Tim Hieu Ve IPTV

8/4/2019 Tim Hieu Ve IPTV

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Khoa in T Vin Thng n chuyn ngnh 2 Tm hiu v IPTV

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1.1. M u :Cc ci tin vut bc trong truy cp mng Internet bng rng v cc k

thut m ha video linh hot nh m ha video ci tin H.264/MPEG-4 dn nvic pht trin ca IPTV. Trong qu kh, IPTV l iu khng tng vi kt ni

quay s Dialup chm chp. Nh vo s m bo v k thut do tin b ca cngngh v vic cnh tranh khc lit trn th trng hin nay thc y nhiu cngty vin thng chuyn t vic cung cp cc dch v d liu n gin sang cung cpcc dch v Triple Play (gm d liu, thoi, v video) hoc thm ch l QuadruplePlay (gm cc dch v Triple Play cng vi dch v di ng) trn c s h tngca h nhm m bo kh nng tn ti v tm kim ngun li nhun mi. T ,IPTV c cc cng ty vin thng xem nh dch v s mang li cc gi tr micho khch hng hin nay ca h v l c s cho php h pht trin cc dch vkinh doanh mi trong tng lai.

IPTV c 2 c im c bn l: da trn nn cng ngh IP v phc v theonhu cu. Tnh tng tc l u im ca IPTV so vi h thng truyn hnh cpCATV hin nay, v truyn hnh CATV tng t cng nh CATV s u theo

phng thc phn chia tn s, nh trc thi gian v qung b n hng (truynt mt trung tm n cc my tivi thu bao). Mng CATV hin nay ch yu dngcp ng trc hoc lai ghp cp ng trc vi cp quang (HFC) u phi chimdng ti nguyn bng tn rt rng. Hn na k thut ghp ni modem cp hinnay u sn sinh ra tp m. So vi mng truyn hnh s DTV th IPTV c nhiu

i mi v dng tn hiu cng nh phng thc truyn b ni dung. Trong khitruyn hnh s thng qua cc menu nh trc (thm ch nh trc hngtun, hoc hng thng) cc user la chn, th IPTV c th cao cht lng

phc v c tnh tng tc v tnh tc thi. Ngi s dng (user hoc viewer) cth t do la chn chng trnh TV ca mng IP bng rng. Vi ngha ng ca

phng tin truyn thng (media) gia server v user.

So vi VOD (video theo yu cu) IPTV c u th l:+ S dng d dng, hin th trn tivi hiu qu cao hn mn my vi tnh,

thao tc trn hp ghp ni v bn phm n gin, thc hin chuyn i nhanhlung cao tc/chng trnh.

+ D qun l, d khng ch, s dng hp kt ni lm u cui nh cung cpdch v tin hnh nh ch i vi hp kt ni khng cn n nghip v an tonv kim tra cht lng. y cng l c s k thut d thu ph.

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IPTV c th thc hin cc dch v multimedia. Cn c vo s la chn cangi dng, IPTV cung cp rt nhiu loi hnh dch v. S dng hp kt ni vitivi, ch nhn ngi trc my n phm iu khin c th xem cc tit mc videoang hot ng, thc hin m thoi IP c hnh, nghe m nhc, tra tm tin tc du

lch trn mng, gi v nhn e-mail, thc hin mua sm gia nh, giao dch triphiu... Nh IPTV cht lng sinh hot gia nh c ci thin rt nhiu.

Mt l do na cng khin IPTV tr nn mnh hn so vi kiu TV truynthng chnh l vic ngy cng c nhiu ngi s dng cc h thng my tnh giitr HTPC (Home Theater PC) thay cho dn gia dng v ti-vi. Chnh v vy m ktni Internet lun c m bo to iu kin thun li cho IPTV xm nhp thtrng. Vi hn 100 triu khch hng trn ton th gii, IPTV ang dn chimlnh th trng. Trong iu kin , th trng Vit Nam, IPTV mi bt u

manh nha xut hin vi tin phong l cng ty truyn thng FPT. Mc d nh cungcp dch v Internet ny mi ch ang xc tin th nghim cho 500 khch hngnhng trn thc t, nhng yu cu ti thiu IPTV c th s dng trn tru lr.

Vn cn nhc n u tin chnh l bng thng rng v pht i nhngni dung cht lng cao, tc ADSL nh hin nay khng th p ng. Ngayc khi FPT h tr kt ni 8Mbps cho nhng khch hng th nghim, vic xemcng lc ni dung ca IPTV trn hai my tnh trong mt gia nh l iu vn cha

th lm c mt cch hon ho. Trong tnh hnh nh vy, ADSL 2+ ang l mttrong nhng gii php c cho l hu hiu v hiu qu nht gii quyt vn ny. ADSL 2+ c kh nng m rng kt ni ca ADSL truyn thng. Bng thngd liu c th ln ti 12Mbps ti xung v 3.5Mbps ti ln tnh t trm tn hiuDSLAM ti my tnh ca ngi dng cui.

1.2. Gii thiu s lc IPTV:IPTV l h thng phn phi cc dch v truyn hnh s dng giao thc IP

trn c s h tng mng. Dch v thng bao gm truyn trc tip (ging pht

qung b) v truyn theo yu cu (VoD Video on Demand). Ni rng hn IPTVl dch v gi tr gia tng s dng mng bng rng IP phc v cho nhiu ngidng (user). s dng c IPTV khch hng cn c my tnh hoc Set TopBox kt ni vi TV. Ni dung video thng c nn (nh MPEG-2 hoc MPEG-4) sau mi gi trong dng truyn MPEG phn phi trn mng theo giao thc

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IP. Ngi ta dng c ch truyn multicasting cho cc chng trnh truyn trctip, v unicasting cho trng hp truyn theo yu cu.

