VII. MACH TAO XUNG VUONG
t re) = 1: f([).o ([-nTs)
Chu ky lay mau kh6ng duqc VUq1. qua nlea chu ky Ian nhat clla tin hi~u ho~c
noi ITI¢t each khac, ran so hiy ITIaU kh6ng duqe nh(') hem 2 Ian uin sci IUn nhM clla tin hi~u dUQ'C lay mau (tfn hi~u dau vil.O luang ttf). Nghia la:
(Ts)max 5 Ta/2= 1/2fa (5-1)
Ts max : Chu ky lay mau Ian nhat
Ta : Chu ky tin hi¢u wang ttf dau vao. fa : Tan so tin hi~u wang tl.! dau vaa (:;-1) co the wang duang fs min = 2fa fs min: Tan so lay mau nh6 nhat.
Ts max gQi la khaang cach (chu ky) Niquist va la khoang thai gian dai nhM duqc dung d~ Ifty m<1u tin hi¢u co bang tan h~n che.
1.2. Luqng til hoa
Luqng tlr hoa Ii qua trlnh thay the' m¢l tin hi¢u wang tl.! drr Ifty mau bang m(ll U).p hu·u hi..tn cae mue bien d¢, tuc Ii qua trinh bien d6i tin hi¢u co bien J¢ lien tl,lc thanh tin hi¢u co bien d¢ rai r<.tc.
Ll!(.mg tu hoa eo hai phl!ong phip eo ban do la ll!qng tu hoi tllyen tlnh hay eon gqi la Ilf(~mg tu hoa dell va Iw;mg tu hmi phi tuyen hay eon gqi la Il!c.rng tit
hoa kh6ng dell.
* LWllIg t/( hod d/I/: Cho phep eae mue Il!c.rng tu eaeh dell nhall. V6i
phl!ong phap Il!\mg tu nay thi khi ph~e hoi tin hi¢ll tl!ang til IU eae xllng ll!qng
IU d phla thu se co sl! chenh I¢eh (sai s6) giUa tin hi¢ll kh6i phl,lc dl!qc va tin hi¢u g6e. SI! ehenh I~ch nay dl!Qe g9i 13. meo I.l!Qng tir ho~c sai s61l!qng 1U.
", Llf(/lIg 1/( /lOcl kh611g d/I/: Luqng tu kh6ng deu duqe thL.te hi~n theo quy
lu~\t logarit, c() nghia la khi muc tin hi¢u nh6 thl sit d~ng mue ll!c.rng tit nh6 va nguQe I~i vai mue tin hi~ll 16n thl su d~ng mue luqng ti'r 16n. Phucmg phap nay thI!e hit;n nen tin hi9Ll 6 phla dicu ehe va ('1 phla thu mu6n thu dU(.1e tin hi¢u thl ph~tj tien hunh theo phuong phap ngW1e I~i, eo nghi<l lit phaj giai nen (gian) tin hi9U. 6 phfa thu si'f dy.ng phu(1ng philp gian tin hi(:u theo quy
lu~\t d6i logarit. 1.3. Mil hOll
MIT ho:t Iii. mOt qua {rlnh so sanh va thay the cae gi,i trj luqng tu rai r<;l.e nh(m duqe (j qua Idnh IU\1ng ti'r hoa bang t~p cae bit nhj phfm; t:}p cae bit nhi ph:ln dU(.1C gQi lu tu rna.
Th6ng thLf<1ng, cae rna nhi phfm duqe si'r dy.ng trong vi¢e rna hoa U\ cae rna nhj phem tl! nhien, cae rna Gray ...
6 phfa thu, cae bit nhj ph:ln duqe sap xep thanh cae xung mau PAM, r6i ehung duqe phy.c hoi bien d(J nha mOt m<;lch 19C thong thap.
Trong cae h~ thelng PCM thl!e te thl b¢ luqng IU hoa va rna hOil ph6i hQp m(JI each e6 hi¢u qUi\. lrong b(J bien doi A/D (Analog/Digital) - Chuyen doi
wang ttl/st).
2. Ma hail va giai rna delta
Dieu ehe delta (DM) la mOt IO<;li dieu xung rna vi sai (DPCM), trong d6, rnoi tir rna ehi e6 m¢t bit nhi phan. U"u di~rn ella dieu ehe delta so v6i cae IO<;li dieu ehc khac la m<;leh dan gian va de dang che t<;Lo cae vi rn~eh CODEC (rna hou va gi,ii rna ... ).
