baigiangtruyenhinh

8/8/2019 BaigiangTRUYENHINH

1/87

n tt nghip Truyn hnh s qua v tinh

1

Li ni u

Trong nhng nm gn y nn kinh t nc ta bc sang mt giai on

mi pht trin mnh m trong mi lnh vc ca cuc sng. iu i hingnh v tuyn truyn hnh cn p ng c nhng thng tin x hi mt cch

y v hon thin hn. p ng c nhng nhu cu trn gp phn vo

y mnh s pht trin ca ngnh truyn thng trn con ng hin i ho.

Trong nhng nm gn y a vo s dng nhiu thit b hin i ph hp

vi tnh hnh thng tin Vit nam trong c truyn hnh s. Nh s pht trin

vt bc ca khoa hc k thut, c bit l s pht trin ca cng ngh s vi s

u vit ca n hn hn vi cng ngh tng t. Do truyn hnh s cng angthay i truyn hnh tng t.

Tc pht trin ca ngnh v tuyn truyn hnh nc ta din ra nhanh

chng t cc i pht quc gia n hu ht cc tnh, thnh ph v u c my

pht hnh cng sut t ln ti nh, vng su vng xa u c cc trm pht li

truyn hnh nh s pht trin ca thng tin v tinh. Truyn hnh l ni hi t

nhng thnh tu khoa hc tin tin cc sn phm v ng dng cng ngh mi

trong lnh vc vin thng - in t - tin hc. Gp phn quan trng vo s phttrin ca truyn thng nc ta.

n, ny mang n ci nhn tng quan v h thng truyn hnh ni

chung v h thng truyn hnh s ni ring. Tuy nhin y l mt ti rt mi

v rng, hn na do thi gian v ti liu tham kho cng vi trnh cn hn

ch nn khng th trnh khi nhng thiu st, em rt mong s gp ca thy c

gio v cc bn.

Em xin chn thnh cm n Ths. Nguyn Xun Quynv cc bn nhittnh gip em hon thnh n ny.


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PHN I: TRUYN HNH TNG T

I. C s l thuyt truyn hnh

I.1 H thng truyn hnh:

H thng truyn hnh l mt h thng bin i hnh nh v m thanh kmtheo thnh tn hiu in, truyn n my thu, ni thc hin bin i tn hiu ny

thnh dng ban u v hin th nn mn di dng hnh nh truyn hnh da trn

cm nhn nh sng ca mt ngi truyn i nhng thng tin cn thit.

Qu trnh gia cng tn hiu hnh nh pha pht v pha thu c minh

ho nh sau:

Hnh I.1 Qu trnh thu pht truyn hnh

Pha pht

Khi 1: Tn hiu hnh nh cn truyn

Khi 2: Camera in t

Khi 3: Trung tm x l (Studio)

Khi 4: Truyn dn tn hiu hnhKhi 5: My pht hnh

Khi 6: ng truyn (cp ng trc)

Khi 7: Anten pht

Pha thu

Khi 1: Anten thu

Khi 2: ng truyn dn t anten thu (cp)

Khi 3:Mch in t trong my thu hnh.

Pha thu

1 2 3 4 56

Pha

Pht

3

1

4

7

2


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Khi 4: n hnh (mn hin hnh)

I.2 Nguyn l h thng truyn hnh:

Hnh nh quang (1) c hin th bng chi phn x B(x,y) c thit

b Camera in t (2) chiu nh ca vt cn truyn i v bin i nh quangthnh tn hiu in, tc l chuyn i nh tn hiu. Tn hiu mang tin tc v

hnh nh gi l tn hiu video. Tn hiu video bin i theo sng ti, mu sc ca

hnh nh quang l tn hiu khng lin tc do s phn tn nh quang thnh tng

im (gi l phn t hnh). Tn hiu hnh tip tc ti trung tm c gia cng (3)

khuych i t tiu chun k thut v qua truyn dn (4) c a ti my

pht hnh. Ti y my pht hnh sng mang, c tn hiu hnh iu ch, qua

ng truyn dn (6) ti anten (7). Ti y tn hiu c bc x vo khng giandi dng sng cao tn.

Ti pha thu th qu trnh ngc li: Anten thu nhn sng t i pht qua

b khuych i ln n mc cn thit ri a n b chuyn i tn hiu - nh

(chuyn i nng lng in thnh nng lng nh sng), i tn, tch sng

phc hi li tn hiu hnh. Sau khi khuych i mc in p cn thit tn

hiu hnh c a ti n hnh. n hnh s bin i tn hiu in thnh hnh

nh quang tng ng lc ban u pht i. ng b cho cc mch qut pha pht v pha thu ngi ta s dng tn

hiu ng b. thc hin c s ng b v ng pha ngi ta dng mt b

to xung ng b. Xung ng b a ti b chuyn i nh tn hiu khng

ch qu trnh phn tch nh ng thi a n b khuych i v gia cng tn

hiu cng vi tn hiu hnh truyn sang pha thu. Tn hiu hnh cng thm xung

ng b gi l tn hiu truyn hnh.

Vy v tuyn truyn hnh l ngnh k thut s dng cc tn hiu in t,tin hc truyn hnh nh, ting ni n ngi xem. Truyn hnh l phng tin

thng tin i chng c s hp dn v mang tnh thuyt phc cao. nc ta, nm

1970 truyn hnh en trng c pht th nghim, nm 1978 pht th nghim

truyn hnh mu h SECAM v t thng 1 nm 1991 pht theo h PAL D,K tc

l pht h mu PAL kt hp h en trng OIRT.

Ngy nay chuyn t k thut Analog sang k thut cng ngh Digital. R

rng l ch c chuyn sang k thut Digital th mi pht huy c nhng thmnh ca cng ngh in t. Rt nhiu cng ngh mi c a vo ng dng


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trong truyn hnh em li hiu qu ln nh iu ch Digital nng cao hiu

sut my pht, nh v tinh em li vng ph sng rng ln v cc cng ngh v

nn tn hiu, k so, kim tra o th nhanh nhy thc hin ngay c khi thit b

vn hnh t ng v iu khin t xa, nng cao cht lng hi phc li tn hium tn, th tn.

I.3 Qu trnh qut:

Nh h thng quang hc m nh ca cc vt trong truyn hnh cn truyn

i trong khng gian c chiu ln mt mt phng. Cc vt trong kh ng gian

c chuyn thnh nh ca chng trn mt phng, ri mi bin i thnh tn

hiu hnh nh (trong truyn hnh l nh phng).

Nu ta chia tm nh thnh cc nh th mi vung l mt phn ttrong mt tm nh (im nh pixels). Mi im nh c chi trung bnh

v mu sc ca n, cng nhiu s im nh tc l nh chia ra cng nh v vy

chi v mu trn ton din ca mi im nh cng ng nht.

Nu ta chia kch thc im nh cng nh m mt thng khng phn

bit c, th ta khng cnphn bit c cc im nh ring r na, lc ta

c cm gic tm nh nh mt khi lin tc lin nhau ch khng phi c ghp

t cc im nh nh li vi nhau. Nh vy nu ta dng nhiu im nh c chi v mu tng ng th ta c th ghp thnh nh. chi ca cc im nh

trn mt dng qut s c chuyn i thnh tn hiu in tng ng ca cc

dng qut .

nh quang hc s c nhn bit bng mt lt qut gm cc dng qut

theo chiu t tri qua phi v t trn xung di. Dng in t c hi t vo 1

im nh v c li ln lt qua nh c hai mnh theo cc dng lin tip. Mi

mnh cha mt na s dng qut ca mt nh. Hai mnh gn nhau c sp xptheo chiu ng sao cho cc dng qut xen k nhau v to thnh mt nh. Tia

qut c iu khin bng hai t trng ngang v ng thng qua cp cun dy

qut ngi ta gi qu trnh l qu trnh qut xen k tuyn tnh. Qu trnh ny

s c lp li cho nh tip theo. Nh vy thng tin v nh s c bin i lin

tc thnh dng tn hiu in theo thi gian trong khong thi gian qut ht 1 tm

nh v quay v dng 1 ca nh tip theo trong thi gian ny, tn hiu hnh c

xo mt (thi gian xo bng thi gian qut ngc ca mt). C nh vy nh sc lin tc qut.


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Xung ng b cng c truyn i t u pht n u thu h thng

qut ti u thu hon ton ng b vi h thng ti u pht nhm bo m ti

to li mt cch chnh xc hnh nh ban u.

S dng qut cng nhiu th cht lng nh cng p. Khong cch ttnht t v tr ngi xem n my thu hnh l:

645

2/

0tg

HD

Trong :

D l khong cch ngi xem n my thu

H l chiu cao ca mn hnh

Xung qut dng

Hnh 1.2 Dng xung qut dng v xung qut mnh

I.4 S nh Truyn trong mt giy v tn hiu video

cho ngi xem c cm gic mt hnh nh chuyn ng lin tc, s

lng nh truyn trong mt giy cng ln th chuyn ng trong nh cng thylin tc m nh khng b nhp nhy. Trong in nh ngi ta chiu 24hnh/s

hnh s chuyn ng lin tc v nh sng khng b chp.

Nhng i vi truyn hnh trnh tnh trng b rung lc hoc c nhng

vt en tri trn mn hnh khi b lc ngun khng bo m cht lng ngi ta

truyn 25 nh trn mt giy di in li c tn s l 50HZ v truyn 30 nh

trn mt giy vi in li c tn s di 60HZ.

loi b cc hin tng chp sng trong truyn hnh, ngi ta s dngphng php qut xen k tc l mi nh c qut hai lt, lt u truyn tt

c cc dng l (mnh l) lt hai truyn tt c cc dng chn (mnh chn).

Khi khi phc li nh, nh trn my thu hnh khng b nhp nhy v tn s

in li 50HZ hoc 60HZ bng tn s nhp nhy ti hn. Tiu chun h truyn

hnh OIRT v CCIR mi giy truyn i 50 mnh. Trong c 25 mnh l v 25

mnh chn, hnh thnh 25 nh. Tuy mi dng ch chp sng 25 ln nhng hai

dng k nhau chp sng thnh 50 ln trong mt giy v rng v khong cchca mi dng l rt b nn mt ngi khng phn bit c.

Xung qut mnh


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I.5.Hnh dng tn hiu Video:

I.5.1.Tn hiu Video.

nh truyn hnh c chia thnh nhiu im nh nh cc im nh ny

mang thng tin v chi v mu sc ca vt. Ti b to nh bin i thng tincha trong cc im nh thnh mt tn hiu in tng ng biu din cho tnh

cht ca im nh l qu trnh bin i nng lng quang thnh nng lng

in. Ti cc tn hiu in tng ng ca cc im nh s c sp xp mt cch

lin tc cho ta mt dng cc tn hiu mang thng tin trn vn v mt bc nh.

y chnh l dng tn hiu video.

Khi tia in t qut ht mt lt quatt c cc dng ca nh t trn xung

di. Sau tia li qut ngc tr li t di ln trn gi l thi gian qutngc ca nh. Trong thi gian ny, tn hiu khng mang theo tin tc ca nh

nn dng lm truyn xung tt (xo) mt nhm tt tia in tca ng thu trong

thi gian qut ngc ca nh. Khong 23 36 chu k ca dng.

I.5.2 Tn hiu ng b

Tn hiu ng b dng khng ch qut trong my thu hnh iu

khin tia in t trong ng thu lm vic ng b v ng pha vi tia in t qut

trong ng pht.Tn hiu ng b c truyn i v c to ra trn knh thng tin cng

vi tn hiu video. Tng hp tn hiu video v tn hiu ng b cho ta thng tin

video v thng tin ng b.

Tn hiu ng b dng dng xc nh cc dng qut v iu khin qu

trnh qut dng. Cc tnh hiu ng b mnh dng iu khin qu trnh qut

mnh v t ti nh xung xo mnh.

Tn hiu ng b cng l cc xung c bin v thi gian xc nh nncn gi l xung ng b dng v xung ng b mnh. Tn hiu ng b mnh

cn mang theo cc xung cn bng gm cc chui xung nm trc v sau xung

ng b mnh trong khong thi gian xo mnh.

I.5.3 Ph tn hiu video:

Xc nh cc thnh phn xoay chiu ca tn hiu chnh l xc nh tn ph

tn hiu hnh. ng vi ccchi tit ln ca nh l tn s thp, vi chi tit nh ca

nh l thnh phn tn s cao ca tn ph tn hiu hnh.


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Tn s cao nht ca ph tn hiu ph thuc vo s dng qut, rng di

tn tn hiu tng ln th s dng qut cng ln, r cng cao v vy s dng

phng php qut xen k s gim i di tn tn hiu. Nu s dng qut xen k th

ta nng tn s mnh ln gp i (50 mnh) do gim tn s tn hiu hnhxung mt na.

c im ca ph tn hiu gia cc nhm ph hai tn s dng tn ti cc

khong trng ta c th li dng khong chng truyn thng tin khc hay p dng

trong truyn hnh mu ph ca tn hiu mu c t vo khong trng ca ph

tnh hiu chi.

II. Truyn hnh mu

II.1 Khi nim tng qut v nguyn l truyn hnh muTt c cc nguyn tc ca truyn hnh en trng u c tn dng

truyn hnh mu, ni cch khc truyn hnh mu trc ht phi lm li tt c cc

cng vic c ca truyn hnh en trng. im khc bit gia truyn hnh en

trng v truyn hnh mu ch ch thay ch quan tm n cng sng ti ca

tng im trn cnh th by gi truyn hnh mu phi quan tm ti tnh cht

mu sc ca tng im 1 trn nh.

