UNG DUNG DAN LOC THICH NGHI
TRONG VIEC KHU TIENG VANG AM THANH
Iran Dire Ian
Tnixxng Dgi hge Cdng nghe, DHQG Hd Ngi Tdni tdt:
Trong cdc phdng hgi nghi, bg phdn nhdn dm thanh thudng Id ngn nhdn cua mdi tnrdng tdn sdc truyen dm. Hien tugng tdn .sdc luiy tgo ra red nhieu tieng vang trcmg thi hieu nhciii. Mudn ndng cao chdt lugng tin hieu nhdn, chiing ta bdt biigc phdi xdy dung cdc phuong phdp nhdm logi bd cdc tieng vang. Bdi bdo ndy trinh bdy mgt phuong phdp hien dang dugc dua veto thirc tien, do Id sir dgng ddn Igc thich nghi. Phuong phdp ndy dd dugc thir nghiem tren cdc mdu dm thanh cho ket qud rdt tot.
Abstract:
In the conference rooms, audio devices usually are victims of dispersion environment.
Dispersion phenomenon creates a lot of echoes in the received signal. In order to improve the quality of received signal, we have to apply the suitable method to remove the echo. One of these methods that were mentioned in this paper is using an adaptive filtering band. This method was tested on audio samples and obtained good results.
I. DAT VAN DE
Khii tieng vang am thanh la mdt vdn de cd tdm thye tien quan trgng, chdng ban khir tidng vang trong cdc phdng hdi nghi hodc cho nhirng ngudi sir dung dien thoai di ddng trong cdc xe hai ma khdng cdn phdi dimg den tay.
Xu ly thfch nghi dugc iing dyng rdt phd bidn trong xir ly tidng ndi, dac biet Id dugc su dung de Igc nhidu am bgc vd khir tidng vang (AEC). Trong thye tien, vide xiir ly thfch nghi tren todn ddi se lam cho he thdng phdi chiu 1 lugng tfnh toan rdt Idn va gay khd khdn khi thye thi vdo cdc phdn ciimg. Dd ciing chfnh la nguyen nhdn rdt khdch quan de ddn tdi vide xii dyng thfch nghi bdng con trong bai todn xir If ty thfch nghi [1].
II. NHIEU TIENG VONG
Thdng thudng, luc mdt tfn hidu chira dyng thdng tin dugc phdt di, d ddu nhdn ngodi phdn tap dm do bd nhan phdt sinh, ta cdn nhdn dugc rdt nhieu tidng vang tir tin hieu ndy tao ra do tfnh tan sdc cua mdi trudng truyen dm.
He thdng sd xir If am thanh thdng thudng phdi Idy mdu khodng 8000 mdu mdi gidy. Do do, luc do tre do hien tugng tdn sdc tao ra kha Idn thi md hinh kenh truyen ciia mdi trudng truyen dm thanh la mdt td hgp tuyen tfnh ciia rdt nhieu he sd tuang iing vdi rdt nhidu do tre khdc nhau, tren tfn hieu dugc phdt di.
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Hinh 1. Hf thong truyen am thanh trong phong hgi nghj
Tfn hieu ciia ngudi phat ngdn vao micro dugc phat ra tii' loa (xem hinh 1). Am thanh phat di do mdi tmdng ciia phong hdi nghj se dugc vang dgi theo nhidu chidu hudng khac nhau,
uw ciuy "lam va ao ire. ngoai la, ucm man i-uu 1111^1-.^ nwv.. ^..«i .wv..© —•-••cs —- — ' 5 o'^y mdt loa? lap am dugc ggi la tap am trdng hodc t?ip am nhiet. Nhu the, neu tfn hieu phdt di la x(t) thi d ddu ra ciia micro, tfn hidu r(t) se c6 dang:
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Hinh 2 md td mdt phan tfch nhieu am hge dan gidn. 0 do, mgt tfn hieu gdc Id tfn hieu hinh sin cd tdn sd 20 Hz dugc Idy mdu d tan so Fs=100 Hz. Sau dd, tfn hi?u nhieu tieng vgngco bien do bdng mot niia dugc cpng vao tfn hi?u goc sau 0.3 giay. Hinh lien ndt cho ta phdp phan tfch plid cep (cepstriim) cua tfn hieu bj vgng va hinh dirt ndt id phd cep cua tfn hieu gdc. Liru y rdng phd cep cua tfn hi?u bi vgng xuat hien mgt dinh tai t = 0.3 gidy.
