Tm hiu cc phng php m ha thoi trong GSMI.GII THIU1. Tng quanTrong thng tin di ng rng di tn c gi tr nh hng ha, do b m ha ting ni ng vai tr quan trng khng ch vi cht lng ting ni m cn phi m bo chim dng t bng tn vi tc bit thp. B m ha tc bit thp lm tng dung lng h thng l yu cuquan trng ca cc nh cung cp dch v.Cc b m ha ting ni cbn cphn thnh 2loi chnh l: Mha theo dngsng v mha ngun (vocoder).
Hnh 1.1 Phn loi m ha ting niM ha dng sng:L k thut m ha v gii m lin tc bn theo dng sng ca tn hiu m thanh, v nguyn tc chng c thit k c lp vi ngun m nn cht lng khng ph thuc vo ngun tn hiu. Kiu m ny chng nhiu tt, khng phc tp, gi r song h s nn tn hiu cha cao nh PCM, ADPCMM ha ngun:Da trn s m phng ngun m c th (ting ni c nghin cu trc cc c tnh khai thc kh nng nn m), k thut ny kh phc tp, c tr cao,gi thnh t b li c h s nn ln v cho tc bit m ha thp.2. S lc qua trnh s ha v m ha trong gsm. u tin, ting ni c microphone bin i sang tn hiu in dng tng t. Microphone bao gm mt mn mng v mt cun dy t trong khe t trng cua mt nam chm. gim lng d liu cn thit tng ng vi sng m, ta cho tn hiu qua b lc thng di trong khong tn s t 300 Hz n 3.4 kHz. Sau , tn hiu ny c bin i sang tn hiu s bng b bin i A/D dng k thut iu xung m PCM vi tn s ly mu l 8kHz v m ho mi mu bng 13 bit. Do , lung tn hiu s sau khi c bin i c tc 104 kbps.Tn hiu s ng ra cua b bin i A/D c tc 104 kbps c nn li bng b m ho ting ni. M ho ting ni l phng php nn tn hiu thoi dng s. Yu cu cua m ho ting ni l phi m bo thi gian thc v cht lng c th chp nhn c. Trong GSM, ngi ta s dng m Vocoder. Nguyn tc cua k thut ny l thay v truyn i lung s t ting ni th ta s truyn i thng s cua c quan pht m ti thi im pht ra ting . Nh vy, chui bit truyn i s ngn hn nn tc s gim xung.
Hnh 1.2
Tn hiu s ng ra cua b bin i A/D c tc 104 kbps c chia thnh tng on c chiu di 20 ms, nh vy mi on cha 2080 bit (tng ng 160 mu). truyn i chui bit ny, ngi ta s thay th thng s cua b lc c chiu di 260 bit. Nh vy, 260 bit mi 20ms tng ng vi tc truyn tht s l 13 kbps.
Hnh 1.3
3. Tm hiu m thanh, ting ni v tnh cht. Ting ni l phng tin ch yu m con ngi s dng lin lc v giao tip hang ngy.Qu trnh to ting ni l s kt hp phc tp ca nhiu b phn trong c th ngi nh thanh qun, khoang ming, khoang mi, li, c hm, mi,..Trong k thut m ha ting ni,ngi ta chia ting ni thnh hai dng m chnh sau da trn s dao ng ca cc dy thanh m: + m hu thanh : c hnh thnh khi cc dy thanh m dao ng ng m lm ngt qung lunkhng kh v s nht qung ny c xem gn nh l tun hon tc ng ln c quan pht m.Theo thc nghim chu k tun hon ny khong t 2 20ms. Do , vi mhu thanh, tn hiukch thch c m hnh ha l cc xung tun hon.+ m v thanh : c to ra khi lung khng kh i qua thanh mn tc ng ln c quan pht m khng theo mt quy lut no c (khng tun hon). Do vi m v thanh, tn hiu kch thch c m hnh ha tng t nh mt nhiu.Dng tn hiu ting ni c mt s tnh cht hu ch c th khai thc c khi tin hnh m ha tn hiu. Tnh cht thng dng nht l s phn b xc sut khng u ca bin ting ni, c s tng quan gia cc mu lin tip, bn cht khng phng ca ph tn hiu ni , s tn ti ca cc thnh phn m ku v m khng ku ca cc tnh hiu ting ni.
II. Qu trnh to ting ni
A- S lc qua trnh to ting ni 1. B phn cung cp ln hi:Bao gm hai l phi, c s tc ng ca cc c ngc, sn, c honh cch m, c bng.
