Abstract:
Methods and apparatus for quickly selecting an optimal excitation waveform from a codebook are presented herein. In encoding schemes that use forward and backward pitch enhancement, storage and processor load is reduced by approximating a two-dimensional autocorrelation matrix with a one-dimensional autocorrelation vector. The approximation is possible when a cross-correlation element is configured to determine the autocorrelation matrix of an impulse response and a pulse energy determination element is configured to determine the energy of a pulse code vector that incorporates secondary pulse positions.

Description:
BACKGROUND 
     1. Field 
     The present invention relates generally to communication systems, and more particularly, to speech processing within communication systems. 
     2. Background 
     The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, personal digital assistants (PDAs), Internet telephony, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. As used herein, the term “cellular” system encompasses both cellular and personal communications services (PCS) frequencies. Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred to collectively herein as IS-95), and proposed high-data-rate systems for data, etc. are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies. 
     Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. Exemplary cellular telephone systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of the present invention and incorporated by reference herein. An exemplary system utilizing CDMA techniques is the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission (referred to herein as cdma2000), issued by the TIA. The standard for cdma2000 is given in the draft versions of IS-2000 and has been approved by the TIA. The cdma2000 proposal is compatible with IS-95 systems in many ways. Another CDMA standard is the W-CDMA standard, as embodied in 3 rd    Generation Partnership Project  “3 GPP ”, Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214. 
     With the proliferation of digital communication systems, the demand for efficient frequency usage is constant. One method for increasing the efficiency of a system is to transmit compressed signals. In a regular landline telephone system, a sampling rate of 64 kilobits per second (kbps) is used to recreate the quality of an analog voice signal in a digital transmission. However, by using compression techniques that exploit the redundancies of a voice signal, the amount of information that is transmitted over-the-air can be reduced while still maintaining a high quality. 
     Typically, conversion of an analog voice signal to a digital signal is performed by an encoder and conversion of the digital signal back to a voice signal is performed by a decoder. In an exemplary CDMA system, a vocoder comprising both an encoding portion and a decoding portion is located within remote stations and base stations. An exemplary vocoder is described in U.S. Pat. No. 5,414,796, entitled “Variable Rate Vocoder,” assigned to the assignee of the present invention and incorporated by reference herein. In a vocoder, an encoding portion extracts parameters that relate to a model of human speech generation. A decoding portion re-synthesizes the speech using the parameters received over a transmission channel. The model is constantly changing to accurately model the time varying speech signal. Thus, the speech is divided into blocks of time, or analysis frames, during which the parameters are calculated. The parameters are then updated for each new frame. As used herein, the word “decoder” refers to any device or any portion of a device that can be used to convert digital signals that have been received over a transmission medium. The word “encoder” refers to any device or any portion of a device that can be used to convert acoustic signals into digital signals. Hence, the embodiments described herein can be implemented with vocoders of CDMA systems, or alternatively, encoders and decoders of non-CDMA systems. 
     Of the various classes of speech coder, the Code Excited Linear Predictive Coding (CELP), Stochastic Coding, or Vector Excited Speech Coding coders are of one class. An example of a coding algorithm of this particular class is described in Interim Standard 127 (IS-127), entitled, “Enhanced Variable Rate Coder” (EVRC). Another example of a coder of this particular class is described in pending draft proposal “Selectable Mode Vocoder Service Option for Wideband Spread Spectrum Communication Systems,” Document No. 3GPP2 C.P9001. The function of the vocoder is to compress the digitized speech signal into a low bit rate signal by removing all of the natural redundancies inherent in speech. In a CELP coder, redundancies are removed by means of a short-term formant (or LPC) filter. Once these redundancies are removed, the resulting residual signal can be modeled as white Gaussian noise, or a white periodic signal, which also must be coded. Hence, through the use of speech analysis, followed by the appropriate coding, transmission, and re-synthesis at the receiver, a significant reduction in the data rate can be achieved. 