pha nh qung b, cc chng trnh TV c m ha MPEG-2 v gi

theo cc dng truyn theo chun DVB truyn ti nh cung cp dch v IPTV(bng v tinh hoc cc phng thc truyn dn khc). headend ca nh cungcp dch v IPTV, cc chng trnh TV c thu nhn v c th c m ha litheo yu cu (v d MPEG-2, H.264). Cc nh cung cp c th can thip vo dngtruyn video chn qung co, chn cc ni dung ca h Ti y, nh cungcp cng kim sot v qun l cc vn khc nh: quyn truy cp, ha n, ccyu cu i knh, truyn phim theo yu cu Cc dng truyn sau c nggi theo giao thc IP v truyn trn mng ca nh cung cp.

pha thu, cc gateway ti nh ng vai tr nh l im cui ca mngInternet v l im bt u ca mng trong nh. N thc hin truyn lu lng dliu t Internet n thu bao v ngc li. Ngi s dng dng Set Top Box hocmy tinh kt ni n mng trong nh (hoc trc tip n gateway ti nh) nhnd liu v gii m ni dung.

Nhn chung, ngi s dng yu cu IPTV phi cung cp dch v nh truynhnh truyn thng. H k vng s m TV, tm knh, xem v khng mun ngtruyn b tm dng, gin on. Hn na, h khng mun i vi giy khi chuyn

knh v nht l kh chu khi phi xem cc chng trnh b git, gin on. y lnhng thch thc chnh i vi nhng nh cung cp dch v IPTV.

Trong khi truyn video, thm ch vi mng t yu cu QoS, c th xy rali. Vi truyn unicasting, STB c th yu cu server gi li cc gi d liu mthoc h hng nhng vi truyn multicasting th iu ny l khng th, nn yu cuquan trng l mng phi c thit k m bo cht lng. Do , nh cung cpdch v ch tp trung quan tm n cht lng ca cc dch v phn phi trnmng ca chnh h, khng quan tm n cc dch v khc ca nhng nh cung cp

th 3 khc trn mng Internet. m bo cht lng QoS, nh cung cp s utin lu lng ng truyn cho cc dch v ca h nhm khc phc tr, h hngtn hiu. Nh cung cp s khim sot end-to-end m bo yu cu cht lngQoS.

1.3. Mng tng th IPTV:

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T ngun ni dung ti u cui ngi dng c h chia lm 4 phn: nghipv cung cp v gii thiu cc ni dung, nghip v chuyn ti, nghip v tip niu cui v nghip v qun tr.S khi biu th cc chc nng ca nghip vIPTV nh hnh sau:

Hnh 1.1. Mng tng th IPTVHay khi n gin hn nh hnh sau:

Hnh 1.2. Mng tng th n gin IPTV1.3.1) Mng ni dung:

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Mng ny cung cp v gii thiu ni dung gm x l ni dung truynhnh trc tip/truyn hnh VOD (theo im) v x l, gii thiu cc ng dnggia tng (phc v tin tc, in thoi c hnh, email, nhn tin...). Ngun nidung truyn hnh trc tip/truyn hnh VOD khng qua h thng x l ni

dung c m ha ph hp vi lung media theo yu cu qua mng chuynti a cc lung ny cung cp ti cc ngi dng u cui.

1.3.2) Mng truyn ti (truyn dn):y l mng cp IP. i vi lung media c hnh thc nghip v khng

ging nhau c th dng phng thc chuyn a hng (multicast) cng c thchuyn theo phng thc n knh. Thng thng, truyn hnh qung b BTVtruyn a hng ti user u cui, truyn hnh theo yu cu VOD thng quamng cp phn pht ni dung CDN (Content Distribution Network) ti a

im ngi dng u cui.

Mng truyn dn chia thnh hai thnh phn chnh gm: mng li (corenetwork) v mng truy cp (access network):

+ Mng li: Mng ny lin kt cc mng truy cp m nhim vic ktni n cc thit b pha ngi s dng. Mng ny c th l mng phn phiquc gia Gigabit Ethernet hoc IP/MPLS cng vi cc mng phn phi vngkhc (c h tr chun carrier-Grade Ethernet). Ni dung c qun l thng

tp trung v x l trong mng phn phi quc gia trc khi c phn phin cc mng truy cp. Tuy nhin, vi kh nng la chn cc ni dung khngc qun l t nhng nh cung cp ni dung th 3, ni dung khng cqun l cng c truyn trn mng li n khch hng thng qua Internet.

+ Mng truy cp: C nhiu phng thc khc nhau kt ni thit b cuica khch hng n mng truyn dn thng qua STB. Cc k thut ph binhin nay ch yu l xDSL, mng lai cp quang v cp ng trc (HFC), mngcp quang nh FTTN (Fiber-To-The-Node) nhm m rng nhiu i tng

khch hng (ch khng ch b buc trong kt ni xDSL hay cp). Do bngthng ca nhng mng truy cp u rt gii hn, phc v cho tt c cckhch hng truy cp ng thi cc knh truyn hnh, k thut multicastingc ng dng rng ri cho php phn phi linh hot d liu video trn ITPV.Thay v dng k thut unicasting phi mt nhiu dng ni dung trc tip truyntrn ton mng, k thut multicasting tit kim bng thng v ti thiu vic sao

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chp khng cn thit cc gi d liu. Ch mt dng truyn cho mi d liuvideo duy nht s c chia x bi mt nhm cc khch hng cng yu cu nidung trc tip ging nhau. D liu ch c sao chp cc nhnh thch hp, cth to thnh cc dng truyn con (substream) cung cp cho mt nhm

khch hng hoc ch mt khch hng

1.3.3) Mng u cui (cn gi l mng cp gia nh):Theo cc nh khai thc vin thng, th mng ny l mng tip ni bng

rng xDSL, FTTx+LAN hoc WLAN. Set top box l thit b pha khch hnggiao tip vi thit b cui ca ngi s dng (nh: TV, PC, hoc laptop) vng DSL hoc ng cp. STB thng ci t phn mm client(middleware) nhn d liu hng dn chng trnh, gii m d liu videoMPEG-2, MPEG-4 v hin th trn mn hnh. Ngoi ra, dng trnh duyt Web

cng c th nhn c d liu hng dn chng trnh t server trung tm.STB c th c tch hp vi modem DSL hoc modem cp, hoc thm chtch hp vi switch IEEE 802.11 dng kt ni mng truy cp Internet trongnh. Tm li hp STB thc hin 3 chc nng sau:

+ Ni tip vo mng bng tn rng, thu pht v x l s liu IP v lungvideo.