Hnh 2.1
Phi gm nhng t bo xp, c co gin ln, to thnh bi nhng ti nh, cc ti ny gin ra cha y khng kh, v co li y khng kh ra ngoi bng cc ph qun. Cc ph qun ny u thng vo kh qun, trng ging nh nhng r cy bm vo gc cy .
S co gin ch ng ca phi l do s h tr tch cc ca lng ngc v honh cch m cng cc c bng: Honh cch m h xung, lng ngc trng ra, lm cho phi gin ra tng thm th tch, to khong trng cho khng kh bn ngoi vo. Honh cch m nng ln, bng hi hp vo, lng ngc bung lng xung, lm cho phi thu nh li, y khng kh ra ngoi.
Chng ta c th v hai l phi nh mt ci b, cung cp dng kh cho c th v thi thn kh ra ngoi. Mi ln th bnh thng, ta ht vo na lt khng kh. Mi pht, ta th khong 15 ln v 15 lt mu c i mi.
Khi ln hi t phi c y ra ngoi, nu tc ng ng cch ln thanh i (dy ting), th s pht ra m thanh. Cht lng ca m thanh pht ra, mt phn ln ph thuc vo ln hi t phi a ln tc ng vo thanh i.
2. B phn pht thanh.- (Ch mi pht ra m thanh ch cha pht ra ting, ra li) gm 2 thanh i nm trong thanh qunThanh qun l mt ng ni tip vi kh qun. Phn gia thanh qun tht li nh c chai. Ch tht li ny l do cc dy c v sn chn ngang hai bn to nn thanh i. Thanh i l b phn ch yu to ra m thanh: Do p lc ca ln hi t phi a ln, thanh i - vi nhng cng khc nhau v hnh dng khc nhau - m ra v ng li nhanh chm khc nhau, ct ln hi thnh nhng sng m c tn s khc nhau, to thnh nhng m thanh c cao khc nhau: Thanh i mng/ngn m ng nhanh hn thanh i dy/di (thanh i ph n v tr em ngn v mng hn n ng, nn ging n v tr em cao hn ging n ng mt bt ). Thanh i mng hn khi c cng ra, hoc thanh i khng m ng ton phn, m ch m ng trn mt phn no ca mnh, lm cho phn thanh i tham gia ct ln hi ngn i, v nh vy to c nhng m thanh cao. cng, hnh dng, kch thc ca thanh i khng ch nh hng n cao , m c m sc na. Cn cng m thanh l do sc mnh ca ln hi a ln
Nh vy m thanh pht ra ph hp hay khng l do s phi hp nhun nhuyn gia ln hi v cc c iu khin thanh i. Do , cn bo v thanh i cho lnh mnh.
3. B phn truyn tng m:Gm ch yu l cung hng (yt hu) thng vi ng ming hoc ng mi.
1. Cc chn ng m thanh do thanh i to ra, c b phn truyn m gom li v dn ra ngoi theo hai hng ming hoc mi. Cung hng v ming khng nhng truyn m, m cn gp phn kh quan trng vo vic tng m (ng vai tr nh mt hp cng hng).
2. Cung hng c bao bc bi mt h thng nim mc, d b kch thch, do cn phi gi gn cung hng khng b vim nhim, nh hng xu n ging (trnh dng ru, c ph, thuc l v thc n thc ung qu nng, qu lnh, qu cay ...)
4. B phn pht m (nh ch):L ming vi cc hot ng ca mi, rng, li, hm di, vm mm (hm ch mm, ca mm, ngc mm,). Chng ta nhn ra c li ca, ting ht vi ngha ca n, l nh vo hot ng ca cc c nng trn.Khi ni n khu hnh l ni n hnh th, hnh dng, c bn ngoi ln bn trong ca ming do hot ng phi hp ca mi, li, hm di, vm mm to ra khi pht m. M khu hnh khng ng cch s nh hng khng ch n cht lng m thanh, m nht l nh hng n vic r li.