Truyn hnh mu c c trng bng cc thng s v sng mu sc v bo ho mu. Mu sc l cm nhn ch quan ca con ngi tuy nhin kt qu

trn mu cho thy mt mu bt k l tp hp li ca 3 thnh phn mu [(,

Lc, Lam) (Red, Green, Blue)}.

bo ho mu c coi nh s m rng cho n khi no 1 mu sc

khc xa mu trng trong iu kin trungtnh. Trong truyn hnh mi nh mu

u c th phn tch v tng hp t cc mu c bn theo nhng t l nht nh

ta s c mu trng v cc mu khc nhau. phn tch hnh nh mu l Cameratruyn hnh cn tng hp nh l cc my thu hnh mu.

II.2 Tnh tng hp gia truyn hnh mu v truyn hnh en trng.

Tnh tng hp thun cho mi tivi en trng c cng tiu chun truyn

hnh khng cn phi thay i iu chnh gi u c th trc tip thu c chng

trnh truyn hnh mu sang nh hin trn mn hnh vn l en trng.

Tnh tng hp ngc ti mi tivi mu c cng tiu chun truyn hnh

cng khng cn thay i iu chnh u c th trc tip thu c truyn hnhen trng. nh hin trn mn my thu hnh mu ch l en trng.


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thc hin c tnh tng hp th i pht truyn hnh mu phi pht

i by thng tin

1. Tn hiu hnh video iu bin 2.

Tn hiu ting, m thanh iu tn

3. Tn hiu xung ng b dng FH4. Tn hiu xung ng b mnh FV5. Tn hiu hiu mu EREy6. Tn hiu hiu mu EB - EY7. Tn hiu ngb mu

Khi pht truyn hnh mu i khng c gy nhiu mu ln nh en trng

do pha i pht khng trc tip gi i tn hiu mu c bn m gi icc tn hiu hiu mu tc l ly tn hiu mu tr i tn hiu chi lm nh vy s

khng gy ra nhiu mu ln nh en trng v tin hiu l s khng

II.3 L thuyt 3 mu:

II.3.1 nh sng v mu sc:

nh sng l sng in t nm trong di tn s t 3,8.10 14HZ 7,8 14

tc truyn lan trong khng gian

C =3.108m/s

C = 300.000km/s

C

Mu sc l mt thuc tnh ca dao ng in t m mt ngi cm nhn

c. Mi nh sng c bc sng khc nhau tc ng ln mt ngi, th mu sc

cm nhn c s khc nhau.

II.3.2 Chn ba mu c bn:

tiu chun ho vic o mu trn th gii ngi ta quy nh ba mu

c bn v ngy nay ang c s dng rng ri trong cng ngh truyn hnh

c coi l h s mu R, G, B/Red, Green, Blue

. Mu , k hiu bng ch R (Red) c bc sng R = 700mm

. Mu lc, k hiu bng ch G (Green) c bc sng G = 546,8mm

. Mu lam, k hiu bng ch B (Blue) c bc sng B = 435,8mm

iu kin chn ba mu c bn l


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. Nu em hai trong ba mu c bn trn vi nhau th khng th cho ra

mu c bn th ba.

. Nu em chn ba mu c bn trn vi nhau theo cc t l khc nhau th

s em cho ra c cc mu trong t nhin.II. 3.3 Ba yu t xc nh mt sc mu.

. chi (Luminance) cho bit mu sng hay ti

. Sc mu (Hue) cho bit mu sc khc nhau

. bo ho mu (Saturation) cho bit nng mu m hay nht.

II.3.4 tch mu:

Bng h thng knh quang hc v knh lc mu ta c th tch t mt

ngun nh sng trng hoc t mt nh mu ra thnh ba mu c bn theo cngthc sau:

Mu c bn + Mu ph = Mu trng

VD: Mu + xanh l = mu trng

Mu lc + tm mn = mu trng

Mu lam + vng = mu trng

II.3.5 Trn mu:

Nu em chiu 3 ngun sng mu c bn c cng cng ln phng

mu trng s phn chiu hon ton th ti nhng vng giao nhau

Nu em trn ba mu c bn trn theo t l c bn khc nhau th ti vng

chnh tm s c cc mu nh t nhin.

II.3.6 Cc nh lut c bn v trn mu:

Nh ton hc ngi c H.Grasman a ra nh lut trn mu nh

sau.

Hnh I.3 trn mu

Vng

Lc

Lam

Xanh

TmMn

Trng


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. Bt k mt mu sc no cng c th to c bng cch chn ba mu c

bn c lp tuyn tnh i vi nhau.

. S bin i lin tc ca cc bc x c th to nn mu khc.

. Mu sc tng hp ca mt s bc x khng phi c xc nh bi ctnh ph ca cc bc x c trn m c xc nh bi mu sc thnh phn ca

cc bc x .

II. 4 Cc thng s c bn ca tn hiu v tuyn truyn hnh:

II.4.1 Tn hiu truyn hnh mu ton phn PAL D/K

Tn hiu mu ton phn c to bi tn hiu chi E/Y, tn hiu mu Em ,

xung ng b mu Esm, xung xo tng hp Exv xung ng b tng hp Es

Biu thc biu th l:Emt = E

/y + Em + Esm + Ex + Es

Trong :

Emt: Tn hiu mu ton phn

E/y = 0,3 E/

R + 0,59 E/G + 0,11E

/B (tn hiu chi)

E/R: tn hiu mu c bn

E/G: tn hiu mu lc c bn

E

/

B: tn hiu mu xanh lam c bnEm = m2(E

/B - E

/y)sin t m1(E

/R - E

/y)cos t(tn hiu mu)

m1 , m2: h s bin tn hiu mu

= 2 fmtn s gc mang mu

fm: tn s mang mu

Esm: ng b mu

Ex: xo tng hp

Es: ng b tng hp ca knh truyn hnh vi PAL D/K l 6,5 MHZII.4.2 Cc thng s ca tn hiu mu:

II.4.2.1 Tn hiu chi EY(Lumirace Signal)

cho my thu hnh en trng lc thu chng trnh truyn hnh mu vn

nhn c nh en trng bnh thng vi cc bc sng chnh xc nh nh

truyn i. Cc h truyn hnh mu i chng, ngoi cc tn hiu phn nh tin tc

mu cn phi to ra v truyn sang pha thu tn hiu chi thng c k hiu l

EY. Tn hiu chi chnh l tn hiu hnh truyn hnh en trng. Cc tn hiutruyn hnh mu c bn u c mang tin tc v chi, v rng khi chi ca


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hnh cn truyn tng hoc gim th bin cc tn hiu mu c bn cng tng

hoc gim theo, nhng t l gia chng khng thay i.

Tn hiu chi c to thnh bi tng hp tuyn tnh cc tn hiu mu c

bn sau khi sa phi tuyn lE/y = 0,3 E

/R + 0,59 E

/G + 0,11E

/B

Trong

E/y: tn hiu chi

E/R: tn hiu mu c bn

E/G: tn hiu mu xanh lc c bn

E/B: tn hiu mu xanh lam c bn

Hin nay cc h NTSC, PAL, SECAM u s dng biu thc ny hnhthnh tn hiu chi b lpm mu. Ch khc nhau gia cc h ny l rng

di tn tn hiu chi. rng di tn tn hiu chi c chn bng rng di

tn tn hiu hnh h truyn hnh en trng cng tiu chun.

II.4.2.2 Cc tn hiu hiu mu (Color dibberence Signal)

Tn hiu chi, v l thuyt cha ton b tin tc v chi ca cnh vt

truyn i. V vy, truyn tt c tin tc v mu sc ca cnh vt, ch cn thm

mt tn hiu na, n ch cha ton b tin tc v tnh mu (c sc mu ln boho mu)

Song trong cc tn hiu mu c bn E/R , E/G ,E

/Bc cha c tin tc v

chi ln tin tc v tnh mu ca cnh vt. V vy nu truyn tn hiu chi v cc

tn hiu mu c bn l khng cn thit. khc phc tnh trng ny. Cc h

NTSC, PAL, SECAM u dng cc tn hiu mu hoc cc t hp tuyn tnh ca

n thay cho cc tn hiu c bn.

kt hp gia truyn hnh mu vi truyn hnh en trng th i phthnh mu pht i cc tn hiu hiu mu ly tn hiu mu tr i tn hiu chi

lm nh vy l khng gy nhiu mu ln mn nh en trng v ti nh trng cc

tn hiu mu l s khng.

Ta c cc tn hiu mu sau:

ERy = EREY = 0,7ER0,59EG0,11EB

EGy = EGEY = - 0,3ER + 0,41EG0,11EB

EBy = EBEY = - 0,3ER + 0,59EG + 0,89EB


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V l thuyt cc tn hiu mu ch cha tin tc v tnh mu, khng cha tin

tc v chi ca cnh truyn i (ta dng cc mch ma trn hnh thnh cc

tn hiu mu).

Nh vy my thu en trng khi ta truyn tn hiu mu vn thu c tnhiu en trng ng ra th phi truyn i tin tc chi EYv ba tn hiu hiu mu

k trn. Trong thc t trong bn tin tc l EYv ba tn hiu hiu mu ta ch cn

truyn i ba tin tc l ta c th suy ra tin tc th 4. Ngi ta chn E REY v

EBEY truyn i cng vi EY (EY l tin tc bt buc phi truyn cho cc my

thu en trng). Ti my thu tin tc cn li E GEYs c suy ra t ba tin tc

trn.

L do khng truyn EGEYcng cng sng chun nh nhau gi trtn hiu EGy nh hn ca EBy v ERy.

ER - Ey = 0,70v

EG - Ey = 0,41v

EB - Ey = 0,70v

Chnh v vy trong khi phi chn hai trong 3 tn hiu hiu mu truyn

i, ngi ta loi EGEY

L do th hai loi EGEYna l v mt kh tinh t vi mu l cy, do di tn i hi ca EGEY cao hn nn kh truyn hn nu so vi di tn ca

EREY v EBEYch vo khong 1,5 MHZ.

Ti my thu EGEY s c suy ra t EREY v EBEY

EGEY = - 0,509(R/- Y/)0,194(B/- Y/)

Gi tr cc tn hiu hiu mu ph thuc vo bo ho mu, bo ho

mu cng cao, gi tr tn hiu mu cng ln.

Kt lun: Dng tn hiu hiu mu thay cho tn hiu mu c bn c cc uim sau.

. Gim r rt nhiu do tn hiu mu sinh ra trn nh truyn hnh en trng

my thu hnh en trng cc mng trng ca nh truyn hnh mu. Bi v lc

truyn mu trng, phi iu chnh sao cho li vo ca b lp m mu t c

iu kin.

E/B = E/G = E

/R cho nn E

/Ry = E

/By = 0


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Cn lc truyn cc chi tit c bo ho mu thp, gi tr cc tn hiu

hiu mu nh (cc cnh thng gp hng ngy c bo ho thp).

. Gim nh nh hng ca nhiu ti chi ca nh truyn hnh. Mt

ngi rt nhy cm vi s sai lch chi (so vi s sai lch mu sc). Dtruyn cc tn hiu E/Y, E/R v E

/Bhay truyn E

/Y, E

/By , E

/Ry c ba tn hiu

u chu nh hng ca nhiu. M trong cc tn hiu E/R v E/Bcha tin tc ca

chi. Cn trong cc tn hiu hiu mu E/Ry v E/By v l thuyt, khng cha

tin tc ny. V vy tc ng ca nhiu ti tn hiu hiu mu ch lm thay i tn

hiu mu chs khng lm thay i chi. Trong khi , nu truyn tn hiu E/R

v E/Bth khng nhng thay i tn hiu mu m cn lm cho chi thay i

nhiu hn.. Thun tin cho vic xy dng mch in my thu hnh mu ch cn

cng tngtn hiu hiu mu vi tn hiu chi s nhn c cc tn hiu mu c

bn E/R , E/G v E

/B

II.4.2.3 Sng mang ph truyn tn hiu hiu mu:

Ta khng th truyn tn hiu chi v hai tn hiu hiu mu theo mt

ng truyn, v ph ca chng u l ph ri rc. pha thu khng th no

tch ring chng. Nn ch c tn hiu chi EYtruyn trc tip, cn li hai tn hiuhiu mu phi dch ph v pha tn s cao nht ca tn hiu chi. Vic la chn

sng mang ph v phng php iu ch hai tn hiu mu l nguyn nhn tn ti

cc h mu ngy nay NTSC, PAL, v SECAM

II.4.2.4 Tn hiu xung ng b mu:

cho mu sc ca nh truyn hnh mu khng sai so vi mu ca nh

quang truyn i, iu kin trc tin l phi m bo cho c tn s v gc pha

ca sng mangph chun c to ra ti my thu hnh mu v ca sng mangph pha pht lun bng nhau. Do gip cho mch tch sng mu my

thu lm vic c tt, phi truyn sang pha thu mt tn hiu c bit, gi l tn

hiu ng b mu thc hin ng bv ng pha cng c bc sng mang

ph chun c to ra my thu hnh mu.

II.5 Mt s h truyn hnh mu chnh c s dng trn th gii hin

nay


14/87


14

Nh trnh by phn trn vic chn sng mang mu iu ch vo

vi tn hiu chi EYta thy c mt vng tn s m y c c tn hiu chi v

tn hiu mu, (tn hiu chi chn ph tn hiu mu v ngc li) y l s xuyn

ln. Vn la chn sng mang mu v phng php iu ch hai tn hiu munh th no cho s xuyn ln trn gim i ti thiu l nguyn nhn sinh ra cc

h mu khc nhau. Nh NTSC, PAL, SECAM, c ba h ny u c mt n th

li km mt kia, khng h no t c tuyt i u im, chnh v vy c ba h

vn song song tn ti.

