Chng II: X l m thanh X l thoi

Tng quan v XL thoi M ha d on: DPCM, ADPCM Vocoder M ha lai Cc tiu chun m ha thoi

X l m thanh M hnh Psychoacoustic v chc nng nghe-

ngi Cc bc c bn trong m ha m thanh cm

nhn M ha m thanh MPEG

X l thoi M ha ting ni l qu trnh biu din tn hiu ting

ni s ha s dng cng t bit cng tt, m boc mc cht lng ting ni hp l. (Nn tingni).

Dch v thoi: dch v vin thng c bn. Cng ngh m ha ting ni : thu ht s quan tm ca

cc nh nghin cu, chun ha, doanh nghip; Nh u vit ca vi in t, tnh kh dng ca cc b XL

c kh nng lp trnh, gi thnh thp, chip nh chuyndng; khc phc nhc im v hn ch.

Chun ha cc phng php m ha ting ni cho ccng dng khc nhau.

Cc ng dng ca MH thoi

Mc tiu m ha thoi

2,4 Kbps, t l 53 :1

Tn s LM=8 kHzS bit/mu=16

Tc bit=8 kHz x 16 bits = 128 Kbps

Mc tiu ca m ha thoi

Cu trc ca h thng m ha thoi

Digital Speech

FocusFocus

Yu cu t ra B m ha thoi Tc bit thp: bng thng truyn dn thp, s dng h

thng hiu qu hn( cht lng thoi). Ty thuc vong dng.

Cht lng thoi cao: cht lng c th chp nhn ctheo ng dng hng n; Cc tham s xc nh: tnh dhiu, tnh t nhin, tnh d chu, kh nng nhn din gingngi ni.

Tnh bn vng: qua cc ngn ng khc nhau, chng nhiu Hiu nng tt i vi tn hiu phi thoi: m thng bo,

nhc. Kch thc b nh thp, phc tp tnh ton thp Tr m ha thp

Cc phng php nh gi cht lng thoi Phng php nh gi theo thang im MOS (Mean Opinion Score)

(khuyn ngh ITU-T P.800): 5-Excellent/4-Good/3-Fair/2-Poor/1-Bad. PCM (64 Kbps lut A, ) c MOS = 4,5 5,0

Phng php nh gi da trn m hnh gic quan PSQM(Perceptual Speech Quality Measurement) (khuyn ngh ITU-T P.861): im PSQM th hin lch gia tn hiu chun v tn hiu

truyn dn. Phng php PESQ (Perceptual Evaluation of Speech Quality):

So snh tn hiu gc X(t) vi tn hiu suy gim Y(t) l kt qu ca vic truyn tnhiu X(t) qua h thng thng tin. u ra ca PESQ l mt c lng v chtlng thoi nhn c ca tn hiu Y(t).

Phng php da trn m hnh nh gi truyn dn E-model (chunETR 250): c lng cht lng thoi hai chiu v tnh n cc yu t nh: ting vng, tr...

Phn loi cc b m ha thoi

Theo tc Theo k thut m ha Theo phng thc s dng

Phn loi cc b MH thoi (theo k thut m ha)

Cht lng thoi & Tc bit ca cc b m ha

M ho dng sng

Trong min thi gian: M ho iu xung m (PCM): mi mu tn hiu

c m ha c lp vi cc mu khc. iu bin xung m vi sai (DPCM):

Cc mu ln cn tng quan vi nhau ng k = s saikhc v bin gia cc mu lin tip l kh nh.

Xy dng m hnh m ha tn dng tnh cht ny gim tc s liu u ra ca ngun: m ha s saikhc gia cc mu lin tip thay v m ha tng muc lp.

M ha d on (LPC, DPCM)

Quan st: Cc mu ln cn c s tng quan vi nhau rt ln.

M ha d on: D on mu hin ti t cc mu trc . Lng t ha,

m ha sai s d on thay v c gi tr mu. Nu d on chnh xc, sai s d on tp trung gn 0 v c

th m ha t bit hn so vi mu ban u. B d on thng c s dng l b d on tuyn tnh:

( ) ( )=

-=P

kkp knxanx

1

S khi b m ha DPCM

S khi b gii m DPCM

V d 1M ha chui mu sau s dng b m ha DPCM:-Chui: {1,3,4,4,7,8,6,5,3,1,}- S dng b d on: d on gi tr hin ti bi gi tr trc :

- S dng b lng t ha 3 mc:

- Vit chui mu c khi phc.- Vit chui bit m ha nh phn u ra, nu t m sau c s dng:

Sai s 0 l 1, sai s 2 l 01, sai s -2 l 00.

( ) ( )1px n x n= -

( )2 10 1

2 1

dQ d d

d

=

iu ch Delta S dng b lng t ha sai s d on gm c 2

mc: . Mi mu m ha 1 bit.

Bi tpBi 3:

M ha chui m sau s dng DM: {1,3,4,4,7,8,6,5,3,1,}. S dng deltabng 2, gi s cho trc mu d on u tin. Biu din chui khi phcv chui m ha nh phn.

Bi 4:Xt h thng d on s dng iu ch DM. B d on d on mu hinti da trn mu khi phc trc . Sai s d on c lng t hatheo b lng t:

Cho chui mu {3,4,5,3,1,}. Tnh gi tr d on, sai s d on, sai s d on lng t ha, gi tr

khi phc i vi mi mu cho,vi mu d on u tin l 3 c sdng c 2 b m ha v gii m. Gii thit 1 biu din e0 v 0 vie

M ho bng con SBC (subband coding)

Tn hiu c chia thnh nhiu di bng hp, tnhiu trong min thi gian ng vi mi di c mha c lp.