+ Tin hnh gii m lung video MPEG-2, MPEG-4, WMV, Real...m bo video VOD hin th ln mn hnh ti vi cc s liu...

+ Phi hp vi bn phm m bo cc thao tc trn HTML mng, tin

hnh gi nhn email Hp STB m nhim cc nhim v trn ch yu davo b vi x l.

1.3.4) B qun tr:Bao gm qun l ni dung, qun l cp truyn, tnh cc ph, qun l

cc thu bao, qun l cc hp ghp ni STB. Ta thy trong mng IPTV c 3dng lung tn hiu: lung qung b BTV, lung truyn n a im theo yucu VOD v lung nghip v gi tr gia tng. Ta xt cc phng thc truyntn hiu th tn. C 3 phng thc truyn trc tip hin trng, truyn qung b

c nh thi gian v truyn ti im VOD. Khi truyn hnh trc tip ng thita ly ni dung ny lu vo b nh pht li vo truyn hnh qung b nhthi gian hoc lm ngun cc tit mc cho truyn hnh VOD. i vi tit mcqung b c nh thi IPTV dng phng php truyn pht a im IP c titkim bng tn tc l phng thc multicast. Phng thc ny thc hin "nht

pht, a thu". Dng phng thc ny, mi tit mc mng cp ch pht mt

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lung s liu thi gian thc (real time) khng lin quan ti s ngi xem titmc ny. Phng thc ny c th truyn pht cho hng nghn thu bao.

IPTV cung cp ng thi hnh nh (video) v m thanh (audio) trn mng

cp. m bo cht lng ca 2 loi tn hiu trn IPTV dng phng php ngb A/V thng qua mt server duy nht thu thp cc d liu ti hin trng, vnbn s dng theo khuyn ngh truyn dn thi gian thc RTP. IPTV dng k thutnn th tn c hiu sut cao nn bng tn truyn dn ti 800kbit/s c th tip cnvi bng tn thu DVD nn to iu kin cho cc nh khai thc d dng pht trincc dch v video. Mng chuyn ti CDN gm nhiu server cache phn b ti cckhu vc tp trung thu bao, Khi c yu cu ca thu bao, cache server chuyn lnVOD server trong mng ngun cung cp, tm ni dung ph hp v chuyn ti chothu bao s hot ng ca cc server trong mng chuyn ti da trn k thut cn

bng ph ti ton cc (GSLB). Trong qu trnh truyn a multimedia IPTV cth dng kha mt m m bo an ton ca ni dung truyn dn.

IPTV p dng cc khuyn ngh quc t v tiu chun, nh khuyn ngh vtruyn dn thi gian thc (RTP), khuyn ngh v khng ch thi gian thc(RTCP)...

IPTV cng cng lm vic vi my tnh dng h iu hnh UNIX,VIC/VAT, Apple v Quick Time.

Hin nay cch thc m ha video ca lung ch ca IPTV theo MPEG-2,MPEG-4, H.264/AVC; Real Microsoft UWMV-9. Trong , MPEG-2 v MPEG-4 c pht trin mnh. H.264 l lut m ha th tn ca ITU-T xut thch hpcho cc h thng cng cng. Do H.264 c kh nng thnh cch m ha chnhca IPTV.

Nh nu trn, nghip v IPTV chnh l phc v cho cc h gia nh.Phng thc tip nhp bng rng ti gia nh thng dng phng php truy nhp

ADSL, nhng v IPTV thit lp ti user nghip v multimedia thi gian thc vtng tc nn ADSL khng tha mn cc yu cu ca IPTV. Cp quang truyndn ti tn nh FTTH c cng nhn l phng thc chuyn ti ti u. Cpquang c bng tn rt rng v c kh nng truyn dn hai hng i xng m

bo c yu cu truyn hnh nh ng theo hai hng vi cht lng cao.

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1.4. Phng thc pht truyn tn hiu ca IPTV:Ni mt cch gin n, trong h thng IPTV hnh nh video do cc phn

cng thu thp theo thi gian thc (real time), thng qua phng thc m ha (nhMPEG 2/4...) to thnh cc lung tn hiu s. Sau , thng qua h thng phn

mm, IPTV pht truyn vo mng cp. u cui ca cc user tip nhn, la chn,gii m v khuch i.

Trong h thng IPTV c 2 phng thc truyn a tn hiu c d nhtrc (scheduled programs). l:

- Pht qung b (broadcasting), truyn pht ti mi ni- Pht n a im theo yu cu (on demand).

Nguyn l hot ng ca h thng qung b, cc chng trnh c v hnh sau:

Hnh 1.3. H thng qung b.Trong MBone (mng xng sng ca h thng a im) chnh l ng

trc Internet. Tuy nhin ngi s dng ch theo lnh ca b qun l ni dung(content manager) c gii thiu ni dung chng trnh hu quan. Chngtrnh c th do rt nhiu b IPTV server thu thp c hoc cng do cc serverca mng MBone cung cp.

