windows. In some embodiments, the short window grouper operates according to the first perceptual entropy threshold value  26 . In other embodiments, it operates according to a second perceptual entropy threshold value  58 . In still other embodiments, it may operate according to both, or according to still other values.  
         [0024]    Perceptual entropy is but one example of a signal characteristic upon which grouping decisions can be based. The invention will be explained with reference to perceptual entropy, but is not limited to such. This skilled reader will appreciate how to utilize this invention in performing grouping based upon threshold determinations with respect to signal characteristics per the needs of the application at hand.  
         [0025]    [0025]FIG. 6 illustrates another embodiment of an encoder  60  according to this invention, and is shown in an architectural format similar to that commonly used in illustrating the MPEG AAC encoder. The encoder includes an adaptive grouping psychoacoustic perceptual model  54  which may, in some embodiments, be constructed as shown in FIG. 5. The encoder further includes an iterative rate control loop, a gain control, a modified discrete transform (MDCT) block, a temporal noise shaping (TNS) block which decreases volume of noise induced during encoding by flattening the spectral envelope, a multi-channel mid/side stereo (M/S) intensity module which encodes two audio channels as sum and difference of signals in the channels and performs joint coding of the high frequency portions of both channels, a predictor (“Predict”), a Z −1  block which takes into account information from the immediately previous encoded block of the signal to facilitate prediction, a scale factor extractor, a quantizer (“Quant”), an entropy encoding module, and a side information coding and bitstream formatting module, as shown.  
         [0026]    [0026]FIG. 7 illustrates one method of operation of the adaptive grouping psychoacoustic perceptual model of this invention. For each of the eight short windows, a perceptual entropy (PE) value is calculated, as represented by the bars labeled 1-8. When the PE value crosses (above or below) the predetermined threshold value (T2), a new window group is started. In the MPEG AAC embodiments, this can be indicated in the bitstream by giving a corresponding value to the seven-bit scale_factor_grouping parameter. Each bit position is a binary value indicating whether the corresponding window is the start of a new group of short windows. Although there are eight short windows, the parameter has only seven bits, because the first short window is always the start of a group; thus, the highest order bit position scale_factor_grouping[6] corresponds to short window  2 , and the lowest order bit position scale_factor_grouping[0] corresponds to short window  8 . The reader will appreciate, of course, that the numbering conventions, the parameter name and size, the number of short windows, and so forth can be changed without departing from the scope of this invention, and that the MPEG AAC example is given only for purposes of illustration. In one embodiment, a 0 indicates the start of a new group and a 1 indicates that the window belongs to the same group as the previous block. The parameter value 1011101 indicates that short windows  1  and  2  are a first group (G1), short windows  3  through  6  are a second group (G2), and short windows  7  and  8  are a third group (G3). A new group is started at short window  3  because the PE of short window  2  was below the threshold T2, but the PE of short window  3  was above the threshold T2. A new group is started at short window  7  because the PE of short window  6  was above the threshold T2, but the PE of short window  7  was below the threshold T2.  
         [0027]    [0027]FIG. 8 illustrates another embodiment of a method of operation of the invention, in which a new group is started for each short window whose PE is above the threshold value T2, and at threshold crossings. Short windows  1  and  2  are a first group (G1). Short window  3  is a new group (G2) because its PE is above the threshold. Short windows  4 ,  5 , and  6  each is a new group by itself, because its PE is still above the threshold. Short windows  7  and  8  are a sixth group (G6) because the PE of short window  6  was above the threshold, but the PE of short window  7  dropped below the threshold.  
         [0028]    [0028]FIG. 9 illustrates another example using the same methodology as in FIG. 7, where new windows are started at threshold crossings.  
         [0029]    [0029]FIG. 10 illustrates another embodiment in which a first threshold value T2 is used for upward crossings, and a second threshold value T3 is used for downward crossings. Short windows  1  and  2  are a first group (G1). Short window  3  starts a new group (G2) because its PE rose above T2. Short window  5  is also in G2 because, even though its PE has fallen below T2, it is still above T3. Short window  6  starts a new group (G3) because its PE has fallen below T3. In other embodiments, the T3 threshold may be above the T2 threshold.  
         [0030]    [0030]FIG. 11 illustrates one embodiment of a method  100  of operation of the adaptive grouping psychoacoustic perceptual model of this invention. The model analyzes ( 101 ) or calculates the psychoacoustic perceptual entropy (PE) of an input audio data block. If ( 102 ) the PE is not above a first threshold (T1), there is not too much entropy (meaning there are no sharp transients), and the block can be handled ( 103 ) as a LONG window. Otherwise, there are transients, and the block should be handled ( 104 ) as a EIGHT SHORT windows. The first window always starts a new block. Beginning with the next ( 105 ) window, the value of the next bit position ( 106 ) of the scale_factor_grouping parameter is determined. If ( 107 ) the PE of the window has crossed the threshold (T2) with respect to the PE of the prior window, the scale_factor_grouping bit is set to 0. Otherwise, it is set ( 109 ) to 1, indicating that the corresponding short window does not begin a new group. If ( 110 ) all eight windows are not analyzed, operation returns to analyze the next window ( 105 ). Otherwise, the method is done ( 111 ).  
         [0031]    The reader will appreciate that this invention may be practiced in a wide variety of applications, not limited to MPEG AAC nor even limited to audio encoding, and that these have been used as examples for illustration only.  
         [0032]    The reader will appreciate that drawings showing methods, and the written descriptions thereof, should also be understood to illustrate machine-accessible media having recorded, encoded, or otherwise embodied therein instructions, functions, routines, control codes, firmware, software, or the like, which, when accessed, read, executed, loaded into, or otherwise utilized by a machine, will cause the machine to perform the illustrated methods. Such media may include, by way of illustration only and not limitation: magnetic, optical, magneto-optical, or other storage mechanisms, fixed or removable discs, drives, tapes, semiconductor memories, organic memories, CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-R, DVD-RW, Zip, floppy, cassette, reel-to-reel, or the like. They may alternatively include down-the-wire, broadcast, or other delivery mechanisms such as Internet, local area network, wide area network, wireless, cellular, cable, laser, satellite, microwave, or other suitable carrier means, over which the instructions etc. may be delivered in the form of packets, serial data, parallel data, or other suitable format. The machine may include, by way of illustration only and not limitation: microprocessor, embedded controller, PLA, PAL, FPGA, ASIC, computer, smart card, networking equipment, or any other machine, apparatus, system, or the like which is adapted to perform functionality defined by such instructions or the like. Such drawings, written descriptions, and corresponding claims may variously be understood as representing the instructions etc. taken alone, the instructions etc. as organized in their particular packet/serial/parallel/etc. form, and/or the instructions etc. together with their storage or carrier media. The reader will further appreciate that such instructions etc. may be recorded or carried in compressed, encrypted, or otherwise encoded format without departing from the scope of this patent, even if the instructions etc. must be decrypted, decompressed, compiled, interpreted, or otherwise manipulated prior to their execution or other utilization by the machine.  
         [0033]    Reference in the specification to “an embodiment,” “one embodiment,” “some embodiments,” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the invention. The various appearances “an embodiment,” “one embodiment,” or “some embodiments” are not necessarily all referring to the same embodiments.  
         [0034]    If the specification states a component, feature, structure, or characteristic “may”, “might”, or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element.  
         [0035]    Those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.