bai giang xu ly tieng noi

7/31/2019 Bai Giang Xu ly tieng noi

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CAO QUYT THNG Trang 1

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TRNG I HC HNG HI VIT NAM

KHOA CNG NGH THNG TIN

BI GING MN HCX L TING NI


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MC LC

M U .................................................................................................................. 2

CHNG I: NHP MN1. TN HIU TING NI ............................................................................. 3

2. X L TN HIU..................................................................................... 4

3. X L TN HIU S .............................................................................. 5

4. X L TING NI BNG S................................................................. 6

CHNG II: C S X L TN HIU S

1. CC H THNG V CC TN HIU THI GIAN RI RC ................... 92. BIU DIN BIN I CA CCH THNG V CC TN HIU......... 11

3. C BN V CC LC S ................................................................... 15

4. LY MU .............................................................................................. 19

CHNG III: CC M HNH S CHO TN HIU TING NI

1. NHP MN .......................................................................................... 22

2. QU TRNH TO TING NI............................................................... 23

3. L THUYT M HC CA VIC TO TING NI ............................. 29

4. CC M HNH NG MT T ................................................................ 40

5. CC M HNH S CHO CC TN HIU TING NI ........................... 48

TI LIU THAM KHO ......................................................................................... 52


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M U

Ting ni l phng tin giao tip c bn nht ca loi ngi, n hnh thnh vpht trin song song vi qu trnh tin ha ca loi ngi. i vi con ngi, s dng lini l mt cch din t n gin v hiu qu nht. u im ca vic giao tip bng tingni trc tin l tc giao tip, ting ni t ngi ni c ngi nghe hiu ngay lptc sau khi c pht ra. Bn cnh , ting ni l cch giao tip c s dng rng r inht bt c ai (d nhin l tr nhng ngi khuyt tt) cng c th ni c. Ngy nay,nh s pht trin ca khoa hc k thut, my mc dn dn thay th cc lao ng taychn. Tuy nhin iu khin my mc, con ngi phi lm kh nhiu thao tc tn nhiuthi gian v cn phi c o to. iu ny gy tr ngi khng t i vi vic s dngcc my mc, thnh tu khoa hc k thut. Trong khi , nu iu khin my mc thit bbng ting ni s d dng hn. Nhu cu iu khin my mc thit b bng ting ni cng

bc thit hn i vi cc thit b cm tay, nh: in thoi di ng, PC, cho my tnh c th nghe c nhiu ngi vt ln vi tn hiu m thanh

trong hn na th k qua trong lnh vc nhn dng ting ni. Qu trnh ny c nhdu bng cc kt qu nghin cu c sc trong lnh vc phn tch v x l ting ni, ccng dng thc t kh hu ch. Nhng d sao, kh nng ca my vn vn cn trongkhong gii hn, cn cn pht trin hn na c th tht s p ng nhu cu thc sca cucsng.


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CHNG 1NHP MN

Trong bi gingny ta s xt cch cc k thut x l tn hiu s c th p dngvo cc bi ton lin quan n vic truynting ni. Do vy, phn nhp mn ny ta sni n cc vn nh bn cht ca tn hiuting ni, cc k thut x l tn hiu sngvai tr th no trong vic hc x l tn hiu ting ni v mt vi lnh vc p dng quantrng ca vic truynting ni m k thut x l tn hiu s c s dng trong .

1. TN HIU TING NI

Mc ch ca ting ni l truyn thng tin. C mt s cch c trng cho victruynting ni. Mt cch tip cn c cht lng cao l dng cc quan im ca l thuytthng tin a ra bi Shannon nm 1968. Theo l thuyt thng tin , ting nic th biu

din di dng ni dung thng bohoc thng tin. Mt cch c trng khc lting nibiu din di dng tn hiu mang thng tin thng bo. Mc d cc quan im l thuytca thng tin ng vai tr ch o trong cc h thng truyn tin phc tp, ta s thy lbiu dinting ni da trn dng sng hoc m hnh tham s c s dng chnh trongcc ng dng thc t.

xt qu trnh thng tinting ni, u tin nn coi thng bo nh mt dng trutng no trong c ngi ni. Qua qu trnh phc tp to m, thng tin trong thngbo ny c chuyn trc tip thnh tn hiu m hc. Thng tin thng bo c th cbiu din di mt s dng khc nhau trong qu trnh toting ni. Chng hn, thng tinthng bo lc ban u c chuyn thnh tp hp cc tn hiu thn kinh iu khin c

ch pht m ( l chuyn ng ca li, mi, dy thanh m, v. v...). B my pht mchuyn ng tng ng vi cc tn hiu thn kinh ny to ra dy cc iu b, m ktqu cui cng l dng sng m cha thng tin trong thng bo gc.

Thng tin c thng bo bngting niv bn cht l ri rc, c th biu dinbi vic dn cc phn t mt tp hp hu hn cc k hiu. Cc k hiu m mi m cth c phn loi ra gi l cc m v(phoneme). Mi ngn ng c tp hp cc m vring ca n,con s mu mc l khong t 30 n 50. V d, ting Anh c th biu dinbng khong 42 m v (chng 3); ting Vit khong 33 m v (a, , , b, c, d, , e, , f, g,h, i, j, k, l, m, n, o, , , p, q, r, s, t, u, , v, w, x, y, z; 12 nguyn m, 21 ph m).

Trong l thuyt thng tinngi ta cn xt tc truyn thng tin. Viting ni, lu

n cc gii hn vt l ca tc chuyn ng ca b my pht m, nh gi th catc thng tin l con ngi to ra ting nivi tc trung bnh khong 10 m v trong1 giy. Nu mi m v biu din bng mt s nh phn th m s 6 bit l qu biudin tt c cc m v ting Anh. Vi tc trung bnh 10 m v trn giy v b qua tngtc gia cc cp m v lin k, ta c c lng 60 bit/giy cho tc thng tin trung bnhca ting ni. Ni cch khc l lng vit ra ca ting nicha thng tin tng ngvi 60 bit/gy tc ni chun. D nhin, cn di ca ni dung thng tin xc thc trong ting nic coi l cao hn tc ny. c lng trn khng tnh n cc nhn t nhtrng thi ca ngi ni, tc ni, m hng cating ni, v. v... .


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Trong h thng truynting ni, tn hiuting nic truyn i, lu gi v x lbng nhiu cch. Cc gii php k thut cho ta nhiu cch biudin tn hiuting ni. C2 cch chnh:

- Lu gi ni dung thng bo trong tn hiu ting ni- Biu din tn hiuting nidi dng thun tin truyn i hoc lu gi, hoc

di dng linh ng c th sa cha m khng nh hng n ni dung th ng bo.Biu din tn hiuting niphi lm sao cho ni dung thng tin c th d dng lnh

hi c bi ngi nghe hoc bng my t ng. Trong bi gingny ta s thy ccbiu din ca tn hiuting ni(ch khng phi l ni dung thng bo) c th yu cu t500 n trn 1 triu bit/gy. Trong vic thit k v x l cc biu din ny, cc phngphp x l tn hiu ng vai tr c bn.

2. X L TN HIU

Cc bi ton chung ca thao tc v x l thng tin c v hnh 1.1. Trongtrng hp cc tnhiuting ni, ngi ta coi ngun thng tin, o c hoc quan st, nichung, l c dng sng m. X l tn hiu bao gm trc ht l nhn c biu din tnhiu da trn m hnh cho v sau l dng bin i mc cao hn t tn hiuvo dng tin dng hn. Bc cui cng ca x l l trch ra v s dng thng tin thngbo. Bc ny c th thc hin hoc bi ngi nghe hoc t ng bng my. Ly v dl h thng c chc nng nhn bit t ng ngi ni t mt tp hp ngi cho, cth s dng biu din ph ph thuc thi gian ca tn hiuting ni. Mt bin i tn hiuc th dng l ph trung bnh mt cu y , so snh ph trung bnh vi ph trungbnh lu tr ca mi ngi ni, ri sau da trn s o tng t ca ph m nh n

bit ngi ni. v d ny, thng tin trong tn hiu dng nhn dng ngi ni.

Hnh 1.1. Cc bi ton thao tc v x l thng tin

Nh vy, x l cc tn hiuting ni, ni chung, gm 2 vic. Th nht l phngtin nhn c biu din tn hiuting nini chung, hoc di dng sng m hocdi dng tham s. Th hai l x l tn hiu, thc hin vic chuyn tn hiu thnh cc

dng khc t tng quan hn nhng thch hp hn cho cc ng dng.

N u n Thn tin

Trch ra v S dng Thng tin

o c hoc Quan st

Biu din tn hiu

Bin i tn hiuhiuTn

lX


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3. X L TN HIU S

Mc ch ca mn hc l khm ph vai tr ca k thut s trong x l cc tn hiu ting ni. X l tn hiu stp trung vo 2 vic l nhn c cc biu din ri rc ca tnhiu v l thuyt, thit k, thc hin cc th tc s x l cc biu din ri rc ny. itng ca x l tn hiu s l nhn bit cc i tng trong x l tn hiu tng t. Vvy, mt cu hi c l l v sao cc k thut x l tn hiu sli c dng nghin cuthng tin ting ni? C th nu ra nhiu l do tr li. u tin v quan trng nht l cchm x l tn hiu phc tp c th thc hin bng cch dng k thut s. Cc thut tons xt trong bi gingl cc thut ton cho cc h thng thi gian ri rc. nhiu trnghp, khng th coi cc h thng ny l h thng xp x ca cc h thng tng t.

Cc k thut x l tn hiu s lc u c dng trong x l ting ni nh mphng cc h thng tng t phc tp. Quan im lc ban u l phi m phng cc hthng tng t trn my tnh trnh vic xydng cc h thng thc nghim. Khicc m phng s ca cc h tng t c s dng, cc tnh ton i hi nhiu thigian, chng hn, cn khong 1 gi x l vi pht ni! n khong gia nhng nm1960 n ra cch mng trong x l tn hiu s. Cc xc tc chnh l s pht trin ca mytnh nhanh hn v cc tin b nhanh trong l thuyt k thut x l tn hiu s. Nh vy, rrng l cc h thng x l tn hiu s c hiu lc hn kh nng m phng cc hthng tng t. Cng thm vi cc pht trin l thuyt, cc pht trin ng thi trongphm vi phn cng s cng lm mnh ln u th ca cc k thut x l tn hiu sso vicc h thng tng t. Cc h thng s ng tin cy v rt cht ch. Cng ngh mngtng th pht trin n trng thi m cc h thng cc k phc tp c th hot ngtrn mt chip n. Cc thnh cng ca lgic l nhanh s ln cc tnh ton thc t

trong nhiu hm x l tn hiu c th thc hin trong thi gian thc v tc mu tingni.C nhiu l do khc dng k thut s trong cc h thng thng tin ting ni.

