[123doc.vn] - truyen-hinh-so-mat-dat-theo-tieu-chuan-dvb-t2-full.pdf

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GVHD: TS Nguyn Cnh Minh SVTH: Nguyn Th Hin

LI M U

Khi i sng vt cht ca ngi dn ngy cng c nng cao, yu cu v cht

lng cc chng trnh truyn hnh, gii tr ngy cng ln. Lnh vc pht thanh truyn

hnh trong my nm tr li y ang c nhng bc tin nhy vt. Vi nhng u

im vt tri ca truyn hnh s so vi truyn hnh analog, trong nhng nm qua,

truyn hnh s mt t pht trin mnh m trn th gii ni chung v Vit Nam

ni ring. Vic thay th hon ton truyn hnh mt t analog bng cng ngh truyn

hnh s mt t trn ton th gii s din ra trong tng lai khng xa.

n nay c nhiu quc gia trn th gii la chn tiu chun DVB-T pht

sng mt t. Tuy nhin, trc cc nhu cu i hi v dung lng, khng li ng

truyn, nng cao tin cy vi cc loi hnh dch v, v gim t s cng sut

nh/cng sut trung bnh, nhu cu v phn chia ph tn ca cc dch v vin thng

khc, cng vi xu th hi t trong cc mi trng truyn dn, s pht trin mnh m

ca truyn hnh phn gii cao HDTV vi dung lng bit ln m DVB-T cha p

ng c.

T cc yu cu thc t t ra , nhm DVB Project pht trin chun truyn

hnh s th h th hai l DVB-T2. Tiu chun ny c xut bn ln u tin vo

thng 6/2008 v c ETSI chun ha t thng 9/2009. Hin nay, Anh, Phn Lan

thng bo trin khai cc dch v HDTV trn ng truyn mt t dng chun DVB-

T2, c, , Ty Ban Nha, Thy in cng ang tin hnh th nghim pht sng

DVB-T2.

T nm 2001, Vit Nam tin hnh th nghim pht sng truyn hnh mt

t. Trc sc p v l trnh chuyn i hon ton sang pht sng s mt t trc

nm 2020 v xu hng pht trin ca truyn hnh s mt t DVB-T2 trn th gii,

n Nghin cu h thng truyn hnh s mt t theo tiu chun DVB-T2

tp trung nghin cu cc gii php k thut ca h thng truyn hnh s mt t DVB-

T, nhng bt cp, tn ti khi trin khai Vit Nam, ng thi nghin cu cu trc v

nhng c tnh k thut c bn, nhng tnh nng u vit ca tiu chun DVB-T2. T

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xut la chn gii php cng ngh hp l nht cho h thng truyn hnh s mt

t ti Vit Nam trong nhng nm ti. Ni dung ca n c th hin qua 4 phn

nh sau:

- Tng quan v h thng truyn hnh k thut s: gii thiu s lc v h

thng truyn hnh, khi nim, c im v cc tiu chun ca truyn hnh s.

- Truyn hnh s mt t theo tiu chun DVB-T: nghin cu, tm hiu v

s khi ca h thng truyn hnh s mt t DVB-T v cc c tnh k thut ca

tiu chun DVB-T.

- Truyn hnh s mt t theo tiu chun DVB-T2: nghin cu, tm hiu v

s khi ca h thng truyn hnh s mt t DVB-T2 v cc c tnh k thut ca

tiu chun DVB-T2.

- Gii php cng ngh cho truyn hnh s ti Vit Nam: tm hiu v kh

nng ng dng DVB-T2 ti Vit Nam v khuyn ngh la chn gii php cng ngh

cho truyn hnh s mt t ti Vit Nam.

Trong thi gian lm ti, em c gng rt nhiu song do kin thc cn hn

ch v thi gian lm ti c hn nn n cn nhiu sai st. Em rt mong nhn

c s ph bnh, cc kin ng gp ca thy c v cc bn n c hon

thin hn.

Em xin chn thnh cm n s gip tn tnh ca TS Nguyn Cnh Minh

cng cc thy c trong b mn k thut vin thng gip em hon thnh n tt

nghip ny.

H Ni, Ngythngnm 2014

Sinh vin thc hin

Nguyn Th Hin

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MC LC

Trang

LI M U ......................................................................................................... 1

MC LC .............................................................................................................. 3

DANH MC CC K T VIT TT ................................................................... 6

DANH MC HNH V .......................................................................................... 9

DANH MC BNG BIU ................................................................................... 11

CHNG I - TNG QUAN V H THNG TRUYN HNH K THUT S 12

1.1 S lc v h thng truyn hnh ........................................................... 12

1.1.1 S lc v lch s pht trin ..................................................... 12

1.1.2 Cng ngh truyn hnh tng t ............................................... 14

1.2 Khi nim truyn hnh k thut s ........................................................ 16

1.3 c im truyn hnh k thut s.......................................................... 17

1.4 X l tn hiu, truyn dn tn hiu truyn hnh k thut s .................... 19

1.4.1 C s truyn hnh k thut s ................................................... 19

1.4.2 S ha tn hiu truyn hnh ...................................................... 21

1.4.3 Chuyn i tng t sang s .................................................... 21

1.4.4 Chuyn i s sang tng t .................................................... 23

1.4.5 Nn tn hiu truyn hnh ........................................................... 24

1.4.6 M ha v iu ch tn hiu s ................................................. 26

1.4.7 Cc phng thc truyn dn tn hiu truyn hnh s................. 28

1.5 Cc tiu chun ca truyn hnh k thut s ........................................... 31

1.6 Kt lun chng I ................................................................................. 34

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CHNG II - TRUYN HNH S MT T THEO TIU CHUN DVB-T ... 35

2.1 Gii thiu v h thng truyn hnh s mt t DVB-T .......................... 35

2.2 S khi h thng truyn hnh s mt t .......................................... 36

2.3 Cc c tnh k thut ca tiu chun DVB-T ........................................ 38

2.3.1 B iu ch DVB-T ................................................................. 38

2.3.2 M ha COFDM trong DVB-T ................................................ 41

2.3.3 Chm sao tn hiu v qu trnh nh x...................................... 49

2.3.4 M sa sai ................................................................................ 54

2.3.5 DVB-T trong mi trng b phn x - Mng n tn (SFN) ..... 54

2.4 Kt lun chng II ................................................................................ 57

CHNG III - TRUYN HNH S MT T THEO TIU CHUN DVB-T2 58

3.1 Yu cu t ra i vi tiu chun truyn hnh s mt t mi ............... 58

3.2 Gii thiu chung v tiu chun truyn hnh s mt t DVB-T2 ........... 59

3.3 M hnh cu trc h thng truyn hnh s mt t DVB-T2 .................. 60

3.4 Cc c tnh k thut ca DVB-T2 ....................................................... 61

3.4.1 Lp vt l ................................................................................ 61

3.4.2 Cu hnh mng ......................................................................... 63

3.4.3 Hiu qu ca vic s dng k thut chm sao quay, chn thi gian

v tn s.. ............................................................................................... 77

3.4.4 iu ch v m sa sai trong DVB-T2 ..................................... 79

3.5 Kt lun chng III .............................................................................. 80

CHNG IV - GII PHP CNG NGH CHO TRUYN HNH S MT T

TI VIT NAM .................................................................................................... 81

4.1 So snh cc tham s c bn ca DVB-T v DVB-T2 ............................ 81

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4.2 Kh nng chuyn i t DVB-T sang DVB-T2 .................................... 82

4.2.1 L trnh s ha truyn hnh s mt t ..................................... 82

4.2.2 Kh nng chuyn i t DVB-T sang DVB-T2 ........................ 83

4.2.3 Kh nng ng dng DVB-T2 ti Vit Nam .............................. 83

4.3 Khuyn ngh la chn gii php cng ngh cho truyn hnh s mt t ti

Vit Nam ........................................................................................................... 84

4.3.1 La chn gii php cng ngh cho truyn hnh s mt t ti Vit

Nam. ............................................................................................... 84

4.3.2 Trin khai DVB-T2 ti Vit Nam ............................................. 85

4.4 Kt lun chng IV .............................................................................. 91

KT LUN ........................................................................................................... 92

NHN XT CA GIO VIN HNG DN V GIO VIN C DUYT . 93

LI CM N ....................................................................................................... 94

TI LIU THAM KHO ..................................................................................... 95

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DANH MC CC K T VIT TT

T vit tt Ting Anh y Ting Vit

ACE Active Constellation Extension M rng chm sao tch cc

AFC Automatic Frequency Control iu khin tn s t ng

ATSC Advanced Television System

Commitee

Hi ng v h thng truyn

hnh ci bin

AVG Audio Visual Global Cng ty c phn nghe nhn ton

cu

BCH Bose-Chaudhuri-Hocquenghem Bose-Chaudhuri-Hocquenghem

BER Bit Error Rate T s li bit

BPSK Binary Phase Shift Keying iu ch pha nh phn

CATV Collective Antenna Telvision Truyn hnh cp dy dn

C/N Carrier/Noise Sng mang/tp m

COFDM Coding Othogonality Fequency

Dvision Mltiplexing

Ghp knh phn chia theo tn s

trc giao c m

DTT Digital Terrestrial Television Truyn hnh s mt t

DiBEG Digital Broadcasting Expert

Group Nhm chuyn gia truyn hnh s

DPCM Differential Pulse Code

Modulation iu ch xung m vi sai

DVB Digital Video Broadcasting Truyn hnh s

EDTV Enhanced Definition Television Truyn hnh phn gii m

rng

ES Elementary Stream Dng c s

FEC Forward Error Correction Sa li trc (thun)

FEF Future Extension Frame Khung m rng dnh cho tng

lai

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FFT Fast Fourrier Transform Bin i Fourrier nhanh

FIR Finite Impulse Responese p ng xung hu hn

GI Guard Intervalman Khong bo v

HDTV High Definitiom Television Truyn hnh phn gii cao

HP High Priority u tin cao

ICI Inter-Carrier Interference Nhiu lin sng mang

IF Intermediate Frequency Trung tn

IFFT Inverse Fast Fourrier Transform Bin i nhanh Fourrier ngc

IPTV Internet Protocol Television Truyn hnh IP

ISDB Integrated Services Digital

Broadcasing

Truyn hnh s cc dch v

tch hp

ISI Inter-Symbol Interference Nhiu lin k t

JPEG Joint Photoghraphic Experts

Group

Nhm chuyn gia nghin cu v

nh tnh

LDPC Low Density Parity Check

(Codes)

M kim tra ngang hnh mt

thp

LDTV Low Definitiom Television Truyn hnh phn gii thp

LP Low Priority u tin thp

MCM Multi Carrier Modulation iu ch a sng mang

MFN Multiple Frequency Network Mng a tn

MI Modulator Interface Giao din iu ch

MISO Multi Input Single Output Nhiu u vo mt u ra

MPEG Moving Pictures Experts Group Nhm chuyn gia nghin cu v

nh ng

NTSC National Television System

Comittee

Hi ng h thng truyn hnh

quc gia M

OFDM Othogonality Fequency Dvision

Mltiplexing

Ghp knh phn chia theo tn s

trc giao

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OIRT Organization Internatinal Radio

and Television

T chc pht thanh truyn hnh

quc t

PAL Phase Alternative Line o pha theo tng dng

PAPR Peak to Average Power Ratio T s cng sut nh/cng sut

trung bnh

PCM Pulse Code Modulation iu ch xung m

PES Packetized Elementary Stream Dng c s ng gi

PLP Physical Layer Pipes ng lp vt l

PRBS Pseudo Random Bit Sequence Chui bit gi ngu nhin

QAM Quadrature Amplitude

Modulation iu ch bin vung gc

QPSK Quadrature Phase Shift Keying Kha dch pha vung gc

RF Radio Frequence Tn s v tuyn

SDTV Standard Definition Television Truyn hnh phn gii tiu

chun

SFN Single Frequence Network Mng n tn

SNR Signal to Noise Ratio T s tn hiu trn tp m

TPS Transmission Parameter

Signalling Bo hiu tham s bn pht

TR Tone Reservation Hn ch m sc

TS Transport Stream Dng truyn ti

UHF Ultra High Frequency Tn s siu cao (Siu cao tn)

VHF Very High Frequency Tn s rt cao

VO Video Object i tng video

VSB Vestigal Side Band iu bin ct

VTC Vietnam Television Corporation Tng Cng ty Truyn thng a

phng tin Vit Nam

VTV Vietnam Television i truyn hnh Vit Nam

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DANH MC HNH V

Hnh 1.1: S cu trc tng qut ca h thng truyn hnh s ........................... 17

Hnh 1.2: So snh cht lng tn hiu s v tng t. ........................................... 19

Hnh 1.3: S tng qut h thng thu v pht truyn hnh s .............................. 20

Hnh 1.4: S mch bin i tng t sang s.................................................... 21

Hnh 1.5: S khi mch bin i video s sang tng t . ................................. 23