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Hlnh 2. Phan tich pho cep ciia tin hieu bi vong
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Hlnh 3. Phan tfch phd tin hif u goc va tin hif u bj nhieu am hoc dung FFT
Rd rdng la vdi cdch phdn tfch phd truyen thdng dimg FFT khdng the cho ta nhan bietve nhieu tieng vgng trong vf dy d trdn (xem hinh 3).
De loai bd tieng vang vd nhidu, ngudi ta dimg mdt bg Igc vdi dau vao Id tin hieu r(t). Bo Igc phdi dugc chgn the ndo de ddu ra cho ta tfn hieu xdp xi tdt nhdt ciia tfn hieu s(t) (vdi mot do suy gidm vd mdt do trd ndo do). Bdi vi cdc dac trung ciia mdi trudng ngau nhien kho bi&
trudc dugc cho nen khdng the nao xay dyng dugc tir trudc bd Igc nay. Tuy nhien, cac he so cua bd Igc nay cd the dugc dieu chinh mgt each ty ddng phy thudc vdo chdt lugng va tinh hudng cung nhu mdi trudng boat ddng cua he thdng. Phuang phdp dieu chinh ty dgng cdc he sd nay dugc ggi la phuang phdp ty thfch nghi.
III. XU* LY TV THICH NGHI THEO BANG CON
Vai trd cua he thdng khir tidng vang Id tir r(t) phdi tdi tao lai cho dugc s(t). Ve mat thye t6, luc tdi tao s(t), ta chdp nhdn mdt do trd vd mgt do suy giam nao dd. Ngoai ra, ciing phdi chip nhdn mdt tfn hieu giao thoa ndo dd do khdng loai dugc todn bg nhieu va tidng vang, miin la tfn hieu giao thoa ndy nhd ban nhidu tfn hieu ta mudn tdi tao. Vdn dd khd khan vd mat toan hge cung nhu vd m^t thidt bj la sd tidng vang M khd Idn (cd thd len ddn vai n^hin). Nhimg thudt todn cd dien de nghj diing mdt bg Igc nhieu he sd vd cac he so nay duac dieu chinh theo mdt sd phuang phdp mang ten ty thfch nghi [1]. Cdu true nay thudng duac goi la cdu true xu ly todn bang. Cdc phuong phdp ndy cd thd sir dyng dd gidi quydt vdn dd khir tidng vang va loc nhieu am bgc. Tuy nhien khdi lugng tfnh todn tuang iing la rdt Idn cho nen khdng thd dp dung vdo thdi gian thye dugc. De gidi quyet vdn dk nay, mudn rut gon thdi gian xu If dd thue hien tren thai gian thye thi ta phdi tim cdch song song hod cdu true (xem hinh 4); cd nghia'la thay
the phuang phap xir If hang nghin he sd lan lugt, thi ta xir If mdt loat nhieu he so cimg mdt luc. Day la khai niem ca ban ciia phuong phap xir If song song. Ve mat thye lien, de thye hidn phuong phap nay ta phai dung ddn khai niem xii' If theo bdng con, tii'c la diing ra phai xu' If Idn lugt chdng han mdt bd Igc gdm 1000 he sd, Ihi ta xir ly ciing mdi liic 20 bd Igc ma mdi bd loc chi chua 50 he sd.
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Hinh 4. Minh hoa phuong phap xii- If theo bang con
Nhu da ndi d tren, mdi tmdng truyen am thanh trong cac phdng hdi nghj hay trong khdng gian kfn nhu dtd thi kenh truyen thudng dugc md hinh hod bdi mdt he thdng chiia hang nghin he sd. De cd the xay dyng mdt he thdng theo thdi gian thye ta phdi diing each xir If song song nhu da dugc de cap d tren.
Chung ta se nhin cy the ban vao khfa canh nay nhu sau: gid sii xet ddn Igc bdng con M ddi vdi cdc bd Igc phiic thi chi cdn xii If tren M/2+1 ddi (coi M chdn). Vdi sd lugng bdng con Idn thi lam gdn dung Id cdn xir ly tren M/2 bdng con. Sd trgng sd trdn mdi bdng con luc nay la N/L. Vdi N la sd trgng sd cua bg Igc thfch nghi todn ddi vd 1/L Id tdc do xii ly gidm tren mdi bang. Nhu vay sd trgng sd phiirc cdn thidt la MN/2L. Nhu vdy ta de dang cd lien he sau:
do phuc tap Iren bang con 2M
do pinic tap Iren loan dai
Cd the de dang thdy do phiic tap ciia bdng con cd the dugc gidm theo mdt he sd Id sd bdng con (neu d day ta chgn M=L) [2, 3].