Chng hn, nu m ho c dng,ting nidi dng s ho c th truyn i mt cchtin cy trn cc knh rt n. Cng vy, nu tn hiu ting ni dng s th n ng nhtvi d liu ca cc dng khc. Do vy, mt li thng tin c th dng truyn c tingni v cc d liu khc m khng cn phn bit chng tr vic gii m. Ngoi ra, v yucu bo mt vic truyn cc tn hiu ging ni, biu din s c u th khc bit so vi cch thng tng t. bo mt, cc bit thng tin c th i i cui cng c th ti hinli ngi nhn. Vi cc l do nu trn v nhiu l do khc na m cc k thut s c

s dng ngy cng nhiu trong cc bi ton truynting ni.


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4. X L TING NIBNG S

Khi xem xt ng dng ca k thut x l tn hiu s vo cc bi ton truyntingni, ta phi ch n 3 ch chnh: biu dincc tn hiuting nidi dng s, thchincc k thut x l phc tp v cc lp cc ng dngda ch yu vo X l tn hiu

s.D nhin, vic biu din cc tn hiuting nidi dng s l ch c bn. V

vic ny, chng ta c hng dn bng nh l ly mu (Sampling Theorem, H. Nyquist,1928) pht biu l: tn hiu gii hn di(bandlimited) c th c biu din bi cc muly tun hon theo thi gian, min l cc mu c ly t l cao. Nh vy, vic xl mu nm trn trong l thuyt v ng dng ca x l ting nibng s. C nhiu cchbiu din ri rc cc tn hiuting ni. Nh biu din hnh v, cc biu din ny c thphn thnh 2 nhm ln gi l biu din dng sng (waveform representation) v biudin tham s (parametric representation). Biu din dng sng, nh tn gi ch ra,quan tm n vic bo ton n gin "dng sng" ca tn hiuting nitng tqua mu v x l v lng. Cc biu din tham s, mt khc, biu din tn hiuting ninh u ra ca m hnh to ting ni. Bc th nht nhn c biu din tham sthng l biu din dng sng bngs, tn hiuting nic ly mu v lng ho, risau c x l tip tc nhn c cc tham s ca m hnh toting ni. Cctham s ca m hnh ny c phn loi thch hp thnh cc tham s kch thch(excitation parameter, lin quan n ngun ca cc mting ni) hoc cc tham s png vt thanh m (vocal tract response parameter, lin quan n cc m ting ninl).

Biu din Tn

hiu ting ni

Cc biu dinDng Sng

Cc biu dinTham s

Tham sKch thch

Tham s png vt Thanh

m

Hnh 1. 2. Cc cch biu din Tn hiu ting ni

Tc d liu (bits/giy)200000 60000 20000 10000 500 75

Cc phng phpPhn tch - Tng hp

Tng hp tVn bn in

(Khng m ho ngun)Biu din Dng Sng

(M ho ngun)Cc biu din Tham s

Hnh 1. 3. Th hng cc tc bits cho mt s kiu biu din ting ni.


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Hnh 1. 3 so snh bng s cc biu din khc nhau ca tn hiu ting ni theo tc d liu. ng ngn cch l tc d liu khong 15000 tch biu din dng sngtc cao vi cc dng tham s tc thp.

Cc ng dng caThng tin ting ni

Truyn vLu gibng s

Tng hpting ni

Kim tra vNhn bitngi ni

Thanhnting

ni

Gip ngiTn tt

Tng cngcht lngtn hiu s

Hnh 1. 4. Vi ng dng ca vic truynting ni.

Hnh 1. 4 cho mt vi trong nhiu lnh vc ng dng ca vic truyn ting ni. Sau y l mt trnh by ngn gn v mi phm vi ny.

4.1. Truyn v lu gi ting ni bng s (Digital transmission and storage of speech):

Mt trong nhng ng dng sm nht v quan trng nht ca x l ting ni l VOCODERhay m ho ting ni (voice coder) a ra bi Homer Dudlay vo nm 1930. Mc chca VOCODER l thu gn rng bng cn thit truyn tn hiuting ni. S cn thitphi thu hp rng di nhiu tnh hung l do rng di c cung cp bi v tinh,bi sng m v cc h thng thng tin quang hc b tng ln.4.2. H thng Tng hpting ni(Speech synthesis system): Ngi ta dnh nhiu ch

cho cc h thng tng hpting nil v cn lu giting nibng s cho cc h thngp ngting nica my tnh (computer voice response) mt cch tit kim. H thng

p ng ny do R. L. Rabiner v R. W. Schafer ngh nm 1976. Mt h thng p ngting ni cbn l mt dch v thng tin t ng, s ho hon ton, c th b kch thchbi ngi dng bn phm hoc d liu v p ng vi thng tin i hi bngting ni.4.3. Cch thng kim tra v nhn bit ngi ni(Speaker verification and indentification

systems): c B. S. Atal d ngh nm 1976. Cc k thut kim tra v nhn bit ngi nidng nhn dngting nihoc nhnra ngi ni trong mt tp hp ln nhng ngini c th c. Khi c mtting nipht ra, ngi ta da vo cc d liu c kim trav nhn bit ngun hoc ngi pht rating ni.

4.4. Cch thng on nhn (recognition) ting ni: c D. R. Reddy ngh nm1976. Vic on nhn ting ni, di dng chung nht ca n, l chuyn i t dngsng m thnh bn vit ca thng tin thng bo. Bi ton on nhn ting ni ph thucrt nhiu vo cc rng buc t cho ngi ni, tnh trng ni v ni du ng thng bo. Ccng dng ln ca cc h thng on nhn ting ni rt nhiu v a dng, chng hn nhmy ch iu khin bng ting ni, thng tin ni vi cc my tnh, v. v...Mt h thngon nhn ting ni kt hp vi mt h thng tng hp ting ni to ra mt h thngtruyn thng c t l bit thp ti a (the ultimate low bit rate communica- tion system).

4.5. Cch thng gip ngi tn tt (Aids-to-the handicapped): ng dng ny tptrung vo qu trnh x l tn hiu ting ni lm thng tin c dng thch hp vi cc ngi


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tn tt, nh ghi m cho ngi m; hin th hnh nh ca TTinting ni dy cho ngiic do H. Levitt ngh nm 1973.4.6. Tng cng cht lng tn hiu (Enhancement of signal quality): nhiu tnh hung,

tn hiuting ni b suy gim theo hng hn ch hiu qu vic truyn i, hoc phi loib ting vang, ting n khini. cc tnh hung ny cc k thut x l tn hiu s cs dng ci thin cht lng ting ni. Cc v d l kh b nhiu (hay ting n, tpm) trong ting nihoc khi phc cc m.


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CHNG 2C S X L TN HIU S

1. CC H THNG V CC TN HIU THI GIAN RI RC

Trong hu nh mi tnh hung x l hoc truyn thng tin, ngi ta phi bt ubng vic biu din tn hiunh mu bin i lin tc. Sng m pht ra cng c bn chtnh vy. V mt ton hc, c th biu din cc mu bin i lin tc nh vy l hm cabin lin tc tbiu din thi gian. Trong bi ging ny, ta s dng k hiu x a(t) cho dngsng thi gian bin i lin tc (hoc tng t). Cng c th biu din tn hiu ting ninh dy cc s. Ni chung, ta dng k hiu x(n) biu din dy s. Nu dy c th coil dy cc mu tn hiu tng t xy ra tun hon vi chu k mu T th ta s dng khiu xa(nT). Hnh 2.1 cho v d tn hiu ting nibiu din c 2 dng tn hiu tng t(analog) v dng dy cc mu (samples) c t l mu 8 kHz.

Hnh 2.1. Cc biu din ca tn hiu tingni.

Khi nghin cu cc h thng x l tn hiu ting ni ta s s dng mt s dy sc v hnh 2.2. Mu n v (unit sample) hay dy xung n v (unit impulse

sequence) c nh ngha (nh ngha) l: (n) = 00

01

n

n

Dy bc n v(unit step sequence) l: u(n) =00

01


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X l tn hiu i hi bin i tn hiu thnh dng mong mun theo mt ngha no. Ta s tp trung xt cc h thng ri rc, hay ni tng ng l cc bin i dy vo

thnh dy ra. Ta s m t cc php bin i y bng lc nh hnh 2.3a.

x(n) y(n)*T[x(n)] x(n)y(n)*T[x(n)]

(a) (b)

Hnh 2.3. Lc biu din: (a) H thng vo/ra n; (b) H thng vo/nhiu ra.

Lp cc h thng bt bin-dch chuyn tuyn tnh (LSI - Linear Shift Invariant)thng c dng trong x l ting ni. Cc h thng ny c c trng hon ton bip ng ca chng cho ci vo mu n v. Vi cc h thng ny, ci ra, y(n), c th tnhc t ci vo, x(n), v p ng ca mu n v, h(n), theo tch chp:

y(n) =k

knhkx )()( = x(n)*h(n), (1)

y * l k hiu tch chp ri rc(discrete convolution). Biu thc tng ng l:

y(n) =k

knxkh )()( = h(n)*x(n),

Cc h thng LSI thng dng lp cc php lc trn cc tn hiu ting ni v,c l quan trng hn l, chng rt c ch cho cc m hnh to ta ting ni.

T[ ]T[ ]T[ ] T[ ]


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2. BIU DIN BIN I CA CC H THNG V CC TN HIU

Phn tch v thit k cc h thng tuyn tnh c thc hin d dng nh ccbiu din min tn s (frequency-domain representation) ca c cc tn hiu v h thng.Do vy, cn xt cc biu din ca bin i Fourier(Fourier Transform, FT) v ca bin iZ(Z - Transform, ZT) ca cc tn hiu v h thng ri rc.

1. Bin i Z(ZT) : Biu din ZT ca dy c xc nh bi 2 phng trnh:

X(z) =n

nznx ).( (2a)

x(n) =C

ndzzX

j

1)(2

1(2b)

"Bin i Z" (ZT) hay "bin i trc tip" ca x(n) c xc nh bi (2a). Tng quan, cth thy X(z) l chui lu tha v hn theo bin z-1, trong dy cc gi tr, x(n), ng vai

tr cc h s trong chui lu tha. Ni chung, cc chui lu tha ny s hi t n gi trhu hn ch vi cc gi tr xc nh ca z. iu kin ca hi t l:

n

nznx )( (3)

Tp hp cc gi tr m chui hi t xc nh mt min trn mt phng phc Z gi l minhi t. Ni chung, min ny c dng:

R1 < z < R2 (4) thy quan h ca min hi t vi bn cht ca dy, ta xt vi v d.

V d 1: Cho x(n) = (n-n0) (xung n v ti n0). Th vo (1a) ta c: X(z) = 0n

z .

