Patent Publication Number: US-6704701-B1

Title: Bi-directional pitch enhancement in speech coding systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on U.S. Provisional Application Ser. No. 60/142,092, filed Jul. 2, 1999. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates generally to speech coding; and, more particularly, it relates to low bit rate speech coding systems that employ pitch enhancement to improve the perceptual quality of reproduced speech. 
     2. Description of Related Art 
     Conventional speech coding systems typically employ only forward pitch enhancement in code-excited linear prediction speech coding systems. This is largely due to the fact that the sub-frame size of conventional speech codecs, having relatively large bandwidth availability, can provide sufficient perceptual quality with forward pitch enhancement alone. However, for lower bit rates within various communication media employed in speech coding systems, the perceptual quality of reproduced speech, after synthesis, fails to maintain a high perceptual quality. 
     For conventional speech coding systems that operate at these decreased bit rates, the pitch lag, that is generated during pitch prediction, is commonly much shorter than the overall subframe size, i.e., it covers a relatively small portion of the overall sub-frame. This characteristic is more accentuated for those speakers having a higher (shorter) pitch, such as females and children. Traditional excitation codebook structures do not afford a sufficient high perceptual quality when operating at low bit rates. This is primarily because the periodicity of the voiced signal is not sufficiently established, or the excitation vector extracted from the codebook is insufficiently rich to generate a synthesized speech signal having a high perceptual quality. 
     As the sub-frame size of speech coding systems becomes larger, as is commonly associated with communication systems that have decreasing bit rates, the fact that pitch enhancement is performed in only the forward direction results in significantly poorer perceptual quality. This is due, among other reasons, to the fact that there is a significant amount of dead space in the sub-frame due to the absence of many pulses. In conventional speech coding systems that operate at higher bit rate, having consequently shorter sub-frames, this effect is not typically audibly perceived by the human ear. This effect of lower perceptual quality is realized in nearly all speech coding systems that deal with speech coding having relatively low available bit rates. 
     Further limitations and disadvantages of conventional and traditional systems will become apparent to one of skill in the art through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings. 
     SUMMARY OF THE INVENTION 
     Various aspects of the present invention can be found in a speech coding system that employs forward pitch enhancement and backward pitch enhancement. In certain embodiments of the invention, the forward pitch enhancement and the backward pitch enhancement are performed in a single portion of the entire speech coding system. For example, in speech coding systems having a speech codec, wherein the speech codec contains an encoder and a decoder, the forward pitch enhancement and the backward pitch enhancement are performed in both the encoder and the decoder of the speech codec. Alternatively, in other embodiments of the invention, the forward pitch enhancement and the backward pitch enhancement are performed only in the decoder of the speech codec. As determined by the specific application, the forward pitch enhancement and the backward pitch enhancement are performed in a distributed manner, each being performed, at least in part, in each one of the encoder and the decoder of the speech codec. 
     In certain embodiments of the invention, the backward pitch enhancement is generated using the forward pitch enhancement itself. The backward pitch enhancement is a mirror image of the forward pitch enhancement that is previously generated; the backward pitch enhancement is generated dependent on the forward pitch enhancement. Alternatively, in other embodiments of the invention, the backward pitch enhancement is generated independent of the forward pitch enhancement; the backward pitch enhancement is generated irrespective of the forward pitch enhancement that has previously been generated. 
     The speech coding system, built in accordance with the present invention, is appropriately geared toward those speech coding systems that operate using communication media having limited or constrained bandwidth availability. Any communication media may be employed within in the invention, without departing from the scope and spirit thereof. Examples of such communication media include, but are not limited to, wireless communication media, wire-based telephonic communication media, fiber-optic communication media, and ethernet. 
    
    
     Other aspects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a system diagram illustrating one embodiment of a speech pitch enhancement system built in accordance with the present invention. 
     FIG. 2 is a system diagram illustrating one embodiment of a distributed speech codec that employs speech pitch enhancement in accordance with the present invention. 
     FIG. 3 is a system diagram illustrating another embodiment of a distributed speech codec that employs speech pitch enhancement in accordance with the present invention. 