Dieu ehe delta la phu<'1ng phap rna hoa lIan gi{m nhat hi~n eo, VI Iu rna chi
eel I bit nen tiin so lay rnau va tan s6 bit nhu nhall. Tuy nhien, de: ph6i hqp cae
h~ thong tu rna bit eao thong thuang phai tang dan tun s6 Ift'y mall mOt cUeh dang k{
So do rna hoa va giai rna delta:
Vito
lUang tl.r ~_~ XCH X1l ~ Lay mall
X=X,,-i
I hilI! 5.31. Sri do kln)'i 11/(1 !u)a -gidi m/i delta
T~i phfa di6ll che, tin hi¢u tuang tt! X(t) dmJc hi..m che hang tfin, dUQC lay mau de' t<.lO ra tIn hi~u PAM X"' tfn hi¢u nay dem so sanh vai trj s6 dl,!' doan X"'
d¢ ehenh I~ch gifra chung ell duqe IUQ"ng tir thanh m¢t trong hai trj so hien d¢
+!l hay -t.. Dau ra he) Iuq'ng IU dUQe dua tai b¢ rna hoa. Ma hoa ('j day Ii Slr
dl,!ng m¢t hit nhi ph an de rna hoa eho m¢t xung lay mau, sau d6 tin hj¢u rna hoa dUQ"e ehuyc'n den phia thu.
T~i phfa thu, cae bit dUQ"e d,!i di~n eho cae xung lay mau ell 0') bien d(l la
±6. (gia Irj ehcnh I¢eh), bien d¢ nay dU"qc c(mg vo'j gi{l t1'j d~r doun lue thai a
dau ra phia thu de khoi phl,!c l<:J.i hien d¢ lhlfe ella cae xung mau. Tin hi¢u xung mAu nay c1uqc 19C thong thap de kh6i phl,!e l<;li bien d¢ (hlnh d<;lng) tin hi¢u giong nhu d~ng tin hi¢u llIang tt! d dau vao phfa dicu ehe.
Cae phuang phap rna hoa va giai rna PCM, rna hoa va giiii rna delta dUQ"e lrng dl:mg trong h¢ thong thu phit tin hi¢u va dCi Ji¢u tho<:ti, thu phc.it tIn hi¢u cW li¢u ... lrong h¢ thong tcing dai.
VIII. !)JEU CHE V A GIAI !)JEU CHE TiN HIEU SO 1. Khai quat chung
Dieu ehe V~l giJ.i dieu che so la m()t phfin clla may phat V~l may thu tin hi¢u so trang h¢ thong thong tin so, Ti.~i phia phi.lt thlfc hi¢n qua t1'lnh dicu ehe so la ky thu~t dung tin hi¢u so de 13m thay deli me)! lrong cae thong s6 eua song mango Trong dieu ehe tin hi~u s6 thmJng e6 eac lo~i dicu ehe ea beln sau day:
- Dieu che bien d¢ so (ASK),
- Dieu che Uln so s6 (FSK).
Ngoai d.c ky thUi}t co ban tren, di~u che s6 con co cac lo,:\i dieu ehe'd<;ic bi~t khae nhu: dicu ehe bien d<.) va pha kct hqp (CASK), dieu che cau phuong .. Moi Im.li deu co uu va nhuqc diem khac nhau.
vc ph!a thu. ky thU(lt tach tin hi¢u s6 ra khai song mang dW1c gQi I~I gia clieu che s() hay con gQi la tach song tin hi~u s6. TLIo'ng ung vai moi ky thu(lt dieu eh6 duqc Slr dl,mg 6 ph!a phat thl ('1 phia thu sc Slr dl,lng ky thu(lt gi .. ii dicu ehe llIong (mg. Vf dl,l t<;li phia phat si'r dJ,lng ky thu~t dieu che bien d(:l so thl phla thu ph,ii dllIlg ky thu(tt giai dieu che bien d(:l s6 de thu l<;ti tin hi¢u nguyen thuy nhu tnrac khi dieu che.
Sau dfly chung ta St kh,io Si.lt m¢l s6 ky thu~t dieu che V21 gi,ii dicu che tin hi¢u s6 co b,ln.