V d 2 : Cho x(n) = u(n) - u(n-N) (bc n v trn on [0, N-1]). Khi X(z) =

1

0

)1(N

n

nz =

11

1

z

zN

.

c hai trng hp ny, x(n) c di hu hn. V vy X(z) l a thc ca bin z-1 vmin hi t l mi ni tr ti z= 0. Tt c cc dy c di hu hn u c min hi t t

nht l min 0 < z < .

V d 3:Gi s x(n) = an.u(n). Khi X(z) =0

n n

n

a z =11

1

az, a < z .

Trong trng hp ny, chu lu tha l chui s nhn c tng. Kt qu ny l mu mc

cho cc dy v hn khc 0 vi n > 0. trng hp tng qut ny min hi t c dng Z> R1.

V d 4 : Gi s x(n) = - bn u(-n-1). Khi X(z) =1

n

nnzb =

11

1

bz, z < b.

y l dy di v hn mu khc 0 vi n< 0, c min hi t ni chung l z < R2.

Trng hp tng qut nht, trong x(n) 0 vi - < n< , c th xem nh tng hp

ca cc trng hp nu v d 3 v v d 4. Nh vy, trong trng hp tng qut, min

hi t c dng R1 < z < R2.Php "bin i Z ngc" (Inverse Z- Transform, IZT) c nh ngha bi tch

phn ng kn (2b), trong C l chu tuyn kn bao quanh gc ca mt phng Z v nmtrong min hi t ca X(Z).


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C nhiu nh l v tnh cht ca biu din ZT tin dng cho vic nghin cu cch thng thi gian ri rc. Danh sch cc nh l quan trng cho trong bng 1. V hnhthc, cc nh l ny ging vi cc nh l tng ng ca bin i Laplace cho cc hmthi gian lin tc. Tuy nhin, iu ny khng c ngha l ZT l mt dng xp x no cabin i Laplace. bin i Laplace l biu din chnh xc ca cc hm thi gian lin tc,cn ZT l biu din chnh xc ca dy cc s. Cc nt tng t lin kt cc biu din lintc v biu din ri rc ca tn hiu th hin nh l mu xt 3.

Dy ZT

1. Tuyn tnh (Linear) ax1(n) + bx2(n) aX1(Z) + bX2(Z)

2. Dch chuyn (Shift) x(n + n0) 0nZ X(Z)

3. Trng s lu tha anx(n) X(a-1Z)

4. Trng s tuyn tnh nx(n)- Z

dZ

ZdX )(

5. o ngc thi gian x(-n) X(Z- )6. Tch chp x(n)*h(n) X(Z)H(Z)

7. Nhn dy x(n)w(n)

C

dz

WXj

1)()(2

1

Bng 1. Cc dy v cc ZT tng ng

2. Bin i Fourier(Fourier Transform, FT): Biu din bin i Fourier (FT) ca tn hiuthi gian ri rc cho bi ccPhng trnh

X(e

j

) = n

nj

enx )( , (5a)

x(n) =C

njjdeeX

j)(

2

1. (5b)

D thy cc phng trnh ny l cc trng hp ring ca (2a, 2b), biu din FTnhn c bng cch hn ch ZT v ng trn n v ca mt phng Z,bng cch t z

= je . Nh v hnh 2.2, bin tn s, , cng c biu din nh gc trong mt phng Z.

iu kin tn ti biu din FT c th nhn c bng cch t z = 1 trong (3), .

nnx )( (6)

Ta c th s dng cc V d 2.1 v thay z = ej trong biu thc cho lm cc Vd FT mu. hai V d u, kt qu r rng l FT v min hi t ca X(Z) cha vng trn

n v. Tuy nhin cc V d 3 v 4, FT ch tn ti nu a < 1 v b > 1 tng ng. Dnhin, cc iu kin ny ng vi cc dy tho mn iu kin (6).

iu quan trng l FT ca dy, X(ej ), l hm tun hon ca vi chu k 2 . D

dng kim tra iu ny bng cch thay + 2 vo (5a). Mt khc, do X(ej ) l hn ch

ca X(z) trn ng trn n v, ta thy l X(ej ) phi lp li mi ln i ht 1 vng quanh

ng trn n v,khi chy qua 2 radian.


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Bng cch thay z = ej trong mi nh l bng 1, ta ctp hp cc nh l choFT. D nhin, cc kt qu ny ch c gi tr nu cc FT xt n tn ti.

2.2.3. Bin i Fourier ri rc(Discrete Fourier Transform, DFT): Cng nh trong trng

hp cc tn hiu tng t, nu dy tun hon vi chu k N, .

~x (n) = ~x (n + N) - < n < (7)

th~

x (n) c th biu din bi tng ri rc ca cc ng hnh sin hn l bi dng tch

phn nh (5b). Cc biu din dng chui Fourier cho dy tun hon l :

~

X (k) =1

0

2~

)(N

n

knN

j

enx (8a)

~

x (n) =1

0

2~

)(1 N

k

knN

j

ekXN

(8b)

l biu din chnh xc ca dy tun hon. Tuy nhin, ngi ta hay dng biu din

khc ca (8). Xt dy di hu hn, x(n), bng 0 ngoi on 0 n N-1. Bini ZTca x(n) l

X(z) =1

0

)(N

n

nZnx (9)

Nu ta nh gi X(z) ti N im cch u nhau trn ng trn n v,z k = ej 2 k/N, k = 0..

(N-1), th c

X(k

Nj

e

2

) =1

0

2

)(N

n

knN

j

enx , k = 0.. (N-1) (10)

Nu ta xy dng dy tun hon l dy v hn cc bn sao ca x(n),~x (n) =

1

0

)(N

n

rNnx (11)

th cc mu X(k

Nj

e

2

), theo (8a) v (10), l cc h s Fourier ca dy tun hon~

x (n)

(11). Nh vy, dy c di N c th biu din chnh xc bng bin i Fourier ri rc(Discrete Fourier Transform, DFT) di dng

X(k) =1

0

2

)(N

n

knN

j

enx , k = 0.. (N-1) (12a)

x(n) =1

0

2

)(1N

k

knNjekXN

, n = 0.. (N-1) (12b)

R rng l gia (12) v (8) ch c khc bit mt cht v k hiu (b du ~ ch s tun

hon) v hn ch vo cc khong hu hn: 0 k N-1 v 0 n N-1. Tuy nhin, iu rtquan trng phi nh khi dng DFT l tt c cc dy c coi l tun hon khi biu dinbi DFT. Nh vy, DFT thc s l biu din ca dy tun hon cho bi (11). Mt quannim khc l khi dng biu din DFT th dy ch s phi c coi l modulo N. iu nyrt ra t s kin l nu x(n) c di l N th

~

x (n) =k

rNnx )( = x( n mod N) = x((n))N.


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K hiu ngoc 2 ln biu din vic tun honni ti (build-in periodicity) ca DFT. Stun hon ni ti ny c tc ng ln cc tnh cht ca biu din DFT. Mt s nh lquan trng c nu bng 2. iu ni bt nht l cc dy dch chuyn theo modulo N.iu ny dn n cc thay i r rng trong php chp ri rc.

Dy DFT N-im

1. Tuyn tnh (Linear) ax1(n) + bx2(n) aX1(k) + bX2(k)

2. Dch chuyn (Shift) x((n + n0))N )(0

2

kXekn

Nj

3. o ngc thi gian (Time Reversal) x((-n))N X*(k)

4. Chp (Convolution)1

0

))(()(N

m

Nmnhmx X(k)H(k)

5. Nhn dy (Multiplication of Sequence) x(n)w(n)1

0

))(()(1 N

r

NrkWrXN

Bng 2. Cc dy v DFT tng ng ca chng.

Biu din DFT vi tt c cc nt ring ca n l quan trng do mt s l do:

Bin i DFT, X(k), c th coi l bn mu ca bin i ZT (hoc bin iFT) ca dy c di hu hn.

Bin i DFT c cc tnh cht rt ging (c cc sa i do s tun honni ti) vi nhiu tnh cht hu ch ca bin i ZT v FT.

N gi tr ca X(k) c th tnh ton rt hiu qu (vi thi gian t l viNlogN) bng tp hp cc thut ton tnh ton c bit chung l bin i

Fourier nhanh(Fast Fourier Transform, FFT).DFT c dng rng ri tnh cc c lng ph (Spectrum estimate), hm

tng quan(Correlation function) v thc hin cc lc s.


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3. C BN V CC LC S

Lc s l h thng bt bin dch chuyn tuyn tnh thi gian ri rc (Discrete-TimeLinear Shift-Invariant System). Nh rng vi h thng nh vy, ci vo v ci ra c quanh theo biu thc tch chp (1). Quanh tng ng gia bin i ZT ca ci vo v cira cho bng 1,

Y(z) = H(z)X(z)Bin i ZT ca p ng mu n v, H(z), c gi l hmh thng (system

function) ca h, bin i FT ca p ng xung n v, H(ej ), c gi l p ng tn s

(frequency response). H(ej ) ni chung l hm phc ca bin , c th vit di dngphn thc v phn o

H(ej ) = Hr(ej ) + j.Hj(e

j )hoc dng mun ( ln, magnitude) v argument (gc pha, phase angle)

H(ej ) = H(ej ) . )](arg[j

eHje .Mt h thng bt bin dch chuyn tuyn tnh nhn qu(causal) l h c h(n) =

0 khi n < 0. Mt h thng n nh (stable) l h thng m mi ci vo b chn sinh ra mtci ra b chn. iu kin cn v h thng bt bin dch chuyn l n nh l

n

nh )( < .

iu kin ny ng nht vi (6) v do vy l tn ti H(ej ). Cng vi biu thc tchchp (1), c th ni l tt c cc h thng bt bin dch chuyn tuyn tnh c dng lmlc c tnh cht l ci vo v ci ra tho mn phng trnh sai phn(difference equation)

tuyn tnh dng

y(n) -N

k

k knya1

)( =M

r

r rnxb0

)( (13)

Ly ZT hai v ca phng trnh ny ta c th chng t rng

H(z) =)(

)(

zX

zY=

N

k

k

k

M

r

r

r

za

zb

1

0

1

(14)

So snh (13) v (14) ta c kt lun hu ch sau: cho phng trnh sai phn dng (13) ta

c th nhn c H(Z) trc tip bng cch t cc h s ca ci vo b lm chm(delayed input) phng trnh (13) (cc br)cng lu tha tng ng ca Z

-1 t s vcc h s ca ci ra b lm chm (cc ai) vi lu tha tng ng ca Z

-1 mu s.

Hm h thng, H(z), ni chung l hm hu t ca z-1. Nh vy, n c c trngbi cc v tr cc(pole) v khng im(zero) trn mt phng z. c bit, H(z) c th vitl

H(z) =N

jk

k

M

i

i

zd

zcA

1

1

1

1

)1(

)1(

(15)


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Khi xt cc ZT, ta bit rng cc h thng nhn qu c min hi t dng z > R1. Nu hthng cng l n nh th R1phi nh hn 1 nn min hi t cha ng trn n v. Dovy, tt c cc cc ca H(z) phi nm trong ng trn n v i vi h thn g n nhv nhn qu.

