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MT S M HNH CC I PTTHTRIN KHAI CNG NGH PHT HNH S DVB-T

KS. L Trng Bng, KS. ng Trn Kin

T 5/1/2003 THVN t b cng ngh k thut analog trong lnh vc truyn qua v tinh,thay vo l cng ngh k thut s, cc chng trnh truyn hnh ca THVN pht qua vtinh l cc chng trnh truyn hnh s.

Sau khi ngi dn sng ti H Ni cng cc tnh ln cn ca ng bng Bc B, ni ngoithnh tp. Hi Phng, ti cc vng ca tnh Thi Nguyn, H Long (Qung Ninh), Thi Bnh vkhu vc Bnh Dng, tp. H Ch Minh, Tin Giang thu c truyn hnh s DVB-T, nhiu iPTTH trao i vi chng ti v gii php trin khai my pht s ti a phng mnh.

Chng ti xin nu mt s m hnh (hay phng n) trin khai my pht s v ang thc

hin cc i PTTH cng tham kho. Chng ti xut pht t nguyn tc tn dng ti a vti u c s h tng: nh t my, h thng in nng, h thng iu ho khng kh, c bitl h thng cp v anten hin c v thc hin u tkinh t nht.

Lu , khi ni n tn s (hay knh sng) my pht s ch c khi nim tn s trung tm.Tn s trung tm ca cc knh pht hnh s (di thng 8MHz) bng tn UHF c tnh bngcng thc: 474MHz + [KD-21]x8MHz. Bt u bng knh 21D (K=knh; D=Digital=S) ctn s trung tm l 474MHz. Knh 26D c tn s trung tm 474MHz + [26-21] x8MHz=514MHz, Knh 29D c tn s trung tm l 474MHz + [29-21] x 8MHz = 546MHz.

1. My pht hnh s cng vi my ph t analog. u t vi mc kinh ph thp nht, phc tp nht, trc ht, chng ti lun ngh ti

vic tn dng h thng cp v anten di rng hin c. H thng thit b cng ngh k thut sgn khng chim nhiu din tch, tiu tn t in nng nht. S tc ng vo h thng thit b(c th l my pht hnh) ang khai thc thp nht, thi gian trin khai nhanh nht.

1.1. M hnh c s dng b iu ch s DVB-T.y l m hnh thc hin cho my pht chuyn tip ti Thi Nguyn, Thi Bnh v Tin

Giang. Chng ti s dng hai b iu ch s DVB-T c tn hiu ra cao tn trn hai knh link bng tn UHF, c th knh 29 v knh 30. Cc khuch i bn dn ca my pht hnh s cdi rng 16MHz cho c hai knh lin k [29+30] hoc [40+41]. Tn hiu pht s di rng16MHz (sau khi qua b lc thng di 16MHz) c cng vi tn hiu analog, thng qua bcng knh (Combiner) s dng chung h thng cp v anten UHF. Hnh 1 m t s khica h thng.

Vic pht trn hai knh lin k 29+30 cha h v khng bao gi thc hin c vi cng

ngh pht hnh analog. y cng l mt li th rt mnh ca cng ngh pht hnh s mt tDVB-T, m ln u tin cng ty VTC thc hin thnh cng ti Vit Nam.

u im ca m hnh ny: mt l, sa mo tuyn tnh v khng tuyn tnh ca ton hthng chuyn tip cc chng trnh truyn hnh s rt d dng, v c s dng cc b iu chs (c phn mm hiu chnh b sa mo). Hin nay, nhiu b iu ch s DVB-T thngc phn mm hiu chnh bn km. Hai l, tn hiu a vo my pht hnh s l dng truyn ti(ting Anh gi l Transport Stream) cha cc chng trnh nn v ghp. Dng truyn ti cth l tn hiu ni tip khng ng b ASI (Asynchronous Serial Interface), hoc c th l tnhiu song song (gi l tn hiu LVDS, Low Voltage Difference Signal). Tn hiu dng truynti ny c u thu s thc hin sa li nh m sa sai.

Nu s dng tn hiu dng truyn ti, th c thm mt li th na, l b i mt chng

trnh no v thay vo bng mt chng trnh khc (ca a phng) tht d dng. Tt nhinphi cn n b nn MPEG-2 (4:2:0 Main Profile @ Main Level) nn chng trnh ca aphng v h thng tch ghp li nhm chng trnh. Hnh 2 m t s khi ca ton b h

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thng. Thit b trong c vin bng l u t thm. M hnh ny th hin cha ghp chngtrnh ca tnh Thi Nguyn vo.

