3GPP TS V ( )

Transcription

1 TS 6.9 V.. (-9) Techncal Specfcaton 3rd Generaton Partnershp Project; Techncal Specfcaton Group Servces and System Aspects; Speech codec speech processng functons; Adaptve Mult-Rate - Wdeband (AMR-WB) speech codec; Transcodng functons (Release ) The present document has been developed wthn the 3 rd Generaton Partnershp Project ( TM ) and may be further elaborated for the purposes of. The present document has not been subject to any approval process by the Organzatonal Partners and shall not be mplemented. Ths Specfcaton s provded for future development work wthn only. The Organzatonal Partners accept no lablty for any use of ths Specfcaton. Specfcatons and reports for mplementaton of the TM system should be obtaned va the Organzatonal Partners' Publcatons Offces.

2 TS 6.9 V.. (-9) Keywords UMTS, GSM, codec, LTE Postal address support offce address 65 Route des Lucoles - Sopha Antpols Valbonne - FRANCE Tel.: Fax: Internet Copyrght Notfcaton No part may be reproduced except as authorzed by wrtten permsson. The copyrght and the foregong restrcton extend to reproducton n all meda., Organzatonal Partners (ARIB, ATIS, CCSA, ETSI, TTA, TTC). All rghts reserved. UMTS s a Trade Mark of ETSI regstered for the beneft of ts members s a Trade Mark of ETSI regstered for the beneft of ts Members and of the Organzatonal Partners LTE s a Trade Mark of ETSI currently beng regstered for the beneft of ts Members and of the Organzatonal Partners GSM and the GSM logo are regstered and owned by the GSM Assocaton

3 3 TS 6.9 V.. (-9) Contents Foreword... 5 Scope... 6 Normatve references Defntons, symbols and abbrevatons Defntons Symbols Abbrevatons... 4 Outlne descrpton Functonal descrpton of audo parts Preparaton of speech samples Prncples of the adaptve mult-rate wdeband speech encoder Prncples of the adaptve mult-rate speech decoder Sequence and subjectve mportance of encoded parameters Functonal descrpton of the encoder Pre-processng Lnear predcton analyss and quantzaton Wndowng and auto-correlaton computaton Levnson-Durbn algorthm LP to ISP converson ISP to LP converson Quantzaton of the ISP coeffcents Interpolaton of the ISPs Perceptual weghtng Open-loop ptch analyss kbt/s mode ,.65, 4.5, 5.85, 8.5, 9.85, 3.5 and 3.85 kbt/s modes Impulse response computaton Target sgnal computaton Adaptve codebook Algebrac codebook Codebook structure and 3.5 kbt/s mode kbt/s mode kbt/s mode kbt/s mode kbt/s mode kbt/s mode kbt/s mode kbt/s mode Pulse ndexng Codebook search Quantzaton of the adaptve and fxed codebook gans Memory update Hgh-band gan generaton Functonal descrpton of the decoder Decodng and speech synthess Hgh-pass flterng, up-scalng and nterpolaton Hgh frequency band Generaton of hgh-band exctaton LP flter for the hgh frequency band kbt/s mode ,.65, 4.5, 5.85, 8.5, 9.85, 3.5 or 3.85 kbt/s modes Hgh band synthess... 4

4 4 TS 6.9 V.. (-9) 7 Detaled bt allocaton of the adaptve mult-rate wdeband codec Homng sequences Functonal descrpton Defntons Encoder homng Decoder homng Bblography... 5 Annex A (nformatve): Change hstory... 5

5 5 TS 6.9 V.. (-9) Foreword The present document descrbes the detaled mappng of the wdeband telephony speech servce employng the Adaptve Mult-Rate (AMR-WB) speech coder wthn the system. The contents of the present document are subject to contnung work wthn the TSG and may change followng formal TSG approval. Should the TSG modfy the contents of ths TS, t wll be re-released by the TSG wth an dentfyng change of release date and an ncrease n verson number as follows: Verson x.y.z where: x the frst dgt: presented to TSG for nformaton; presented to TSG for approval; 3 Indcates TSG approved document under change control. y the second dgt s ncremented for all changes of substance,.e. techncal enhancements, correctons, updates, etc. z the thrd dgt s ncremented when edtoral only changes have been ncorporated n the specfcaton;

6 6 TS 6.9 V.. (-9) Scope Ths Telecommuncaton Standard (TS) descrbes the detaled mappng from nput blocks of 3 speech samples n 6-bt unform PCM format to encoded blocks of 3, 77, 53, 85, 37, 365, 397, 46 and 477 bts and from encoded blocks of 3, 77, 53, 85, 37, 365, 397, 46 and 477 bts to output blocks of 3 reconstructed speech samples. The samplng rate s 6 samples/s leadng to a bt rate for the encoded bt stream of 6.6, 8.85,.65, 4.5, 5.85, 8.5, 9.85, 3.5 or 3.85 kbt/s. The codng scheme for the mult-rate codng modes s the so-called Algebrac Code Excted Lnear Predcton Coder, hereafter referred to as ACELP. The mult-rate wdeband ACELP coder s referred to as MRWB-ACELP. Normatve references Ths TS ncorporates by dated and undated reference, provsons from other publcatons. These normatve references are cted n the approprate places n the text and the publcatons are lsted hereafter. For dated references, subsequent amendments to or revsons of any of these publcatons apply to ths TS only when ncorporated n t by amendment or revson. For undated references, the latest edton of the publcaton referred to apples. [] GSM 3.5: " Dgtal cellular telecommuncatons system (Phase ); Transmsson plannng aspects of the speech servce n the GSM Publc Land Moble Network (PLMN) system" [] TS 6. : "AMR wdeband speech codec; Frame structure". [3] TS 6.94: "AMR wdeband speech codec; Voce Actvty Detecton (VAD)". [4] TS 6.73: "AMR wdeband speech codec; ANSI-C code". [5] TS 6.74: "AMR wdeband speech codec; Test sequences". [6] ITU-T Recommendaton G.7 (988): "Codng of analogue sgnals by pulse code modulaton Pulse code modulaton (PCM) of voce frequences". 3 Defntons, symbols and abbrevatons 3. Defntons For the purposes of ths TS, the followng defntons apply: adaptve codebook: The adaptve codebook contans exctaton vectors that are adapted for every subframe. The adaptve codebook s derved from the long-term flter state. The lag value can be vewed as an ndex nto the adaptve codebook. algebrac codebook: A fxed codebook where algebrac code s used to populate the exctaton vectors (nnovaton vectors). The exctaton contans a small number of nonzero pulses wth predefned nterlaced sets of potental postons. The ampltudes and postons of the pulses of the k th exctaton codevector can be derved from ts ndex k through a rule requrng no or mnmal physcal storage, n contrast wth stochastc codebooks whereby the path from the ndex to the assocated codevector nvolves look-up tables. ant-sparseness processng: An adaptve post-processng procedure appled to the fxed codebook vector n order to reduce perceptual artfacts from a sparse fxed codebook vector. closed-loop ptch analyss: Ths s the adaptve codebook search,.e., a process of estmatng the ptch (lag) value from the weghted nput speech and the long term flter state. In the closed-loop search, the lag s searched usng error mnmzaton loop (analyss-by-synthess). In the adaptve mult-rate wdeband codec, closed-loop ptch search s performed for every subframe. drect form coeffcents: One of the formats for storng the short term flter parameters. In the adaptve mult-rate wdeband codec, all flters whch are used to modfy speech samples use drect form coeffcents.