By gi ta nh ngha hai lp h thng bt bin dch chuyn tuyn tnh (LinearShift -Invariant). l lp cc h thng p ng xung di hu hn (Finite durationImpulse Response, FIR) v lp cc h thng p ng xung di v hn (Infinite duration

Impulse Response, IIR). Cc lp ny c cc tnh cht c bit c tm tt sau y.

1. Cch thngFIR: Nu tt ccc h s aktrong (13) bng 0 th phng trnh sai phnl

y(n) =M

r

r rnxb0

)( (16)

So snh (16) vi nh nghatch chp ta thy

h(n) =.,00

,0,Mnn

Mnbn

Cc h thng FIR c hai tnh cht quan trng. u tin, ta lu l H(z) l a thctheo z-1v do vy H(z) ch c cc 0, khng c cc khc 0. Mt khc, cc h thng FIR chc th c pha ng tuyn tnh (exactly linear phase). Nu h(n) tho mn

h(n) = h(M-n) (17)

th H(ej ) c dng

H(ej ) = A(ej )e-j (M/2x)

trong , A(ej ) hoc l thc hoc thun o ph thuc vo vic (17) tho mn vi du +hoc du - tng ng.

Kh nng pha ng tuyn tnh thng rt hu ch trong x l ting ni v vicdng thi gian chnh xc l ct yu. Tnh cht ny ca cc lc FIR cng c th lm rtn gin bi ton xp x v n ch cn tp trung vo vic xp x p ng bin mongmun. Hnh pht phi chu thit k cc lc c p ng pha ng tuyn tnh l chcn phi ko di p ng xung rng xp x tt cc lc c ngng chnh xc.

Da trn cc tnh cht ca lc FIR pha tuyn tnh, ngi ta pht trin 3phng php thit k xp x tp hp cc c trng bt k vi lc FIR, l:

1. Thit k Windows. 2. Thit k mu tn s. 3. Thit k ti u.Ch c phng phpth nht trong cc k thut ny l k thut thit k gii tch, c tp

hp dng ng cc phng trnh c th gii c nhn c cc h s lc. Ccphng php thit k th hai v th ba l cc phng php ti u dng cch tip cnbng lc nhn c lc mong mun (ch khng c dng ng). Mc d phng php Windows d p dng, phng php th ba cng c dng nhiu. iu ny mt phnth hin cc nghin cu tnh cht ca cc lc FIR ti u, phn khc l nhiu ti liuthit k chng trnh dng xp x cc tp hp cc ch dn mong mun.

2. Cch thngIIR Nu hm h thng (15) c cc cc cng nh c khng im th phng trnh sai phn (13) c th vit l:

y(n) =N

k

k knya1

)( +M

r

r rnxb0

)( (13b)


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y l mt cng thc quy c th s dng tng bc tnh cc gi tr ca dy ci rat cc gi tr qu kh ca n v t cc gi tr hin ti v qu kh ca ci vo. NuM < N trong phng trnh(15) th H(z) c th khai trin thnh tng cc phn thc ti gin

H(z) =N

k k

k

zd

A

1

1

1

. (15b)

Vi cc h thng nhn qu, c th chng minh

h(n) =N

k

n

kk nudA1

)()(

l p ng xung. Nh vy h(n) c di v hn. Tuy nhin, theo cng thc quy (13b),thng c th thc hin mt lc IIR xp x cc ch nh cho hiu qu hn (dng t tnhton hn) so vi lc FIR. iu ny ng ring cho cc lc c ngng chnh xc.

Nhiu phng phpthit k c th dng cho cc lc IIR. Cc phng php thitk cho cc lc la chn tn s (thng thp (lowpass), thng di (bandpass), v.v...)nichung l cc bin dng ca cc th tc thit k tng t c in thc hin trc tip. Ccth tc ny l:

1. Cc thit k Butterworth (bin phng cc i(maximally flat amplitude)).2. Cc thit k Bessel (lm chm nhm phng cc i (maximally flat group

delay)).3. Cc thit k Tsebysev (r rm u (equiripple) hoc thng di hoc thngdng(stopband)).4. Cc thit k Elliptic (r rm u c thng di v thng dng).

Tt c cc phng php trn c bn cht l gii tch v u c dng rng ri thit k cc lc s IIR. Ngoi ra cn c nhiu phng php ti u IIR c xt n

cho cc loi thit k gn ng khng thch hp vi mt trong nhng phng php nutrn.iu khc bit chnh gia cc lc FIR v IIR l ch cc IIR khng c thitk

c pha ng tuyn tnh, trong khi FIR c tnh cht y. Tuy nhin, lc IIR thng spxp bin hiu qu trong vic thc hin cc lc ngng chnh xc (sharp cutoff filter)hn l cc lc FIR.

C s yn chuyn r rng khi thc hin cc h thng IIR. Biu din ny thngc gi l thc hin dng trc tip(direct form implementation). Vic tng qut cho M vN tu l hin nhin.Phng trnh sai phn (13b) c th vit nhiu dng tng ng,

thng dng l tp hp cc phng trnh

w(n) =N

k

k knwa1

)( + x(n);

y(n) =M

r

r rnwb0

)( . (13c)

Tp hp cc phng trnh ny c th thc hin nh hnh 2.6b c lu tr b nh cnthit lu cc gi tr dy lm chm.

phng trnh (15) chng t l H(z) c th biu din nh tch ca cc cc v cckhng im. Cc cc v cc khng im ny xut hin trong cc cp s phc lin hp v

cc h s ak, br l cc s thc. Bng cch nhm cc cc v khng im lin hp phcvo, c th biu din H(z) l tch cc hm h thng bc hai s cp


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H(z) =K

k kk

kk

zaza

zbzbA

12

2

1

1

2

2

1

1

1

1

trong K l phn nguyn ca2

1N.

Vic khai trin phn thc ti gin (15b) gi mt cch tip cn khc. Bng cch gp ccs hng cha cc cc lin hp phc, H(z) c th vit l

H(z) =K

k kk

kk

zaza

zcc

12

2

1

1

1

10

1.

Cng thc ny gi cch x l dng song song nh v hnh 2.7b cho N = 4. Tt c cc cch thc hin nu u c dng trong X L TING NI . Ni

chung, vi cc ng dng lc tuyn tnh, dng xp chng (cascade form) th hin cch xl cao cp cho n gt da(roundoff noise), cho cc khng chnh xc ca h s v chos n nh.


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4. LY MU

dng ccphng php x l tn hiu strn tn hiu tng t nhting ni,cn biu din tn hiu l dy cc s. iu ny c thc hin bng cch ly mu tn hiutng t(sampling the analog signal), k hiu l xa(t). Mu ny sinh ra dy s tun hon

x(n) = xa(nT), - < n < (16)trong n ch nhn gi tr nguyn. Hnh 2.1 cho dng sng ting niv tp hp cc mutng ng vi chu k T = 1/8000 giy.

1. nh l ly mu (Sampling Theorem): Nu tn hiu xa(t) c FT gii hn di Xa(j ) m

Xa(j ) = 0 khi 2 FN, th xa(t) c th xy dng li duy nht t cc mu tun hon

xa(nT), - < n < , nu 1/T > 2FN.nh l ny c suy ra t lp lun sau: Nu FT ca xa(t)

Xa(j ) = ( )j t

ax t e dt

v FT ca dy x(n) xc nh (5a), ta c: nu X a(ej ) c tnh cho cc tn s = T,

th X(ej T) c tnh t Xa(j ) theo cng thc (xem A.V. Oppenheim and R.W. Schafer,Digital Signal Processing, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1975):

Xa(ej T) =

1 2( )a

k

X j j kT T

(17)

thy r cng thc (17), ta gi s Xa(j ) c dng hnh 2.8a,gi s Xa(j ) = 0 vi >

N = 2 FN. Tn s FNgi l tn s Nyquist. Theo (17), Xa(ej T) l tng ca v hn cc

mu ca Xa(j ), mi mu c tm ti bi s nguyn ln2

T

. Hnh 2.8b m t trng hp

1/T > 2FNm cc nh ca FT khng vo thng thp < 2 FN. Hnh 2.8c cho trng

hp 1/T < 2FN. Trong trng hp ny, nh c tm ti 2 /T vo thng thp. iu kinny, khi tn s cao c v chim phn ca tn s thp, c gi l ly b danh (aliasing).R rng c th trnhc vic ly b danh nu FT gii hn di v nu tn s mu (1/T)bng t nht l 2 ln tn s Nyquist (1/T > 2FN).

Vi iu kin 1/T > 2FNth FT ca dy cc mu t l vi FT ca tn hiu tng ttrong di c s,

Xa(ej T

) =1

( )ak

X jT , < T .

S dng kt qu ny c th chng minh rng tn hiu gc c th lin quan n dy ccmu bng cng thc ni suy

xa(t) =sin[ ( )

( )( ) /

a

n

t nT T x nT

t nT T (18)

Nh vy, cc mu ca tn hiu tng t gii hn di ly ti t l t nht 2 ln tn sNyquist cho, c th dng xy dng li tn hiu tng t gc theo cng thc (18).Cc my chuyn i s thnh tng t (Digital - to - Analog Converter, DAC) u tmcch xp x (18).


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2. Ct b (decimation) v thm vo(interpolation) cc dng sng mu: nhiu v d sxt, ta phi thay i t l mu ca tn hiu thi gian ri rc. Chng hn, khi ting ni lmu dng lng t vi phn 1-bit ti t l mu cao (iu bin delta) c chuyn thnhbiu din iu bin m xung (Pulse Code Modulation, PCM) a bit t l mu thp, lc phi gim t l mu. Tri li, khi tham s no ca tn hiu ting ni l mu t lthp cho m ho hiu qu v sau phi dng t l cao xy dng li tn hiu, lc phi tng t l mu. Qu trnh tng v gim t l mu nh vy gi l ct b v thm vo.