Hnh 1.6: M hnh h thng nn video. .................................................................. 24

Hnh 1.7: Bn phm vi ng dng cc tiu chun truyn hnh s trn th gii.... 31

Hnh 2.1: S khi h thng truyn hnh s mt t DVB-T. ............................... 36

Hnh 2.2: S khi b iu ch s DVB-T. .......................................................... 38

Hnh 2.3: Cc bc ca qu trnh phn tn nng lng, m ha ngoi v ghp xen

ngoi. .................................................................................................................... 40

Hnh 2.4: Khi iu ch COFDM trong DVB-T ..................................................... 42

Hnh 2.5: Ph tn hiu OFDM vi s sng mang N=16 ......................................... 43

Hnh 2.6: Biu din chm sao ca iu ch QPSK, 16 QAM, v 64 QAM . ............ 45

Hnh 2.7: Phn b sng mang ca DVB-T (cha chn khong bo v ................... 46

Hnh 2.8: Phn b cc pilot ca DVB-T. ............................................................... 47

Hnh 2.9: Phn b cc pilot ca DVB-T trn biu chm sao. ............................ 47

Hnh 2.10: Phn b sng mang khi chn thm khong thi gian bo v. ............... 48

Hnh 2.11: Cc tia sng n trong khong thi gian bo v ................................. 49

Hnh 2.12: nh x 16 QAM v 64 QAM chm sao u vi =1. ........................... 51

Hnh 2.13: Chm sao iu ch phn cp DVB-T.................................................... 52

Hnh 2.14: nh x 16 QAM v 64 QAM khng ng u vi = 4. ...................... 53

Hnh 3.1: M hnh cu trc h thng DVB-T2. ...................................................... 60

Hnh 3.2: Lp vt l. .............................................................................................. 61

Hnh 3.3: M hnh MISO. ...................................................................................... 65

Hnh 3.4: Mu hnh Pilot phn tn i vi DVB-T (tri) v DVB-T2 (phi). ......... 66

Hnh 3.5: Khong bo v (GI) i vi 8K 1/32 v 32K 1/128. ............................... 68

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Hnh 3.6: Cu trc khung DVB-T2. ....................................................................... 69

Hnh 3.7: S tng hp khung hnh trong MPEG-4 ................................................. 71

Hnh 3.8: ng i ca tn hiu trong mng n tn cng ngh DVB-T2. ............. 74

Hnh 3.9: Kin trc c bn ca h thng mng n tn SFN DVB-T2. .................. 74

Hnh 3.10: Chm sao 16 QAM quay ...................................................................... 77

Hnh 3.11: Thnh tch ca chm sao quay so vi khng quay. ............................... 78

Hnh 3.12: th chm sao 256 QAM. .................................................................. 79

Hnh 4.1: Cu hnh tng th h thng mng SFN theo tiu chun DVB-T2 ca AVG.

.............................................................................................................................. 87

Hnh 4.2: Bn ph sng ton quc ca AVG. .................................................... 88

Hnh 4.3: Bn ph sng DVB-T2 ti H Ni ca VTV....................................... 90

Hnh 4.4: Bn ph sng DVB-T2 ti Tp. HCM ca VTV. .................................. 91

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DANH MC BNG BIU

Bng 2.1: Khong thi gian bo v cc ch pht khc nhau. ......................... 56

Bng 3.1: Khong bo v tng ng ca DVB-T2 (theo EBU-TECH 3348) .......... 75

Bng 3.2: Mc C/N ti thiu trong knh tn 8MHz tng ng vi cc ch iu ch

khc nhau . ............................................................................................................ 76

Bng 3.3: C/N tng ng vi cc phng php thu trong DVB-T2. ....................... 77

Bng 4.1: DVB-T2 s dng ti UK so vi DVB-T. ................................................. 81

Bng 4.2: Dung lng d liu DVB-T2 so vi DVB-T trong mng SFN................. 82

Bng 4.3: Cc tham s pht sng ca truyn hnh An Vin. ................................... 86

Bng 4.4: Cc tham s pht sng ca i truyn hnh Vit Nam . ......................... 90

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CHNG I - TNG QUAN V H THNG TRUYN HNH K

THUT S

Truyn hnh l mt h thng bin i hnh nh v m thanh km theo thnh

tn hiu in, truyn n my thu, ni thc hin bin i tn hiu ny thnh dng ban

u v hin th ln mn hnh di dng hnh nh. Trong nhng nm gn y, cng

ngh truyn hnh ang chuyn sang mt bc ngot, l qu trnh chuyn i t

cng ngh Analog sang Digital. Qu trnh ny l mt cuc cch mng lm thay i

cch suy ngh, cch tip xc, phng thc sn xut chng trnh truyn hnh, nng

cp cht lng tn hiu, tnh linh hot v kh nng hi nhp vo mi trng thng tin

chung. Vit Nam, cng ngh s v ang c ng dng v trin khai mnh m.

Trong mt tng lai khng xa, trn th gii ni chung v Vit Nam ni ring, tin

ti mt h thng truyn hnh hon ton k thut s l mt tt yu [2].

1.1 S lc v h thng truyn hnh

1.1.1 S lc v lch s pht trin

Truyn hnh c mi lin h mt thit vi mt s loi hnh truyn thng hay

ngh thut khc nh pht thanh, in nh Tuy nhin, ch sau mt vi thp k s

khai, truyn hnh tin nhng bc tin di v thc s tch ra khi cc loi hnh

khc, tr thnh phng tin truyn thng c lp v c sc mnh to ln trong vic

to dng v nh hng d lun. Vic pht sng truyn hnh u tin M c bt

u t nhng nm 1930, v truyn hnh ch thc s ph bin t nhng nm 1950.

Nhng i pht thanh nh NBC, CBS, ABC sau khi pht trin thm h thng truyn

hnh thc s ln mnh v tr thnh nhng tp on pht thanh truyn hnh tm c

th gii.

Trn thc t, s hnh thnh v pht trin ca truyn hnh gn lin vi cc s

kin khoa hc - cng ngh cng nh cc s kin chnh tr - x hi khc. Ngay t u

nhng nm 1920, ngi ta ch n truyn hnh do h nhn thc c vai tr ca

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truyn hnh trong vic tuyn truyn, qung b trn cc mt kinh t, chnh tr, x hi

c th k n mt s mc quan trng trong lch s truyn hnh nh sau:

- Nm 1839: Bee Quell tm ra hin tng quang in.

- Nm 1898: Volsske tm ra h thng truyn hnh khng dy dn (truyn hnh

bng sng in t).

- Thng 3/1899: lin lc v tuyn quc t u tin ra i Anh v Php, di

46Km.

- Nm 1941: M chp nhn chun 525 dng qut vi b phn gii ca mnh.

- Trong v sau chin tranh th gii th II: cc cng quc chy ua gay gt

pht cc chng trnh truyn hnh nhm vn ng nhn dn ng h cc chin lc

qun s v kinh t ca mnh.

- Nm 1957: Php xut hin h truyn hnh mu SECAM do Henry De

France nghin cu v thc hin.

- Nm 1962: gio s Walter Bruce Ty c cng b h truyn hnh PAL.

C hai h SECAM v PAL v nguyn l chung thng nht vi h NTSC.

- Nm 1966: Na Uy tin hnh hi ngh CCIR chn h truyn hnh mu

thng nht cho c Chu u, tin cho vic trao i chng trnh truyn hnh mu

gia cc nc.

Kt qu mt s nc chn h SECAM, mt s nc dng h PAL, M v Nht

s dng h NTSC. Vit Nam chn h PAL tiu chun OIRT (Organization

Internatinal Radio and Television - T chc pht thanh truyn hnh quc t).

- Nm 1994: M nghin cu v th nghim truyn hnh s, n thng 12 nm

1996 ban hnh tiu chun ATSC.

- Nm 1997: Nht Bn ban hnh tiu chun ISDB - hay cn gi l tiu chun

DiBEG.

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- Nm 1997: tiu chun DVB-T ca Chu u ra i. Nhiu nc Bc u, mt

s nc Chu trong c Vit Nam v nhiu nc khc la chn tiu chun

ny v d kin pht sng s hon ton vo nm 2010 - 2015.

Vit Nam t nhng nm 1997 n nay c mt s n v k thut c nghin

cu v tip cn vi cng ngh s, cho n nay nhiu cng on trong sn xut chng

trnh, truyn dn c s ha, nhiu ti nghin cu truyn hnh s v ang

c nghin cu th nghim chnh v vy m n mang tnh khoa hc v thc tin cao

nhm cng ngy cng nng cao cht lng cho vic pht hnh s ti Vit Nam.

Nh vy, c th thy, lch s pht trin ca truyn hnh lun nm trong v cng

song hnh vi lch s tin b nhn loi. Truyn hnh ngy mt ln mnh l do nhu

cu thng tin ca cng chng ngy cng cao, khoa hc k thut pht trin v xut

hin nhu cu giao lu quc t. Chnh bn thn cc vn s kin chnh tr, x hi

cng gp phn thc y truyn hnh phi t pht trin v pht huy hn na nhng u

th ca mnh, t dn to nn nhng c trng ring bit trong h thng cc phng

tin truyn thng i chng hin nay.

1.1.2 Cng ngh truyn hnh tng t

Truyn hnh en trng l bc m u cho vic truyn cc hnh nh i xa. N

c nghin cu v ch to vo nhng nm 60 ca th k XX vi nhng ng thu hnh

Vidicon. Truyn hnh en trng tng c s dng hu ht cc quc gia trn th

gii, cng vi s pht trin nhanh chng ca n in t th cc thit b ca truyn

hnh en trng c n nh hn, cht lng tt hn. Tuy nhin truyn hnh en trng

li c nhc im rt ln l khng c kh nng truyn i cc hnh nh c mu sc

nh trong thc t. Do , nm 1957, h truyn hnh mu u tin ra i ti Php,

m ra cuc cch mng i vi nn cng nghip truyn hnh.

Hin nay, c 3 loi cng ngh truyn hnh tng t chnh c s dng trn

ton th gii da trn tiu chun m ha hnh nh NTSC, SECAM v PAL v s dng

iu ch RF iu ch tn hiu sng mang VHF hoc UHF. Mi khung hnh ca

mt hnh nh truyn hnh bao gm cc dng qut trn mn hnh. Cc dng c sang

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khc nhau, tp hp ton b dng c v mt cch nhanh chng mt ngi cm

nhn n nh l mt hnh nh. Cc khung hnh tip theo tun t c hin th, cho

php m t chuyn ng. Cc tn hiu truyn hnh tng t c thng tin v thi gian

v ng b my thu c th ti to li mt hnh nh hai chiu chuyn ng t mt

tn hiu mt chiu bin thin theo thi gian.

- Tiu chun NTSC c pht trin ti M v ln u tin s dng trong

nm 1954, NTSC hin l tiu chun truyn hnh lu i nht. N bao gm qut dng

l 525 v tn s qut hnh l 60Hz.

- Tiu chun SECAM c pht trin Php v ln u tin c s dng vo

nm 1967. N s dng qut dng 625 v tn s qut hnh l 50Hz. Cc loi tiu chun

khc nhau s dng bng thng video v thng s k thut cung cp dch v m thanh

khc nhau.

- Tiu chun PAL c pht trin c v ln u tin c s dng vo nm

1967. L mt bin th ca NTSC, chun PAL s dng qut dng 625 v tn s qut

hnh l 50Hz. Cc tiu chun khc nhau s dng bng thng video v thng s k

thut cung cp dch v m thanh khc nhau.

Trong thi i truyn hnh tng t, cc tiu chun u c nhng phng tin

truyn dn tn hiu cho ring mnh. Cc thit b mng c gi thnh cao nn cc mng

truyn hnh mt t u trc thuc nh nc hoc chnh ph. Cc chun khng c

kh nng giao tip vi nhau to nn i hi cc quc gia phi c h thng truyn hnh

mt t ring cho mnh. H thng truyn hnh tng t bao gm h thng sn xut

chng trnh truyn hnh (quay hu trng, chnh sa, hon thin v lu tr video),

pht sng (to ra composite video, iu ch, khuch i, pht sng) v tip nhn (thu

tn hiu t anten, gii iu ch ca my thu truyn hnh v hin th hnh nh v m

thanh n ngi xem) vi tt c cc tn hiu l tng t.

Ngy nay, thng tin k thut s c to ra trong phng thu. Vi truyn hnh

k thut s, tt c cc qu trnh l k thut s, cc hnh nh, m thanh v tt c cc

thng tin b sung c to ra, truyn i v nhn c tn hiu k thut s. iu ny

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cho php nh dng tt nht cho hnh nh v m thanh, hnh nh rng hn so vi bn

gc (mn hnh ton cnh), vi mc cao hn v phn gii ( phn gii cao) v

m thanh stereo.

