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B GIO DC V O TO TRNG I HC BCH KHOA H NI ---------------------------------------

H Ni 2006 Nguyn Nh Nht

NGUYN NH NHT IN T VIN THNG 2004-2006

LUN VN THC S KHOA HCNGNH: IN T -VIN THNG

TI

TRUYN HNH S DVB VIT NAM V XU HNG PHT TRIN

NGUYN NH NHT

H NI 2006 Lun vn thc s

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MC LC TM TT LUN VN ....................................................................................... 4 M U ............................................................................................................... 5 CHNG 1 TNG QUAN ................................................................................. 7 CHNG 2 TO DNG MPEG-2 V CU TRC MPEG-2 TS ................ 10 2.1. C IM CA TN HIU TRUYN HNH ......................................... 10 2.2. CHUYN I S I VI TN HIU VIDEO ...................................... 10 2.2.1. Qu trnh s ho tn hiu Video tng hp (composite signal). ............. 10 2.2.2. Qu trnh s ho tn hiu Video thnh phn (component signal) ......... 11 2.3. NN VIDEO CHUN MPEG................................................................... 14 2.3.1. Mt s tiu chun nn. ........................................................................ 14 2.3.2. Cu trc dng bt Video MPEG-2 ....................................................... 15 2.3.3. Nn theo khng gian (Nn intra) - nh I.............................................. 16 2.3.5. Profile & Level ................................................................................... 20 2.4. NN AUDIO MPEG -2. ........................................................................... 22 2.4.1. H nn audio MPEG:.......................................................................... 22 2.4.2. S nn audio MPEG. ..................................................................... 23 2.4.3. c im nn Audio MPEG. .............................................................. 24 2.5. CU TRC DNG TRUYN TI MPEG-2 TS ...................................... 25 2.5.1. Cu trc phn lp h thng ................................................................. 26 2.5.2. Cu trc dng truyn ti: .................................................................... 32 2.5.4. Thng tin c t chng trnh (PSI- Program Specific Information) ... 37 2.5.5. c im ca dng MPEG-2 ............................................................. 40 2.5.6. ng dng dng truyn ti MPEG-2 TS ............................................... 41 CHNG 3 TRUYN HNH S MT T DVB-T ..................................... 42 3.1. C IM CHUNG CA TRUYN HNH S MT T ................... 42 3.2. TRUYN HNH S MT T DVB-T .................................................. 43 3.2.1. c im tiu chun DVB-T .............................................................. 44 3.2.2. S khi .......................................................................................... 45 3.2.3. Cu trc khung OFDM. ...................................................................... 59 3.2.4. Cht lng ng truyn tiu chun DVB T .................................... 67 CHNG 4 TRUYN HNH S QUA V TINH DVB-S .............................. 70 4.1. C IM CA TRUYN HNH V TINH .......................................... 70Nguyn Nh Nht Lun vn thc s

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4.2. TIU CHUN DVB-S (EN-300421) ........................................................ 73 4.2.1. M ho knh truyn ........................................................................... 74 4.2.2. Phn iu ch...................................................................................... 78 4.2.3. Cht lng ng truyn .................................................................... 81 4.3. MT S TIU CHUN KHC. .............................................................. 81 4.3.1. Tiu chun DVB-S (TR 101 198). ...................................................... 81 4.3.2. Tiu chun DVB-DSNG (EN 301 210)............................................... 81 4.4. TNH TON CHT LNG NG TRUYN TRONG DVB-S ........ 82 CHNG 5 TRUYN HNH S QUA CP DVB-C .................................... 87 5.1. TIU CHUN DVB-C (EN 300 429). ...................................................... 87 5.1.1. S khi .......................................................................................... 87 5.1.2. M ha knh truyn ............................................................................ 88 5.1.3. Phn iu ch...................................................................................... 89 5.2. MT S TIU CHUN KHC ............................................................... 91 CHNG 6 TRUYN HNH S DVB VIT NAM V XU HNG PHT TRIN ..................................................................................................... 92 6.1 TNG LAI CA CNG NGHIP TRUYN HNH .............................. 92 6.2. TRUYN HNH S DVB VIT NAM HIN NAY ............................. 93 6.2.1. Truyn hnh s qua v tinh DVB-S ..................................................... 94 6.2.2. Truyn hnh s qua cp DVB-C .......................................................... 99 6.2.3. Truyn hnh s mt t DVB-T .......................................................... 99 6.3. XU HNG PHT TRIN CC DCH V DVB TI VIT NAM ..... 104 6.4. KT LUN ............................................................................................. 107 MT S THUT NG ................................................................................... 109 TI LIU THAM KHO................................................................................ 116

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TM TT LUN VNCng ngh Truyn hnh s ang pht trin rt mnh m trn ton th gii, v Truyn hnh Vit Nam cng khng th ng ngoi xu th . Do vy trong thi gian qua Truyn hnh Vit Nam nghin cu, tm hiu, so snh v cn nhc la chn mt chun truyn hnh s trong ba chun hin nay ang s dng trn th gii: ATSC, DVB, ISDB-T. Nm 2001 truyn hnh Vit Nam chn chun DVB c s dng trn lnh th Vit Nam. T bn lun vn ny trnh by v Truyn hnh k thut s DVB Vit Nam v xu hng pht trin bao gm: - Gii thiu chung v nguyn l truyn hnh. - Cch to ra dng tn hiu truyn hnh s c nn MPEG-2 hin ang c s dng rng ri trn th gii. - Cc phng thc truyn dn tn hiu truyn hnh s ti ngi xem gm: - Truyn dn pht sng s trn mt t DVB-T. - Pht sng tn hiu truyn hnh s qua v tinh DVB-S. - Pht tn hiu truyn hnh s qua mng cp DVB-C. - Gii thiu cc cu trc, s khi m cc mng truyn dn ny s dng, tp trung tm hiu chi tit mt s k thut quan trng s dng trong mi phng thc truyn dn ny. - Trnh by v xu th hi nhp gia truyn hnh v cng ngh thng tin cng vi cc dch v s c cung cp trong tng lai. - Trnh by v hin trng cc mng truyn dn tn hiu truyn hnh s ti Vit Nam hin nay, cng nh kh nng chuyn i t mng pht hnh tng t sang s. Cc dch v s c cc i truyn hnh, cc nh cung cp pht trin trong thi gian ti. - Mt s kin ngh. Trn trng cm n!

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M UTruyn hnh ngy nay tr thnh mt nhu cu khng th thiu trong x hi. Ngoi nhng nhu cu tm hiu tin tc, truyn hnh v s mang li cho ngi xem cc chng trnh gii tr b ch v cc dch v thng tin khc nh nhiu loi dch v tng tc theo hai chiu nh video on demand, gameshow on demand, eshopping on demand, new on demand, webbrosing on demand... Cng vi s pht trin ca khoa hc cng ngh, nht l cng ngh thng tin to ra s thay i ln rong lnh vc truyn hnh v em li nhiu dch v mi trong lnh vc truyn hnh. Trong nhng nm gn y cng ngh truyn hnh chuyn sang mt bc pht trin mi - l qu trnh chuyn i t cng ngh truyn hnh tng t sang cng ngh truyn hnh s. Vic chuyn i ny khng ch xy ra trong lnh vc sn xut cc chng trnh truyn hnh m cn trong c lnh vc truyn dn v pht sng tn hiu truyn hnh. Nhiu nh nghin cu cho rng bt u mt thi i mi- thi i ca Digital Video, Digital Television. Truyn hnh tng t ngy cng t c nhc ti v i dn vo qun lng nhng ng cho truyn hnh k thut s. T thp k 80 cc nc pht trin trn th gii nghin cu, trin khai th nghim truyn dn v pht sng truyn hnh s qua v tinh, qua cp v mng pht sng mt t. Cc cng ngh ny v ang c hon thin trong nhng nm cui th k 20 v u th k 21 nhng nm ti y s chuyn sang pht sng s chnh thc. Cng vi s pht trin cng ngh nn tn hiu Video, m ra kh nng pht trin truyn hnh s trong vic lu tr chng trnh cng nh vic truyn dn v qung b tn hiu truyn hnh s n ngi xem. Truyn hnh s c nhng u im ni bt so vi truyn hnh tng t nh: C th truyn nhiu chng trnh trn cng mt bng tn. C cht lng cao t b nh hng bi tp m ng truyn. C th lu tr chuyn i m khng lm gim cht lng. C kh nng truyn c chng trnh truyn hnh phn gii cao (HDTV) trong bng tn 8MHz. C th kt hp cc dch v truyn hnh v cc dch v vin thng khc. V vy vic nghin cu v truyn hnh s v tip cn vi khoa hc cng ngh ang thay i ht sc nhanh chng l mt vic ht sc quan trng v cn c y mnh a Truyn Hnh Vit Nam (THVN) tin kp vi cc nc trong khu vc v trn th gii, trnh tt hu v lng ph. Vit Nam k thut truyn hnh s cng c quan tm t nhng nm 1990, cc trung tm nghin cu pht trin v ng dng qun l truyn hnh cng c nhiu ti v truyn hnh s. i THVN (Truyn Hnh Vit Nam) cng aNguyn Nh Nht Lun vn thc s

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ra l trnh chuyn dch t truyn hnh tng t sang truyn hnh s bt u t nm 1999. Sau nhiu hi ngh khoa hc v vic la chn chun truyn hnh s no (ATSC, DVB, ISDB-T) c s dng ti Vit Nam. Nm 2001 i THVN ra quyt nh chn chun DVB. Trc nhu cu thc t nh vy v bn thn l k s ang lm vic trong ngnh truyn hnh, c th l ti i THVN, nn ti chn ti Truyn hnh s DVB Vit Nam v xu hng pht trin nghin cu, tm hiu nhm c c nhng ng gp nht nh cho truyn hnh Vit Nam. Ni dung lun vn cp n:

C s truyn hnh s Cc phng thc truyn dn tn hiu truyn hnh s DVB Truyn hnh s DVB Vit Nam v Xu hng pht trinCng ngh truyn hnh s l mt mt lnh vc mi v phc tp n hp nhiu kin thc mi ca cng ngh thng tin v truyn thng, cho nn trong bn lun vn ny s khng trnh khi nhng thiu st. V vy ti mong mun nhn c nhng kin ng gp ca cc thy, c v cc bn ng nghip bn lun vn ny c c nhng tc dng nht nh. Qua y ti xin chn thnh cm n s gip , hng dn ca thy, tin s Hong Tin tn tm gip , hng dn ti hon thnh cun lun vn ny. H NI, thng 10 - 2006

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CHNG 1 TNG QUANH thng truyn hnh l mt tp hp cc thit b cn thit m bo qu trnh pht v thu cc tin tc hnh nh. Lch s pht trin truyn hnh n nay cho ra i nhiu tiu chun truyn hnh khc nhau, nhng d th no th cng phi c s khi nh hnh 1.1ng knh Cnh vt AB chuyn i nh - tn hiuB khuch i v gia cng tn hiu

Knh truyn

B kh.i v x l tn hiu

B chuyn i tn hiu - nh

B to xung ng b

B tch xung ng b

Hnh 1.1: S khi tng qut h thng truyn hnh en trng Truyn hnh en trng ra i t nhng nm u ca thp k 20 vi nhiu tiu chun truyn hnh khc nhau: A, B, C, D, K, E, F, G, H, J, K, L, M, N. Tip theo l s ra i ca cc h truyn hnh mu vi 3 h : NTSC, PAL, SECAM xut hin vo thp k 50 ca th k trc to nn mt bc ngot mi trong qu trnh pht trin ca cng ngh truyn hnh. C 3 h truyn hnh ny u s dng cc tn hiu thnh phn l tn hiu chi v 2 tn hiu mu (Y, R-Y, B-Y). iu khc nhau c bn gia cc h mu l phng php iu ch v ghp knh tn hiu mu vi tn hiu chi. Do s pht trin nhanh chng ca cng ngh in t vi s ra i ca cc vi mch c ln, cc b x l tn hiu s vi tc cao, cc b nh dung lng ln v nht l s bng n ca cng ngh thng tin trong nhng nm gn y, video s, truyn hnh s hon ton mang tnh kh thi v tng bc tr thnh hin thc. S ha tn hiu truyn hnh trn thc t khng ch l s bin i tn hiu truyn hnh tng t sang tn hiu s m cn l qu trnh x l tn hiu gim cc thng tin khng cn thit m bo tn hiu truyn hnh s c th truyn c trn bng tn ca truyn hnh tng t. So vi h thng truyn hnh tng t (analog), truyn hnh s c nhng u im nh: - Tn hiu s t nhy cm vi cc dng mo xy ra trn ng truyn. - C kh nng ti to, khng suy hao cht lng khi truyn dn cng nh khi lu tr tn hiu. - C tnh linh hot, mn do trong qu trnh x l tn hiu. - Hiu qu s dng bng tn cao khi truyn dn tn hiu truyn hnh s c nn.Nguyn Nh Nht Lun vn thc s

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- C kh nng phn cp chng trnh khi s dng h truyn hnh s mt t DVB-T hay ISDB-T. - C kh nng truyn dn tn hiu truyn hnh c phn gii cao trn 1 knh truyn hnh tng t. - C kh nng kt hp vi nhiu dch v thng tin khc nhau. - Kho m n gin. - Cng ngh truyn hnh s c nhng kh nng m truyn hnh tng t khng th thc hin c. S tng qut h thng truyn hnh s nh hnh 1.2 u vo ca thit b truyn hnh s s nhn tn hiu truyn hnh tng t. Trong thit b m ha (bin i A/D) tn hiu truyn hnh analog s c bin i thnh tn hiu truyn hnh sTn hiu truyn hnh analog Thit b pht M ha iu ch knh

Bin i A/D

Nn, ghp knh

Knh truyn Thit b thu Gii m knh

Tn hiu truyn hnh analog

Bin i D/A

Tch knh gii nn

Gii iu ch

Hnh 1.2 S khi tng qut h thng truyn hnh s Cc tham s v c trng ca tn hiu ny c xc nh t h thng truyn hnh chn. Tn hiu truyn hnh s c a ti thit b pht, c chuyn ti knh thng tin v ti thit b thu. Ti bn thu tn hiu c bin i ngc li vi qa trnh x l ti pha pht, gii m tn hiu truyn hnh, thc hin bin i tn hiu truyn hnh s thnh tn hiu truyn hnh tng t. H thng tn hiu truyn hnh s s xc nh trc tip cu hnh ca b m ha v gii m. Khi truyn qua knh thng tin tn hiu truyn hnh s c m ha knh chng li. Hin nay trn th gii tn ti 3 tiu chun truyn hnh s l ATSC do M xut, DVB do chu u xut v ISDB-T do Nht Bn xut. Truyn hnh Vit Nam trong qua trnh s ho phi chn cho mnh mt l trnh thch hp ph hp vi hon cnh t nc v k tha thnh tu ca cng ngh truyn hnh s trn th gii. Do vy trong khong hn 10 nm tr li y TruynNguyn Nh Nht Lun vn thc s

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hnh Vit Nam tp trung tm hiu, so snh cc u nhc im ca cc h thng truyn hnh s trn th gii v sau nhiu hi ngh khoa hc chng ta chn tiu chun DVB l chun truyn hnh s duy nht c s dng Vit Nam vo nm 2001. thi im chun DVB s dng tn hiu truyn hnh s c nn theo chun MPEG-2. T n nay Truyn hnh Vit Nam tng cng u t cc trang thit b theo chun DVB MPEG-2 nhm tng bc s ho ngnh cng ngh truyn hnh trong nc. Trong nhng nm u vic s ho c thc hin khu sn sut cc chng trnh truyn hnh, n nay ang s ho dn cng on truyn dn v pht sng tn hiu truyn hnh. Hin nay ton b h thng truyn hnh s Vit Nam u s dng chun nn MPEG-2 ca DVB. Trong khi hin nay trn th gii pht trin nhiu cng ngh mi nh nn Video MPEG-4 c kh nng gim c tc bit xung thp hn m cht lng hnh nh khng i. Tuy nhin cc sn phm thng mi ca cng ngh mi ny cha ph bin hn na s lng thit b u t theo chun MPEG-2 ti Vit Nam l rt ln v s tip tc pht trin trong mt thi gian di na. Do lun vn ny ch tp trung vo lnh vc Truyn dn tn hiu truyn hnh s theo chun nn MPEG-2.