Khi ni v c hai trng hp, gi s ta c dy cc mu x(n) = xa(nT), (19)

y hm tng t xa(t) c FT gii hn di m Xa(j ) = 0 khi > 2 FN. Khi , theonh l ly mu, nu 1/T > 2FNth FT ca x(n) tho mn

X(ej T) =1

( )a

X jT

, > R r), ZL( ) Rr. Nng lng tiu tn(energy dissipated) do pht x t l vi phn thc

ca tr khng pht x. Nh vy, ta c th thy l vi H thng to rating niy (Bmy pht m v tn x), cc mt mt do tn x l quan trng nht cc tn s cao. nh gi ln ca hiu ng ny, cc phng trnh (10), (11c) v (13) c gii ngthi cho trng hp ng bt bin thi gian c thnh mm, cc mt mt nhit v ma st,cng mt mt pht x tng ng vi tm chn phng v hn. Hnh 3.21 v p ng tnskt qu

Va(j ) =( , )

( )G

U

U


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vi ci vo U(0, t) = UG( )ej t. Hiu ng chnh trn rng gii cng hng xy ra

cc tn s cao. rng di cng hng (tng cng) u tin c xc nh ch yu ldo mt mt ca thnh, cn cc rng di tng cng cao hn c xc nh ch yu ldo mt mt pht x.C th ni l rng di tng cng th hai v th ba c xcnh bng cch t hp hai c ch mt mt ny.

p ng tn s lin h vn tc m ti mi vi vn tc m vo ti mi. Quan hgia p lc ti mi v vn tc m ti thanh mn l iu ng ch , c bit khi ng nicm p lc(pressure sensitive microphone) c dng chuyn sng m thnh sng

in t. V P(, ), U(, ) lin h vi nhau bi phng trnh (13a) th hm truyn p lc(pressure transfer function) c dng n gin

Ha( ) =( , )

( )G

P

U

=

( , )

( , )

P

U

( , )

( )G

U

U

= ZL( ).Va( )

C th thy l cc hiu ng chnh c nhn mnh tn s cao v ci vo l 0 ti = 0.

3.5.Cc hm truyn

(transfer function)cab mypht m cho cc nguyn m:

Cc

phng trnh xt cc phn 3 v 4 to nn m hnh chi tit cho vic truyn m v phtx trong vic to rating ni. Dng cc k thut tch phn s dng min thi gian (bin

t) hoc min tn s (bin ) c th gii c cho nhiu loi hm p ng cab mypht m. Cc li gii ny cho php hiu bn cht ca qu trnh to ting niv tn hiuting ni.

V d Portnoff 1973 dng cc phng trnh min tn s (10), (11c), (12) v (13) tm ra cc hm p ng tn scho mt tp hp cc hm din tch, V d cc hm din

tch cab my pht m v cc p ng tn stng ng (( , )

( )G

U

U

) cho cc nguyn m

tng Nga /a/, /e/, /i/ v /u/. Cc hm ny cng minh ho cc hiu ng ca tt c cc cch mt mt ni phn 3 v 4.

Tng kt, ta c th kt lun qua cc v d ny v cc v d cc phn trcnhng im sau:

1. H thng pht m c c trng bi tp hp cc cng hng (cc tngcng) ph thuc trc ht vo hm din tch ca b mypht m, mc d c nng lncht t do mt mt, khi so snh vi trng hpng mt t.

2. Cc rng di ca cc tn s tng cng thp nht (tn s u v tn s thhai) ph thuc trc ht vo mt mt ca thnhb mypht m.

3. Cc rng di ca cc tn s tng cng cao hn ph thuc trc ht vo mast nht v mt mt nhit trongb mypht mv mt mt tn x.

3.6. Hiu ng ni ng mi(nasal coupling): Khi to cc m mi /m/, /n/, v / /, vm mingmm h xung nh ca sp ni ng mi vi c hng. Khi , mt bao ng hon tonc to ra (V d mi cho /m/). Cu hnh ny c th v nh hnh 3.27a c hainhnh, mt nhnh hon ton ng. Ti im r nhnh p lc m l nh nhau u ming, vn tc m hng l tng ca cc vn tc m ti li vo khoang mi v ming.

Vi cc ph m mi, pht x ca m xy ra trc ht l l mi. Nh vy, ngmi c kt thc vi tr khng pht x thch ng vi c m ng mi. B my minghon ton ng. Cc nguyn m m mi ho c to ra vi cng h thng c b my


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ming kt thc nh cho nguyn m. Khi , tn hiu ting ni s l cc ci ra mi vming chng ln nhau.

M hnh ton hc cho cu hnh ny gm 3 tp hp cc phng trnh vi phn ohm ring c cc iu kin bin to ra bi dng kch thch thanh mn, cc kt thc ca bmy pht m mi v mm, v cc quanh lin tc cc ch ni. y l tp hp ccphng trnh phc tp, nhng v nguyn tc l gii c v cho cc s o chp nhnc ca cc hm din tch cho c 3 ng. Hm truyn ca h thng c nhiu nt chungvi cc v d trc. l h thng c c trng bi cc cng hng hoc cc tngcng ph thuc vo hnh dng v di ca 3 ng. Mt khc bit quan trng rt ra ts kin l khoang ming ng c th chn nng lng cc tn s no , ngn cn cctn s ny xut hin li ra mi. Kt qul vi cc m mi, hm truyn cab my phtm c c trng bi cc phn-cng hng (cc khng (0)) cng nh cc cng hng.Ngi ta cng thy rng cc tng cng ng mi c di rng hn cc m hu thanhkhng ng mi (non-nasal voiced sounds). l thuc tnh ca ma st nht ln hnv mt mt nhit ph thuc din tch b mt khoang mi.

3.7. Kch thch m (Excitation of sound) b mypht m:Cc mc trc xt cchdng cc nh lut vt l m t vic truyn v pht x m trong vic to rating ni.By gi, hon thin vic nghin cu cc nguyn l m hc, ta phi xt c ch to racc sng m h thng pht m. Nh li rng khi xt tng quan vic to rating ni3.1 ta ch ra 3 c ch kch thch chnh l:

1. Lung khng kh t phi c iu chnh bi rung ng ca dy thanh m tora kch thch nh l xung gn tun hon (quasi-periodic pulse-like excitation).

2. Lung khng kh t phi tr nn hn lon ging nh khng kh chuyn qua mtch tht li b my pht m to ra kch thchnh l ting n (noise-like).

3. Lung khng kh to ra p lc sau mt im ng hon ton b my pht m.Vic gii phng nhanh p lc ny bng cch di chuyn ch tht li gy ra mt kch thchtm thi.M hnh chi tit kch thch m b my pht m bao gm h thng di thanh mn (phi,cung phi v kh qun), thanh mn vb my pht m. Thc vy, mt m hnh y trong tt c cc chi tit cn thit cng hon ton c kh nng kch thch th cng nh tora ting ni ! ([2]). Flanagan l ngi u tin (1968) c c gng lp m hnh chi tit chovic to m trong H thng pht m. Cc nghin cu sau (xem J. F. Flanagan, K.Ishizaka, and K. L. Shipley, "Synthesis of Speech from a Dynamic Model of the VocalsCords and Vocal Tract, Bell Sys. Tech. J., Vol. 54, No. 3, pp. 485-506, March, 1975) a ra m hnh p hn, biu din rt chi tit qu trnh to ra c m v thanh ln m huthanh. M hnh ny, da trn c hc c in v c hc cht lng, vt ra ngoi phm vitho lun y.Tuy nhin, mt trao i ngn gn v cc nguyn l c bn ca vic tom s c ch trong vic ch hng cho cc m hnh n gin, c s dng rng ri cs ca x lting ni.

S dao ng ca cc dy thanh m trong vic to cc nguyn m c th gii thchbng cch xt biu din lc ca h thng pht m. Cc dy thanh m lm tht ling dn t phi nb my pht m. Do p lc t phi tng, lung khng kh ra tphi v qua ca gia cc dy thanh m (thanh mn). nh l Bernoulli ni rng khi dng

cht lng qua mt l(orifice), p lc nh i ch tht li hn ch khc. Nu s cng cc dy thanh m c iu chnh hp l, p lc gim cho php cc dy thanh m i


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cng nhau, lm tht li hon ton dng kh. (iu ny c v bng cc nt t hnh).Kt qu l p lc tng pha sau cc dy thanh m, buc dy thanh m m ra v cho phpkhng kh li i qua thanh mn. Mt ln na p lc khng kh thanh mn li gim vqu trnh lp li. Nh vy, cc dy thanh m a ra iu kin dao ng duytr(sustainedoscillation). Mc thanh mn ng v m c iu khin bi p lc khng kh phi,s cng v cng vng ca dy thanh m, v din tch ca m thanh mn. C nhiutham s iu khin m hnh vn hnh dy thanh m chi tit. M hnh nh vy phi chac cc hiu ng cab my pht m v vic thay i p lc trongb my pht m nhhng n vic thay i p lc thanh mn.

3.8. Cc m hnh da trn l thuyt m hc:Ton b phn ny xt tng i chi titcc nt quan trng ca l thuyt m hc trong vic to rating ni. Cc m hnh chi titv vic to ra, lan truyn v pht x m, v nguyn tc c th gii c vi cc gi trkch thch v gi tr cc tham s cab my pht m thch hp, a ra dng sng m.Ngi ta coi l cch hiu qu nht tng hp cc m ni t nhin. Tuy nhin, do

nhiu l do m cch lm chi tit ny l khng thc t v khng cn thit. Trong cctrng hp y, l thuyt m hc cho ta cch tip cn n gin m hnh ho cc tnhiuting ni. Hnh 3.31 cho ta s khi biu din m hnh c s cho vic x l.

Hnh 3.31. M hnh h thng ngun ca vic to rating ni.

Cc m hnh ny c im chung l cc kch thch c tch ra khi b my pht m vpht x.b my pht m v cc hiu ng pht x c coi l h thng tuyn tnh bin itheo thi gian. Mc ch ca iu ny l m hnh ho cc hiu ng cng hng m ta ni n. My pht kch thch to ra tn hiu hoc nh l dng cc xung (thanh m) hoc lcc n bin i ngu nhin. Cc tham s ca ngun v h thng c chn la to raci ra c cc tnh cht gingting nimong mun. Nu c th lm c iu th mhnh c th x dng cho vic x l. phn sau ca chng ny ta s xt mt s m hnh

dng ny.

EXCITATIONGENERATOR

(my phtkch thch)

TIME-VARYINGLINEAR SYSTEM

(HTTTnh bin i theoThi gian)

SPEECHOUTPUT

(ci ra ting ni)


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4. CC M HNH NG MT T

Cc din tch mt ct khng i {A k} c chn xp x hm din tch, A(x), ca bmy pht m. Nu dng nhiu ng c di ngn, ta c th hy vng l cc tn s cnghng ca cc ng ni li s gn vi tn s cng hng ca ng c hm din tch thayi lin tc. Tuy nhin, v cch xp x ny b qua cc mt mt do ma st nht, dn nhitv rung ng ca thnh ng, nn ta cng c th hy vng hp l l rng di(bandwidth)

ca cc cng hng khc vi rng di cacc m hnh chi tit c tnh n cc mtmt y. D sao th cc mt mt cng c th tnh cho thanh mn v mi (hai u ca bmy pht m), v ta s thy y l c th lm iu biu din chnh xc cc tnhcht cng hng ca tn hiuting ni.

iu quan trng hn l cc m hnh ng mt t cung cp cch chuyn i thuntin gia cc m hnh thi gian lin tc v m hnh thi gian ri rc.

