Abstract:
There are provided short term enhancement methods and systems to improve perceptual quality in reproduced speech. According to one aspect, a method of enhancing a speech signal includes processing said speech signal to generate a plurality of frames, wherein each of said plurality frames includes a plurality of subframes, coding a previous subframe of said plurality of subframes using Code-Excited Linear Prediction to generate a previous excitation signal, and applying short term enhancement on said previous excitation signal to enhance a current excitation signal for a current subframe.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application claims the benefit of U.S. Provisional Application No. 60/233,042, filed Sep. 15, 2000, which is incorporated by reference herein. 
   U.S. patent application Ser. No. 09/663,242, “SELECTABLE MODE VOCODER SYSTEM,”, filed on Sep. 15, 2000. 
   U.S. patent application Ser. No. 09/755,441, “INJECTING HIGH FREQUENCY NOISE INTO PULSE EXCITATION FOR LOW BIT RATE CELP,”, filed on Sep. 15, 2000. 
   U.S. patent application Ser. No. 09/771,293, “SHORT TERM ENHANCEMENT IN CELP SPEECH CODING,” , filed on Sep. 15, 2000. 
   U.S. patent application Ser. No. 09/761,029, “SYSTEM OF DYNAMIC PULSE POSITION TRACKS FOR PULSE-LIKE EXCITATION IN SPEECH CODING,”, filed on Sep. 15, 2000. 
   U.S. patent application Ser. No. 09/782,791, “SPEECH CODING SYSTEM WITH TIME-DOMAIN NOISE ATTENUATION,”, filed on Sep. 15, 2000. 
   U.S. patent application Ser. No. 09/782,383, “SYSTEM FOR ENCODING SPEECH INFORMATION USING AN ADAPTIVE CODEBOOK WITH DIFFERENT RESOLUTION LEVELS,”, filed on Sep. 15, 2000. 
   U.S. patent application Ser. No. 09/663,837, “CODEBOOK TABLES FOR ENCODING AND DECODING,”, filed on Sep. 15, 2000. 
   U.S. patent application Ser. No. 09/662,828, “BITSTREAM PROTOCOL FOR TRANSMISSION OF ENCODED VOICE SIGNALS,”, filed on Sep. 15, 2000. 
   U.S. patent application Ser. No. 09/781,735, “SYSTEM FOR FILTERING SPECTRAL CONTENT OF A SIGNAL FOR SPEECH ENCODING,”, filed on Sep. 15, 2000. 
   U.S. patent application Ser. No. 09/663,734, “SYSTEM OF ENCODING AND DECODING SPEECH SIGNALS,”, filed on Sep. 15, 2000. 
   U.S. patent application Ser. No. 09/663,002, “SYSTEM FOR SPEECH ENCODING HAVING AN ADAPTIVE FRAME ARRANGEMENT,”, filed on Sep. 15, 2000. 
   U.S. patent application Ser. No. 09/940,904, “SYSTEM FOR IMPROVED USE OF PITCH ENHANCEMENT WITH SUB CODEBOOKS,”, filed on Sep. 15, 2000. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   This invention relates to speech communication systems and, more particularly, to systems for digital speech coding. 
   2. Related Art 
   One prevalent mode of communication is by communication systems that include both wireline and wireless radio systems. Data and voice transmissions within a wireless system occur within a bandwidth of an allowed frequency range. Due to increased wireless communication traffic, reduced bandwidth of transmissions to improve capacity with the system is desirable. 
   Voice and data are transmitted digitally in wireless telecommunications due to noise immunity, reliability, compactness of equipment, and the ability to implement sophisticated signal processing functions using digital techniques. One form of digital transmission is accomplished using digital speech processing systems. Waveforms representing analog speech signals are sampled and then digitally encoded. The number of bits of the encoded signal can be expressed as a bit rate that specifies the number of bits to describe one second of speech. Over the years, significant variations and enhancements have been applied to waveform matching techniques in an effort to improve the quality of the synthesized speech and increase the speech compression. 