Hot ng ca h thng VOD IPTV c miu t nh hnh sau:

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Hnh 1.4. H thng VoD ca IPTV

Cc bc thc hin VOD nh sau:1. Mt thu bao c chng nhn nhp mng v chu s qun l ca b

qun l EPG2. Thu bao mun yu cu mt ni dung no . Thu bao gi yu cu n

EGP3. EGP cho bit a ch ca server cn tm4. Thu bao gi yu cu ti server .5. Server da theo yu cu ca thu bao m cung cp ni dung.Trn y ch l mt th d n gin nht. Hin nay cc nhu cu nghip v

ca IPTV rt a dng nn cu trc mng phc tp hn nhiu.

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2.1. Gii thiu v MPEG:MPEG l tn vit tt ca hi phim nh th gii (The Moving Picture

Experts Group) l mt sn phm nhm mang tnh ISO/IEC c pht trin cho ccthit b m thanh v hnh nh bng cch nn d liu chun. Chun MPEG ln u

tin c ra mt vo thng 5 nm 1988 ti Ottawa, Canada. Cho n ngy nay,MPEG pht trin hn 350 thnh vin t cc hi nghi trn tt c cc lnh vccng nghip, cc khu nghin cu, n cc trng i hc. Tiu chun chnh thcca MPEG l ISO/IEC JTC1/SC29 WG11. MPEG c chun ha theo nh dngnn v cc chun ph thuc nh sau:

Hnh 2.1. Phn loi MPEG. MPEG-1: Vit tt ca chun video v audio nn. Sau c s

dng nh l chun dnh cho a VCD (Video CD) v cha c m thanhthng thng lp 3 (MP3) l dng m thanh nn. Mpeg-1 c th nn tnhiu video ti 1.5Mbit/s vi cht lng VHS v m thanh lp th (stereoaudio) vi tc 192 kbit/s. N c dng lu tr video v m thanhtrn CD-ROM.

MPEG-2: Truyn ti, hnh nh v m thanh trn cc knh truynhnh, qung b cht lng cao. c s dg cho cc bui pht hnh ATSC,DVB v ISDB, cc dch v truyn hnh k thut s v tinh, truyn hnh capv dnh cho nh dng a DVD.

MPEG-3: Nguyn bn c thit k dnh cho HDTV, nhng bloi b khi pht hin ra MPEG-2 (vi tn ui) ph hp vi HDTV (ngnhm vi MP3, c nh ngha l MPEG-1 lp 3)

MPEG-4: Pht trin t MPEG1 vi s h tr v m thanh, hnhnh, vt th, v ni dng 3 chiu, m ha bitrate thm v h tr Qun lquyn s ha. C mt vi chc nng mi hn (MPEG-2) vi hiu qu m

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thah v hnh nh chun. (Cha c cc nh dng MPEG-2 video thay th)Mpeg-4 tr thnh mt tiu chun cho nn nh k thut truyn hnh s, ccng dng v ho v Video tng tc hai chiu (Games, VideoConferencing) v cc ng dng Multimedia tng tc hai chiu (World

Wide Web hoc cc ng dng nhm phn pht d liu Video nh truynhnh cp, Internet Video...). Mpeg-4 tr thnh mt tiu chun cng nghtrong qu trnh sn xut, phn phi v truy cp vo cc h thng Video. N gp phn gii quyt vn v dung lng cho cc thit b lu tr, giiquyt vn v bng thng ca ng truyn tn hiu Video hoc kt hpc hai vn trn.

MPEG-7: cha c t thng tin, giao din cho vic tm kim thngtin.

Mpeg-1 v Mpeg-2 c chun ho trong khi Mpeg-4 v Mpeg-7 angc pht trin.

D liu Mpeg bao gm 2 lp:+Lp h thng (System Layer) cha thng tin v thi gian (Timing)

v cc thng tin khc cn thit cho vic tch cc dng d liu Video vAudio ng thi ng b ho Video v Audio trong qu trnh pht(Playback).

+Lp d liu nn (Compression Layer) bao gm cc dng d liuVideo v Audio.

Hnh 2.2. Gii m MPEGH thng gii m s tch cc thng tin v thi gian t d liu h thng

Mpeg v gi n n thit b h thng khc (Vic ng b ho s cn thm nhiuthng tin v thi gian). H thng gii m cng tch cc d liu Video v Audio td liu ban u. sau gi chng n tng b gii m thch hp.

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B gii m Video (Video Decoder) v Audio (Audio Decoder) gii nn ccdng d liu Video v Audio. Hu ht cc s nn Mpeg u dng k thut lymu b xung (Subsampling) v lng t ho (Quantization) trc khi m ho. Lymu b xung nhm mc ch lm gim kch thc khung hnh u vo theo c

theo chiu ngang v chiu dc, nh vy s gim s lng cc im nh trc mho. Trong mt s trng hp ngi ta cn ly mu b xung theo thi gian lmgim s lng cc khung hnh trong cnh trc khi m ho. y c xem nh lmt k thut rt c bn nhm loi b s d tha da vo kh nng lu nh ca mtngi cm th.

Chun Mpeg nh ngha mt kiu phn cp cu trc trong d liu Videonh sau:

-Cnh (Video Sequence): bt u vi mt Sequence Header, bao

gm mt hoc mt nhm khung hnh v kt thc vi m End-of-Sequence.-GOP (Group of Picture): Mt dy lin tip cc nh (Picture) trongcnh.

-Khung hnh (Picture): l thnh phn m ho chnh. Thng thngchng ta c th phn bit s thay i v sng ca nh (Brightness) tthn so vi s thay i v mu (Chrominance). Do trc ht cc s nn Mpeg s tin hnh chia khung hnh thnh cc thnh phn sng Y vthnh phn mu Cb, Cr (mt thnh phn v sng v hai thnh phn v mu). Mt khung hnh s gm c 3 ma trn ng vi cc thnh phn v

sng (Y) v hai thnh phn v mu Cb v Cr.

Hnh 2.3. Video Frame trong MPEG.