     The coding parameters for a given frame of speech are determined by first determining the coefficients of a linear prediction coding (LPC) filter. The appropriate choice of coefficients will remove the short-term redundancies of the speech signal in the frame. Long-term periodic redundancies in the speech signal are removed by determining the pitch lag, L, and pitch gain, g p , of the signal. The combination of possible pitch lag values and pitch gain values is stored as vectors in an adaptive codebook. An excitation signal is then chosen from among a number of waveforms stored in an excitation waveform codebook. When the appropriate excitation signal is excited by a given pitch lag and pitch gain and is then input into the LPC filter, a close approximation to the original speech signal can be produced. Thus, a compressed speech transmission can be performed by transmitting LPC filter coefficients, an identification of the adaptive codebook vector, and an identification of the fixed codebook excitation vector. 
     An effective excitation codebook structure is referred to as an algebraic codebook. The actual structure of algebraic codebooks is well known in the art and is described in the paper “Fast CELP coding based on Algebraic Codes” by J. P. Adoul, et al., Proceedings of ICASSP Apr. 6-9, 1987. The use of algebraic codes is further disclosed in U.S. Pat. No. 5,444,816, entitled “Dynamic Codebook for Efficient Speech Coding Based on Algebraic Codes”, the disclosure of which is incorporated by references. 
     Due to the intensive computational and storage requirements of implementing codebook searches for optimal excitation vectors, there is a constant need to increase the speed of codebook searches. 
     SUMMARY 
     Novel methods and apparatus for implementing a fast code vector search in coders are presented. In one aspect, a method is presented for selecting a code vector in an algebraic codebook wherein a pre-computed Toeplitz autocorrelation matrix, stored as single dimensional vector of the weighting filter impulse response, and pitch-sharpened pulses are used for a fast codebook search that greatly saves the storage memory required for conducting the codebook search. 
     In another aspect, an apparatus is presented for selecting an optimal pulse vector from a pulse vector codebook, wherein the optimal pulse vector is used by a linear prediction coder to encode a residual waveform. The apparatus comprises: an impulse response generator for outputting an impulse response vector; a correlation element configured to receive the impulse response vector and a plurality of target signal samples, to output an autocorrelation value based on the impulse response vector, and to output a cross-correlation vector based on a composite impulse response vector and the plurality of target signal samples, wherein the composite impulse response vector is determined using the impulse response vector; and a pulse energy determination element configured to generate an energy value using a pulse vector from the pulse vector codebook, a composite pulse vector that is determined using the pulse vector, and the autocorrelation value, wherein the energy value and the autocorrelation value are used by a metric calculator to determine a ratio value that is used to select the optimal pulse vector. 
     In another aspect, a method for selecting an optimal pulse vector from a codebook of pulse vectors is presented. The method comprises: determining an autocorrelation value associated with an impulse response vector; determining a cross-correlation value associated with a target signal and a pitch-sharpened impulse response vector, wherein the pitch-sharpened impulse response vector is determined from the impulse response vector; determining an energy value for each pulse vector from a plurality of pulse vectors, wherein the energy value is determined using each pulse vector and a pitch-sharpened pulse vector associated with each pulse vector; and using the plurality of energy values and the cross-correlation value to determine a plurality of ratios, wherein the residual waveform is encoded by using the pulse vector that is selected as having the highest ratio of the plurality of ratios. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an exemplary communication system. 
     FIG. 2 is a block diagram of a conventional apparatus for performing codebook searches. 
     FIG. 3 is a block diagram of an apparatus for performing slow codebook searches in a coder that uses pitch enhanced impulse responses. 
     FIG. 4 is a block diagram of an apparatus for performing fast codebook searches in a coder that uses pitch enhanced impulse responses. 
     FIG. 5 is a flow chart of method steps for performing a fast codebook search. 