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CHNG 2 TO DNG MPEG-2 V CU TRC MPEG-2 TS2.1. C IM CA TN HIU TRUYN HNH Thit b truyn hnh s dng trong truyn chng trnh truyn hnh l thit b nhiu knh, ngoi thit b truyn hnh cn c cc thng tin km theo l cc knh m thanh v cc thng tin ph nh tn hiu in bo thi gian chun, tn s kim tra hnh nh tnh. Tt c cc tn hiu ny c ghp vo cc khong trng ca ng truyn nh b ghp knh . Truyn tn hiu truyn hnh s c thc hin khi c s tng quan gia cc knh tn hiu. Thng tin ng b s c truyn i ng b cc tn hiu . kim tra v tnh trng ca thit b truyn hnh s s dng cc h thng o kim tra tng t nh h thng truyn hnh tng t thng qua o kim tra tn hiu chun. Sau y l mt s c im ca truyn hnh s so vi truyn hnh tng t - Tn hiu s t nhy cm vi cc dng mo xy ra trn ng truyn. - C kh nng ti to, khng suy hao cht lng khi truyn dn cng nh khi lu tr tn hiu. - C tnh linh hot a dng trong qu trnh x l tn hiu. - Hiu qu s dng bng tn cao khi truyn dn tn hiu truyn hnh s c nn. - C kh nng phn cp chng trnh khi s dng h truyn hnh s mt t DVB-T hay ISDB-T. - C kh nng truyn dn tn hiu truyn hnh c phn gii cao trn 1 knh truyn hnh tng t. - C kh nng kt hp vi nhiu dch v thng tin khc nhau. - Kho m n gin. - Cng ngh truyn hnh s c nhng kh nng khng th thc hin c trong truyn hnh tng t. 2.2. CHUYN I S I VI TN HIU VIDEO 2.2.1. Qu trnh s ho tn hiu Video tng hp (composite signal). i vi tn hiu video tng hp (NTSC hay PAL) tn s ly mu tn hiu Video c ly bng 4 ln tn s sng mang tn hiu mu. Theo tiu chun Nyquist tn s ly mu phi ln hn hoc bng 2 ln so vi tn s cc i ca tn hiu. Trong thc t khi chuyn i A/D i vi tn hiu video tng hp, qua nhiu ln th nghim chng t rng vic ly mu vi tn s c gi tr bng n fSC l c hiu qu hn c. i vi tn hiu truyn hnh mu h PAL c tn s ly mu c la chn l 4fSC = 17,734475 MHz. i vi tn hiu h mu NTSC c tn s ly mu l 4fSC = 14,32818 MHz. i vi tn hiu video tng hp, cc pixel c lng t ho 8 hayNguyn Nh Nht Lun vn thc s

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10 bit. Do cht lng ca tn hiu Composite thp v vy tn hiu ny t c s dng. Thay vo l vic chuyn i tng t - s theo khuyn co CCIR 601 i vi cc tn hiu mu thnh phn. 2.2.2. Qu trnh s ho tn hiu Video thnh phn (component signal) Tn hiu Video c c trng ring, nn ngoi vic tha mn nh l ly mu Nyquist, qa trnh ly mu cn phi tun theo cu trc ly mu, tnh tng thch gic cc h thng ... a) Ly mu Cc chun ly mu C nhiu chun ly mu tn hiu Video thnh phn s, im khc nhau ch yu t l gia tn s ly my v phng php ly mu tn hiu chi v tn hiu mu. Trong bao gm: Tiu chun 4:4:4; 4:2:2; 4:2:0; 4:1:1. Cc nh dng s video c nn ch ly mu cho cc dng tch cc ca tn hiu Video Trong cc tiu chun , thng c s dng trong thit b Studio l 4:2:2 v trong cc thit b truyn dn pht sng l 4:2:0. nh dng tn hiu Video s CCIR- 601 4:2:2. Tn hiu Video s CCIR-601 4:2:2 l tn hiu Video s thnh phn ph hp vi 2 h qut 525/60 hay 650/50 dng trong pht sng. Cc thng s ly mu: Ly mu trc giao 4:2:2 (4 mu Y : 2 mu Cr : 2 mu Cb) im u ly mu c Y,Cr,Cb , im k tip ch ly mu Y(Khi gii m tn hiu mu ca im ny c suy ra t im trc ). i vi c 2 tiu chun qut mnh 50/60 th tn s ly mu i vi cc tn hiu chi v mu l: i vi tn hiu chi Y: fs = 13,5 MHz. i vi tn hiu mu Cr hay Cb : fs = 6,75 MHz. Tc dng bt khi ly mu vi h PAL (625x720), c 576 dng tch cc Khi m ho 8 bit/mu, tc : (720 + 360 + 360) x 576 x 8 x 25 =165,888Mbit/s Khi m ho 10bit/mu, tc : (720 + 360 + 360) x 576 x 10 x 25 = 207,36Mbit/s Cc v tr ly mu theo tiu chun 4:2:2 c m t trong hnh v 2.1:

Luma sample Cb sample Cr sample

Hnh 2.1: Cc v tr ly mu theo chun 4:2:2

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b) Lng t ho Cc tn hiu c lng t ho theo cc mc: i vi cc tn hiu chi Y Cc mc Cc mc bo v cao Mc lng t cao nht Mc trng Mc xo Mc lng t thp nht Mc bo v thp i vi tn hiu mu Cr hay C b Cc mc Cc mc bo v cao Mc lng t cao nht Mc dng ln nht Mc xo Mc m ln nht Mc lng t thp nht Mc bo v thp 8 bit FF FE F0 80 10 01 00 10 bit 3FC-3FF 3FB 3C0 200 040 004 000-003 8 bit FF FE FB 10 01 00 10 bit 3FC-3FF 3FB 3AC 040 004 000-003

Dng s liu Video s c ghp t tn hiu chi v cc tn hiu mu nn dng s liu ny c tc ln gp 2 ln tc ly mu tn hiu chi Y hay gp 4 ln tc ly mu tn hiu mu Cr hay Cb. Do cc ng h knh c tn s gp 2 ln tn s ly mu tn hiu chi Y tc l : 13,5 x 2 = 27 MHz. Xung ng b trong tn hiu video tng t c thay th bng tn hiu chun thi gian (TRS - Timing Reference Signal) cc tn hiu chun thi gian l: - Bt u tn hiu Video (SAV- Start of Active Video). - Ht tn hiu Video (EAV- End of Active Video). Tn hiu chun thi gian TRS gm 4 t m. Trong trng hp 8 bit, m ho h HEX bao gm 4 t m c xp xp theo th tNguyn Nh Nht Lun vn thc s

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FF 00 00 XY Ba t u c gi tr c nh, t m XY mang nhng thng tin: mnh chn hoc l, bt u hoc kt thc xung xo mnh, bt u hoc kt thc xung xo dng. Trong trung hp 10 bit 4 t m nh sau: 3FF 000 000 XYZ. 3 t bit u 3FF 000 000 l c nh , cc gi tr 3FF v 000 d phng nhn dng thi gian v nhn dng thng tin ng b SAV v EAV. Cc bit XYZ mang thng tin v nhn dng field, trng thi xo mt(mnh), trng thi xo dng. Cc d liu ph (Ancillary data): Vi h qut 625/50 trong thi gian xo dng c th chn 280 t d liu (HANC- Horiz Ancillary Data) v bt u bng 3 t bit 000 3FF 3FF. Trong thi gian xo mnh, (ngoi tr mt s dng c bit) c th chn c 1440 t bit d liu (VANC- Vert. Ancillary Data) v cng bt u bng 000 3FF 3FF.

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2.3. NN VIDEO CHUN MPEG Trong tt c cc loi tn hiu th tn hiu truyn hnh l chim di tn ln nht cho mt knh thng tin V d nh dng bit dng thc video ITU- R 601 4:2:2 vi mc lng t ho 10 bit c tc l 270 Mbit/s ( trng hp s ho c cc dng khng tch cc), nh vy di ph cn thit truyn tn hiu ny khng di (3/4)x270 = 180 MHz di ph ny l phi thc t do vy nn tn hiu Video l khng th thiu trong truyn hnh s. Vic nn tn hiu Video trc ht l loi b cc tn hiu d tha trong dng bit Video: d tha v kh nng cm nhn hnh nh ca th gic ca con ngi, d tha v mt khng gian (cc im nh k cn nhau trong mt nh thng c gi tr gn ging nhau), d tha v thi gian (cc nh k cn trong mt chui nh thng ging nhau), d tha bit xt v mt xc sut thng k. l cc nn khng suy hao, khng lm thay i cm nhn ca mt i vi hnh nh trc v sau khi nn (Lossless compression). tng cng t s nn, ngi ta s loi b cc thng tin t quan trng v chp nhn mt mc suy gim cht lng nht nh l cc k thut nn c suy hao. 2.3.1. Mt s tiu chun nn. Tiu chun JPEG (Join Photographic Expert Group) ra i khong nhng nm 1980: l mt tiu chun dng nn cc nh tnh (nh mu v en trng) dng lu tr v s dng trong mt s lnh vc nh Fax mu, truyn nh bo ch, nh cho y hc, camera s... T chun JPEG ngi ta cn a ra chun M-JPEG ch s dng trong khu xn sut chng trnh v hu k, khng dng c cho pht sng v tc bt cn cao. Tiu chun MPEG (Motion Picture Expert Group): tiu chun MPEG do nhm chuyn gia v nh ng nghin cu v xut vi mc ch nn tn hiu Video v Audio. Trong MPEG gm cc chun sau: Chun MPEG-1: Tiu chun nn nh ng c kch thc 320x240 v tc bt cn t 1- 1, 5 Mbit/s dng cho ghi hnh trn bng t v a quang (CD), ng thi truyn dn trong cc mng, nh mng my tnh. Chun MPEG-2: L tiu chun nn c s dng cho cc ng dng cao hn vi tc bt cn 10 Mbit/s truyn tn hiu truyn hnh s thng thng. L tiu chun tip theo ca MPEG -1 nhm vo cc ng dng cho cc chng trnh truyn hnh trong h thng truyn hnh s mt t, cp v v tinh. Chun MPEG-3: L tiu chun nn tn hiu s xung cn 50 Mbit/s truyn tn hiu truyn hnh s c phn gii cao. Nm 1992 MPEG -3 c kt hp vi MPEG- 2 dng cho truyn hnh thng thng v truyn hnh phn gii cao gi chung l MPEG-2.Nguyn Nh Nht Lun vn thc s

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Chun MPEG- 4: Chun ny cho php nn nh Video vi t khung hnh v yu cu lm ti chm nn tc d liu cn rt thp, nhm mc ch pht trin cc tiu chun m ha mi c tc bt thp. Chun MPEG-7: c thit k cho mng truyn thng a phng tin. Chun MPEG-21: Cng c thit k cho truyn thng a phng tin. MPEG l chun nn kt hp gia nn trong nh v nn lin nh l phng php nn c tn hao da trn bin i DCT v b chuyn ng. 2.3.2. Cu trc dng bt Video MPEG-2 Cu trc dng bit MPEG-2 Video gm 6 lp: Lp chui (on chng trnh Video): on Video gm thng tin u on, mt s nhm nh (GOP), thng tin kt thc on. Thng tin u on Video cha kch thc mi chiu ca nh, kch thc ca im nh, tc bt ca dng Video s, tn s nh v b m cn c. on Video v thng tin u on to thnh mt dng bt c m ha gi l dng c bn (Elementary Stream) Nhm nh (GOP): L t hp ca nhiu khung nh I,P v B, gm nhiu nh, c bt u bng mt khung I, thng tin u gm 25 bt cha m nh thi v iu khin. Lp nh: Gm nhiu on nh (slice) cho php b gii m xc nh loi ca nh c m ha l nh I,P hay B, thng tin u dng ch th t khung truyn b gii m c th xp xp li nh theo mt th t ng, ngoi ra header ca nh cn cha cc thng tin v ng b, phn gii v phm v ca vc t chuyn ng. Lp Slice: Gm nhiu Macroblock, mi Macroblock gm mt s block cho cc tn hiu mu v chi, ph thuc vo chun ly mu. V tr: v tr bt u ca Slice. Thang lng t (QScale): bng lng t ho ca Slice. Lp Macroblock (MB): gm nhiu Block. a ch tng( Address Increase): s MB nhy ti. Loi MB: m t cu trc MB c s dng vc t b tr chuyn ng. Thang lng t (QScale): bng lng t ho ca MB. Block: Gm 8x8 Pixel. Tu theo dng thc ly mu m cc Macroblock c cu trc khc nhau. i vi dng thc CCIR 601 4:2:0, mi MB s gm 4 Block Y, 1 Block Cr, 1 Block Cb. i vi dng thc CCIR 601 4:2:2, mi MB s gm 4 Block Y, 2 Block Cr, 2 Block Cb.