Trong m ha ting ni, di tn s thp cha phnln nng lng ca tn hiu, ng thi nhiu lngt nh hng n tai rt thp. Do vy, tn hiu bng tn thp c m ha nhiu bit hn tn hiu min tn cao.

M ho bin i thch nghi ATC (Adaptive Transform Coding)

Ti pha pht: chia cc mu tn hiu ca ngun thnhtng khung Nf mu, s liu trong mi khung cchuyn sang min tn s m ha truyn i.

Ti pha thu mi khung ph cc mu tn hiu cchuyn i ngc li trong min thi gian v tn hius c tng hp li t cc mu.

m ha hiu qu, ta dng nhiu bit cho cc thnhphn ph quan trng, v t bit cho cc thnh phn phkhng quan trng.

Cc php bin i c chn sao cho cc mu phkhng tng quan vi nhau: KLT (Karhunen-Love)(ti u nhng phc tp), DCT.

M ho dng sng Khi phc c tn hiu sng ging nh tn hiu gc.

phc tp, gi thnh, tr cng sut tiu th thp. Ch to c ting ni cht lng cao ti cc tc

ln hn 16kbps.

Khng to c ting ni cht lng cao ti tc nh hn 16kbps.

Lm cch no gim tc bit hn na?

ADPCM khng th cho cht lng tt khitc bit thp hn 16 Kbps.

gim tc bit hn na, m hnh tora ting ni cn c khai thc: m hada trn m hnh hoc m ha vocoder.

Cc phng thc m ha khng da trnm hnh c gi l m ha dng sng.

M ha ngun Tn hiu thoi c to ra t 1 m hnh (m

hnh AR) (c iu khin bi 1 s cc thams): Trong qu trnh m ha, cc tham s cam hnh c d on t tn hiu thoi u vo,m ha truyn n b gii m.

i vi ting ni, b m ha ngun c gi lvocoder: Hot ng da trn m hnh c quan pht m, c

kch thch bi mt ngun nhiu trng i vi onting ni v thanh hoc bi mt dy xung c chu kbng chu k pitch vi on hu thanh.

Thng tin c gi ti b gii m l cc thng s kthut ca b lc..

M hnh AR Phng php chung ca m hnh ha cc tn hiu ngu nhin: biu din tn

hiu lm u ra ca b lc tuyn tnh ton cc (all-pole filter). Khi c:

trong : ph cng sut u ra ca b lc l tch ca ph nhiu trng nhnvi bnh phng bin hm truyn t ca b lc.

Qua vic chn b lc c a thc mu s thch hp c th c c trng phmong mun cho cc tn hiu ngu nhin.

Cho chui cc gi tr x[n], x[n-1],, x[n-M] l th hin tin trnh t ngngc (AR) bc M, tha mn phng trnh vi phn:

trong a1, a2 ,, aM : l cc tham s ca AR, v(n) l tin trnh nhiu trng.hay:

Nhn thy rng: gi tr hin ti ca tin trnh x[n] l t hp tuyn tnh cc gi tr qukh ca tin trnh: x[n-1],, x[n-M] , cng vi v(n). Tin trnh x[n] c th hiu l csuy ngc li trn chnh cc gi tr trc ca n. Nn gi l t ng ngc (AR).

Hm truyn t ca b phn tch AR Ly bin i Z cng thc 3.41, ta c hm truyn t ca b phn

tch AR (b lc ton khng-FIR), vi u vo b lc l x[n] v u ra l v[n]:

Nh vy, b phn tch AR bin i tin trnh AR ti u vo thnh nhiu trng ti u ra.

Hm truyn t ca b tng hp AR Vi tn hiu vo l nhiu trng v[n] v s dng hm truyn t c

biu din nh sau:

to ra tin trnh AR, x[n] u ra. B tng hp tin trnh AR l b lc ton cc c p ng xung v

hn (IIR). B tng hp bin i ly nhiu trng lm u vo v to ra tn hiu AR u ra. Cng thc 3.44 cho thy hm truyn t ca b phn tch l nghch o hm truyn t ca b tng hp.

trong p1, p2,, pM l cc im cc ca Hs(z) v l nghim ca phng trnh c tnh:

B lc tng hp

Kch thch b lc tng hp (hm truyn t Hs(z)) s dng tn hiu nhiu trng, u ra ca b lc s c PSD gn ging vi tn hiu gc

D on tuyn tnh ng vai tr quan trng trong cc thut ton m ha ting ni. Trong mt khung tn hiu, cc trng s (h s d on tuyn tnh)

c s dng tnh ton t hp tuyn tnh c tm qua vic tithiu ha sai s d on bnh phng trung bnh.

ng thi, cc h s ny c s dng biu din li khung tnhiu .

Thnh phn c bn ca phng php d on l m hnh AR.Phn tch d on tuyn tnh l qu trnh d on tm cc thams AR da trn cc mu tn hiu (Gi thit ting ni c m hnhha l tn hiu AR).

LP cng c xem l phng php c lng ph: phn tch LPcho php tm c cc tham s AR (xc nh PSD ca chnh tnhiu). Qua vic tnh ton cc h s LPC ca khung tn hiu c thto ra c mt tn hiu khc c ni dung ph gn ging vi phca tn hiu gc.