Anten thu

K26

u thus DVB-T iu ch

s DVB-TK kch vKcng sut,b lc16MHz

Mypht

Analog(knh32)

angkhai thc

B cng

knh

RF knh 29

iu chs DVB-T

u thus DVB-T

K34

RF knh 32analog

Anten

ph#t

RFRF knh 30

Hnh 1. M hnh thc hin 2/2003: my pht s DVB-Tcng vi my pht hnh analog knh 32 ti Thi Nguyn

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RF k#nh

u thu sDVB-T

Nn MPEG-2ra ASI

Btchghp

iu ch s

My tnh vphn mmiu khin

HUB

Khuch i v lcdithng 16MHz

My phthnh analogang khai

thc

iu ch s

RF knh s n

u thu sDVB-T

B cngCombiner

RF knh 32

Anten thu

RF knh s n+1ASI-2

Anten phtRF knh s n v n1

ASI-3ASI-1

LVDS

Hnh 2. M hnh s dng hai b iu ch s DVB-Tv c thm h thng tch ghp chng trnh pht chng trnh a

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Trn hnh 2 c 4 dng truyn ti: LVDS cng l mt dng truyn ti do u thu s mt t chora, y l dng song song cha 8 chng trnh. Dng ASI-1 l dng do u thu s DVB-T(loi chuyn dng) cho ra, y l dng ni tip cha 8 chng trnh. Dng ASI-2 l dng dob nn MPEG-2 to ra, y l dng cha mt chng trnh ca Thi Nguyn (thc hin t15/3/2003). Dng ASI-3 l dng sau b tch ghp chng trnh cha 8 chng trnh (c bitl trong dng ny c thm chng trnh a phng thay th mt chng trnh no trong8 chng trnh c trong dng ASI-1.

Nhc im ca m hnh ny l phi u t hai b iu ch s DVB-T, b nnghp chngtrnh. Tuy vy, y l m hnh rt hu ch cho a phng (nu mun tip pht cc chngtrnh truyn hnh s do VTC hay Bnh Dng pht). V khng nhng chng trnh ca aphng c pht vi cng ngh k thut s, m ngi dn s thu c rt nhiu chng trnhtruyn hnh s qua h thng pht hnh s DVB-T ny.

1.2. M hnh chuyn tip qua trung tn 36,15MHz.M hnh ny theo cc ti liu quc t, thng s dng cho my pht chuyn tip, thng

qua trung tn 36,15MHz, ph hp cu hnh Gapfile c cng sut rt nh (20-30W) cho ccvng lm sng. Hnh 3 m t s khi. B dao ng ni LO s trn vi trung tn 36,15MHzcho ra tn hiu cao tn knh pht s (Digital=D).

u im ca m hnh ny l khng phi u t cc b iu ch s, gi thnh u t thp.

RF k#nh D

u thu sDVB-T

LO

My phthnh analog(A) hin c

B cngknh s Dv knh A

IF

Khuch ikch v Kcn sut

RF knh A

Hnh 3. M hnh chuyn tip qua trung tn 31,15MHz

Nhc im l bin php hiu chnh b sa mo do cc khuch i gy ra phi thc hinbng phn cng v hiu qu khng cao nh thc hin bng phn mm. Kh nng sa li bits km hiu qu hn l chuyn tip tn hiu c bn (dng truyn ti). Hn na, phi mua loiu thu s DVB-T c cho ra trung tn 36,15MHz (vi mc -15dBm). Thng thng ti u thus c hai gi tr tn s trung tn: 36,15MHz v 4,57MHz; y chn 36,15MHz. Gapfile lmy chuyn tip trn cng knh sng v thng c cng sut rt thp. Nhc im na, vchuyn tip trung tn, nn khng th a chng trnh ca a phng vo pht s c.

2. B sung thm phn pht s vo my pht hnh analog hin c.c bit c mt m hnh m mt s ti liu quc t cp, l b sung thm phn

pht s vo my pht hnh analog hnh ting chung. Gi l phn pht s v n khng phi lmt my pht s hon chnh, m ch tn dng cc khuch i ca my pht hnh analog hinc vn pht analog bnh thng v pht lun c cc chng trnh truyn hnh s, tm gi lmy hai trong mt xem hnh 4.

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A/VChng trnh

a ph

ng

u thu sDVB-T

iu ch sRF s knh n1

Anten thu LVDS/ASI

Chuyn IFln RF

Khuch ikch v cng

sut

B lc(16 MHz)

B iuch analog

I

RF analog knh n

Anten

Hnh 4.M hnh b sung phn pht s vo my pht hnh analog hin c

- m hnh hai tron m t

Cn c s ny, ch cn u t thm (trn hnh v th hin bng c vin bng):- Mt u thu hnh s.- Mt b iu ch s DVB-T c ra cao tn;- Mt b cng hai knh (cng cao tn);- Hiu chnh m rng di thng ca b lc t 8MHz ln 16MHz. Nu khng m rngc, th phi u tmua b lc mi c di thng 16MHz.Knh pht s s l knh lin k vi knh pht hnh hin c; v d, a phng ang pht

knh 7 c th pht s trn knh 8 hoc knh 6. Tuy nhin, chn knh pht s knh lin k trnvi knh analog s c nhiu u im hn, hu nh khng gy can nhiu sang nhau, v phpht s nm gn vi ph mang ting (thp hn ph mang hnh 10dB).

u im ca hai trong mt chnh l kinh ph u t thp v trin khainhanh. Khngcn mua my pht s DVB-T hon chnh, tn dng cc khuch i ca my pht hnh analoghin c. My pht hai trong mt lm nhim v ng thi pht mt chng trnh analog vpht cc chng trnh truyn hnh s.

i vi cc i xa khng thu c truyn hnh s mt t, chng ti xin nu gii php trin khai (theo hnh 5). M hnh trn hnh 5 vn l hai trong mt vi gii php ghpknh a phng vo ( m t hnh 2). S hnh 5 s rt thch hp vi cc thit b mypht hnh ting chung u t nhiu nm qua ti vng lm v mt s tnh.