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K thut nn i vi tn hiu Video theo tiu chun MPEG-2 s dng c hai loi nn khng suy gim (nn theo khng gian) v nn c suy gim (nn theo thi gian). 2.3.3. Nn theo khng gian (Nn intra) - nh I S tng qut v nn intra hnh 2.2

Y Cr Cb Block & MB formating 2D DCT

DPCM Huff man

Quanti zer

Zig-zag Scaning

RLC

VLC

Buffer

Scaling Factor Quantizer

Hnh 2.2: S nn Intra frame

Nguyn l chung nh sau Block v MB formating: trc khi thc hin DCT, c nh c chia thnh cc khi ln ring bit khng chng nhau MB, mi MB gm 4 block Y, 2 block Cr , 2 block Cb , s lng cc block mu ph thuc vo tiu chun ly mu. Chuyn i 2D - DCT: Cc block Y, Cr, Cb vi kch thc 8 x 8 mu vi phn gii lng t 8 bit s c bin i ( 8 x 8 ) 2D - DCT to ra ( 8 x 8 ) h s DCT c phn gii 11 bit. Nng lng thng dng tp trung thnh phn mt chiu DC v cc thnh phn tn s thp nn cc h s ny thng c gi tr ln hn, cc thnh phn tn s cao t tn ti ng thi nh hng t n cm nhn ca mt nn c th loi b trong trung hp cn gim thng tin trong khi lng t ho. DPCM: Cc h s DC ca cc block c m ho DPCM so vi h s DC ca block tng ng trc . Lng t ho (Quantizer): cc thnh phn tn s khc nhau c lng t ho vi cc bc lng t khc nhau: - Lng t ho tinh vi cc thnh phn tn s thp. - Lng t ho th vi cc thnh phn tn s ln.

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Qu trnh lng t ho c thc hin bng cch chia 8 x 8 h s cho bng lng t ho (gm 8 x8 gi tr tng ng). Gi tr ca cc thnh phn tn s ln sau khi lng t ho s nh v c lm trn v 0. MPEG s dng cc bng lng t ho khc nhau khi lng t ho Y,Cr,Cb. Tu theo tnh trng y hay cn ca b m, cc h s thang lng t (Scaling factor) s c iu chnh li. Qu trnh ny c thc hin bng cch chia 8 x 8 h s lng t ho cho h s thang lng t. H s thang lng t c p dng cho c MB, do cn xt tnh cht phn b nng lng ca cc Block xc nh h s thang lng t ph hp nht. Qut zigzag: (zig- zag scanning): chuyn (8 x 8) h s DCT lng t ho (dng ma trn - 2 chiu) thnh mt chiu truyn dn tn hiu vi 1 dy 64 phn t (1 chiu) theo 1 th t nht nh. C 2 ch qut khc nhau l: Zigzag: p dng cho nguyn nh (frame). Alternate: p dng cho loi nh field. RLC + VLC (run length coding v variable length coding): M c di chy v m c di bit i dng loi b cc thng tin d tha xt v mt thng k. B m (Buffer): iu chnh lng bit u ra lun c tc bit khng i. Cn c vo tnh trng y hay cn ca b m c s dng iu chnh li h s thang lng t cho b lng t ho pha trc. Cc loi MB trong nh I: cn phn bit 2 loi Macroblock c s dng trong nh I: Intra: ch s dng bng lng t mc nh. Intra A (Adaptive): s dng bng lng t mc nh kt hp vi h s thang lng t.

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2.3.4. Nn theo thi gian (Nn Inter -nh P v B)Nn nh P: Hnh 2.3 trnh by s khi nguyn l nn nh PSai s d on

nh hin ti

S bnh d on

e = b- b S

Motion Estimator

Motion Compensation Predictor

Picture Memory nh trc

Vector chuyn ng

Hnh 2.3 S nn nh P

nh gi chuyn ng (Motion estimator): i vi mi MBb trong nh th K, tm trong vng xung quanh ng vi nh th K-1 xem c MB no gn ging nht vi MBb (gi s l MCc), t xc nh c vec t chuyn ng d. d = ( dx , dy ) D on b chuyn ng (Motion Compensation Predictor): to ra MB d on b nh sau: b = c, vi c l c trong nh K-1 khi khi phc li. Nh vy, thay v truyn i ton b thng tin MB s c thay th bng vc t chuyn ng v thng tin sai s d on s c DCT, lng t ho, qut zig-zag v m ho RLC, VLC, tng t nh trong nh I Vc t chuyn ng d c m ho DPCM so vi vct chuyn ng ca MB trc , ri ghp knh chung li truyn i. Cc loi MB trong nh P: MB l n v nh gi chuyn ng v b chuyn ng. Trong nh P, c 4 loi MB khc nhau c phn loi da vo 4 quyt nh: - C b chuyn ng hay khng (MC/ No MC) - Nn kiu intra hay khng (intra / non - intra ) - Sai s d on c c m ho hay khng ( coded / not coded) - C s dng h s thang lng t hay khng (quant / no quant) .C 8 loi MB trong nh P l:

Intra: s dng bng lng t mc nh. Intra A: s dng bng lng t mc nh v h s thang lng t. Inter D: c t nht 1 block trong MB c m ho.Nguyn Nh Nht Lun vn thc s

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Inter DA: c t nht 1 block trong MB c m ho v c s dng h s thang lng t. Inter F: ch truyn vector chuyn ng. Inter FA: truyn i vector sai s d on v c s dng h s thang lng t. Skipped: b qua MB ny v n ging y nh MB v tr tng ng trong nh trc . Nn nh B nh gi chuyn ng: i vi mi MBb trong nh th K, tm trong vng xung quanh tng ng trong nh th K-1 v th K+1, xem c MB no gn ging nht vi MBb (gi s MBc1 trong nh K-1 v MBc2 trong nh K+1 ). T xc nh c 2 vc t chuyn ng d 1 d2. D on b chuyn ng: to ra MB d on b nh sau: b = a1c1 + a2c2 Cc loi MB trong nh B: trong nh B, c tt c 12 loi MB khc nhau : Intra: ging nh trong nh I, P. Intra A: ging nh trong nh I,P. Inter F: ging nh trong nh P. Inter FDA: ging nh trong nh P. Inter B : ch truyn i vc t chuyn ng ngc . Inter BD: ch truyn i vc t chuyn ng ngc v sai s d on. Inter BDA: ch truyn i vc t chuyn ng ngc, sai s d on v c s dng h s thang lng t Inter I : ch truyn i vc t chuyn ng thun v ngc. Inter ID : ch truyn i c 2 vc t chuyn ng v sai s d on . Inter IDA: ch truyn i 2 vc t chuyn ng, sai s d on v c s dng h s thang lng t. Skipped : b qua MB ny, ging nh trong nh P. Hnh 2.4 a ra nguyn l chung cho s nn MPEG video, trong : A: sai s d bo im im (Pixel-by-pixel prediction errors). B: h s DCT sai s d bo (Prediction errors DCT co-efficients). C: h s DCT lng t (DCT co-efficients in quantized form).

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D: H s DCT sai s d bo lng t (Quantized prediction error DCT coefficients). E: Sai s d bo im im (Pixel-by-pixel prediction errors). F: Gi tr im khi phc li (Reconstructed pixel values). G: Gi tr d bo b chuyn ng (Motion compensated predicted pixel values). H: Vc t chuyn ng (Motion vectors).F E IDCT D Inverse quantizer

+Video to be coded

+

A

DCT

B

Quantizer

C

+

G Motion estimator H Motion compensator

Bitstream coder

Coded video bitstream

Anchor frame storage (2)

Hnh 2.4: S nn MPEG i vi nn nh I, sau khi ly mu tn hiu c truyn n khi bin i DCT cho cc Macroblock c lp, sau n b lng t ho (Quantizer) v m ho entropy. Tn hiu ra t b lng t c a ti b lng t ho ngc (Invert Quantizer) v bin i DCT ngc sau lu vo b nh nh. B nh nh bao gm nh xut hin trong b gii m sau khi gii m nh truyn loi I. i vi loi nh P mch nn hot ng v lm vic trn c s so snh cc nh c trong b nh, sau s xc nh vc t chuyn ng, d bo nh c bin i DCT lng t ho v m ho entropy. Cng nh i vi nh I, cc tn hiu ra t b lng t ho s c gii lng t ho, bin i DCT ngc kt hp vi nh d bo ang xt v lu gi trong b nh. 2.3.5. Profile & Level M ho co dn: tnh co dn ca dng bit MPEG-2 l kh nng gii m c 1 phn dng bit mt cch c lp vi phn cn li ca dng bit nhm khi phc li video vi cht lng hn ch (hn ch v phn gii khng gian, hn ch v phn gii thi gian hay hn ch v SNR..).Nguyn Nh Nht Lun vn thc s

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Theo tnh co dn, dng bit c phn thnh 2 hay nhiu lp : - Lp c bn: tp con nh nht ca dng bit c th c gii m 1 cch c lp. - Lp nng cao: mt hay nhiu lp cn li. C nhiu loi co dn khc nhau c trin khai: - Co dn khng gian: h tr cc phn gii khng gian khc nhau. - Co dn SNR: h tr cc mc SNR khc nhau trong cng phn gii khng gian. - Co dn thi gian: h tr cc phn gii thi gian khc nhau. - Co dn phn chia d liu hay co dn tn s: h tr cc mc r nt ca hnh nh khc nhau. - Co dn lai: kt hp 2 trong cc loi co dn trn. MPEG-2 ch mi h tr hai loi co dn l co dn khng gian v co dn SNR. Profile v Level : Level: c trng cho cc mc phc tp ca dng thc video v phn gii khng gian v tc khung hnh, c 4 level. Low Level : 352 x 240 x 30. 352 x 288 x 25. Main Level: 720 x 480 x 30. 720 x 576 x 25. High 1440 Level: 1440 x 1080 x 30. 1440 x 1125 x 25. High Level : 1920 x 1080 x 30. 1920 x 1152 x 25. Profile: c trng cho cc mc phc tp v cng c nn (k thut nn c s dng) cc Profile cao hn s bao gm cc cng c ca Profile thp hn. Khi MPEG-2 ra i, mi ch c 5 Profile nh sau: Simple Profile: Nn nh I, P 4:2:0. Main Profile: Nn nh I, P, B 4:2:0. SNR Scalable Profile: Nn nh I, P, B 4:2:0 kt hp vi co dn SNR. Spatially Scalable Profile: Nn nh I, P, B 4:2:0 kt hp vi co dn SNR v co dn khng gian. High Profile: Nn nh I, P, B 4:2:0 v 4:2:2 kt hp vi co dn SNR v co dn khng gian. Cc t hp Profile v Level:Nguyn Nh Nht Lun vn thc s

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phn gii352x240x30 352x288x25 720x480x30 720x576x25 1440x1080x30 1400x1152x25 1920x1080x30 1920x1152x25

Simple Low

Main

NRZ Scalable

Spatially Scalable

High

MP@LL

SNRP@ LL

Main

SP@ML

MP@ML

SNRP@ ML

HP@ML

High 1440 High

MP@ H14L MP@HL

SSP@ H14L HP@HL

Trong cc dng thc trn dng thc c la chn s dng trong truyn dn pht sng l: 4:2:0 MP@ML. Cc thng s chnh ca 4:2:0 MP@ML nh sau: phn gii khng gian: 720 x 576 hay 720 x 480. Tc bit ti a: 15 Mbit/ s. Nn I, P, B. p ng cc yu cu v sn xut chng trnh truyn hnh, MPEG-2 b xung thm 4:2:2 P @ ML. Cc thng s chnh ca 4:2:2 P @ ML nh sau: Cu trc ly mu: 4:2:2. phn gii khng gian: 720 x 576 hay 720 x 480. Tc bit ti a: 50 Mbit/ s. Nn I, P, B. 4:2:2 P @ ML ni chung cho cht lng nh cao hn MP @ ML nhng phi tr gi bng tc bit ln. Tnh tng hp ni b MPEG: b gii m ng vi Profile v Level no c kh nng gii m c dng bit ca Profile thp hn hoc Level thp hn. 2.4. NN AUDIO MPEG -2. 2.4.1. H nn audio MPEG: Cc tn hiu Audio nn theo tiu chun MPEG -2 c th c nhiu nh dng khc nhau v c m t trong hnh 2.5

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MPEG-2

n knh

a knh

Layer Layer Layer I II III

Layer Layer Layer I II III

Hnh 2.5: Nn Audio MPEG

2.4.2. S nn audio MPEG.Audio Time to frequency mapping (filter bank) Bit/Noise Allocator Quantizer Code

Mux

Psychoacoustic model

Ancillary data

Hnh 2.6: S nn Audio MPEG

Filter Bank: Lc a di dng phn b m tn vo cc di con (subband), c phn thnh 3 loi chnh: *Subband Bank (di con): 32 Subband Bank c rng bng nhau (MPEG Layer I, II). S dng PQMF (Polyphase Quandrature Mirror Filters). Th d: trong MPEG Layer II, 1 audio frame gm 1152 mu c phn vo 32 Subband, mi Subband 36 mu. *Transform Bank (di bin i): Thng dng MDCT (Modified DCT). S lng bng MDCT cao ( thng t 256 n 1024). Hybrid Filter Bank (Di lc lai):Nguyn Nh Nht Lun vn thc s

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*Kt hp Subband Filter v MDCT: Th d: trong MPEG Layer III, c 32 bng SB, mi bng SB li chia thnh 18 bng MDCT, do c tng cng 32 x 18 = 576 bng con. c im: Filter Bank phn gii thp (t bng con): Phi m ho mi Subband nn gim hiu qu ca hiu ng che lp, n gin, phn gii thi gian tt. Filter Bank phn gii cao (nhiu bng con): khng nht thit phi m ho mi Subband, tt vi c tnh tai nghe hn (Critical band), phc tp hn, phn gii thi gian xu (khng thch hp vi tn hiu qu ). V vy trong thc t, ngi ta c th s dng loi Filter Bank thch nghi theo tng loi tn hiu. Psychoacoustic Model: tnh ton nng lng ph tn hiu audio, xc nh ng cong che lp (da trn m hnh thnh gic). Bit/Noise Allocator, Quantizer, Coder: trong tng di con, t S/M (hay S/N cho php) xc nh s lng bit cn thit lng t ho v m ho. Multiplexer: ghp s liu sau khi c lng t ho, m ho vi cc thng tin ph khc. 2.4.3. c im nn Audio MPEG. Nn Audio MPEG - 1: c im Layer I: Tc bit: 32 384 Kb/s (tng cng). 384 mu / frame / 1 knh. 8 ms framing @ 48 KHz (12 x 32 x 20.83 ms = 8 ms). 32 subband u, mi Subband gm 1 block 12 mu (32 x 12 = 384 mu). ng dng: Philips Digital Compact Cassette (DCC) vi 192 Kb/s / 1 knh. c im Layer II: Khi cht lng tt hn vi tc bit thp hn, khong 128 Kb/s. Tc bit: 32 384 Kb/s (tng cng). 1157 mu / frame / 1 knh. 24 ms framing @ 48 KHz (384 x 3 x 20.83 ms = 24ms). 32 Subband u, cc block 36 mu (32 x 36 = 1152 mu). ng dng : CD - ROM, DVB, DAB, DBS, Multimedia. c im Layer III: Cho cht lng tt nht vi tc bit thp, khong 64 Kb/s: Tc bit: 32 320 Kb/s. 1152 mu / frame / 1 knh.Nguyn Nh Nht Lun vn thc s

