DO TIM BIEN THIEN DOT NGOT CIJA TIN HIeU BANG PHAN
T I C H ' S O N GCON
Nguyen Linh Lan'"', Nguyen Cdng Phuvng"", Vu Todn TlUiiig'"
(a), (b)BiVI Ky thudt Do & Tin hoc CN - Khe)a Dien, Tmng tdm nghien cim qudc te eta phuvng lien, truyen thdng vd irng dung (MICA), Triivng DII Bekh Khoa lid Nen
(c)Plu)ng Phdt trien He thdng diin, Vien Ndng Lire/ng
Tom tdt:
Tremg .xir ly tin hieu .sel tem tgi mat dgng tin hieu tien dinh phirc tgp trong etd tdn .so thay ddi lien tuc. Tremg tin hieu neiy tdn .so ciia nd theiy etdi theo the/i gian, vd viec sir elung phep biin etdi Fentrier cho tin hieu neiy klu'mg thich hgp vi nd hum kem theo nhirng elao etdng ggn .se'mg vd bien do khe'mg bdng nhau. Bed beio gie'd thieu cdch phdn tich tin hifu khe'mg dirng theo tdn .so. dej Id phuong pheip eld tim bien thien etgt ngdt cua tin hiiu bdng sdng cem (weivelet) bao genu thudt toe'in vd phdn mem eld tim tren Mealeib.
Abstract:
\. GIOI THIEU
Trong xir ly sd tfn hidu ta da bidt den cdc loai tfn hieu tien djnh, gan tien dinh va tfn hieu ngau nhien. Cac ifn hieu tidn djnh va gdn lien dinh bao gdm tfn hieu ca ban va tfn hidu phirc tap.
Ddi vdi tin hidu tidn dinh tudn hoan phirc lap trong thue td cd mdi dang tfn hieu co tdn sd thay ddi lien tuc theo thdi gian ggi la tin hieu tudn hodn phirc tgp khdng dirng theo tdn sd. Viec sir dung phep bidn ddi Fourier cho tfn hieu nay trong mien tdn sd X{f) ludn kem theo nhiing dao ddng ggn sdng va bidn do lai khdng gidng nhau. Dieu dd cd nghTa la sir dung phep bidn doi Fourier cho tfn bidu tudn hoan phirc tap khdng dimg theo tan sd kiwng thich hgp. Bien ddi Fourier (FT) cbi cho ta bidt trong tfn hieu da cho cd tdn tai bao nhieu tdn sd ma khdng cho biet khi ndo tdn sd dd xudt hien.
De khdc phuc nhugc diem trdn, phep bidn ddi Fourier thdi gian ngdn - STFT dugc de xudt.
Chi cd mdt khac bidt nhd giira STFT vd FT Trong bien ddi STFT, tfn hieu dugc chia thanh cac khodna nhd va tron? khodng dd tfn hidu tfn hidu duoc sid dinh la tfn hidu dn dinh. De thirc hien ky thudt nay cdn chgn mdt ham cira sd vv sao cho do dai ciia cua sd dimg bdng cac khodng tfn hidu phan chia. Vdi phep bidn ddi STFT, chiing ta cd the thu dugc dap ung tdn sd - thdi gian ciia tfn hidu ddng thdi ma vdi phep bidn ddi FT la khdng thye hien dugc.
Tuy nhien, sir dung bidn ddi Fourier thdi gian ngdn lai co nhugc diem, neu ham cira sd hep thi cd do phan gidi thdi gian tdt nhung do phdn gidi tdn sd kem, cdn neu ham ciia sd rdng thi do phdn gidi thdi gian kem ma do phan gidi tdn sd tdt.
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II. WAVELET VA BifeN D 6 I WAVELET
Wavelet [1] d day dugc hieu Id tin hi^u dao dgng d$c bi?t cd bien dg suy gidm rat nhanh. D6 la dieu ki?n dl hdm ndy cd chieu ddi him ban. Thuat ngir Wavelet d day chi dieu ki?n ham dao dgng vd tdt din rit nhanh.