II.5.1 H truyn hnh mu NTSC (National television System Committee

T chc h thng truyn hnh quc gia)

II.5.1.1 H ny c cc c im chnh sau:H truyn hnh mu NTSC l h truyn hnh mu c tnh tng hp u

tin trn th gii vo nm 1950 c hnh thnh ti nc M. Theo h NTSC tn

hiu chi c to ra t ba tn hiu mu c bn v pht i trong ton di tn

dnh cho h thng truyn hnh en trng thng thng.

Tn chi c xc nh theo biu thc

Y/= 0,299R/+ 0,587G/+ 0,114B/

C rng ri tn 0 4,2MHZTrong Y/ , R/ , G/ , B/l cc gi tr in p tn hiu chi v ba mu c

bn sau hiu chnh gamma.

H ny truyn ng thi cng mt lc hai tn hiu mu E I v EQcng vi

tn hiu chi.

H NTSC cho php dng mt tn hiu mu c di tn rng v mt tn hiu

mu c di tn hp hn, phi hp r mu ca nh truyn hnh v chng li

hin tng nhiu gia cc tn hin mu sau mch tch sng ng b. EI = 0,73(EREY)0,27(EBEY)c rng di tn l 1,5 MHZ

EQ = 0,48(EREY) + 0,41(EBEY)c rng di tn l 0,5MHZ

iu bin nn vung gc EI v EQvo sng mang mu ph c tn s c

chn l f = 3,58 MHZ

Tn s qut dng fH = 15750 HZ

Tn s qut mnh fV = 60 HZ

Khong cch hnh cch ting: 4,5 MHZII.5.1.2 Dng th vc t mu gii thch tn hiu mu EIv EQ


15/87


15

Hnh I.4 th gii thch tn hiu EI v EQ h NTSCRing tn hiu EQv mt ngi khng nhy cm khng phn c cc chi

tit nh ca hnh nh. Do ch cn gi tn hiu i trong mt di thng hp t (0

0,5)MHZ l ,di khng hp gim bt cc thit b.

. Gii thch gc vung.

Ta em dch sng mang mu 3,58 MHZ ly mt gc dng (+450) vt

trc mang tn hiu EIv mt gc li sau m (-450) mang tn hiu EQ cho hai

tn hiu khi ln vo nhau.c im ca sng iu bin nn l khi tn hiu mu cn gi i khng c

(bng khng) th sng mang mu cng b trit tiu ht (bng khng).

Bin ln nht ca sng iu bin nn cng ch bng cao ca tn hiu

mu.

Vi hai c im trn ngi M chn phng thc iu bin nn

mang tn hiu mu i nhm khng gy nhiu mu ln nh en trng.

II. 5.1.3 S khi mch to m mu pha pht

) 330

330 (

(ER EY)

(EB EY)0

EQ

EI

0 1,5MHZ

0 1,5MHZ

0 0,5MHZ

EI


16/87


16

II.5.1.4 S khi mch gii m mu pha thu

Knh

Tn hiuvideomu

Mch cnghng3,58 MHZ

- Chn lc- Khuych iTrung tn mu

ER- EY(EB - EY)

Tch sngiu bin

nnER- EYEG - EY(EB - EY)

MchmatrnG - Y

FH

Xung qut

dng

CngBursh

Sopha

To lisng mangmu 3,58

MHZ

Dch pha450

Hnh I.5 s khi mch to m mu pha pht

ante

nhmucn

truyni

ER

EG

EB

Tn hiu chi Ey

Hai tn hiu hiumu(ER - EY)(xoay 33 )

EI

(EB - EY)(xoay 330)

Mchma

trn

EQ

Khuych i EYEY

Tn hiumu C

Mch to m mu

(cch gi tn hiu mu i).

Dng mt sng mang ph

3,58MHZ iu bin nn v

vung gc mang hai tn

hiu EI v EQri t hp thnh1 tn hiu mu C

u

f

MHZ

0

C

Y

3,5

4

iubinAM

fA

My pht

sngmang

hnh nhfA

ftTn hiu

Audio m

My phtsng mangm thanhftiu tn

em tn hiu mu C lngvo trong ph tn ca tn

hiu chi EY

C


17/87


17

Hnh I.6: S khi mch gii m mu pha thu

Tn hiu video mu a n mch gii m mu trc ht c mch cng

hng 3,58 MHZ tip nhn, chn lc ly trung tn mu ri khuych i cho

trung tn mu mnh ln v a n mch tch sng iu bin nn (cn c gi

l mch tch sng ng b). Mun tch c sng iu bin nn ly ra tn

hiu mu th trc ht my thu phi to li sng mang mu 3,58 MHZ ri cho sopha ng b vi sng mang mu pha pht gi n lm chun v c xung

qut dng m cng ly xung ng b mu do i pht gi n. Sng mang

mu 3,58 MHZ c dch pha 450ri em cng vi sng iu bin nn do i

pht gi n thnh sng iu bin thng thng ri dng i t tip im cao

tn tch sng iu bin ly ra hai tn hiu hiu mu (EREY) v (EBEY) ri

cho qua mch ma trn G Y cho ra ba tn hiu hiu mu.

Tn hiu mu NTSC c cha tng cng 7 tin tc: 4 tin tc c sn truynhnh en trng EY, 2 tin hin mu EI v EQ, tn hiu ng b mu. Trong tn

hiu ng b mu l tin tc v pha gc ca sng mang ph cn thit phi truyn

i tch sng iu bin nn.

H truyn hnh NTSC tn ti mt s nhc im trong quan trng hn

c tn hiu mu rt nhy cm vi mo pha vi sai. Do s bin i pha sng mang

mu ph, lm cho mu sc ca nh khi phc thiu chnh xc. i hi cc thng

s k thut ca cc thit b truyn hnh phi c chnh xc cao, sai lch lrt nh.

II.5.2 H truyn hnh mu PAL (Pluse Alterntion line: pha thay i

theo dng)

khc phc nhc im ca h NTSC nhiu h truyn hnh mu ln

lt ra i v c nhng khc bit so vi h NTSC. H truyn hnh PaL l h

truyn hnh mu c Cng ho Lin bang c nghin cu v c xem l h

tiu chun t nm 1966. y l h truyn hnh ng thi, n ng thi truyn tnhiu chi v hai tn hiu hiu mu.


18/87


18

II.5.2.1 Dng th vc t mu gii thch nguyn l sa mo pha

trong h PAL

Trong h truyn hnh mu PAL ngi ta cng truyn tn hiu chi Y/ v

hai tn hiu hiu mu U v V.Y/: 0,299 R/+ 0,587 G/+ 0,114 B/

V = 0,877 (RY)

U = 0,493 (BY)

Di tn tn hiu video h PAL rng 5 MHZ (tng thch vi chun qut

625 dng/50. Hai tn hiu mu U v V c rng di tn bng nhau v bng 1,3

MHZ. H PAL dng mt sng mang ph mang ng thi hai tn hiu hiu mu

U v V, dng phng php iu ch vung gc v c thnh phn sng mang phmang tn hiu V o pha theo tng dng qut. Vic o pha ny xy ra trong thi

gian qut ngc ca dng.

Vic o pha thnh phn sng mang ph tn hiu hiu mu V ca h PAL

nhm gim nh hng ca mo pha tn hiu mu n cht lng nh mu c

khi phc.

th gii thch qu trnh ny nh sau:


19/87


19

Hnh I.7: th gii thch nguyn l sa mo pha h PAL

Gi s ti dng th n th pha i pht gi i mt tn hiu mu l vc t

OM1 c gc pha l v c to l + EV v EUnhng n pha thu nhn c

th b sai pha thnh vc t OM/1. n dng tip theo n + 1 th my pht c pht

i mt mu gi l vc t OM2c to l - EV v + EU th my thu nhn c

cng b sai pha thnh vc t OM/

2nhng y l mu gi my thu li phi ongc pha li thnh mu tht, do thnh vc t OM//2. By gi em cng

hai vc t mu ca hai dng k lin k nhau li v bo l mt mu ca mt

dng ta c vc t tng hp l M/1 + M/2chnh l vc t OM1 ko di ra.

Vy l gc pha tr v gc pha ban u khng b mo pha na, ch l bin

ln ln gn gp i ta dng chit p gim xung mt ln l xong.

EV

+2EV

+2EV

EV

+EU

M1+ M//

2

M/

M//2

M 1

0 EU

(EB EY)

M/2

M2

- EV

- 2EV


20/87


20

II.5.2.2 S khi mch to m mu h PAL pha pht

Tn hiu chi EY

Hai tn hiu(ER - EY)(x h s)

EV

(EB - EY)(x h s)

Tnhiuvideomu

EM

Khuych i EYEY

Tn hiumu C

Mch to m mu

(cch gi tn hiu mu i)

Dng mt sng mang ph 4,43 MHZ

iu bin nn v vung gc mang hai

tn hiu Ev v EMtrong ring EVc

lin tc o pha theo dng k

Ri t hp thnh mt tn hiu mu C

u

f (MHZ)0

C

EY

Y

4,436

iubin AM

anten

fA

Mypht

hnh fA

em tn hiu mu C lng vo trong ph tn ca tn hiu chi EY

Hnh I.8: S khi mch to m mu h PAL pha pht

C


21/87


21

II.5.2.3 S khi mch phi gii m mu h PAL pha thu

Cc tn hiu mu sau tch sng v tn hiu chi sau b tr c a vo

ma trn to li tn hiu mu c bn ban u. C th nhn thy, phn gii

mu theo chiu dc h PAL bng mt na phn gii ca thng tin chi. II.5.2.4 H PAL c cc c im chnh sau:

Tn hiu chi vn xc nh nh h NTSC nhng di tn rng 5 MHZ

tng thch vi tiu chun qut 625/50. Tn hiu mu c ghp knh theo tn

s cng tn hiu chi truyn i. Hai tn hiu hiu chnh mu l:

V = 0,877(RY)

U = 0,493 (BY)

Hai tn hiu mu U, Y c rng di tn bng 1,3 MHZTn s qut dng: fH = 15625 HZ

Tn s qut mnh: fV = 50HZ

Khong cch hnh cch ting: 5,5 MHZ (PAL D/K: 6,5 MHZ)

H PAL dng mt sng mang ph mang ng thi hai tn hiu mu U v

V dng phng php iu ch vung gc v c thnh phn sngmang ph mang

tn hiu V o pha theo tng dng qut, o pha trong thi gian qut ngc ca

dng.

Hnh I.9: S khi mch to m mu h PAL pha thu

(ER- EY)

(EG - EY)

(EB - EY)

Trctip

Tr64MS

opha1800

+2EU+

2EV+

(ER- EY)

(EB - EY)

Tch

sngiu

binnn

Mchmatrn

G - Y

Knh mu

tn hiuvideo mu

Mch cnghng 4,43MHZ

- Chn lc- KhuychiTrung tnmu

CngBurst Soha To li sngman mu Dchpha 450

fH

Xungqutdng


22/87


22

Tn s sng mang ph phi min tn s cao ca ph tn tn hiu chi fsc

= 4,43 MHZ

C hai h PAL v SECAM u dng chung mt lun c cho rng hai

dng k lin k nhau th tn hiu hnh nh v mu sc hon ton ging nhau, coinh l mt. Do , h PAL ly tn hiu mu ca dng trn cho i chm li

64MS l mt chu k qut dng ng thi xut hin cng vi mu ca dng

sau n ri em cng hai dng mu li v bo l mu ca mt dng sa

c mo pha.

Cn h SECAM th ly tn hiu mu dng trn cho i chm li cng

64MS ng thi xut hin cng vi mu ca dng di n v bo l hai

mu ca mt dng mc d pha i pht ch pht i mi dng k c mt mu (lnlt v mu).

II.5.3 H truyn hnh mu SECAM

(Squentiel Couleur Ammoire): l h truyn hnh mu ng thi ln lt.

Sau nhiu nm hon thin nm 1967 h cn c tn SECAM IIIB h ny c tnh

chng nhiu tng i cao, km nhy vi mo pha, mo pha visai, mo bin

visai.

II.5.3.1 Tn hiu mu v phng php iu ch:Trong h truyn hnh SECAM ngi ta truyn i tn hiu chi Y/ v hai

tn hiu hiu mu D/R v D/

B

Trong Y/= 0,299 R/+ 0,587 G/+ 0,114 B/

D/R = - 1,9 (RY)

D/B = 1,5 (BY)

Tn hiu chi Y/c rng bng tn l 6MHZ, hai tn hiu hiu mu c

rng di tn bng nhau v bng 1,3 MHZ, du tr th hin cc tnh ca (R Y) tn hiu ch Y/ng truyn tt c cc dng, cn hai tn hiu mu D/R v

D/Btruyn ln lt theo dng qut trn hai sng mang ph c tn s trung tm l

fOR v fOBtheo phng php iu tn.

fOR = 282 x fH = 282 x 15,625 = 4,40625 MHZ

fOB = 272 x fH = 272 x 15,625 = 4,25 MHZ

Ngi ta chn fOR v fOB khc nhau lm tng tnh chng nhiu m

khng gim h thng tng hp, v tn s cao lm gim mo giao thoa giacc tn hiu my thu hnh


23/87


23

II.5.3.2 Tin nhn tn cao pha pht v gii nhn tn cao pha thu

V h SECAM mang tn hiu mu i bng phng thc iu t n m

sng iu tn c bin rt ln khng h b suy gim nn h SECAM phi b

xung thm hai k thut tin nhn v gii nhn (khc vi h NTSC v PAL)Dng mch cng hng c c tuyn tn s dng chung nga pha

pht suy gim sng mang mu xungcc tiu nhm khng gy nhiu mu ln

nh en trng v dng chung xp pha thu khi phc li sng mang mu,

nh chung cng hng c chnh ng tn s bng 4,286 MHZ.