5. B phn di m:Gm tt c cc khong trng trong thn th, ch yu l cc khong trng trn u, trn mt, gi l xoang cng minh (cng hng). Ngoi khoang hng v khoang ming va tng m va di m, th cc xoang mi, xoang vm mt, xoang trn v.v... ch yu c tnh cch di m, tc l lm cho m thanh c cng hng, m vang y n, giu m sc nh cc bi m (ho m) m chng to ra. V th, khi ht cn phi phng m thanh ln pha trc to c v tr di m trc mt. V tr trc mt m ln hi phi hng ti khng ging nhau i vi mi ngi. Mi ngi s tm tng v tr cho tng cao , nhng ni chung chng nm khong gia hm trn v trn (ngay c khi ni, nu ta bit ni m thanh ra pha trc, th hng ta s mt v ting ni ta vn r rng m khng tn sc)
Hnh 2.2B- B lc ngun m1. Dng b lc ngun d dng phn tch tn hiu thoi , hu ht b m ha ting ni u c dng vocal tract. Dng ny thng c dung hu ht qu trnh m ha v gii m .Khi m ha cc kiu thng s c xc nh miu t chnh xc tn hiu thoi ng vo . i vi gii m cng c cu to tng t v da vo ti to chnh xc li m thanh. Mt dng to thoi thng c s dng nht l dng b lc ngun. B lc ngun ny c dng ging nh vocal tract. Ngun tn hiu cung cp cho b lc ngun ny l tn hiu kch thch.
a. Vocal tract C hng, mi, li v ming l hc cng hng khng kh to nn ting ni cua con ngi. Vocal tract c cu trc khc nhau th s c cc tn s cng hng khc nhau. Tn s cng hng cng vi tn hiu kch thch l hai h s chnh iu khin vocal tract to ra cc m v.
b. Kch thchi vi ting ni m hu thanh, dng sng tun hon to kch thch n vocal tract. Dng sng tun hon t cc xung thanh mn s lm cho dy thanh s rung. Dng n gin v hay dng cho m v thanh l nhiu trng. Nhiu trng thng ngu nhin v c ph bng phng mi tn s c cng cng sut. Gi s nhiu trng c to ra khi khng kh i qua b phn co tht. Mt s m nh m /z/ c to ra va bi mt kch thch tun hon v vocal tract co tht khng kh. iu ny c gi l kch thch pha trn. V vy, nhim v chnh cua m ho thoi l phi phn bit u l m hu thanh, m v thanh hay l pha trn cua n.c. Dng b lc ngun tng qut Hnh 2.3 Dng b lc ngun tng qutThng c 2 dng, b lc ngun s kt hp quyt nh cng m hu thanh/m v thanh i vi mi on thoi. Trong trng hp ny, chc nang cua khi quyt nh trn nh mt chuyn mch vi kch thch l m hu thanh/m v thanh. Thng tin vocal tract c cung cp vo khi vocal tract to ra mt b lc vocal tract. B lc s lm cho ph cua kch thch ging nh cua tn hiu thoi gc. Thc t, thng tin vocal tract c to ra bng mt s phng php bao gm mt d on tuyn tnh v gi tr Fourier. Kch thch c lc bi vocal tract to ra thoi tng hp n tai ngi nghe sao cho ging tn hiu thoi ban u nht.
III. Cc Phng Php C S M Ha Ting Ni 3.1 Cc c trng ca ting ni Ting ni c mt s tnh cht c th c th a vo thit k b m ha l:Di tn gii hn ( hm c gi tr ln nht khi bin = 0 v bng khng vi bin v cng ln. Dng phn b ca hm mt xc sutc s dng lm c s cho k thut lng t ha phi tuyn.
3.1.2) Hm T Tng Quan (AFC)Mt tnh cht ca ting ni l tn ti s tng quan khc 0 gia cc mu khi tinhnh ly mu bin ting ni. C ngha mt mu trong ting ni chng ta c th d ontrc t mt s mu trc vi mt li ngu nhin nh, tnh cht ny c c lngbi hm:
Trong x[k] biu din mu ting ni th k, c ly chun vi ting ni trong khong [-1,1] . Thng thng C(0) =1 v C(1) xp x 0.85-0.9
3.1.3 Hm mt ph cng sut Hm mt ph cng sut trung bnh theo thi gian cho thy cc thnh phn tn scao ch chim mt phn nng lng nh, cc thnh phn tn s thp chim nng lng caohn. c tnh khng bng phng ca hm mt ph cng sut ca ting ni to nn khnng nn tn hiu trong vng tn s, bng cch m ha vi cc h s khc nhau trong cckhong tn s khc nhau. Bn cht khng bng phng ny l biu hin tnh cht t tngquan trong vng tn s. S khng bng phng ca ph c thnh gi qua hm SFM (nh l ts ca trungbnh s hc/ trung bnh hnhhc cacc mu khily mupsd). Skl mu tn s th k ca psd. Thng thng tn hiu ting ni trong thi gian di c SFM = 8 v dao ng t 2 n 500 trong khong thi gian ngn.