7 7 TS 6.9 V.. (-9) fxed codebook: The fxed codebook contans exctaton vectors for speech synthess flters. The contents of the codebook are non-adaptve (.e., fxed). In the adaptve mult-rate wdeband codec, the fxed codebook s mplemented usng an algebrac codebook. fractonal lags: A set of lag values havng sub-sample resoluton. In the adaptve mult-rate wdeband codec a sub-sample resoluton of /4th or /nd of a sample s used. frame: A tme nterval equal to ms (3 samples at an 6 khz samplng rate). Immttance Spectral Frequences: (see Immttance Spectral Par) Immttance Spectral Par: Transformaton of LPC parameters. Immttance Spectral Pars are obtaned by decomposng the nverse flter transfer functon A(z) to a set of two transfer functons, one havng even symmetry and the other havng odd symmetry. The Immttance Spectral Pars (also called as Immttance Spectral Frequences) are the roots of these polynomals on the z-unt crcle. nteger lags: A set of lag values havng whole sample resoluton. nterpolatng flter: An FIR flter used to produce an estmate of sub-sample resoluton samples, gven an nput sampled wth nteger sample resoluton. In ths mplementaton, the nterpolatng flter has low pass flter characterstcs. Thus the adaptve codebook conssts of the low-pass fltered nterpolated past exctaton. nverse flter: Ths flter removes the short term correlaton from the speech sgnal. The flter models an nverse frequency response of the vocal tract. lag: The long term flter delay. Ths s typcally the true ptch perod, or ts multple or sub-multple. LP analyss wndow: For each frame, the short term flter coeffcents are computed usng the hgh pass fltered speech samples wthn the analyss wndow. In the adaptve mult-rate wdeband codec, the length of the analyss wndow s always 384 samples. For all the modes, a sngle asymmetrc wndow s used to generate a sngle set of LP coeffcents. The 5 ms look-ahead s used n the analyss. LP coeffcents: Lnear Predcton (LP) coeffcents (also referred as Lnear Predctve Codng (LPC) coeffcents) s a generc descrptve term for the short term flter coeffcents. mode: When used alone, refers to the source codec mode,.e., to one of the source codecs employed n the AMR-WB codec. open-loop ptch search: A process of estmatng the near optmal lag drectly from the weghted speech nput. Ths s done to smplfy the ptch analyss and confne the closed-loop ptch search to a small number of lags around the open-loop estmated lags. In the adaptve mult-rate wdeband codec, an open-loop ptch search s performed n every other subframe. resdual: The output sgnal resultng from an nverse flterng operaton. short term synthess flter: Ths flter ntroduces, nto the exctaton sgnal, short term correlaton whch models the mpulse response of the vocal tract. perceptual weghtng flter: Ths flter s employed n the analyss-by-synthess search of the codebooks. The flter explots the nose maskng propertes of the formants (vocal tract resonances) by weghtng the error less n regons near the formant frequences and more n regons away from them. subframe: A tme nterval equal to 5 ms (8 samples at 6 khz samplng rate). vector quantzaton: A method of groupng several parameters nto a vector and quantzng them smultaneously. zero nput response: The output of a flter due to past nputs,.e. due to the present state of the flter, gven that an nput of zeros s appled. zero state response: The output of a flter due to the present nput, gven that no past nputs have been appled,.e., gven that the state nformaton n the flter s all zeroes. 3. Symbols For the purposes of ths TS, the followng symbols apply:

8 A( z) A ( z) ( ) H z = A z ( ) 8 The nverse flter wth unquantzed coeffcents The nverse flter wth quantzed coeffcents The speech synthess flter wth quantzed coeffcents TS 6.9 V.. (-9) a a m W( z) γ T β The unquantzed lnear predcton parameters (drect form coeffcents) The quantfed lnear predcton parameters The order of the LP model The perceptual weghtng flter (unquantzed coeffcents) The perceptual weghtng factor The nteger ptch lag nearest to the closed-loop fractonal ptch lag of the subframe The adaptve pre-flter coeffcent (the quantfed ptch ga Hh ( z) Pre-processng hgh-pass flter w ( LP analyss wndow L Length of the frst part of the LP analyss wndow w ( L Length of the second part of the LP analyss wndow w ( r (k) The auto-correlatons of the wndowed speech s'( wlag ( ) Lag wndow for the auto-correlatons (6 Hz bandwdth expanso f f s The bandwdth expanson n Hz The samplng frequency n Hz r '( k) The modfed (bandwdth expanded) auto-correlatons E ( ) The predcton error n the th teraton of the Levnson algorthm k ( ) a j The th reflecton coeffcent The jth drect form coeffcent n the th teraton of the Levnson algorthm F ( z) F ( z) Symmetrc ISF polynomal Antsymmetrc ISF polynomal F ( z) Polynomal F ( z) F ( z) Polynomal F ( z) wth roots z = and = z elmnated

9 9 TS 6.9 V.. (-9) q q ( n q ) The mmttance spectral pars (ISPs) n the cosne doman An ISP vector n the cosne doman The quantfed ISP vector at the th subframe of the frame n ω T ( m x ) The mmttance spectral frequences (ISFs) A mth order Chebyshev polynomal f( ), f( ) The coeffcents of the polynomals F ( z ) and F ( z) ' ' f ( ), f ( ) The coeffcents of the polynomals F ( z) and F ( z) f ( ) The coeffcents of ether F ( z) or F ( z) ( ) C x x Sum polynomal of the Chebyshev polynomals Cosne of angular frequency ω λ k Recurson coeffcents for the Chebyshev polynomal evaluaton f t [ f f ] f6 The mmttance spectral frequences (ISFs) n Hz f = The vector representaton of the ISFs n Hz z ( The mean-removed ISF vector at frame n r ( The ISF predcton resdual vector at frame n p( n ) ( n ) The predcted ISF vector at frame n r ˆ The quantfed resdual vector at the past frame k rˆ The quantfed ISF subvector at quantzaton ndex k d The dstance between the mmttance spectral frequences f + and f ( ) h n The mpulse response of the weghted synthess flter H ( z) W ( z) The weghted synthess flter T s'( The nteger nearest to the fractonal ptch lag of the prevous (st or 3rd) subframe The wndowed speech sgnal sw ( The weghted speech sgnal s ( n ) ( ) x n Reconstructed speech sgnal The target sgnal for adaptve codebook search x (, x t The target sgnal for algebrac codebook search

