III. DiA LUll TRU THONG TIN 1 each 16 chuc Ihong lin Iren dTa
1. Cae thc.nh phon cua <> dia cung
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Hhlh 1-25. Minh hO{1 m(J/ (/ dla cling
4.£)Ia CD
4.1. Via CD-ROM
Hi~n nay, diu CD-ROM dugc Slr dl,lng nit f(lng raL M~c du duqc thiet k€ de lUll tn) dfr li¢u tren dTa CD nhung cong ngh¢ cua m¢t ilia CD-ROM hoan
toan khac v6i cong ngh¢ ella m¢t 6 dTa cung. Kh,fc vOi dla til, thong tin a day dugc lUll tn] kh6ng phiij tren cae duang tron dong Him rna thea hlnh xoan ac til diem dAu den diem cu6i va ch<:ty lien tl;lc tir tam trl,lc fa ngoai ria. Yang xoan ac chua cae sector co chieu dai banK nhau va co kha nang lUll tru mQl kh6i lugng dfr li¢u nhu nhau.
4.1.1. Cii'u tf!.o dia quang
Dia quang (Dia CD) Hi rnl)t dia nhva co duong kinh 120 mm, day 1,2 mm. Duong kfnh 16 trl,lc quay la 15 mm. L6 thong tin (pit) co duang kfnh 0,6 j..Lm, sau 0,12 j..Lm. Cae ranh (Track) each nhau 1,6 j..Lrn, m~t d¢ ilia VaG khoang 16000 TPI (Track Per Inch). Khac v6i ilia tir, dfr Ii~u ghi tren dia quang di tU troug fa ngoai thea hlnh xoun oc. M¢t dia CD co 22188 d~a. Du li~u tren CD- ROM duqc chia thanh lung khoi wang tv nhu trong dia cung.
M6i khoi bao gam 12 Byte dong bO, 4 byte <lia chi kMi, 2048 byte dii li~u, 288 byte rna sua 16i.
Toc d9 d'lc co so cua mQt rna quang hi 150 Kbyte/giay. Cae 6 dia dQc nhanh co toc di) Iii h¢i s6 cita t6e d¢ co sO nay (Vi d~ 24X, 32X, 36X, ... ,52X). Thai gian truy nh~p m¢t dia CO - ROM nhanh han 150 ms.
CD - ROM chi e6 the doc v1 thong tin duqe ghi bang cong ngh~ dot chay. Thong tin nay chi viet vao dla m¢t Ian v1 bc m:}t ella dia lhl;fe sl! duqc nhting dc du liGu. Be m~lt CD - ROM luu tru c<ic du li~u duoi d<~ng d.c hoc 16m va nhung cho bttng phrtng. C{IC h6e 16m d~i di¢n cho bit 0 va cho btmg phang d<;ti di~n cho bit l. Kho[lng e{leh giua de vong xoan oc xap Xl 1,6 micromet. Voi kfch thuOe n\1(1 nhu v<~y, mai dia CD - ROM chua khoang 16000 ranh tren mbi inch sit han so vC1i dia mcm va di<l cung.
4.1.2. Nguyen tdc hOtJt dljllg dl)c dji'li¢u
CAe di<l dung tia laser h()i 1l,l r[it manh va b0 ph<!.n phiit hi~n tia laser de biet eho nan co, eh/) nao kh6ng e6 hoc 16m. Nguan tia V~l b¢ ph<~n phat hi0n tia dU(1e lllP Ifen m()t d.i gia c11<.~y dQc theo ban kinh eua dia CD. Chuyen dQng ttt trong dill fa ngoai di<l cua m~lt LASER c¢ng voi chuy~n d0ng quay tron cua dia CD sc 1.:.10 len v¢t d()c hlnh XOrtn ()e tu trang ra ngoai dia. Toc d¢ quay clla dia phl,l thuQc Van vi trf cua dau dQc. Dfiu doc cang gfin t[lm dia thl dia quay cang nhanh, elm fa ngo~li vilnh dia thi toe d¢ quay e<'lOg gillm. Tia laser huang vao m~t dudi eua did xuycn qua hon lmm ncn plastic tntOe khi chicu Icn m(l\ philn X<~. Khi tia laser d(l.p vao m9t dinh bttng se philn X<.l It.ti qua cae h¢ th{(u kfnh ve d.c "diot nh(\O" nit m~nh (Wong ling muc logic I). Khi tia laser d~p vao hoc, anh sang phan .x~ b! phan tan nen hfiu het nang luqng laser bj tan X~\ ra moi huang va tin hi~u quay tro ve cae diot nh(tn rat yeu ( mue logic 0).
CD - ROM dU9c str dl}ng de dqc d.c du li~u tren ma CD - ROM vai d() tin c(ly eao.
4.2. Din CD ghi du'!c (CD-R) vii dla CD ghi I~i duuc (CD - RW)
CD ghi duqc hi Im.1i dia quang tung dUQ'e g9i la WORM (Write once, read multiple). Dia nay c6 cftu true va hOi;lt d¢ng lUang W nhu CD thuang. Lqi die'm cua CD-R Iii dung du'lc lrong bflt ky 6 dia CD nao. Tuy nhicn no c6 nhl1'lc di6m la khong ghi chong duqc len du li~u cO (chi ghi duqe m¢t I<'in).
Di<l CD - R bao gam cae lop sau:
• Lop nhan hi~u (kh6ng nhf(t thiet cfin den). • Lop phu chong xuoc (kh6ng nhat thict can den). • Lap phim bao v¢ tia lU ngo~i
• Lap phim philn X';l (vang hay h(.1p kim mau b~c, 50-;.-100 nm) • Lap mau polymer huu co
• Lap palycarbonat trong suot.
Lop mau Polymer la 16p chua dCr li¢u va nam ngay sat m~t chua nhan hi¢u. VI Y(ly luu y tranh him xuac mi.~1 n:1Y khi dung CD - R. Khi hj tia Laser dot chay, 16p mau nay chuy~n sang m~tu den ',la dong vai tro dc lb dO' li¢u (PIT) ella CD thuang.