Trn hnh 5 c 3 dng ASI khc nhau: v d dng ASI-1 cha cc chng trnh[VTV1+VTV2+VTV3]; dng ASI-2 c mt chng trnh a phng (P); dng ASI-3 scha c 4 chng trnh truyn hnh nn v s ho.

Hnh 4 v hnh 5 c bn l ging nhau. Ch khc u thu cho dng ASI-1: s hnh

4 l u thu s mt t DVB-T ( thu cc chng trnh pht s mt t t Lc Trung- H Nihay t Bnh Dng), trn hnh 5 l u thu s v tinh ( thu cc chng trnh ca THVN).

Nhm thng s pht hnh s trong my hai trong mt c th chn: 2k; 16-QAM; khongbo v 1/32, t l m sa li 3/4; khi vn tc dng truyn ti s t 18 Mbit/s ( pht 4chng trnh). Hin nay 3 chng trnh VTV1+VTV2+VTV3 truyn qua bng C chim13,5Mbit/s, nh vy, cn 4,5Mbit/s dnh cho chng trnh ca a phng. Thc t cho thy thu s tt, mc trng ca s cn thp hn ca thu analog khong 15-18dB. Nu pht vinhm thng s nu trn, anten pht l anten di rng, m bo vng ph sng s v analogtng ng nhau, th cng sut pht s s thp hn cng sut pht analog rt nhiu, v dcng sut pht analog 2kW, th cng sut pht s ch cn khong 350-400W, cng sut phtanalog 5kW th cng sut my pht hnh s khong 500-600W. Mc cng sut pht s s xc

nh thng qua iu chnh mc cao tn ra ca b iu ch s DVB-T.

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V pht s khng cn cng sut ln, nn in nng dnh cho my pht s s khng tngthm nhiu, mt bng khng thm bao nhiu (b nn v ghp rt b), nhng pht c thmcc chng trnh truyn hnh s bn cnh 1 chng trnh analog.

Vn tc dng truyn ti v gi tr t s C/N (lin quan ti cng sut pht s) thay i theonhm thng s pht c th tm thy trong ti liu [2].

A/Va phng

u thu v tinh radng ASI

iu ch s

Chuyn IFln RF

Khuch ikch v

cng sutiu chAnalog

B lc(16 MHz)

NnMPEG-2ra ASI

B ghp

ASI-2

RF knh s n 1

RF knh analog n

A/Va phng

ASI-3

ASI-1

Anten thu v tinh

Hnh 5. M hnh my pht hai trong mt

3.Ph pht s DVB-T v pht analogTrong bi Cc c im c bn ca my pht s DVB-T cp ti ph ca my

pht s DVB-T. Trong phn ny, chng ti ch nu ph pht s trn hai knh lin k, c th lph pht s trn hai knh 29+30. Hnh bn tri (chp ti Hi Phng) cho thy ph pht analogknh 28 bn cnh ph pht s knh 29+30. Hnh bn phi (chp ti H Long-Qung Ninh)cho thy ph pht analog knh 31 bn cnh ph pht s hai knh lin k 29+30. Nhn vo haihnh chp nh ph cho thy, ph pht analog ch c 3 thnh phn: thnh phn ti sng manghnh, mang mu v mang m thanh; trong khi pht s cc thnh phn ph dy c trongton b di thng 16 MHz (khi pht trn hai knh lin k).

Hnh 6. Ph ph t s trn knh lin k

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4. Kt lunCng ngh k thut s ni chung, pht s DVB-T ni ring c nhng u im v li th

tht s so vi cng ngh analog c. Cht lng hnh nh v m thanh ca cc chng trnh capht s DVB-T vt tri hn pht analog. Ngi dn nhit tnh n nhn. Trn th gii cnhiu nc theo h DVB-T, nn cc hng sn xut v bn cc h thng thit b DVB-T. Cnc vo thc trng my pht hnh hin c ca tnh mnh, cc i PTTH c th s trin khaingay pht s DVB-T vi mc kinh ph u t khng ln, sm mang li cho ngi dn c

hng thnh qu ca cng ngh mi rt u vit ny.

5. Ti liu tham kho[1] TR 101 190 V1.1.1(1997-12) DVB Technical Report, pages 69-72.[2] KS.L Trng Bng, . Phm c Bi, KS. Anh T, Cc c im c bn ca my phts DVB-T; TS.

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