10 TS 6.9 V.. (-9) reslp ( n ) ( ) c n ( ) v n The LP resdual sgnal The fxed codebook vector The adaptve codebook vector y( = v( h( The fltered adaptve codebook vector yk ( The past fltered exctaton ( ) u n The exctaton sgnal u '( n ) T op t mn t max The gan-scaled emphaszed exctaton sgnal The best open-loop lag Mnmum lag search value Maxmum lag search value R( k) Correlaton term to be maxmzed n the adaptve codebook search R( k R ) ( k) t The nterpolated value of for the nteger delay k and fracton t A k C k Correlaton term to be maxmzed n the algebrac codebook search at ndex k The correlaton n the numerator of A k at ndex k E D k The energy n the denomnator of A k at ndex k t d = H x The correlaton between the target sgnal x ( n ) and the mpulse response h ( fltered target,.e., backward H The lower trangular Toeplz convoluton matrx wth dagonal h ( ) and lower dagonals h( ),, h( 63) = h( The matrx of correlatons of Φ H t H d( The elements of the vector d φ(, j ) c k The elements of the symmetrc matrx Φ The nnovaton vector C The correlaton n the numerator of A k m ϑ N p The poston of the th pulse The ampltude of the th pulse The number of pulses n the fxed codebook exctaton

11 TS 6.9 V.. (-9) E D The energy n the denomnator of A k ( resltp The normalzed long-term predcton resdual ( ) b n The sgnal used for presettng the sgns n algebrac codebook search sb ( The sgn sgnal for the algebrac codebook search d ( n ) Sgn extended backward fltered target φ ' (, j ) The modfed elements of the matrx Φ, ncludng sgn nformaton z t, z( n ) The fxed codebook vector convolved wth h( n ) ( ) E n E ~ E( n ) The mean-removed nnovaton energy (n db) The mean of the nnovaton energy The predcted energy [ b b b3 b4 ] The MA predcton coeffcents R ( k ) E I The quantfed predcton error at subframe k The mean nnovaton energy R( The predcton error of the fxed-codebook gan quantzaton E Q The quantzaton error of the fxed-codebook gan quantzaton e( The states of the synthess flter A ( z ) ew ( The perceptually weghted error of the analyss-by-synthess search η g c g c g c g p The gan scalng factor for the emphaszed exctaton The fxed-codebook gan The predcted fxed-codebook gan The quantfed fxed codebook gan The adaptve codebook gan g p The quantfed adaptve codebook gan = g g A correcton factor between the gan g c and the estmated one γ gc c c g c γ gc The optmum value for γ gc γ sc Gan scalng factor

12 TS 6.9 V.. (-9) 3.3 Abbrevatons For the purposes of ths TS, the followng abbrevatons apply. ACELP Algebrac Code Excted Lnear Predcton AGC Adaptve Gan Control AMR Adaptve Mult-Rate AMR-WB Adaptve Mult-Rate Wdeband CELP Code Excted Lnear Predcton FIR Fnte Impulse Response ISF Immttance Spectral Frequency ISP Immttance Spectral Par ISPP Interleaved Sngle-Pulse Permutaton LP Lnear Predcton LPC Lnear Predctve Codng LTP Long Term Predctor (or Long Term Predcto MA Movng Average MRWB-ACELP Wdeband Mult-Rate ACELP S-MSVQ Splt-MultStage Vector Quantzaton WB Wdeband 4 Outlne descrpton Ths TS s structured as follows: Secton 4. contans a functonal descrpton of the audo parts ncludng the A/D and D/A functons. Secton 4. descrbes nput format for the AMR-WB encoder and the output format for the AMR-WB decoder. Sectons 4.3 and 4.4 present a smplfed descrpton of the prncples of the AMR-WB codec encodng and decodng process respectvely. In subclause 4.5, the sequence and subjectve mportance of encoded parameters are gven. Secton 5 presents the functonal descrpton of the AMR-WB codec encodng, whereas clause 6 descrbes the decodng procedures. In secton 7, the detaled bt allocaton of the AMR-WB codec s tabulated. Secton 8 descrbes the homng operaton. 4. Functonal descrpton of audo parts The analogue-to-dgtal and dgtal-to-analogue converson wll n prncple comprse the followng elements: ) Analogue to unform dgtal PCM - mcrophone; - nput level adjustment devce; - nput ant-alasng flter; - sample-hold devce samplng at 6 khz; - analogue-to-unform dgtal converson to 4-bt representaton. The unform format shall be represented n two's complement. ) Unform dgtal PCM to analogue - converson from 4-bt/6 khz unform PCM to analogue; - a hold devce; - reconstructon flter ncludng x/sn( x ) correcton; - output level adjustment devce;

13 3 TS 6.9 V.. (-9) - earphone or loudspeaker. In the termnal equpment, the A/D functon may be acheved - by drect converson to 4-bt unform PCM format; For the D/A operaton, the nverse operatons take place. 4. Preparaton of speech samples The encoder s fed wth data comprsng of samples wth a resoluton of 4 bts left justfed n a 6-bt word. The decoder outputs data n the same format. Outsde the speech codec further processng must be appled f the traffc data occurs n a dfferent representaton. 4.3 Prncples of the adaptve mult-rate wdeband speech encoder The AMR-WB codec conssts of nne source codecs wth bt-rates of , 9.85, 8.5, 5.85, 4.5,.65, 8.85 and 6.6 kbt/s. The codec s based on the code-excted lnear predctve (CELP) codng model. The nput sgnal s pre-emphaszed usng the flter H pre-emph (z)= µz. The CELP model s then appled to the pre-emphaszed sgnal. A 6th order lnear predcton (LP), or short-term, synthess flter s used whch s gven by: H( z) = A = ( z) m + a z =, ( ) where â,=,,m are the (quantzed) lnear predcton (LP) parameters, and m = 6 s the predctor order. The long-term, or ptch, synthess flter s usually gven by: = B( z) T g pz, ( ) where T s the ptch delay and g p s the ptch gan. The ptch synthess flter s mplemented usng the so-called adaptve codebook approach. The CELP speech synthess model s shown n Fgure. In ths model, the exctaton sgnal at the nput of the short-term LP synthess flter s constructed by addng two exctaton vectors from adaptve and fxed (nnovatve) codebooks. The speech s syntheszed by feedng the two properly chosen vectors from these codebooks through the short-term synthess flter. The optmum exctaton sequence n a codebook s chosen usng an analyss-by-synthess search procedure n whch the error between the orgnal and syntheszed speech s mnmzed accordng to a perceptually weghted dstorton measure. The perceptual weghtng flter used n the analyss-by-synthess search technque s gven by: where A(z) s the unquantzed LP flter, H de emph =.68z weghtng flter uses the unquantzed LP parameters. W ( z) = A( z / γ ) H ( z), ( 3 ) de emph, and γ =.9 s the perceptual weghtng factor. The The encoder performs the analyss of the LPC, LTP and fxed codebook parameters at.8 khz samplng rate. The coder operates on speech frames of ms. At each frame, the speech sgnal s analysed to extract the parameters of the CELP model (LP flter coeffcents, adaptve and fxed codebooks' ndces and gans). In addton to these parameters, hgh-band gan ndces are computed n 3.85 kbt/s mode. These parameters are encoded and transmtted. At the decoder, these parameters are decoded and speech s syntheszed by flterng the reconstructed exctaton sgnal through the LP synthess flter.