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24 ms frarming @ 48 KHz (384 x 3 x 20.83 ms = 24 ms). 32 Subband u, mi Subband chia thnh 18 bng MDCT (tng cng 576 bng con). i vi tn hiu qu , ch s dng 192 bng con. Nn audio MPEG - 2: Audio MPEG - 2 l s m rng ca audio MPEG - 1 p ng yu cu ca cc ng dng mi nh: Di cht lng audio tri rng t tc bit thp n cao: 32 Kb/s 1066 Kb/s. Mi frame ca dng bit audio MPEG - 2 c chia lm 2 phn: Phn tng hp vi MPEG-1. Phn m rng. M ho c ti 6 knh audio k c 1 knh LFE h tr m thanh Surroud: Left, Right, Center, L Surround, R Suround v LFF ( tn thp) H tr cc tc ly mu phn na: 16 KHz, 22,05 KHz, 24 KHz. Cc c tnh m rng ny u c p dng cho mi Layer. Ngoi ra trong nn Audio ngi ta cn dng mt s chun nn khc nh AC-3 c cht lng cao hn, truyn c nhiu ng ting hn. Tuy vy cc thit b gii m ang s dng hin nay ch yu l cc thit b MPEG-2 Layer II. Do vy vic pht sng truyn hnh s i vi knh audio l MPEG-2 Layer II, tc i vi mt knh Stereo nn chn t 182 192Kb/s. 2.5. CU TRC DNG TRUYN TI MPEG-2 TS truyn tn hiu truyn hnh s hin nay s dng 3 phng php l truyn qua v tinh, truyn qua cp, v truyn (pht sng) trn mt t. Vic pht trin c ba phng thc l rt thc t v chng b xung cho nhau. Truyn hnh qua v tinh c th ph sng mt khu vc rng ln vi nhiu chng trnh c th ln n hng trm chng trnh. Truyn hnh s trn mt t nhm vo mt s lng ngi thu khng ln v ch ph sng trong mt khu vc nht nh. Nhng ng thi ngoi vic thu bng ng ten c nh trn mi nh truyn hnh s mt t cn c th p ng cho nhu cu thu di ng, thu sch tay. Truyn hnh cp phc v thun li cho cc khu vc dn c ng c thnh ph khng c iu kin thu v tinh (ch yu gi thnh v tinh cn cao ). thc hin vic truyn dn nh trn, hin nay trn th gii xut hin cc tiu chun truyn hnh s nh DVB ca Chu u, ATSC ca M v ISDBT ca Nht Bn. Nhng trong ni dung ti ny ch quan tm n truyn dn tn hiu truyn hnh Vit Nam - chn theo chun DVB ca chu u. Tn hiu truyn hnh s c truyn dn l dng bt Video c s ho v nn theo chun

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MPEG-2. Do vy trc khi tm hiu phn truyn dn chng ta cn tm hiu chi tit v dng truyn ti MPEG-2. 2.5.1. Cu trc phn lp h thng Cu trc phn lp h thng ca MPEG-2 c tm tt nh hnh 2.7 Dng d liu MPEG c phn chia lm 4 loi:

nh dng ngun

Gii nh dng ngun

Lp nn

Nn/ M ho dng c bn ES

Gii nn/ gii m

ng gi

M gi

dng Lp h thng Ghp knh dng chng trnh chng trnh Ghp knh dng chng trnh

ng gi dng truyn ti

M gi dng truyn ti

Mi trng rruyn dn

Hnh 2.7 Cu trc phn lpNguyn Nh Nht Lun vn thc s

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1.Dng c s (ES): Dng c s v c bn l dng tn hiu gc ti u ra ca b m ho v ch cha nhng thng tin cn thit gip b gii m ti to li hnh nh hoc m thanh ban u - dng video nn MPEG-2 hoc dng audio nn MPEG-2 hoc dng d liu. C php dng c s nh hnh 2.8Quantizing Matrices Motion Vector Sync Pattern I,P,B Time Open/Close Picture rate

Profile

Level

Block of Coefficients

Macro block

Slice

Picture

Group of Picture Chroma Type

Global Vector

Aspect ratio Picture Progressive Size Interlatce

Hnh 2.8 C php dng c s

n v c s ca mt hnh nh l khi DCT, mi khi c 8x8 pixel Block: Gm 8x8 Pixel. Tu theo dng thc ly mu m cc Macroblock c cu trc khc nhau. i vi dng thc CCIR 601 4:2:0, mi MB s gm 4 Block Y, 1 Block Cr, 1 Block Cb. i vi dng thc CCIR 601 4:2:2, mi MB s gm 4 Block Y, 2 Block Cr, 2 Block Cb. Lp Macroblock (MB): gm nhiu Block. M t cu trc MB c s dng vc t b tr chuyn ng. Thang lng t (QScale): bng lng t ho ca MB. Lp Slice: gm nhiu Macroblock, mi Macroblock gm mt s cho cc tn hiu mu v chi, ph thuc vo chun ly mu. V tr: v tr bt u ca Slice. Thang lng t (QScale): bng lng t ho ca Slice. Lp nh (Picture): gm nhiu on nh (Slice). Loi khung: I, P, hay B frame. Tham s b m: dng gii m ti phn thu. Lp nhm nh (GOP - Group of Picture): gm nhiu nh (Picture). Thi gian m ho (time code): thi gian nn khung (hours, minutes, seconds, frames). Tham s nhm nh: m t cu trc ca GOP. Lp chui (Sequence): gm nhiu GOP. Cc thng tin v chui bao gm:Nguyn Nh Nht Lun vn thc s

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Thng s video: rng, chiu cao t l Pixels, tc hnh nh. Thng s dng bit: tc , kch thc b m, tham s flag. Loi khung: nn trong nh (I frames) hay nn d bo nh (P hay B frames). 2. Dng c s ng gi (PES): Dng c bn video, audio hoc d liu sau khi c ng thnh gi (Packet). Dng c bn ng gi PES c to t dng c s bng cch ng gi dng c s ES thnh cc gi (packet) vi chiu di tu , chiu di ca gi c th n vi trm Kbytes, chiu di cc i ca gi PES l 216 byte (c th di n 8 Kbytes). Cu trc dng c s ng gi MPEG-2 nh hnh 2.9

Hnh 2.9 Cu trc dng PES

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Cu trc gi PES nh trong hnh 2.10

8 PES header

Kbytes data payload

SC

SI

PL

BS

Hnh 2.10 Cu trc gi c bn PES

Trong PES header gm SC: M bt u gi (Start Code - 3 bytes). SI: Nhn dng gi (Stream Identification - 1 byte). PL: di gi (Packet Length - 2 bytes). BS: kch thc b m (Buffer Size - 2 bytes). D liu header c th phn bit c 16 chng trnh Video v 32 chng trnh Audio, Tng chng trnh trong s 48 chng trnh (video v audio) u c th km theo d liu v ngi dng (user). D liu v ngi dng c s dng cho nhiu mc ch khc nhau ca nh cung cp. C php chi tit hn ca dng PES nh hnh 2.11

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Page 30 of 116 PES Start Code Prefix Stream ID PES Packet length PES Header Stuffing byte PES packet data

10

PES PES ScrambingPriority control

Data Alinment Indicator

Copy Right

Orginal or copy

7 Flag

PES Header Optional Data Field Length

PTS ES ES DT CR rate S

Additional DSM Copy Trick Mode information

PES CRC

PES extension

5 Flag

Optional

PES Private Data

Packet Header field

Program Pecket Sqe Couter

P-STD Buffer

PES Extension Field

Hnh 2.11 C php dng c s ng gi PES PES Packet Data Length cho thng tin v s Byte c trong d liu bt u tu chn Cc Flags ch r mnh tu chn (option Fields) tn ti hay khng tn ti, thng tin c m ho hay khng m ho, yu cu v bn quyn, s c mt hay khng c mt ca PTS , V DTS, hai thng tin ny oc s dng m bo s ng b mch gii m, DTS m bo th t nh khi gii m cn PTS m bo th t nh c hin th. Tip theo l 2 bit mang gi tr 10 PES Scrambling Control (2 bit): Gi tr 1 ch th xo trn bt nu c PES Priority (1 bit): ch th s u tin ca gi Data alignment Indicator (1bit): gi tr 1 ch th mt trng Payload bt u vi mt m m u Video hay t ng b audio Copy Right(1bit): Gi tr 1 ch th trng Payload gi PES c bo v bn quyn. Original or Copy (1bit): gi tr 2 ch th trng payload gi PES l nguyn gc. PTS-DTS Flags (2bit): gi tr 2(tc l 10), v 3 ch th c PTS hin din pha sau, ngay trong header ny, gi tr 1 ch th c DTS hin din cn gi tr 0 ch th khng c c DTS v PTS hin din. ES-CR (1bit): gi tr 1 ch th chun ng h hin din trong header gi PESNguyn Nh Nht Lun vn thc s

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ES-Rate(1bit): Gi tr 1 ch th tc bt hin din trong headergois PES DSM Trick Mode (1bit): gi tr 1 ch th s hin din ca trng ch k xo di 8 bit pha sau. Additional Copy information Flag(1bit): gi tr 1 ch th s hin din ca thng tin bn quyn PES CRC Flag(1bit): gi tr 1 ch th CRC(Cyclic Redundancy Check- mt loi m kim tra chn l bo v chng li) xut hin trong gi PES 3.Dng chng trnh (PS) Cch ghp knh to dng chng trnh c thit lp trong tiu chun MPEG-1 v vn tip tc c vn dng trong MPEG-2. Loi ghp knh ny to ra mt chng trnh n vi mc ch lu tr trong mi trng lu tr s, v d nh ng dng trong cng ngh CD-ROM. Mt dng chng trnh l kt qu ca vic ghp knh mt vi dng c s ng gi PES s dng cng mt h thng xung nhp thi gian (Time Clock). Dng chng trnh c th l mt dng video c km audio hoc mt chng trnh nhiu knh audio Dng chng trnh c thit k s dng trong mi trng khng nhiu v n rt d b li. iu ny c gii thch bi 2 l do sau: - Dng chng trnh bao gm s ni tip cc gi tng i di v di ny li lun lun thay i. Mi gi bt u bng mt header. Mt li xy ra trong phn Header c th lm mt thng tin ca ton gi. V cc gi ca chng trnh c th cha vi chc Kbyte s liu, s mt mt thng tin ca mt gi c th lm mt hoc gin on c mt khung nh. - di gi khng c nh khin cho b gii m khng d on c khi no gi chm dt v mt gi mi bt u. Thay vo , b gii m phi c v gii li thng tin v di gi cha trong mi Header. Nu thng tin v di gi ny b li, b gii m s mt ng b v nh vy s lm mt thng tin t nht l mt gi . Dng ghp knh video, audio v d liu (nu c) ca 1 chng trnh, thng c s dng trong lnh vc lu tr hay truyn dn tn hiu mi trng cht lng cao (t nhiu). Dng chng trnh cha thng tin ca 1 hay nhiu dng gi c bn ghp knh chung thnh 1 dng s liu, s dng chung thi gian gc (time base), bao gm nhiu gi (packet) lin tip. Cc gi PES c di tu , khng quy nh s lng v th t cc gi. Tuy nhin cc gi c truyn theo th t theo thi gian xy ra. Dng chng trnh c th cha n 32 dng audio, 16 dng video, 16 dng d liu, s dng chung thi gc.Nguyn Nh Nht Lun vn thc s

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Dng chng trnh rt nhy vi li ng truyn v thng dng trong thu multimedea, phn phi mng cc b. Pack headerSC SCR MR

Video

Audio1

Audio2

Data

....

Data

PES header

Video payload

PES Pack # nHnh 2.12 Cu trc dng chng trnh SC: M bt u gi ( Start Code). SCR: Chun ng h h thng (System Clock Reerence). MR: Tc ghp (Multiplexed Rate).

4. Dng truyn ti (TS) Dng ghp knh ca 1 hay nhiu chng trnh, mi chng trnh gm nhiu ngun video, audio, d liu, thng c s dng trong lnh vc truyn dn (mi trng c nhiu). 2.5.2. Cu trc dng truyn ti: Dng truyn ti cha thng tin ca 1 hay nhiu chng trnh c thi gc khc nhau hoc ch l s kt hp cc PES. Thng tin c chia thnh cc gi truyn ti (transport packet) thng c chiu di 188 bytes c th sa jitter m bo tin cy trn ng truyn c nhiu chp nhn s mt mt thng tin khi gii m. Ngoi ra, kch thc gi truyn ti c nh nn d dng chuyn i dng truyn ti TS thnh cc t bo (cell) ca mng ATM (Asynchronous Transper Mode). Hnh 2.13 Minh ho cch ghp knh to dng truyn tiNguyn Nh Nht Lun vn thc s

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Program 1VideoAudio 1 Audio 2

Program 2data Video Audio

Program 3Video Audio1

Header 188 bytes

Data payload

Hnh 2.13 Ghp knh dng truyn ti

Minh ho cch chia cc gi PES thnh cc gi truyn ti nh hnh 2.14

Hnh 2.14: Chia cc gi PES thnh cc gi TSNguyn Nh Nht Lun vn thc s

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Trong dng truyn ti cc Gi PES t cc dng gi s cp khc nhau c phn nh v gn vo cc phn Payload ca gi TS, quy trnh ny phi tho mn hai iu kin sau -Byte u tin ca mi gi PES phi tr thnh byte u tin ca phn Payload - Bt k gi TS no cng ch mang phn d liu ly t cng mt gi PES Tt c cc dng gi s cp PES ghp knh chung vi nhau to nn mt hay nhiu chng trrnh s c chuyn thnh cc gi theo cng mt cch thc nu trn. Cc gi TS c xp xp tun t to ra dng truyn ti TS. Cc gi TS cha thng tin dch v cng nh cc gi TS rng c s dng lp y knh truyn. Th t cc gi c th xp xp tu min sao m bo c iu kin cc gi TS ca cng mt dng s cp ng gi PES phi c truyn theo th t thi gian.