Cc h thng truyn hnh s vi nhng u im vt tri so vi truyn hnh

tng t kh nng chng nhiu cng nh tng hiu qu bng thng ang ngy cng

pht trin v c nghin cu nhiu quc gia. Do , xu hng hin nay l chuyn

i sang truyn hnh s.

1.2 Khi nim truyn hnh k thut s

Truyn hnh s l tn gi mt h thng truyn hnh m s dng phng php

s to, lu tr v truyn tn hiu ca chng trnh truyn hnh trn knh thng tin,

m ra mt kh nng c c bit rng ri cho cc thit b truyn hnh lm vic theo

cc h truyn hnh c nghin cu trc.

So vi tn hiu tng t, tn hiu s cho php to, lu tr, ghi c nhiu ln

m khng lm gim cht lng nh. Tuy nhin, khng phi tt c cc trng hp, tn

hiu s u t c kt qu cao hn so vi tn hiu tng t (b lc l mt v d c

th). Mc d vy, xu hng chung cho s pht trin ca cng nghip truyn hnh trn

th gii nhm t c mt h thng nht chung l h thng truyn hnh hon ton

bng k thut s c cht lng cao v d dng phn phi trn knh thng tin. H

truyn hnh k thut s v ang c pht trin trn ton th gii, to nn mt

cuc cch mng tht s trong cng nghip truyn hnh.

Nguyn l cu to ca h thng v cc thit b truyn hnh s c a ra nh

trn hnh 1.1. u vo ca thit b truyn hnh s s tip nhn tn hiu truyn hnh

tng t. Trong thit b m ha (bin i A/D), tn hiu truyn hnh tng t s c

bin i thnh tn hiu truyn hnh s, cc tham s v c trng ca tn hiu ny c

xc nh t h thng truyn hnh c la chn. Tn hiu truyn hnh s c a ti

thit b pht. Sau tn hiu truyn hnh s c truyn ti bn thu qua knh thng

tin. Ti bn thu, tn hiu truyn hnh s c bin i ngc li vi qu trnh x l

ti pha pht. Qu trnh gii m tn hiu truyn hnh thc hin bin i tn hiu truyn

-17-


hnh s sang tn hiu truyn hnh tng t. H thng truyn hnh s s trc tip xc

nh cu trc m ha v gii m tn hiu truyn hnh.

Hnh 1.1: S cu trc tng qut ca h thng truyn hnh s [2].

Khi truyn qua knh thng tin, tn hiu truyn hnh s c m ha knh. M

ha knh m bo chng cc sai st cho tn hiu trong knh thng tin. Thit b m

ha knh phi hp c tnh ca tn hiu s vi knh thng tin. Khi tn hiu truyn

hnh s c truyn i theo knh thng tin, cc thit b bin i c gi l b iu

ch v b gii iu ch.

Ti bn thu, tn hiu truyn hnh s c bin i ngc li vi qu trnh x

l ti pha pht. H thng truyn hnh s s trc tip xc nh cu trc m ha v gii

m tn hiu truyn hnh [2].

1.3 c im truyn hnh k thut s

Cc h thng truyn hnh ph bin hin nay nh: NTSC, SECAM, PAL u l

cc h thng truyn hnh tng t. Tn hiu truyn hnh tng t l hm lin tc theo

thi gian v t khu to dng, truyn dn, pht sng n khu thu tn hiu u chu

Tn hiu truyn

hnh tng t

Tn hiu truyn

hnh tng t Bin i

A/D

Thit b pht

Thit b thu

Bin i

D/A

M ho

ngun

Gii m ho

ngun

M ho

knh

Gii m

ho knh

iu ch s

Gii iu

ch s

Knh

thng tin

-18-


nh hng ca nhiu yu t (nhiu v can nhiu t ni b h thng v t bn ngoi)

lm gim cht lng hnh nh. khc phc nhng hin tng , ngi ta m ha

tn hiu truyn hnh dng s x l. Do h thng truyn hnh s c nhng u

im sau:

- C kh nng pht hin li v sa sai.

- C th tin hnh rt nhiu qu trnh x l trong trung tm truyn hnh m t

s S/N (t s tn hiu/tp m) khng gim. Trong truyn hnh tng t th vic ny

gy ra mo tch ly (mi khu x l u gy mo).

- Thun li cho qu trnh ghi c: c th ghi c v hn ln m cht lng

khng b gim.

- C kh nng lu tr tn hiu s trong cc b nh n gin v sau c n

vi tc ty .

- Kh nng truyn trn c ly ln: tnh chng nhiu cao (do vic ci m sa li,

chng li, bo v).

- Kh nng thu di ng tt. ngi xem d i trn t, tu ha vn xem c

cc chng trnh truyn hnh. S d nh vy l do x l tt hin tng Doppler.

- D to dng, ly mu tn hiu, do d thc hin vic chuyn i h truyn

hnh, ng b t nhiu ngun khc nhau, thc hin nhng k xo trong truyn

hnh.

- C kh nng thu tt trong truyn sng a ng. Trong h thng truyn hnh

tng t, thng xy ra hin tng bng ma do tn hiu truyn n my thu theo

nhiu ng. Vic trnh nhiu ng knh trong h thng thng tin s cng lm gim

i hin tng ny trong truyn hnh qung b.

- Tit kim c ph tn nh s dng cc k thut nn bng tn, t l nn c

th ln n 40 ln m hu nh ngi xem khng nhn bit c s suy gim cht

lng. T c th truyn c nhiu chng trnh trn mt knh sng, trong khi

truyn hnh tng t mi chng trnh phi dng mt knh sng ring.

-19-


- C kh nng truyn hnh a phng tin, to ra loi hnh thng tin hai chiu,

dch v tng tc, thng tin giao dch gia im v im. Do s pht trin ca cng

ngh truyn hnh s, cc dch v tng tc ngy cng phong ph a dng v ngy

cng m rng.

- Bo ton cht lng: cht lng ca tn hiu s v tn hiu tng t trong

qu trnh truyn t my pht n my thu c th hin nh hnh 1.2.

Hnh 1.2: So snh cht lng tn hiu s v tng t.

Tuy nhin, truyn hnh s cng c mt vi nhc im ng quan tm nh:

Di thng ca tn hiu tng do rng bng tn ca thit b v h thng

truyn ln hn nhiu so vi tn hiu tng t.

Vic kim tra cht lng tn hiu s mi im ca knh truyn thng

phc tp hn (phi dng mch chuyn i s - tng t).

1.4 X l tn hiu, truyn dn tn hiu truyn hnh k thut s

1.4.1 C s truyn hnh k thut s

Theo hnh 1.3, mi mt chng trnh truyn hnh cn mt b m ha MPEG-

2 ring trc khi bin i tn hiu t tng t sang s. Khi c nn gim ti

d liu, cc chng trnh ny s ghp li vi nhau tp thnh dng bit lin tip. Lc

Khong cch gia my pht v my thu

Tn hiu s

Tn hiu tng t

Cht lng

-20-


Audio

CT n CT 1

Video

M ha

MPEG-2

A/D A/D

Gii iu ch iu ch

Gii m ha

Gii MPEG-2

D/A

Video Video Audio Audio

DVB-T

DVB-C

MPEG-2

Khi s ha tn

hiu truyn

Pha thu Pha pht

Khi nn

video s

DVB-S

Ghp knh chng trnh Tch knh chng trnh

ny chng trnh sn sng c truyn i xa, cn c iu ch pht i theo

cc phng thc:

Hnh 1.3: S tng qut h thng thu v pht truyn hnh s.

Truyn hnh s v tinh DVB-S (QPSK)

Truyn hnh s cp DVB-C (QAM)

Truyn hnh s mt t DVB-T (COFDM)

-21-


Pha thu sau khi nhn c tn hiu s tin hnh iu ch ph hp vi phng

php iu ch, sau tch knh ri gii nn MPEG-2, bin i ngc li s sang

tng t, gm 2 ng hnh v ting ri n my thu hnh.

1.4.2 S ha tn hiu truyn hnh

Video s l phng tin biu din dng sng video tng t dng mt dng

d liu vi cc u im:

- Tn hiu video s khng b mo tuyn tnh, mo phi tuyn v khng b nhiu

gy ra cho qu trnh bin i tng t sang s (ADC) v s sang tng t (DAC).

- Thit b video s c th hot ng hiu qu hn so vi thit b video tng

t.

- Tn hiu video s c th tit kim b lu tr thng tin hn nhng b nn tn

hiu.

1.4.3 Chuyn i tng t sang s

Qu trnh chuyn i nhn chung c thc hin qua 3 bc c bn l: ly

mu, lng t ha v m ha. Cc bc lun kt hp vi nhau thnh mt qu trnh

thng nht (hnh 1.4).

Hnh 1.4: S mch bin i tng t sang s [2].

- Ly mu

Ly mu tn hiu tng t l qu trnh ri rc theo thi gian bng tn s ly

mu (fS), kt qu cho ta mt chui cc mu. Ly mu l bc u tin th hin tn

Lc thng

thp

Tn

hiu

vo

Ly

mu M ha

Lng

t ha

Xung ly mu + xung

ng h

Tn

hiu

ra

-22-


hiu tng t sang s, v cc thi im ly mu c chn s ch ra ta ca cc

im . Qu trnh bin i ny phi tng ng v mt tin tc. C ngha l tn hiu

sau khi ly mu phi mang thng tin ca dng tn hiu vo. Bin tn hiu tng

t c ly mu vi chu k TS, thu c mt chui cc xung hp vi tn s ly mu

c tnh bng:

fS =Ts

1 (1.1)

Trong : - fS: tn s ly mu

- T: chu k ly mu

i vi tn hiu tng t VI th tn hiu ly mu VS sau qu trnh ly mu c

th khi phc tr li VI mt cch trung thc nu tha mn iu kin:

fS 2fImax (1.2)

Trong : - fS: tn s ly mu

- fImax: gii hn trn ca di tn s tng t

V mi ln chuyn i in p ly mu tn hiu s tng ng u cn c mt

thi gian nht nh nn phi nh mu trong mt khong thi gian cn thit sau mi

ln ly mu. in p tng t u vo c thc hin chuyn i A/D trn thc t l

gi tr VI i din, gi tr ny l kt qu ca mi ln ly mu [2].

- Lng t ha

Bc tip theo trong qu trnh bin i A/D l lng t ha. Trong qu trnh

ny, bin tn hiu c chia thnh cc mc - gi l mc lng t. Khong cch

gia hai mc lin k nhau gi l bc lng t. Cc mu c c t qu trnh ly

mu s c bin bng cc mc lng t. Tn hiu s nhn c l mt gi tr xp

x ca tn hiu ban u, nguyn nhn do qu trnh lng t ha xc nh cc gi tr s

ri rc cho mi mu.

C hai phng php lng t ha: lng t ha tuyn tnh c cc bc lng

t bng nhau v lng t ha phi tuyn c cc bc lng t khc nhau. Trong hu

-23-


ht cc thit b video s cht lng studio, tt c cc mc lng t u c bin

bng nhau, v qu trnh lng t ha c gi l lng t ha ng u. y l qu

trnh bin i t mt chui cc mu vi v hn bin sang cc gi tr nht nh, v

vy qu trnh ny gy ra sai s, gi l sai s lng t. Sai s lng t l mt ngun

nhiu khng th trnh khi trong h thng s. Cc gi tr lng t c th cha sai s

trong phm vi 2

1Q (trong , Q l bc lng t).

- M ha

M ha l khu cui cng ca b bin i A/D. M ha l mt qu trnh bin

i cu trc ngun m khng lm thay i tin tc, mc ch l ci thin cc ch tiu

k thut cho h thng truyn tin. D liu sau m ha c u im: tnh chng nhiu

cao hn, tc hnh thnh tng ng kh nng thng qua ca knh.

1.4.4 Chuyn i s sang tng t

Qu trnh tm li tn hiu tng t t N s hng (N bit) bit ca tn hiu s

vi chnh xc l mt mc lng t (1 LBS). Qu trnh ny c thc hin nh

hnh 1.5.

Hnh 1.5: S khi mch bin i video s sang tng t [2].

ly c tn hiu tng t t tn hiu s dng nguyn tc nh hnh 1.4 trn.

Chuyn i s sang tng t khng phi l php nghch o ca chuyn i tng t

sang s, v khng th thc hin c php nghch o ca qu trnh lng t ha.

Theo s 1.5, th qu trnh chuyn i s sang tng t l qu trnh tm li tn hiu

tng t c ly mu.

D/A Ly mu

Xung ly

mu

Lc thng

thp

Video s Mch logic

Video

tng t Khuch i

-24-


1.4.5 Nn tn hiu truyn hnh

X l video, audio s c u im l cht lng cao v hnh nh v m thanh.

