Trng i Hc Bch Khoa H NiVIN CNG NGH THNG TIN VA TRUYN THNG =======&&&=======

Bo co n ti: Tng cng cht lng tn hiu ting ni

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: PGS.TS Trnh Vn Loan20071458 : KTMT K52

SV thc hin : Nguyn Hng LP

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Ngi Nhn Ngi Gi

: PGS.TS Trnh Vn Loan:

Nguyn Hng

ti:

Tng Cng Cht Lng Tn Hiu Ting Ni

I. Tng cng cht lng tn hiu ting ni l g: Tng cng cht lng tn hiu ting ni lin quan n vic ci thin cm nhn ca ngi nghe vi ting ni b suy gim cht lng do s nh hng ca nhiu c trong ting ni. Trong hu ht cc ng dng th vic tng cng cht tn hiu ting ni chnh l ci thin v cht lng v tnh d nghe ca ting ni b suy gim do nhiu gy ra. Vic x l m tt th s gip cho ngi nghe d nghe hn. K c trong mi trng c mc nhiu cao v lin tc trong thi gian di. Trong thc t c rt nhiu ngun nhiu nh l nhiu trn tu ha,trn my bay, trong phng , trong bnh vinTy vo mi mi trng s c yu cu tng cng cht lng v x l khc nhau. V d : vi thng tin thoi th chu nh hng ca nhiu nn t t, nh hng...khi truyn ti ich. V th thut ton tng cng cht lng tn hiu c th c x l ngay ti im thu, trong cc khi tin x l. Tuy nhin nu xt v phng din thc t th cc thut ton Speed Enhancement ch c th ci thin c cht lng ca ting ni. N c th gim c nhiu nn trong ting ni. Tuy nhin n s lm tng mo ca ting ni. Do yu cu chnh ca vic thit k mt thut ton l vic m bo nn c nhiu v khng c gy ra mo trong s cm nhn tn hiu ting ni. Gii php a ra cn ph thuc vo ng dng chng ta s dng. Cc vn nh l ngun nhiu, giao thoa nhiu, mi lin h gia nhiu v ting ni sch.

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II. L thuyt v nhiu: II.1.Ngun nhiu : Ngun nhiu tn ti mi ni, trn ph, vn phng, nh hng, cc bn xe, khu vui chi gii tr, cc cng trng xy dngN tn ti di nhiu hnh dng v hnh thc khc nhau. Nhiu c th hnh thnh mt ni c nh v khng thay i theo thi gian. V d ting n pht ra t qut my tnh. Nhiu c th nhiu ch khc nhau, nh trong cc qun n, nh hng. Cc c tnh v ph trong nh hng thay i khng theo quy lut nn vic nn nhiu trong cc mi trng c nhiu thay i nh vy s gp nhiu kh khn. Cc c tnh c bit khc nhau ca cc loi nhiu l hnh dng ca ph v s phn b ca nng lng nhiu trong min tn s. V d nhiu gy ra bi gi th nng lng n tp trung tn s thp hn 500 Hz. Nhng i vi nhiu trong nh hng,trn xe lath n li phn b trn mt di tn s rng.

Hnh1 : Dng v s phn b nng lng trung bnh nhiu trn xe

Hnh 2.Dng v s phn b nng lng trung bnh trn tu.

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Hnh 3.Dng v s phn b nng lng trung bnh trong nh hng. 2.2.Nhiu v mc tn hiu ting ni trong cc mi trng khc nhau: im ti hn trong vic thit k cc thut ton ca Speed Enhancement l s nhn bit s bin thin ca ting ni v mc cng nhiu trong mi trng thc t. T chng ta c th m t min bin thin ca mc t s tn hiu trn nhiu (SNR) c bt gp trong mi trng thc t. Mc ca ting ni v nhiu c o bng mc m thanh. Php o y l o mc p sut ca m thanh tnh bng dB SPL(Sound Pressure Level). Khong cch cng nh hng ti cng m thanh. Khong cch c trng trong giao tip mt gip mt l 1m. Khi khong cch tng gp i th mc cng m gim i 6 dB.

Hnh 4:Mc nhiu v ting ni(c o bng SPL dB) trong cc mi trng khc nhau Hnh trn l s tng hp v mc m trung bnh gia ting ni v nhiu trong cc mi trng khc nhau. Mc nhiu nh nht trong cc mi trng phng hc trong nh 4

trong bnh vin v trong cc ta nh. Trong cc mi trng khc nhau th mc nhiu nm trong khong 50-55 dB SPL. V mc ca ting ni l 60-70 dB SPL. V ta a ra mc t s tn hiu trn nhiu l 5-15 dB. Mc m ca nhiu cao trong cc mi trng tu in, trn my bay n t 70-75 dB SPL. Do mc t s SNR() l bng 0 dB.III. Mt s phng php nh gi cht lng tn hiu ting ni. III.1

. nh gi ch quan

III.1.1 .Phng php nh gi tuyt i ACR: -MOS (Mean Opinion Scores ): l php o cht lng thoi ni ting. Mang tnh cht ch quan. C 2 phng php kim tra l nh gi m thoi v nh gi nghe. Score 5 4 3 2 1 Quality of the Speed Excellent Good Fair Poor Bad Level of Distortion Imperceptible Just perceptible,but not annoying Perceptible and slight annoying Annoying but not Objectinable Very annoying and Objectionable