     FIG. 4 is a system diagram illustrating another embodiment of an integrated speech codec that employs speech pitch enhancement in accordance with the present invention. 
     FIG. 5 is a diagram illustrating a speech sub-frame depicting forward and backward predicted pulses to perform pitch enhancement in accordance with the present invention. 
     FIG. 6 illustrates a functional block diagram illustrating an embodiment of the present invention that generates backward speech pitch enhancement using forward speech pitch enhancement in accordance with the present invention. 
     FIG. 7 illustrates a functional block diagram illustrating an embodiment of the present invention that performs backward speech pitch enhancement independent of forward speech pitch enhancement in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF DRAWINGS 
     FIG. 1 is a system diagram illustrating one embodiment  100  of a speech pitch enhancement system  110  built in accordance with the present invention. The speech pitch enhancement system  110  contains, among other things, pitch enhancement processing circuitry  112 , speech coding circuitry  114 , forward pitch enhancement circuitry  116 , backward pitch enhancement circuitry  118 , and speech processing circuitry  119 . The speech pitch enhancement system  110  operates on non-enhanced speech data or excitation signal  120  and generates pitch enhanced speech data  130 . The pitch enhanced speech data or excitation signal  130  contains speech data having pitch prediction and pitch enhancement performed in both the forward and backward directions with respect to a speech sub-frame. The speech pitch enhancement system  110  operates only on an excitation signal in certain embodiments of the invention, and the speech pitch enhancement system  110  operates only on speech data in other embodiments of the invention. 
     In certain embodiments of the invention, the speech pitch enhancement system  110  operates independently to generate backward pitch prediction using the backward pitch enhancement circuitry  118 . Alternatively, the forward pitch enhancement circuitry  116  and the backward pitch enhancement circuitry  118  operate cooperatively to generate the overall pitch enhancement of the speech coding system. A supervisory control operation, monitoring the forward pitch enhancement circuitry  116  and the backward pitch enhancement circuitry  118 , is performed using the pitch enhancement processing circuitry  112  in other embodiments of the invention. The speech processing circuitry  119  includes, but is not limited to, that speech processing circuitry known to those having skill in the art of speech processing to operate on and perform manipulation of speech data. The speech coding circuitry  114  similarly includes, but is not limited to, circuitry known to those of skill in the art of speech coding. Such speech coding known to those having skill in the art includes, among other speech coding methods, code-excited linear prediction, algebraic code-excited linear prediction, and pulse-like excitation. 
     FIG. 2 is a system diagram illustrating one embodiment of a distributed speech codec  200  that employs speech pitch enhancement in accordance with the present invention. A speech encoder  220  of the distributed speech codec  200  performs pitch enhancement coding  221 . The pitch enhancement coding  221  is performed using both backward pulse pitch prediction circuitry  222  and forward pulse pitch prediction circuitry  223 . As described above in another embodiment of the invention, the pitch enhancement coding  221  generates pitch prediction and pitch enhancement in both the forward and backward directions within the speech sub-frame. The speech encoder  220  of the distributed speech codec  200  also performs main pulse coding  225  of a speech signal including both sign coding  226  and location coding  227  within a speech sub-frame. Speech processing circuitry  229  is also employed within the speech encoder  220  of the distributed speech codec  200  to assist in speech processing using methods known to those having skill in the art of speech processing to operate on and perform manipulation of speech data. Additionally, the speech processing circuitry  229  operates cooperatively with the backward pulse pitch prediction circuitry  222  and forward pulse pitch prediction circuitry  223  in certain embodiments of the invention. The speech data, after having been processed, at least to some extent by the speech encoder  220  of the distributed speech codec  200  is transmitted via a communication link  210  to a speech decoder  230  of the distributed speech codec  200 . The communication link  210  is any communication media capable of transmitting voiced data, including but not limited to, wireless communication media, wire-based telephonic communication media, fiber-optic communication media, and ethernet. Any communication media capable of transmitting speech data is included in the communication link  210  without departing from the scope and spirit of the invention. The speech decoder  230  of the distributed speech codec  200  contains, among other things, speech reproduction circuitry  232 , perceptual compensation circuitry  234 , and speech processing circuitry  236 . 