Nhc im ca x l video v audio l phi thc hin mt s lng ln cc file d

liu trong khi tnh ton v cc ng dng truyn dn. Gii php nn cho php ngi

s dng la chn mt trong cc phm vi thay i cc thng s ly mu v cc t s

nn, cc lin kt thch hp nht cho mc ch s dng. Nn v c bn l mt qu trnh

trong lng s liu (data) biu din lng thng tin ca mt nh hoc nhiu nh

c gim bt bng cch loi b nhng s liu d tha trong tn hiu video. Cc

chui nh truyn hnh c nhiu phn nh ging nhau, vy tn hiu truyn hnh c cha

nhiu d liu d tha ta c th b qua m khng lm mt thng tin nh. l cc

phn xa dng, xa mnh, vng nh tnh hoc chuyn ng rt chm, vng nh nn

ging nhau, m cc phn t lin tip ging nhau hoc khc nhau rt t. Thng

th chuyn ng trong nh truyn hnh c th c d bo, do ch cn truyn cc

thng tin v chuyn ng. Cc phn t ln cn trong nh thng ging nhau, do

ch cn truyn cc thng tin bin i. Cc h thng nn s dng c tnh ny ca tn

hiu video v cc c trng ca mt ngi l km nhy so vi sai s trong hnh nh

c nhiu chi tit v cc phn t chuyn ng. Qu trnh sau nn nh l dn nh to

li nh gc hoc xp x nh gc.

1.4.5.1 M hnh nn nh

M hnh ca h thng nn nh c th hin nh hnh 1.6.

Hnh 1.6: M hnh h thng nn video.

Biu din lng t

Lng t ha

Gn t

m

Gii

lng t ha

M ha video Gii m video

Biu din

thun li

Gii t m

X l knh

-25-


tng u tin ca b m ha video, tn hiu video c trnh by di dng

thun tin nn c hiu qu nht. im ct yu l ch xc nh ci g c m

ha. S biu din c th cha nhiu mu thng tin m t tn hiu hn l chnh tn

hiu, nhng hu ht thng tin quan trng ch tp trung trong mt phn nh ca s m

t ny. Trong cch biu din c hiu qu, ch c phn nh d liu cn thit truyn

cho vic ti to tn hiu video.

Hot ng th hai ca b m ha l lng t ha, gip ri rc ha thng tin

c biu din. truyn tn hiu video qua mt s knh s, nhng thng tin biu

din c lng t ha thnh mt s hu hn cc mc.

Hot ng th ba l gn cc t m. Cc t m ny l mt chui bit dng

biu din cc mc lng t ha.

Cc qu trnh s ngc li trong b gii m video.

Mi hot ng c gng loi b phn t d tha trong tn hiu video v tn

dng s gii hn ca h thng nhn ca mt ngi. Nh b i cc phn d tha, cc

thng tin ging nhau hoc c lin quan n nhau s khng c truyn i. Nhng

thng tin b i m khng nh hng n vic nhn cng khng c truyn i.

1.4.5.2 Cc phng php nn

X l tn hiu s ha hn thay th tt c cc phng php tng t (c) v tc

dng, tc mnh, NTSC, PAL, SECAM, HDTV v cui cng l tp trung vo

HDTV s bng rng.

- K thut tng t: nn thng tin video bng cch gim rng bng tn mu

< 1,2 MHz.

- K thut gim (nn) d liu video: cc h thng nn s liu l s phi hp

ca rt nhiu k thut x l nhm gim tc bit ca tn hiu s m vn m bo

cht lng hnh nh ph hp ng vi mt ng dng nht nh. Nhiu k thut nn

mt v khng mt thng tin (loss/lossless data reduction techniques) c pht

trin trong nhiu nm qua.

-26-


Nn khng tn tht: cho php phc hi li ng tn hiu ban u sau khi gii

nn. y l mt qu trnh m ha c tnh thun nghch. H s nn ph thuc vo chi

tit nh c nn. H s nn ca phng php nn khng mt thng tin nh hn 2:1.

Trong truyn hnh, phng php nn khng tn tht c kt hp trong cc

phng php nn c tn tht s cho t l nn tt m khng gy mt mt v phn

gii.

Nn c tn tht: chp nhn mt mt mt t thng tin gia tng hiu qu nn,

rt thch hp vi ngun thng tin l hnh nh v m thanh. Nh vy, nn c tn tht

mi tht s c ngha vi truyn hnh. N c th cho t l nn hnh nh cao truyn

dn, pht sng. ng thi cho mt t l nn thch hp cho x l v lu tr nh trong

studio.

Nn video tn tht DPCM - iu xung m vi sai:

- y l mt phng php nn quan trng v hiu qu, nguyn l c bn ca

n l: ch truyn ti tn hiu vi sai gia mu cho v d bo (c to ra t cc mu

trc ).

- Cng ngh DPCM thc hin loi b tnh c nh v cc thng tin d tha ca

ngun tn hiu bng mt b lc c bit c p ng u ra l tn hiu s gia mu u

vo v gi tr d bo ca chnh n. Rt nhiu gi tr vi sai ny gn bng 0 nu cc

im nh bin i khng ng u. Cn vi nh c nhiu chi tit gi tr sai s d bo

c th ln. Khi c th lng t ha chng bng mc lng t cao hn do c im

ca mt ngi khng nhy cm vi nhng chi tit c tng phn cao, thay i

nhanh. S gim tc bit y thu c t qu trnh lng t ha v m ha.

- Hu ht cc cch thc nn nh u s dng vng lp DPCM.

1.4.6 M ha v iu ch tn hiu s

Trong k thut truyn hnh tng t, ngi ta s dng phng php iu bin

(AM) hoc iu tn (FM). Ti u thu s c gii iu ch ti to li thng tin v

hnh nh, m thanh ban u.

-27-


Mt u im ca truyn hnh s so vi truyn hnh tng t l trn mi knh

thng tin c th truyn c nhiu chng trnh sau khi qua h thng ghp knh dng

truyn ti. truyn dn c tn hiu s, phi s dng phng php m ha v iu

ch tn hiu s m bo tn hiu c truyn dn u thu mt cch trung thc.

- M ha: c th truyn vi tin cy cao, cc dng c s video, audio

c ng li thnh dng c s ng gi PES (Packetized ES) tng ng vi cc gi

c di thay i. Mi gi PES bao gm mt header v s liu cc dng c s. Cc

gi PES ny c ghp knh vi nhau to ra dng tuyn ti TS (Transport Stream)

hoc dng chng trnh.

- Gii m: Gm cc quy trnh ngc li, dng truyn ti c phn knh tr

li cc dng c s video v dng s cp audio. Sau c gii m video, audio

to li tn hiu.

- ng b: ng b l mt vn quan trng. i vi cc b ghp knh phn

chia theo thi gian, thi gian tr gia b m ha v b gii m ha thng c gi tr

khng i. i vi cc b ghp knh gi MPEG-2, thi gian tr phi thay i theo s

thay i di gi (nu c) v tn s ghp knh. ng b trong h thng ghp knh

MPEG-2 c thc hin thng qua cc nhn thi gian (Time Stamps) v cc chun

ng h (Clock Reference).

Khi mt lung s trn knh v tuyn, cc tn hiu bng gc s phi c bin

i thnh cc tn hiu bng tn v tuyn, qu trnh ny gi l iu ch. Qu trnh ti

to li tn hiu s t cc tn hiu trong bng tn v tuyn gi l gii iu ch.

Qu trnh iu ch bao gm vic kha chuyn bin , tn s, pha ca sng

mang, ti ba phng php iu ch truyn dn s l iu bin, iu tn v iu

pha. Mi phng php iu ch u c u im ring. Vic la chn ph thuc vo

tim nng thng tin, cng sut pht v rng knh. Vic la chn phi m bo:

- Tc s liu cc i

- Xc sut li cc tiu

-28-


- Cng sut pht cc tiu

- rng knh cc tiu

- Kh nng chng nhiu cc i

- Mc phc tp cc tiu

Trong k thut truyn hnh s, vic s dng iu ch kha chuyn pha v kha

chuyn bin kt dng c gi l iu ch cu phng hay bin vung gc (QAM).

1.4.7 Cc phng thc truyn dn tn hiu truyn hnh s

1.4.7.1 Truyn hnh s v tinh

Knh v tinh (khc vi knh cp v knh pht sng mt t) c trng bi

bng tn rng v s hn ch cng sut pht. Khuch i cng sut ca Transponder

lm vic gn nh bo ha trong cc iu kin phi tuyn. Truyn hnh s v tinh c

nhng c im nh sau:

- C ly lin lc ln: mt ng truyn v tinh c th truyn i cc tn hiu vi

khong cch rt xa, nh vy c th t hiu qu cao cho cc ng truyn di cng

nh cho dch v im - im.

- ng truyn v tinh khng b nh hng bi iu kin a hnh, a vt, v

mi trng truyn dn rt cao so vi b mt qu t. Truyn hnh v tinh c th

thc hin qua i dng, rng rm, ni cao cng nh cc a cc.

- Vic thit lp ng truyn qua v tinh c thc hin trong thi gian ngn,

iu ny c ngha quan trng trong vic thu thp tin tc, mt cng vic i hi thi

gian thit lp nhanh chng.

- V tinh cng s dng cho cc h thng im - a im. Vi mt v tinh, c

th t v s trm thu trn mt t, rt thun li cho h thng CATV cng nh cho

dch v truyn hnh trc tip n tn tng gia nh DTN. Ngoi ra, truyn hnh v

tinh cn c kh nng phn phi chng trnh vi cc h thng lin kt khc.

Tuy nhin phng thc ny cn tn ti nhiu nhc im:

-29-


- Cng sut trn v tinh l hu hn, ng th c ly thng tin ln nn suy gim

ng truyn rt ln.

- D b nh hng ca ma, nht l bng tn Ku

- T s SNR thp (10dB hoc t hn)

- Hiu sut s dng bng thng khng cao so vi cc phng php truyn dn

khc.

1.4.7.2 Truyn hnh s cp

iu kin truyn cc tn hiu s trong mng cp tng i d hn, v cc knh

l tuyn tnh vi t s cng sut sng mang trn tp m (C/N) tng i ln. Tuy

nhin rng bng tn knh b hn ch (8MHZ). i hi phi dng cc phng php

iu ch s c hiu qu cao hn so vi truyn hnh s qua v tinh. Truyn hnh cp

c nhng u im nh:

- t chu nh hng ca nhiu cng nghip, m bo cht lng cho tn hiu.

- Khng b nh hng ca thi tit do c kh nng cch ly v chng nhiu tt

ca cp.

- Khng chim dng ph tn s v tuyn.

- Khng gy can nhiu cho cc trm pht sng nghip v khc do tn hiu

truyn trn si cp c cch ly v chng nhiu tt.

- C kh nng cung cp tt dch v truyn hnh s v cc dch v hai chiu

khc: di thng ln ca mng truyn hnh cp hu tuyn s cho php khng ch cung

cp cc dch v truyn hnh tng t m cn cho php cung cp nhiu chng trnh

truyn hnh s, truyn hnh tng tc v c bit l kh nng cung cp cc dch v

vin thng hai chiu, truy cp Internet, truyn s liu tc cao.

- C th s dng cc knh k nhau truyn tn hiu trong tt c cc phm vi

m khng xut hin hin tng nhiu ng knh.

Phng thc ny cng cn mt s nhc im nh:

-30-


- Cht lng hnh nh, m thanh cha thc s nt do c tp m do suy hao

trong qu trnh truyn ti d liu.

- Do phi i dy cp nn d xy ra cc trng hp mt tn hiu v hin tng

pht x tn hiu t cc thit b h tr ca truyn hnh cp.

1.4.7.3 Truyn hnh s mt t

Truyn hnh s mt t c din ph sng hp hn so vi truyn qua v tinh,

song d thc hin hn so vi mng cp. Cng b hn ch bi bng thng nn s dng

phng php iu ch OFDM nhm tng dung lng dn qua mt knh sng v khc

phc hin tng nhiu truyn hnh mt t tng t. Phng php ny c nhng

c im:

- Trong phm vi ph sng, cht lng n nh, khc phc c cc vn

phin toi nh hnh nh c bng, can nhiu, tp nhiu, tp m

- My thu hnh c th c lp t d dng cc v tr trong nh, c th xch

tay hoc lu ng ngoi tri.

- C dung lng ln, cha m thanh (nh m thanh nhiu ng, lp th, bnh

lun) v cc d liu.

- C th linh hot chuyn i t pht chng trnh c hnh nh v m thanh

cht lng cao (HDTV) sang pht nhiu chng trnh cht lng thp hn v ngc

li.

Tuy nhin, phng thc truyn hnh s mt t cng c mt s nhc im:

- Knh b gim cht lng do hin tng phn x nhiu ng (multipath).

- Gi tr tp do con ngi to ra l cao.

- Do phn b tn s kh dy trong ph tn i vi truyn hnh, giao thoa gia

truyn hnh tng t v s l vn phi cn xem xt.