Bi ton d on tuyn tnh D on tuyn tnh l bi ton nhn dng vi cc tham s AR c

c lng t chnh tn hiu AR (hnh 4.1). Tn hiu nhiu trng x[n] clc bi b tng hp tin trnh AR cho u ra s[n] tn hiu AR- vi cctham s AR l . B d on LP c s dng d on s[n] datrn M mu trc :

trong ai l nhng d on ca tham s AR v l h s LPC.

Sai s d on:

Ti thiu ha sai s Bi ton nhn dng h thng: d on cc tham s AR t s[n], vi

cc d on l cc h s LPC. thc hin d on, phi thit lpc tiu chun. Trong trng hp ny: sai s d on bnhphng trung bnh:

c ti thiu ha qua vic chn cc h s LPC. (J l hm bc 2ca cc LPC). Thy c s ph thuc ca J v cc h s LPC.

Tm cc h s LPC ti u qua vic ly o hm J theo ak:

Khi phng trnh 4.4 tha mn, th cc h s LPC = tham s AR.Do khi tm c cc h s LPC, h thng s s dng cc thams ny to ra tn hiu AR (b tng hp AR).

Ti thiu ha sai s T 4.4, vit li:

Hoc:

Phng trnh 4.6 nh ngha cc h s LPC ti u theo cc t tng quan Rs[l] ca tn hiu x[n]. Biu din 4.6 dng ma trn:

Biu din 4.6 dng ma trn:

Trong :

Nu tn ti ma trn nghch o ca ma trn tng quan Rs, tm c cc h s ti u LPC:

li d on (t s gia phng sai ca tn hiu vo vi phng sai ca sai s d on): nh gi hiu nng ca b d on

Sai s d on bnh phng trung ti thiu T hnh 4.1, khi ; sai s d on bng nhiu trng

(c s dng to tn hiu AR l s[n]): e[n]=v[n]. Khi , sai sbnh phng trung bnh l nh nht ( li d on ln nht):

iu kin t ti u: bc ca b d on bc ca b tng hpAR. Trong thc t, M khng bit trc. Do , phi lm cho lid on l hm theo bc d on. Bng cch ny, c th xc nhc bc d on khi li bo ha.

Nu bit c bc d on M, J t ti thiu khi .Cc tham s AR c s dng to tn hiu s[n]:

Kt hp 4.16 v 4.9:(0 l vector khng Mx1)

Phn tch LP i vi tn hiu khng dng Tn hiu thoi: tnh cht ng. Cc h s LPC phi c tnh mi

khung. Gi thit tnh thng k khng i trong mi khung . Tnhcc h s LPC t N im d liu kt thc ti thi im m: s[m-N+1],s[m-N+2],, s[m]. Vector LPC (M: bc d on) l:

T 4.9 vit li dng ph thuc vo thi gian:

Cc c ch d on C 2 k thut c bn: d on trong v d on ngoi.

D on trong: cc h s LPC c tnh t cc gi tr t tng quan clng c s dng d liu ca khung thoi x l cho chnh khung thoi .

D on ngoi: cc h s LPC tm c c s dng trong khung tng lai(sau ). D on ngoi c s dng v tnh thng k ca tn hiu thay ichm theo thi gian. Nu khung khng qu ln, nhng tnh cht thng k c thc ly t cc khung trc khng xa.

Khung c di in hnh: 160 n 240 mu. Phi s dng ca sc kch thc hu hn ly ra cc mu. Khung di hn: phc tp tnh ton t, tc bit thp hn, v vic tnh ton v

truyn cc h s LPC t thng xuyn hn. Song tr m ha ln hn v h thngphi ch tp hp cc mu.

Khng cho li d on cao.

Khung ngn hn: biu din chnh xc hn, nhng ti tnh ton v tc bit caohn

Gii thut Levinson-Durbin Phng trnh 4.9 c th gii theo li gii 4.13, nhng nhn chung l

phc tp. Hai gii thut Levison-Durbin (LD) v Leroux-Gueguen (LG) l hai

gii thut rt ph hp cho vic phn tch LP ca cc h thng trin khai trong thc t.

Xt phng trnh:

Mc tiu: Tm cc h s ai theo cc gi tr t tng quan cho trc.

Cc gi tr tng quan c c t vic c lng cc mu tn hiu. J l sai s d on trung bnh bnh phng ti thiu (thc t khng bit trc).

Thut ton LD: tm li gii ca b d on bc M t b d on bc M-1 ( quy lp).

Thut ton da trn tnh cht c bn bt bin ca ma trn tng quan:

B d on bc 0:

M rng chiu ca 4.29:

Gii thut Levinson-DurbinB d on bc 1 (tip):

V a1=0, nn iu kin ti u khng t c, a thm cn bngphng trnh v c xc nh:

T tnh cht ca ma trn tng quan, 4.30 tng ng vi:

Phng trnh 4.30 v 4.32 c s dng cho bc tip theo. B d on bc 1:

Tm li gii cho:

Trong , 2 bin cn tm cho phng trnh 4.34 :

: l h s d on ca b d on bc 1. J1 l sai s d on bnh phng trung bnh ti thiu c th t c s dng b d on

bc 1.