    
    
     DETAILED DESCRIPTION 
     As illustrated in FIG. 1, a wireless communication network  10  generally includes a plurality of remote stations (also called mobile stations or subscriber units or user equipment)  12   a - 12   d , a plurality of base stations (also called base station transceivers (BTSs) or Node B)  14   a - 14   c , a base station controller (BSC) (also called radio network controller or packet control function  16 ), a mobile switching center (MSC) or switch  18 , a packet data serving node (PDSN) or internetworking function (IWF)  20 , a public switched telephone network (PSTN)  22  (typically a telephone company), and an Internet Protocol (IP) network  24  (typically the Internet). For purposes of simplicity, four remote stations  12   a - 12   d , three base stations  14   a - 14   c , one BSC  16 , one MSC  18 , and one PDSN  20  are shown. It would be understood by those skilled in the art that there could be any number of remote stations  12 , base stations  14 , BSCs  16 , MSCs  18 , and PDSNs  20 . 
     In one embodiment the wireless communication network  10  is a packet data services network. The remote stations  12   a - 12   d  may be any of a number of different types of wireless communication device such as a portable phone, a cellular telephone that is connected to a laptop computer running IP-based, Web-browser applications, a cellular telephone with associated hands-free car kits, a personal data assistant (PDA) running IP-based, Web-browser applications, a wireless communication module incorporated into a portable computer, or a fixed location communication module such as might be found in a wireless local loop or meter reading system. In the most general embodiment, remote stations may be any type of communication unit. 
     The remote stations  12   a - 12   d  may be configured to perform one or more wireless packet data protocols such as described in, for example, the EIA/TIA/IS-707 standard. In a particular embodiment, the remote stations  12   a - 12   d  generate IP packets destined for the IP network  24  and encapsulate the IP packets into frames using a point-to-point protocol (PPP). 
     In one embodiment, the IP network  24  is coupled to the PDSN  20 , the PDSN  20  is coupled to the MSC  18 , the MSC  18  is coupled to the BSC  16  and the PSTN  22 , and the BSC  16  is coupled to the base stations  14   a - 14   c  via wirelines configured for transmission of voice and/or data packets in accordance with any of several known protocols including, e.g., E1, T1, Asynchronous Transfer Mode (ATM), IP, Frame Relay, HDSL, ADSL, or xDSL. In an alternate embodiment, the BSC  16  is coupled directly to the PDSN  20 , and the MSC  18  is not coupled to the PDSN  20 . In another embodiment, the remote stations  12   a - 12   d  communicate with the base stations  14   a - 14   c  over an RF interface defined in the 3 rd    Generation Partnership Project  2 “3 GPP 2”, “Physical Layer Standard for cdma2000 Spread Spectrum Systems,” 3GPP2 Document No. C.P0002-A, TIA PN-4694, to be published as TIA/EIA/IS-2000-2-A, (Draft, edit version 30) (Nov. 19, 1999), which is fully incorporated herein by reference. In another embodiment, the remote stations  12   a - 12   d  communicate with the base stations  14   a - 14   c  over an RF interface defined in  3   rd    Generation Partnership Project “3 GPP ”, Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214. 
     During typical operation of the wireless communication network  10 , the base stations  14   a - 14   c  receive and demodulate sets of reverse-link signals from various remote stations  12   a - 12   d  engaged in telephone calls, Web browsing, or other data communications. Each reverse-link signal received by a given base station  14   a - 14   c  is processed within that base station  14   a - 14   c . Each base station  14   a - 14   c  may communicate with a plurality of remote stations  12   a - 12   d  by modulating and transmitting sets of forward-link signals to the remote stations  12   a - 12   d . For example, as shown in FIG. 1, the base station  14   a  communicates with first and second remote stations  12   a ,  12   b  simultaneously, and the base station  14   c  communicates with third and fourth remote stations  12   c ,  12   d  simultaneously. The resulting packets are forwarded to the BSC  16 , which provides call resource allocation and mobility management functionality including the orchestration of soft handoffs of a call for a particular remote station  12   a - 12   d  from one base station  14   a - 14   c  to another base station  14   a - 14   c . For example, a remote station  12   c  is communicating with two base stations  14   b ,  14   c  simultaneously. Eventually, when the remote station  12   c  moves far enough away from one of the base stations  14   c , the call will be handed off to the other base station  14   b.    