Nhn chung c 3 phng php truyn dn truyn hnh s trn u c nhng

u, nhc im ring v chng s b sung, h tr ln nhau. Nu truyn hnh qua v

-31-


tinh c th ph sng mt khu vc rt ln vi s lng chng trnh ln n hng trm

th tn hiu s trn mt t dng chuyn cc chng trnh khu vc, nhm vo mt

s lng khng ln ngi thu.

ng thi, ngoi vic thu bng anten c nh trn mi nh, truyn hnh s mt

t cn cho php thu c bng anten nh ca my tnh xch tay, thu trn di ng

(trn t, my bay). Truyn hnh s truyn qua mng cp phc v thun li cho i

tng l c dn cc khu ng c, khng c iu kin lp anten thu v tinh hay

anten mt t.

1.5 Cc tiu chun ca truyn hnh k thut s

Chun truyn dn truyn hnh s s dng qu trnh nn v x l s c kh

nng truyn dn ng thi nhiu chng trnh truyn hnh trong mt dng d liu,

cung cp cht lng nh khi phc thy theo mc phc tp ca my thu. Trn th

gii c 5 nhm tiu chun truyn hnh s chnh:

Hnh 1.7: Bn phm vi ng dng cc tiu chun truyn hnh s trn th gii

[10].

-32-


ATSC (Advanced television System Committee): Tiu chun Bc M/Hn

Quc

DVB (Digital Video Broadcasting): Tiu chun Chu u

ISDB (Intergrated Services Digital Broadcasting): Tiu chun Nht Bn

ISDTV/ISDB-Tb (Brazilian International Standard for Digital Television):

Tiu chun Brazil/M Latinh

DTMB (Chinese Standard for Digital Television): Tiu chun Trung Quc

Cc im tng ng gia cc h thng l s dng cng mt bng tn s, ci

thin phn gii theo chiu dc v ngang c bit, nng cao phn hin th mu sc

v nh dng vi t l 16:9, h tr m thanh a knh trung thc cao v truyn d

liu.

Vi truyn hnh s mt t, hin ti hu ht cc nc trn th gii ang s

dng cc chun truyn hnh sau:

1. Tiu chun ATSC

Nm 1996, FCC chp nhn tiu chun truyn hnh s ca M da trn tiu

chun gi d liu quc t 188 byte MPEG-2. Cc ch tiu k thut c th c quy

nh bi ATSC. ATSC cho php 36 chun video t HDTV n cc dng thc video

tiu chun SDTV khc vi cc phng thc qut (xen k, lin tc) v cc t l khun

hnh khc nhau.

ATSC c cu trc dng lp, tng thch vi m hnh lin kt h thng m

(OSI) 7 lp ca mng d liu. Mi lp ATSC c th tng thch vi cc ng dng

khc cng lp. ATSC s dng dng thc gi MPEG-2 cho c video, audio v d liu

ph. Cc n v d liu c di c nh ph hp vi sa li, ghp dng chng

trnh, chuyn mch, ng b, nng cao tnh linh hot v tng thch vi dng thc

ATM.

-33-


2. Tiu chun ISDB-T

ISDB-T cn c gi l tiu chun DiBEG ca Nht Bn, ban hnh vo nm

1997, s dng k thut ghp knh on di tn BTS (Band Segmened) OFDM v

cho php s dng cc phng thc iu ch tn hiu s khc nhau i vi tng on

d liu nh QPSK, DQPSK, 16QAM hoc 64QAM.

ISDB-T s dng tiu chun m ha MPEG-2 trong qu trnh nn v ghp knh.

H thng s dng phng php ghp knh phn chia theo tn s trc giao OFDM cho

php truyn a chng trnh phc tp vi cc iu kin khc nhau, truyn dn phn

cp, thu di ng c th s dng cho cc knh truyn 6, 7 v 8MHz.

3. Tiu chun Chu u (DVB-T)

DVB dng iu ch ghp knh phn chia tn s trc giao c m (COFDM),

tc bit ti a 24Mbps (di thng 8MHz).

im ni tri nht ca COFDM l ch dng d liu cn truyn ti c phn

phi cho nhiu sng mang ring bit. Mi sng mang c x l ti mt thi im

thch hp v c gi l mt COFDM Symbol. Cc sng mang ring bit c

iu ch QPSK, 16QAM hoc 64QAM. T l m ha thch hp ca m sa sai cng

gp phn ci thin cht lng h thng [8].

Nh vy, im ging nhau ca c ba tiu chun trn l s dng chun nn

MPEG-2 cho tn hiu video. im khc nhau c bn l phng php iu ch.

ATSC s dng k thut iu ch iu bin ct ca nhng nm 1980. iu

bin ct 8-VSB (Vestigal Side Band) cho t s tn hiu trn tp m tt hn nhng li

khng c kh nng cho thu di ng, khng khc phc hin tng phn x v khng

thit lp c mng n tn nh gii php ca h thng ISDB-T v DVB-T.

DiBEG c tnh phn lp cao, cho php a loi hnh dch v, linh hot mm

do, tn dng ti a di thng, c kh nng thu di ng nhng khng tng thch vi

cc dch v truyn hnh qua v tinh, truyn hnh cp.

-34-


DVB-T vi phng php iu ch COFDM t ra c nhiu c im u vit,

nht l i vi cc nc c a hnh phc tp, c nhu cu s dng mng n tn v

c bit l kh nng thu di ng.

1.6 Kt lun chng I

S dng cng ngh truyn hnh s em li nhiu li ch cho ngi s dng,

hiu qu cao cho nh cung cp dch v. Cng ngh truyn hnh s khng ch tng s

knh truyn m cn cho php cc nh cung cp dch v m rng kinh doanh ra cc

dch v mi m vi cng ngh tng t khng th thc hin c. Hin nay truyn

hnh s pht trin ht sc a dng v loi hnh dch v, phng thc truyn dn v

pht sng. Trong 3 tiu chun pht sng truyn hnh s, truyn hnh s mt t DVB-

T s dng phng php iu ch COFDM, m ha theo tiu chun MPEG-2 t ra

c nhiu u im ni bt v c nhiu nc trn th gii la chn trong c Vit

Nam.

-35-


CHNG II - TRUYN HNH S MT T THEO TIU CHUN

DVB-T

Chun truyn hnh s s dng qu trnh nn v x l s c kh nng truyn

dn ng thi nhiu chng trnh truyn hnh trong mt dng d liu, cung cp cht

lng nh khi phc ty theo mc phc tp ca my thu. Chun truyn hnh s l

mt s thay i ng k trong nn cng nghip sn xut v qung b cc sn phm

truyn hnh. N mang li tnh mm do v hiu qu s dng cao do c nhiu dng

thc hnh nh khc nhau trong nn s. Hin nay, trn th gii tn ti song song ba

tiu chun truyn hnh s. Trong DVB-T l tiu chun ang c ng dng v s

dng rng ri nht nhiu quc gia.

2.1 Gii thiu v h thng truyn hnh s mt t DVB-T

DVB-T (Digital Video Broadcast Terrestrial) l tiu chun truyn hnh s

c t chc ETSI (European Telecommunications Standards Institute) cng nhn

vo thng 2 nm 1997.

DVB-T s dng k thut COFDM (Code Orthogonal Frequency Division

Multiplexing). COFDM l k thut c nhiu c im u vit, c kh nng chng li

phn x nhiu ng, ph hp vi cc vng dn c c a hnh phc tp, cho php

thit lp mng n tn (SFN Single Frequency Network) v c kh nng thu di

ng, ph hp vi cc chng trnh c nt cao HDTV.

DVB-T l thnh vin ca mt h cc tiu chun DVB, trong bao gm cc

tiu chun truyn hnh s qua v tinh, truyn hnh s mt t v truyn hnh s cp.

Phm vi ca tiu chun:

- Tiu chun ny m t h thng truyn dn cho truyn hnh s mt t. N

xc nh h thng iu ch, m ha knh dng cho cc dch v truyn hnh s mt t

nhiu chng trnh nh: LDTV/SDTV/EDTV/HDTV.

- Tiu chun m t chung h thng c bn ca truyn hnh s mt t.

-36-


- Tiu chun xc nh cc yu cu ch tiu chung, v cc c im ca h

thng c bn, mc ch t c cht lng dch v.

- Tiu chun xc nh tn hiu c iu ch s cho php vic tng thch

gia cc phn thit b c sn xut bi cc nh sn xut khc nhau. t c iu

ny bng cch m t chi tit tn hiu x l pha cc module, trong khi th vic

x l cc my thu l m cho cc gii php thc hin khc nhau

2.2 S khi h thng truyn hnh s mt t

S khi ca h thng truyn hnh s mt t theo tiu chun DVB-T c

th hin nh hnh 2.1:

Hnh 2.1: S khi h thng truyn hnh s mt t DVB-T [3].

1. Pha pht

- Tn hiu Video/Audio ngun:

Tn hiu ngun l tn hiu s hay tng tc bin i thnh cc d liu s.

Cc chun tn hiu s c nh dng sao cho tng thch vi h hng m ha.

Tn hiu video c tc bit rt ln, chng hn chun CCIR 601 th tc bit

tng ln n 270 Mbps. cc knh truyn hnh qung b c rng 8 MHz c th

Video,

Studio s

M ha ngun M ha truyn dn

(knh)

a hp/sa li

iu ch

Chuyn i D/A

Gii m

ngun

Gii m truyn dn Gii a hp/sa li

Gii iu

ch TV

Pha pht

Pha

thu

TX

RX

-37-


p ng cho vic truyn tn hiu s, cn phi gim tc bit bng nhiu cch nn tn

hiu video.

- M ha ngun d liu (Source Coding)

M ha ngun d liu thc hin nn s cc tn s nn khc nhau. Vic nn

c thc hin bng b m ha MPEG-2. Vic m ha da trn c s nhiu khung

hnh nh cha nhiu thng tin vi s sai khc rt nh.

Do , MPEG lm vic bng cch ch gi i nhng s thay i ny v d liu

lc ny c th gim t 100 n 200 ln. Vi tn hiu Audio cng vy, vic nn da

trn nguyn l tai nghe ngi kh phn bit m thnh trm nh so vi m thanh ln

khi chng c th b i v khng c s dng.

M ha ngun ch lin quan n cc c tnh ca ngun. Phng tin truyn

pht khng nh hng n m ha ngun.

- M ha knh:

Gi v a hp Video, Audio v cc d liu ph vo mt dng d liu ph

y l dng truyn ti MPEG-2. Nhim v ca m ha knh l lm cho tn hiu truyn

pht sng ph hp vi knh truyn.

Trong truyn hnh s mt t m c s dng l m Reed-Solomon. M

Reed-Solomon c s dng rng ri trong h thng thng tin ngy nay do c kh

nng sa li rt cao.

- iu ch:

iu ch tn hiu pht sng bng dng d liu, qu trnh ny bao gm c m

ha truyn dn, m ha knh v cc k thut h thp xc sut li, chng li cc suy

gim cht lng do pha inh, tp nhiu [3].

2. Pha thu

Bn thu s thc hin qu trnh m gi, gii m, hin th hnh v a ra my

thu.

-38-


2.3 Cc c tnh k thut ca tiu chun DVB-T

2.3.1 B iu ch DVB-T

B iu ch DVB-T c th hin qua hnh 2.2 di y:

Hnh 2.2: S khi b iu ch s DVB-T [5].

- Khi phn tn nng lng

Dng s vo ca h thng c t chc thnh cc gi c di c nh chnh

l u ra ca b ghp truyn dn cc dng MPEG-2. Chiu di tng cng ca mi gi

sau b ghp l 188 byte, trong c mt byte chc t m ng b. Th t x l

pha pht lun lun theo th t t bit MSB (bit 0) ca byte cha t m ng b.

m bo cc chuyn i nh phn c thc hin chnh xc th d liu ca khi ghp

knh MPEG-2 u vo h thng c m ha.

D liu vo: 1011100xxxxxxxx

Dy PRBS: 00000011

a thc b to chui PRBS l 1 + X14 + X15

Vic np dy 100101010000000 vo thanh ghi PRBS c bt u ti u ca

mi lt gi truyn dn. cung cp tn hiu khi to cho b gii nhiu tn hiu

(descrambler), byte ng b ca gi truyn dn u tin trong nhm 8 gi cc bit

c o li. Ton b qu trnh c gi l qu trnh thch nghi ghp truyn dn.

Tch sng

MPEG

Phn

tn

nng

lng

nh x

(Mapper)

Ghp

xen

ngoi

M ha

ngoi

M ha

trong

Ghp

xen

trong

Chn

khong

bo v

Thch ng

khung IFFT

Lc

FIR IF/RF

Khuych

i

Lc

Band Pass

-39-


Bit u tin ti u ra b to PRBS s l bit u tin (MBS) ca byte u tin

sau byte t m ng b c o. h tr thm cc chc nng ng b, trong

thi gian ca cc byte ng b ca 7 gi truyn dn ngay sau , chui PRBS vn

c to nhng li khng ly ra khin cc byte ng b ny khng c ngu nhin

ha. V th chu k ca PRBS l 1503 byte.