[ ]10 R=D0D

B d on bc 1 (tip)Tm c h s phn x k1 , h s d on ca b lc bc 1, v J1:

Gii thut:

Bi tp1. Cho mt khung d liu thoi c cc t tng quan l R(0)=1; R(1)=0,866; R(2)=0,554

v R(3)=0,225. Tm cc h s ai=? (i=1,2,3).2. Cho h thng LPC c cc h s d on a1=1,793; a2=-1,401; a3=0,566; a4=-0,147.

Bit li thu G=2, di chu k pitch=60; gi thit l m hu thanh. Vi cc iukin u =0 ti thi im bt u ca chu k pitch, tng hp 10 mu u tin?

3.a. Cho s khi ca mt m hnh d on tuyn tnh ca tn hiu x(m). Vit phng trnh m t quan h vo-ra ca m hnh trn (min thi gian). Ly bin i Z phng trnh vo/ra tm hm truyn t ca m hnh (m hnh cc) Vit phng trnh b lc d on tuyn tnh ngc. Tm sai s bnh phng ti thiu i vi cc h s d on tuyn tnh

b. Cho 3 h s t tng quan u tin ca tn hiu l: r(0)=1; r(1)=0,865; r(2)=0,521

Tm cc h s ca m hnh d on tuyn tnh bc 2, biu din cc h s dng cc. S dng m hnh ny tnh p ng tn s ca qu trnh v biu din ph ca b d on.

M ha ngun

C nhiu m hnh c xut: m hnh da trn don tuyn tnh (thnh cng nht):b lc bin i theothi gian. S dng to ra ting ni ting ni.

Tc bit in hnh: 2 n 5 kbps. C cc loi: m ha d on tuyn tnh (LPC), m ha

d on tuyn tnh kch thch hn hp (MELP).

M hnh ha h thng pht m

Cc cu trc gii phu to nn h thng pht m ca con ngi.

Velum: vm ming (ngc mm)Pharyngal cavity: Khoang huLarynx: Thanh qunTrachea: Kh qun

Ting ni (sng m) pht ra tmi, ming khi khng kh t phithot ra. Cc khoang mi+khoangming+khoang hu = b lc mthanh c bn c p ng tn sthay i theo thi gian, c kchthch bi khng kh. C quan pht m = khoang hu+ khoang mi tn s cng hng ca cquan pht m = tn s formant:ph thuc vo hnh dng v kchthc ca c quan pht m.

M hnh ha h thng pht m

Cc cu trc gii phu to nn h thng pht m ca con ngi.

Velum: vm ming (ngc mm)Pharyngal cavity: Khoang huLarynx: Thanh qunTrachea: Kh qun

Dy thanh (bn trong thanhqun ): ng/m nhanh trong khipht m. m hu thanh: khi dy thanhrung lm cho lung khng kh tphi b ngt theo chu k, to rachui xung kch thch c quanpht m. m v thanh: Khng kh thotra khng lm rung dy thanh,khng c tnh chu k, hn lon Trong min thi gian,m huthanh c tnh chu k rt mnh, vitn s c bn = tn s pitch.

M hnh ha h thng pht m(M hnh AR: autoregressive)

Phi: to khng kh = nng lng kch thch c quanpht m biu din bi ngun nhiu trng S dng k thut nhn dng (d on tuyn tnh) d on cc thng s ca b lc bin i theo thigian da trn tn hiu quan st c.

M hnh thoi c quan pht m

u ra ca blc s (b lcLPC): tn hiuthoi s

u vo l chuixung hoc chuinhiu trng.

Quan h gia 2 m hnh:

B m ha vocoder

Thng tin a n b gii m: Cc tham s c trng cho b lc; m v thanh/hu thanh; Nhng thay i cn thit ca tn hiu kch thch, chu k

m thanh.

Phng trnh biu din quan h vo/ra ca b lc c th hin phng trnh sai phn tuyn tnh:

Hm truyn t ca b lc:

M ha vocoder M hnh b lc c biu din di dng vector:

A thay i theo chu k 20ms (theo tnh cht khng dng ca tn hiuthoi), ti tn s ly mu 8000 Hz, chu k 20 ms tng ng vi 160mu. Do vy tn hiu thoi c phn chia thnh cc khung c di20 ms (50 khung/sec)

M hnh ny tng ng vi

Nh vy, 160 gi tr ca S c i din cho 13 gi trca A

2 kiu bi ton: Tng hp (Synthesis): Cho A, to S. Phn tch (Analysis): Cho S, tm A tt nht

B m ha ngun Thc hin:

Tm cc thng s ca b lc = phn tch d ontuyn tnh, c th to ra khung tn hiu c ni dungph ging vi khung ban u, vi m thanh gnging.

V vy, khung c th biu din qua vic s dng 10thng s b lc + h s nh c (tnh t mc cngsut ca khung gc). Tng s bit: 45 bit (40 bit chocc thng s, 5 bit: h s nh c)

LPC Vocoder 2,4Kbps S :

Hot ng vi tc khong 2,4 Kbps hoc thp hn To ra thoi c m thanh d hiu nhng khng trung thc so vi ting

ni t nhin ca con ngi. Cc h s LPC c biu din l cc tham s cp ph vch (line

spectrum pair (LSP)). LSP tng ng 1-1 v mt ton hc vi LPC LSP c tnh nh sau:

LPC Vocoder 2,4Kbps Phn tch thnh tha s cc phng trnh trn:

l cc tham s LSP LSP c bc v bin: LSP tng quan t khung ny n khung khc hn LPC Kch c khung l 20 msec (C 50 frames/sec, tc 2400 bps = 48

bits/frame). Cc bit ny c gn nh sau:

LPC Vocoder 2,4Kbps 34 bit cho LSP c gn

nh B1. tng ch G, c m

ha s dng b lng tha v hng khng u 7bit (b lng t ha vec t1 chiu).

i vi m hu thanh, lcc gi tr t 20-146. V/UV,T c m ha nh B2.