4.1. Truyn sng cc ng mt t ni li (Wave propagation in concatenated lossless

tubes): V mi ng hnh 3.32 mt t nn vic truyn m mi ng c m t bi ccphng trnh (2) vi cc gi tr thch hp ca din tch mt ct. Nh vy, nu xt ng thkvi din tch mt ct Akth p lc v vn tc m ng ny c dng

pk(x, t) =k

c

A[ ( )k

xu t

c+ ( )k

xu t

c] (14a)

uk(x, t) = ( )kx

u tc

+ ( )kx

u tc

(14b)

trong x l khong cch o c t im kt thc bn tri ca ng th k (0 x k),

cn ku () v ku () l cc sng truynv pha dng v v pha m trong ng th k. Quan

h gia cc sng truyn cc ng k nhau(adjacent tube) c th nhn c bng cchs dng lut vt l l p lc v vn tc m phi lin tc c theo thi gian v khng gianmo ni, mi lc trong h thng. iu ny cho cc iu kin bin c hai u mi ng.

Xt ring vic ni ng th kv th k+1. p dng cc iu kin lin tc ti onni, ta c

pk(k, t) = pk+1(0, t) (15a)uk(k, t) = uk+1(0, t) (15b)

Th cc phng trnh (14) vo (15), ta c

1k

k

A

A

[ ( )k ku t + ( )k ku t ] = 1( )ku t + 1( )ku t (16a)

( )k k

u t + ( )k ku t = 1( )ku t + 1 ( )ku t (16b)

trong k =k

c

l thi gian sng i qua chiu di ng th k. Ta thy phn sng v pha

dng n mi ni c truyn v pha phi, cn phn kia i v pha tri. Nh vy, nu

ta gii 1 ( )ku t v ( )k ku t theo 1( )ku t v ( )k ku t ta c th thy cch cc sng ngc

v xui truyn trong ton b h thng. Gii phng trnh (16b) tm ( )k ku t v th vo

(16a) c


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1( )

ku t = 1

1

2 k

k k

A

A A( )

k ku t + 1

1

k k

k k

A A

A A1

( )ku t (17a)

Th phng trnh (16b) vo phng trnh (16a) c

( )k ku t = -1

1

k k

k k

A A

A A ( )k ku t +

1

2 k

k k

A

A A1( )ku t (17b)

C th thy t (17a) l i lng

rk = 1

1

k k

k k

A A

A A(17c)

bng lng 1( )ku t phn x ti ch ni. Do vy rkc gi l h s phn x (reflection

coefficient) ch ni th k. V cc din tch u dng nn ta c -1 rk 1. S dng rk

cc phng trnh (17) c dng

1( )ku t = (1 + rk) ( )k ku t + rk 1( )ku t (18a)

( )k ku t = - rk ( )k ku t + (1 - rk) 1( )ku t (18b)

Cc quy c ho dng tn hiu (xem A.V. Oppenheim and R.W. Shafer, DigitalSignal Processing (X l Tn hiu S), Prentice-Hall, Inc., Englewood Cliffs, N.J., 1975)c s dng biu din cc php cng v nhn trong cc phng tr nh (3.41). Hinnhin l mi ch ni ca H thng hnh 3.32 c th biu din nh H thng hnh 3.34.Nh vy, m hnh 5 ng hnh 3.32 phi c 5 tp hp cc tr vo, ra v 4 ch ni, mich c biu din bi mt h s phn x. hon thin vic biu din s truyn sngtrong H thng cc ng mt t ni li, ta phi xt cc iu kin bin mi v thanh m

(2 u ca h thng).4.2. Cc iu kin bin:Gi s c N on ni, c nh s t 1 n N, bt u t thanh

m. Khi , iu kin bin ti mi s lin kt p lc, p N(N, t), v vn tc m, uN(N, t), tiu ra ca ng th N vi p lc pht x v vn tc m. Nu dng cc quan h min tns 3.4 ta c biu thc quan h

PN(N, ) = ZL.UN(N, )Nu gi thit tm thi ZLl s thc th ta c quan h min thi gian

N

c

A[ ( )N Nu t + ( )N Nu t ] = ZL[ ( )N Nu t - ( )N Nu t ] (19)

(Nu ZLl s phc th (18) c thay bngPhng trnh vi phn lin h pN(N, t) v uN(N,t).) Gii ra ( )N Nu t c

( )N Nu t = - rN ( )N Nu t (20)

trong h s phn x ti mi(reflection coefficient at the lips) l

rL =L

N

L

N

cZ

A

cZ

A

(21)

Vn tc m ra ti mi l

uN(N, t) = ( )N Nu t - ( )N Nu t = (1 + rL) ( )N Nu t


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Hiu ng ca kt thc ti mi c biu din (20), (21) v c v hnh 3.35.

Ch rng nu ZLl s phc th c th chng minh rng (21) cn ng v, d nhin, lc rLcng l s phc v phi thay (20) bng phng trnh trong min tn s.

Cc quan h min tn s vi gi thit ngun kch thch l tch c tuyn tnh t b my pht m xt 2.6. Dng gi thit ny cho p lc v vn tc m ti u voca ng th nht ta c

U1(0, ) = UG( ) = - P1(0, )/ZG.Li gi s ZGl s thc th

1( )u t - 1 ( )u t = uG(t) = -

1

c

A

1 1( ) ( )

G

u t u t

Z

Gii ra1

( )u t ta c

1( )u t =

(1 )

2

Gr

uG(t) + rG 1 ( )u t (23)

y h s phn x ti thanh mn (glotal reflection coefficient) l

rG = 1

1

G

G

cZ

A

cZ

A

(24)

phng trnh (23) c th biu din hnh 3.36.

Cng nh trong trng hp kt thc pht x, nu Z Gl s phc th phng trnh

(23) vn ng. Tuy nhin, rGphi l s phc v phng trnh (22) phi thay bi phngtrnh tng ng min tn s. Bnh thng, cc tr khng ZG v ZLc ly l sthc cho n gin.

Tc m mi c xc nh l uL(t) = u2(2, t). Vit cc phng trnh cho Hthng ny min tn s, p ng tn sca H thng l

Va( ) =( )

( )

L

G

U

U=

1 2

1 2 1 2

( )

1

2 2 2( )

1 1

0.5(1 )(1 )(1 )

1

j

G L

j j j

G L L G

r r r e

r r e r r e r r e

ng ch n mt s thnh phn ca Va( ). u tin l nhn t 1 2( )j

e t s.

Nhn t ny biu din vic lm chm lan truyn tng th H thng t thanh mn ti

mi. c hm h thng, ta thay j bi sv c

Va(s) =1 2

1 2 1 2

( )

1

2 2 2( )

1 1

0.5(1 )(1 )(1 )

1

s

G L

s s s

G L L G

r r r e

r r e r r e r r e

Cc cc ca Va(s) l cc tn s cng hng phc ca h thng. Ta thy c s v hncc cc v snm m.

4.3. Quan h vi cc lc s (Relationship to digital f ilters): Dng Va(s) ca m hnh 2 ng

cho thy cc m hnh ng mt t c nhiu tnh cht chung vi cc lc s. thy iu

ny, ta xt H thng gm N ng mt t, mi ng di x =N

, trong l di tng th

cab my pht m. H thng nh vy v hnh 3.38 vi N = 7.


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Vic truyn sng trong H thng ny c th biu din nh hnh 3.34 (mc 1), trong

tt c cc lm chm(delay) u bng =x

c, thi gian truyn dc mt ng. u tin ta

xt ca H thng vi ngun xung n v uG(t) = (t). Xung truyn theo dy cc ngc phn x tng phn v c truyn tng phn cc ch ni. Nghin cu chi titqu trnh ny a n kt lun l X (. tc m ti mi theo xung ti thanh mn) cdng

va(t) = a0 (t - N ) +1

( 2 )kk

a t N k .

R rng, sm nht l xung c th n li ra sau N giy. Cc xung tip sauph thuc vo

phn x ti cc ch ni s n li ra ti bi ca 2 giy tip sau. Lng 2 l thi gianphi c truyn theo c 2 hng trong mt ng. Hm H thng ca H thng ny cdng

Va(s) =( 2 )

0

s N k

k

k

a e = e - s N 2

0

sk

k

k

a e

Nhn t e - s N ng vi thi gian tr cn thit truyn qua N phn. i lng

( )a

V s = 2

0

sk

k

k

a e

l hm H thng ca mt H thng tuyn tnh c X n gin ( )av t = va(t+N ). Phn

ny biu th cc tnh cht cng hng ca h thng. Hnh 3.39a biu din s khi ca

m hnh ng mt t v tch bit H thng ( )av t khi b phn lm chm. p ng tn s

( )a

V l

( )aV s =

2

0

j k

k

k

a e .

D dng chng minh l

2( )

2aV = ( )aV .

D nhin, iu ny rt gi nh n p ng tn s ca H thng thi gian ri rc. Tht

vy, nu ci vo (. kch thch) ca H thng l gii hn gii(band limited) n cc tn sdi

2th ta c th ly mu ci vo vi chu k T = 2 v lc tn hiu mu vi lc s c

X l

( )v x =, 0;

0, 0.

nn

n

Vi chu k mu T = 2 , vic lm chm N giy ng vi thay i v tr(shift) N/2 mu. Hthng thi gian ri rctng ng cho cc tn hiu vo gii hn di v hnh 3.39b.Ch rng nu N chn, N/2 l s nguyn v vic lm chm c th thc hin bng cch

thay i v tr dy ci ra ca H thng th nht. Nu N l th phi ni suy c cc mu


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ci ra ca hnh 3.39a. Vic lm chm ny ging nh l b qua mt cch no (xem di) v n khng li hu qu lm trong mt s ng dng ca cc m hnh ting ni.

ZT ca ( )v n l ( )aV s vi e

STthay bi z. Nh vy

( )a

V z =0

k

kk

a z

Mt cch tng t, biu lung tn hiu cho H thng thi gian ri rctng ng cth nhn c t biu ca H thng tng t. c bit, mi nt bin trong H thng

tng t c thay bi dy cc mu tng ng. Cng nh vy, mi lm chm giy

c thay bi lm chm1

2mu v =

2

T.

Cc lm chm1

2mu hnh 3.40b ko theo php ni suy na ng(interpolation half-

way) gia cc gi tr mu. Ni suy nh vy khng th thc hin c chnh xc. C th

thu c mt cu hnh ng mong i hn bng cch xt cu trc c dng ci thang(ladder), c cc yu t lm chm ch phn trn v di. Cc tn hiu truyn sang phi nhnh trn v sang tri nhnh di. Ta c th thy l lm chm vng quanh bt cnhnh ng no s c bo ton nu cc lm chm nhnh di c chuyn chonhnh trc tip trn tng ng. Lm chm ton b t ci vo n ci ra lc s lsai, nhng iu ny t c ngha trong thc t v v mt l thuyt c th n b bngcch gn mt lng tin ng (tng quan l z N/2). (Ch l ta cng c th chuyn tonb lm chm cho nhnh di. Khi , lm chm qua h thng c th sa c bng

cch a vo mt lm chm2

N mu). u im ca dng ny l cc phng trnh vi

phn c th vit cho h thng v cc phng trnh vi phn ny c th s dng lp li tnh cc mu ca ci ra qua cc mu ca ci vo.