     If the transmission is a conventional telephone call, the BSC  16  will route the received data to the MSC  18 , which provides additional routing services for interface with the PSTN  22 . If the transmission is a packet-based transmission, such as a data call destined for the IP network  24 , the MSC  18  will route the data packets to the PDSN  20 , which will send the packets to the IP network  24 . Alternatively, the BSC  16  will route the packets directly to the PDSN  20 , which sends the packets to the IP network  24 . 
     As discussed above, a speech signal can be segmented into frames, and then modeled by the use of LPC filter coefficients, adaptive codebook vectors, and fixed codebook vectors. In order to create an optimal model of the speech signal, the difference between the actual speech and the recreated speech must be minimal. One technique for determining whether the difference is minimal is to determine the correlation values between the actual speech and the recreated speech and to then choose a set of components with a maximum correlation property. 
     FIG. 2 is a block diagram of an apparatus in a conventional encoder for selecting an optimal excitation vector from a codebook. This encoder is designed to minimize the computational complexity involved when convolving an input signal with the impulse response of a filter, said complexity being further increased by the need to convolve multiple input signals in order to determine which input signal results in the closest match to a target signal. To reduce the complexity, this encoder convolves a group of input signals with an impulse response that has been extended with zero-values. This extension results in an impulse response that is stationary. The autocorrelation matrix for a stationary impulse response has a Toeplitz form. 
     A frame of speech samples s(n) is filtered by a perceptual weighting filter  230  to produce a target signal x(n). The design and implementation of perceptual weighting filters is described in aforementioned U.S. Pat. No. 5,414,796. An impulse response generator  210  generates an impulse response h(n). Using the impulse response h(n) and the target signal x(n), a cross-correlation vector d(i) is generated at computation element  290  in accordance with the following relationship:            d        (   i   )       =       ∑     j   =   1     M                       x        (   i   )            h        (     i   -   j     )             ,       for                 j     =     1                 to                   M   .                                
     The impulse response h(n) is also used by computation element  250  to generate an autocorrelation matrix:            φ        (     i   ,   j     )       =       ∑     n   =   j     M                       h        (     n   -   i     )            h        (     n   -   j     )             ,       for                 i     ≥   j                            
     The autocorrelation matrix φ becomes a Toeplitz matrix if the analysis window is extended from M samples to M+L−1 samples, wherein the extra samples are zero-valued. A Toeplitz matrix is a square matrix whose entries are constant along each diagonal. Hence, the Toeplitz autocorrelation matrix can be represented by a one-dimensional vector, rather than a two-dimensional matrix. 
     The entries of the autocorrelation matrix φ are sent to computation element  240 . Pulse codebook generator  200  generates a plurality of pulse vectors {c k , k=1, . . . , M}, which are also input into computation element  240 . An excitation waveform codebook, alternatively referred to as a pulse waveform codebook or a pulse codebook herein, can be generated in response to a plurality of pulse position signals, {p i , i=1, . . . , M} (not shown in figure), wherein i is the position of a unit pulse in the pulse vector. N p  is a value representing the number of pulses in a pulse vector. Computation element  240  filters the pulse vectors with the autocorrelation matrix φ in accordance with the following formula:          E   yy     =         ∑     i   =   0         N   p     -   1                       φ        (       p   i     ,     p   j       )         +     2   ·       ∑     i   =   0         N   p     -   1                         ∑     j   =     i   +   1           N   p     -   1                           c   k          (     p   i     )              c   k          (     p   j     )              φ        (       p   i     ,     p   j       )       .                                      
     The pulse vectors {c k , k=1, . . . , M} are also used by computation element  290  to determine a cross-correlation between d(n) and c k (n) according to the following equation:          E   xy   2     =         (       ∑     i   =   0         N   p     -   1                           c   k          (     p   i     )       ·     d        (     p   i     )           )     2     .                            
     Once values for E yy  and E xy  are known, a computation element  260  determines the value T k  using the following relationship:          T   k     =           (     E   xy     )     2       E   yy       .                            
     The pulse vector that corresponds to the largest value of T k  is selected as the optimum vector to encode the residual waveform. 