B1

B2

Tng qut ha: Cu trc ca b MH ngun

S b m ha LPC

S b gii m LPC

B m ha ngun Qu trnh m ha: (theo tng khung)

Ti pha pht: Tm cc h s ca b lc t khung thoi. Tm h s nh c t khung thoi. Gi cc h s b lc v nh c ti pha thu.

Pha thu: To ra chui nhiu trng. Nhn cc mu nhiu trng vi h s nh c. Xy dng b lc s dng cc h s b lc nhn c t

pha pht v lc chui nhiu trng nh c. u ra ca blc chnh l thoi tng hp.

u nhc im ca Vocoder

Cht lng ph thuc vo m hnh thoi.

Cc Vocoder c th pht m kh gi to.

Cht l-ng km cc vocoder rt nhy cm vi li.

C th cung cp thoi s vi tc < 2 Kbps.

M ho Hybrid (lai)

S dng lai ghp 2 cng ngh m ho sng vm ho Vocoder

C th t -c cht l-ng thoi tt ti cc tc bt 2-16kbps

M ha lai ph bin nht l m ho phn tchbng cch tng hp AbS (Analysis-by-Synthesis): RPE-LTP(Regular-Pulse-Excited-Long-Term Prediction), CELP, ACELP, CS-CELPvv

M ha lai To ra cc m thanh t nhin hn, tn hiu kch thch l

ty , c chn sao cho dng sng ting ni c tora cng ging vi dng sng tht cng tt.

B m ha lai: s dng m ha m hnh b lc v tnhiu kch thch nh mt dng sng.

B m ha d on kch thch m (CELP): chn tn hiukch thch t cc t m trong bng m c thit ktrc.

Nguyn l ny cho php cht lng tn hiu thoi c thchp nhn c trong di tc 4,8 16 kbps trong cch thng in thoi v tuyn.

M ho phn tch bng cch tng hp AbS C ch ti u ha vng kn (closed-loop) : chn tham s tt nht

nh x tn hiu thoi tng hp cng ging cng tt tn hiu gc. Tn hiu c tng hp trong qu trnh m ha cho mc ch phn

tch gi l AbS.

M ho phn tch bng cch tng hp AbS (Analysis-by-Synthesis)

Cng s dng m hnh c quan pht m ca conngi.

Thay v s dng cc m hnh tn hiu kch thch ngin th tn hiu kch thch c chn sao cho c gngt c dng sng ting ni ti to cng ging vidng sng ting ni ban u cng tt.

Thut ton tm ra dng sng kch thch quyt nh phc tp b m ha.

c s dng ph bin trong cc chun m ha tingni cho mng di ng.

Vng Phn tch bng cch tng hp trong CELP

B lc d on thi gian di Vic s dng s liu ting ni thc cho thy bc ca b lc phi

ln c th c t nht 1 chu k pitch m hnh ha c tn hiuhu thanh.

B d on tuyn tnh bc 10 khng chnh xc m hnh hatnh chu k ca tn hiu hu thanh c chu k pitch=50.

Khi tng bc b d on, tnh chu k trong sai s d on khngcn, dn n tng li d on.

Song nu bc b on cao s lm tng chi ph thc hin, tc bitv cn nhiu bit biu din cc h s d on, tng thm vic tnhton trong qu trnh phn tch. Cn phi c gii php va n gianli va c th m hnh ha tn hiu chnh xc.

Quan st thc nghim: li d on tng ch yu 8-10 h sd on u tin, cng thm h s ti chu k pitch l 49. Cn cch s bc 11-48 v ln hn 49 khng ng gp vo vic ci thin li d on (Hnh 4.9).

B lc d on thi gian di

B d on ngn hn c bc d on M tng i thp (M=8-12):loi b s tng quan gia cc mu ln cn. B d on thi giandi hng n s tng quan gia cc mu cch nhau 1 chu k.

Hm truyn t ca b lc thi gian di:

Hai tham s cn xc nh: chu k pitch T v li d on b (Biton phn tch LP thi gian di).

Cu trc khung/khung con ng dng trong phn tch LP thi gian ngn i vi khong tng

i di (khung- 240 mu). Khung c chia thnh cc khung con (60 mu) (khong thi gian

ngn hn). Vic phn tch LP thi gian di c thc hin trn cckhung con ny. (B m ha CELP).

B m ha CELP S dng cc m hnh d on tuyn tnh di hn v ngn hn

tng hp ting ni, trnh vic phn loi m hu thanh v v thanhca LPC. Sau kt hp vi bng m kch thch (c truy vntrong qu trnh m ha), tm ra chui kch thch tt nht.

B lc tng hp pitch to ra tn hiu c tnh chu k vi tn s c bnpitch, a n b lc tng hp formant to ng bao ph.

Bng m: c nh hoc thch ng, cha cc xung xc nh hocnhiu ngu nhin.

n gin: bng m c nh, cha cc mu nhiu trng.

Ch s kch thch s chn ra chui nhiu trng a vo cc b lc.