Hnh 10: c tnh dng chung

V h SECAM dng phng thc iu tn mang cc tn hiu hiu mu.

Do trong khi iu tn. Nhiu lt vo pha tn s cao ca tn hiu mu l rt

nhiu hn nhiu vng tn s thp khi cng bin . V vy trc khi a vo

iu ch ngi ta c khuych i cho tn hiu mu mnh hn ln pha tn

s cao trc khi a vo iu tn vn cng c nhiu v c gi l tin

nhn tn cao. pha thu, sau khi iu tn ly ra tn hiu mu th phi suy gimtn s cao xung cho tr li bnh thng v gi l gii nhn tn cao.

Pha thu

Pha pht

U

4,250 4,286 4,406

FoB FoR

F(MH)0

K

0 F(MHZ)

Tin nhn pha pht

Gii nhn pha thu


24/87


24

Hnh 11. Tin nhn tn cao pha pht, gii nhn tn cao pha thu

II.5.3.3S khi mch to m mu h SECAM:

Tn hiumu C

anten

em tn hiu mu C lng votrong ph tn ca tn hiu chi

EY

Hnh 12:s khi mch to m mu h SECAM

Tn

hiuvideo

mu

C

EY

Tn hiu chi EY

Hai tn hiu hiu

(ER - EY)(xh s) DR

(EB - EY)(xh s) DB

Khuych i EYEY

Mch to m mu

(cch gi tn hiu mu i).

Dng th

n

Dng th

n +1

Dng th

n +2

Dng th

n +3

Ey

DR

fOB = 4,06

MHZ

iu tn

Ey

DB

fOB = 4,250

MHZ

iu tn

Ey

DR

fOB = 4,06

MHZ

iu tn

Ey

DB

fOB = 4,250

MHZ

iu tn

u

f (MHZ)0

Y

4,406

6

iu binAM fA

My phthnh

C

4,250

C

4,406


25/87

n tt nghip Truyn hnh s

Nguyn Quang Luyn Khoa TVT - HBKHN 25

Mch cng

hng

4,286MHZ

dng chung

xp.

-Chn lc

-Khuych i

Trung tn mu

PERMUTALOR

Tr64 S

DR

DB DR

DB

DB

DR DB

DR

Trc tip

Tn hiu mu l nlt ln ln

DR

DR

DB

DB

Tn hiumu ngthi ring

bit

Tch sng FMf0= 4,406MHZ

phi nhn tncao

Tch sng

F.Mf0B=4,250MH

Z gii nhntn cao

DB

DR DR

DB DB

DR

Mch

ma

trn

G-Y

(EREy)

(EBEy)

(EREy)

(EGEy)

(EBEy)Knh

mu

Tn hiu

Mch cng hng

4,250MHZ nhn

dng mu (Ident

gate)

Trit

mu

Killer

Flip Flop

fH xung qut dng

fH xung qut dng

Hnh.13: Mch gii m h secam pha thu

II.5.3.4.MchgiimmuhSecamphathu

Chuyn mchSECAM


26/87


26

Tn hiu video mu a n mch gii m SECAM trc ht i qua mch

cng hng dng xung chung xp iu chnh ng tn s 4,286 MHZ chn

lc v khuych i trung tn mu ri chia ba ng nh sau: Ng 1: Trc tip a tn hiu mu n u vo ca chuyn mch SECAM

(permutator)

Ng 2: Cho tn hiu mu i qua dy tr sng 64ms lm chm li thi gian

mt dng k ri cng a n u vo ca chuyn mch SECAM. Vy l u

vo ca chuyn mch SECAM ch t mt dng u tin ra, cn t dng th hai

tr i th dng no vo cng ng thi c hai mu v lam nhng tn hiu mu

y l ln lt v mu vn ln ln v ng i. Khi thng qua chuyn mchSECAM th tn hiu mu c hng i cho ng tuyn nn u ra cu

chuyn mch SECAM .th mi dng k u ng thi c c hai mu v lam

v ring bit tch ra thnh hai ng. Chng c dn n hai mch tch sng

iu tn ly ra tn hiu hiu mu ly ra mu , mch tch sng iu tn

phi chnh theo tn s f0R= 4,406 MHz ly ra mu lam, mch tch sng u

mang chnh theo tn s FOB= 4,250 MHZ cc tn hiu mu ny c gii nhn

tn cao cho tr li bnh thng ri a n mch ma trn G Y ly ra ba tnhiu hiu mu.

Ng 3: Trung tn mu c a n mch cng hng 4,250 MHZ

nhn dng mu m ca nhn dng v dng xung qut dng fH m ca nhn

dng. Nu khng nhn dng ra c tn hiu mu th n s iu khin cho mch

trit mu kho chuyn mch SECAM lm tt knh mu khng gy nhiu mu

ln nh en trng.

Cn vic iu khin lp trng thi ca chuyn mch SECAM l ly xungqut dng thng qua mch Flp Flp chia i tn s i vo iu khin.

II.5.3.5 Mt vi c im chnh h SECAM

Tn hiu chi vn xc nh nh hai h trn nhng rng di tn 6 MHZ.

Truyn hnh ng thi ln lt tng dng mt hai tn hiu mu DR v DB

DR = - 1,9 (EREY)

DB = 1,5 (EBEY)

C mt dng truyn i DR: 1 dng DBri li DR v DBHai tn hiu ny c rng di tn bng 1,3 MHZ.


27/87


27

iu tn DR, DBvo hai sng mang mu ph ring c tn s

fOR = 4,406 MHZ

fOB = 4,25 MHZ

Tn hiu mu SECAM c 8 tin tc c sn en trng, tin tc th 5 tn tcth 6 ca fMR v fMBch xut hin ci n hoc ci kia ti mi thi im

fMR = fOB = 280 KHZ

fMB = fOB = 230 KHZ

Tin tc th 7 l nhn dng dc, tin tc th 8 l nhn dng ngang ch s

dng mt trong hai ti my thu.

u im ni bt ca h SECAM: tn hiu iu tn t nhy cm vi mo

pha nn h SECAM m bo c mu sc ca nh truyn hnh c n nhcao.


28/87


28

PHN II: TRUYN HNH S

CHNG I: TNG QUAN TRUYN HNH S.

I.Gii thiu chung.

Truyn hnh mu vi ba h: NTSC. PAL, SECAM xut hin vo nhngthp k 50, 60 to nn mt bc ngot ln trong qu trnh pht trin ca cng

ngh truyn hnh. C ba h s dng cc tn hiu thnh phn l tn hiu chi v

hai tn hiu mu (Y, R-Y, B-Y). iu khc nhau c bn gia ba h ny l

phng php iu ch tn hiu hiu mu, tn s sng mang mu v phng thc

ghp knh.

Trong nhng nm gn y, cng ngh truyn hnh ang chuyn sang mt

bc ngoc mi qu trnh chuyn i tcng ngh Analog sang Digital. Nhiunh nghin cu cho rng Thi i Digital Video, Digital Television bt u,

cng ngh Analog sp ht vai tr lch s. Truyn hnh Analog ngy cng a

vo s qun lng, nhng ng cho truyn hnh s. Cng ngh truyn hnh

ang tri nghim mt s thay i ln lao mt s thay i v cht!.

Tht vy, thit b Analog truyn thng nh my ghi hnh, Camera, Mixer,

k xo... c Digital ho. Vi nm trc y, thit b s dng cng ngh

Digital mi ch l nhng hn o Digital nh trong i dng Analogmnh mng. Ngy nay nhng hn o Digital nh b t khng nh

mnh v tr thnh chu lc Digital rng ln, nhanh chng chim lnh mi

trng cng ngh truyn hnh.

Trn thc t, cc nh sn xut gim vic thit k cc thit b truyn

hnh Analog. V vy thit b Analog ngy cng vng bng trn th trng. Trong

mt tng lai khng xa, cc thit b sn xut chng trnh, cc my pht hnh,

cc thit b vi deo, Audio ni chung s c thay th bng thit b Digital.Cng khng c g qu, nu ta gi qu trnh Digital ho l mt cuc cch

mng v cng ngh Digital s lm thay i tn gc cch suy ngh cch tip cn ,

phng thc sn xut chng trnh truyn hnh, l cha k n vic nng

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

tin chung nh Fax, Telephone, My tn .. s hi nhp ny to iu kin khng

ch c ngha v mt kinh t, k thut m cn to cho ngnh truyn hnh nhng

kh nng ng dng mi.


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Vi cng ngh Digital, tn hiu Video trong cc trung tm truyn hnh s

khng cn xut hin trn mn hnh hin sng vi nhng thnh phn quen thuc

mc trng, mc en... Th h k tip, trong mt vi nm ti s ch cn bit

n tn hiu truyn hnh nh nhng chui bits (bits stremam) hoc th mtEYDGRAM). Tn hiu Video thc s tr thnh nhng chui s liu hon ton

tng thch v c kh nng truyn ti, x l trong mng my tnh.

Trong cc Studio Digital, vic truyn v x l Video s c thay th

vic truyn v x l Data. Vi s pht trin ca cng ngh thng tin ngy nay,

truyn hnh Digital trong tng lai s lm thay i v cn bn dy chuyn cng

ngh, phng thc sn xut chng trnh truyn hnh.

Cng ngh truyn hnh s ang bc l th mnh tuyt i so vi cngngh tng t trn nhiu lnh vc.

*Tn hiu s t nhy cm vi cc dng mo xy ra trn ng truyn.

*C kh nng pht hin li v sa sai.

*Tnh a nng v linh hot trong qu trnh x l tn hiu.

*Hiu qu s dng di thng cao v c kh nng truyn nhiu chng

trnh trn mt knh RF.

*Tnh phn cp. V d: mt dng d liu c th c truyn mt chngtrnh c phn gii cao duy nht hoc mt vi chng trnh c phn gii

tiu chun.

*Kh nng ti nhiu dng thng tin khc nhau.

*Tit kim nng lng, vi cng mt cng sut pht, din ph sng ln

hn cng ngh tng t. Cng sut my pht s ch bng 1/4 cng sut my pht

tng t.

*Truyn hnh s vi cng ngh mng n tn (SFN) c th tit kim cti nguyn tn s qu him ca quc gia.

*D dng thch nghi vi cc bc pht trin tip theo sang truyn hnh

phn gii cao hoc pht thanh vi cht lng CD trong tng lai.

*Th trng a dng, c kh nng cung cp nhiu loi hnh dch v.

*Chi ph khai thc thp.

*Hon ton c kh nng ho nhp vo xa l thng tin.

Tuy nhin vic chuyn i tn hiu truyn hnh t tng t sang s cnggp phi nhiu vn cn xem xt nghin cu.


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Qu trnh s ho tn hiu phi qua ba cng on: ly mu, lng t ho, s

ho. Tn hiu Video theo tiu chun OIRT c tn s nh hn hoc bng 6MHz,

m bo cht lng tn s ly mu phi ln hn 12MHz, vi s ho 8 bit/s,

truyn ti y thng tin mt tn hiu Video thnh phn vi phn giitiu chun, tc bit phi ln hn 200 bit/s. i vi truyn hnh phn gii

cao (HDTV) tc bit phi ln hn 1 Gbit/s.

Dung lng ny qu ln, cc knh truyn hnh thng thng khng c kh

nng truyn ti. C mt s vn mu cht cn xem xt trong qu trnh s ho

tn hiu truyn hnh, trong bao gm:

*Tn s ly mu.

*Phng thc ly mu.*T l gia tn s ly mu tn hiu chi v tn hiu hiu mu (trong

trng hp s ho cc tn hiu thnh phn)

*V nn tn hiu Video c th truyn tn hiu truyn hnh s trn cc

knh truyn hnh thng thng trong khi vn m bo cht lng tn hiu theo

mc ch s dng.

I.1.c im ca truyn hnh s:

1.1.Yu cu v bng tn.Tn hiu s gn lin vi yu cu bng tn rng hn rt nhiu so vi tn

hiu tng t . V d i vi tn hiu tng hp yu cu tn s ly mu phi bng

4fscnh i vi h NTSC l 14,4 MHz. Nu thc hin m ho 8bit/mu, tc

dng bit s l 115,2 Mbit/s, rng bng tn khong 58 MHz. Trong khi tn

hiu tng t cn mt bngtn l 4,2 MHz. Bin php khc phc nn tn hiu.

1.2.T l tn hiu trn tp m (Signall/Noise).

*T l : Signal/noise Nhiu, tp m trong tn hiu tng t c tnh chtcng. T l S/N ca ton b h thng l do tng cng cc ngun nhiu thnh

phn gy ra v vy S/N ca ton b h thng < S/N c t l thp nht nhiu

ln.

*i vi tn hiu s nhiu l cc bit li c khc phc bng mch sa

li. Khi c qu nhiu bit li, s nh hng ca nhiu c lm gim bng cch

che li, t s S/N ca h thng s gim rt t hoc khng i tr trng hp t

l li qu ln lm cho mch sa li mt tc dng khi dng bit khng cn ngha tin tc.


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* ngha: c im c ch cho vic sn xut chng trnh truyn hnh vi

cc chc nng bin tp phc tp , cn nhiu ln c v ghi, vic truyn tn hiu

qua nhiu chng cng c thc hin rt thun li vi tn hiu s m khng lm

suy gim cht lng tn hiu hnh. 1.3.Mo phi tuyn.

Tn hiu s khng b nh hng bi mo phi tuyn trong qu trnh ghi v

truyn. Cng nh i vi t l S/N tnh cht ny rt quan trng trong qu trnh

ghi c nhiu ln.

1.4.Chng ph (Aliasing).