2.5.3. Cu trc gi truyn ti TSGi truyn ti c kch thc c nh v bng 188 byte chia thnh 2 phn l header v payload, hnh 2.15

Hnh 2.15 Cu trc gi truyn ti MPEG 2-TS

Cu trc gi truyn ti gm 2 phn Header v d liu. Cc thng tin quan trng nht ca phn Header trong dng truyn ti l thng tin nhn dng gi (PID Packet Identification) v chun ng h chng trnh (PCR - Program ClockNguyn Nh Nht Lun vn thc s

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Reference). PID c s dng khi phc li cc thnh phn s liu trong qa trnh tch knh chng trnh. PCR cha cc thng tin nh thi ng b xung ng h 27 MHz ti b gii m. di ti thiu ca Header l 4 bytes. di ca Header c th ln hn 4 bytes khi cc Header c cha thng s PCR. Thng s cc trng Header: Sync byte (Byte ng b khung - 8 bit): c gi tr 0x47 to c s nhn dng bt u mi gi. Transport Error Indicator (Bit ch th li truyn TEI - 1 bit): dng bo hiu c li xy ra trn ng truyn (t khu iu ch, gii ch, phn knh). Payload Unit Start Indicator (Bit ch th khi ng n v s liu truyn ti PUSI - c 1 bit): bo cho ta bit byte u tin ca phn Payload chnh l byte u tin ca gi PES mi (i vi s liu Audio, Video) hay l phn u ca 1 bng (i vi cc gi thng tin c t chng trnh PSI). Transport Priority (u tin truyn ti TP - 1 bit): gi tr 1 ch th gi TS c quyn u tin hn cc gi khc. Nu khng c gi TS u tin no th bit ny c b qua phn thu. Packet Identifcation (Nhn dng gi PID - 13 bit): ch th nhn dng s liu lu tr trong gi. Transport Scrambling Control (Ch th m ho dng TS TCC- 2 bit). Adaptation Field Control (iu khin trng thch nghi AFC - 2 bit): ch th cho bit phn trng thch nghi c xut hin hay khng. Continuity Counter ( 4 bit) Gi tr ny s tng ln 1 theo cc gi TS k tip nhau thuc v cng mt gi s cp PES (cng PID) Ngoi ra cn c th s dng Adaptation field(trong phn payload) Adaptation field: S hin din ca phn Adaptation field c bo hiu bi thng tin v Adaptation field control trong header. Phn ny gm cc thng tin c s dng cho cc chc nng gii m bc cao hn. Phn Adaptation field c di khng c nh mc ch l lp y cc gi TS khng c thng tin hu ch t PES. Cu trc trng thch nghi nh hnh 2.16 Cc thng tin PCR c ghi trong trng PCR gm 48 bit, trong c 33 bit u cha cc thng tin PCR, 6 bit tip theo d phng 9 bit cui cng cha cc thng tin PCR m rng. Trng PCR c th b thay i trong qua trnh x l tn hiu ti phn pht.

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Page 36 of 116 OPCR flag Splicing point flag Transport data flag PCR flag Adap ter field length Adaption extention flag Optional field Stuffing Bytes

Discontinuity indicator Random access indicator elementary stream priority indicator

PCR 48 bit

OPCR 48 bit

Splice countdown 8 bit

Transport Private data 48 bit

Adaptaton fld. Extention length 48 bit

Hnh 2.16: Cu trc trng thch nghi

Tng t nh vy cc thng tin OPCR (Orignal program Clock Reference) cha cc thng tin ng b ca n chng trnh v c cu trc tng t trng PCR. Khi mi chng trnh c nhiu dng s cp (Video, audio, data) th cc dng s cp dng chung mt ng h ch (chung PCR). Cc thng tin nh thi ring s uc suy ra t ng h ch. Vic kt hp cc gi truyn ti ca nhiu dng truyn ti vi nhau s tao ra dng truyn ti a chng trnh, mi mt chng trnh s dng mt chun ng h chng trnh (OPCR) c lp. Khi tc ca dng a chng trnh khng i nhng c th thay i tc ca cc dng s cp.

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2.5.4. Thng tin c t chng trnh (PSI- Program Specific Information) Thng tin c t chng trnh bao gm cc loi thng tin l PAT, PMT, NIT, CAT. Mt dng truyn ti c th gm mt s cc chng trnh, mi chng trnh li bao gm mt s dng s cp. Do h thng cn phi theo di tt c cc dng s liu khc nhau v cc PID lin h. iu ny c thc hin bng hai c ch nh x nh sau: Bng t chc chng trnh (PAT - Program Association Table): Danh sch tt c cc chng chnh cha trtong dng truyn ti s c ghi trn PAT. Mi dng truyn ti phi c 1 PAT c s PID lun lun bng 0. Cn phi c bng PAT nhn dng cc s PID ca mi chng trnh (Cc s PID ny thng ch n cc bng PMT ca mi chng trnh). Bng nh x chng trnh (PMT - Program Map Table): Mi chng trnh trong dng truyn ti u c 1 bng PMT ring. Bng ny dng xc nh cc s PID ca mi dng s cp thuc chng trnh . C th ghi thm cc Descripter (b m t) vo PMT. Descripter mang cc thng tin chi tit v chng trnh cng nh v cc dng s cp thnh phn nh: cc thng s m haVideo, cc thng s m ha Audio, nhn dng ngn ng, chi tit v truy cp c iu kin, thng tin v bn quyn... Bng thng tin mng (NIT - Network Information Table): Trong PAT chng trnh s 0 c dnh ring ch n NIT. Cc chng trnh c nh s theo th t lin tip, bng thng tin mng (NIT) l ty chn v ni dung ca bng cng mang tnh ring t (c nh ngha bi ngi s dng, ch khng phi bi MPEG-2). MPEG-2 khng yu cu mt s PID c bit no dng ring cho NIT (Tuy nhin mt h thng c th no li c th yu cu iu ny). NIT thng cung cp cc thng tin v mng vt l dng truyn dng truyn ti nh : tn s knh, chi tit v b pht p v tinh, c tnh iu ch... Bng truy cp c iu kin (CAT - Conditional Access Table ): nu c dng s cp no trong dng truyn ti c xo trn, th CAT phi hin din cung cp thng tin chi tit v h thng xo trn c s dng v cung cp cc gi tr PID ca gi cha thng tin v qun l vic truy cp c iu kin. nh dng ca loi thng tin ny khng c quy nh bi MPEG-2, m l thuc vo loi h thng xo trn c s dng. H thng MPEG c 2 cp truy nhp c iu kin l: Kho m ti dng truyn ti TS, v kho m ti gi dng c s PES. Cc a ch nhn dng gi c m t trong bng sau: TableNguyn Nh Nht

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PAT CAT TSDT

0000 0001 0002

Ngoi cc bng thng tin theo tiu chun MPEG l PAT, PMT, CAT, hay TSDT cc thng tin c t cn c th bao gm nhiu bng thng tin c t khc ph thuc vo khi ghp knh (multiplexer). V d nh trong tiu chun DVB ngoi cc gi k trn cc thng tin c t cn c cc gi: Bng m t dch v (SDT- Service Description Table) c PID l 0011. Bng thng tin s kin (EIT - Event information table) : c PID l 0012. Cc gi tr v cu trc bng thng tin c t ghp thm dng truyn t theo DVB c trnh by trong tiu chun ETSI EN 300768 V1.4.1. Cc thng tin c t chng trnh PSI l cc bng PAT, PMT, CAT, NIT c minh ho trong hnh 2.17.

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PAT ( Bng phn phi chng trnh ) Chng trnh 0 17Chng trnh 1 Chng trnh 2

( PID

22 33 .... 55 ....

........Chng trnh K

S liu mng ring

.........

NIT ( Bng thng tin mng )

Dng 1 Dng 2 Dng 3

Video Audio Audio

54 48 49 .... 55 ....

....Dng K

....Data

....

....

PMT ( Bng bn chng trnh ) Dng 1 Video 16 Dng 2 Audio 81Dng 3 Audio

CATS liu truy cp c iu kin

....Dng K

....Data

82 .... 88 ....

....

....

Chng trnh 1 Audio 1

Chng trnh 2 Video

Chng trnh 1 PMT

Chng trnh 2 Audio 1

Chng trnh 1 Video

PAT

EMM

PID 48

PID 16

PID 22

PID 81

PID 54

PID 0

PID 1

Hnh 2.17 V d v PSI

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2.5.5. c im ca dng MPEG-2 u im ca dng truyn ti: Cp pht dung lng ng: Cc gi TS vi di c nh to kh nng linh hot trong vic cp pht dung lng knh gia cc dng s cp video, audio cng nh cc s liu ph khc. Mi gi TS c nhn dng bi mt s PID (Packet Identification = nhn dng gi ) c trong TS header. Dung lng ton b ca knh cng c th c cp pht li khi phn phi s liu. Kh nng ny cng c th c ng dng phn phi cc kho m trong tng giy n ngi xem, trong cc chng trnh truyn hnh c thu tin. Kh nng co gin: Mt knh di thng rng hn c th c khai thc ti a bng cch s dng nhiu dng s cp ES ti u vo b ghp knh hoc ghp knh thm cc dng s cp ES ny vo dng bit ban u ti mt b ghp knh th 2. Tnh cht ny c gi tr khi phn phi trn mng cng nh cung cp kh nng lin vn hnh (Inter-operation). Kh nng m rng: Cu trc dng truyn ti cho php m rng c kh nng phc v cc dch v tng lai. Cc dng bit s cp mi c th ghp thm vo dng truyn ti m khng cn thay i cu to phn cng pha pht m ch cn gn thm vo cc PID mi. Kh nng tng hp ngc vn c bo m, ngha l b gii m h thng hin nay vn gii m c dng truyn ti tng lai nhng b qua cc gi tng ng vi cc PID mi Kh nng chng li sai v ng b: Cc gi TS vi di khng i to c s cho vic kim sot li gy ra bi ng truyn v vic khi phc li ng b gia cc dng s cp video, audio ang ghp knh vi nhau (da vo cc thng tin cn thit ghi trong Header ). Thit b thu r tin: Cc gi TS c di khng i cng s cho php ch to b gii m h thng n gin r tin. Tnh linh hot ca dng truyn ti Cc chng trnh c th c nh ngha nh l bt k s kt hp no ca cc dng s cp. Mt dng s cp c th xut hin trong mt hoc nhiu chng trnh khc nhau (th d nh hai dng video khc nhau c th cng kt hp vi mt dng audio to ra hai chng trnh khc nhau). Cc chng trnh cng c th c sa i ph hp vi mt s yu cu c bit (th d nh cng mt chng trnh truyn hnh nhng phn audio li c thay i theo tng vng ngn ng khc nhau) Nhiu chng trnh khc nhau c th ghp knh trong cng mt dng truyn ti, ti pha thu (pha gii m ) tng chng trnh ring s c tch ra rt d dng.

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2.5.6. ng dng dng truyn ti MPEG-2 TS Dng truyn ti MPEG-2 c th c truyn trn tt c h thng thng tin v h thng truyn ti MPEG-2 cng c th truyn cc dng bit c to ra bi cc h thng thng tin khc. Mt cch tng qut, khng c g ngn tr vic truyn cc dng truyn ti MPEG-2 truyn hnh trn cc h thng truyn dn khc nhau. Tuy nhin, trong mt s h thng nh truyn hnh cp, v tinh, ATM, dng truyn ti MPEG-2 c truyn d dng hn, thun tin hn so vi mt s h thng khc (v d nh mng my tnh s dng Protocol FDDI, IEE 802,6). C th s dng dng truyn ti MPEG-2 truyn hnh ni ni b trong studio cng nh truyn ti cc thit b, cc h thng khc thng qua mt s phng php sau: Phng php truyn s liu ni tip SDI (Serial Digital Interface) theo khuyn ngh 656. Phng php truyn s liu gi ni tip SDDI (Serial Digital Interface). Phng php truyn SDH/ SONET (Synchronous Digital Hierachy / Synchronous Optical Network) Phng php truyn s liu gi ATM (Asynchronous Tranfer Mode). Mt kha cnh lin vn hnh khc ca dng truyn ti hnh l kh nng truyn cc dng bit khc (khng phi l bit truyn hnh) trn cc h thng truyn hnh s da theo tiu chun MPEG-2. iu ny thc hin c khi cc dng bit ny bao gm cc gi TS nhn dng c. Tuy nhin cng cn phi x l thm i cht mc h thng. c im ny cho php ta xy dng mt ng truyn dn a dch v dng v tinh ph sng ton quc rt tin li. Kt lun Dng truyn ti MPEG -2 c th kt hp vi nhiu dch v khc nhau v c th truyn dn trong nhiu mi trng truyn dn khc nhau. Vic s dng dng truyn ti ny i vi truyn hnh cn phi nghin cu truyn dn trong 3 mi trng ch yu l truyn hnh s mt t, truyn hnh qua v tinh v h thng truyn hnh cp. Khi s dng i vi h thng nhiu chng trnh nn s dng h thng ghp knh thng k.

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CHNG 3 TRUYN HNH S MT T DVB-T3.1. C IM CHUNG CA TRUYN HNH S MT T So vi cc phng thc truyn dn khc, phng thc truyn dn truyn hnh s mt t c nhng c im nh: Cht lng ng truyn gim do hin tng phn x nhiu ng (multipath) do b mt mt t cng nh cc to nh cao tng. Truyn dn tn hiu trong mi trung tp m cao do con ngi to ra. Giao thoa gia h thng truyn hnh tng t v truyn hnh s do phn b tn s kh dy trong bng tn phn b cho truyn hnh mt t. Tuy vy truyn hnh s c nhng im thun li. Truyn dn c knh truyn hnh c phn gii cao HDTV hay nhiu chng trnh truyn hnh tiu chun. Cc my thu c th t trong nh, xch tay hay thu lu ng. C ba tiu chun truyn hnh s mt t l 8VSB pht trin bi ATSC (M), DVB - T pht trin bi ETSI (Chu u) v ISDB - T (Nht Bn). Hin nay Vit Nam chn chun DVB, do trong bn lun vn ny ch cp n DVB-T. H thng truyn hnh s mt t c khi qut nh hnh 3.1 Qu trnh truyn dn tn hiu truyn hnh s mt t bao gm cc khi chnh nh sau: Khi m ho ngun (Soure coding): y l khi nn tn hiu Video v Audio nhm loi b cc thng tin d tha. y chnh l khi m ho tn hiu (Encoder) theo tiu chun MPEG 2 i vi tn hiu truyn hnh s. Khi ghp knh(Multiplexer): C nhim v ghp knh Video nn, Audio nn, Data, cc thng tin c t ca mt hay nhiu chng trnh truyn hnh to thnh dng truyn ti MPEG 2. Khi M ho ng truyn v iu ch (channel coding & modulation): c nhim v m ho sa li ng truyn. iu ch truyn dn tn hiu. Sau s n khi i tn, khuch i cng sut v ghp a ra anten pht sng ti my thu.