     In certain embodiments of the invention, the speech processing circuitry  229  and the speech processing circuitry  236  operate cooperatively on the speech data within the entirety of the distributed speech codec  200 . Alternatively, the speech processing circuitry  229  and the speech processing circuitry  236  operate independently on the speech data, each serving individual speech processing functions in the speech encoder  220  and the speech decoder  230 , respectively. The speech processing circuitry  229  and the speech processing circuitry  236  include, but are not limited to, that speech processing circuitry known to those having skill in the art of speech processing to operate on and perform manipulation of speech data. The main pulse coding circuitry  225  similarly includes, but is not limited to, circuitry known to those of skill in the art of speech coding. Examples of such main pulse coding circuitry  225  include that circuitry known to those having skill in the art, among other main pulse coding methods, code-excited linear prediction, algebraic code-excited linear prediction, and pulse-like excitation, as described above in another embodiment of the invention. 
     FIG. 3 is a system diagram illustrating another embodiment of a distributed speech codec  300  that employs speech pitch enhancement in accordance with the present invention. A speech encoder  320  of the distributed speech codec  300  performs main pulse coding  325  of a speech signal including both sign coding  326  and location coding  327  within a speech sub-frame. Speech processing circuitry  329  is also employed within the speech encoder  320  of the distributed speech codec  300  to assist in speech processing using methods known to those having skill in the art of speech processing to operate on and perform manipulation of speech data. The speech data, after having been processed, at least to some extent by the speech encoder  320  of the distributed speech codec  300  is transmitted via a communication link  310  to a speech decoder  330  of the distributed speech codec  300 . The communication link  310  is any communication media capable of transmitted voiced data, including but not limited to, wireless communication media, wire-based telephonic communication media, fiber-optic communication media, and ethernet. Any communication media capable of transmitting speech data is included in the communication link  310  without departing from the scope and spirit of the invention. A speech decoder  330  of the distributed speech codec  300  performs pitch enhancement coding  321 . The pitch enhancement coding  321  is performed using both backward pulse pitch prediction circuitry  322  and forward pulse pitch prediction circuitry  323 . As described above in various embodiments of the invention, the pitch enhancement coding  321  generates pitch prediction and pitch enhancement in both the forward and backward directions within the speech sub-frame. Speech processing circuitry  336  is also employed within the speech decoder  330  of the distributed speech codec  300  to assist in speech processing using methods known to those having skill in the art of speech processing to operate on and perform manipulation of speech data. Additionally, the speech processing circuitry  339  operates cooperatively with the backward pulse pitch prediction circuitry  322  and forward pulse pitch prediction circuitry  323  in certain embodiments of the invention. 
     In certain embodiments of the invention, the speech processing circuitry  329  and the speech processing circuitry  336  operate cooperatively on the speech data within the entirety of the distributed speech codec  300 . Alternatively, the speech processing circuitry  329  and the speech processing circuitry  336  operate independently on the speech data, each serving individual speech processing functions in the speech encoder  320  and the speech decoder  330 ; respectively. The speech processing circuitry  329  and the speech processing circuitry  336  include, but are not limited to, that speech processing circuitry known to those having skill in the art of speech processing to operate on and perform manipulation of speech data. The main pulse coding circuitry  325  similarly includes, but is not limited to, circuitry known to those of skill in the art of speech coding. Examples of such main pulse coding circuitry  325  includes that circuitry known to those having skill in the art, among other main pulse coding methods, code-excited linear prediction, algebraic code-excited linear prediction, and pulse-like excitation, as described above in another embodiment of the invention. 