Tat nhien Id vdi xii ly bang con thi chung ta phdi tfnh them nhirng tfnh todn can thiet trdn cdc dan Igc phan tfch vd tdng hgp [4]. Cdng thirc dugc thiet lap cho dan Igc kieu DFT Id:
A^„+2A^+2Mlog2M A^,+2Ay+2Mlog,M
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Vdi Na vd Ns la sd trgng sd cua bd Igc tren ddn Igc phan tfch vd tdng hgp. Neu dua vdo nhiing tfnh todn cu thd dk so sdnh vd do phiic taj) thi phuang phdp xii ly ty thfch nghi bdng con se tidt kiem dugc tdi 60% md van ddm bdo chdt lugng [5].
Cd rdt nhieu phuang phdp xu ly ty thfch nghi bang con, n]iu mdt cdch rat ty nhien va rat true quan, he thdng ma bdi bdo quan tam dugc thye hien theo md hinh trong hinh 5. Mgt diem dd nhan thdy khi quan sdt hinh 5 Id nhugc didm cua xii ly ty thfch nghi bang con: thii nhdt Id se gay nen trd. (do ddn Igc) vd chdng phd (do gidm mau). d do ldi chdng phd la khd nghiem trgng va thye td cd rdt nhidu tdc gid dua ra cdc cdch khdc phyc khdc nhau nhu Idy mdu tdc do rdt cao hay su dyng cdc each thidt kd ddn Igc tdt ban.
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Hinh 5. ling dung cua AEC bang con
Trong bai bao ndy, cdc kdt qud thu dugc tuang irng vdi cdc he thdng xu If ty thfch nghi bang con khdc nhau Id M = 2, 4, 8. RLS (Recursive Least Square) vd LMS (Least Mean Square) la cdc phuang phdp giiip ta didu chinh cdc he so ciia cdc bang con the ndo de sai sd tai tao nhd nhdt. Thdng thudng phuang phap LMS dugc ua chugng ban vi dg phirc tap thdp hon.
Cung nhin tren hinh 5 minh hoa vd ddn Igc M kenh, luu y rdng vdi mdi bg Igc thfch nghi thi Cj (i =1,2,..M) la ldi udc lugng tren moi bg Igc thfch nghi bang con se dugc dung dl dieu khidn tirng bg Igc thfch nghi ndy. Tren thye te, c6 nhidu cdch dd didu chinh van tdc hgi tu ciia cdc bd Igc thfch nghi, trong do co cdch Id dieu khien bdng tfn hieij tdng hgp cua cdc Cj ndy.
Trong chuang trinh thir nghiem ndy, ddn Igc phdn tfch vd tdng hgp 4 bang con dugc su dyng (xem hinh 6). Kdt qud thye thi dugc trinh bdy tren hinh 6 vdi 1 tfn hieu vd 2 tieng vang da dugc ghi dm tir micro. Tat nhien rdng cdu true ndy cd the md rdng cho bat ky cac thdng so ndo ma ngudi sir dyng mong mudn.
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Hinh 6. Dap ung bien dq ciia mgt dan Ipc bang con 4 kenh phan tfch
Kdt qud trinh bay tren hinh 7 tuang irng vdi cdc tfn hieu gdc, tin hieu bj nhilu tidng vang vd tfn hieu sau khi khir tieng vang. Rat dd nhdn thdy rdng tfn hi?u bi nhidu tidng vang bj sai khdc rat nhieu so vdi tfn hieu gdc vd co thd cam nhdn rd rdng khi nghe tryc tidp. Nhd sii dung ty thfch nghi theo bdng con md tfn hieu khdi phuc rat tot tuy rdng chiing ta phai chip nhan mgt khodng trd nhdt djnh.
tin hien am thanh ydc
thanh phin 8 tiing vang ,
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Hinh 7. Ket qua khu- tieng vang dung cau true 4 bang con ty thfch nghi voi 2 tieng vang Hinh 8 md td sy hdi tu ciia he so bd Igc thfch nghi tren bdng con co ddi thdng thdp nhdt. 6 day budc nhay thfch nghi dugc lya chgn la 0.002 vd chidu dai bd loc Id 40.