Tren co sd tiep can biln d6i STFT, biln d6i wavelet dugc phdt triln dl gidi quyet van de d^
phan gidi tin hi?u (miln thdi gian hoae tin so), md STFT vin cdn ban che. Biln doi Wavelet dugc thye hien nhu sau. Tin hi?u dugc nhan vdi hdm wavelet (tuong tu nhu nhan vdi cdc ham cua sd trong biln doi STFT) roi thye hi?n bien doi rieng re cho cdc khodng tin higu khdc nhau trong mien thdi gian tai cdc tin so khac nhau. Cdch tilp can nhu vay cdn pgi la phan tich da phan gidi - MRA (MuUi Resolution Analysis, phan tich tin hi?u d cdc tdn so khdc nhau vd cho cdc do phdn gidi khdc nhau [2]).
MRA khi phan tich tin hi$u cho phep phan gidi thdi gian tot vd phan piai tan so kem d cdc tan sd cao. Phdn gidi tin sd tdt va phdn gidi thdi gian kdm d cac tin sd thap. Nhu vay ky thuat nay rit thich hgp vdi nhirng tin hieu c6 thanh phin tan sd cao, xuat hien trong thdi gian ngan.
Thoi gian
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I ' l l ' ' ' ' »Thdi gian Phan tich Wavelet Hinh 2. Bien doi Wavelet
Chiing ta co thi dl ddng nhan thiy rang phan tich wavelet khdng diing mgt mien thdi gian-tan
so, ma la miln thai gian-ty le. FFT (Fast Fourier Transform*- Bien ddi Fourier nhanh) la mgt
thuat todn rit phd biln, cho phep tinh dugc cdc thanh phin tin sd va do Idn ciia chiing trong
mgt tin hi$u chu ky. Nhung yi true hoanh cua dd thi FFT la tin sd nen nd khdng cho thdng tin
vl thdi dilm thay doi tin sd. Trong khi dd, DWT (Discrete Wavelet Transform - Biln ddi
song con rdi rac) cd true hoanh la thdi gian, cho phep biln doi nay cd thi cung cip thdng tin
vl thdi dilm thay ddi tin so ([3], frang 32). Do dd phuang phap DWT dugc lya chgn dk do
tim thdi dilm biln thien tin sd cua tin hieu.
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Hlnh 3. Tin hi$u bien thiSn dgt ng$t IIL THUAT TOAN DO TIM :: _,
Tron^ chuang trudc ta da nhac ddn tin hieu bien thien dot nggt Id tin hieu khdng dimg theo tin sd. Chiing ta bdt diu vdi mdt tm hieu S dugc tao ra nhu sau.
Sir dung Matlab tao ra mgt tin hieu do nhu hinh ve 3. Tin hieu biln thien Id tdng cua 3 doan tin hieu dugc xdu chudi vdi nhau vdi tin sd lin lugt Id f =50 Hz,
f= 200 Hz vd f^= 50 Hz Khi cd sy biln thien dot ngdt thi tai thdi diem dd c6 tin sd thay ddi nhu brong hinh 3.
Sir dyng phep phan tich wavelet rdi rac mgt chilu dl xay dyng cac xap xi (Approximation) vd chi tidt (Detail) tir cdc he so.
Cho tin hieu S di qua 2 bg Igc thdng thip va thdng cao cd dac tinh bu nhau va phdn tach thdnh 2 tin hieu, dd la cac chi tiet vd cac xip xi. Phuang phap cy till hom Id su dung ham wavelet. C6 mdt sd hg song con nhu Haar, Sym, Bior vd Daubechies (Db), tuy nhien viec phan tich theo "Db " cho ta kit qud thuan tien nhit trong viec dd tim thdi dilm dot biln vi tai thdi dilm cd biln thien dot ngdt se lam cho gia trj cua chi tilt khac khdng, cdn tai cdc thai diem khdc thi gia tri ciia cdc chi tiet xdp xi bang khdng nen ta chgn sdng con "Db" "Db "
cd 10 bae. Cac bae cua "Db" su dyng d l phan tich cdc tin hieu, cac hg "Db3, 5 ,7"
dlu CO thi phdn tich dugc tin hieu biln thien tin sd tuong ddi chinh xac. Ta chgn
"2)65" dk thye hien viec dd tim tin hieu biln thien tin sd bang sdng con. Phep phan tich Wavelet Daubechies 'Db5" ciia tin hieu S theo tiiuat todn minh hga d hinh 7.