     The search for the optimum pulse vector using the above scheme is efficient due to the simplification of the autocorrelation matrix φ. However, the apparatus of FIG. 2 cannot be implemented in the new generation of voice encoders, such as the Enhanced Variable Rate Codec (EVRC) and the Selectable Mode Vocoder (SMV). In the apparatus of FIG. 2, the simplification of the autocorrelation matrix φ is possible by extending the window of the speech frame with zero values so that impulse response h(n) becomes stationary. Accordingly, the entries of autocorrelation matrix φ are such that φ(i, j)=φ(i−j). 
     However, in some of the new vocoders, such as the ones mentioned above, the windows of the speech frame cannot be extended with zero values due to the incorporation of non-zero valued contributions from pitch periodicity. In these vocoders, the pitch periodicity contribution of the codebook pulses is enhanced by incorporating a gain-adjusted forward and backward pitch sharpening process into the analysis frame of the speech signal. 
     An example of pitch sharpening is the formation of a composite impulse response {tilde over (h)}(n) from h(n) in accordance with the following relationship:            h   ~          (   n   )       =         g   p     P   -   1            h        (     n   -       (     P   -   1     )        L       )         +   …   +       g   p   3          h        (     n   -     3      L       )         +       g   p   2          h        (     n   -     2      L       )         +       g   p          h        (     n   -   L     )         +     h        (   n   )       +       g   p          h        (     n   +   L     )         +       g   p   2          h        (     n   +     2      L       )         +       g   p   3          h        (     n   +     3      L       )         +   …   +       g   p     P   -   1            h        (     n   +       (     P   -   1     )        L       )                                  
     in which P is the number of pitch lag periods (whole or partial) of length L contained in the subframe, L is the pitch lag, and g p  is the pitch gain. 
     FIG. 3 is a block diagram of an apparatus for searching an excitation codebook in which the impulse response of the filter has been pitch enhanced. A frame of speech samples s(n) is filtered by a perceptual weighting filter  330  to produce a target signal x(n). An impulse response generator  310  generates an impulse response h(n). The impulse response h(n) is input into a pitch sharpener element  370  and yields a composite impulse response {tilde over (h)}(n). The composite impulse response {tilde over (h)}(n) and the target signal x(n) are input into a computation element  390  to determine a cross-correlation vector d(i) in accordance with the following relationship:            d        (   i   )       =       ∑     j   =   1     M                       x        (   i   )              h   ~          (     i   -   j     )             ,       for                 j     =     1                 to                   M   .                                
     The composite impulse response {tilde over (h)}(n) is also used by computation element  350  to generate an autocorrelation matrix:            φ        (     i   ,   j     )       =       ∑     n   =   j     M                         h   ~          (     n   -   i     )                         h   ~          (     n   -   j     )             ,       for                 i     ≥     j   .                              
     The entries of the autocorrelation matrix φ are sent to computation element  340 . Pulse codebook generator  300  generates a plurality of pulse vectors {c k , k=1, . . . , M}, which are also input into computation element  340 . Computation element  340  filters the pulse vectors with the autocorrelation matrix in accordance with the formula:          E   yy     =         ∑     i   =   0         N   p     -   1                       φ        (       p   i     ,     p   j       )         +     2   ·       ∑     i   =   0         N   p     -   1                         ∑     j   =     i   +   1           N   p     -   1                           c   k          (     p   i     )              c   k          (     p   j     )              φ        (       p   i     ,     p   j       )       .                                      
     The pulse vectors {c k , k=1, . . . , M} are also used by computation element  390  to determine a cross-correlation between d(n) and c k (n) according to the following equation:          E   xy   2     =         (       ∑     i   =   0         N   p     -   1                           c   k          (     p   i     )       ·     d        (     p   i     )           )     2     .                            
     Once values for E yy  and E xy  are known, a computation element  360  determines the value T k  using the following relationship:          T   k     =           (     E   xy     )     2       E   yy       .                            
     The pulse vector that corresponds to the largest value of T k  is selected as the optimum vector to encode the residual waveform. Since the composite impulse response {tilde over (h)}(n) is no longer stationary, the autocorrelation matrix cannot be simplified to a single-dimensional matrix, and the total number of elements required to store the φ matrix remain large. 