Hinh 8. Sy hoi tu ciia bp loc thfch nghi tren bang con co dai thong thap nhat
Vdi myc dfch so sdnh chdt lugng cua tin hieu tren todn ddi va tren bang con, chung ta tien hdnh so sanh ldi dugc tfnh bang trj tuyet ddi ciia hieu giira tfn hieu khdi phyc vd tfn hieu gdc.
Ket qua dugc the hien trdn hinh 9. Nhdn xet rang tfn hieu ldi do phuang phdp bang con liic dau suy gidm nhanh ban nhung sau khi dn djnh thi phuang phdp todn ddi thdng lai cho Idi nhd horn. Khi cd hai he thdng hdi tu, dn djnh thi rd rdng cdc hieu ling gay bdi cdc ddn Igc phan tich va tdng la khdng the tranh khdi.
Quay trd lai vdi he thdng Igc thfch nghi 4 bang con. Chiing ta cdn xet tdi bai todn tdi uu chgn L va mu nhu thd ndo dk loi la tdi thidu (d day bdi todn gid su dang nham tdi tdi uu thdng sd nay).Ket hgp vdi thuat toan ndi suy cue bd se cho phep xdc Sinh chfnh xdc dugc gid trj tdi uu cua L va mu ( d day L=32 vd mM=0.055) (xem hinh 10). Tuang ty nhu vdy neu chiing ta can tdi uu mdt thdng sd ndo khdc thi chiing ta lai tien hdnh xay dyng thuat todn phii hgp vd tdi uu thdng sd mong mudn.
Vd mat ly thuydt, bdi bdo ndy kdt hgp phuang phdp luan ciia phuang phdp xii ly ty thfch nghi va cdu tnic bdng con. Nhirng ket qua thu lugm dugc cho thdy cau true xir ly ty thfch nghi todn bang va cdu triic xii ly ty thfch nghi theo bang con cho cdc ket qud cd chdt lugng tuang duomg. Tuy nhien, cau triic bang con cho phep rut ngan thdi gian xu ly cho ndn tuang ddi thfch hgp vdi cac tinh hudng thye te. Ket qud thu dugc cho thay xii If ty thfch nghi theo bdng con thfch hgp ban rdt nhidu cho nhung tinh hudng md tidng vang la ngudn nhieu chfnh ma bd xir ly phai tryc dien.
Hinh 9. So sanh chat luQiig ciia tfn hi9u tren toan dai va tren bang con.
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Hinh 10. Sy phu thupc ciia loi vao budc nhay mu, moi dudiig irng vdi 1 gia trj L khac nhau
IV. KET LUAN
Bdi bdo nay trinh bay mgt cdu true khir tieng vang am thanh. Nhung ket qud thye n^hifm cho thdy chdt lugng ciia cdu true ndy khd tdt; vd vdi thdi gian tfnh todn riit ggn ta thay ciu triic ndy cd the dugc thiet kd de dua vdo nhirng dp dyng trong dd cdc tfn hieu phai dugc xur ly theo thdi gian thye. Hai dp dyng quan trgng nhu da ggi y Id khu tieng vang am thanh trong cac phdng hgi nghj Idn, vd nghe dien thoai di dgng trong dtd ma khdng cdn phai dung tay cam didn thoai de dua dng nghe den tan tai. Day Id hai iing dung thye tien quan trgng ma cdu triic nay se ddng vai trd thiet ydu.
TAI LIEU THAM KHAO
[1] FARHANG-BOROUJENY, B, Adaptive filters: theory and applications', John Wiley and Sons Ltd., Chichester, 1998.
[2] WEISS, S., LAMPE, L., and STEWART, R.W.: 'Efficient implementation of complex and real valued filter banks for comparative subband processing with an application to adaptive filtering' Proceedings of Intemational Symposium on Communication systems und digital signal processing, Sheffield UK, April 1998, pp. 32-35.
[3] RUPP, M.: A Family of Adaptive Filter Algorithms with Decor-relating Properties, IEEE.
Trans. Signal Processing 46 No. 3 (1998), 771-775
[4] HARTENECK, M., WEISS, S., and STEWART, R.W.: 'Design of near perfect reconstruction oversampled filter banks for subhand adaptive filters', IEEE Tmns Circuits Syst. II, Analog Digit. Signal Process., 1999,46, (E), pp. 1081-1085
[5] COURVILLE, M.jDUHAMEL, P.: Adaptive Filtering in Sub-bands Using a Weighted Critenon, IEEE. Trans. Signal Process-ing 46 No. 9 (1998), 2359-2371.