III.1.2 Cc phng php nh gi tng i III.1.2.1 nh gi bng phng php so snh mu tn hiu l thch nghe mu no hn. Hay thng gi l so snh nh gi theo tng cp tn hiu. Phng php ny th ngi nghe s c nghe 2 mu thoi v s nh gi thch mu no hn. Mt bin th ca phng php ny l Theshold Test. Phng php ny th so snh tn hiu qua x l vi tn hiu gc chun m suy gim ca n c th c kim sot. Rating Quality of speech 3 Much better 2 Better 1 Slightly Better 0 About the Same -1 Slightly Worse -2 Worse -3 Much Worse III.1.2.2 Phng php nh gi theo suy gim cht lng Degradation Category Rating (DCR) l s gim st v cht lng ca tn hiu qua x l so vi tn hiu cht lng cao cha c x l5

Rating 1 2 3 4 5 III.2 nh gi Khch quan

Degradation Very Annoying Annoying Sightly annoying Audible but not annoying Inaudible

L phng php nh gi da trn cc php o thuc tnh ca tn hiu

3.2.1 PESQ L 1 trong cc phng php nh gi khch quan.N c phc tp tnh ton nht. c khuyn ngh bi ITU-T cho vic nh gi cht lng ting ni vi tn s 3.2kHz (narrow-band) my thu pht in thoi cm tay v narrow-band speech codecs.Thut ng PESQ c tnh nh phng trnh tuyn tnh kt hp gia gi tr nhiu lon trung bnh Dind v gi tr nhiu lon bt i xng Aind nh cng thc bn di:

PESQ=a0 + a1Dind + a2Aind

(1)

Khi m a0 = 4.5 , a1 = -0.1 v a2 = -0.0309. Gi tr a0, a1, a2 l optimized cho vic x l ting ni trn mng v khng cho vic x l ting ni bi cc thut ton nn. Mt cch khng mong mun PESQ lien quan ti 3 tiu ch cho cc phng php o lng l: mo tng ni, mo ting n, v ton b cht lng. Chng ta hy ti u ha phng php PESQ bng vic quan tm ti 1 trong 3 tiu ch trn. ng vi mi tiu ch s c 1 tp (a0, a1, a2 ) khc nhau. Nhiu b hi quy tuyn tnh thng quyt nh tham s a0, a1 v a2 . Gi tr Dind , Aind c tnh ton nh gi tr c lp trong phn tch hi quy. Thc t th im ch quan cho 3 tiu ch c s dng trong phn tch hi quy. 3.2.2 o t s tn hiu trn nhiu tng khung: o SNR trn tng khung trong min thi gian l phng php nh gi v mt ton n gin nht. Quan trong l tn hiu gc v qua x l phi trong cng min thi gian, lch pha hin thi phi c hiu chnh chnh xc. Cng thc:6

SNRseg =Trong :x(n) l tn hiu gc( tn hiu thu c c nhiu). l tn hiu c tng cng loi nhiu. N : l chiu di khung. M : l s khung ca tn hiu. Mt cch khc xc nh SNRseg do Richards xut:

(2)

SNRsegR =

)

(3)

Nh vy c th trnh c cc gi tr sai lch ln trong sut cc khong lng ca tn hiu ting ni. Ch rng gi tr nh nht c th t c by gi l 0 tt hn nhiu so vi cc gi tr m v cng. u im chnh ca vic xc nh trc phn on SNR l trnh c vic phi lm r rng gia cc khong ting ni v khong lng. o SNR cho tng khung c th m rng trong min tn s:

fwSNRseg =trong : Bj l trng lng ti di tn th j K: s di tn. M: tng s khung tn hiu.

(4)

F(m,j): Dy tn hiu gc qua b lc c khuch i ti di ln th j v khung th m. : dy tn hiu c tng cng qua lc khuch i cng mt di tn vi F(m,j).

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u im chnh ca vic s dng SNRseg trn min tn s thay v min thi gian tng thm tnh linh ng ca vic phn b trng s ca ph khc nhau cho nhng di tn khc nhau. 3.2.3 o khong cch da trn LPC 3.2.3.1.Phng Php LLR c nh ngha theo cng thc sau.

dLLR (

p

,

c

) = log(

)

(5)

trong c l vector LPC ca khung tn hiu ban u. p l vector LPC ca khung tn hiu c tng cng v Rc l ma trn t tng quan ca tn hiu ting ni ban u. Ch 1 khong nh 95% gi tr ca khung LLR l c s dng tnh gi tr trung bnh LLR, gi tr LLR c gii hn trong [0, 2] gim bt gi tr ngoi l. 3.2.3.2 Phng php IS(Itakura Saito) L phng php o khong cch da trn LPC ca 2 khung tn hiu tng ng.

dIS (

p

, c) =

(

) + log(

) -1

(6)

trong l s gia tng LPC ca tn hiu sch v tn hiu c tng cng tng ng. Gi tr IS c gii hn trong khong [0,100] . l cn thit cc tiu ha gi tr ngoi l.3.2.3.3

Phng php Cepstrum distance provides(CEP)

L 1 s c lng log spectral distance gia 2 ph. N tn ti quy t h s tin on LPC { am } s dng biu thc bn duwois. c(m)= am + c(k) am-k 1 1, frame l c ting ni Else frame l khng c ting ni

(39)

Trong cng thc ny th E r l biu din cho nng lng ca frame nhiu, khi k. E r l ngng c s dng trong s pht hin ting ni. C h s t l k cho php mt di an ton cho vic sa li Er ( sa li gi tr ngng ). S khc bit nng lng c bn VAD chnh l s khac nhau cp nht ngng. Er new = (1-p) .Er old + p.Esilence (40)

Er new l c cp nht ca ngng, Er old .. Esilence l nng lng ca hu ht frame khng phi l ting ni, p l hng s ( 0< p =1.25 th p=0.25.Nu t s >=1.1 v