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By Hoang Le Uyen Thuc , Electronics And Telecommunication Department , Nng University Of Technologies , Vit Nam

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GII THIU X L TN HIU SChng ny nu tng qut cc vn lin quan n mn hc. Ni dung chnh chng ny l: Gii thch cc khi nim nh: Tn hiu, Tn hiu s, X l tn hiu, X l tn hiu s... Cc khu c bn trong h thng x l tn hiu s Nu mt s ng dng ca x l tn hiu s So snh x l tng t v x l s Gii thch khi nim Tn s Cc bc c bn chuyn i tn hiu t tng t sang s Cc bc c bn chuyn i tn hiu t s sang tng t

1.1 TN HIU, H THNG v X L TN HIU hiu X l tn hiu l g, ta s tm hiu ngha ca tng t. Tn hiu(signal) dng ch mt i lng vt l mang tin tc. V mt ton hc, ta c th m t tn hiu nh l mt hm theo bin thi gian, khng gian hay cc bin c lp khc. Chng hn nh, hm: x(t ) = 20t 2 m t tn hiu bin thin theo bin thi gian t. Hay mt v d khc, hm:

s ( x, y ) = 3 x + 5 xy + y 2 m t tn hiu l hm theo hai bin c lp x v y, trong x v ybiu din cho hai ta khng gian trong mt phng. Hai tn hiu trong v d trn thuc v lp tn hiu c th c biu din chnh xc bng hm theo bin c lp. Tuy nhin, trong thc t, cc mi quan h gia cc i lng vt l v cc bin c lp thng rt phc tp nn khng th biu din tn hiu nh trong hai v d va nu trn.

Hnh 1.1 V d tn hiu ting ni Ly v d tn hiu ting ni- l s bin thin ca p sut khng kh theo thi gian. Chng hn khi ta pht m t away, dng sng ca t c biu din trn hnh 1.1. Mt v d khc l tn hiu in tm (ECG)- cung cp cho bc s nhng tin tc v tnh trng tim ca bnh nhn, hay l tn hiu in no (EEG) cung cp tin tc v hot ng ca no. Cc tn hiu ting ni, ECG, EEG l cc v d v tn hiu mang tin c th biu din l hm theo bin thi gian. Thc t c nhng tn hiu l hm theo nhiu bin c lp. V d nh tn -1-

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hiu nh (image)- l s thay i ca cng nh sng theo khng gian, c th xem l hm sng theo hai bin khng gian. Tt c cc tn hiu u do mt ngun no to ra, theo mt cch thc no . V d tn hiu ting ni c to ra bng cch p khng kh i qua dy thanh m. Mt bc nh c c bng cch phi sng mt tm phim chp mt cnh/ i tng no . Qu trnh to ra tn hiu nh vy thng lin quan n mt h thng, h thng ny p ng li mt kch thch no . Trong tn hiu ting ni, h thng l h thng pht m, gm mi, rng, li, dy thanh... Kch thch lin quan n h thng c gi l ngun tn hiu (signal source). Nh vy ta c ngun ting ni, ngun nh v cc ngun tn hiu khc. C th nh ngha h thng (system) l mt thit b vt l thc hin mt tc ng no ln tn hiu. V d, b lc dng gim nhiu trong tn hiu mang tin c gi l mt h thng. Khi ta truyn tn hiu qua mt h thng, nh b lc chng hn, ta ni rng ta x l tn hiu . Trong trng hp ny, x l tn hiu lin quan n lc nhiu ra khi tn hiu mong mun. Nh vy, x l tn hiu (signal processing) l mun ni n mt lot cc cng vic hay cc php ton c thc hin trn tn hiu nhm t mt mc ch no , nh l tch ly tin tc cha bn trong tn hiu hoc l truyn tn hiu mang tin t ni ny n ni khc. y ta cn lu n nh ngha h thng, khng ch n thun l thit b vt l m cn l cc phn mm x l tn hiu hoc l s kt hp gia phn cng v phn mm.V d khi x l s tn hiu bng cc mch logic, h thng x l y l phn cng. Khi x l bng my tnh s, tc ng ln tn hiu bao gm mt lot cc php ton thc hin bi chng trnh phn mm. Khi x l bng cc b vi x l- h thng bao gm kt hp c phn cng v phn mm, mi phn thc hin cc cng vic ring no . 1.2 PHN LOI TN HIU Cc phng php ta s dng trong x l tn hiu ph thuc cht ch vo c im ca tn hiu. C nhng phng php ring p dng cho mt s loi tn hiu no . Do vy, trc tin ta cn xem qua cch phn loi tn hiu lin quan n nhng ng dng c th. 1.2.1 Tn hiu nhiu hng v tn hiu nhiu knh Nh ni trong mc 1.1, tn hiu c th c m t l hm theo mt hoc nhiu bin c lp. Nu tn hiu l hm theo mt bin, ta gi l cc tn hiu mt hng (one-dimention signal), nh tn hiu ting ni, ECG, EEG. Ngc li ta gi l tn hiu nhiu hng (multidimention signal), v d nh tn hiu nh trng en, mi im nh l hm theo 2 bin c lp.y