     FIG. 4 is a system diagram illustrating another embodiment  400  of an integrated speech codec  420  that employs speech pitch enhancement in accordance with the present invention. The integrated speech codec  420  contains, among other things, a speech encoder  422  that communicates with a speech decoder  424  via a low bit rate communication link  410 . The low bit rate communication link  410  is any communication media capable of transmitting voiced data, including but not limited to, wireless communication media, wire-based telephonic communication media, fiber-optic communication media, and ethernet. Any communication media capable of transmitting speech data is included in the low bit rate communication link  410  without departing from the scope and spirit of the invention. Pitch enhancement coding  421  is performed in the integrated speech codec  420 . The pitch enhancement coding  421  is performed using, among other things, backward pulse pitch prediction circuitry  422  and forward pulse pitch prediction circuitry  423 . As described above in various embodiments of the invention, the backward pulse pitch prediction circuitry  422  and the forward pulse pitch prediction circuitry  423  operate cooperatively in certain embodiments of the invention, and independently in other embodiments of the invention. 
     As shown in the embodiment  400 , the backward pulse pitch prediction circuitry  422  and the forward pulse pitch prediction circuitry  423  are contained within the entirety of the integrated speech codec  420 . If desired, the backward pulse pitch prediction circuitry  422  and the forward pulse pitch prediction circuitry  423  are both contained in each of the speech encoder  422  and the speech decoder  424  in certain embodiments of the invention. Alternatively, either one of the backward pulse pitch prediction circuitry  422  or the forward pulse pitch prediction circuitry  423  is contained in only one of the speech encoder  422  and the speech decoder  424  in other embodiments of the invention. Depending on the specific application at hand, a user can select to place the backward pulse pitch prediction circuitry  422  and the forward pulse pitch prediction circuitry  423  in only one or either of the speech encoder  422  and the speech decoder  424 . Various embodiments are envisioned in the invention, without departing from the scope and spirit thereof, to place various amounts of the backward pulse pitch prediction circuitry  422  and the forward pulse pitch prediction circuitry  423  in the speech encoder  422  and the speech decoder  424 . For example, a predetermined portion of the backward pulse pitch prediction circuity  422  is placed in the speech encoder  422  while a remaining portion of the backward pulse pitch prediction circuitry  422  is placed in the speech decoder  424  in certain embodiments of the invention. Similarly, a predetermined portion of the forward pulse pitch prediction circuitry  423  is placed in the speech encoder  422  while a remaining portion of the forward pulse pitch prediction circuitry  423  is placed in the speech decoder  424  in certain embodiments of the invention. 
     FIG. 5 is a coding diagram  500  illustrating a speech sub-frame  510  depicting forward pitch enhancement and backward pitch enhancement performed in accordance with the present invention. A main pulse M 0    520  is generated in the speech sub-frame  510  using any method known to those having skill in the art of speech processing, including but not limited to, code-excited linear prediction, algebraic code-excited linear prediction, analysis by synthesis speech coding, and pulse-like excitation. Using various methods of speech processing, including those methods described above that are employed in various embodiments of the invention, a forward predicted pulse M 1    530 , a forward predicted pulse M 2    540 , and a forward predicted pulse M 3    550  are all generated and placed within the speech sub-frame  510 . As described above, the generation of the forward predicted pulse M 1    530 , the forward predicted pulse M 2    540 , and the forward predicted pulse M 3    550  is performed using various processing circuitry in certain embodiments of the invention. In addition, a backward predicted pulse M −1    560  and a backward predicted pulse M −2    570  are also generated in accordance with the invention. 
     In certain embodiments of the invention, the backward predicted pulse M −1    560  and the backward predicted pulse M −2    570  are generated using the forward predicted pulse M 1    530 , the forward predicted pulse M 2    540 , and the forward predicted pulse M 3    550 . Alternatively, in other embodiments of the invention, the backward predicted pulse M −1    560  and the backward predicted pulse M −2    570  are generated independent of the forward predicted pulse M 1    530 , the forward predicted pulse M 2    540 , and the forward predicted pulse M 3    550 . An example of independent generation of the backward predicted pulse M −1    560  and the backward predicted pulse M −2    570  is an implementation within software wherein the time scale of the speech sub-frame  510  is reversed in software. The main pulse M 0    520  is used in a similar manner to generate both the forward predicted pulse M 1    530 , the forward predicted pulse M 2    540 , and the forward predicted pulse M 3    550 , and the backward predicted pulse M −1    560  and the backward predicted pulse M −2    570 . That is to say, the process is performed once in the typical forward direction, and after the speech sub-frame  510  is reversed in software, the process is performed once again in the atypical backward direction, yet it employs the same mathematical method, i.e., only the data are reversed with respect to speech sub-frame  510 . 