3.2 K thut lng t Lng t l php din t mt di lin tc cc gi tr ca bin tn hiu bng mt tphu hn cc gi tr bin ri rc. Lng t tn hiu s lm mo tn hiu trong phm vi mngi thit k c th nh trc. B lng t n bit c th c L = 2^n mc bin ri rc.3.2.1 ) Lng t ha tuyn tnh Trong lng t tuyn tnh cc gi tr lng t phn b u trong ton b tn hiu,khong cch gia hai gi tr gn nhau nht c gi l bc lng t. mo tn hiu lng t t l vi bnh phng bc lng t, bc lng t t l nghch vi s gi tr ri rc. Mo lng t c tnh l li trung bnh bnh phng nh sau:
Trong :x : l bin tn hiu c ly mu ti thi im t.f_Q(x) : biu din gi tr lng t gn x(t) nht.Mo lng t (n lng t) v cht lng ca b lng t c nh gi qua t s cng sut tn hiu / n lng t (S/N). Nu php lng t l tuyn tnh th ta c cng thc lin h nh sau:(S/N)dB = 6.02n + .Trong : = 4.77 20lg(V/x_max) y V l gi tr nh ca b lng t ha v x_max l gi tr hiu dng ca tn hiu vo. T phng trnh trn ta thy l c thm mt bit lng t th SQNR c tng thm 6dB.
3.2.2) Lng t ha khng u Mo lng t c th c gim thiu nu phn b cc gi tr lng t mt cch thchhp (khng phi cch u nh lng t tuyn tnh). Phn b ny da trn hm pdf nh sau:
-Lng t ha theo lut A- Lng t ho theo lut
3.3) Cc phng php m ho ting niV c bn b m ha ting ni c 2 loi:M ha dng sng (waveform).M ha ngun ( source).Nguyn l ca m ha dng sng l tm cch s ha dng sng ca ting ni theo cch thch hp.Ti pha pht,b m ha s nhn cc tn hiu ni tng t lin tc v chuyn thnh tn hiu s trc khi truyn i.Ti pha thu s lm nhim v ngc li khi phc tn hiu ting ni.Khi khng c li truyn dn th dng sng ca ting ni khi phc rt ging vi dng sng ca ting ni gc.u im ca loi m ha ny l: phc tp,gi thnh thit k, tr v cngsut tiu th thp.B m ha dng sng n gin nht l iu ch xung m (PCM),iu ch Delta(DM) Tuy nhin , nhc im ca b m dng sng l khng to c ting ni cht lng cao, tc di 16kbit/s.B m ha ngun khc ph c nhc im ny. Nguyn l ca m ha l m ha kiu pht m(vocoder),v d nh b m ha bng d on tuyn tnh (Linear PredictionCoding-PLC) . Cc b m ha ny c th thc hin c ti tc bit > 1kbps. Hn ch ch yu ca m ha kiu pht m PLC l vic m phng ngun kch thch cn n gin nn ting ni ti to c l ting ni dng tng hp , cht lng khng cao v kh c th nhn ra ging ngi ni chuyn. Vo nm 1982,Atal xut mt m hnh mi v kch thch,c gi l kch thch axung.Trong m hnh ny,khng cn bit trc xem l m hu thanh hay v thanh.S kchthch c m hnh ha bi 1 s xung c bin v v tr c xc inh bng vic cc tiu hasai lch,c tnh n trng s th cm,gia ting ni gc v ting ni tng hp.Vic a ra mhnh ny gy ch v l m hnh u tin ca mt th h mi ca cc b iu ch tingni phn tch bng tng hp(Analisis-by-synthesis).Chng c kh nng cho ting ni cht lngcao ti tc quang 10kbps v c th ti tn 4,8kbps.Tn hiu kch thch s c ti u ha 1cch k lng v ngi ta s dng k thut m ha dng sng m ha tn hiu kch thch nymt cch c hiu qu.