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Page 43 of 116Service Multiplex and Transport RF/Transmission System

Video Subsystem Video Source Coding and Compression

Video

TransportAudio Subsystem Audio

Channel Coding

Audio

Source Coding and Compression

Service Multiplex

Modulation Ancillary Data

Control Data

Receiver Characteristics

Hnh 3.1: Cu trc h thng truyn hnh s mt t

3.2. TRUYN HNH S MT T DVB-T T nm 1995 cc nc chu u tin hnh th nghim pht sng truyn hnh s mt t (DTTB). n thng 2 nm 1997 tiu chun truyn hnh s mt t DVB-T chnh thc c cng nhn v ban hnh bi T chc ETSI (European Telecommunications Standards Institute). Tiu chun DVB-T thng c vit l DVB-T-ETS 300-744. DVB-T da trn phng php ghp knh theo tn s trc giao c m ho COFDM (Coded Orthogonal Frequency Division Multiplexing). K thut COFDM thc cht l iu ch OFDM kt hp vi k thut m ho knh truyn (channel Coding). H thng DVB-T c thit k da trn tng: - a ra m hnh h thng pht sng truyn hnh s mt t. - Xc nh cc yu cu v cht lng tn hiu v kh nng tng thch cho cc loi dch v khc nhau. - Quan tm ti vn x l tn hiu my thu m rng cc gii php thc hin khc nhau. - Nhm vo mc ch chng phn x nhiu ng, chng pha inh la chn tn s, chng hiu ng Doppler. - To kh nng thu i ng. - To kh nng thit lp mng n tn.Nguyn Nh Nht Lun vn thc s

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- To kh nng kt hp vi truyn thng a phng tin. - Kh nng m rng cho d n Mobile Multimedia- a phng tin di ng. 3.2.1. c im tiu chun DVB-T Tiu chun DVB -T khc vi tiu chun 8-VSB (h M), DVB-T c 2 ch khc nhau l mode 2k hoc 8k-OFDM. Trong mode 2k c 1705 sng mang v trong 8k mode c 6817 sng mang ghp knh theo tn s trc giao. Trong mi mode c th truyn ti c ti a 2 cp khc nhau trong khu truyn dn dng truyn ti MPEG-2. i vi ch khng phn cp, cc chng trnh khng c s u tin trong khu truyn dn tn hiu do vy ch c cc vng c cng trng t gi tr ln hn mc ngng mi c th thu c v ch c mt gi tr. i vi cc my pht hot ng ch c phn cp, khi c cc mc ngng khc nhau. Mc ngng 1 cho php thu c ton b chng trnh c chng trnh u tin v khng u tin. Mc ngng 2 ch cho php thu c chng trnh c u tin trong khu truyn dn. Vic u tin trong khu truyn dn bao gm 2 phn, u tin v phn m ho ng truyn v u tin trong khu iu ch tn hiu. Ch phn cp ti iu ch ch thc hin c khi iu ch 16 QAM hay 64 QAM. iu ch khng phn cp : (a = 1). iu ch phn cp : a = 1; a = 2; a = 4 iu ch sng mang : QPSK, 16QAM, 64QAM. Khong bo v : 1/4, 1/8, 1/16, 1/32. T l m xon : 1/2; 2/3; 3/4; 5/6/ 7/8. M khi : RS204/188. Tn hiu vo: : Dng truyn ti MPEG-2. Tc d liu vo : 4,9831,67Mb/s. rng knh RF : 6, 7, 8MHz. u im ca tiu chun DVB-T C kh nng tng thch vi cc loi hnh dch v DVB-C, DVB-S, ISDN v INTERNET. C th hot ng c cc knh RF : 6, 7, 8 MHz, do vy c th s dng vi bng tn 8 MHz m truyn hnh mt t Vit Nam hin nay ang s dng. C th pht c nhiu tc bit t 4,9831,67Mb/s tu theo vic la chn m ng truyn, khong bo v v phng php iu ch cho mi sng mang. S dng iu ch phn cp trong khu pht sng, cho php ng thi cng lc c th truyn c 2 chng trnh vi cht lng khc nhau C th pht c 45 chng trnh SDTV hoc mt chng trnh HDTV cng cc d liu ph.Nguyn Nh Nht Lun vn thc s

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C th s dng m thanh MPEG-2 Layer II hoc Dolby. C th s dng m thanh MPEG-2 Layer II hoc Dolby AC3. Kh nng chng can nhiu cng knh v knh ln cn tt. C th pht sng cng knh - mng n tn SFN. C th s dng cc knh ln cn. Kh nng chng phn x a hng, pha inh la chn v hiu ng Doppler rt tt. C kh nng thit lp my pht thu di ng. H thng cn nhiu kh nng m rng cho cc d n trong tng lai nh a phng tin di ng Mobile multimedia. Nhc im tiu chun DVB-T Ngng thu cao hn so vi tiu chun ATSC. T s C/N phi t 19,1dB ti tc bit l 19,35Mb/s cao hn so vi tiu chun ATSC (15dB) l 4dB. Do hiu sut ph sng ca tiu chun DVB-T nh hn ATSC 4dB. Cc sng mang c cng sut thp nn d b nh hng ca pha inh la chn tn s. khc phc hin tng ny my thu DVB-T phi s dng thm k thut cn bng thch nghi, tnh ton knh v sa li. Gi thnh my thu do cng tng ln. T s cng sut nh trn cng sut trung bnh c 10dB cao hn ATSC. Ph thuc vo kiu iu ch v sc xut thi gian h s nh ca tn hiu COFDM nm trong di t 915dB. y cng chnh l tht li cng sut u ra ca my pht COFDM. Cu hnh ton b h thng phc tp v t tin. 3.2.2. S khi H thng pht c th chia lm 2 phn chnh l b EXCITER DVB-T v khuch i cng sut vi phi - anten. Nhng phn khuch i cng sut vi phi v anten l hon ton ging trong cc my pht hnh cng ngh tng t nn y khng cp n na. B EXCITER DVB-T li c th chia lm 2 phn c bn l m ho knh truyn v iu ch OFDM. Theo s khi th tn hiu vo l dng truyn ti MPEG -2 c a ti khi thch ng ghp knh dng truyn ti MPEG-2 v phn tn nng lng (Mux Adaptation Energy Dispersal), sau i qua khi m ngoi (Outer coder) n khi xo trn ngoi (Outer Interleaver), khi m trong (Inner Coder), khi xo trn trong (Inner Interleaver), ri n khi iu ch OFDM.

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Programme

MUXTransport

VideoCoder Audio Coder1

MUX

Splitter

Data

nMPEG-2 Source Coding and Multiplexing

To AerialMux Adaptation Energy Dispersal

Outer Coder

Outer Interleaver

Inner Coder

Inner Interleaver

Mapper

Frame Adaptation

OFDM

Guard Interval Insertion

D/A

Front end

Mux Adaptation Energy Dispersal

Outer Coder

Outer Interleaver

Inner Coder

Pilot &TPS Signals

Terrestrial Channel Adapter

Hnh 3.2 S khi my pht DVB-T

Phn iu ch OFDM bao gm khi iu ch tn hiu (Mapper), khi thch ng khung OFDM (Frame Adaptation), khi bin i IFFT v iu ch s I/Q, khi chn khong bo v (Guard interval insertion), khi bin i D/A. Sau tn hiu iu ch OFDM c a n b i tn BB/IF, b i tn IF/RF ri n tng khuych i cng sut, lc hi ra h thng phi - anten. u vo my pht DVB-T l dng truyn ti MPEG-2 a chng trnh c tc bit t 4,98Mb/s31,67Mb/s . Khi hot ng ch c phn cp dng truyn ti c phn chia thnh 2 dng truyn ti, dng c u tin cao (tc thp) v u tin thp (tc cao) cp vo my pht.Nguyn Nh Nht Lun vn thc s

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3.2.2.1. Phn m ho knh truyn Khi Mux Adaptation Energy Dispersal - Thch ng ghp knh v phn tn nng lng. Dng MPEG-2 TS a chng trnh cn phi c thch ng vi c tnh v ch lm vic ca my pht. Do khi u tin l module SFN v thch ng tc bit, ti y c th chn ch lm vic ca my pht l a tn (MFN) hay n tn (SFN), ng thi c nhim v xc nh gi ca dng truyn ti MPEG-2. Sau khi chn ch lm vic v cc thng s cn thit, dng d liu vi tc chn c a vo mch ngu nhin ho. di gi tng cng ca gi ghp knh truyn ti MPEG-2 l 188bytes v bao gm 1 byte cho t m ng b (47 HEX). Th t x l ti u pht lun bt u t bit c gi tr ln nht MSB (bit 0) ca byte ng b (0100 0111). Byte ng b c tch ra nh l chun nh thi gian to xung nhp (clock). m bo s chuyn dch nng lng hp l dng d liu MPEG-2 cn c ngu nhin ho tri ph gim chnh lch gia cc bit 1 v bit 0 trnh tp trung nng lng ti mt tn s no . Nguyn l ngu nhin ho l mch cng modul 2 (EXOR) ca chui d liu vo vi chui nh phn gi ngu nhin PRBS (Pseudo Random Binary Sequence). S mch ngu nhin ho, hnh 3.3.

Hnh 3.3 S b m ho ngu nhin

Biu thc ton ca chui gi ngu nhin ho PRBS l : 1 + x14 + x15 Qu trnh np chui khi ng 100 101 010 000 000 vo cc thanh ghi dch nh sau

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Khi nhn tn hiu byte ng b cc gi tr ca thanh ghi dch s c np gi tr 100101010000000 v vic to tn hiu ngu nhin c thc hin vi chu k l 8 gi dng truyn ti MPEG - 2. Khi nhn dng c byte ng b ca gi tip theo, cc thanh ghi dch li c np li gi tr trn v thc hin mt chu k mi. Trong qu trnh phn tn nng lng vi chu k tn hiu gi ngu nhin l 8 gi dng truyn ti, ch c byte ng b gi u tin l khng b thay i cn 7 byte ng b ca 7 gi tip theo u b bin i do tc ng ca tn hiu gi ngu nhin. Cc byte ng b khng b nh hng k hiu l: SYNC Cc byte ng b b nh hng k hiu l: SYNC Ti phn thu tn hiu gi ngu nhin c to ra t mt mch hon ton ging vi phn pht. ng b gia pht v phn thu, khi ta thu v nhn dng c byte ng b gi (SYNC - byte ng b khng b bin i bi tn hiu gi ngu nhin) cc gi tr ca thanh ghi dch s c np gi tr 100101010000000 v vic to tn hiu ngu nhin c thc hin vi chu k l 8 gi. Khi kt thc 8 gi th cc gi tr ca thanh ghi l c Reset li. Trong qa trnh ngu nhin gi tr cc byte ng b gi b thay i nhng c th xc nh c ti pha thu gi tr ny thng qua gi tr byte ng b v v tr ca n trong chu k chui gi ngu nhin. h tr cc chc nng ng b ho th trong khong thi gian ca cc bytes ng b ca 7 gi truyn ti tip theo, chui PRBS vn c pht nhng u ra khng c chui bit ra, do vy cc bytes ny khng b ngu nhin ho. Qu trnh ngu nhin ho vn c tip din ngay c khi khng c d liu vo, hoc khi n khng tng thch vi nh dng dng truyn ti MPEG-2 (v d: 1 byte ng b + 187 bytes gi d liu). Khi Outer coder - M ho ngoi. M ho ngoi c thc hin trn cu trc ca gi truyn ti. Cc gi u vo ca dng truyn ti l cc gi c di c nh 188 byte trong c 1 byte ng b. Sau khi i qua khi m ho ngu nhin, gi tr cc byte ng b b thay i, vic xc nh v tr gi c thng qua mt ng h nhp cho ton b h thng. M ngoi trong tiu chun truyn hnh s DVB -T l m Reed-Solomon RS (204, 188, t=8). M Reed-Solomon RS (204, 188, t=8) l m ngn c suy ra t m khi h thng gc RS (255, 239, t=8). c p dng cho mi gi truyn ti c ngu nhin ho (188 bytes) kt hp vi 16 byte tng suy to ra gi chng li c di l 204 byte cho php sa c 8 bytes li ngu nhin trong 204 bytes thu c. a thc to m : g(x) = (x+l0) (x+l1)... (x+l15) vi l = 2 HEX. a thc to trng : P(x) = x8 + x4 + x3 + x2 + 1.

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M Reed-Solomon lm ngn c th c thc hin bng cch thm 51 bytes rng (tt c thit lp v 0) trc cc bytes thng tin u vo b m ho RS (255, 239, t=8). Sau khi m ho RS cc bytes rng ny s b loi b to thnh t m RS vi di l 204 bytes. Khi Outer interleaver - Xo trn ngoi Cng nh cc phong php truyn dn tn hiu s khc, tng cng hiu qu ca phng php m khi v phn b li ng u trn cc gi, tiu chun DVB-T cng thc hin bin php xo trn byte sau khi m khi. Sau y l b xo trn byte theo tiu chun DVB-T vi su I=12 nhnh v p dng cho cc gi chng li. Phng php Interleaver l phng php m ho kt hp vi phng php RS 204,188 khc phc nhc im khng sa c cc li chm trong cng mt gi. Khi c li chm xy ra, cht lng ng truyn suy gim t ngt do iu kin ng truyn dn thay i trong mt khong thi gian rt ngn, khi c th xy ra hin tng li chm trn mt gi bt k. Nu gp hin tng li chm th phng php m sa sai RS 204,188 khng th khc phc c v thng tin trn gi s dn ti s sai lch trong qu trnh gii m li tn hiu. khc phc hin tng , b xo trn (Interleaver) c nhim v lm xo trn cc byte trong cc gi khc nhau vi nhau theo mt quy lut nht nh sao cho cc byte lin nhau thuc cc gi khc. Ti pha thu, vic xo trn c lm ngc li so vi phn pht, khi c li chm xy ra trn ng truyn th cc li phn u trn cc gi m khng tp trung ti mt gi, nh m khi ng truyn b li chm vn c th khc phc c trong mt gii hn nht nh no . Vic xo trn tn hiu c thc hin thng qua vic i ch cc byte trong cc gi khc nhau qua 12 nhnh, cc nhnh c cu trc l cc thanh ghi dch (FIFOvo trc ra trc) v c di l jxM (cell). j: ch s nhnh thanh ghi dch M: di thanh ghi nh nht (M = N/I =17). N: di cc gi (204). I : tng s nhnh (12). Mi (cell) thanh ghi dch gm mt byte (8 bit). S hnh3.4 ch r s hot ng ca khi Outer Interleaver.