Qu trnh ngu nhin ha cng trng thi tch cc khi dng bit u vo b

iu ch khng tn ti hoc khng cng nh dng vi dng MPEG-2.

- Khi m ngoi

M ngoi v tro ngoi s c thc hin trn cu trc gi u vo theo hnh

v (hnh 2.3a). m rt ngn Reed-Solomon R-S c thc hin vi tng gi v

c ngu nhin ha (188 byte) nh hnh 2.3b to ra gi c tnh chng li (hnh

2.3c. M R-S cng c thc hin vi byte ng b. Gi d liu c th c o

hoc khng o.

a) Gi truyn ti MPEG-2.

b) Gi truyn ti ngu nhin ha: Cc byte ng b v cc byte d liu ngu

nhin ha.

SYNC

1byte MPEG-2 Transport MUX data (187 byte)

8 transport MUX packet

PRBS period = 1503 bytes

SYNC 8

Randomized

Data SYNC

1 SYNC

2

SYNC 1

Randomized

Data Randomized

Data Randomized

Data

-40-


c) Gi chng li vi m Reed-Solomon R-S (204, 188,8).

d) Cu trc d liu sau khi ghp xen ngoi.

Hnh 2.3: Cc bc ca qu trnh phn tn nng lng, m ha ngoi v ghp xen

ngoi [5].

- Khi ghp xen ngoi

Khi ny thc hin ghp xen k vi su I = 12 cc gi d liu (Outer

Interleave) c bo v li R-S. Cc gi c bo v khi li R-S c chiu di l

204 byte c chia thnh 17 khi vi 12 byte mi khi. Mi phn t ghi dch FIFO

c cu trc thnh byte. Cc chuyn mch u vo v u ra c ng b vi nhau.

Cc byte ng b ca MPEG-2 lun chy khng tr qua nhnh 0 ca b ghp xen.

Qu trnh ghp xen k s phn b cc li cm (burst error) qua mt s cc khi lm

cho vic sa sai c th thc hin mt cch hiu qu.

B chn xen k c 12 nhnh kt ni vng trn i vi dng byte u vo. Mi

nhnh c cha mt thanh ghi dch vo trc ra trc FIFO (First In First Out) c

chiu su l MJ.

Trong : M: l tr c bn = N/J = 17

N: l chiu di khung = 204 bytes

J: l tham s chy t 0 11

SYNC 1 or

SYNC n

Reed-Solomon

16 bytes MPEG-2 transport MUX data (187 bytes)

204 bytes

SYNC 1 or SYNC n

203 bytes SYNC 1 or SYNC n

203 bytes SYNC 1 or

SYNC n

-41-


Qu trnh sa li c chia thnh 2 phn: m ngoi (Outer Coder) v m trong

(Inner Coder). M trong s dng phng php m ha xon (Convolution Code).

- Khi m trong

M trong da trn c s b m ha vng xon gc c tc 1/2 vowis 64 trng

thi. H thng DVB-T cho php tao cc t l m khc nhau bng cch tch trch

(Puncturing) d liu sau b m ha xon. iu cho php la chn kh nng sa

sai cng nh tc d liu trong cc mode truyn phn cp v khng phn cp.

- Khi ghp xen trong

Khi ghp xen trong (Inner Interleaving) bao gm khi ghp xen bit v khi

ghp xen symbol.

Ghp xen bit

u vo b ghp xen trong c th c ti hai u vo, c tch thnh 2, 4 hoc

6 dng con theo th t iu ch v loi iu ch. Trong mode khng phn cp, dng

bit vo duy nht c tch ra thnh v dng bit con, v = 2 khi iu ch QPSK, v = 4

khi iu ch 16 QAM v v = 8 khi iu ch 64 QAM. Trong mode iu ch phn cp,

c hai dng u tin thp thnh 4 dng con. iu ny c thc hin vi c hai ch

iu ch QAM ng nht v khng ng nht.

Ghp xen symbol

Mc ch ca ghp xen symbol l nh x cc t m v bit ln 1512 sng mang

(trong mode 2K) hay 6048 sng mang (trong mode 8K) thuc mt symbol OFDM.

B ghp xen symbol thc hin ghp xen cc khi 1512 hay 6048 symbol d liu.

2.3.2 M ha COFDM trong DVB-T

S khi ca b iu ch COFDM c minh ha nh hnh 2.3. C s ca

iu ch COFDM da vo thut ton bin i ngc Fourrier nhanh IFFT (Inverse

Fast Fourrier Transform).

-42-


Hnh 2.4: Khi iu ch COFDM trong DVB-T.

2.3.2.1 Nguyn l COFDM

Nh chng ta bit h pht s DVB-T s dng k thut COFDM (ghp tn

s trc giao c m) nh mt phng thc iu ch d liu. OFDM l mt dng c

bit ca h thng iu ch a sng mang. Cc sng mang da trn nguyn tc phn

chia lung d liu thnh cc lung d liu con ln cc sng mang. Cc sng mang

c iu ch vi tc bit thp v vi s lng sng mang ln s mang c lung

d liu c tc bit cao.

tng u tin ca OFDM xut pht t khi xem xt s suy yu xy ra trong

pht sng cc knh mt t. p ng ca knh khng tng ng vi tng di tn

nh do c nhiu tn hiu nhn c (tn hiu chnh + tn hiu phn x), ngha l s

khng cn nng lng thu hoc s thu c nhiu hn mt tn hiu. c th

lp li c nhng d liu mt bn thu, cn m ha d liu trc khi pht.

a. Tnh trc giao ca cc sng mang

Vic s dng s ln sng mang tng nh khng c trin vng lm trong thc

t v khng chc chn, v s cn rt nhiu b iu ch, gii iu ch v cc b lc i

km theo. V cng c v nh s cn mt di thng ln hn cha cc sng mang

ny. Nhng vn trn c gii quyt khi cc sng mang m bo iu kin

c t u n cch nhau mt khong fU = 1/TU vi TU l khong symbol hu ch

(u: useful). y chnh l iu kin trc giao ca cc sng ang trong h thng ghp

knh phn chia tn s trc giao. Hnh 2.4 biu din hnh nh ca ph tn hiu OFDM

vi 16 sng mang trc giao nhau trong di thng knh truyn dn v ph tn hiu RF

B x l

trc

IFFT

K/8K 2

D/A Chuyn i

D/A

I/Q

B iu ch

Data

R (Real)

I (Image)

I

Q

IF Output Input

Chuyn i

-43-


ca my pht s DVB-T c di thng 8 MHz. Cc thnh phn ph ca my pht s

DVB-T (gm hng ngn sng mang) chim ht di thng 8 MHz.

Hnh 2.5: Ph tn hiu OFDM vi s sng mang N=16.

- V m ton hc, vic trc giao s nh sau: sng mang th k c biu din:

tjk

KUet )( (2.1)

Vi = 2/TU, v iu kin trc giao m sng mang phi tha mn l:

0)()( *

dttt L

T

K

U

, k L (2.2)

= TU, k = l

- V ngha vt l: khi gii iu ch tn hiu cao tn ny, b gii iu ch

khng nhn thy cc tn hiu cao tn kia, kt qu l khng b cc tn hiu co tn khc

gy nhiu.

- V phng din ph: im ph c nng lng cao nht ca mt sng mang

roi vo im bng khng ca sng mang khc. V cc sng mang c t gn nhau

nn tng cng di ph cng ch nh iu ch sng mang n nu chng c

iu ch vi tt c d liu v s dng b ct nh l tng.

C nhiu nguyn nhn gy ra suy gim tnh trc giao v do s gy ra nhiu

tng h gia cc sng mang (ICI). Chng c th l cc li xy ra trong vic ly mu

tn s ca my thu hay phase-noise trong cc b to dao ng ni. Tuy nhin trong

thc t, nhng nh hng ny c th c gi mc gii hn c th chp nhn c.

-44-


b. Bin i IFFT v iu ch tn hiu

K thut iu ch a sng mang phi s dng nhiu mch iu ch cu phng

v cc b lc nhng chng ta trnh c iu ny da vo thnh cc sng mang

c iu ch theo mt kiu no trn min thi gian lin tc. Ty thuc vo

kiu iu ch mi t hp bit trong chui bit u vo c gn cho mt tn s sng

mang, v vy mi sng mang ch ti s lng bit c nh. Nh b nh x (Mapper) v

iu ch M-QAM, sng mang sau khi iu ch QAM l mt s phc v c xp vo

biu chm sao theo quy lut m Gray trn trc Re (trc thc) v trc Im (trc o).

V tr ca mi im tn hiu (s phc) trn biu chm sao phn nh thng tin v

bin v pha ca cc sng mang. Qu trnh bin i IFFT s bin i cc s phc

biu din cc sng mang trong min tn s thnh cc s phc biu din cc sng

mang trong min thi gian ri rc. Trong thc t, cc thnh phn Re v Im c biu

din bng chui nh phn c b iu ch IQ s dng iu ch sng mang cng

c biu din bng mt chui nh phn. Chui nh phn sau iu ch IQ c bin

i D/A nhn li tn hiu trong bng tn c bn.

Qu trnh x l pha thu s thc hin bin i FFT to cc im iu ch

phc ca tng sng mang ph trong symbol OFDM, sau khi gii nh x (Demapping)

xc nh biu bit tng ng cc t hp bit c cng li khi phc dng d

liu truyn.

c. La chn iu ch c s

Ti mi symbol, mi sng mang s c iu ch bi mt s phc ly t tp

chm sao. Ty thuc vo kiu iu ch c s c chn l QPSK, 16 QAM hay 64

QAM mi sng mang s vn chuyn c s bit d liu l 2,4 hoc 6 bit. Tuy nhin

vi cng sut pht c nh, khi c nhiu bit d liu trong mt symbol th cc im

trong chm sao cng gn nhau hn v kh nng chng li s b gim. Do vy cn c

s cn i gia tc v mc li.

Vi m hnh iu ch khng phn cp lung s d liu u vo c tch

thnh cc nhm c s bit ph thuc vo kiu iu ch c s. Mi nhm bit ny mang

-45-


thng tin v pha v bin ca sng mang v tng ng vi mt im trn biu

chm sao. Hnh 2.5 biu din cc chm sao ca iu ch QPSK (4 QAM), 16 QAM

v 64 QAM khng phn cp.

Hnh 2.6: Biu din chm sao ca iu ch QPSK, 16 QAM, v 64 QAM [5].

Trong m hnh iu ch phn cp, hai lung s liu c lp s c truyn

trong cng mt thi im. Lung d liu c mc u tin cao (HP) c iu ch

QPSK v lung c mc u tin thp c iu ch 16 QAM hoc 64 QAM.

2.3.2.2 S lng, v tr v nhim v ca cc sng mang

Tn hiu truyn i c t chc thnh cc khung (Frame). C 4 khung lin tip

to thnh mt siu khung. L do vic to ra cc khung l phc v t chc mang

thng tin tham s bn pht (bng cc sng mang bo hiu tham s bn pht

Transission Parameter Signalling TPS carriers). L do ca vic hnh thnh cc siu

khung l chn va mt s nguyn ln gi m sa sai Reed-Solomin 204 byte

trong dng truyn ti MPEG-2 cho d ta chn bt k cu hnh thm s pht, iu ny

trnh vic phi chn thm cc gi m khng cn thit. Mi khung cha 68 symbol

OFDM trong min thi gian (c nh du t 0 n 67). Mi symbol ny cha hng

ngn sng mang (6817 sng mang vi ch 8K, v 1705 sng mang vi ch 2K)

nm dy c trong di thng 8MHz (Vit Nam ch di thng 8MHz, c nc chn

7MHz). Hnh 2.6 biu din phn b sng mang ca DVB-T theo thi gian v tn s.

000010 64 QAM

00

4QAM 16QAM 0010

-46-


Hnh 2.7: Phn b sng mang ca DVB-T (cha chn khong bo v).

Nh vy, trong mt symbol s cha:

- Cc sng mang d liu (video, audio) c iu ch M-QAM. S lng

cc sng mang d liu ny ch c 6048 vi 8K v 1512 vi 2K.

- Cc pilot (sng mang) lin tc: bao gm 177 pilot vi 8K v 45 pilot vi 2K.

Cc pilot ny c v tr c nh trong di tn 8MHz v c nh trong biu chm sao

u thu sa li tn s, t ng iu chnh tn s (AFC) sa li pha.

- Cc pilot (sng mang) ri rc (phn tn): bao gm 524 pilot vi 8K v 131

pilot vi 2K c v tr c nh trong biu chm sao. Chng khng c v t c nh

trong min tn s, nhng c tri u trong di thng 8MHz. Bn my thu khi nhn

c cc thng tin t cc pilot ny s t ng iu chnh t c p ng knh

tt nht v thc hin vic hiu chnh (nu cn).

- Khc vi sng mang cc chng trnh, cc pilot khng iu ch QAM, m

ch iu ch BPSK vi mc cng sut ln hn 2.5 dB so vi cc sng mang khc.