   A reduction in the quality of the synthesized (or reconstructed) speech may occur with respect to the original speech. This divergence in the quality of the synthesized speech is due in part to the failure to closely replicate perceptual aspects of the original speech with the bits of data available to describe the signal. Poor replication of the perceptual aspects could result in noise, loss of clarity and the failure to capture recognizable characteristics such as tone, pitch and magnitude. These characteristics allow a listener to recognize who the speaker is, as well as providing other perception based features, such as, intelligibility and naturalness of the speech. 
   Accordingly, there is a need for systems of speech coding that are capable of minimizing the bandwidth of original speech, while providing synthesized speech that closely resembles the original speech and captures the perceptually important features of the speech. 
   SUMMARY 
   This invention provides a system for an improved excitation enhancement system that uses short term prediction to enhance the excitation signal. As speech data applications continue to operate in areas having intrinsic bandwidth limitations, the perceptual quality of reproduced speech data in typical speech coding systems suffers. The invention employs short term enhancement to improve perceptual quality in reproduced speech. 
   Speech coding systems may operate using communication media having limited or constrained bandwidth availability. Any communication media may be employed. 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 systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
       FIG. 1  is an illustration of a waveform illustrating an exemplary speech signal. 
       FIG. 2  is a block diagram illustrating one embodiment of a speech excitation enhancement system. 
       FIG. 3  is a block diagram illustrating one embodiment of a speech codec that employs excitation enhancement. 
       FIG. 4  is a block diagram illustrating another embodiment of a speech codec that employs excitation enhancement. 
       FIG. 5  is a block diagram illustrating one embodiment of an integrated speech codec that employs excitation enhancement. 
       FIG. 6  is a diagram illustrating a speech sub-frame depicting excitation enhancement. 
       FIG. 7  is a functional block diagram illustrating an embodiment of this invention that generates short term enhancement. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A system is provided that utilizes short term enhancement to enhance coded data that, when decoded, produces a synthesized speech signal that resembles an original speech sample. The system is typically used to enhance speech signals transmitted via a wireless radio telecommunications network. Mobile cellular standards, such as the Adaptive Multi-Rate (AMR) and Selectable Mode Vocoder (SMV) standards, define digital transmission in wireless radio telecommunications. An SMV system is utilized to describe the invention. However, those skilled in the art will appreciate that other systems could be used with the invention. 
   In  FIG. 1 , speech coding circuitry (also described in  FIG. 2 ) utilizes prediction to separate a redundant part of a speech signal  100  from an excitation part of the signal  100 . The redundant part of the speech signal  100  is an approximately periodic part of the speech signal  100  and the excitation part of the signal describes variations in the speech signal  100 . The excitation part of the signal typically may be coded by an encoder and transmitted to a decoder to be converted into synthesized speech (the encoder and decoder are described in  FIG. 3 ). The signals may be coded using a linear predictive coding (LPC) filter. A frame-based algorithm stores sampled input speech signals into blocks of samples called frames  110 . An exemplary SMV system operates at a frame size of twenty milliseconds (ms) or one hundred sixty samples per frame. Other sized frames may be used. For signal processing purposes, the frames  110  may be divided into sub-frames  120  that are typically forty samples in size. 
   Short term enhancement may be used to enhance the excitation signal per sub-frame  120 . Short term enhancement utilizes pitch lag information to enhance the excitation signal. Pitch  130  is the approximately periodic part of the speech signal  100 , and lag is a measure of the pitch delay in samples. The general shape of the speech signal  100  evolves relatively slowly as a function of time, facilitating pitch prediction and interpolation. By determining information of lag and gain of a sample from a past sub-frame, the information can be scaled and added to a current sub-frame  140  to enhance the limited amount of data generally used to describe the signal for the current sub-frame  140 . Thus, a first approximation of the excitation for peak P1 in the current sub-frame  140  is advantageously determined using a scaled segment of the previously sampled value for peak P2. Short term enhancement, further described below with regard to  FIG. 6 , samples signals within the pitch  130  of a previous sub-frame to approximate corresponding excitation signals in the current sub-frame  140 . 
     FIG. 2  shows a system diagram illustrating one embodiment of an excitation enhancement system  200 . The excitation enhancement system  200  may include, among other things, speech enhancement processing circuitry  210 , speech coding circuitry  212 , long term enhancement circuitry  214 , short term enhancement circuitry  216 , and speech processing circuitry  218 . The speech coding circuitry  212  can include fixed and adaptive codebooks as are known in the art. The speech excitation enhancement system  200  operates on non-enhanced excitation  220  and generates enhanced excitation  230 . The speech excitation enhancement system  200  is implemented, for example, on one or more integrated circuits (IC), digital signal processors (DSP) or general processors. 