Tin hieu S c6 chilu ddi N. S se tao ra 2 tdp he sd: cdc he sd xip xi CAi vd cdc he sd chi tilt CDi. Cac vector nay dugc nhan nhd nhan chap S vdi bd Igc thdng thip Lo;
_D dk tao ra xip xi va nhan chap Hi_D dk tao ta chi tilt, theo bdi sy phdn tach dyadic (ha mdu).
Nlu /i=length(s) thi cdc tin hieu F va G
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: Ha mauHinh 7. Sa do tinh cac he so xap xi va chi tilt
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Trong dd fioor(x) la mdt phep lay xap xi
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lam trdn xudng. Neu chung ta muon tinh cdc he so xap xi vd chi tilt d mire tiep theo thi ta chi can thay S d so dd tren bdng CAi vd lam hodn todn tuomg ty ta se tinh dugc cdc chi tiet vd xap xi CA2 vd CD2....
Nhiing d ddy bai todn ciia chiing ta ddt ra la viec dd tim tin hidu bien thien dgt ngdt.
Do dd ta chi cin tinh din cdc h? so xdp xi vd chi tiet d miic 1 Id du. Sau khi da tinh dugc cdc he sd xap xi vd chi tiet mire 1 ta tinh ra dugc cdc xap xi vd chi tiet mire 1 Id Al vd Dl. Hinh 8 the hi?n cdc xap xi vd cdc chi tilt.
Trong dd 'a' d ddy Id cdc xap xi vd 'd' Id cdc chi tiet. Cdc chi tilt miic I (DI) chi ra sy dgt bien rat rd rdng. Khi c6 sy biln thien dgt nggt, Iiic dd cd thdnh phin tin so eao xudt hien trong tin hieu lam cdc chi tilt tai thdi dilm do khdc 0.
Ta sii dung thudt todn de xdc djnh chinh xdc thdi diem bien thien dgt ngdt tin so.
Trong chuong trinh, sai sd £ dugc lya chgn de xdc djnh cdc thdi diem biln thien dot ngdt. Sy lya chgn £ Id do:
D Dya vao tdn sd Idy mdu cua tin hieu dya vdo djnh ly Idy mdu.
D Qua phep phdn tich wavelet tao ra chi tiet Dl cd nhirng sai sd do tinh todn nen edc gid trj ciia Di xip xi bdng 0.
N I U nhu tai thdi diem T; (s) ciia tin hieu Dl md cd gid trj Idn ban hodc bdng € thi Ti chmh Id thdi dilm biln thien dgt ngdt.
Cdn nlu khdng cd gid yj ndo ciia Tj thda man dieu kiln thi kit qud se cho ta Id tin hieu cd tan sd ludn dn dinh.
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Hlnh 8. Dang cua cac chi tiet vd xip xi sau ph^p phan tich Wavelet
Hinh 9. Thuat toan do tim tin hieu bien thien dot
IV. Xay dyng phan mem
Su dung phin mim Matlab vd giao dien GUI (Graphical User Interface) dk xay dyng phin mem do tim bien thien dot ngdt cua tin hieu. Trong phin Tool box cua Matlab [5] dd tich hgp mgt sd ham sir dyng biln ddi wavelet vd c6 sdn mdt so ham giao tilp vdi vi dm thanh. Ta thidt lap so do thii nghiem thye tl [6] gdm cac thilt bj nhu hinh 10, frong do mdy phdt sdng cd nhiem vu tap ra tin hieu bien thien tdn sd, tin hieu chudn hda di vao vi dm thanh, vi dm thanh nay ddng vai yd nhu mgt vi thu thap sd lieu (DAQ) dugc cdi sdn trong may PC, phin mim
"Dd tim bien thien dgt nggt ciia tin hieu" dugc cdi dat frong PC se liy tin hieu tir vi am thanh xir ly, dd tim vd hien thj kit qud.
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Hinh 10. Sor do thiir nghiem
Phan mem gdm cac phin hiln thj dang sdng diu vao, hiln thj kit qud phan tich, hiln thj thdi diem bien tiiien, cd thi lya chgn giira tin hieu gid lap hodc thye hien tiiu tin hieu tir mdy phat song de thye hien dd tim tin hieu biln thien tin sd.
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Hinh 11. Chuomg trinh do tim bien thien tan so dot ngot
HiSn nay chuang trinh hoat ddng dya tren tin hieu thu dugc tir vi dm thanh. Vi thd tin sd liy