B m ha CELP

B m ha RPE-LTP(Regular-Pulse-Excited-Long-Term Prediction)

L b m ha ADPCM, trong b d on thc hin tnh ton ttn hiu, tm sai s d on v lng t sai s ny s dng c chthch nghi.

C 2 b d on thi gian ngn v thi gian di, tng c lid on trung bnh.

B m ha: Cc tham s ca mi khung/khung con c ly ra v c ng gi to thnh

lung bit. Chia cc mu ting ni u vo thnh cc khung (160 mu 20ms), t cc

khung chia thnh cc khung con (40 mu). Khi tin x l: s dng b lc thng cao loi b thnh phn DC. Phn tch LP: c thc hin trn tng khung, s dng bc d on l 8. 9 gi

tr t tng quan c tnh ton t khung s dng ca s hnh ch nht. Ccgi tr tng quan c s dng tm 8 h s phn x.

Phn tch d on thi gian di, lc v khi m ha

B gii m RPE-LTP

Nhn xt Hu ht tt c cc b m ha lai u da

trn m hnh LPC, tu theo cch to ra tnhiu kch thch m ngi ta a ra cc loim ho lai khc nhau nh:

- M ho a xung MPE-LTP- M ho xung u RPE-LTP- M ho kch thch bng m CELP,ACELP,CS-

ACELP..- M ho kch thch vect tng VSELP.vv

Cc b m ha lai khc phc nhc imca LPC v cho dch v thoi tc thp vcht lng tng i tt.

Cc tiu chun m ha thoi

M ha m thanh (audio coding)

m thanh M hnh b m ha v gii m m thanh B m ha cm nhn

m l hc (psychoacoustics) Hiu ng che (auditory masking)

Che min tn s Che min thi gian

Chun nn m thanh MPEG

m thanh (Sound) m thanh l mt tn hiu lin tc c to ra bi s nn gin khng

kh. S thay i p sut khng kh lm cho mng nh (eardrum) rung

ng. Di tn s t 16Hz -20000Hz c gi l di tn s m thanh.

Bc sng ca m thanh trong di m tn l t 21.25m n0.017m.

Nhng m c tn s nh hn 16Hz gi l sng h m Nhng m c tn s ln hn 20000 Hz gi l sng siu m

c tnh m thanh Tn s m thanh:

S ln dao ng ca khng kh truyn dn m trong mt n vthi gian l 1 giy.

Tn s biu th cao (pitch) ca m thanh. Tn s cng ln th m thanh cng cao v ngc li.

Cng ( mnh: intensity): L lng nng lng c sng m truyn i trong mt n v thi

gian qua mt n v din tch t vung gc vi phng truyn m. n v: W/m2 Mc m thanh c cm nhn bi con ngi c cp n

nh m lng (loudness). c o bi n v m lng: phon L cng cm gic ti 1000 Hz (gi tr cng m chun)

Cng sut: L nng lng m thanh i qua mt din tch S trong thi gian

mt giy. Cht lng quality (m sc: timbre):

m sc c bit n nh l "cht lng" m thanh hay "mu sc" ca mthanh; gip phn bit nhng loi nhc c khc nhau.

Truyn tn hiu audio s

Him khi n knh (monoaural sound) CD: 2 knh (stereo). DVD: 7.1 knh (surround sound) (7 knh

normal + 1 knh hiu ng tn s thp LFE -

M ha m thanh m nhc c bng tn rng hn v a knh. M ha dng sng m bo c cht lng

m thanh t nhin S dng nhng c tnh ca tai ngi xc

nh s mc lng t ha trong cc di tn skhc nhau. Mi thnh phn tn s c lng t ha vi kch c

bc ph thuc vo ngng nghe. Khng m ha thnh phn tn s m tai ngi khng

th nghe c.

M ha m thanh Cht lng m thanh cao hn tc ly mu nhanh

hn, nhiu bit/ mu hn, v nhiu knh hn. Tc truyn tn hiu audio Nch knh:

B0 = b (s bit/mu). Fs. Nch DVD-Video: 48 kHz x 24 bit/mu = 1.152 kbps/ 1 channel; 2.304

kbps/2 channles; 6.912 kbps/5.1; 9.216 kbps/7.1; Bng thng yu cu ln, phn pht n khch hng qua

mt s phng tin truyn thng c dung lng hn ch(wireless: yu cu ln hn 36 ln so vi bng thngknh c gn).

Gii php: Tng dung lng knh truyn (chi ph ln, ko th thc hin c) Hoc gim yu cu bng thng (gim tc bit: m ha m thanh

s).

S m ha m thanh

Yu cu t bitT s nn: r=B0/B

(B: tc bit yu cu truyn bn rt gn)

B m ha knh, b iu ch, knh vt l, b gii iu ch, Pht sinh li bit.

Khng tn tht: tn hiu m thanh khi phc ging vi tn hiu m thanh ngun.

Tn tht: bn gn ging, mt s thng tin b mt, tn hiu m thanh mo (khng cm nhn c)

tng L thuyt thng tin: tc bit trung bnh ti thiu cn

thit truyn tn hiu ngun l entropy H ca n (xcnh bi xc sut phn b ca tn hiu ngun).