Mt tn hiu s c ly mu theo c chiu ngang v chiu dc nn c kh

nng xy ra chng ph theo c hai hng. Theo chiu dc, chng ph trong hthng tng t v s l nh nhau. ln ca mo chng ph theo chiu ngang

ph thuc vo thnh phn tn s vt qu tn s ly mu gii hn Nyquist.

ngn nga hin tng chng ph theo chiu ngang c th thc hin bng cch s

dng tn s ly mu bng hai ln tn s cao nht trong h thng tng t.

1.5.X l tn hiu

Tn hiu s c th c chuyn i v x l tt cc chc nng m h

thng tng t khng lm c hoc gp nhiu kh khn. C th thao tc cccng vic phc tp m khng lm gim cht lng hnh nh. Kh nng ny c

tng cng nh kh nng lu tr cc bit trong b nh v c th c ra vi tc

nhanh. Cng vic tn hiu s c th thc hin d dng l: sa li gc thi gian ,

chuyn i tiu chun, dng hu k, gim rng bng tn.

1.6.Khong cch gia cc trm truyn hnh ng knh.

Tn hiu s cho php cc trm truyn hnh ng knh thc hin mt

khong cch gn nhau hn nhiu so vi h thng tng t m khng b nhiu.Mt phn v tn hiu s t chu nh hng ca nhiu ng knh, mt phn l do

kh nng thay th xung ho v xung ng b bng cc t m, ni m h thng

tng t gy ra nhiu ln nht.

1.7.Hin tng Ghosts (bng ma)

Hin tng ny xy ra trong h thng tng t do tn hiu truyn n my

thu theo nhiu ng. Vic trnh nhiu ng knh ca h thng s cng lm

gim i hin tng ny trong truyn hnh qung b.I.2.S khi h thng thu pht hnh s.


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S dng phng php s to, lu tr v truyn tn hiu ca chng

trnh truyn hnh trn knh thng tin m ra kh nng c bit rng ri cho cc

thit b truyn hnh.

*Nguyn l cu to ca h thng v cc thit b truyn hnh s.

Hnh 1. S khi h thng thu pht hnh s.

I.3.Nguyn l hot ng.

u vo ca thit b truyn hnh s s tip nhn nhng tn hiu truyn

hnh Analog , qua b bin i A/D tn hiu truyn hnh 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 n thit b pht sau qua knh thngtin tn hiu truyn hnh s c a ti thit b thu qua gii m, b chuyn i,

b bin i tn hiu Digital thnh tn hiu truyn hnh tng t. (B gii m). H

thng truyn hnh s s trc tip xc nh cu trc m ho v gii m tn hiu

truyn hnh.

M ho knh thng tin m bo chng cc sai st cho tn hiu trong knh

thng tin. Cc thit b bin i trn c gi l b iu ch v gii iu ch. Ty

theo phng php bin i tn hiu s dng tn hiu Video tng hp hoc tnhiu Video thnh phn m c im ca h thng truyn hnh cng thay i theo.

M ho

knh

Bin itn hiu

Thit b pht

Bin iA/D

T/h truynhnh

Digital

Knhthng tin

Gii m Bin itn hiu

Thit b thu

Bin iA/D

T/h truynhnh Analog


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II.S ho tn hiu truyn hnh.

II.1. M u.

Cc h truyn hnh mu c s dng hin nay (NTSC, PAL, SECAM) l

nhng h truyn hnh tngt. Tn hiu Video l hm lin tc theo thi gian .Tn hiu truyn hnh tng t (t khu to, truyn dn pht sng n khu thu)

chu 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.

Tn hiu Video s c to t tn hiu Video tng t. Tn hiu Video s

c hai trng thi logc 0 v 1. Tp hp cc gi tr logic 0 v 1 biu din

tn hiu Video s. Tn hiu Video s c tnh chng nhiu cao, c nhiu u im

hn tn hiu Video tng t. Ngy nay nhiu nc s dng truyn hnh sthay th truyn hnh tng t trong nhiu cng on ca h thng truyn hnh.

II.2.Bin i tng t sang s.

Qu trnh bin i tn hiu tng t sang tn hiu s bao gm ba giai on:

*Ly mu tn hiu :L qu trnh gin on theo thi gian bng tn s ly

mu (fsa) . Kt qu cho ta chui cc mu (sample).

*Lng t ho tn hiu : L qu trnh ri rc ho bin tn hiu, c ngha

l chia bin ra nhiu mc khc nhau v mi mc khc nhau s c gn mt gi tr bin .

*M ha tn hiu: l qu trnh bin i tn hiu c lng t ho

thnh tn hiu s bng cch sp xp cc mc tn hiu tng ng theo h m

thp phn ( theo h m nh phn 0 v 1 )

III.C s bin i tn hiu truyn hnh.

III.1.Bin i tn hiu Video.

Bin i tn hiu Video tng t thnh tn hiu Video s l qu trnh bini thun, cn qu trnh bin i Video s thnh Video tng t l qu trnh bin

i ngc. Trong h thng truyn hnh s c s dng rt nhiu b bin i

thun ADC v cc b bin i ngc DAC.

Trong truyn hnh mu tin tc hnh nh truyn c biu din bng ba tn

hiu mu c bn: (R), Lc (G), v Lam (B). N c th c truyn bng ba

kh nng sau:

*Ba tn hiu bng rng R, G , B.*Mt tn hiu chi bng rng Y v hai tn hiu bng hp I,Q.


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*Mt tn hiu Video mu tng hp.

Nh vy, c th chia tn hiu Video thnh hai loi chnh: tn hiu Video

mu tng hp v tn hiu Video thnh phn. V th qu trnh bin i Video

tng t thnh tn hiu Videos s c hai cch chnh.*Bin i tn hiu Video mu tng hp.

*Bin i tn hiu Video mu thnh phn (tn hiu chi Y v hai tn hiu

hiu mu R-Y v B-Y hoc ba tn hiu mu c bn R, G, B).

III.2.Tn s ly mu tn hiu Video.

Tn s ly mu c chn sao cho hnh nh c cht lng cao nht, tn

hiu truyn c tc bit nh nht, tc bng tn nh nht v mch thc hin

n gin. vic ly mu khng gy mo, phi chn tn s ly mu tha mnnh l Nyquist: fsa (2fgh). Kt qu nghin cu cho thy, khi tn s ly mu tin

gn ti 13MHz th cht lng khi phc nh rt tt, cn nu nh hn 13MHz,

cht lng nh gim r rt ( khi ly mu tn hiu Video mu tng hp). V d

nh gim nt conture ( nho) cc chi tit nh theo chiu ng. Trong

trng hp ly mu tn hiu thnh phn , nu gim tn s ly mu tn hiu mu

t 1/2fsaca tn hiu chi xung 1/4fsac th gy ra mo di dng dng nhp

nhy hoc xut hin cc vng theo dng. Vic chn tn s ly mu ti u vi tng loi tn hiu nh sau:

III.2.1.Tn hiu Video tng hp.

i vi tn hiu Video tng hp (NTSC v PAL), tn s ly mu tn hiu

Video s (fsa) thng gp nhiu ln tn s sng mang mu fSC, v d fSa = 2fSC;

fSa= 3fSC hoc fSa= 4fSC.

H SECAM dng hai ti tn mang mu, v s dng iu tn nn khng

dng c mt tn s fSacho cc tn hiu hiu s mu v chn tn s ly muti u rt kh.

h NTSC, tn s ly mu (fSa), nu mun c th c mi quan h cht ch

vi c tn s dng (fH) v tn s sng mang mu fSC. Nhng iu ny khng th

thc hin c h PAL v SECAM.

Nu chn fSa= 3fSCi vi h NTSC tng ng 10.74 MHz l gi tr

cha ln. Nu chn fSa= 4fSCtng ng 14.3MHz s cho cht lng hnh

nh rt tt. Vic chn tn s ly mu cng ln s lm cho vic s dng cc blc trnh chng ph (c t trc b ADC) d dng hn v b lc ti to cng


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a li mt c tuyn tt hn, tuy nhin n s lm tng tc bit, dn n lng

ph di thng.

tin ti xu hng chun ho cho cc thit b Video s, ngi ta a

ra hai tiu chun v tn s ly mu l 4fSC NTSC v 4fSCPAL, tn s ly mubng 4 ln tn s sng mang mu, v s dng 8 bit hoc 10 bit biu din cc

mu.

Tn hiu Video s tng hp cn mang y nhng nhc im ca tn

hiu Video tng t, nht l hin tng can nhiu gia tn hiu chi v tn hiu

mu.

Trong nhng nm gn y ngi ta thng s dng phng php s ho

tn hiu Video thnh phn.Tn s ly mu ln i hi thit b, ng truyn v cc b nh c dung

lng ln. Chi ph cho ton b h thng do vy tng ln gp nhiu ln. Tn s

ly mu thch hp nht nm trong khong 12 14MHz.

S ho tn hiu Video tng hp c u im l tc bit thp so vi s

ho tn hiu Video thnh phn, iu cng c ngha l lng bng t dng

trong cc my ghi hnh hn ch hn.

Tuy nhin tn hiu s tng hp bc l, nhiu nhc im trong qu trnhx l s, to k xo, dng hnh...

III.2.2.Tn hiu Video thnh phn.

Vi tn hiu Video thnh phn, tn s ly mu thng c biu th thng

qua t l tn s ly mu tn hiu chi v tn s ly mu cc tn hiu hiu mu. V

d 14:7:7 l t l ly mu ng vi tn s ly mu tn hiu chi l 14MHz v i

vi hai tn hiu hiu mu l 7MHz. H thng dng tiu chun 14:7:7 vi 8

bit/mu cho ta tc bit bng 244 Mbit/s ln hn tc bit h 12:4:4 40% vh 12:6:6 17%.

4:2:2 l tiu chun trong t s ly mu gia tn s ly mu ca tn hiu

Video thnh phn Y/CB/CR= 4:2:2 v cc tn s ly mu tng ng vi cc tn

hiu thnh phn bng (vicu trc ly mu trc giao):

fs(Y) = 13,5 MHz; fs(CB) = 6,75 MHz; fs(CR) = 6,75 MHz

Gi tr 13,5 MHz l mt s nguyn ln tn s dng cho c hai tiu chun

525 v 625 dng:13,5 MHz = 864fHi vi tiu chun 625 (fH = 15625 Hz)


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13,5 MHz = 858 fHi vi tiu chun 525 (fH = 15750 Hz)

Tn s ly mu quyt nh b rng ti a ca di ph tn hiu. Theo

Shanno v Nyquist, tn s ly mu ti thiu phi bng hai ln tn s cao nht ca

tn hiu. Nh vy vi tn s ly mu nh trn, b rng di ph ca tn hiu thnh phn theo l thuyt s bng 6,75 MHz i vi tn hiu chi, v 3,75 MHz

i vi tn hiu hiu mu CB, CR.

Video s thnh phn c coi l phng php s ho c s dng hin

ti cng nh trong tng lai ti cc studio hon ton s (All Digital Studio). Tn

hiu Video s thnh phn c th d dng c x l, ghi, dng trong cc cng

on hu k ti cc studio. Tn hiu Video s thnh phn loi b c nhng

nh hng do sng mang mu trong tn hiu Video tng hp gy nn.Trong qu trnh s ho tn hiu Video s thnh phn khi nim tn s ly

mu phi l bi s ca sng mang mu tr nn khng ph hp. Tn s ly

mu, nh trn trnh by, ngoi yu cu c gi tr trong khong t 12414 MHz

l bi s ca tn s dng cn phi t iu kin l tn s ly mu chung cho c

hai tiu chun truyn hnh 525 v 625 dng, c th tin ti mt tiu chun

Video s chung cho ton th gii. Loi b c nhng iu phin phc gy nn

bi tnh trng a h truyn hnh tng t.Tm li, tn s ly mu p ngc nhng yu cu trn c cc t

chc pht thanh truyn hnh (SMPTE, EBU, CCIR) thng nht la chn l:

FSa= 13,5 MHz chung cho c hai tiu chun 525 v 625 dng. C hai tiu

chun s dng chung cng mt tn s ly mu, cng mt lng mu trong thi

gian tch cc ca mt dng.

Thi gian mt dng ca hai h 625 v 525 bng 64 s v 63,56 s (h 625

c thi gian ln hn), trong khi thi gian tch cc ca mt dng trong h 625 l52 s. Nu c hai h u ly thi gian tch cc bng 52 s, thi gian xo dng

tng ng vi tng h l 12 s v 11,59 s thng s c bn i vi mi h l:

Tng s mu mi dng:

H 625 dng: 64 x 13,5 =864 mu.

H 525 dng: 63,56 x 13,5 = 858 mu

S mu trong thi gian tch cc ca mi dng:

H s 625 dng: 52 x 13,5 = 702mu


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H 525 dng : 52 x 13,5 = 702 mu

S mu trong thi gian xo dng:

H 625 dng : 12 x 13,5 = 162 mu

H 525 dng : 11,56 x 13,5 = 156 mu.III.2.3.Cu trc mu (sample)

Tn hiu hnh nh t camera v c hin th trn nh hnh cha thng tin

v ng b theo mnh v dng, l nh hai chiu. V vy khi phc chnh

xc nh, tn s ly mu phi c lin quan n tn s dng. Cc kt qu nghin

cu cho thy, tn s ly mu phi l bi s ca tn s dng. Khi im ly mu

trn cc dng qut s thng hng vi nhau, do trnh c cc hiu ng mo

ng bin.Nh vy, vic ly mu khng nhng ph thuc vo thi gian m cn ph

thuc vo to cc im ly mu. V tr cc im ly mu hay cu trc ly mu

c xc nh theo thi gian, theo cc dng v theo cc mnh. Hm ly mu c

th bin i dng xq(t,x,y). Tn s ly mu ph hp vi cu trc ly mu s cho

php khi phc hnh nh tt nht. V vy tn s ly mu phi thch hp theo c 3

chiu t,x,y. Tuy nhin, trong cc cu trc ly mu ph bin, ta ch xt cc mu

theo 2 chiu x,y. C 3 dng cu trc c dng ph bin cho tn hiu Video: III.2.3.1.Cu trc trc giao.