Cc mng s c th dng tnh cc mu ca tn hiu ting nitng hp t tnhiu ca cc mu kch thch thch hp. cc ng dng ny, cu to ca biu din mngxc nh s phc tp ca cc php ton cn thit tnh mi mu ra. Ta thy mi chni phi tnh 4 php tnh nhn v 2 php tnh cng. Nh vy, phi lm 4N php tnh nhnv 2N php tnh cng h c N ng ni li. Do php tnh nhn cn nhiu thi gian hnphp tnh cng th cn tm mt cu trc khc c th gim s php tnh nhn. C th ara cc cu trc nh vy nh hnh 3.41a. Cc phng trnh vi phn biu din s ny

l( )u n = (1 + r) ( )w n + r ( )u n (25a)

( )w n = - r ( )w n + (1 - r) ( )u n (25b)

Cc phng trnh ny c th vit di dng

( )u n = ( )w n + r ( )w n + r ( )u n

( )w n = - r ( )w n - r ( )u n + ( )u n

Ch l cc s hng r ( )w n v r ( )u n c mt c hai phng trnh, hai ci ra ca 4

php tnh nhn trong cc phng trnh (25) c th loi ra nh v hnh 3.41b. Cu hnh


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ny yu cu 2 php tnh nhn v 4 php tnh cng. Mt cch thc hin khc l nhmcc s hng c rnh

( )u n = ( )w n + r[ ( )w n + ( )u n ]

( )w n = ( )u n - r[ ( )w n + ( )u n ]

V s hng r[ ( )w n + ( )u n ] c mt c hai phng trnh th cu hnh ny ch c 1 php

tnh nhn v 3 php tnh cng.Khi s dng m hnh ng mt t tng hpting ni, vic chn cu trc tnh ton

ph thuc vo tc thc hin cc php tnh nhn v tnh cng, cng vi vic d dngkim tra tnh ton.

4.4. Hm truyn ca m hnh ng mt t (Transfer function of the lossless tube model): hon thin vic nghin cu cc m hnh thi gian ri rcca ng mt t dng tota phi a ra biu thc tng qut cho hm truyn theo cc h s phn x. Cc phngtrnh loi ny c nghin cu vo nhng nm 1970 di dng phn tch d on

tuyn tnh (linear predictive analysis) ting ni.

Vn y l tm hm truyn V(z) =( )

( )

L

G

U z

U z. lm iu , thun tin nht l

biu din UG(z) theo UL(z) v gii tm t s trn.Cc phng trnh ZT cho ch ni l

1( )

kU z = (1 + rk)z- 1/2 ( )

kU z + rk 1( )kU z

( )k

U z = - rkz- 1 ( )

kU z + (1 - rk)z

- 1/21 ( )kU z

Gii ra ( )kU z v ( )kU z , ta c

( )kU z =

1

2

1 k

z

r1 ( )kU z -

1

2

1

k

k

r z

r1 ( )kU z (26a)

( )k

U z =

1

2

1

k

k

r z

r1

( )kU z -

1

2

1k

z

r1

( )kU z (26b)

Cc phng trnh ny cho php ta lm vic ngc t ci ra ca m hnh ng mt t nhn c UG(z) theo UL(z).

thu c kt qu cht ch hn, ta phi biu din iu kin bin ti mi theo

cng cch nh tt c cc ch ni trong h thng. imcui ny, ta nh ngha UN+1(z)l ZT ca ci vo ca ng tng tng(fictitious tube) th N+1 di v hn nn khng csng i v pha m ng th N+1. Mt cch xt tng ng l coi ng th N+1 c

kt thc cng hng c trng (characteristic impedance) ca n, tc l 1( )NU z =

UL(z) v 1 ( )NU z = 0. Khi , t cc phng trnh tnh rk (17c) v h s phn x ti mi

rL (21), ta thy l nu AN+1 =L

c

Zth c th nh ngha rN = rL.

Cc phng trnh (26) c th vit di dng ma trn l

Uk = Qk Uk+1 (27)vi


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Uk =( )

( )

k

k

U z

U z

v

Qk =

11

22

1 1

2 2

1 1

1 1

k

k k

k

k k

r zzr r

r z z

r r

.

Bng cch lp li (27), d dng chng t rng cc bin u vo ca ng th nht cth biu din qua cc bin u ra bng tch ma trn

U1 = Q1 Q2 ... QN UN+1 =1

N

k

k

Q UN+1

T hnh 3.36 c th thy rng iu kin bin thanh mn c th vit lUG(z) =

2

1 Gr 1 ( )U z -

2

1

G

G

r

r 1 ( )U z = [

2

1 Gr, -

2

1

G

G

r

r] U1.

Nh vy, t

UN+1 =( )

0

LU z =1

0UL(z)

ta c

( )

( )

G

L

U z

U z=

22,

1 1

G

G G

r

r r.

1

N

k

k

Q .1

0=

1

( )V z.

xt cc tnh cht ca V(z) ta vit Qkdng

Qk =1

2z1 1

1

1 1

1 1

k

k k

k

k k

r

r r

r z z

r r

=1

2z . kQ (28)

Khi

1

( )V z =

1

2

z .

22

,1 1

G

G G

r

r r . 1

N

kk

Q.

1

0 (29)

Do cc phn t ca cc ma trn k

Q hoc l hng s, hoc l t l vi z - 1 th tch ca

chng l a thc bc N ca z - 1. V d khi tnh vi N = 2 ta c

1

( )V z=

1 1 2

1 2 1 2

1 2

2(1 )

(1 )(1 )(1 )

G G

G

r r z r r z r r z z

r r r

hay

V(z) =1

1 2

1 2

1 2 1 2

0,5(1 )(1 )(1 )

1 ( )

G

G G

r r r z

r r r r z r r z.


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Tng qut, t (27), (28) ta thy hm truyn(transfer function) cho m hnh ng mt t cth vit l

V(z) =

2

1

0,5(1 ). (1 ).

( )

NN

G k

k

r r z

D z

(30a)

trong D(z) l a thc ca z - 1vit di dng tch cc ma trn

D(z) = [1, - rG].1

1 1

1

1 r

r z z...

1 1

1 N

N

r

r z z.

1

0(30b)

. D(z) c dng

D(z) = 1 -1

Nk

k

k

z .

Ni cch khc, hm truyn ca m hnh ng mt t c lm chm(delay) tng ng vi scc phn ca m hnh, khng c cc khng m ch c cc cc. D nhin l cc cc ny

xc nh cc cng hng hoc cc tng cng(formant) ca m hnh ng mt t.Trong trng hp c bit, rG = 1 (zG= ), a thc D(z) c th xc nh bng cch

dng cng thc quy suy ra t (30b). Nu ta bt u tnh tch cc ma trn t bn tri, taphi nhn ma trn hng 12 vi ma trn 22, ri cui cng phi nhn vi ma trn ct 21.C th a ra cng thc quy bng cch tnh mt vi tch cc ma trn. K hiu

P1 = [1, -1].1

1 1

1

1 r

r z z= [(1 + r1 z

- 1), - (r + z - 1)]

vD1(z) = 1 + r1 z

- 1

th cP1 = [D1(z), - z

- 1.D1(z)].Tng t, k hiu P2l ma trn hng

P2 = P1.2

1 1

2

1 r

r z z,

thc hin php nhn, ta cP2 = [D2(z), - z

- 2.D2(z- 1)],

trong D2(z) = D1(z) + r2z

- 2D1(z- 1).

Bng quy np ta c

Pk = Pk-1. 1 11 k

k

r

r z z= [Dk(z), - z

- k.Dk(z- 1)],

trong Dk(z) = Dk-1(z) + rk.z

- k.Dk-1(z- 1).

Cui cng, a thc D(z) cn tm l

D(z) = PN.1

0= DN(z).

Nh vy, ta c th thy l khng cn phi lm tt c cc php nhn ma trn m ch cn

tnh theo quy


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D0(z) = 1;Dk(z) = Dk-1(z) + rk.z

- k.Dk-1(z- 1), k = 1..N;

D(z) = DN(z).


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5. CC M HNH S CHO CC TN HIU TING NI

Ta thy 3 l c th a ra cc biu din ton hc kh chi tit ca l thuytm hc toting ni. Mc ch ca chng ta khi xt l thuyt ny l nghin cu cc ntc bn ca tn hiu ting niv ch ra quan h v mt vt l ca chng vi vic to ra ting ni. Ta thy l m c sinh ra bng 3 cch, mi cch cho mt kiu ci ra khcnhau, ng thib my pht m buc cc cng hng tun theo cc kch thch to racc m ting ni khc nhau.

Cch tip cn ng n biu din tn hiu ting nil dng m hnh "tng t phncui" nh v hnh "M hnh h thng ngun ca vic toting ni" (3.7). lh thng tuyn tnh m ci ra c tnh cht ging ting ni (speech-like) khi c kim trabng tp hp cc tham s c quan h xc nh vi qu trnh to ting ni. M hnh nytng ng vi m hnh vt l kt thc ( ci ra), nhng c cu trc bn trong khngging m hnh vt l ca vic to ting ni. c bit, chng ta ch n cc m hnhtng t phn cui, c thi gian ri rc, biu din cc tn hiuting ni mu.

to ra tn hiu gingting ni, kiu kch thch v cc tnh cht cng hng cah thng tuyn tnh phi thay i theo thi gian. c bit, dng sng m cho thy cc tnhcht ca tn hiuting nithay i tng i chm theo thi gian. Vi nhiu m ting ni,c th gi s l cc tnh cht chung ca kch thch v b my pht m khng thay itrong cc khong t 10 n 20 mili giy. Nh vy, m hnh tng t phncui(terminalanalog) to ra mt h thng tuyn tnh bin i chm theo thi gian c kch thch bitn hiu kch thch c bn cht c bn thay i t cc xung gn tun hon cho ting nihu thanh n ting n ngu nhin cho ting ni v thanh.

M hnh thi gian ri rc ng mt t mc trc l mt v d in hnh cho iu

ni trn. Hy nh rng h thng cab my pht m c c trng bng tp hp ccdin tch, hay tng ng l cc h s phn x. Ta chng minh l quan h gia civo v ci ra c th biu din bng hm truyn V(z) c dng

V(z) =k

kz

G

1

(31)

trong G v { k} ph thuc vo hm din tch.