14 4 TS 6.9 V.. (-9) The sgnal flow at the encoder s shown n Fgure. After decmaton, hgh-pass and pre-emphass flterng s performed. LP analyss s performed once per frame. The set of LP parameters s converted to mmttance spectrum pars (ISP) and vector quantzed usng splt-multstage vector quantzaton (S-MSVQ). The speech frame s dvded nto 4 subframes of 5 ms each (64 samples at.8 khz samplng rate). The adaptve and fxed codebook parameters are transmtted every subframe. The quantzed and unquantzed LP parameters or ther nterpolated versons are used dependng on the subframe. An open-loop ptch lag s estmated n every other subframe or once per frame based on the perceptually weghted speech sgnal. Then the followng operatons are repeated for each subframe: - The target sgnal x( s computed by flterng the LP resdual through the weghted synthess flter W ( z) H ( z) wth the ntal states of the flters havng been updated by flterng the error between LP resdual and exctaton (ths s equvalent to the common approach of subtractng the zero nput response of the weghted synthess flter from the weghted speech sgnal). - The mpulse response, h( of the weghted synthess flter s computed. - Closed-loop ptch analyss s then performed (to fnd the ptch lag and ga, usng the target x( and mpulse response h(, by searchng around the open-loop ptch lag. Fractonal ptch wth /4th or /nd of a sample resoluton (dependng on the mode and the ptch lag value) s used. The nterpolatng flter n fractonal ptch search has low pass frequency response. Further, there are two potental low-pass characterstcs n the the adaptve codebook and ths nformaton s encoded wth bt. - The target sgnal x( s updated by removng the adaptve codebook contrbuton (fltered adaptve codevector), and ths new target, x (, s used n the fxed algebrac codebook search (to fnd the optmum nnovato. - The gans of the adaptve and fxed codebook are vector quantfed wth 6or 7 bts (wth movng average (MA) predcton appled to the fxed codebook ga. - Fnally, the flter memores are updated (usng the determned exctaton sgnal) for fndng the target sgnal n the next subframe. The bt allocaton of the AMR-WB codec modes s shown n Table. In each ms speech frame, 3, 77, 53, 85, 37, 365, 397, 46 and 477 bts are produced, correspondng to a bt-rate of 6.6, 8.85,.65, 4.5, 5.85, 8.5, 9.85, 3.5 or 3.85 kbt/s. More detaled bt allocaton among the codec parameters s gven n tables a-. Note that the most sgnfcant bts (MSB) are always sent frst.

15 5 TS 6.9 V.. (-9) Table : Bt allocaton of the AMR-WB codng algorthm for ms frame Mode Parameter st subframe nd subframe 3rd subframe 4th subframe total per frame VAD-flag 3.85 kbt/s ISP 46 LTP-flterng 4 Ptch delay Algebrac code Codebook gan HB-energy Total 477 VAD-flag 3.5 kbt/s ISP 46 LTP-flterng 4 Ptch delay Algebrac code Gans Total 46 VAD-flag 9.85 kbt/s ISP 46 LTP-flterng 4 Ptch delay Algebrac code Codebook gan Total 397 VAD-flag 8.5 kbt/s ISP 46 LTP-flterng 4 Ptch delay Algebrac code Gans Total 365 VAD-flag 5.85 kbt/s ISP 46 LTP-flterng 4 Ptch delay Algebrac code Gans Total 37 VAD-flag 4.5 kbt/s ISP 46 LTP-flterng 4 Ptch delay Algebrac code Gans Total 85 VAD-flag.65 kbt/s ISP 46 LTP-flterng 4 Ptch delay Algebrac code Gans Total 53 VAD-flag 8.85 kbt/s ISP 46 Ptch delay Algebrac code 8 Gans Total 77 VAD-flag 6.6 kbt/s ISP 36 Ptch delay Algebrac code 48 Gans Total 3

16 6 TS 6.9 V.. (-9) 4.4 Prncples of the adaptve mult-rate speech decoder The sgnal flow at the decoder s shown n Fgure 3. At the decoder, the transmtted ndces are extracted from the receved btstream. The ndces are decoded to obtan the coder parameters at each transmsson frame. These parameters are the ISP vector, the 4 fractonal ptch lags, the 4 LTP flterng parameters, the 4 nnovatve codevectors, and the 4 sets of vector quantzed ptch and nnovatve gans. In 3.85 kbt/s mode, also hgh-band gan ndex s decoded. The ISP vector s converted to the LP flter coeffcents and nterpolated to obtan LP flters at each subframe. Then, at each 64-sample subframe: - The exctaton s constructed by addng the adaptve and nnovatve codevectors scaled by ther respectve gans. - The.8 khz speech s reconstructed by flterng the exctaton through the LP synthess flter. - The reconstructed speech s de-emphaszed. Fnally, the reconstructed speech s upsampled to 6 khz and hgh-band speech sgnal s added to the frequency band from 6 khz to 7 khz. 4.5 Sequence and subjectve mportance of encoded parameters The encoder wll produce the output nformaton n a unque sequence and format, and the decoder must receve the same nformaton n the same way. In table a-, the sequence of output bts and the bt allocaton for each parameter s shown. The dfferent parameters of the encoded speech and ther ndvdual bts have unequal mportance wth respect to subjectve qualty. The output and nput frame formats for the AMR wdeband speech codec are gven n [], where a reorderng of bts take place. 5 Functonal descrpton of the encoder In ths clause, the dfferent functons of the encoder represented n Fgure are descrbed. 5. Pre-processng The encoder performs the analyss of the LPC, LTP and fxed codebook parameters at.8 khz samplng rate. Therefore, the nput sgnal has to be decmated from 6 khz to.8 khz. The decmaton s performed by frst upsamplng by 4, then flterng the output through lowpass FIR flter H decm (z) that has the cut off frequency at 6.4 khz. Then, the sgnal s downsampled by 5. The flterng delay s compensated by addng zeroes nto the end of the nput vector. After the decmaton, two pre-processng functons are appled to the sgnal pror to the encodng process: hgh-pass flterng and pre-emphaszng (and sgnal down-scalng). (Down-scalng conssts of dvdng the nput by a factor of to reduce the possblty of overflows n the fxed-pont mplementaton.) The hgh-pass flter serves as a precauton aganst undesred low frequency components. A flter at a cut off frequency of 5 Hz s used, and t s gven by z z H h ( z) =. ( 4 ).97888z z (Both down-scalng and hgh-pass flterng are combned by dvdng the coeffcents at the numerator of H h (z) by.) In the pre-emphass, a frst order hgh-pass flter s used to emphasze hgher frequences, and t s gven by emph ( z) =.68z H pre ( 5 )