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Hnh 3.4 Khi Outer interleaver v khi Outer Deinterleaver

Khi nhn ra tn hiu byte ng b gi (SYNC hay SYNC), khi bt u thc hin vic xo trn cc byte gia cc gi vi nhau. Trong nhp u tin, byte ng b khng b tr c i qua nhnh 0. Sau nhp th hai bt u, byte th 2 (byte tip sau byte ng b) c np vo u tin ca nhnh 1 ng thi c s liu ra ti cui cng ca nhnh 1. tr ca byte c ra (khong thi gian t lc vo nhnh n lc ra khi nhnh) c xc nh nh sau: T1 = chu k np * di nhnh 1. T1 = I*M = 12*17 = 204 (nhp). Khi nhp th 3 bt u th byte tip theo c np vo nhnh 2 ng thi c ra byte v tr cui cng nhnh 2 v c nh vy cho n ht nhnh 11 khi Interleaver s tr v nhnh 0 v tip tc cc chu k mi. tr ca cc byte khi i qua nhnh j c xc nh nh sau: Tj = I * M * j Tj = 12 *17 * j tr ln nht l: Tmax = 12 *17 * jmax Tmax = 12 *17 * 11 = 204 * 11. Khi ra khi khi Interleaver c dng nh sau:SYNC 1 SYNC n 203 Bytes SYNC 1 SYNC n 203 Bytes SYNC 1 SYNC n 203 Bytes

S khc bit ca tn hiu ra so vi tn hiu vo l s liu trong mi gi u ra s l s liu ca 11 gi khc nhau u vo. Cc byte ng b gi khng b thay i v tr. Khi c li chm xy ra ti 1 gi trn ng truyn th cc li s phnNguyn Nh Nht Lun vn thc s

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chia trn 11 gi khc nhau khi c khi phc li ti pha thu trc khi qua b gii m ngoi (Outer Decoder), v do lm tng kh nng sa li ca phng php RS 204,188. pha thu, tn hiu s c xo trn ngc li nh hnh v, cc byte phn pht c tr t s c tr nhiu ln v ngc li sao cho tng tr ca c phn thu v pht ca tt c cc byte l 17*12*11 = 2244 nhp v khi ra khi khi De Interleaver cc byte c th t nh trc khi vo khi Interleaver. B xo trn ngoi (outes interleaver) c t hp t 12 nhnh v c kt ni thnh vng khp kn vi lung byte u vo nh mt chuyn mch u vo. Mi nhnh J l mt thanh ghi dch FIF0 vi chiu di l JxM (cell) (M=17=N/I; N = 204bytes). Cc (cell) ca thanh ghi dch FIF0 s cha 1 byte v cc chuyn mch u vo v ra phi ng b vi nhau. Vi mc ch m bo ng b ho, cc bytes ng b gi ( k c cc byte ng b b m ho ngu nhin) s lun c dn ti nhnh 0 ca b xo trn (tng ng vi khng c tr). Ti my thu, b gii xo trn ngoi cng c nguyn l tng t nh b xo trn, nhng cc ch s ca nhnh l ngc li, nhnh J=0 tr t nht phn pht s tr nhiu nht, nhnh J = 17 s khng b tr m bo cc byte d liu s ng u tr khi i qua c phn pht v thu. Vn ng b ho cho b gii xo trn c thc hin bng cch a byte ng b (SYNC) vo nhnh 0. Nhng cn phi bit rng cc byte ng b c cc gi tr khc nhau do b m ho ngu nhin vi chu k 8 gi ti phn pht. M trong (Inner Coding) H thng DVB-T s dng m xon (Convolutional code) da trn m xon gc t l 1/2 vi 64 trng thi. N cho php la chn mc sa li thch hp nht cho mt dch v nht nh hoc mt tc s liu ch truyn dn phn cp v khng phn cp . a thc sinh ca b m ca m gc l G1=171 oct cho u ra x v G2=133 oct cho u ra y. Nu s dng phng thc truyn dn phn cp 2 mc th mi mt trong 2 b m ho knh song song c th c t l m ho ring. Ngoi t l 1/2 ca m gc h thng cn cung cp cc t l m ho khc nh 2/3; 3/4; 5/6 v 7/8. Code Rates Puncturing Transmitted sequence pattern (after parallel-to-serialconversion) 1/2 X:1 X1 Y1 Y:1 2/3 X:10 X1 Y1 Y2 Y:11Nguyn Nh Nht Lun vn thc s

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3/4 5/6 7/8

X:101 Y:110 X:10101 Y:11010 X:1000101 Y:1111010

X1 Y1 Y2 X3 X1 Y1 Y2 X3 Y4 X5 X1 Y1 Y2 Y3 Y4 X5 Y6 X7

Khi inner interleaving -Xo trn trong B xo trn trong bao gm b xo trn bit v tip theo l b xo trn symbol. C 2 loi xo trn ny u da trn c s khi (block). Xo trn bit (Bit interleaving). u vo gm ti a 2 dng bit c tch thnh 2, 4, 6 lung con tu theo cp v kiu iu ch. ch khng phn cp, lung bit n c tch thnh v lung con vi vi v=2 cho QPSK, v=4 cho 16-QAM v v= 6 cho 64-QAM. ch phn cp, c 2 dng bit vo gi l dng u tin cao (HP) v u tin thp (LP) c tch thnh 2 lung con cho iu ch 16-QAM. Cn vi iu ch 64-QAM th lung HP c tch thnh 2 lung con, dng LP c phn thnh 4 lung con. iu ny p dng cho c 2 kiu iu ch QAM ng nht (uniform) v khng ng nht (non-uniform). Vic tch phn lung c xem nh l mt s sp xp cc bit u vo Xdi thnh cc bit u ra be,do. Vi kiu khng phn cp: x di = bdi (mod) v, di (div) v Vi kiu phn cp: x'di = bdi (mod) 2, di (div) 2 (Hight - Priority) x''di = b[di (mod) (v-2)] + 2, di (div)(v-2) (Low Priorty)Trong : xdi : dng bit u vo b phn knh kiu khng phn cp. x'di : dng bit vo b phn knh kiu phn cp vi u tin cao.

x''di : dng bit u b phn knh kiu phn cp vi u tin thp. di : s bit u vo. be,do: u ra b phn knh. e : s dng bit c phn knh (0 e < V-1). d 0 : s bit ca dng bit vo u ra b phn knh Mi lung con t b phn knh c x l nh mt b xo trn bit ring bit. Ph thuc vo s lung v c ti 6 b xo trn c gn nhn t Io ti I5. Io v I1 s dng cho iu ch QSPK, IoI3 cho 16-QAM v IoI5 cho 64-QAM.Nguyn Nh Nht Lun vn thc s

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Xo trn bit ch c thc hin trn s liu c ch, kch thc khi (block) l nh nhau i vi mi b xo trn, nhng th t xo trn l khc nhau trong mi trng hp. Kch c ca khi xo trn l 126 bit. Qu trnh xo trn khi c lp li mt cch chnh xc 12 ln trong 1 symbol OFDM cho d liu c ch ch 2k v 48 ln ch 8k. Vi mi b xo trn bit vc t bit u vo c xc nh nh sau: B(e) = (be,0 , be,1 , be,2 ... be,125) Trong e thay i t 0 n v-1. Vc t u ra c xc nh nh sau: A(e) = (ae,0 ; ae,1 ; ae,2 ... ae,125).b0,0; b0,1 x'0; x1;x2 DEMUX b1,0; b1,1 a1,0; a1,1 Symbol Interleaver a2,0; a2,1 Y 0; Y 1 Mapping

Bit Interleaver I0

a0,0; a0,1 Re (z) convey y0,0 ; y2,0

Bit Interleaver I1

b2,0; b2,1 x0 ; x1;x2 DEMUX b3,0; b3,1

Bit Interleaver I2

Im (z) convey16QAM y1,0; y3,0 a3,0; a3,1

Bit Interleaver I3

x0; x1;x2

DEMUX

b0,0; b0,1

Bit Interleaver I0

a0,0; a0,1

b1,0; b1,1

Bit Interleaver I1

a 1,0; a1,1

Re (z) convey y0,0 ; y2,0; y4,0

b2,0; b2,1

Bit Interleaver I2

a2,0; a2,1

Symbol Interleaver Y 0; Y 1

Mapping

b3,0; b3,1 x0 ; x1;x2 DEMUX

Bit Interleaver I3

a3,0; a3,1

64QAM

Im (z) convey y1,0 ; y3,0; y5,0

b4,0; b4,1

Bit Interleaver I4

a 4,0; a4,1

b5,0; b5,1

Bit Interleaver I5

a5,0; a5,1

Hnh 3.5: S khi khi xo trn trong h thng c phn cp.

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Trong ae,W = be , He(w) y w = 0,1,2....125 vi He(w) l hm hon v v l khc nhau vi mi b xo trn. He(w) c xc nh nh sau: I0 : H0(w) = w I1 : H1(w) = (w+63) mod 126 I2 : H2 (w) = (w+105) mod 126 I3 : H3 (w) = (w+42) mod 126 I4 : H4 (w) = (w+21) mod 126 I5 : H5 (w) = (w+84) mod 126

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b0,0 ; b0,1 x0; x1;x2 Bit Interleaver I0 DEMUX b1,0 ; b1,1 Bit Interleaver I1

a0,0; a0,1 Symbol Interleaver Mapping Y0; Y1

Re (z) convey y0,0

a1,0; a1,1

QPSK

Im (z) convey y1,0

b0,0; b0,1 b1,0; b1,1 x0; x1;x2 DEMUX b2,0; b2,1 b3,0; b3,1

Bit Interleaver I0 Bit Interleaver I1 Bit Interleaver I2 Bit Interleaver I3

a0,0; a0,1 Re (z) convey y0,0 ; y2,0 a1,0; a1,1 Symbol Interleaver Mapping Y0; Y1 Im (z) convey y1,0; y3,0

a2,0; a2,1

a3,0; a3,1

16QAM

b0,0; b0,1 b1,0; b1,1 b2,0; b2,1 b3,0; b3,1 b4,0; b4,1 b5,0; b5,1

Bit Interleaver I0

a0,0 ; a0,1

Bit Interleaver I1

a1,0 ; a1,1

Re (z) convey y0,0 ; y2,0 ; y4,0

x0; x1;x2

Bit Interleaver I2

DEMUX

a2,0; a2,1 a3,0 ; a3,1

Symbol Interleaver

Mapping Y0; Y1 Im (z) convey y1,0; y3,0; y5,0

Bit Interleaver I3

64QAM Bit Interleaver I4 a4,0 ; a4,1

Bit Interleaver I5

a5,0; a5,1

Hnh 3.6: S khi khi xo trn trong h thng khng phn cp.

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Cc u ra ca cc b xo trn v bit c nhm thnh cc symbol d liu s. Mi symbol ca v bit s gm chnh xc 1 bit t mi b xo trn. Do u ra t cc b xo trn l 1 t m v bit c a ti u ra. I0 l bit c ngha nht, tc l : y(w) = (a0,W , a1,W ... , av-1 ,w). Xo trn symbol (symbol interleaving). Mc ch ca vic xo trn symbol l nh v cc t m v bit vo 1512 sng mang (ch 2k) hay 6048 sng mang ( mode8k) cho 1 symbol OFDM. B xo trn symbol lm vic trn cc khi gm 1512 (2k) hay 6048 (8k) symbol d liu (t m v bit). Do vy, ch 2k, 12 nhm gm 126 t m s liu (mi nhm) t b xo trn bit c c tun t vo 1 vc t: Y= y0 , y1 .... y1511 ). Tng t nh vy, ch 8k, 48 nhm 126 t s liu c c tun t vo vc t Y= (y1 , y1... y6047 ) sau b xo trn symbol to nn vc t xo trn: Y = (y0 , y1 .... ymax-1 ) Trong : yH(q)= yq cho cc symbol chn vi q = 0,1... , Nmax-1. yq = yH(q) cho cc symbol l vi q = 0,1... , Nmax-1. y: N max = 1512 ch 2k Nmax= 6048 ch 8k Ch s ca symbol chn, l xc nh v tr ca symbol OFDM hin hnh trong khung OFDM. H(q) l hm hon v v c xc nh nh sau: Vi q = 0 Vi (i=0, i< Nmax, i=i+1); H(q) =( i mod 2).2Nr-1+ Ri( j ).2 jj =0 Nr -2

Nu H(q) < Nmax th q = q + 1 Xc nh Ri: Mt t nh phn Ri gm (Nr - 2) bit c nh ngha vi Nr = log2Mmax Mmax = 2048 2k mode Nr= log22048 = 11 Mmax = 8192 8k mode Nr= log28492 = 13Ri ly cc gi tr nh sau:

i = 0,1 : Ri [Nr-2, Nr-3, , 1, 0] = 0, 0, , 0, 0 i = 2 : Ri [Nr-2, Nr-3, , 1, 0] = 0, 0, , 0, 1 2 < i < N max{Ri [Nr-3, Nr-4, , 1, 0] = Ri [Nr-2, Nr-3, , 2, 1]; 2k mode: Ri [9] = Ri-1 [0] # # # Ri-1 [3] 8k mode: Ri [11] = Ri-1 [0] # # # Ri-1 [1] # # # Ri-1 [4] # # # Ri-1 [6]}

Vect Ri nhn c t vect Ri nh hon v bit theo bng sau ch 2KNguyn Nh Nht Lun vn thc s

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Ri bit positions (j)Ri bit (rule[j]) positions

9 0

8 7

7 5

6 1

5 8

4 2

3 6

2 9

1 3

0 4

ch 8K.11 Ri bit positions (j) Ri bit positions 5 (rule[j]) 10 11 9 3 8 0 7 10 6 8 5 6 4 9 3 2 2 4 1 1 0 7

S khi ca thut ton dng to hm hon v Hnh 3.7 (2k mode) v Hnh 3.8 (8k mode).

ToggleRControl Unit R

Wires Permutation

MSBSkipAddress Check

10 11

H(q)Hnh 3.7 : S to a ch xo trn symbol ch 2K.

ToggleRControl unit

Wires Permutation MS SkipAdress Check

R

12 13 H(q)

Hnh 3.8: S to a ch xo trn symbol ch 8K.

Tng t nh y, y gm V bit: yq = (y0,q, y1,q, , yv-1,q)Nguyn Nh Nht Lun vn thc s

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Vi q l ch s symbol iu ch u ra b trn symbol. Cc gi tr ny ca y c s dng nh v s liu vo cc chm sao tn hiu nh b Mapper. 3.2.2.2. Phn iu ch Khi iu ch QAM (Mapper) H thng DVB-T s dng phng thc truyn dn ghp knh theo tn s trc giao (OFDM). Tt c cc sng mang s liu trong 1 khung OFDM c th c iu ch QPSK, 16-QAM v 64-QAM. Cc kiu iu ch 16-QAM v 64-QAM c th l ng nht (a=1) hoc khng ng nht (a=2 ; a=4). Qu trnh nh v cc t hp bit yq vo cc sng mang d liu c ch s dng php nh x Gray. Mi t hp bit cnh nhau ch khc nhau 1 bit. C 3 kiu iu ch QPSK, 16-QAM v 64-QAM u s dng nh x Gray. iu ch khng phn cp: Trong trng hp iu ch khng phn cp ch s dng 1 dng d liu a chng trnh. Dng s liu ti u ra ca b xo trn symbol gm cc t m v bit a vo b nh v (Mapper) tn hiu. Biu cc chm sao tn hiu l tp hp cc s phc cho 3 kiu iu ch QPSK, 16-QAM v 64-QAM ng nht (uniform) vi a=1

64QAM 16QAM 4QAM00

000010

0010

iu ch phn cp:Hnh 3.9: Biu chm sao tn hiu c bn ca DVB-T khng phn cp

Trong ch iu ch c phn cp cn s dng 2 dng d liu c tc khc nhau gi l dng u tin cao HP v dng u tin thp LP. Dng HP c bo v bi m xon mnh hn (v d t l 1/2). Cn dng LP c bo v bi m xon yu hn (v d t l 5/6), c 2 dng HP v LP c iu ch ng thi theo kiu 16QAM. Cc bit u tin cao trong cc im tn hiu cnh nhau cng c u tin bng khong cch thng qua h s a.Nguyn Nh Nht Lun vn thc s

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Vi iu ch 16-QAM phn cp: Cc bit u tin cao l cc bit yo,q v y1,q ca cc t m u ra ca b xo trn symbol. Cc bit u tin thp l cc bit y2,q v y3,q ca cc t m u ra ca b xo trn symbol. Nh vy mi t hp 4 bit c tch thnh 2 cp 2 bit, Ni cch khc, mi mt im tn hiu iu ch bao gm thng tin ca c 2 dng HP v LP kt hp li.