Hnh 2.7 biu din phn b sng mang pilot ri rc v lin tc vi mc cng sut ln

hn cc sng mang d liu 2.5 dB.

-47-


Hnh 2.8: Phn b cc pilot ca DVB-T.

- Cc sng mang thng s pht TPS (Transmission Parameter Signalling) cha

nhm thng s pht c iu ch BPSK v th trn biu chm sao, chng nm

trn trc thc. Sng mang TPS bao gm 68 sng mang trong ch 8K v 17 sng

mang trong ch 2K. Cc sng mang TPS ny khng nhng c v tr c nh trn

biu chm sao, m cn hon ton c nh cc v tr xc nh trong di tn 8MHz.

Hnh 2.8 biu din v tr cc pilot v sng mang TPS c iu ch BPSK.

Hnh 2.9: Phn b cc pilot ca DVB-T trn biu chm sao [5].

Sng mang d liu

-48-


2.3.2.3 Chn khong thi gian bo v

Trong thc t khi khong t hp thu c tri theo 2 symbol th khng ch c

nhiu gia cc symbol (ISI) m cn c nhiu tng h gia cc sng mang (ICI).

trnh iu ny ngi ta chn thm khong bo v (Guard Interval Duration) Tg trc

mi symbol m bo cc thng tin l n t cng mt symbol v xut hin c nh

(hnh 2.9).

Hnh 2.10: Phn b sng mang khi chn thm khong thi gian bo v.

Khong bo v l thi gian thit b thu ch i, trc khi x l tn hiu. Tg

cng ln, khong cch ti a gia cc my pht hnh cng ln. Tuy nhin, v gc

l thuyt thng tin, Tg c gi tr cng nh cng tt, bi l Tg l khong thi gian khng

c s dng trong knh truyn. Tg ln s lm gim dung lng ca knh truyn.

Mi khong symbol c ko di thm v th n s vt qu khong t hp

ca my thu TU. Nh vy on thm vo ti phn u ca symbol to nn khong

bo v s ging vi on c cng di ti cui symbol. Min l tr khng vt qu

on bo v, tt c thnh phn tn hiu trong khong t hp s n t cng mt symbol

v tiu chun trc giao c tha mn. ICI v ISI ch xy ra khi tr vt qu khong

bo v.

-49-


di khong bo v c la chn sao cho ph hp vi mc thu a ng

(multipath) ca my thu. Vic chn khong thi gian bo v c thc hin ti pha

pht. Khong thi gian bo v Tg c cc gi tr khc nhau theo quy nh ca DVB:

1/4TU, 1/8TU, 1/16TU, 1/32TU.

Hnh 2.11: Cc tia sng n trong khong thi gian bo v [5].

Khi chnh lch thi gian ca cc tia sng n u thu khng vt qu khong

thi gian bo v Tg, th my thu hon ton khc phc tt hin tng phn x (hnh

2.10). Thc cht, khong thi gian bo v Tg l khong thi gian trng khng mang

thng tin hu ch, v vy, cng ch pht, Tg cng ln, thng tin hu ch s cng t,

s lng chng trnh s gim. Nhng Tg cng ln kh nng khc phc cc tia sng

phn x t xa n cng hiu qu. Vi s dng k thut ghp a tn trc giao v vi

thng s khong thi gian bo v ny to tin cho vic thit lp mng n tn

DVB-T. Cc my pht thuc mng n tn u pht cng mt knh sng, rt thun

li cho quy hoch v tit kim ti nguyn tn s.

2.3.3 Chm sao tn hiu v qu trnh nh x

thc hin pht sng trn mt t, h thng DVB-T s dng k thut iu

ch a sng mang MCM (Multi Carrier Modulation) vi tng sng mang c iu

g t

t

t

t

t

t

-50-


s QPSK hoc QAM. Cc sng mang n l c truyn i ng thi bng phng

php ghp knh phn chia tn s trc giao OFDM. Cc sng mang c iu ch s

qua php nh x (Mapping) da trn c s biu chm sao (constellation) ca k

thut iu ch s. Php nh x thc hin gn cc gi tr symbol u vo ln cc im

ca chm sao tng ng vi pha v ln ca cc vecto bao gm phn thc (Real)

v phn o (Image). Trong mi symbol l t hp ng thi ca gi tr iu ch

trc giao I (Inpha) v Q (Quadrature) to nn im chm sao. Khi nh x (Mapping)

thc hin tip ni theo sau khi m ha xon (Convolutional Coder). Php nh x cn

ph thuc cc mode truyn phn cp hoc khng phn cp (Hierachical, Non-

Hierachical) c p dng cho vic truyn ghp knh a sng mang phn chia trc

giao OFDM).

2.3.3.1 Mode truyn khng phn cp (Non-Hierachical)

Mode truyn khng phn cp s dng cc gi truyn ti TS qua m ha

knh v m ha chn xen k v s c nh x ln mu chm sao thch hp. iu

ny c ngha l tt c cc gi truyn MPEG-2 s c b iu ch x l nh nhau v

s c mnh nh nhau khi truyn. Tc l th tc m ha knh khng phn cp s

khng phn bit cc bit c bit trong cc gi truyn MPEG-2 c nh x n nhng

v tr c bit trong biu chm sao. Do , truyn khng phn cp s s dng b

iu ch thng thng (cn bng ng u) vi h s =1 nh hnh 2.11 [6].

-51-


Hnh 2.12: nh x 16 QAM v 64 QAM chm sao u vi =1.

2.3.3.2 Mode truyn phn cp (Hierachical)

H thng DVB cho php truyn phn cp vi dng truyn ti MPEG-2 qua

khi phn tch (Splitter) c hai dng truyn ti MPEG: u tin cao HP (High

Priority) v u tin thp (Low Priority). Vi mode phn cp d liu dng truyn

MPEGtachs bit s c chn tin hnh m ha knh phn cp (Hierachical

Coding). D liu s c nh x vo nhng im ca chm soa khng u (Non-

uniform). B iu ch s khng ng u vi h s > 1 c p dng cho mode

phn cp s cho mnh cao trong qu trnh truyn. iu ch phn cp thc ra l s

dch bin ca 16 QAM v 64 QAM thng qua 4 QAM nh trn hnh 2.12.

-52-


Hnh 2.13: Chm sao iu ch phn cp DVB-T [6].

iu ch phn cp c xem nh l s phn tch knh RF thnh hai knh o,

mi knh o truyn cc d liu vi tc , t l m ha khc nhau v do vy cho cc

vng ph sng khc nhau. Vic nh x dng bit trc tin p dng chm sao quay 4

Qam. Mi trng thi bin v pha ca chm sao 4 QAM xc nh gc phn t m

sng mang chim gi trong chm sao . Nu d liu th 2 c nh x bi cp 2 bit

th chm sao phn cp chnh l iu ch 4 QAM qua 4 QAM v biu chm sao lc

ny ging nh 16 QAM. Cn nu d liu th hai c nh x bi 4 bit th chm sao

phn cp chnh l iu ch 16 QAM qua 4 QAM v biu chm sao lc ny ging

nh 64 QAM. S cc biu chm sao ca iu ch s khng ng u vi > 1

c cho hnh 2.13 vi nhng m nh x trn biu chm sao.

Dng d liu th nht lun lun s dng iu ch 4 QAM c gi l dng d

liu u tin cao (HP) v dng th hai c gi l dng u tin thp (LP).

0

1

0

HP LP

101

001

1

0 1101 10 1101

10

01

1001

1101

10

4QAM over 4QAM 16QAM over 4QAM

HP LP

-53-


Hnh 2.14: nh x 16 QAM v 64 QAM khng ng u vi = 4.

-54-


H s trong iu ch phn cp l t l gia khong cch ca 2 im gn nht

ca 2 gc phn t cn k v khong cch gia 2 im gn nhau nht trong cng mt

gc phn t ca chm sao. H s cng ln th cng li cho iu ch HP nhng iu

ch LP li chu nhiu km hn.

Nh vy, dng d liu tc thp cn c gi l dng c u tin cao HP.

N mang d liu cho php nhn c ngay khi iu kin truyn pht xu (c nhiu

cao hoc thu di ng). Hai dng c u tin khc nhau s cho vng ph sng khc

nhau.

2.3.4 M sa sai

H thng DVB-T s dng m sa sai R-S (Reed-Solomon) v m cun. M

Reed-Solomon l mt m sa li thuc mt lp con ca m BCH - lp m BCH

khng nh phn. M BCH (m Bose, Chaudhuri v Hocquenhem) l mt loi m sa

li vng ngu nhin quan trng, c kh nng sa c nhiu li v c ng dng

rt rng ri. B m R-S nhn mt khi thng tin ri thm vo cc bit d. Li xut

hin trong khi truyn hoc lu tr c th do nhiu nguyn nhn (nh tp m). B gii

m R-S s x l tng khi d liu v c gng sa li khi phc li d liu ban u.

S lng v kiu li c th sa c ph thuc vo cc c tnh ca m R-S .

u im ca vic s dng m R-S v m cun l xc sut tn ti ca mt li

cn li trong d liu c m ha thng l nh hn rt nhiu so vi xc sut ca

mt li khi khng s dng m R-S. Kh nng ny thng c gi l tng ch iu

ch.

2.3.5 DVB-T trong mi trng b phn x - Mng n tn (SFN)

H pht s DVB-T s dng ghp a tn trc giao OFDM c nhiu u im.

Trong c mt u im rt ni tri v quan trng (hn hn h ATSC ca M) l

cho kh nng thit lp mng n tn. Khi thit lp mng n tn, tt c cc my pht

thuc mng n tn u pht cng knh sng, rt thun li cho quy hoch v tit

kim ti nguyn tn s. Mng n tn tun th 3 iu kin:

-55-


- Cc my pht cng mt dng truyn ti TS: cc my pht thuc mng n

tn ch pht ng mt dng truyn ti duy nht (c v ni dung, c v thi gian). V

ni dung, c ngha l ti bt k my pht no cng khng c lm mt tnh thng

nht ca dng TS .

- Pht cng tn s: cc my pht phi pht cng tn s, nh bit cc my

pht s DVB-T khng c b dao ng hnh sin to cc sng mang nhu cc h thng

truyn hnh tng t. Hng ngn sng mang trong b iu ch s c to ra l do

tn hiu ca dng TS sau khi chia nh ra c bin i Fourier ngc to nn.

chnh xc ca tn s lin quan cht ch ti chnh xc ca dng TS. Nh vy, vic

ng b cc dng TS cng ng ngha vi vic thc hin ng b tn s khu iu

ch ca cc my pht.

- Pht cng thi im: cc my pht pht cng thi im mc ch

nhn mnh tnh kht khe ca ng b: cng thi im pht gi u tin ca cng

mt Mega-frame ra khng trung tt c cc my pht trong mt mng n tn, khng

c s nhanh chm hn nhau, ni cch khc s chnh lch thi gian thi gian pht gi

ny ti tt c cc my pht trong mt mng n tn phi bng khng. y chnh l

vn ct li ca qu trnh ng b.

V vy, mng n tn hot ng hiu qu cn thc hin tt vic thit lp v

hiu chnh ng b gia cc my pht

Trong DVB-T cn p ng yu cu v khong phng v, m bo n cn

phi gi khong cch gia cc my pht phi m bo cc iu kin khong bo v,

tc l khng c vt khong cch cc i cho php vi nhau.

2.3.5.1 DVB-T trong mi trng b phn x nh l mng n tn t nhin

Phn x l hin tng chung v ph bin ca truyn sng in t.

Trong mi trng thc t, chng ta ang chu hu qu ca hin tng phn x

sng khi thu cc chng trnh truyn hnh. i vi cng ngh analog, nhiu sng n

-56-


anten thu v gy ra nhiu hnh trn tivi, to nn bng ma lem nhem, thm ch cc hnh

nh ph nhau lm mt ng b v khng th xem c.

Sng ca my pht hnh s cng chu quy lut phn x, nhng do k thut

ghp a tn trc giao v nh c thng s khong thi gian bo v ca DVB-T, nn

cc thit b thu s DVB-T c th khc phc hiu qu hin tng thu phn x.

2.3.5.2 iu kin thu tt trong mi trng c phn x

H thng DVB-T c mt thng s lin quan n vic chng hin tng phn

x, l khong thi gian bo v Tg. Khong thi gian bo v Tg c cc gi tr khc

nhau theo quy nh ca DVB (xem bng 2.1) v ngi pht sng c th la chn mt

trong cc gi tr . Bng 2.1 cho thy, mt ch pht 8K hoc 2K, c th chn

mt trong 4 gi tr cho khong thi gian bo v. Ch pht 8k cho khong thi gian

bo v ln gp 4 ln ch 2K (nu cng t l Tg/Tsymbol).

Bng 2.1: Khong thi gian bo v cc ch pht khc nhau [5].