     FIG. 3  shows exemplary speech coding circuitry (e.g., speech coding circuitry  212  from  FIG. 2 ) that utilizes enhancement coding  322  at the encoder  320  to perform short term excitation enhancement and long term pitch prediction. A system diagram  300  illustrates one embodiment of a speech codec (e.g., IC with encoder/decoder) that employs speech enhancement in accordance with the invention. A speech encoder  320  of the speech codec  300  performs enhancement coding  322 . The enhancement coding  322  is performed using both long term enhancement circuitry  324  and short term enhancement circuitry  326 . The enhancement coding  322  generates prediction and enhancement within the speech sub-frame  120 . 
   The speech encoder  320  of the speech codec  300  also may perform main pulse coding  328  of the speech signal  100  including both sign coding  330  and location coding  332  within the speech sub-frame  120 ,  FIG. 1 . Speech processing circuitry  334  also is employed within the speech encoder  320  of the speech codec  300  to assist in speech processing using methods known to those having skill in the art 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 speech codec  300  is transmitted via a communication link  340  to a speech decoder  350  of the speech codec  300 . The communication link  340  may be any communication media capable of transmitting voice data, including but not limited to, wireless communication media, wire-based telephonic communication media, fiber-optic communication media, and Ethernet. 
   The speech decoder  350  of the speech codec  300  may include, among other things, excitation reconstruction circuitry  352 , post perceptual compensation circuitry  354 , and speech reconstruction circuitry  356 . In certain embodiments, the transmit speech processing circuitry  334  and the receiver speech processing circuitry  356  operate cooperatively on the speech data within the entirety of the speech codec  300 . Alternatively, the transmit speech processing circuitry  334  and the receiver speech processing circuitry  356  may operate independently on the speech data, each serving individual speech processing functions in the speech encoder  320  and the speech decoder  350 , respectively. 
   The speech processing circuitry  334  and  356  and the main pulse coding circuitry  328  may include, but are not limited to, circuitry and associated algorithms known to those of skill in the art of speech coding. Examples of such main pulse coding circuitry  328  include Code-Excited Linear Prediction (CELP), eXtended CELP (eX-CELP), algebraic CELP and pulse-like excitation. An example of an eX-CELP based speech coder system is described in commonly assigned U.S. patent Application, “SYSTEM OF ENCODING AND DECODING SPEECH SIGNALS,” by Yang Gao, Adil Beyassine, Jes Thyssen, Eyal Shlomot and Huan-Yu Su, previously incorporated by reference. 
     FIG. 4  illustrates a system diagram of another embodiment of a speech codec  400  that employs excitation enhancement at the speech decoder  450  in accordance with the preferred embodiments. Because the excitation enhancement is performed using data from past sub-frames  120 ,  FIG. 1 , the enhancement is accomplished without increasing bandwidth. The speech encoder  410  of the speech codec  400  performs main pulse coding  420  of the speech signal  100  including both sign coding  422  and location coding  424  within the speech sub-frame  120 . Speech and excitation processing circuitry  430  also may be employed within the speech encoder  410  of the speech codec  400  to assist in speech processing using methods known to those having skill in the art to operate on and perform manipulation of speech data, examples of which have been previously identified. 
   The speech data, after having been processed, at least to some extent by the speech encoder  410  of the speech codec  400  may be transmitted via a communication link  440  to a speech decoder  450  of the speech codec  400 . The speech decoder  450  of the codec  400  performs excitation enhancement coding  460 . The enhancement coding  460  may be performed using both long term enhancement circuitry  462  and short term enhancement circuitry  464 . In other embodiments, only short term enhancement is performed. The enhancement coding  460  generates prediction and enhancement within the speech sub-frame  120 . The speech decoder  450  of the speech codec  400  may also contain speech reproduction circuitry  470 , post perceptual compensation circuitry  480 , and excitation reconstruction circuitry  490 . 