y1

I(x1,y1)

x x1

Hnh 1.2 V d tn hiu nh mu (2 hng- 3 knh) -2-

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Trong mt s ng dng, tn hiu c to ra khng phi t mt m l nhiu ngun hay nhiu b cm bin. Cc tn hiu nh vy c gi l tn hiu a knh (multi-channel signal). Bc nh trn hnh 1.2 l mt v d v tn hiu 2 hng, 3 knh. Ta thy sng I(x,y) mi mt im l hm theo 2 bin khng gian c lp, sng ny li ph thuc vo sng ca 3 mu c bn red, green v blue. Mt v d khc, tn hiu nh TV mu l tn hiu 3 hng- 3 knh, c th biu din bi vector sau :

I r (x, y, t) I(x, y, t) = Ig (x, y, t) I (x, y, t) b Trong gio trnh ny, ta tp trung xt tn hiu mt hng- mt knh, bin l bin thi gian (mc d thc t khng phi lc no bin cng l bin thi gian) 1.2.2 Tn hiu lin tc v tn hiu ri rc Tn hiu lin tc (continuous-time signal) hay cn gi l tn hiu tng t l tn hiu c xc nh ti tt c cc gi tr thi gian. V mt ton hc, c th m t tn hiu ny l hm ca mt bin lin tc, v d tn hiu ting ni. Tn hiu ri rc (discrete-time signal) ch c xc nh ti mt s thi im no . Khong cch gia cc thi im ny khng nht thit phi bng nhau, nhng trong thc t thng l ly bng nhau d tnh ton. C th to ra tn hiu ri rc t tn hiu lin tc bng 2 cch. Mt l ly mu tn hiu lin tc, hai l o hay m mt i lng vt l no theo mt chu k nht nh, v d cn em b hng thng, o p sut khng kh theo gi... Tn hiu x(t n ) = e n , n = 0, 1, 2, 3,... l mt v d v tn hiu ri rc. Ta c th dng bin nguyn n thay cho bin thi gian ri rc tn. Lc ny, tn hiu tr thnh mt hm theo bin nguyn, v mt ton ta c th biu din tn hiu ri rc l mt dy s (thc hoc phc). Ta s dng k hiu x(n) thay cho x(tn), ngha l tn = nT vi T l hng s- khong cch gia hai thi im ri rc cnh nhau. Hnh 1.3 l mt v d v tn hiu ting ni ri rc.t

Hnh 1.3 V d tn hiu ri rc 1.2.3 Tn hiu bin lin tc v tn hiu bin ri rc Bin ca c tn hiu lin tc v ri rc u c th lin tc hay ri rc. Nu tn hiu c tt c cc gi tr trong mt di bin no th ta gi l tn hiu bin lin tc (continuous-valued signal). Ngc li, nu tn hiu ch ly mt s gi tr no (cn gi l mc) trong mt di bin th l tn hiu bin ri rc (discrete-valued signal). -3-

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Khong cch gia cc mc bin ny c th bng nhau hay khng bng nhau. Thng th ta biu din cc mc bin ny bng mt s nguyn, l bi s ca khong cch gia hai mc bin cnh nhau. Tn hiu ri rc theo c thi gian v bin c gi l tn hiu s (digital signal). Hnh 1.4 l mt v d v tn hiu s.