17 7 TS 6.9 V.. (-9) 5. Lnear predcton analyss and quantzaton Short-term predcton, or LP, analyss s performed once per speech frame usng the autocorrelaton approach wth 3 ms asymmetrc wndows. An overhead of 5 ms s used n the autocorrelaton computaton. The frame structure s depcted below. wndowng frame n- wndowng frame n frame n (4 x 5 ms) The autocorrelatons of wndowed speech are converted to the LP coeffcents usng the Levnson-Durbn algorthm. Then the LP coeffcents are transformed to the ISP doman for quantzaton and nterpolaton purposes. The nterpolated quantzed and unquantzed flters are converted back to the LP flter coeffcents (to construct the synthess and weghtng flters at each subframe). 5.. Wndowng and auto-correlaton computaton LP analyss s performed once per frame usng an asymmetrc wndow. The wndow has ts weght concentrated at the fourth subframe and t conssts of two parts: the frst part s a half of a Hammng wndow and the second part s a quarter of a Hammng-cosne functon cycle. The wndow s gven by: πn w( = cos, L π ( n L ) = cos, 4 L n =,, L n = L,, L + L, ( 6 ) where the values L =56 and L =8 are used. The autocorrelatons of the wndowed speech s'(,n=,,383 are computed by 383 r( k) = = n k s' ( s'( n k), k =,,6, ( 7 ) and a 6 Hz bandwdth expanson s used by lag wndowng the autocorrelatons usng the wndow [] w lag πf ( ) = exp, =, 6, f s ( 8 ) where f =6 Hz s the bandwdth expanson and f s =8 Hz s the samplng frequency. Further, r() s multpled by the whte nose correcton factor. whch s equvalent to addng a nose floor at -4 db. 5.. Levnson-Durbn algorthm The modfed autocorrelatons r '() =.r() and r '( k) = r( k) w lag ( k), k =, 6, are used to obtan the LP flter coeffcents a k,k=,,6 by solvng the set of equatons. 6 k = ( k ) = r' ( ), =,,6. ak r' ( 9 ) The set of equatons n (9) s solved usng the Levnson-Durbn algorthm []. Ths algorthm uses the followng recurson:

18 8 TS 6.9 V.. (-9) The fnal soluton s gven as E() = r' () For = to 6 do k = r' ( ) + a ( ) For = k E( ) = j = to do a ( ) j = a ( ) j j = a ( k ) E( ) (6) a j = a j,j=,,6. j + k a r' ( j) / E( ) ( ) j The LP flter coeffcents are converted to the ISP representaton [4] for quantzaton and nterpolaton purposes. The conversons to the ISP doman and back to the LP flter doman are descrbed n the next two sectons LP to ISP converson The LP flter coeffcents a k, k=,,6, are converted to the ISP representaton for quantzaton and nterpolaton purposes. For a 6th order LP flter, the ISPs are defned as the roots of the sum and dfference polynomals and ' 6 f ( z) = A( z) + z A( z ) ( ) ' 6 f ( z) = A( z) z A( z ) ( ) respectvely. (The polynomals f' (z) and f' (z) are symmetrc and antsymmetrc, respectvely). It can be proven that all roots of these polynomals are on the unt crcle and they alternate each other [5]. f' (z) has two roots at z = (ω=) and z = - (ω = π). To elmnate these two roots, we defne the new polynomals and ' z f ( z) = f ( ) ( ) ' f ( z) = f ( z) /( z ± Polynomals f (z) and f (z) have 8 and 7 conjugate roots on the unt crcle ( e j ω ) polynomals can be wrtten as ( [ ]) ( + a 6 q z + z ) ). ( 3 ) respectvely. Therefore, the F = ( 4 ) ( z) =,,...,4 and ( [ ]) ( a 6 q z + z ) F ( z) = ( 5 ) =,3,...,3 where q =cos(ω ) wth ω beng the mmttance spectral frequences (ISF) and a[6] s the last predctor coeffcent. ISFs satsfy the orderng property ω < ω < < ω < π. We refer to q as the ISPs n the cosne doman. < 6 Snce both polynomals f (z) and f (z) are symmetrc only the frst 8 and 7 coeffcents of each polynomal, respectvely, and the last predctor coeffcent need to be computed. The coeffcents of these polynomals are found by the recursve relatons for = to 7

19 9 TS 6.9 V.. (-9) f ( ) = f ( ) = a a + a a m m, + f ( ). ( 6 ) f ( 8) = a where m=6 s the predctor order, and f ) = f ( ). ( = 8 The ISPs are found by evaluatng the polynomals F (z) and F (z) at ponts equally spaced between and π and checkng for sgn changes. A sgn change sgnfes the exstence of a root and the sgn change nterval s then dvded 4 tmes to better track the root. The Chebyshev polynomals are used to evaluate F (z) and F (z) [6]. In ths method the roots are found drectly n the cosne doman {q }. The polynomals F (z) and F (z) evaluated at z = e jω can be wrtten as wth 7 j8ω ( x j7ω ( x F ω) = e C ( ) and F ω) = e C ( ) ( 7 ) C( x) = f( ) T8 ( x) + f(8)/, and C( x) = f( ) T8 ( x) f(7)/, = + ( 8 ) = where T m =cos(mω) s the mth order Chebyshev polynomal, f() are the coeffcents of ether F (z) or F (z), computed usng the equatons n (6). The polynomal C(x) s evaluated at a certan value of x = cos(ω) usng the recursve relaton: for k = n b k = xb end C( x) = xb f - down to k + b b k + + f ( n + f ( n f f 6 ) /, k) where n f =8 n case of C (x) and n f =7 n case of C (x), wth ntal values b nf =f() and b nf+ =. The detals of the Chebyshev polynomal evaluaton method are found n [6] ISP to LP converson Once the ISPs are quantzed and nterpolated, they are converted back to the LP coeffcent doman { a k }. The converson to the LP doman s done as follows. The coeffcents of F (z) and F (z) are found by expandng Equatons (4) and (5) knowng the quantzed and nterpolated ISPs q =,=,,m-, where m=6. The followng recursve relaton s used to compute f (z) for = to m / end f ( ) = q for j = down to f( j) = f( j) q end f () = f () q f ( ) + f ( ) f ( j ) + f ( j ) wth ntal values f ()= and f ()=-q. The coeffcents f () are computed smlarly by replacng q - by q - and m/ by m/-, and wth ntal condtons f ()= and f ()=-q. Once the coeffcents f (z) and f (z) are found, F (z) s multpled by -z -, to obtan F' (z); that s ' f ( ) ' = f ( ) f f ( ) = f ( ) ( ), =,, m /, =,, m / ( 9 )