Ma trn Y c s hng v ct bng nhau (ma trn vung). Ma trn Cb v Crc s hng v ct bng na ma trn Y. Hnh cho thy quan h v v tr ca Y vcc thnh phn Cb v Cr. Lu rng c 4 gi tr Y li c 2 gi tr kt hp mt caCb v mt ca Cr (V tr ca gi tr Cb v Cr l tng ng).

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-Slide: Dy cc Macroblock, th t ca cc Macroblock bn trongSlide c xc nh t tri qua phi, trn xung di. Slide rt quan trngtrong vic nh li. Nu dng d liu (Bitstream) c cha li, b gii m cth b qua v tip tc Slide k tip. Nhiu Slide trn dng d liu cho

php che du li tt hn v c dng ci thin cht lng hnh nh.

-Macroblock: 16 pixel trong 16 hng ca thnh phn Y tng ngvi 8 pixel trong 8 hng ca thnh phn Cb v Cr. Trong hnh cho thykhng gian ca thnh phn Y v Cb, Cr. Mt Macroblock cha 4 Y Blockvi 1 Cb Block v 1 Cr Block nh trong hnh vi cc con s ng vi th tca Block trong dng d liu.

-Block: l tp hp 8 pixel trong 8 hng cc gi tr ca thnh phn Yhoc Cb, Cr.

Chun MPEG cng nh ngha cu trc phn cp d liu c chp nhn,gii m v xut ra Audio s. D liu Mpeg Audio cng nh Mpeg Video gmhng lot cc gi (Packet). Mi gi cha Audio Packet Header v dy cc AudioFrame.

Mi Audio Packet Header cha cc thng tin sau:+Packet Start Code: xc nh gi l Audio Parket.+Packet Length: xc nh lng thng tin trong Audio Packet.

Hnh 2.4. Audio Packet trong MPEG.Mi Audio Frame cha cc thng tin:

+Audio Frame Header: cha ng b, ID, nn thng tin (Bit rate),thng tin v tn s ly mu.

+Error Checking Code: cha thng tin cho vic kim tra li.+Audio Data: cha thng tin.

+Ancillary Data (d liu ph thuc): cha d liu do ngi dng xcnh.

Nn Mpeg l s kt hp hi ho ca bn k thut c bn: Tin x l(Preprocessing), on trc s chuyn ng ca cc khung hnh (Picture) b m

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ho (Temporal Prediction), b chuyn ng b gii m (Motion Compensation)v m lng t ho (Quatization Coding).

Cc b lc tin x l s lc ra nhng thng tin khng cn thit t tn hiu

Video v nhng thng tin kh m ho nhng khng quan trng cho s cm th camt ngi. K thut on chuyn ng da trn nguyn tc l cc khung hnhtrong mt cnh Video (Video Sequence) dng nh c lin quan mt thit vinhau theo thi gian: Mi khung hnh ti mt thi im nht nh s c nhiu khnng ging vi cc khung hnh ng ngay pha trc v ngay pha sau n. Cc bm ho s tin hnh qut ln lt tng phn nh trong mi khung hnh gi lMacroblock, sau n s pht hin Macroblock no khng thay i t khung hnhny ti khung hnh khc. B m ho s tin on trc s xut hin ca ccMacroblock khi bit v tr v hng chuyn ng ca n. Do ch nhng s thay

i gia cc Macroblock trong khung hnh hin ti v cc Macroblock c tinon mi c truyn ti bn pha thu. Pha bn thu tc b gii m lu tr snnhng thng tin m khng thay i t khung hnh ny ti khung hnh khc trong

b nh m ca n v chng c dng in thm mt cch u n vo cc vtr trng trong nh c khi phc.

Nn tn hiu Video c thc hin nh vic loi b c s d tha v khnggian (Spatial Coding) v thi gian (Temporal Coding). Trong Mpeg, vic loi bd tha v thi gian (nn lin khung hnh) c thc hin trc ht nh s dng

cc tnh cht ging nhau gia cc khung hnh lin tip (Inter-Picture). Chng ta cth s dng tnh cht ny to ra cc khung hnh mi nh vo nhng thng tin tnhng khung hnh gi trc n. Do vy pha b m ho, ch cn gi nhngkhung hnh c thay i so vi nhng khung hnh trc trc, sau dng phng

php nn v khng gian (Spatial Coding) loi b s d tha v khng giantrong chnh khung hnh sai khc ny. Nn v khng gian da trn nguyn tc l

pht hin s ging nhau ca cc im nh (pixel) ln cn nhau (Intra-Picture).

2.2. M ha MPEG-II:MPEG-2 l mt tiu chun m ha nn(thng c gi tt l chun nn)

trong b tiu chun MPEG dng m ha lung d liu hnh c kt hp vi ccthng tin v m thanh. y l mt phng thc m ha d liu c tn hao cho

php lu tr v truyn phim nh trn nn h thng v bng thng hin thi. Mpeg-2 c m rng da trn chun Mpeg h tr vic nn d liu truyn Video

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s cht lng cao. hiu c ti sao nn Video l rt quan trng, ta cn tmhiu bng thng (Bandwidth) cn thit truyn cc khung hnh Video s khngnn.

PAL (Phase Alternate Line) l chun truyn tn hiu TV tun t(Analog) c s dng kh nhiu nc trn th gii. Khung hnh TV dng PALkhng nn i hi bng thng rt ln ti 216 Mbps, ln hn rt nhiu kh nngca truyn sng radio. Mt s nc dng h thng Analog TV l NTSC. H thngny cung cp cc thng tin v mu sc km trung thc hn vi t l truyn cckhung khc nhau. Tn hiu NTSC khng nn i hi dung lng ng truynthp hn khng ng k mc 168 Mbps. TV phn gii cao HDTV (HighDefinition TV) yu cu bng thng ti thiu l 1 Gbps.