S sai khc: R= B0 H, d tha thng k. M ha m thanh kiu lossless: remove d tha

thng k t tn hiu ngun cng nhiu cng tt, saocho B cng gn H cng tt. (Hnh 1.2)

M ha entropy: l k thut m ha g b phn d tha thng k

Nhn xt:T l nn: hn ch (2:1), ko tha mn yu cu thc t (36:1), vimc ny mt s thng tin trong tn hiu ngun s b mt, khngchuyn i ngc li c (b gii m)

tng Thng tin mt khng chuyn ngc li c gy mo

tn hiu audio khi phc ti u ra b gii m. Vn : Thit k c b m ha m bo vic tai ko

cm nhn c mo, hoc c th cm nhn cnhng cha n mc phin phc (annoying).

Phn thng tin trong tn hiu ngun gy ra mo nhngko nh hng n cm nhn hoc khng phin phc lthng tin ko lin quan n cm nhn (ngoi cm nhn:perceptual irrelevant) c th loi b khi tn hiungun, gim ng k tc bit (B m ha lossy).

B m ha lossy: remove nhng thng tin ko nh hngn s cm nhn + d tha thng k. (Hnh 1.3)

M hnh d liu Lm cho m ha m thanh hiu qu hn, nhng cng

phc tp. Cc tn hiu m thanh c s tng quan rt ln v c

cc cu trc bn trong c th th biu din qua cc mhnh d liu.

V d: tn hiu sin 1000 Hz, c LM vi tc 48 kHz,biu din 16 bit/mu, tnh chu k chng minh rng n cs tng quan ln. M hnh ha tnh chu k b i stng qua l d on tuyn tnh hoc chuyn itrc giao (gii tng quan) (chuyn tn hiu vo thnhcc h s ko tng quan, c nng lng c chuynvo mt s t cc h s: DCT), hoc chuyn i lapped(m ha bng con)

M hnh cm nhn

M hnh xc nh mc ti u c th loi b an ton thng tin khng lin quan n cm nhn (perceptual irrelevance).

c thit k trong min tn s (ngng che).

Kin trc c bn (P.15)- B m ha m thanh

Kin trc c bn (P.15)- B gii m m thanh

B m ha m thanh cm nhn(Perceptual Audio Coder)

Di b lc phn tch v tng hp Di b lc:

Thnh phn quan trng trong hu ht cc b m ha video. Chuyn i t min thi gian dang min tn s v ngc li.

Gim tc ly mu (Down-Sampling)

Hot ng gim tc ly mu i N ln m t qu trnh gi li cc mu th nN.

V d Di b lc phn tch :N=1024 b lcTn s ly mu: fs=44100 HzTn s Nyquist: fg=22050 Hz

Tng tc ly mu (Up-sampling) Hot ng tng tc ly mu ln N ln m t vic chn vo

N-1 mu 0 gia cc mu u vo.

Di b lc tng hp

Di b lc iu ch Ci thin di b lc: To ra kch c ca s ln hn Hnh dng ca s khc nhau V d:

m l hc (Psychacoustic) S khi ca b m ha m thanh cm nhn

Cu to ca tai

Tin x l m thanh trong h thng ngoi bin

La chn tn s ca mng nn

X l m thanh trong h thng thnh gic Mm nn=Di b lc

c tai

X l m thanh trong h thng thnh gic

H thng thnh gic c th c m hnh nh di b lc, gm 25 blc bng thng chng ln, t 0 n 20 KHz.

Tai khng th phn bit cc m thanh xut hin ng thi trongcng mt di bng.

Mi di bng c gi l bng c bn. Bng tn ca mi bng c bn khong 100 hz i vi cc tn hiu

di 500 Hz, v tng tuyn tnh sau 500 Hz n 5000 Hz. 1 bark = rng ca 1 bng c bn.

Tai ngi: di b lc

2

/100, 500ar

9 4log ( /100), 500f f Hz

B kf f Hz

= + >

Cm nhn m thanh Tn s v di tn s m thanh

Ngng ngheq Ngng nghe l mt hm ca tn s m thanh.

q Khi cc thnh phn tn s thp di mc ngng th cc m thanh c tn s ny s khng nghe c.

q Tai ngi nhy nht trong phm vi tn s t 2 4KHz.

Che min tn sTn hiu c p sut cao hn mc ngng nghe vn c th bche khut bi cc tn hiu c p sut ln hn v tr gn tnhiu trong min tn s tn hiu tn s ny s khng nghec. Tn hiu che khut lm dch ngng nghe.

Che min thi gian

M ha m thanh cm nhn Phn tch tn hiu thnh cc di tn s ring bit qua

vic s dng di b lc. Phn tch nng lng tn hiu trong cc di khc nhau

v xc nh ngng che tng ca mi di bi cc tnhiu trong di khc.

Lng t ha cc mu trong cc di khc nhau c t lchnh xc vi mc che.

Mt tn hiu no di mc che khng cn m ha. Tn hiu trn mc che c lng t ha v cc bit

c gn qua cc di sao cho mi bit thm vo c thgim ti a mo cm nhn.

Cc tiu chun MPEG MPEG: nhm chuyn gia nh ng ca t chc tiu chun quc t

(ISO). MPEG-1: nh ngha cc chun m ha v m thanh v video, cch

thc gi ha cc bit m thanh v video ng b thi gian. Tc tng: 1,5 Mbps. Video (352x240 pels/frame, 30 frame/s): 30 Mbps n 1,2 Mbps. m thanh ( 2 knh, 48 K samples/s, 16 bit/sample): 2*768 kbps n < 0,3 Mbps. ng dng: web movies, MP3 audio, video CD.