20 TS 6.9 V.. (-9) Then F' (z) and F' (z) are multpled by +q m- and -q m-, respectvely. That s ' f ( ) = (- q ' f ( ) = (+ q m- m- ) f ' ( ), ) f ' ( ) =,, m /, =,, m / Fnally the LP coeffcents are found by a =.5 f ( ) +.5 f ( ),.5 f ( ).5 f ( ),.5 f ( m / ), q ' ', m ' ' ' =,, m /, = m / +,, m, = m /, = m. ( ) ' Ths s drectly derved from the relaton A ( z) = ( F ( z) + F ( z)) /, and consderng the fact that F' (z) and F' (z) are symmetrc and antsymmetrc polynomals, respectvely Quantzaton of the ISP coeffcents The LP flter coeffcents are quantzed usng the ISP representaton n the frequency doman; that s ' f S f = arccos( q ), π f S = arccos( q ), 4π =, 4, = 5, ( ) where f are the ISFs n Hz [,64] and f s =8 s the samplng frequency. The ISF vector s gven by [f f,,f 5 ], wth t denotng transpose. t f = A st order MA predcton s appled, and the resdual ISF vector s quantfed usng a combnaton of splt vector quantzaton (SVQ) and mult-stage vector quantzaton (MSVQ). The predcton and quantzaton are performed as z n denote the mean-removed ISF vector at frame n. The predcton resdual vector r( s gven by: follows. Let ( ) r ( z( p( = ( ) where p( s the predcted LSF vector at frame n. Frst order movng-average (MA) predcton s used where: where ˆ( n ) r s the quantzed resdual vector at the past frame. p ( = rˆ ( n ), ( 3 ) 3 The ISF resdual vector r s quantzed usng splt-multstage vector quantzaton S-MSVQ. The vector s splt nto subvectors r ( and r ( of dmensons 9 and 7, respectvely. The subvectors are quantzed n two stages. In the frst stage r ( s quantzed wth 8 bts and r ( wth 8 bts. For 8.85,.65, 4.5, 5.85, 8.5, 9.85, 3.5 or 3.85 kbt/s modes, the quantzaton error vectors () r = r rˆ, =, are splt n the next stage nto 3 and subvectors, respectvely. The subvectors are quantzed usng the bt-rates descrbed n Table.

21 TS 6.9 V.. (-9) Table. Quantzaton of ISP vector for the 8.85,.65, 4.5, 5.85, 8.5, 9.85, 3.5 or 3.85 kbt/s modes. UNQUANTIZED 6-ELEMENT-LONG ISP VECTOR. STAGE ( r ) 8 bts. STAGE ( r ) 8 bts 3. STAGE r ) () (, 6 bts 3. STAGE r ) () (,3 5 7 bts 3. STAGE r ) () (,6 8 7 bts 3. STAGE r ) () (, 5 bts 3. STAGE r ) () (,3 6 () For 6.6 kbt/s mode, the quantzaton error vectors r = r rˆ, =, are splt n the next stage nto and subvectors, respectvely. The subvectors are quantzed usng the bt-rates descrbed n Table 3. 5 bts Table 3. Quantzaton of ISP vector for the 6.6 kbt/s mode. UNQUANTIZED 6-ELEMENT-LONG ISP VECTOR. STAGE ( r ) 8 bts. STAGE ( r ) 8 bts 3. STAGE r ) () (, 4 7 bts 3. STAGE r ) () (,5 8 7 bts 3. STAGE r ) () (, 6 A squared error ISP dstorton measure s used n the quantzaton process. In general, for an nput ISP or error resdual k subvector r,=, and a quantzed vector at ndex k, rˆ, the quantzaton s performed by fndng the ndex k whch mnmzes n [ ] k r rˆ = m where m and n are the frst and last elements of the subvector Interpolaton of the ISPs 6 bts E =, ( 4 ) The set of quantzed (and unquantzed) LP parameters s used for the fourth subframe whereas the frst, second, and thrd subframes use a lnear nterpolaton of the parameters n the adjacent frames. The nterpolaton s performed on the ( ) ISPs n the q doman. Let ˆ n ( n ) q 4 be the ISP vector at the 4th subframe of the frame, and q ˆ 4 the ISP vector at the 4th subframe of the past frame n-. The nterpolated ISP vectors at the st, nd, and 3rd subframes are gven by qˆ qˆ qˆ ( ( ( 3 =.55ˆ q4 ( n =.qˆ 4 ( =.4qˆ ( n ) ) n ) 4 ( +.45ˆ q4 ( +.8ˆ q4, ( +.96ˆ q The same formula s used for nterpolaton of the unquantzed ISPs. The nterpolated ISP vectors are used to compute a dfferent LP flter at each subframe (both quantzed and unquantzed) usng the ISP to LP converson method descrbed n Secton Perceptual weghtng The tradtonal perceptual weghtng flter W ( z) = A( z / γ ) / A( z / γ ) has nherent lmtatons n modellng the formant structure and the requred spectral tlt concurrently. The spectral tlt s more pronounced n wdeband sgnals due to the wde dynamc range between low and hgh frequences. A soluton to ths problem s to ntroduce the preemphass flter at the nput, compute the LP flter A(z) based on the preemphaszed speech s(, and use a modfed flter W(z) by fxng ts denomnator. Ths structure substantally decouples the formant weghtng from the tlt., 4.

22 TS 6.9 V.. (-9) A weghtng flter of the form W ( z) = A( z / γ ) H ( z) s used, where de emph H de emph = β z and β =.68. Because A(z) s computed based on the preemphaszed speech sgnal s(, the tlt of the flter /A(z/γ ) s less pronounced compared to the case when A(z) s computed based on the orgnal speech. Snce deemphass s performed at the decoder end, t can be shown that the quantzaton error spectrum s shaped by a flter havng a transfer functon W - (z)h de-emph (z)=/a(z/γ ). Thus, the spectrum of the quantzaton error s shaped by a flter whose transfer functon s /A(z/γ ), wth A(z) computed based on the preemphaszed speech sgnal. 5.4 Open-loop ptch analyss Dependng on the mode, open-loop ptch analyss s performed once per frame (each ms) or twce per frame (each ms) to fnd two estmates of the ptch lag n each frame. Ths s done n order to smplfy the ptch analyss and confne the closed loop ptch search to a small number of lags around the open-loop estmated lags. Open-loop ptch estmaton s based on the weghted speech sgnal sw ( whch s obtaned by flterng the nput speech sgnal through the weghtng flter W ( z) = A( z / γ ) H de emph ( z), where H de emph = and β =.68. βz That s, n a subframe of sze L, the weghted speech s gven by s 6 w( = = s( + aγ s( n ) + β sw( n ), n =,..., L. ( 5 ) The open-loop ptch analyss s performed to a sgnal decmated by two. The decmated sgnal s obtaned by flterng sw ( through a fourth order FIR flter H decm ( z ) and then downsamplng the output by two to obtan the sgnal s wd ( kbt/s mode Open-loop ptch analyss s performed once per frame (every ms) to fnd an estmate of the ptch lag n each frame. The open-loop ptch analyss s performed as follows. Frst, the correlaton of decmated weghted speech s determned for each ptch lag value d by: C 8 ( d ) s ( s ( n d ) w( d ), d = 7,, 5 = n= wd wd, ( 6 ) where w(d) s a weghtng functon. The estmated ptch-lag s the delay that maxmses the weghted correlaton functon C(d). The weghtng emphasses lower ptch lag values reducng the lkelhood of selectng a multple of the correct delay. The weghtng functon conssts of two parts: a low ptch lag emphass functon, w l (d), and a prevous frame lag neghbourng emphass functon, w n (d): The low ptch lag emphass functon s a gven by: w ( d ) w ( d ) w ( d ) =. ( 7 ) l n ( d ) cw( d ) w l = ( 8 ) where cw(d) s defned by a table n the fxed pont computatonal descrpton. The prevous frame lag neghbourng emphass functon depends on the ptch lag of prevous speech frames: ( T d + ), cw old 98 v >.8, wn ( d ) = ( 9 )., otherwse,