     FIG. 5  is a system diagram that illustrates another embodiment of an integrated speech codec  500  that employs speech and excitation enhancement. The integrated speech codec  500  may contain, among other things, a speech encoder  510  that communicates with a speech decoder  520  via a low bit rate communication link  530 . The low bit rate communication link  530  may be any communication media capable of transmitting voice data, including but not limited to, wireless communication media, wire-based telephonic communication media, fiber-optic communication media, and Ethernet. 
   Excitation enhancement coding  540  is performed in the integrated speech codec  500 . The enhancement coding  540  may be performed using, among other things, both long term enhancement circuitry  542  and short term enhancement circuitry  544 . The long term enhancement circuitry  542  and the short term enhancement circuitry  544  operate cooperatively in certain embodiments, and independently in other embodiments. As shown, the long term enhancement circuitry  542  and short term enhancement circuitry  544  may be arranged within the entirety of the integrated speech codec  500 . Depending on the specific application at hand, a user can select to place the long term enhancement circuitry  542  and short term enhancement circuitry  544  in only one or both of the speech encoder  510  and the speech decoder  520 . Various embodiments are envisioned, without departing form the scope and spirit of the invention, to place various amounts of the long term enhancement circuitry  542  and the short term enhancement circuitry  544  in the speech encoder  510  and the speech decoder  520 . For example, a predetermined portion of the short term enhancement circuitry  544  may be placed in the speech encoder  510  and the remaining portion of the short term enhancement circuitry  544  may be placed in the speech decoder  520 . 
     FIGS. 1 and 6  illustrate short term enhancement of the invention. Short term enhancement uses the previous excitation signal to enhance the excitation signal of the current sub-frame  140 . The past excitation, weighted by a current weighting filter, may be used to estimate correlation peaks at a distance within the current sub-frame  140 . Those skilled in the art will appreciate that an algorithm, similar to that used for long term prediction of pitch lag, can be used to estimate short term correlation of the speech signal  100 . In one embodiment, to evaluate short term correlation of the speech signal  100 , typically less than five peaks and gains per sub-frame  120  are determined from the past excitation. Those skilled in the art will appreciate that more or less correlation peaks and gains can be determined, depending on the application. 
     FIG. 6  illustrates a diagram of two pulses I 3  and I 4  shown at distances R 1  and R 2  from pulse I 2 , which correlate to peaks P 3 , P 4  and P 2 , respectively on  FIG. 1 . I 2  indicates the main pulse, I 3  and I 4  indicate pulses generated by short term enhancement and Pitch indicates a pulse generated by long term enhancement or short term enhancement where the true pitch lag is incorrectly determined. The excitation pattern P(n) is constructed as 
               P   ⁡     (   n   )       =       C   ⁢       ∑   i     ⁢     Gi   ·     δ   ⁡     (     n   -   Ti     )             +     δ   ⁡     (   n   )           ,         
where Gi is the gain and Ti is the distance for the ith peak. Regarding  FIG. 6 , T 0  could equal R 1 , T 1  could equal R 2  and T N  could equal the distance from the main pulse I 2  to Pitch. G 0 , G 1  and G N  can correspond to the magnitudes of I 3 , I 4  and Pitch respectively. The gains Gi and the distance Ti may be determined using methods know to those skilled in the art of speech processing. Gains and distances can be calculated, for example, by maximizing correlations of past synthesized signals in a weighted speech domain. The value C is a coefficient typically between 0 and 0.5, and may be a constant or an adaptive value related to the stability of the speech signal. P(n) accounts in part for the fact that the excitation pattern may cover a long term correlation in which the true pitch lag is shorter than the sub-frame size, while the detected pitch lag may be double or triple the true pitch lag.
 
     FIG. 7  is a functional block diagram illustrating an embodiment that generates long term and short term excitation enhancement. In a block  710 , a speech signal  100  is processed. In a block,  720 , an excitation is coded. In block  730 , long term enhancement is performed, and in a block  740 , short term enhancement is performed. Additional pulses to the current excitation, as determined by the short term enhancements can be added to the excitation by performing a convolution operation of the excitation pattern P(n) with excitation signals, for example, from a fixed codebook of the speech coding circuitry  512 , as known to those of skill in the art. In a block  750 , the speech data information is transmitted via a communication link. In a block  760 , the speech signal is reconstructed/synthesized. 
   While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.