Cc mu c sp xp trn cc dng k nhau thng hng theo chiu ng.

Cu trc ny c nh theo mnh v theo nh (hai mnh). Trong trng hp ny

tn s ly mu tho mn nh l Nyquist do tc bit rt ln c s dng.

Dng 1, mnh 1

Hnh II.2 Cu trc trc giao

Dng 1, mnh 2

Dng 2, mnh 2

Dng 2, mnh 1


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III.2.3.2.Cu trc quincunx mnh.

i vi cu trc quincunx mnh, cc mu trn cc dng k nhau thuc

mt mnh xp thng hng theo chiu ng (trc giao), nhng cc mu thuc

mnh mt li dch i na chu k ly mu so vi cc mu ca mnh th hai.

Hnh II.3. Cu trc quinncunx mnh

Phn b ph tn cu trc quinncunx mnh rt c ngha vi mnh 1, n

cho php gim tn s ly mu theo dng. Ph tn ca cu trc ni trn ca mnh

2 so vi ph ca mnh 1 b dch i v c th lng vi ph tn c bn, gy ra

mo cc chi tit nh (khi hnh nh c cc sc ng).

III.2.3.3.Cu trc quinncunx dng.

Cc mu trn cc dng k nhau ca mt mnh s lch nhau na chu k lymu, cn cc mu trn mt dng ca mnh 1 s lch so vi mu ca dng tip

theo ca mnh 2 mt na chu k ly mu.

y khng xy ra trng hp lng ph bin vi ph chnh v khng b

mo. iu cho php s dng tn s ly mu nh hn 25% tn s Nyquist, v

tit kim c ph bin ca tn hiu s.

Hnh II.4. Cu trc quincunx dng.

Dng 1, mnh 2

Dng 2, mnh 2

Dng 2, mnh 1

Dng 1, mnh 1

Dng 1, mnh 2

Dng 2, mnh 2

Dng 2, mnh 1

Dng 1, mnh 1


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Tu theo cu trc ly mu, s xut hin mo nh c trng. i vi cu

trc trc giao, phn gii ca nh s gim. i vi cu trc quincunx mnh s

xut hin nhp nhy cc im nh.Ngc li cu trc quincunx dng s xut hin

cc vng trn theo chiu ngang (mo ng bin). Tm li cu trc trc giao chocht lng nh co nht, vi vi mt ngi th phn gii thp d chu hn

l hai loi mo nu trn.

II.2.4. Cc chun ly mu tn hiu Video.

Qu trnh ly mu l bc u tin ca vic s ho tn hiu Video , trc

ht ta tm hiu v mt vi tiu chun ly mu. C nhiu tiu chun Video s

thnh phn, im khc nhau c bn gia chng t l gia tn s ly mu v

phng php ly mu tn hiu chi v cc tn hiu mu, trong bao gm: tiuchun 4:4:4, 4:2:2, 4:4:0, 4:1:1. Di y ta xt phng thc tng chun. Tc

ly mu da trn c s tn s chun l 3,375 MHz.

III.2.4.1.Chun 4:4:4

Mu tn hiu ch c ly i vi cc phn t tch cc ca tn hiu Video .

Vi h PAL, m hnh c chia lm 625 x 720 im (pixel).

Hnh II.5. Tiu chun 4:4:4.

Cc tn hiu chi (Y), tn hiu hiu mu (CR, CB) c ly mu ti tt c

cc im ly mu trn dng tch cc ca tn hiu Video. Cu trc ly mu l cu

trc trc giao, v tr ly mu nh trnh by trong hnh v trn y:

Tiu chun 4:4:4 c kh nng khi phc hnh nh cht lng tt nht

trong cc tiu chun, thun tin cho vic x l tn hiu. Tiu chun ny c th

dng trong trng hp x l tn hiu chi v tn hiu mu RGB. N c th c

dng trong studio nhm ri rc ho tn hiu. Tuy nhin tiu chun ny s i hi

tc bit rt cao. Cc t chc tiu chun quc t thng nht v ch tiu tn s

ly mu cho truyn hnh s theo tiu chun ny vi tn gi l CCIR-601.

im ly mu tn hiu chi Y

im ly mu mu CR

im ly mu mu lam CB


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Vi chun 4:4:4 tc dng d liu (v d cho h PAL) c tnh nh

sau:

*Khi ly mu 8 bit: (720 + 720 + 720) x 576 x 8 x 25 = 249 Mbit/s

*Khi ly mu 10 bit: (720 + 720 + 720) x 576 x 10 x 25 = 311 Mbit/sIII.2.4.2.Chun 4:2:2.


im u ly mu ton b ba tn hiu : chi (Y) v hiu mu (C R, CB)im k tio ch ly mu tn hiu chi Y, cn hai tn hiu hiu mu khng

ly mu. Khi gii m mu suy ra t mu ca im nh trc.

im sau na l ly mu ba tn hiu Y, C R, CB

Tun t nh th, c 4 ln ly mu tn hiu chi Y, th hai ln ly mu CR,

hai ln ly mu CBto nn c cu 4:2:2.

i vi h PAL tc dng d liu theo chun ny c tnh nh sau:

*Khi ly mu 8 bit: (720 + 360 + 360) x 576 x 8 x 25 = 166 Mbit/s *Khi ly mu 10 bit: (720 + 360 + 360) x 576 x 10 x 25 = 207 Mbit/s

Tiu chun 4:2:2 l tiu chun c bn ca truyn hnh s. Cht lng hnh

nh ca tiu chun ny cao hn sau tiu chun 4:4:4. N cho php x l tn hiu

mt cch thun li.

III.2.4.3.Chun 4:2:0.

Theo chun ny, tn hiu Y c ly mu ti tt c cc im nh ca

dng, cn tn hiu hiu mu th c cch mt im s ly mu cho mt tn hiuhiu mu. Tn hiu hiu mu c ly xen k, nu hng chn ly mu cho tn

hiu hiu mu CRth dng l s ly mu cho tn hiu hiu mu CB.



im ly mu mu CR

im ly mu mu lam CB


im ly mu mu CR

im ly mu mu lam CB


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i vi h PAL tc dng d liu theo chun ny c tnh nh sau:

*Khi ly mu 8 bit: (720 + 360) X 576 X 8 X 25 = 124,4 Mbit/s

*Khi ly mu 10 bit: (720 + 360) X 576 X 10 X 25 = 155,5 Mbit/s

III 2.4.4. Chun 4:1:1 im nh u ly mu Y, CR, CB: ba im nh tip sau ch ly Y khng

ly mu CR, CB. Khi gii m mu ca ba im nh sau phi suy ra t im nh

u.

Tun t nh th, c bn ln ly mu Y, c mt ln ly mu CB,mt ln

ly mu CBy l c cu 4:1:1

Hnh II.8 Tiu chun 4:1:1

i vi h PAL, tc dng d liu theo chun ny c tnh nh sau:

*Khi ly mu 8 bit: (720 + 180 + 180) X 576 X 8 X 25 = 124,4 Mbit/s *Khi ly mu 10 bit: (720 + 180 + 180) X 576 X 10 X 25 = 155,5 Mbit/

Tiu chun ny cho cht lng hnh nh tng i thp, thng c

dng cho in thoi truyn hnh.

Chng II. Nn nh s.

I.Mc ch nn.

Vi k thut s ho truyn hnh, tc truyn v do vy di thng truynl qu ln. Do vy i hi rng knh truyn l rt ln. V d: tn hiu Video

s thnh phn (s ho 8 bit 625 dng ) c tc bit bng 216 Mbit/s. Di ph

cn thit truyn tn hiu ny c b rng khng di (3/4)x216 162 MHz .

Nu truyn qua v tinh bng C vi tiu chun 40 MHz/ 1 Transponder th

phi cn 4 Transponder ghp lin nhau.

Trong Studio truyn tn hiu bng cp, rc ni cht lng cao v vi

khong cch ngn, vic nn di tn ch mang tnh kinh t, nn vic truyn tn


im ly mu mu CR

im ly mu mu lam CB


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hiu c th thc hin c m khng cn nn. Song s rt kh khn, thm ch

khng th thc hin c vic truyn tn hiu Video s qua v tinh vi rng

di tn mt knh 27 MHz hoc qua h thng truyn hnh qung b trn mt t

vi tiu chun 7 8 MHz cho mt knh truyn hnh tiu chun. Do vy nnVideo l cng on khng th thiu c khc phc nhng kh khn v yu

cu di bng tn cho php.

II.Thc cht ca nn Video.

Bn cht ca nn Video 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 t ging nhau. Vy tn hiu truyn hnh c cha nhiu d liu dtha, ta c th b qua m khng lm mt thng tin hnh nh. l qu trnh xo

dng ,xo mnh, vng nh tnh hoc chuyn ng rt chm, vng nh nn ging

nhau hoc khc nhau rt t. Thng th chuyn ng trong nh truyn hnh c th

d bo, do ch cn truyn cc thng tin chuyn ng. Cc phn t ln cn

trong nh ging nhau, do ch cn truyn cc thng tin bin i. Cc h th ng

nn s dng c tnh ny ca tn hiu Video v cc c trng ca mt ngi (l

km nhy vi sai s trong hnh nh c nhiu chi tit v cc phn t chuynng). Qu trnh sau nn l gin nh to li nh gc hoc mt xp x nh gc.

II.1.M hnh nn nh.

Tng u tin ca b m ho Video , tn hiu Video c trnh by di

dng hiu qu nn hiu qu nht. im ct yu l phi xc nh ci g c

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

chnh l bn thn tn hiu,nhng hu ht cc thng tin quan trng ch tp trung

trong mt phn nh ca s m t ny. Trong cch biu din hiu qu, ch cphn nh d liu l cn thit truyn cho vic ti to li tn hiu Video.

Hnh II.9. M hnh h thng nn Video

Biu dinthun li

Lngt ho

Gn tm

Ngun X lknh

Gii tm

Giilng t

Biu dinthun li

Gii m VideoM m Video

Video

khi

phc


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Hot ng th hai ca b m ho l lng t ho gip ri rc ho thng

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

biu din c lng t ho thnh mt s hu hn mc. cng an ny xy

ra sai s, sai s ny c gi l sai s h thng.Hot ng th ba l gn cc t m . Cc t m ny l mt chui cc bt

dng biu din cc mc lng t ho. Cc qu trnh s ngc li trong b gii

m Video.

Mi hot ng c gng loi b phn d tha trong tn hiu Video v li

dng sgii hn ca h thng nhn ca mt ngi, Nh b i 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 s khng ctruyn i.

II.2. d tha d liu.

Nn s liu l qu trnh gim lng s liu cn thit biu din cng mt

lng thng tin cho trc. Cn phi phn bit gia s liu v thng tin . Thc t

gia s liu v thng tin khng ng nht vi nhau. S liu (v do l tn hiu)

ch l phng tin truyn ti thng tin. Cng mt lng thng tin cho trc c

th biu din bng cc lng s liu khc nhau, n bao gm s liu hoc tkhng cho thng tin thch hp ln xc nh bit.

d tha s liu l trung tm trong nn nh s. d tha d liu

khng phi l mt khi nim tru tng m l mt thc t c th nh lng

c bng ton hc.

Nu r1 v r2l lng s liu trong hai tp hp s liu cng c dng

biu din mt lng thng tin cho trc th d tha s liu tng i ca t p

hp s liu th nht so vi hp s liu th hai c th c nh ngha nh sau:RD= 1- 1/CN

Trong CN thng c gi l t s nn v CN = r1/r2.

Trong trng hp r1=r2 th CN= 1 v RD= 0, c ngha l so vi tp hp s

liu th hai th tp hp s liu th nht khng d tha. Khi r2


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c th thay i ty theo c im ca hnh nh ngun v ni dung nh. nh

gi cht lng nh, ngi ta tnh s bit cho mt im trong nh nn (Nb). N

c xc nh l tng s bit nh nn chia cho tng s im:

Nb= S bit nn/ s imTrong nn nh s c ba loi d tha s liu.

II.2.1.D tha m (Coding Redundancy).

Nu cc mc ca tn hiu Video c m ho bng cc Symbol nhiu hn

cn thit th kt qu c d tha m. gim d tha m, trong nn nh

thng s dng cc m c di thay i (VLC) nh m Huffman, m RLC..

Lng thng tin v hnh nh c xc xut cao s c m ho bng t m t bit

hn so vi lng thng tin c xc xut thp hn.II.22.D tha trong pixel (Interpixel Redundancy).

V gi tr ca bt k pixel no , cng c th c d bo t cc pixel ln

cn n, nn thng tin t cc pixel ring l tng i nh. S tham gian ca mt

pixel ring vo nh l d tha.

Nhiu tn (bao gm: d tha khng gian, d tha hnh hc, d tha trong

nh) c t ra phn bit s ph thuc ny ca pixel. Ta dng d tha

pixel ch tt c cc tn trn. gim d tha trong pixel ca mt nh, dy pixel hai chiu dng cho

vic nhn v ni suy phi c bin i thnh mt dng c hiu qu hn. Trong

cc phng php nn nh ngi ta thng s dng phng php bin i Cosin

ri rc (DCT) bin i pixel t min khng gian sang min tn s, bng cch

ny s gim c d tha d liu trong pixel min tn s cao.

II.2.3.D tha tm sinh l.