23 3 TS 6.9 V.. (-9) where T old s the medan fltered ptch lag of 5 prevous voced speech half-frames and v s an adaptve parameter. If the frame s classfed as voced by havng the open-loop gan g>.6, then the v-value s set to. for the next frame. Otherwse, the v-value s updated by v=.9v. The open loop gan s gven by: 7 wd n= n= ( s ( n d ) g = ( 3 ) 7 s s wd 7 wd max ( s ( n d ) wd n= where d max s the ptch delay that maxmzes C(d). The medan flter s updated only durng voced speech frames. The weghtng depends on the relablty of the old ptch lags. If prevous frames have contaned unvoced speech or slence, the weghtng s attenuated through the parameter v ,.65, 4.5, 5.85, 8.5, 9.85, 3.5 and 3.85 kbt/s modes Open-loop ptch analyss s performed twce per frame (every ms) to fnd two estmates of the ptch lag n each frame. The open-loop ptch analyss s performed as follows. Frst, the correlaton of decmated weghted speech s determned for each ptch lag value d by: C 63 max ( d ) s ( s ( n d ) w( d ), d = 7,, 5 = n= wd wd, ( 3 ) where w(d) s a weghtng functon. The estmated ptch-lag s the delay that maxmses the weghted correlaton functon C(d). The weghtng emphasses lower ptch lag values reducng the lkelhood of selectng a multple of the correct delay. The weghtng functon conssts of two parts: a low ptch lag emphass functon, w l (d), and a prevous frame lag neghbourng emphass functon, w n (d): w ( d ) w ( d ) w ( d ) The low ptch lag emphass functon s gven by: =. ( 3 ) ( d ) cw( d ) l n w l = ( 33 ) where cw(d) s defned by a table n the fxed pont computatonal descrpton. The prevous frame lag neghbourng emphass functon depends on the ptch lag of prevous speech frames: ( T d + ), cw old 98 v >.8, wn ( d ) = ( 34)., otherwse, where T old s the medan fltered ptch lag of 5 prevous voced speech half-frames and v s an adaptve parameter. If the frame s classfed as voced by havng the open-loop gan g>.6, then the v-value s set to. for the next frame. Otherwse, the v-value s updated by v=.9v. The open loop gan s gven by: n= ( s ( n d ) g = ( 35) wd n= s s wd 63 wd max ( s ( n d ) wd n= where d max s the ptch delay that maxmzes C(d). The medan flter s updated only durng voced speech frames. The weghtng depends on the relablty of the old ptch lags. If prevous frames have contaned unvoced speech or slence, the weghtng s attenuated through the parameter v. max

24 4 TS 6.9 V.. (-9) 5.5 Impulse response computaton = γ ˆ( s computed each The mpulse response, h(, of the weghted synthess flter H ( z) W ( z) A( z / ) H ( z) / A z) de emph subframe. Ths mpulse response s needed for the search of adaptve and fxed codebooks. The mpulse response h( s computed by flterng the vector of coeffcents of the flter A(z/γ ) extended by zeros through the two flters / ˆ( z) H z A and ( ) de emph. 5.6 Target sgnal computaton The target sgnal for adaptve codebook search s usually computed by subtractng the zero-nput response of the weghted synthess flter H ( z) W ( z) = A( z / γ ) Hde emph ( z) / Aˆ( z) from the weghted speech sgnal s w (. Ths s performed on a subframe bass. An equvalent procedure for computng the target sgnal, whch s used n ths codec, s the flterng of the LP resdual sgnal r( through the combnaton of synthess flter / A ˆ( z) and the weghtng flter A( z / γ ) H de emph ( z). After determnng the exctaton for the subframe, the ntal states of these flters are updated by flterng the dfference between the LP resdual and exctaton. The memory update of these flters s explaned n Secton 5.. The resdual sgnal r( whch s needed for fndng the target vector s also used n the adaptve codebook search to extend the past exctaton buffer. Ths smplfes the adaptve codebook search procedure for delays less than the subframe sze of 64 as wll be explaned n the next secton. The LP resdual s gven by 5.7 Adaptve codebook 6 r( = s( + aˆ s( n ), n =,...,63. ( 36 ) = Adaptve codebook search s performed on a subframe bass. It conssts of performng closed loop ptch search, and then computng the adaptve codevector by nterpolatng the past exctaton at the selected fractonal ptch lag. The adaptve codebook parameters (or ptch parameters) are the delay and gan of the ptch flter. In the search stage, the exctaton s extended by the LP resdual to smplfy the closed-loop search. In.65, 4.5, 5.85, 8.5, 9.85, 3.5 or 3.85 kbt/s modes, n the frst and thrd subframes, a fractonal ptch delay s used wth resolutons /4 n the range[34, ], resolutons / n the range [8, 59 ], and ntegers only n the range [6, 3]. For the second and fourth subframes, a ptch resoluton of /4 s always used n the range [T - 8, T ], where T s nearest nteger to the fractonal ptch lag of the prevous (st or 3rd) subframe. In 8.85 kbt/s mode, n the frst and thrd subframes, a fractonal ptch delay s used wth resolutons / n the range [34, 9 ], and ntegers only n the range [9, 3]. For the second and fourth subframes, a ptch resoluton of / s always used n the range [T -8, T +7 ], where T s nearest nteger to the fractonal ptch lag of the prevous (st or 3rd) subframe. In 6.6 kbt/s mode, n the frst subframe, a fractonal ptch delay s used wth resolutons / n the range [34,9 ], and ntegers only n the range [9, 3]. For the second, thrd and fourth subframes, a ptch resoluton of / s always used n the range [T -8, T +7 ], where T s nearest nteger to the fractonal ptch lag of the frst subframe. Closed-loop ptch analyss s performed around the open-loop ptch estmates on a subframe bass. In 8.85,.65, 4.5, 5.85, 8.5, 9.85, 3.5 or 3.85 kbt/s modes, n the frst (and thrd) subframe the range T op ±7, bounded by ,