5.1. B my pht m (Vocal Tract): Cc cng hng (tng cng) ca tin ni ng vi

cc cc ca hm truyn V(z). Mt m hnh ton cc(all-pole) l biu din rt tt cc hiung cab my pht m cho nhiu m ting ni. Tuy nhin, l thuyt m hc li ni lcc m mi v m xt yu cu c cng hng (resonance) v phn cng hng(anti-resonance) (c cc v khng im). Trong cc trng hp ny ta c th a cc khngim vo hm truyn hoc gii thch nh Atal (B.S. Atal & S.L. Hanauer, Speech analysisand Synthesis by Linear Prediction of the Speech Wave, J. Acoust. Soc. Am., Vol. 50 2(Part 2) pp. 637-655, August 1971) l hiu ng ca khng im ca hm truyn c th thuc bng cch a vo nhiu cc hn. Trong a s trng hp cch tip cn ny ca chung hn.

V cc h s mu s ca V(z) (29) l s thc th cc nghim ca a thc

mu s hoc l s thc hoc l cc cp nghim phc lin hp


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sk,*

ks = - k j2 Fk (32)

Cc cc lin hp phc tng ng trong biu din thi gian ri rc l

zk,*

kz =Tke . TFj ke 2

= Tke cos(2 FkT) jTke sin(2 FkT) (33)

rng di ca cng hng cab my pht m gn bng 2 kv tn s trung tm l

2 Fk (theo A.M. Bose & K.N. Stevens, Introductory Network Theory, Harper & Row, New

York 1965). mt phng Z, bn knh t gc n cc xc nh rng di, tc l

zk =Tke (34a)

v gc z-mt phng l

k = 2 FkT (34b)

Nh vy, nu mu s ca V(z) c phn tch ra tha s th cc tn s tng cngtng t v cc rng di tng ng s tm c theo cc cng thc (34). Nh v hnh, cc tn s t nhin phc cab my pht m ca con ngi u nm na tri

ca mt phng sv n l h thng n nh.

4 ta thy m hnh ng mt t c hm truyn dng (29). C th chng minh(xem B.A. Atal & S.L. Anauer; J.D. Market & A.H. Gray, Linear Prediction of Speech,Springer - Verlag, New York 1976) l trong iu kin cc din tch ca m hnh ngdng th tt c cc cc ca hm V(z) nm trong vng trn n v. Ngc li, c thchng minh l cho hm dng (29) th c m hnh ng mt t nhn n l hm truyn. Nhvy, mt cch s dng hm truyn cho l dng mt cu trcxp tng ni tip.

Cch tip cn khc l dng mt trong nhng cu trc thc hin lc s tiu chun cho chng 2.

Tri li, ta c th biu din V(z) nh mt cu trc xp tng ni tip cc h thng bc hai(cc cng hng), tc l

V(z) = ,)(1

M

k

k zV

trong Ml s nguyn ln nht nh hn2

1Nv

Vk(z) =221

2

)2cos(21

)2cos(21

zzzTFz

zTFz

kkk

kkk

T s ca Vk(z) c chn sao cho tch c tnh cht nh m hnh ng mt t. Ch rngti tn s 0 (z = 1) th Vk(1) = 1.

Cnch rng cc m hnh cu trc xp tng ni tip v cu trc xp tng songsong lc u c xt nh cc m hnh tng t. Lm nh vy c nhiu hn ch v cch thng cp hai tng t (cc cng hng) c cc p ng tn s tt dn cng tn s.iu ny dn ti vic Fant [1] a ra cc "lng sa cc cp cao" cng vi cc tn stng cng tng t thu c cn bng ph tn s c bit cao. Khi cc tng t sc dng, Gold v Rabiner (xem B. Gold & L.R. Rabiner, Analysis of Digital and AnalogFormant Synthesizers, IEEE Trans. Audio and Electroacoustics, Vol. AU-16, pp 81-94,

March 1968) nhn thy l cc cng hng s c thc hin tn s cao ng n do s


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tun hon ni ti ca chng. Thc t, ta thy iu ny trng hp m hnh ng mtt. Nh vy, khng c mng "lng sa cc cp cao" c dng trong cc tng t s.

5.2. Pht x (Radiation): Cho n y ta xt hm truyn V(z) lp quan h vn tc m

thanh mn vi vn tc m mi. Nu ta mun nhn c m hnh cho p lc ti mi th

phi a vo cc hiu ng pht x. Ta thy trong m hnh tng t, p lc v vn tcm c quan h theo phng trnh

P(, ) = ZL( ).U(, ).Theo , ta a ra quan h ZT tng t ca PL(z) v UL(z):

PL(z) = R(z).UL(z).Nm 1975, trong cun sch "Digital Signal Processing", Oppenheim A.V. v Shafer R.W. chng minh c rng mt xp x thch hp cho cc hiu ng pht x l

R(z) = R0(1 - z- 1).

M hnh tng t phn cui bao gm cc hiu ng pht x c th biu din di dng

y V(z) c th thc hin mt cch thch hp v cc tham s cn thit phi ph hpvi cu hnh chn, V d l hm din tch cho m hnh ng mt t hoc cc tn s tngcng v rng di cho m hnhting ni.

5.3. Kch thch(excitation): hon thin m hnh tng t phn cui ta phi xt thmcc cch to ra ci vo thch hp cho h thng pht x cab my pht m. Ta bit la s cc mting nic th phn lp, hoc l m hu thanh, hoc m v thanh, v ccm ny c th to ra hoc l sng m c xung gn tun hon, hoc l sng m n ngunhin.

Nm 1971, trong mt nghin cu hiu ng ca xung thanh mn tc ng ln cht

lngting ni, A.E. Rosenberg chng t l dng sng xung thanh mn c th thaybng dng sng xung tng hp c dng

g(n) =

.laitrai0

;NNnNkhi]N2

)Nn(cos[

;Nn0khi)]N

ncos(1[

2

1

211

2

1

1

1

Do g(n) c di hu hn th ZT G(z) ca n ch c cc khng im.Vi cc m v thanh, m hnh kch thch n gin hn nhiu. Ch cn c ngun n

ngu nhin v tham s iu khin l cng ca kch thch v thanh.

5.4. M hnh y : Kt ni tt c cc thnh phn vi nhau ta c m hnh y .Bng cch chuyn gia cc my pht kch thch hu thanh v v thanh ta c th m hnhho vic thay i phng thc kch thch. B my pht mc th c m hnh ho theonhiu cch nu. mt s trng hp nn kt hp xung thanh mn v cc m hnhpht x thnh mt h thng. Thc t, chng ta thy phn tch d on tuyn tnh(Linear Predictive Analysis) vic kt hp xung thanh mn, pht x v tt c cc thnhphn cab my pht m vi nhau, v biu din chng thnh mt hm kiu ton cc

H(z) = G(z)V(z)R(z)

l iu c li.


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Mt cu hi t nhin t ra y l m hnh nh vy c hn ch g? Chc chn lm hnh cn khc xa vi h thng biu din bi cc phng trnh vi phn o hm ringt ra ban u. Rt may l khng mt thiu st no ca m hnh ny lm hn ch khnng ng dng ca n. Trc ht l cu hi t ra v vic bin i theo thi gian ca cctham s. cc m xt nh l cc nguyn m, cc tham s thay i rt chm v m hnhlm vic rt tt. Vi cc m ngn(transient sound) nh cc ph m tc, m hnh khngtht tt nhng cng thch hp. Phi nhn mnh l vic dng cc hm truyn v cc hmp ng tn sca chng ta c yu cu cht ch l ta c th biu din tn hiuting nitrn mt nn "thi gian ngn" (short-time). iu ny c ngha l cc tham s ca m hnhc gi thit l khng i (hng s) trong nhng khong thi gian di chng 10 -20miligiy. Khi hm truyn V(z) c dng xc nh cu trc ca m hnh c cctham s thay i rt chm theo thi gian. Hn ch th hai l thiu d phng cho cckhng im nh yu cu l thuyt cho cc m mi v cc m xt. y l hn ch nhtnh cho cc m mi nhng khng qu cht ch cho cc m xt. Cc khng im c tha vo m hnh nu cn thit. Th ba l vic phn i ca kch thch nguyn m-ph ml khng tho ng vi cc m xt hu thanh. Vic thm vo mt cch n gin cc kchthch nguyn m v ph m l khng tho ng v vic xt tng quan vi cc nh cadng thanh mn. Mt m hnh phc tp hn cho cc m xt hu thanh c nu ra(xem L.R. Robiner, Digital Formant Synthesizer for Speech Synthesis Studies, J. Acoust.

Soc. Am., Vol. 43, 4, pp. 822-828, April 1968) v c th s dng khi cn thit. Cuicng, c mt nhn xt nh l m hnh hnh 3.50 yu cu cc xung thanh mn cngn cch bi bi s nguyn ca chu k mu, T. G. Winham & K. Steiglitz ( InputGenerators for Digital Sound Synthesis, J. Acoust. Soc. Am., Vol. 47, 2, pp. 665-666,February 1970) xt cc cch loi b hn ch ny trong cc tnh hung phi iu khin

cao ca ging(pitch) mt cch chnh xc.5.5. Tm lc:Chng ny xt 3 lnh vc chnh: cc m ting ni, c trng vt l cavic toting niv cc m hnh thi gian ri rc cho vic toting ni. Vic xem xt licch pht m m hc v l thuyt m hc ca vic toting nic hi di, nhng cngcha . Mc ch y l cung cp kin thc cn thit v cc tnh cht tng quan catn hiuting ni, cng nh ra v gi cc m hnh thch hp x lting ni.

Cc m hnh tho lun 4, 5 l c s nghin cu sau ny theo hai cch: phntch ting ni (Speech Analysis) v tng hp ting ni(Speech Synthesis). V mt phn

tch ting ni, ta s xt cc k thut c lng cc tham s ca m hnh t cc tnhiuting nit nhin c gi thit l ci ra ca m hnh. V mt tng hp ting ni, tas s dng m hnh to ra tn hiuting nitng hp bng cch iu khin m hnhbng cc tham s thch hp. Hai quan im ny s kt hp vi nhau trong nhiu trnghp v s pht trin nhiu lnh vc ca vn . C s ca cc tho lun sau ny l ccm hnh nu chng ny.


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TI LIU THAM KHO

1. LAWRENCE R. RABINER, RONALD W. SCHAFER, Digital Processing of

Speech Signals, Prentice-Hall, Inc. Englewood Cliffs, NewJersey, 1978.2. LAWRENCE R. RABINER, BIING-HWANG JUANG, Fundamentals of Speech

Recognition, Prentice-Hall PTR, Englewood Cliffs, NewJersey, 1992.

3. GORDON E. PELTON, Voice Processing, McGraw-Hill, Inc. 1991.

4. LAWRENCE R. RABINER, Acoustics Research Laboratory, Bell TelephoneLaboratories, Murray Hill, NewJersey, 1999

5. Collins Cobuild English Language Dictionary, Collins London and Glasgow,1st Published 1987 Reprinted 1988.

6. T in TingVit, Vin Ngn ng hc, H Ni, 1992.

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