Mpeg-2 cung cp cch nn cc tn hiu Video s thnh cc mc c th qunl c. Kh nng nn Video ca Mpeg-2 lit k theo bng sau:

Hnh 2.5. Bng so snh i hi bng thng ca cc chun MPEG.

Do chun Mpeg-2 cung cp kh nng nn rt cao bng cch dng cc thut

ton tiu chun, n tr thnh chun cho TV s vi cc c tnh:+Nn Video tng thch vi Mpeg-1.+Ch Full-screen kt hp vi ci tin cht lng Video (cho TV

v mn hnh PC).+Ci tin m ho Audio (cht lng cao, mono, stereo...).+Truyn phi hp nhiu thnh phn.

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+Cc dch v khc.

Cc h thng s dng Mpeg-2 ang rt pht trin nh: TV s, VoD, DigitalVersatile Disc (DVD)... Thut ton nn Video Mpeg-2 t c kh nng nn cao

nh li dng s d tha in thng tin Video. Mpeg-2 loi b c d tha v khnggian v d tha v thi gian trong cc cnh Video.

D tha thi gian xut hin khi cc khung Video lin tip hin th hnh nhca nhng hnh nh ging nhau. N cha cc hnh nh gn nh khng i hocthay i rt nh gia cc khung hnh lin tip. D tha khng gian xy ra khi mt

phn ca nh c ti to li (vi thay i khng ng k) trong mt khung Video.D liu t cc Macroblock cn c m ho s c a n c b tr

(Subtractor) v b on chuyn ng (Motion Estimator). B on chuyn ng s

so snh cc Macroblock mi c a vo ny vi cc Macroblock c avo trc v c lu li dng tham kho. Kt qu l b on chuyn ngs tm ra cc Macroblock trong khung hnh tham kho gn ging nht viMacroblock mi ny. B on chuyn ng sau s tnh ton Vector chuynng (Motion Vector), Vector ny s c trng cho s dch chuyn theo c haichiu dc v ngang ca Macroblcok mi cn m ho so vi khung hnh thamkho. Lu rng Vector chuyn ng c phn gii bng mt na do thc hinqut xen k.

Hnh 2.6. D on chuyn ng.B on chuyn ng cng ng thi gi cc Macroblock tham kho c

gi l cc Macroblock tin on (Predicted Macroblock) ti b tr tr viMacroblock mi cn m ho. T ta s c cc sai s tin on (ErrorPrediction) hoc tn hiu d, chng s c trng cho s sai khc gia Macroblockcn tin on v Macroblock thc t cn m ho.

Tn hiu d hay sai s tin on ny s c bin i DCT, cc h s nhnc sau bin i DCT s c lng t ho lm gim s lng cc bits cntruyn. Cc h s ny s c a ti b m ho Huffman, ti y s bits ctrng cho cc h s tip tc c lm gim i mt cch ng k. D liu t u ra

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ca m ho Huffman s c kt hp vi Vector chuyn ng v cc thng tinkhc (thng tin v I, P, B-picture) gi ti b gii m.

i vi trng hp P-picture, cc h s DCT cng c a n b gii m

ni b (nm ngay trong b m ho). Tn hiu d hay sai s tin on c bin ingc li dng php bin i IDCT v c cng thm vo khung hnh ngtrc to nn khung hnh tham kho (tin on). V d liu khung hnh trong

b m ho c gii m lun nh vo b gii m ni b ngay chnh bn trong bm ho, do c th thc hin thay i th t cc khung hnh v dng cc phng

php tin on trn.

Qu trnh khi phc li khung hnh ti b gii m l hon ton ngc li.T lung d liu nhn c u vo, Vector chuyn ng c tch ra v a

vo b b chuyn ng (Motion Compensator), cc h s DCT c a vo bbin i ngc IDCT bin tn hiu t min tn s thnh tn hiu min khnggian. i vi P-picture v B-picture, Vector chuyn ng s c kt hp vi ccMacroblock tin on to thnh cc khung hnh tham kho.

Khng cn thit phi lun nn mi khung hnh Video cng mt mc ,mt phn ca Clip c th c d tha khng gian thp (v d cc hnh nh phctp) trong khi cc phn khc ca Clip li c d tha thi gian thp (v d cccnh chuyn ng nhanh). V th d liu Video ng nhin s cc t l nn (Bit

rate) thay i trong khi vic truyn d liu thng yu cu tc c nh. Chakho iu khin tc truyn l trt t d liu nn trong b m(Buffer).Vic nn c th c tin hnh vi vic loi b mt vi thng tin cla chn. nh hng nh nht i vi cht lng ton b khung hnh c th tc bng cch b bt cc thng tin chi tit. iu ny m bo gii hn t l nnd liu trong khi cht lng ca khung hnh suy gim ti thiu.

Hnh 2.7. C ch nn MPEG II.

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Mpeg-2 bao gm c ch nn trong mt phm vi rng. Mt b m ho vic ch nn phi ph hp vi mt hoc on cnh ring bit. Ni chung b m hort phc tp, n phi la chn c c ch nn thch hp nht bi vy tng chtlng khung hnh i vi t l nn d liu truyn. B gii m Mpeg-2 cng c

nhiu kiu, kh nng a dng v cc la chn khi kt ni.

S lng cc Level v Profile c nh ngha cho vic nn Video Mpeg-2.H thng Mpeg-2 c pht trin trn mt tp no cc Level v Profile:

+Profile: cht lng ca Video.+Level: phn gii ca Video.

H thng c bn vi tn MP@ML (Man Profile Man Level) nn d liuVideo t 1-15 Mbps. Cc Level khc nhau nh: High Level, High Level 1440,Low Level v cc Profile nh: Simple, SNR, Spatial, 4:2:2 & High.