25 5 TS 6.9 V.. (-9) s searched. In 6.6 kbt/s mode, n the frst subframe the range T op ±7, bounded by , s searched. For all the modes, for the other subframes, closed-loop ptch analyss s performed around the nteger ptch selected n the prevous subframe, as descrbed above. In.65, 4.5, 5.85, 8.5, 9.85, 3.5 or 3.85 kbt/s modes, the ptch delay s encoded wth 9 bts n the frst and thrd subframes and the relatve delay of the other subframes s encoded wth 6 bts. In 8.85 kbt/s mode, the ptch delay s encoded wth 8 bts n the frst and thrd subframes and the relatve delay of the other subframes s encoded wth 5 bts. In 6.6 kbt/s mode, the ptch delay s encoded wth 8 bts n the frst subframe and the relatve delay of the other subframes s encoded wth 5 bts. The closed loop ptch search s performed by mnmzng the mean-square weghted error between the orgnal and syntheszed speech. Ths s acheved by maxmzng the term 63 x( y ( ) n= k n T k =, ( 37 ) 63 y ( y ( n= k where x( s the target sgnal and y k ( s the past fltered exctaton at delay k (past exctaton convolved wth h(). Note that the search range s lmted around the open-loop ptch as explaned earler. The convoluton y k ( s computed for the frst delay n the searched range, and for the other delays, t s updated usng the recursve relaton k y ( = y ( n ) + u( k) h( ( 38 ) k k where u(,n= (3+7),,63, s the exctaton buffer. Note that n search stage, the samples u (, n =,, 63, are not known, and they are needed for ptch delays less than 64. To smplfy the search, the LP resdual s coped to u( n order to make the relaton n Equaton (38) vald for all delays. Once the optmum nteger ptch delay s determned, the fractons from to 4 wth a step of around that 4 nteger are tested. The fractonal ptch search s performed by nterpolatng the normalzed correlaton n Equaton (37) and searchng for ts maxmum. Once the fractonal ptch lag s determned, v'( s computed by nterpolatng the past exctaton sgnal u( at the gven phase (fracto. (The nterpolaton s performed usng two FIR flters (Hammng wndowed snc functons); one for nterpolatng the term n Equaton (34) wth the snc truncated at ±7 and the other for nterpolatng the past exctaton wth the snc truncated at ±63). The flters have ther cut-off frequency (-3 db) at 6 Hz n the oversampled doman, whch means that the nterpolaton flters exhbt low-pass frequency response Thus, even when the ptch delay s an nteger value, the adaptve codebook exctaton conssts of a low-pass fltered verson of the past exctaton at the gven delay and not a drect copy thereof. Further, for delays smaller than the subframe sze, the adaptve codebook exctaton s completed based on the low-pass fltered nterpolated past exctaton and not by repeatng the past exctaton. In order to enhance the ptch predcton performance n wdeband sgnals, a frequency-dependant ptch predctor s used. Ths s mportant n wdeband sgnals snce the perodcty doesn t necessarly extend over the whole spectrum. In ths algorthm, there are two sgnal paths assocated to respectve sets of ptch codebook parameters, wheren each sgnal path comprses a ptch predcton error calculatng devce for calculatng a ptch predcton error of a ptch codevector from a ptch codebook search devce. One of these two paths comprses a low-pass flter for flterng the ptch codevector and the ptch predcton error s calculated for these two sgnal paths. The sgnal path havng the lowest calculated ptch predcton error s selected, along wth the assocated ptch gan. The low pass flter used n the second path s n the form B LP (z)=.8z z -. Note that bt s used to encode the chosen path. Thus, for.65, 4.5, 5.85, 8.5, 9.85, 3.5 or 3.85 kbt/s modes, there are two possbltes to generate the adaptve codebook v(, v ( = v ( n the frst path, or v( = b ( + ) LP v ( n + ) n the second path, where b LP =[.8,.64,.8]. The path whch results n mnmum energy of the target sgnal x ( defned n Equaton (4) s selected for the fltered adaptve codebook vector. For 6.6 and 8.85 kbt/s modes, v( s always = = v ( = b ( + ) v ( n + ). LP

26 6 TS 6.9 V.. (-9) The adaptve codebook gan s then found by 63 x( y( n= g p, bounded by g., 63 p ( 39 ) y( y( = n= where y( = v( h( s the fltered adaptve codebook vector (zero-state response of H ( z) W ( z) to v (). To nsure stablty, the adaptve codebook gan g p s bounded by.95, f the adaptve codebook gans of the prevous subframes have been small and the LP flters of the prevous subframes have been close to beng unstable. 5.8 Algebrac codebook 5.8. Codebook structure The codebook structure s based on nterleaved sngle-pulse permutaton (ISPP) desgn. The 64 postons n the codevector are dvded nto 4 tracks of nterleaved postons, wth 6 postons n each track. The dfferent codebooks at the dfferent rates are constructed by placng a certan number of sgned pulses n the tracks (from to 6 pulses per track). The codebook ndex, or codeword, represents the pulse postons and sgns n each track. Thus, no codebook storage s needed, snce the exctaton vector at the decoder can be constructed through the nformaton contaned n the ndex tself (no lookup tables). An mportant feature of the used codebook s that t s a dynamc codebook consstng of an algebrac codebook followed by an adaptve preflter F(z) whch enhances specal spectral components n order to mprove the synthess speech qualty. A preflter relevant to wdeband sgnals s used whereby F(z) conssts of two parts: a perodcty enhancement part /(-.85z -T ) and a tlt part ( β z - ), where T s the nteger part of the ptch lag and β s related to the vocng of the prevous subframe and s bounded by [.,.5]. The codebook search s performed n the algebrac doman by combnng the flter F(z) wth the weghed synthess flter pror to the coddedbook search. Thus, the mpulse response h( must be modfed to nclude the preflter F(z). That s, h( h( * f (. The codebook structures of dfferent bt rates are gven below and 3.5 kbt/s mode In ths codebook, the nnovaton vector contans 4 non-zero pulses. All pulses can have the ampltudes + or -. The 64 postons n a subframe are dvded nto 4 tracks, where each track contans sx pulses, as shown n Table 4. Table 4. Potental postons of ndvdual pulses n the algebrac codebook, 3.85 and 3.5 kbt/s Track Pulse Postons, 4, 8,, 6,, 4, 8,, 6,, 4, 8, 3 36, 4, 44, 48, 5, 56, 6, 5, 9, 3, 7,, 5, 9, 3, 7,, 5, 9, 33, 37, 4, 45, 49, 53, 57, 6 3, 6,, 4, 8,, 6,, 4, 8,, 6, 3, 34, 38, 4, 46, 5, 54, 58, 6 4 3, 7,, 5, 9, 3 3, 7,, 5, 9, 3, 7, 3, 35, 39, 43, 47, 5, 55, 59, 63 The sx pulses n one track are encoded wth bts. Ths gves a total of 88 bts (+++) for the algebrac code kbt/s mode In ths codebook, the nnovaton vector contans 8 non-zero pulses. All pulses can have the ampltudes + or -. The 64 postons n a subframe are dvded nto 4 tracks, where each of the frst two tracks contans fve pulses and each of the other tracks contans four pulses, as shown n Table 5.