Bng trc quan ta thy, s thu nhn cng sng ca mt ngi thay ich gii hn trong mt phm vi nht nh. Hin tng ny xut pht t s tht

l mt khng p ng vi cng nhy ca cc thng tin nh thy. Thng tin

n gin c tm quan trng t hn thng tin khc trong vng nhn thy. Thng

tin ny c gi l d tha tm sinh l nhn. N c th c loi b m

khng nh hng ng k n cht lng nh.

Khc vi d tha m v d tha trong pixel, d tha tm sinh l

c lin quan n thng tin nh hng, n quan trng ti vic lng t ho.iu c ngha l nh x mt khong cch rng cc gi tr u vo ln mt s


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hu hn cc gi tr u ra. l ton t khng o ngc (mt thng tin) cho

kt qu nn s liu c tn hao.

III.Cc phng php nn.

Cc h thng nn l s phi hp ca rt nhiu cc k thut x l nhmgim tc bit ca tn hiu s m vn m bo cht lng nh. C 2 k thut

nn l nn khng tn hao v nn c tn hao.

III.1.Nn khng tn hao.

Nn khng tn hao l qu trnh nn khng mt thng tin, cho php phc

hi li ng tn hiu ban u sau khi gii nn. y l qu trnh m ho c tnh

thun nghch. H s nn ph thuc vo chi tit nh c nn. H s nn ca

phng php ny lun nh hn 2:1Cc k thut nn khng mt thng tin bao gm:

III.1.1.M ha vi di t m thay i (VLC)

Phng php ny cn c gi l m ho Huffman v m ho entropy,

da trn tnh thng k catn hiu, kh nng xut hin ca cc gi tr bin

trng hp trong mt bc nh v thit lp mt t m ngn cho cc t m c tn

sut hin cao nht, v t m di cho cc gi tr cn li. Khi thc hin gii nn,

cc thit lp m trng hp s c s dng to li gi tr tn hiu ban u. Mho v gii m Huffman c th c thc hin mt cch d dng bng cch s

dng cc bng tm kim.

III.1.2.M ho vi di ca t m ng (RLC)

Phng php ny da trn s lp li ca cng mt gi tr mu to ra

cc m c bit biu din s bt u v kt thc ca gi tr c lp li. Ch c

cc mu c gi tr khc 0 mi c m ho. S mu c gi tr bng 0 s c

truyn i dc theo cng dng qut. Cui cng, cc chui 0 c to ra bng qutrnh gii tng quan nh phng php DCT hay DPCM.

III.1.3.S dng khong xo dng v mnh.

Vng thng tin xo c loi b khi dng tn hiu truyn i vng

thng tin tch cc ca nh. Theo phng php ny, thng tin xo dng v mnh

s khng c ghi gi v truyn i. Chng c thay bng cc d liu ng b

ngn hn tu theo ng dng.

II.1.4.Bin i cosin ri rc (DCT)


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Qu trnh DCT thun nghch c coi l khng mt thng tin nu di

t m h s 13 hoc 14 bng tn i vi dng Video s s dng 8 bitbiu din

mu. Nu di t m h s nh hn h s ca php bin i DCT nh hn th

qu trnh ny tr nn c mt thng tin.Trong k thut truyn hnh, nn c hiu qu ngi ta thng kt hp

hai phng php nn khng tn hao vi nn c tn hao.

III.2.Nn c tn hao.

Nn c tn hao chp nhn mt mt mt t thng tin gia tng hiu qu

nn, n rt thch hp vi ngun thng tin l hnh nh v m thanh. N cho t l

nn nh cao c th truyn dn, pht sng. ng thi cng cho mt t l nn

thch hp cho x l v lu tr nh trong studio.Nn tn hao thc hin theo 3 bc:

-Bc 1: bin i tn hiu t min thi gian sang min tn s bng cch s

dng cc thut ton bin i cosin ri rc DCT. Bc ny thc hin vic gim

d tha ca pixel trong nh, tuy nhin qu trnh ny khng gy tn hao tn

hiu.

-Bc 2: Thc hin lng t ho cc h s DCT, lm trn cc s liu.

Vic mt mt s hiu xy ra giai on lm trn ny.-Bc 3: Nn s liu bin i v lm trn bng cch m ho entropy,

y dng cc m khng tn hao nh m Huffman, RLC...

IV.Cc loi m dng trong nn.

I.1.M RLC (Runlength Coding).

RLC l loi m xy dng trn c s s dng s ln lp li ca cc im

nh. RLC tch chui cc gi tr ging nhau v biu din nh l mt tng . Khi

dn gi tr ny to li s ln biu din tng. K thut ny ch c th p dng chocc chui symbol tuyn tnh. Do , khi p dng cho s hiu nh hai chiu, hnh

nh c tch thnh mt chui cc dng qut.

Mt loi ci bin ca m RLC l m VLC (m c di thay i c),

m ny dng biu din cc gi tr symbol cng nh di chy. Cch thc

hin l tnh phn b xc sut ca cc di chy v cc gi tr symbol.

RLC khng thch hp trc tip cho cc nh t nhin c tng nh lin tc

c m ho bng mt qu trnh s ho tuyn tnh, v RLC phi nn tp s liuc bc chy tng i ln ging nhau, trong khi nh c tng lin tc c tn


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s khng gian thp, vic thay i cc vng rng c lm y bng cc gi tr

pixel ging nhau l tng i nh. RLC t n s khng cho cc mc nn c

ngha.

RLC c dng kt hp vi cc k thut khc nh cc chun nn JPEG,MPEG s cho kt qu nn tt. RLC c dng cho cc h s lng t ho tt

hn l dng trc tip cho s liu nh.

II.2.M Shannon-Fano.

M ny da trn xc sut xut hin ca nhng symbol ngun bn trong

mt lng thng t in. Phng php ny s dng nhng t m c di thay i

m ho cc symbol ngun thng qua xc sut ca chng. Vi nhng symbol

c xc sut xut hin cng ln th t m dng m ho chng cng ngn. II.3.M Huffman.

M Huffman thuc loi m entropy hoc m thng k. T tng chnh l

dng m VLC, s dng t bit m ho cc gi tr hay xy ra, v nhiu bit

m ho cc gi tr xy ra. T tc bits gim ng k.

Ni chung, m Huffman s t hiu sut thp nu xc sut xut hin cc

phn t gn nhau. Ngc li hiu sut cao nu xc sut xut hin cch xa nhau.

Trn thc t m Huffman c pht trin trn mt tp nh, nhng sau c p dng trn cc nh khc, mi nh c phn b xc sut symbol ring ca

n. Do , m Huffman khng cn ti u cho nh c bit no.

III.4.M d on (DPCM)

Phng php m d on hay cn gi l iu xung m vi sai (DPCM).

Phng php ny khng m ho bin thng tin mi mu, m ch m ho

cc thng tin c bin chnh lch gia mu cho v mu d bo. Do phn b

chnh lch v bin im nh c th hnh chung xung quanh im 0. Nu da trn cc c trng thng k nh, th s khc nhau ny l khng

ln lm, do vy m ho n ch cn gim mt s t bit l .

Phng php m ho d on cn s dng c im ca mt ngi v cho

php dng c trng phi tuyn v lng t ho.

Sau y l s m ho v gii m DPCM.


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S b m ho:

S b gii m

Nhm trnh cc li c th xut hin trong khi truyn, mt mu y

c gi i theo chu k nht nh, cho php cp nht c cc gi tr chnh xc.

M ho DPCM cng s dng thm cc k thut d on v lng t ho thch

nghi hon thinthm k thut nn ny.II.4.1.DPCM trong mnh (intraframe DPCM).

Tn hiu d bo c to t cc mu nm trong cng mt mnh, cc mu

c bin i nm trn cng mt dng qut (m gia cc pixel) v cng vi cc

dng qut ln cn (m gia cc dng).

Vic to tn hiu d bo ca DPCM trong mnh da trn mi lin kt gia

cc im nh. Cc cng trnh nghin cu cho thy h s lin kt gim nhanh

khi i xa im nh quan st theo chiu ngang (cc pixel trn cng mt dng) vtheo chiu ng (cc pixel ca dng k nhau). Do khi to tn hiu d bo cn

ch n cc mu tn hiu nm k nhau.

Tn hiu d bo c th l hm tuyn tnh hoc c th l hm phi tuyn ca

cc mu ln cn. Vic to tn hiu d bo phi tuyn i hi thit k mch phc

tp hn, do trong thc t khng dng cho tn hiu Video.

III.4.2. DPCM gia cc mnh.

Tn hiu d bo c to trn c s cc mu nm cc mnh k nhautrc . Phng php DPCM gia cc mnh tn dng quan h cht ch gia

Blng t

M hoentropy

Knh

Bd on

Tn hiuVideo s ee

+ -

P V +

-

Gii mentropy

u ragii m

Bd on

Knh tnhiu

eq+

-

P

+ V


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cc mnh k nhau ca nh v c trng ca mt (nhy mo vi nh tnh hn nh

ng). Nghin cu thng k v cu trc nh cho thy rng s khc nhau gia

cc nh k nhau l khng ln lm. to li mt nh trung thc ch cn truyn

cc im nh lm bin i v khi phc li (ti phi thu) tt c cc im nh cnli t mnh trc hoc nh trc trong b nh.

Phng php DPCM gia cc mnh thun li cho truyn hnh thoi. V tn

hiu truyn hnh thoi c c im l nh chuyn ng t, tc chuyn ng

chm, do vng nh rng, t hay i gia ccmnh k nhau.

Phng php c bn ca DPCM gia cc mnh l m ho trc tip vi sai

gia cc mnh k nhau.

c trng lng t l phi tuyn nn s chnh lch nh gia cc mc sc khi phc vi chnh xc cao hn so vi mc chnh lch ln. iu

khng nh vic khi phc mt cch chnh xc cc vng nh rng, chuyn ng

chm (m trn tt c cc mo rt d nhn bit).

Ngc li, ti vng nh c nhiu chuyn ng, khi khi phc li s km

chnh xc hn v do c im ca mt ngi, phn gii ca nh c th thp

hn.

Cc loi DPCM thng dng nht:III.4.2.1.Intraframe DPCM.

y l phng php ly mu t phn nh chuyn ng, ch truyn cc

phn nh chuyn ng v khi phc cc im nh cn li bng tr trung bnh cc

nh c truyn.

III4.2.2.Phng php lm y c chn lc (Selective replenishment)

Phng php an cho cc mnh cn gi l phng php lm y c

chn khi nh truyn t chuyn ng v tc thp. Phng php ny da trnnguyn tc ch truyn phn im nh c chn mi mnh v nhn nh c

chiu sng lu.

III.4.2.3 Phng php chia thnh nhng phn nh chuyn ng v tnh.

Phng php ny l s kt hp ca phng php lm y c chn lc v

phng php ly mu tng phn trong nh chuyn ng cng vi DPCM gia

mnh vi d bo 1 phn t. Mi nh c chia thnh phn t chuyn ng v

phn t tnh, i vi phn nh tnh dng phng php lm y c chn lc; ivi cc phn t nh ng th s dng phng php ly mu tng phn.


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III.5.M chuyn v (Transform Coding).

i vi vic m ho ring r tng im mt s khng t c hiu qu

bi vkhng tn dng ht c mi quan h gi cc khi im trong nh s.

Phng php m chuyn v l mt cch c hiu qu trong vic m ho khiim thng qua bin i tuyn tnh cc im ny thnh cc h s chuyn v v

m ho cc h s chuyn v .

Trong h thng PCM (tuyn tnh) v DPCM ngi ta cn c vo bin

tn hiu thc hin, cn trong m chuyn v th da vo vic chuyn i tin

hiu t min thi gian sang min tn s. y khng dng php bin i

Fourier v n khng ph hp, m ngi ta dng php bin i tuyn tnh.

Phng php gm cc bc nh sau:Chia nh thnh cc khi tiu nh vi kch thc nxn.

Bin i tuyn tnh cc phn t (mu) ca tn hiu mi tiu nh.

M ho cc phn t ca tn hiu.

H thng m ho v gii m m chuyn v:

Trong m chuyn v, bc u tin l phi chn mt chuyn i thch hp

gim s i lp gia cc im trong khi mt cch hiu qu. Mc d bn thn

qu trnh chuyn i ny khng lm gim tc bit i cht no nhng n c

hiu qu trong vic gim d liu trong qu trnh lng t ho. Hay ni cch

khac, qu trnh chuyn i thnh m chuyn v khng h xy ra nn d liu m

ch l bc chun b cho vic gim d liu trong qu trnh lng t ho. Sau khi

chn c bin i trc giao th bcquan trng nht l xc inh bit ch nh v

To nhnxn nh

Bin ithun

Lngt ho

Symbolencoder

nh

nn

nh nn

NxN

Symboldecoder

Bin ingc

nh nn To hp

nxn nh

nh gii nn

a)M ho

b)Gii m


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thit k h thng lng t ha cho cc h s ny ton b qu trnh nn d

liu c thc hin ti y. Bc cui cng l s dng m entropy gim tc

d liu.

Mc ch ca vic chia nh thnh cc nh con nxn l tch lin kt ccpixel ca tng nh con hoc gi cng nhiu thng tin v mt s t cc h s bin

i.

Sau trng thi lng t ho tch c chn lc hoc lng t ho th hn

cc h s mang tin t nht, Cc h s ny t nh hng ti cht lng nh con

c khi phc. Qu trnh m ho xc nh cc h s lng t ho c thc

hin bng m VLC.

Chn chuyn v (bin i): cc h thng m chuyn v (bin i) trn cs bin i Karhuman Loeve (KLT), DFT, DTC, Walsh hadamard (WHT).

Vic chn mt chuyn v c th cho 1 ng dng nht nh ph thuc vo mc sai

s khi phc ca n v c th tnh bng my tnh. C th nn trong khi lng t

ho cc h s bin i (nhng ngoi thi gian chuyn v).