Cc b gii m in hnh:+ 720 x 576 x 25 fps (PAL CCIR 601).+ 352 x 576 x 25 fps (PAL Half-D1).+ 720 x 480 x 30 fps (NTSC CCIR 601).+ 352 x 480 x 30 fps (NTSC Half-D1).

Hu ht cc b gii m u h tr Mpeg-1:+ 352 x 288 x 25 fps (PAL SIF).

+ 352 x 240 x 30 fps (NTSC SIF).

Chun Mpeg-2 nh ngha mt s phi hp m ho Audio. Audio s c thc m ho trong cc dng m ho khc nhau cc t l nn khc nhau.Mpeg-2cng cung cp cc h tr cho vic truyn d liu. Mpeg-2 phn bit hai kiu dliu:

+ Service Information: thng tin v Video, Audio v Data truyn biMpeg-2.

+ Private Data: thng tin ngi s dng hoc thit b thu.

2.3. Chun m ha MPEG-IV:MPEG-4 l 1 tiu chun nn m thanh/ hnh nh c thit l p bi t chc

Moving Picture Experts Group gi tt l MPEG. Chun ny gm nhiu tiu chunnh, gi l cc phn ca MPEG-4. Cc phn ng ch l:

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+Part 2: codec cho d liu trc quan(hnh nh, nh ng,) C tnthng gi l MPEG-4 ASP. in hnh nh DivX, Xvid...

+Part 3: codec m thanh AAC. in hnh: Nero AAC, CTAAC(winamp xi ci ny), Real AAC,..

+Part 10: 1 codec hnh khc. Tn thng dng l MPEG-4 AVC. Cit: Nero Avc, MainConcept, Elecard, x264(min ph)+Part 12: container cha hnh/ting, .mp4.

Nu 1 file mpeg-4 chun ISO th container phi l mp4, m thanh l AAC,hnh th mpeg-4 ASP hoc l mpeg-4 AVC. Hin nay, c rt nhiu phn mm cth chuyn i cc nh dng khc sang mp4. Thc s, cc phn mm ny thngxut ra file AVI, video th xi DivX, m thanh th thng l mp3. AVI khng phil container chnh thc ca chun MPEG-4.

AVI :- Khng h tr VFR (variable frame rate, tc hnh thay i),- Khng th cha m thanh AAC( nht nh thi nay).- N cng khng h tr MP3 VBR. C 1 s phn mm hack v c

th nht c: VirtualDubMod, Mencoder. Nhng ng tic l khng cnhiu phn mm nghe nhc h tr dng AVI nh vy.

Vy sao th trng vn dng AVI nhiu? V n thm nin. Mp4 mi ra i

nn cha c ph bin m AVI c pht trin v h tr bi Microsoft.

Cc bn AAC:+Nero AAC: min ph, h tr ABR, VBR,... m thanh 5.1+CT AAC: coding technology AAC, h tr 5.1 nhng khng c VBR.

c dng trong winamp, min ph.+Real AAC: cng min ph h tr VBR v 5.1

Cc bn MPEG4- AVC:

+x264: min ph.+Vorbis: cng min ph.

GUI encode m thanh:+Belight: GUI ca Besweet.+BeHappy: m file bng DirectShow.

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GUI encode video:+Xvid, x264: Megui.

2.4. Tiu chun m ha MPEG-4 part 10 (H.264):H.264 l chun nn m c cng b chnh thc vo nm 2003, hin lchun h tr cng ngh nn tin tin v hiu qu nht hin nay. V n ang dnc a vo thnh chun nn tiu chun ca ngnh cng ngh an ninh gim st

bng hnh nh. H.264 (cn c gi l chun MPEG-4 Part 10/AVC for AdvancedVideo Coding hay MPEG-4 AVC) n k tha nhng u im ni tri ca nhngchun nn trc y. ng thi s dng nhng thut ton nn v phng thctruyn hnh nh mi phc tp, phng php nn v truyn hnh nh m chunH.264 s dng lm gim ng k d liu v bng thng truyn i ca video.

Vi cch nn v truyn thng tin bng chun H.264 lm gim n 50%bng thng v kch thc file d liu lu tr so vi cch nn thng thng hinnay (chun nn thng thng hin nay ang c s dng rng ri l MPEG-4Part 2) v gim ti hn 80% bng thng v kch thc file d liu lu tr so vinn bng chun Motion JPEG. iu cho chng ta thy vi cng mt h thngnu chng ta s dng chun nn mi chng ta c thi gian lu tr gp i v bngthng mng gim i mt na, li ch m chng ta thy ngay l chi ph cho lutr d liu video gim mt na so vi dng h thng c chun nn thng thng.

Ngoi ra vic truyn hnh nh chim bng thng gim mt na, v vy chi phdnh cho thu bng thng mng cng gim ng k. Hoc chng ta c th tngcht lng hnh nh gim st ln gp i nhng vn m bo c bng thng vthi gian lu tr nh trc y. y l mt li th rt ln, bi vi mt h thng anninh ln, gii quyt vn bng thng mng v thi gian lu tr l rt phc tp.Vi chun nn H.264 n gii quyt c rt nhiu nhng kh khn nh vy.

Vi vic gim c bng thng ca chun nn H.264 thc y cho dngcamera nt cao (hay cn gi Camera Megapixel) c c hi pht trin mnh m.

Vi nhng h thng gim st quan trng cn hnh nh r nt th la chn cccamera nt cao v u ghi h tr chun nn H.264 l hon ton hp l.

Chun nn H.264 c k vng l cn gi mi, to thm sinh kh thcy s pht trin ca ngnh an ninh gim st pht trin mnh m hn.

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2.4.1) Gii thiu chung:K t khi mi xut hin vo u nhng nm 90, chun nn video

MPEG-2 hon ton thng lnh th gii truyn thng. Cng trong thp kny, chun nn MPEG-2 c ci tin v nhiu mt. Gi y n c tc

bit thp hn v vic ng dng n c m rng hn nh c cc k thut nhon chuyn ng, tin x l, x l i ngu v phn b tc bit ty theotnh hung thng qua ghp knh thng k.