     FIG. 6 illustrates a functional block diagram illustrating an embodiment  600  of the present invention that generates backward speech pitch enhancement using forward speech pitch enhancement in accordance with the present invention. In a block  610 , a speech signal is processed. In a block  620 , a main pulse of the speech data is coded. In an alternative process block  655 , the speech data information is transmitted via a communication link. The alternative process block  655  is employed in embodiments of the invention wherein the forward pitch enhancement and backward pitch enhancement are performed after the coded speech data is transmitted for speech reproduction. In a block  630 , forward pitch enhancement is performed, and in a block  640 , backward pitch enhancement is performed. The backward pitch enhancement of the block  640  is a mirror image of the forward pitch enhancement that is generated in the block  630  in certain embodiments of the invention. In other embodiments, the backward pitch enhancement of the block  640  is not a mirror image of the forward pitch enhancement that is generated in the block  630 . In an alternative process block  650 , the speech data information is transmitted via a communication link. The alternative process block  650  is employed in embodiments of the invention wherein the forward pitch enhancement and backward pitch enhancement are performed prior to the coded speech data being transmitted for speech reproduction. In a block  660 , the speech signal is reconstructed/synthesized. 
     In certain embodiments of the invention, the backward pitch enhancement performed in the block  640  is simply a duplicate of the forward pitch enhancement performed in the block  650 , i.e., backward pitch enhancement of the block  640  is a mirror image of the forward pitch enhancement generated in the block  630 . For example, after the forward pitch enhancement is performed in the block  650 , the resultant pitch enhancement is simply copied and reversed within a speech sub-frame to generate the backward pitch enhancement performed in the block  640  using any method known to those skilled in the art of speech processing for synthesizing and reproducing a speech signal. 
     FIG. 7 illustrates a functional block diagram illustrating an embodiment  700  of the present invention that performs backward speech pitch enhancement independent of forward speech pitch enhancement in accordance with the present invention. In a block  710 , a speech signal is processed. In a block  720 , a main pulse of the speech data is coded. In an alternative process block  755 , the speech data information is transmitted via a communication link. The alternative process block  755  is employed in embodiments of the invention wherein the forward pitch enhancement and backward itch enhancement are performed after the coded speech data is transmitted for speech et- reproduction. In a block  730 , forward pitch enhancement is performed, and in a block  740 , backward pitch enhancement is performed. The backward pitch enhancement of the block  740  is performed after the speech data is reversed; the backward pitch enhancement of the block  740  is performed independently of the forward pitch enhancement that is performed in the block  730 . This particular embodiment differs from that illustrated in the embodiment  600 , in that, the speech data are reversed and the backward pitch enhancement of the block  740  is generated as if an entirely new set of speech data were being processed. Conversely, in the embodiment  600 , the resulting pitch enhancement itself is utilized, but it extended in the reverse direction. In certain embodiments of the embodiment  700 , it is as if two sets of speech data are being processed for each sub-frame; one set of data is processed to generate the pitch prediction in the forward direction in the block  730 , and one set of data is processed to generate the pitch prediction in the backward direction in the block  740 , yet they are both operating on the same sub-frame of speech data. In an alternative process block  750 , the speech data information is transmitted via a communication link. The alternative process block  750  is employed in embodiments of the invention wherein the forward pitch enhancement of the block  730  and backward pitch enhancement of the block  740  are performed prior to the coded speech data being transmitted for speech reproduction. In a block  760 , the speech signal is reconstructed/synthesized. 
     In view of the above detailed description of the present invention and associated drawings, other modifications and variations will now become apparent to those skilled in the art. It should also be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the present invention.