     The embodiments described below address the need for more efficient computational schemes within the new generation of coders, which are designed to enhance the contribution of pitch periodicity. The embodiments describe a methodology that may be considered counterintuitive to one skilled in the art, but appropriate choices in certain pitch period values can result in a beneficial result. In particular, a widely held belief in the art is that the number of pulses in the pulse code vector should remain small in order to minimize the number of bits needed to represent the vector. A pulse code vector is a vector with unit pulses in designated spaces, wherein the remaining spaces are designated as zero-valued. An example of a pulse vector with a small number of pulses is one with less than 14% of the available spaces occupied by a unit pulse. 
     The embodiments described herein deliberately increase the number of pulses within a code vector. In the coders that enhance the pitch of the impulse response, forward and backward lag values are folded into the window frame that is currently under analysis to form a composite impulse response. In these coders, the autocorrelation matrix φ is determined based on the composite impulse response. 
     The embodiments described herein avoid using the composite impulse response to determine the autocorrelation matrix φ. Rather than using a composite impulse response, the embodiments determine composite pulse codebook vectors, wherein the forward and backward lag values of a pulse code vector are folded back into the code vector. This incorporation of lag values increases the number of pulses in the code vector, which in turn, violates the commonly held belief that the number of code vector pulses should remain minimal. If a composite pulse code vector is used, the need to determine an autocorrelation matrix φ based on the composite impulse response no longer exists due to the following relationship: 
     
       
         c{tilde over (h)}={tilde over (c)}h. 
       
     
     The above equation states that the result of convolving a pulse code vector with a pitch-sharpened impulse response is equivalent to the result of convolving the pitch-sharpened pulse code vector with the impulse response. 
     If the impulse response rather than the composite impulse response is used to determine the autocorrelation matrix φ, then the embodiments herein implicitly assume that the impulse response could be extended with zero values. This assumption is contrary to the practice of folding non-zero lag values back into the impulse response as stated above. Using this assumption, the embodiments approximate the two-dimensional autocorrelation matrix φ with a one-dimensional autocorrelation matrix in order to perform a fast search for an optimal excitation or pulse waveform in coders that use pitch-sharpened impulse responses. 
     FIG. 4 is a block diagram of an apparatus that will perform a fast codebook search using composite pulse vectors. In one embodiment, the pulse vectors in the codebook are 80 samples long and the unit pulse can be located at any of the 80 sample positions. The number of unit pulses in each code vector should remain small, e.g., either 1 or 2 if there are 80 sample positions. Vectors with more pulses could be used in larger sized analysis windows. For each pulse, p i , a corresponding sign s i  is assigned to the pulse. The resulting code vector, c k , is given by the equation below            c   k          (   j   )       =       ∑     i   =   0         N   p     -   1                         s   i            δ        (     j   -     p   i       )       .                                
     A frame of speech samples s(n) is filtered by a perceptual weighting filter  430  to produce a target signal x(n). An impulse response generator  410  generates an impulse response h(n). The impulse response h(n) is input into a pitch sharpener element  470  and yields a composite impulse response {tilde over (h)}(n). The composite impulse response {tilde over (h)}(n) and the target signal x(n) are input into a computation element  490  to determine a cross-correlation vector d(i) in accordance with the following relationship:            d        (   i   )       =       ∑     j   =   1     M                       x        (   i   )              h   ~          (     i   -   j     )             ,       for                 j     =     1                 to                   M   .                                
     The impulse response h(n) is also used by computation element  450  to generate a single dimensional autocorrelation matrix:          φ        (   i   )       =       ∑     n   =   0       M   -   1                         h        (   n   )              h        (     n   -   i     )       .                                
     The entries of the autocorrelation matrix φ are sent to computation element  440 . Pulse codebook generator  400  generates a plurality of pulse vectors {c k , k=1, . . . , M}, which are altered by pitch sharpening element  420  to form composite pulse vectors in accordance with the following formula: 
     
       
           p   i   k   =p   i   0   +kL, k=−k   1   , −k   1 +1, . . . ,0,1,2 , . . . , k   2 , 
       
     
     where k 1 , and k 2  are chosen to be maximum in the range 0≦k 1 ,k 2 &lt;M such that 0≦p i   k &lt;M. Each primary pulse p i   0  will have 0 or more secondary pulses depending on the primary pulse position in the vector, and the pitch lag. For example, for lag L=33, vector size M=80, and the primary position of the i th  pulse being p i   0 =46, the secondary pulse positions are p i   −1 =13, and p i   1 =79. Hence, the composite pulse vector comprises primary pulses and secondary pulses. 