Hnh 1.4 V d tn hiu s vi 6 mc bin khc nhau x l tn hiu, trc ht phi thu ly c tn hiu. V d ta thu ly tn hiu m thanh bng microphone, chuyn i tn hiu m thanh sang tn hiu in. Hay nh tn hiu nh, ta c th thu ly bng my nh. Trong my nh tng t chng hn, tn hiu nh sng iu khin cc phn ng ha hc trn mt tm phim nh. V bn cht, cc tn hiu t nhin u l tng t, c s mc bin v s thi im u l v hn. Do vy, tn hiu tng t khng ph hp x l bng cc h thng s. x l s, tn hiu tng t c ly mu vo cc thi im ri rc, to thnh tn hiu ri rc, sau lng t ha bin ca n thnh mt tp cc mc bin ri rc. Qu trnh lng t ha (quantization) tn hiu, v c bn l mt qu trnh xp x ha. N c th c thc hin d dng bng cch lm trn hay ct gt. V d tn hiu c gi tr l 8.62 c th c xp x ha thnh 8 (nu lng t ha bng cch ct gt) hay l 9 (nu lng t ha bng cch lm trn) 1.2.4 Tn hiu xc nh v tn hiu ngu nhin Qu trnh phn tch ton hc v x l tn hiu yu cu phi m t c tn hiu. S m t ny lin quan n mt m hnh tn hiu. Da vo m hnh tn hiu, ta c mt cch phn loi tn hiu khc. Cc tn hiu c th c m t duy nht bng mt biu din ton hc r rng nh l th, bng d liu... c gi l tn hiu xc nh (deterministic signal). T xc nh mun nhn mnh l ta bit r v chc chn cc gi tr ca tn hiu trong qu kh, hin ti v tng lai. Tuy nhin trong nhiu ng dng thc t, c nhng tn hiu khng th biu din chnh xc bng cc cng thc ton hc hay nhng m t ton nh vy l qu phc tp. Ta khng th on trc s bin thin ca cc gi tr ca loi tn hiu ny. Ta gi y l tn hiu ngu nhin (random signal). V d tn hiu nhiu l tn hiu ngu nhin. Ta cn lu rng vic phn loi tn hiu thc thnh xc nh hay ngu nhin khng phi lc no cng r rng. i khi, xem tn hiu l xc nh hay ngu nhin u dn n nhng kt qu c ngha. Nhng i khi, vic phn loi sai s dn n kt qu b li, bi v c nhng cng c ton ch c th p dng cho tn hiu xc nh, trong khi cc cng c khc li ch p dng cho tn hiu ngu nhin. iu ny s tr nn r rng hn khi ta kim tra cc cng c ton c th. 1.3 H THNG X L TN HIU 1.3.1 Cc khu c bn trong mt h thng x l s tn hiu Nh ni trn, hu ht cc tn hiu bt gp trong khoa hc v k thut u l tng t. C th x l trc tip cc tn hiu bng mt h thng tng t thch hp. Trong trng hp -4-

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ny, ta ni tn hiu c x l trc tip dng tng t, nh minh ha trn hnh 1.5. C tn hiu vo v ra u l tn hiu tng t.T/h tng t vo B x l tn hiu tng t T/h tng t ra

Hnh 1.5 X l tn hiu tng t X l s l mt phng php khc x l tn hiu tng t, nh minh ha trn hnh 1.6. Tn hiu tng t phi c chuyn i thnh dng s (A/D) trc khi x l. iu khng may l qu trnh chuyn i tng t/ s ny khng bao gi hon ho, ngha l tn hiu s khng phi l biu din chnh xc cho tn hiu tng t ban u. Khi tn hiu tng t c chuyn thnh tn hiu s gn ng nht, qu trnh x l s c thc hin bng mt b x l tn hiu s DSP (Digital Signal Processor), to ra mt tn hiu s mi. Trong hu ht cc ng dng, tn hiu s cn c chuyn i ngc li thnh tn hiu tng t (D/A) cui qu trnh x l. Tuy nhin, cng c nhng ng dng lin quan n phn tch tn hiu, trong khng cn chuyn i D/A. Hnh 1.6 l s khi mt h thng x l tn hiu bng phng php s. B x l tn hiu s DSP c th l mt mch logic, mt my tnh s hoc l mt b vi x l lp trnh c.T/h tng t vo B chuyn i A/D T/h s vo B x l tn hiu s DSP T/h s ra B chuyn i D/A T/h tng t ra

Hnh 1.6 X l s tn hiu 1.3.2 u im ca x l s so vi x l tng t C nhiu nguyn nhn khc nhau khin cho x l s c a chung hn l x l trc tip tn hiu tng t. Trc tin, h thng s c th lp trnh c, to ta tnh mm do trong vic cu hnh li cc hot ng x l bng cch n gin l thay i chng trnh, trong khi cu hnh li h tng t, ta phi thit k li phn cng, ri kim tra v thm nh xem cc hot ng c ng khng. chnh xc cng ng mt vai tr qua trng trong vic la chn b x l tn hiu. sai lch ca cc linh kin tng t khin cho cc nh thit k h thng v cng kh khn trong vic iu khin chnh xc ca h thng tng t. Trong khi , vic iu khin chnh xc ca h thng s li rt d dng, ch cn ta xc nh r yu cu v chnh xc ri quyt nh la chn cc b chuyn i A/D v DSP c di t thch hp, c kiu nh dng du phy tnh hay du phy ng. Tn hiu s d dng lu tr trn cc thit b bng a t m khng b mt mt hay gim cht lng. Nh vy tn hiu s c th truyn i xa v c th c x l t xa. Phng php x l s cng cho php thc hin cc thut ton x l tn hiu tinh vi phc tp hn nhiu so vi x l tng t, nh vic x l c thc hin bng phn mm trn cc my tnh s. Trong mt vi trng hp, x l s r hn x l tng t. Gi thnh thp hn l do cc phn cng s r hn, h