Hnh 3.1 .M hnh tng qut ca iu ch ting ni theo phng php LPC
Ch tiu nh gi thut ton m ho: 3.4) Cc phng php m ha theo dng sng3.4.1 PCM (Pulse sssCode Modulation) iu ch theo m: l phng php thng dng nht chuyn i cc tn hiu analog sang dng digital ( v ngc li) nn c th vn chuyn qua mt h thng truyn dn s hay cc qu trnh x l s. S bin i ny bao gm 3 tin trnh chnh: ly mu, lng t ho v m ho. Phng php ny c ng dng trong m ha ting ni mt cch c hiu qu, dng n gin l iu ch xung m PCM , ngoi ra cn c cc thut ton khc gim tc bit hn na. trnh hin tng chng ph , ting ni tng t s c lc trc khi a vo b m ha . Ta cho tn hiu qua b lc thng di trong khong tn s t 300 Hz n 3.4 kHz. Sau , tn hiu ny c bin i sang tn hiu s bng b bin i A/D dng k thut iu xung m PCM vi tn s ly mu l 8kHz. Sau tin hnh lng t ha theo hai cch*) Lng t ha u: u vo b lng t l tn hiu c a qua b ly mu. Vi mt b lng t dung N bit t m, min gi tr s chia lm mc , mi t m N bit tng ng 1 t m. Khong cch gia cc mc ln t l nh nhau . T d thy rng nu ta ly N qu b th s mc lng t s ln dn n sai s s ln, cn nu N ln th bc lng t s b dn ti nhiu nh hng n tn hiu d gy sai lnh. V vy cn phi chon N ph hp, thng cn N c 11bit tr ln m bo c cht lng ca ting ni. iu ny lm tc bit ca n ln nn thc t t c dung.
*) Lng t ha khng u : Mc tiu ca phng php ny l duy tr t s SNR c c nh, t thay i trong phm vi gi tr bin . Thay v lng t ha gi tr ly mu, ta s tnh ton gi tr logarit ca tng gi tr ri mi tin hnh ln t ha , mc ch ca vic ny l nhm to ra cc bc lng t khc nhau ph thuc vo bin , gi tr bin cng ln th bc lng t cng ln v ngc li, t lm cho t s SNR t b thay i trong vng bin tn hiu nh cng nh ln. Hai tiu chun c dung ph bin cho phng php lng t ha ny l lut A v lut . 3.4.2 DM (Delta Modulation) L mt trong nhng phng php iu ch vi sai ,da trn tnh cht l tn hiu ting ni tai thi im c t nhiu phu thuc vo tn hiu cc thi im trc ,v th ta c th d on gi tr ti thi im hin ti, v ch cn lu tr ci gi tr sai khc gia gi tr hin ti v gi tr d on, t s tit kim c s bit biu din => tit kim bng thng. tng ca phng php iu ch delta l ch truyn i gi tr thay i tuyt i ca tn hiu. Da vo s khc nhau ca tn hiu ti thi im k nhau m t ta tnh c tn hiu truyn i. Phng php ny ch s dng 1 bit biu din s sai khc , tc l cho bit tn hiu ti thi im t +1 l ln hn hay b hn ti thi im t.3.4.3 DPCM (Differential PCM) y l phng php cng da trn nguyn tc ch truyn i s khc nhau gia hai thi im t v t+1 . Khc vi DM ch dung 1bit gii m , DPCM dung N bit c th biu th s mc sai khc ny . Cht lng iu ch kh l tt vi lng bit t hn nhiu so vi PCM. 3.4.4 ADPCM (Adaptive Differential PCM)-G.726 B iu ch PCM khng loi b s d tha thng tin trong tn hiu ting ni (do tnhtng quan khc 0). B ADPCM khai thc iu ny gim tc bit. Do c tnh ttng quan cao m cc mu bin cnh nhau t khi thay i ln nn di ng trong tnhiu vi phn nh hn nhiu di ng ca bn thn tn hiu. M ha tn hiu vi phn s chotc bit ch l 32Kbps (=1/2 chun PCM) vi cng cht lng tn hiu (khi gii m dngb tchphn). Trn thc t ADPCM c kt hp vi thut ton tin on tn hiu, thay cho vicm ha s sai khc gia hai mu cnh nhau l vic m ha s sai khc gia mu tht vmu d on n (da trn mt nhm cc mu tht trc , sai khc ny cn gi l li don). Hnh 3.2 B m ha ADPCM
3.5) Phng php m ha theo ngun m VocoderC ch chung ca mt ngun m bao gm c ngun kch thch v b lc thng minhv loa.Ting ni ca chng ta c phn thnh 2 loi:Voiced l kt qu dao ng gn tun hon ca c quan pht m.Unvoice c to nn bi s thi kh qua khe hp ca mi v li.Cc thng s ca m hnh ny l cc tn s chnh (tn s pitch