     The composite pulse vectors, the pulse vectors, and the autocorrelation matrix φ are input into computation element  440 . Computation element  440  filters the pulse vectors and the composite pulse vectors in accordance with the following formula:          E   yy     =         ∑     i   =   0         N   p     -   1                         ∑     v   =     -     k   1           k   2                         g   p        v               φ        (   0   )             +     2   ·       ∑     i   =   0         N   p     -   1                         ∑     w   =     -     k   1           k   2              ∑     j   =     i   +   1           N   p     -   1                         ∑     v   =     -     k   1           k   2              g   p        w               g   p        v                 c   k          (     p   i   0     )              c   k          (     p   j   0     )              φ        (            p   i   w     -     p   j   v            )       .                                          
     The pulse vectors {c k , k=1, . . . , M} are also used by computation element  490  to determine a cross-correlation between d(n) and c k (n) according to the following equation:          E   xy   2     =         (       ∑     i   =   0         N   p     -   1                c   k          (     p   i     )       ·     d        (     p   i     )           )     2     .                            
     Once values for E yy  and E xy  are known, a computation element  460  determines the value T k  using the following relationship:          T   k     =           (     E   xy     )     2       E   yy       .                            
     The pulse vector that corresponds to the largest value of T k  is selected as the optimum vector to encode the residual waveform. The above computation of E yy  has the advantage of incorporating the forward, and backward pitch sharpening into the codebook search in a low complexity method, thereby reducing the memory requirements to just M values for storing a single-dimensional φ(i) vector, unlike the existing requirement of a M×M values of a two dimensional matrix φ(i, j). 
     In an alternative configuration, a cross-correlation element  401  can be implemented that performs the function of generating the autocorrelation matrix φ and the cross-correlation value E xy . In another embodiment, the energy value E yy  can be generated using a pulse energy determination element  402  configured to generate a codebook and a composite representation of the codebook, and to compute the energy value using a received autocorrelation matrix. Alternatively, the pitch sharpener  470  could be implemented separately from the pulse code determination element  402 . In yet another embodiment, a single processor and memory can be configured to perform all functions of the individual components of FIG.  4 . 
     FIG. 5 is a flow chart illustrating a method for performing a fast codebook search in a coder that uses pitch-enhanced impulse responses. A processor and memory can be configured to perform the method steps. At step  500 , a primary pulse vector is generated. At step  502 , a composite pulse vector is generated comprising primary pulses and secondary pulses. At step  504 , a speech signal s(n) is filtered to produce a target signal x(n). At step  506 , an impulse response h(n) is generated. At step  508 , the impulse response h(n) is used to generate a pitch-enhanced composite impulse response {tilde over (h)}(n). At step  510 , a cross-correlation value d(i) is determined based on the composite impulse response {tilde over (h)}(n) and the target signal x(n). At step  512 , a single dimensional autocorrelation matrix φ is determined using the impulse response h(n). At step  514 , a value E xy  is determined using the cross-correlation value d(i) and the pulse vector. At step  516 , an energy value E xy  is determined using the autocorrelation matrix φ, the composite pulse vector, and the primary pulse vector. At step  518 , a maximal criterion T k  is determined using E xy  and E yy . At step  520 , the process is repeated for the next pulse vector of the codebook until all pulse vectors are exhausted. At step  522 , the pulse vector with the largest maximal criterion T k  is selected as the optimal excitation waveform to encode the speech signal within the analysis frame. 
     The method steps described above can be interchanged without affecting the scope of the embodiment described herein. For example, it is clearly possible to determine the value E yy  before the value E xy  without affecting the calculation for T k . 
     Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. 
     The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. 
     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.