Hu ht cc h thng m chuyn v trn thc t uda trn c s DCT,

n cho php phi hp tt gia kh nng gi thng tin v phc tp tnh ton.

Mt c tnh quan trng ca DCT so vi cc bin i sin khc l: nunh bin i DFT s dn n lin kt khng lin tc trong ni dung cn chuyn

v cc thnh phn tn s cao v nu cc h s ny c trung chuyn, lng t

ho th s gy ra hin tng Gibbs v gy ra sai s. Cn DCT khng to ra s ri

rc trong lin kt, do n hn ch c sai s do Gibbs gy ra.

Phng php ny tp trung vo mt s cc h s chuyn v m khng phi

l cc im nh ca nh gc v lng thng tin ch trong mt s t cc h s

chuyn v. Nh vy s bit dng cho qu trnh chuyn v s t i. Mt l do na l do h thng th gic ca con ngi khng nhn bit c

hon ton cc chi tit ca nh khi cc chi tit bin i nhanh so vi cc chi

tit bin i chm, do vy, m ho cc h s chuyn v tn s cao ta ch cn

mt s it bit m cht lng hnh nh vn cao.

Trong h chuyn v hai chiu cho cc nh s, da trn c s tng quan

gia cc mu trn dng hnh. Trc tin mt khi c chia thnh nhiu khi

hnh ch nht vi kch thc pxq. Sau cc khi ny c chuyn v v cc hs thu c sau qu trnh chuyn v c lng t ho v m ho. Phng php


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chuyn v xut hin mo do vic chia nh thnh cc khi nh con, kt qu gy

nhe v phn x cc khi nh con.

V.Nn trong nh.

1.Nguyn l nn trong nh.S nguyn l qu trnh nn trong nh.

S nn trong nhNn trong nh l loi nn nhm lm gim bt thng tin d tha trong min

khng gian. Nn trong nh s dng c hai qu trnh nn c tn hao v nn

khng tn hao gim bt d liu trong mt nh. Qu trnh ny khng s dng

ca cc nh trc v sau nh ang xt.

Thut ng nh y cn c hiumt cch chnh xc, bi v trong k

thut nn nh cho php s dng hoc mnh (fied) hoc nh (frame) nh mt nh

gc. Nu k thut nn dng mnh th nn trong nh s to ra hai nh trong minh.

2.Tin x l.

Trc khi thc hin bin i DCT, c nh c chia thnh cc khi ln

ring bit khng chng nhau (khi MB). Mi MB bao gm 4 block cc mu tn

hiu chi (Y) v 2:4 hoc 8 block cc mu tn hiu s mu (CR, CB). S cc

block ph thuc vo cc loi chun Video.

Tinx l

DCTthun

Lngt ho

M hoentropy

Mchtrn

Khuychi m

Tn hiunh nn

Ngunnh

Bng lng t

iu khin tc

0

2

1

34 5

0

2

1

3

4

6

5

7

0

2

1

3

4

6

8

10

5

7

9

11

a)4:2:0 (4:1:1) b.4:2:2 c)4:4:4


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a)Cu trc cc block tn hiu chi (qut lin tc)

b)Cu trc cc block tn hiu chi (qut xen k)

Tt c cc block c cng kch thc v mi block l mt ma trn im nh

kch thc 8 x 8 c ly t mt nh mn hnh theo chiu t tri sang phi, t

trn xung di.

Cu trc ca MB cng ph thuc vo loi qut nh. Nu qut lin tc thcc block bao gm cc mu t cc dng lin tc (lc ny qut theo nh frame).

Ngc li trong trng hp qut xen k, trong mt block ch gm cc mu ca

mt na nh (nn theo mnh- field. Vic chia thnh nh con ch thc s c

nghia cho bc chuyn v tip theo.

3.Bin i cosin ri rc (DCT).

Cng on u tin ca hu ht cc qu trnh nn l xc nh cc thng s

tin d tha trong min khng gian ca mt mnh hocmt nh ca tn hiu video. Nn khng gian c thc hin bi php bin i DCT (Discrete Cosine

Tranform). DCT bin i d liu di dng bin thnh d liu di dng tn

s. Mc ch ca qu trnh bin i l tch lin kt ca tng nh con, hoc gi

cng nhiu nng lng ca nh con vo mt phn nh cc h s hm truyn.

Vic m ho v truyn ch thc hin i vi cc h s nng lng ny, v c th

cho kt qu tt khi to li tn hiu Video c cht lng cao.

DCT tr thnh tiu chun quc t cho cc h thng m chuyn v bin c c tnh gi nng lng tt, cho kt qu l s thc v c thut ton nhanh

thc hin chng.

Cc php tnh DCT c thc hin trong phm vi cc khi 8 x8 mu tn

hiu chi Y v cc khi tng ng ca tn hiu mu. Vic chia hnh nh thnh

cc block c thc hin khi tin x l.

Hiu qu ca vic chia ny rt d thy v l mt trong cc hng quan

trng ca m ho c cht lng cao. Nu tnh ton DCT trn ton b frame thta xem ton b frame c d tha l nh nhau. Nh c tnh thay i ca cc


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nh khc nhau v cc phn khc nhau ca cng mt nh, ta c th ci thin mt

cch ng k vic m ho nu bit tn dng. V vy, s dng cc c tnh thay

i ca cc phn t d tha trn c s m rng khng gian ca nh, DCT c

tnh trn cc MB v mi MB c x l ring bit. hiu bit ng dng ca DCT trong nn nh, ta tm hiu thut ton bin

i cosin ri rc. DCT l ton t dng tnh cc thnh phn tn s ca mt tn

hiu c ly mu ti mt tc ly mu.

4.Lng t ho.

Bc tip theo ca qu trnh nn l lng t ho cc h s F(u,v) sao cho

gim c s lng bit cn thit. V cc h s c gi tr khc nhau, cho nn cc

h s ring c th tnh bng cc phng php khc nhau.

Cc h s ng vi tn s thp c gi tr ln hn, v nh vy n cha phn

nng lng chnh ca tn hiu, do phi lng t ho vi chnh xc cao.

Ring h s mt chiu i hi chnh xc co nht, v n biu hin chi trung

bnh ca tng khi phn t nh.

Bt k mt s sai st no trong qu trnh lng t ho, h s mt chiuu c kh nng nhn bit d dng, v n lm thay i chi trung bnh ca

khi. Ngc li, vi cc h s tng ng vi tn s cao v c cc gi tr nh, th

biu hin bng tp cc gi tr nh hn hn cc gi tr cho php. Qu trnh ny c

th thc hin bng cch cho nhng khong cch gia cc bc lng t ho thay

i theo h s.

Lng t ho c thc hin bng vic chia cc h s F(u,v) cho cc h

s v tr tng ng trong bng lng t Q(u,v) biu din s ln nh hncc gi tr cho php ca h s DCT. Cc h s c tn s thp c chia cho cc

...

.... .

Bnglng t

8x8

Lng tho theo

trng s

F0,0/Q0,0

Fq(u,v)

Q(u,v)

F0,0F0,

F1,0F1,

....

....

....

F(u,v)


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gi tr nh, cc h s ng vi tn s cao c chia cho cc gi tr ln hn. Sau

cc h s c lm trn (b i cc phn thp phn).

v,uQ

2/)v,u(Qv)F(u,nhtgnnnguytrgi

)v,u(Q

)v,u(F)v,u(Fq

Cc gi tr F(u,v) s c m ho trong cc cng on tip theo.

Cn phi xc nh l trong qu trnh lng t c trng s c xy ra mt

thng tin, gy tn hao. y l bc gy tn hao duy nht trong k thut nn.

Mc tn hao ph thuc vo gi tr cc h s trn bng lng t. Nhiu lng

t i vi tn hiu mu kh nhn thy hn i vi tn hiu chi, cho nn c th

thc hin lng t ha th tin hiu mu.

Nh vy, khi lng DCT ng vai tr quan trng trong qu trnh lngt ho khi thit k h thng nn Video v n nh hng trc tip n vic khi

phc cht lng hnh nh tt hay xu. Vic dng cc m bin i sao cho thu

c kt qu tt tu thuc vo thnh phn tn s bin i chi tit nh sng tng

vng trong min khng gian nh. Cc nh cng chi tit th h s thnh phn tn

s cao cng ln, nu h s trong cc bng lng t tng ng qu thp c th

dn n trn b m. Mt thng tin hi tip v b m cho php iu khin mt

h s cn bng, qua iu khin ti u trng thi ca b m. H s cn bngny ch a vo cc thnh phn AC m thi. H s cn bng l mt hng s

trong mt cu trc khi c bn cha ng hai khi mu v mt s thay i v

khi chi ph thuc vo cu trc nh ban u.

5.M ho entropy.

Cc gi tr lng t ho c th ch biu din nh cc t m c di c

nh hay ng u, tc l cc gi tr lng t ho biu din cng mt s bit. Tuy

nhin hiu qu m ho khng cao. ci thin ngi ta dng m ho entropy.M ho entropy dng c tnh thng k ca tn hiu c m ho.

Khi dng m ho entropy c hai vn c t ra:

-Th nht : m ho entropy lm tng phc tp v yu cu b nh ln

hn so vi m di c nh.

-Th hai: m ho entropy gn lin vi tnh khng n nh ca tn hiu

Video s lm tc bit thay i theo thi gian. Do cn c mt c cu iu

khin b m khi m ho ngun tc bit bin i c ghp vi knh c tc khng i.


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VI.Nn lin nh.

1.M hnh.

Mt tnh cht ca tn hiu Video l cha thng tin d tha trong min

thi gian. C ngha l, trong mt chui nh lin tc, lng thng tin cha trongmi nh s thay i rt t t nh ny sang nh khc. Tnh ton s dch chuyn v

tr ca ni dung hnh nh l mt phn rt quan trng trong k thut nnlin nh.

M hnh nn lin nh nh sau:

Mt chui Video l mt chui nh tnh c hin ra vi tc nhanh scho cm gic chuyn ng lin tc. Mc d cc frame c s khc nhau, cn thit

phi c mt tc frame cao t c cm gic chuyn ng thc s. T

to ra nhiu d tha tm thi gia cc frame k nhau, s b chuyn ng

chnh l loi b phn d tha ny. Sau qu trnh b chuyn ng, tng hiu

qu nn cn s dng k thut nn trong nh x l d tha trong khng

gian trong phn tha ca b chuyn ng.

2 Xp x v b chuyn ng.Nhiu thay i v cng sng t frame ny n frame k tip l do

chuyn ng ca i tng. Trong m b chuyn ng, frame hin ti c d

bo t frame trc bng cch xp x chuyn ng gia hai frame v b

chuyn ng . S khc nhau gia frame hin ti v frame trc gi l phn

d tha ca b chuyn ng v phn d tha ny s c m ho.

i vi mt chui Video bnh thng , c trng v nng lng ca phn

d tha thp hn rt nhiu so vi nng lng trong tn hiu Video gc do loib thnh phn d tha tm thi. M ho phn d tha thay v m ho Video gip

m bo thng tin d tha tm thi khng b m ho lp li.

Nh vy, vic xc nh phn nh ng l xp x chuyn ng. Qu trnh

khi phc mt nh bng cch dng cc phn t nh trc cng vi thng tin v

chuyn ng chnh l b chuyn ng. S nh gi chuyn ng c th l ton

frame. thc hin ti u, chia mi frame thnh cc khi (8x8 pixel) sau mi

nh gi chuyn ng cho tng khi. Vic nh gi ton b chuyn ng frame s gy ra hai vn sau:

B chuynng

Nn trongnh

nh nnNgun nh


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-Thi gian dng theo di mt vng nh rng ca nh cho vector chuyn

ng c tnh ton.

-Cc khi chuyn ng dn n vic xc nh ci g t vo khng

gian trng (do khi chuyn ng). S dng xp x v b chuyn ng l thay th hai yu cu trong h

thng m/ gii m. u tin b gii m phi lu nh trc trong khi to nh tip

theo. Sau , b m ha phi to li mi nh sau khi m ho n d bo cho

b gii m to nh nh th no. iu ny cn thit v b gii m khng c bt k

mt nh gc no c th to li khi b chuyn ng.

Mt phng php d on tm ra cc chi tit nh thay th gia hai

khung hnh lin nhau v to ra mt vector chuyn ng ch v tr ca chi titnh ny trong khung hnh tng ng. Theo , vector chuyn ng s phi hp

vi tt c cc khi c nn trong khung hnh trc m khi cc im nh

ny c lp li ti mt v tr mi.

Vector chuyn ng gia 2 khung hnh lin nhau

C nhiu phng php khc nhau xc nh vector thay th, mt trongs cc phng php ny c tn gi l phi hp cc khi (block matching). Khi

s liu im nh c la chn , gi l khi tham chiu, trong khung hnh hin

ti chuyn ng trong khung hnh trc . Cc gi tr DCT tham chiu c so

snh vi cc gi tr khi 8x8 im trong vng tm kim tm ra khi thch hp

nht, tng ng khi s sai khc l nh nht. Khi vector xc nh khong

cch gia v tr tham chiu v v tr phi hp tm thy c to ra. Thng tin

s liu v vector chuyn ng c truyn ti cng vi khi h s DCT sai lch.

V tr khi trc

Vng tmkim

V tr khihi n t i

Vectorchuyn

V tr khi 8x8 pixel


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gim qu trnh tnh ton vector chuyn ng, phn gii ca nh c

th c gim xung theo c 2 chiu ngang v chiu ng. Bng cch ny c

th gim s bit cn x l v d on cc phn chnh c thay th trong khung

hnh. V d mt nh vi phn gii y l 720 x 48

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