HP 10

LP

HP10

LP

01

1101

4QAM over 4QAM

16QAM over 4QAM

10 1101 1001

Hnh 3.10: Biu chm sao tn hiu DVB-T phn cp.

3.2.3. Cu trc khung OFDM. Tn hiu COFDM truyn i c t chc thnh cc khung mi khung c mt thi gian l Tf v gm 68 symbol OFDM, c 4 khung to nn 1 siu khung (super frame). Mi symbol OFDM c to nn bi tp hp k=6817 sng mang ch 8k, 1705 sng mang ch 2k v c truyn trong thi gian Ts = Tu + D. Trong : Tu - chu k hu ch ca symbol OFDM. D - khong bo v (Guard interval). Khong bo v D c chn vo trc phn Tu ca mi symbol v n l bi s ca chu k c bn T=7/64 hoc 8/64 tnh bng ms. Gi tr ca khong bo v c s dng nh l:Nguyn Nh Nht Lun vn thc s

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D = 1/4.Tu ; D= 1/8.Tu ; D= 1/16.Tu ; D= 1/32.Tu Cc symbol trong 1 khung OFDM c nh s t 0 n 67 tt c cc symols u cha s liu v cc thng tin bo hiu Mi khung OFDM cha cc thng tin bo hiu ngoi dng truyn ti MPEG-2 sau khi m ho ng truyn: - Cc sng mang pilots phn tn. - Cc sng mang pilots lin tc. - Cc sng mang TPS (thng bo thng s truyn dn). Cc sng mang pilots c s dng cho ng b khung, ng b tn s, ng b thi gian, nh gi knh, nhn dng ch truyn dn, v cng c th c s dng rt theo nhiu pha. Ti pha thu cn c vo cc sng mang pilot thu c s ni suy cc thng s v sai pha, tn s ca tn hiu trn ng truyn vic gii iu ch cc sng mang c chnh xc hn. Cc sng mang c nh du bng ch s k thuc bng tn [kminkmax], v c xc nh bng kmin =0 v kmax=1704 ch 2k v kmax=6816 ch 8k. Vi knh RF l 8MHz th rng hu ch l 7,61Mhz. S lng cc sng mang: mode 2k mode 8k Pilots phn tn 31 524 Pilots lin tc 45 177 Sng mang TPS 17 68 Sng mang d liu 1512 6048 Mt vi tham s chnh: Parameter 8k mode 2k mode S lng sng mang k 6817 1705 S hiu sng mang kmin 0 0 S hiu sng mang kmax 6816 1704 Thi gian hu ch Tu 896 ms 224 ms Khong cch gia cc sng mang 1/Tu 1116 Hz 4464 Hz Khong cch gia 2 sng mang kmin v 7,61 MHz 7,61 MHz kmax Tn hiu COFDM truyn i c m t bng phng trnh sau: 67 Kmax (t) Y J2P. fc.t . cm,l,k. m,l,k S(t) = Re e m=0Y

l=0

k=Kmin

m,l,k =

(t)

e

J2P k (t - D -l .Ts-68m.Ts) Tu vi (l+68.m)Ts = t = (l+68.m +1).Ts 0 vi t khc.Lun vn thc s

Nguyn Nh Nht

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Trong : k - Ch s ca sng mang. l - Ch s ca symbol OFDM. m - Ch s ca khung truyn dn. K - Tng s sng mang truyn dn. Ts - Chu k ca symbol OFDM. Tu - Chu k hu ch ca symbol OFDM. D - Khong bo v. Fc -Tn s trung tm ca sng mang cao tn RF. k - Ch s ca sng mang tng i so vi tn s trung tm. Cm, l, k Bin phc cho sng mang k ca symbol s liu th l trong khung th m. Cc gi tr Cm, l, k l cc gi tr iu ch c chun ho ca cc im phc trn biu chm sao ng vi h s a (anpha) dng cho s liu. Cc tn hiu pilot chun. Nh ta trnh by trn, cc sng mang khc nhau trong 1 khung OFDM c iu ch vi cc thng tin chun, m gi tr pht i c my thu nhn bit. Cc sng mang ny c pht vi mc cng sut tng cng 2,5dB so vi cc sng mang d liu, cc thng tin c truyn i trong cc sng mang ny l cc pilots phn tn v pilot lin tc. C 4 symbols OFDM th c mt pilot lin tc trng v tr vi pilot phn tn. Cc pilots phn tn v lin tc c di u trong bng tn t kmin n kmax. Cc sng mang s liu l c nh t symbol ny n symbol khc l 1512 sng mang s liu hu ch ch 2k v 6048 sng mang s liu ch 8k. Gi tr thng tin ca cc pilots phn tn v lin tc c ly t chui nh phn gi ngu nhin (PRBS) l mt lot cc gi tr, mi gi tr cho 1 sng mang pilot. Cc pilots lin tc v phn tn c iu ch bi chui PRBS vi Wk tng ng vi ch s sng mang k ca chng. Chui ny cng qun l khng ch pha khi u ca thng tin TPS. Chui PRBS hot ng theo a thc sau: G (x) = x11 + x2 + 1. Chui PRBS c khi to sao cho bit ra u tin trng vi sng mang u tin kmin. Mt gi tr mi c to ra nh chui PRBS trn mi sng mang s dng (sng mang c th l pilot hay khng phi l pilot). Cc pilot phn tn. Cc pilots phn tn (scattered pilots) vi s lng l 131 trong symbol 2k v 524 trong symbol 8k c a vo b to khung COFDM ti khi thch ng khung (frame adaptation).Nguyn Nh Nht Lun vn thc s

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Cc pilots phn tn l thng tin ch dn cho my thu nh gi tnh trng ca knh truyn tin hnh sa li. Thng tin chun ly t chui chun PRBS c truyn cc pilots phn tn trong tng symbol OFDM vi mc cng sut tng cng. iu ch cc pilots phn tn nh sau: Re ( cm,k,l ) = 4/3 . 2 ( -1/2 wk). Im ( cm,k,l ) = 0. C ngha l sau khi iu ch cc pilots phn tn ch c phn thc v khng c phn o. m L ch s khung COFDM. k L ch s ca sng mang c iu ch. l L ch s ca symbol OFDM. Vi l t 067 cc sng mang ch s k thuc {k = Kmin + 3*( l mod 4 ) + 12p}. p 0 v l s nguyn. k Trc thuc [Kmin ; Kmax] l cc pilots phn tn. Cc pilot lin tc. Thm vo cc pilots phn tn trn y, 45 sng mang pilots ch 2k v 177 sng mang pilots lin tc (continous pilots) ch 8k c chn vo khung COFDM. Tt c cc pilots lin tc l thng tin ch dn cho my thu hnh DVB-T thc hin ng b khung, ng b tn s v ng b thi gian. Chng cng c ri u trong mi symbol. Cc pilots lin tc c iu ch bi chui chun PRBS trn v c truyn vi mc cng sut tng cng thm 2,5dB so vi cc d liu. Cc d liu c chun ho E (c . c*) = 1. Cc pilots lin tc c iu ch nh sau: Re (cm,k,l) = 4/3 . 2 ( -1/2 wk). Im (cm,k,l) = 0. C ngha l, cng ging nh cc sng mang phn tn, sau khi iu ch sng mang lin tc ch c phn tht Re m khng c phn o Im.

Nguyn Nh Nht

Lun vn thc s

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V tr cc sng mang pilot lin tc Continual pilot carrier positions (index number k) 2k mode 8kmode 0 48 54 87 141 0 48 54 87 141 156 192 201 255 279 282 333 432 450 156 192 201 255 483 525 531 618 636 714 759 765 780 804 873 888 918 279 282 333 432 939 942 969 984 1050 1101 1107 1110 1137 1140 1146 450 483 525 531 1206 1269 1323 1377 1491 1683 1704 1752 1758 1791 618 636 714 759 1845 1860 1896 1905 1959 1983 1986 2037 2136 2154 765 780 804 873 2187 2229 2235 2322 2340 2418 2463 2469 2484 2508 888 918 939 942 2577 2592 2622 2643 2646 2673 2688 2754 2805 2811 969 984 1050 2814 2841 2844 2850 2910 2973 3027 3081 3195 3387 1101 1107 1110 3408 3456 3462 3495 3549 3564 3600 3609 3663 3687 1137 1140 1146 3690 3741 3840 3858 3891 3933 3939 4026 4044 4122 1206 1269 1323 4167 4173 4188 4212 4281 4296 4326 4347 4350 4377 1377 1491 1683 4392 4458 4509 4515 4518 4545 4548 4554 4614 4677 1704 4731 4785 4899 5091 5112 5160 5166 5199 5253 5268 5304 5313 5367 5391 5394 5445 5544 5562 5595 5637 5643 5730 5748 5826 5871 5877 5892 5916 5985 6000 6030 6051 6054 6081 6096 6162 6213 6219 6222 6249 6252 6258 6318 6381 6435 6489 6603 6795 6816

Cc sng mang TPS (Transmission Parameter Signalling) c s dng cho mc ch hin th cc thng s c lin quan ti s iu ch v m ho knh truyn. TPS c truyn ng thi trn 17 sng mang TPS ch 2k v trn 68 sng mang TPS ch 8k. Mi sng mang TPS trong cng 1 symbol OFDM mang cng 1 bit thng tin m ho vi sai ging nhau. Cc ch s ca sng mang TPS c b tr nh trong bng sau: Carrier indices for TPS carriers 2k mode 8k mode 34 50 209 34 50 209 346 413 346 413 569 1219 1262 1286 1469 595 688 790 2050 2117 2273 2299 901 1073 1219 2966 3173 3298 3391 1262 1286 1469 3977 4003 4096 4198 1594 1687 4877 5002 5095 5146Nguyn Nh Nht

596 595 688 790 901 1594 1687 1738 1754 2392 2494 2605 2777 3442 3458 3617 3754 4309 4481 4627 4670 5162 5321 5458 5525

1073 1913 2923 3821 4694 5681Lun vn thc s

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5707 5800 5902 6013 6185 6331 6374 6398 6581 6706 6799 Cc sng mang TPS truyn ti cc thng tin v: Kiu iu ch: Bao gm gi tr ca h s a ca biu chm sao QAM. Gi tr a xc nh khong cch gn nht gia 2 trc tht v o ti cc im tn hiu gn nht trong 4 gc phn t ca mt phng phc. N cho php chun ho cc s iu ch ng nht v khng ng nht k c QPSK, 16-QAM v 64-QAM. Thng tin v iu ch phn cp: Khng phn cp hay c phn cp. Khong bo v khng dng cho vic thu nhn ban u, m dng cho vic h tr khi u ca my thu trong trng hp lp li cu hnh. T l m xon(FEC). Ch truyn 2k hay 8k (cng khng dng cho vic thu nhn ban u m dng cho vic h tr khi u ca my thu trong trng hp lp li cu hnh). S khung trong 1 siu khung. Cc sng mang TPS c xc nh trn 68 symbol OFDM lin tc, c 4 khung lin tc tng ng vi 1 siu khung. Chui chun ng vi cc sng mang TPS ca symbol u tin ca mi khung OFDM c s dng khi to iu ch TPS trn mi sng mang TPS. Mi symbol OFDM ch mang 1 bit TPS. Mi khi TPS (ng vi 1 khung OFDM) cha 68 bit v c xc nh nh sau: 1 bit khi u. 16 bit ng b. 37 bit thng tin bo hiu 14 bit d tr cho vic sa li. Trong 37 bit thng tin th hin nay s dng 23 bit, cn li 14 bit d tr cho tng lai v c thit lp gi tr l 0 nh dng truyn dn TPS: S sp xp (nh v) mi mt thng s truyn dn nh (kiu iu ch chm sao, gi tr a, tc m xon, khong bo v v s khung trong 1 siu khung) c thc hin nh trnh by di y. Thng tin TPS truyn trong siu khung th m vi cc bit t S25 n S39 lun p dng cho siu khung tip theo m + 1, trong khi cc bit khc th cp ti siu khung th m. Bit khi u: L bit S0 khi u vic iu ch B-PSK vi sai (DBPSK). iu ch bng bit S0 c suy ra t chui chun PRBS u trn v s gii thiu phn sau.

Nguyn Nh Nht

Lun vn thc s

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Ch th di ca TPS: 6 bit u tin ca thng tin TPS c s dng nh mt b ch th di ca TPS (m nh phn) biu th s bit s dng ca TPS. B ch th ny c gi tr t cc bit S17 S22 = 01 01 11. ng b: Cc bit t S1 S16 l t m ng b. Khi TPS th 1 v th 3 trong mi siu khung c t m ng b nh sau: S1 S16 = 00 11 01 01 11 10 11 10. Khi TPS 2 v 4 c t m ng b nh sau: S1 S16= 11 00 10 10 00 01 00 01 Th t khung trong siu khung: C 4 khung to nn 1 siu khung. Cc khung trong 1 siu khung c nh s t 0 3. Th t khung trong 1 siu khung c biu th bng 2 bit S23 v S24 . Bits S23, S24 00 01 10 11 S khung Frame number 1 in the super-frame Frame number 2 in the super-frame Frame number 3 in the super-frame Frame number 4 in the super-frame

Kiu iu ch (lp chm sao). Chm sao c biu th bng 2 bit S25 v S26 nh trong bng sau: Bits S25, S26 Constellation characteristics 00 QPSK 01 16-QAM 10 64-QAM 11 reserved iu ch phn cp: Thng tin v phn cp xc nh khi no truyn dn l c phn cp, vy th h s a l bao nhiu? H s a c biu th bng 3 bit nh trong bng sau. Cc bit S27, S28, S29 c s dng thit lp thng tin ny. Bits S27,S28,S29 a value 000 Non hierarchical 001 a=1 010 a=2 011 a=4 100 reserved 101 reserved 110 reserved 111 reservedNguyn Nh Nht Lun vn thc s

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T l m FEC: 3 bit S30, S31, S32 c s dng biu th t l m xon cho dng HP v 3 bit S33, S34, S35 cho dng LP. Th t hin th bt u cho dng HP v kt thc dng LP nh trong bng di y. Bits S30, S31, S32 (Hp Stream) Code rateS33, S34,