MPEG-2: cho m thanh v video cht lng tt hn. Video: 720x480 pels/frame, 30 frames/s: 216 Mbps n 3-5 Mbps. Audio (5.1 knh), m ha m thanh tin tin (AAC).

MPEG-4: hng n s a dng v cc ng dng, c di chtlng v tc bit rng, nhng cht lng c ci thin ch yu tc bit thp. Cho ng dng internet audio video streaming

Chun m ha MPEG Tip cn ca MPEG: MPEG ch chun ha khun dng lung bit v

b gii m (khng a ra khuyn ngh v thut ton m ha).

MPEG-1: Tc bit t 32 kb/s n 448 kb/s. Ba lp:

Lp 1: phc tp thp nht Lp 2: phc tp v cht lng tng Lp 3: phc tp cao nht, t cht lng cao nht tc bit thp.

Cc tc hng n: 384 kbps; 256 kbps;

Thut ton m ha m thanh MPEGq B Lc bng con

q Che bng bi bng gn s dng m hnh m l hc(khoa hc tm sinh l nghe (Psychoacoustics) )

q Loi b nhng bng c p sut nm di ngng che.

q Lng t ha/ Gn bit/ M ha

q nh dng lung bit

Cc bc c bn trong m ha m thanh MPEG-1

S dng cc b lc tch chp chia tn hiu m thanh thnh 32bng con: lc bng con.

Xc nh mc che i vi mi bng da trn tn s ca n (ngngche tuyt i threshold in quiet), v nng lng ca bng ln cnv tn s v thi gian (che min tn s v che min thi gian).

Nu nng lng trong mt bng nm di ngng che, khng mha n.

Ngc li, xc nh s bit cn thit biu din h s trong bngny sao cho tp m sinh ra do lng t ha nm di hiu ng che(khi thm vo 1 bit gim c tp m lng t ha i 6 dB).

nh dng lung bit: chn cc tiu thch hp, m ha thng tinpha pht nh lng t ha cc h s t l cho cc bng khc nhauv m ha (s dng m ha di thay i: Huffman).

Cc Lp trong MPEG-1 Lp 1:

di khung: 384 mu (8ms vi fs=48kHz) phn gii tn s: 32 bng con Lng t ha: nn khi (12 mu), bin ca cc mu bng

con c ch th qua h s nh c SCF, phn gii 2dB. Lp 2:

di khung: 1152 mu (24ms vi fs=48kHz) phn gii tn s: 32 bng con Lng t ha: nn khi (12 mu), s dng h s nh c (SCF)

chn thng tin. Lp 3:

di khung: 1152 mu (24ms vi fs=48kHz) phn gii tn s: 576/192 bng con Lng t ha: khng u vi m ha, s dng h s nh c

(SCF) chn thng tin.

MPEG-1 Lp 1 S khi

MPEG-1 Lp 1 Cu trc khung

MPEG-1 Lp 3 MP3 = ISO/IEC IS 11172-3 (MPEG-1 lp 3) v 13818-3

(MPEG-2 lp 3). Khun dng file khng c tiu , khng cn thit c tiu . Tr nh nht ti b M ha/Gii m l 59 ms.

MPEG-1 Lp 3 Cu trc c bn ging nh lp 2: di khung (24ms, 48kHz), di b lc

nhiu pha. im khc:

Di b lc lai ghp (MDCT)(32x18=576 bng con hoc 32x6=192 bng con). (Hnh v)

Lng t ha khng u. M ha Huffman. Cu trc phn tch da trn tng hp. H tr bit thay i

Cu trc b gii m MPEG-1

Qu trnh gii m Lp1 v 2

Qu trnh gii m MPEG-1 Lp 3

Qu trnh gii m MPEG-Tng hp ca b lc bng con

Bi tp 81. Nu 3 c im nghe c bn xc nh mc thp nht ca mt

m thanh c th nghe c?2. Gi s tn hiu m thanh c chia thnh 16 bng tn c nng

lng trong cc bng khc nhau nh sau:---------------------------------------------------------------------------------------Bng 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16Mc (db) 0 8 12 10 6 2 20 60 14 20 15 2 3 5 3 1Gi s rng nu mc ca bng th 8 l 60dB, n cho php che 12dB bng 7, 15 dB bng 9. Xc nh s bit cn thit m habng 7 v 9? Bit rng tn hiu gc c biu din vi 8bit/mu/bng.

3. Nu 3 bc c bn trong m ha m thanh cm nhn? V s khi ch ra 3 thnh phn ny.

4. Nu nhng im khc bit gia cc lp ca audio MPEG4 khacnh cc k thut c s dng v cht lng m thanh cng tc bit?

Li giiBi 1: Ngng nghe tuyt i, che min tn s, che min thi gian. Ngng nghe tuyt i cho bit mc thp nht (ngng) c th nghe

c khi ch c mt tn s m thanh n v ngng ph thuc tn s.Bi 2: V nng lng bng 7 l 20 dB, ln hn 12 dB, cn phi m ha. Do mc

che 12 dB, c th m ha vi tp m lng t 12 dB hay gim tc bit i2 bit. Do ch cn 6 bit.

i vi bng 9, nng lng tn hiu l 14dB, thp hn 15 dB. Khng cn mha bng 9.

Bi 3 Vic lc bng con xut pht t cc tn hiu bng con trong cc di tn s

khc nhau. Tnh ton mc che da trn nng lng ca cc bng con khc nhau. Gn bit trong s cc bng con da trn mc cho ca mi bng con.