Tg/Tsymbol

Khong thi gian bo v Tg [s]

v chnh lch v qung ng i tng ng D=c.Tg

Ch 8K

Tsymbol = 896 s

Ch 2K

Tsymbol = 224 s

1/4 224 s; 67,2 km 56 s; 16,8 km

1/8 112 s; 33,6 km 28 s; 8,4 km

1/16 56 s; 16,8 km 14 s; 4,2 km

1/32 28 s; 8,4 km 7 s; 2,1 km

Khi chnh lch thi gian ca cc tia sng n u thu khng vt qu khong

thi gian bo v Tg, th my thu hon ton c th khc phc tt hin tng phn x.

-57-


2.4 Kt lun chng II

La chn tiu chun truyn hnh s mt t DVB-T ca Chu u l mt s la

chn ng n xy dng h thng truyn hnh s mt t Vit Nam. Truyn hnh

s mt t c nhiu u im hn hn so vi cng ngh truyn hnh tng t.

Nhu cu ngi xem truyn hnh ngy cng tng cao c v thi lng pht sng,

cht lng chng trnh v cht lng hnh nh. Vi xu th hi t trong lnh vc a

phng tin, v s pht trin mnh m ca cng ngh k thut truyn hnh cc dch

v truyn hnh mi nh: HDTV, 3DTV ra i ang v s c nhiu ngi la

chn do tiu chun DVB-T cn phi nhanh chng b sung thm cc tnh nng mi.

-58-


CHNG III - TRUYN HNH S MT T THEO TIU CHUN

DVB-T2

Chun DVB-T c nhiu quc gia la chn trin khai cng nh khng

nh c u th r rt trn th gii. Tuy nhin, qua cc h thng truyn hnh s ang

khai thc v nhu cu thc t t ra v i hi ngy cng tng v dung lng, khng

li ng truyn, nng cao tin cy vi cc loi hnh dch v, v gim t s cng

sut nh/cng sut trung bnh, nhu cu phn chia ph ca cc dch v vin thng

khc, cng xu th hi t trong lnh vc truyn dn, truyn hnh di ng, s pht trin

mnh m ca truyn hnh phn gii cao HDTV v 3DTV vi dung lng bit ln

m DVB-T cha p ng c. Trc nhng nhu cu t ra trn, cui nm 2008,

nhng ni dung c bn ca tiu chun truyn hnh s mt t th h th hai DVB-T2

c ban hnh. Cng vi vic a ra cc chun nn mi nh MPEG-4/H246 kt

hp vi cc u im vt tri ca DVB-T2, nht l khi s dng vi mng n tn v

nhiu knh lin k s l xu hng pht trin cho truyn hnh s mt t trn th gii.

3.1 Yu cu t ra i vi tiu chun truyn hnh s mt t mi

Nhng yu cu c bn ca tiu chun DVB-T2 c th c tm tt nh sau:

- DVB-T2 phi tun th tiu ch u tin c tnh nguyn tc l tnh tng quan

gia cc chun trong h DVB. iu c ngha l s chuyn i gia cc tiu chun

DVB phi thun tin cao nht n mc c th.

- DVB-T2 phi k tha nhng gii php tn ti trong cc tiu chun DVB

khc.

- Mc tiu ch yu ca DVB-T2 l dnh cho cc u thu c nh v di chuyn

c. Do vy, DVB-T2 phi cho php s dng c cc anten thu hin ang tn ti

mi gia nh v s dng li cc c s anten pht hin c.

- Trong cng mt n v truyn sng, DVB-T2 phi t c dung lng cao

hn th h u (DVB-T) t nht 30%.

-59-


- DVB-T2 phi c c ch nng cao tin cy i vi tng loi hnh dch v

c th. iu c ngha l DVB-T2 phi c kh nng t c tin cy cao hn

i vi mt vi dch v so vi cc dch v khc.

- DVB-T2 phi c tnh linh hot i vi bng thng v tn s.

- Nu c th phi gim t s cng sut nh/cng sut trung bnh ca tn hiu

gim thiu gi thnh truyn sng.

Trn c s nhng tiu ch trn, t thng 6/2007 n thng 3/2008, trn 40 t

chc tp trung nghin cu tiu chun DVB-T2, thng qua nhiu bui hi tho, hi

ngh qua mng v Email, cui cng cui nm 2008, nhng ni dung c bn ca tiu

chun DVB-T2 c ban hnh.

3.2 Gii thiu chung v tiu chun truyn hnh s mt t DVB-T2

Tiu chun truyn hnh s mt t th h th hai DVB-T2 c nhm DVB

Project cng b thng 6 nm 2008. Vic trin khai v pht trin cc sn phm mi

cho tiu chun mi ny cng bt u. DVB-T2 k tha nhng thnh cng ca

DVB-T vi nhiu ci tin v vic gia tng dung lng truyn dn. Kh nng gia tng

dung lng l mt trong nhng u im chnh ca DVB-T2. So snh vi chun truyn

hnh s DVB-T. Hin nay, tiu chun DVB-T2 gia tng dung lng ti thiu 30%

trong cng iu kin thu sng v s dng cc anten thu hin c. Thc t c th gia

tng dung lng ln n gn 50%. Vi cng ngh s dng chun DVB-T2, dung

lng d liu t c ti UK ln hn khong 50% so vi DVB-T, ngoi ra DVB-

T2 cn c kh nng chng li phn x nhiu ng (multipath) v can nhiu t bin

tt hn nhiu so vi DVB-T. iu ny cng thun li cho vic trin khai cc dch v

qung b mi vi i hi nhiu dung lng hn nh HDTV hay 3DTV.

Hn na, tnh cnh tranh ca truyn hnh s mt t i vi cc mi trng

truyn dn khc cng l yu t c bit quan trng phi xt n. Trong bi cnh c

nhiu s cnh tranh t cc mi trng truyn dn khc (cp, IPTV, v tinh), cuc ua

v gia tng bng thng s khng kt thc v iu ny s dn n hnh thnh k thut

truyn dn th h k tip u vit hn. Hin nay, mi trng truyn dn v tnh v cp,

-60-


cc chun DVB-S2 v DVB-C2 cng c gii thiu v ng dng, cho php gia

tng ng k so vi nhng chun DVB-S v DVB-C trc . Do , DVB-T2 cng

l s pht trin ph hp gip cho mi trng pht sng mt t co v tr cnh tranh

tch cc so vi cc mi trng truyn dn khc.

3.3 M hnh cu trc h thng truyn hnh s mt t DVB-T2

H thng DVB-T2 c chia thnh 3 khi chnh pha pht (SS1, SS2, SS3)

v 2 khi chnh pha thu (SS4, SS5) nh trnh by di hnh 3.1.

Hnh 3.1: M hnh cu trc h thng DVB-T2 [7]..

- SS1: Khi m ha v ghp knh

Khi m ha v ghp knh c chc nng m ha tn hiu video, audio cng cc

tn hiu ph tr km theo nh thng tin v chng trnh/thng tin dch v PSI/SI hoc

tn hiu bo hiu lp 2 (L2 Signalling) vi cng c iu khin chung nhm m bo

tc bit khng i i vi tt c cc dng bit. Khi ny c chc nng hon ton

ging nhau i vi tt c cc tiu chun DVB. u ra ca khi l dng truyn ti

MPEG-2TS.

SS2:

Basic-

Gateway

SS5:

B gii m

MPEG

SS4:

B gii

iu ch

SS3:

B iu

ch T2 Tn hiu

u vo

Tn

hiu ra

SS5:

B gii m

MPEG

SS5:

B gii m

MPEG

SS4:

B gii

iu ch

SS4:

B gii

iu ch

SS1:

M ha v

ghp knh

video/audio

SS3:

B iu

ch T2

SS3:

B iu

ch T2

-61-


- SS2: Basic T2 Gateway

u ra ca khi l dng T2-MI. Mi gi T2-MI bao gm Baseband Frame, IQ

vector hoc thng tin bo hiu (LI hoc SFN).

- SS3: B iu ch DVB-T2

B iu ch DVB-T2 s dng khung c s (Baseband Frame) v T2-Frame

mang trong dng T2-MI u vo to ra DVB-T2 Frame.

- SS4: B gii iu ch DVB-T2

B gii iu ch DVB-T2 nhn tn hiu cao tn (RF Signal) t mt hoc nhiu

my pht (SFN Network) v cho mt dng truyn ti (MPEG-TS) duy nht ti u

ra.

- SS5: B gii m dng truyn ti

B gii m dng truyn ti nhn dng truyn ti (MPEG-TS) ti u vo v cho

tn hiu video/audio ti u ra.

3.4 Cc c tnh k thut ca DVB-T2

3.4.1 Lp vt l

M hnh lp vt l ca DVB-T2 c trnh by trong hnh 3.2. u vo h

thng bao gm mt hoc nhiu dng truyn ti MPEG-TS hoc dng GS (Generic

Stream). u ra ca lp vt l tn hiu cao tn RF. Tn hiu u ra cng c th c

chia thnh hai ng cung cp cho anten th 2, thng l mt my pht khc.

Hnh 3.2: Lp vt l.

Tro bit nh x

Frame iu ch

D liu

u vo

X l dng

d liu vo

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Vic x l dng d liu vo v FEC phi c la chn sao cho c kh nng

tng thch vi c ch s dng trong DVB-S2. iu c ngha, DVB-T2 phi c

cng cu trc Baseband-Frame, Baseband-Header, gi 0 (Null packet) LDPC/BCH

FEC v ng b dng d liu nh DVB-S2.

Cc thng s COFDM ca DVB-T2 cng c m rng so vi DVB-T, trong

bao gm:

- FFT: 1K, 2K, 4K, 8K, 16K, 32K

- Khong bo v: 1/128, 1/32, 1/16, 1/8, 19/128, 1/4

- Pilot phn tn: 8 bin th khc nhau ph hp vi cc khong bo v khc

nhau

- Pilot lin tc: tng t nh DVB-T, tuy nhin ti u hn

- Tro: bao gm tro bit, tro t bo, tro thi gian v tro tn s

Vic c mt khong la chn rng hn cc thng s COFDM cng vi m sa

sai mnh hn, cho php DVB-T2 t c dung lng cao hn DVB-T gn 50% i

vi mng a tn MFN v thm ch cn ln hn i vi mng SFN.

DVB-T2 cn c mt s tnh cht mi gp phn ci thin cht lng h thng.

- Cu trc khung (Frame Structure), trong c cha symbol nhn din c

bit c s dng qut knh (Channel Scanning) v nhn bit tn hiu nhanh hn.

- Chm sao xoay, nhm to nn tnh a dng trong iu ch tn hiu, h tr

vic thu tn hiu c t l m sa sai ln.

- Cc gii php k thut c bit nhm gim t s gia mc cng nh v mc

cng sut trung bnh ca tn hiu pht.

- Ty chn i vi kh nng m rng khung d liu trong tng lai (Future

Extension Frame).

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3.4.2 Cu hnh mng

Cc c tnh k thut DVB-T2 ch php pht trin ti a hiu qu cc ng dng

trong mng n tn. So vi chun DVB-T, cc mode sng mang mi c cng thm

ci thin hiu sut ca mng SFN v gia tng chu k symbol. Ni cch khc, vic

gia tng chu k symbol cho php gim kch thc khong bo v theo t l trong khi

vn m bo s l cc phn x a hng (Multipath Reflection) Vic h tr thm

mode m ha Alamouti l ty chn h tr thm kh nng thu sng trong mng SFN

(ni my thu c th thu c ng thi nhiu tn hiu t nhiu my pht). Nh cc

c im b sung trn, ngi ta c tnh dung lng mng SFN c th tng ln 67%

so vi cc mode tng t nhng dng DVB-T.

So vi chun DVB-T, bng cch s dng cc k thut TR (Tone Reservation)

v ACE (Active Constellation Extension), DVB-T2 cho php gim mc cng sut

khuch i nh (xt trong mt trm pht sng) vi t l gim c th t di 25%

(y l mt tng lng cng sut thc s ng k i vi mt trm pht cng sut

cao).

Ngoi ra, DVB-T2 nh ngha mt profile kt hp kh nng time-slicing

(nhng khng dng TFS Time Frequency Slicing). Cc c im h tr kh nng

TFS thc thi trong tng lai (vi cc my thu c 2 tuner/front-end). Trong tng lai,

TFS dng cho ghp knh cc tn hiu tri rng thm mt vi tn s lin kt v do

, gia tng ng k dung lng kt qu do ng dng ghp knh thng k v tng

li thit lp mng. Cc phn tch v DVB cho rng TFS c th cho php gia tng v

dung lng ln xp x 20% v li thit lp mng (Network Planning) ln 3-4 dB.

c tnh k thut DVB-T2 h tr kh nng tng cng sc mnh cho tn hiu

chng li nhng nh hng bn ngoi nh cc tc ng ca a l, thi tit, v cc

ta nh cao c. iu ny t c thng qua s dng k thut chm sao quay (Rotated

Constellation), tro thi gian v tn s.

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