Tuy nhin, chun nn MPEG-2 cng khng th c pht trin mt

cch v hn nh. Thc t hin nay cho thy chun nn ny t n ht giihn ng dng ca mnh trong lnh vc truyn truyn hnh t sn xut tin kn hu k v lu tr Video s. Bn cnh , nhu cu nn Video li ang ngymt tng cao km theo s pht trin mnh m ca mng IP m tiu biu l

mng Internet. Khi lng ni dung m cc cng ty truyn thng cng nh ccnh cung cp dch v thng tin c th mang li ngy cng ln, ngoi ra h cnc th cung cp nhiu dch v theo yu cu thng qua h thng cp, v tinh vcc h tng vin thng t bit l mng Internet.

Cc tiu chun m ho Video ra i v pht trin vi mc tiu cung cp

cc phng tin cn thit to ra s thng nht gia cc h thng c thitk bi nhng nh sn xut khc nhau i vi mi loi ng dng Video; Nhvy th trng Video c iu kin tng trng mnh. Chnh v l do ny nn

nhng ngi s dng b gii m cn c mt chun nn mi i tip chngng m MPEG-2 b d.

Hip hi vin thng quc t (ITU) v t chc tiu chun quc t/ Uban k thut in t quc t (ISO/IEC) l hai t chc pht trin cc tiu chunm ho Video. Theo ITU-T, cc tiu chun m ho Video c coi l cckhuyn ngh gi tt l chun H.26x (H.261, H.262, H.263 v H.264). Vi tiuchun ISO/IEC, chng c gi l MPEG-x (nh MPEG-1, MPEG-2 vMPEG-4).

Nhng khuyn ngh ca ITU c thit k dnh cho cc ng dng

truyn thng Video thi gian thc nh Video Conferencing hay in thoitruyn hnh. Mt khc, nhng tiu chun MPEG c thit k hng ti mctiu lu tr Video chng hn nh trn a quang DVD, qung b Video s trnmng cp, ng truyn s DSL, truyn hnh v tinh hay nhng ng dng

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truyn dng Video trn mng Internet hoc thng qua mng khng dy(wireless).

Vi i tng truyn dn Video l mng Internet th ng c vin

hng u l chun nn MPEG-4 AVC, cn c gi l H.264, MPEG-4 part10, H.26L hoc JVT.

2.4.2) Tnh k tha ca H.264:Mc tiu chnh ca chun nn H.264 ang pht trin nhm cung cp

Video c cht lng tt hn nhiu so vi nhng chun nn Video trc y.iu ny c th t c nh s k tha cc li im ca cc chun nn Videotrc y. Khng ch th, chun nn H.264 cn k tha phn ln li im cacc tiu chun trc l H.263 v MPEG-4 bao gm 4 c im chnh nh

sau: Phn chia mi hnh nh thnh cc Block (bao gm nhiu im

nh), do vy qu trnh x l tng nh c th c tip cn ti mc Block.

Khai thc trit s d tha v mt khng gian tn ti gia cchnh nh lin tip bi mt vi m ca nhng Block gc thng qua d onv khng gian, php bin i, qu trnh lng t v m ho Entropy (haym c di thay i VLC).

Khai thc s ph thuc tm thi ca cc Block ca cc hnh nh

lin tip bi vy ch cn m ho nhng chi tit thay i gia cc nh lintip. Vic ny c thc hin thng qua d on v b chuyn ng. Vi

bt k Block no cng c th c thc hin t mt hoc vi nh m hotrc hay nh c m ho sau quyt nh vc t chuyn ng,cc vc t ny c s dng trong b m ho v gii m d on ccloi Block.

Khai thc tt c s d tha v khng gian cn li trong nh bngvic m cc block d tha. V d nh s khc bit gia block gc v Blockd on s c m ho thng qua qu trnh bin i, lng t ho v mho Entropy.

2.4.3) C ch nn hnh nh ca H.264:

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+Nn theo min thi gian:Khi b m ho ang hot ng ch "gia khi" (inter), khi ny

s phi qua cng on hiu chnh chuyn ng. Qu trnh ny s pht hinra bt k chuyn ng no din ra gia khi v mt khi tng ng

mt hoc hn mt nh tham chiu c lu tr t trc, sau to ramt khi "chnh lch" hoc "li". Thao tc ny s gim bt d liu trongmi block mt cch hiu qu do ch phi trnh by chuyn ng ca n mthi. Tip n l cng on bin i csin ri rc (DCT) bt u nntheo min khng gian. Khi b m ho hot ng ch "trong khi"(intra), khi ny s b qua cng on hiu chnh chuyn ng v ti thngcng on DCT.

Hnh 2.8.S khi m ho MPEG, ng t nt c trng chophn b sung ca MPEG-4 AVC trong vic nn theo min khng gian.

+Nn theo min khng gian:Cc khi thng c cha cc im nh tng t hoc thm ch ging

ht nhau. Trong nhiu trng hp, cc im nh thng khng thay imy (nu c). Nh vy c ngha l tn s thay i gi tr im nh trongkhi ny l rt thp. Nhng khi nh th c gi l khi c tn s khnggian thp. B lp m li dng c im ny bng cch chuyn i cc gitr im nh ca khi thnh cc thng tin tn s trong cng on bin icosin ri rc.

* Bin i cosin ri rc:Cng on DCT bin i cc gi tr im nh ca khi thnh mt ma

trn gm cc h s tn s ngang, dc t trong khng gian tn s. Khi khiban u c tn s khng gian thp, DCT s tp hp phn ln nng lngtn s vo gc tn s thp ca mng. Nh vy, nhng h s tn s thp gc s c gi tr cao hn.

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Mt s lng ln cc h s khc cn li trn ma trn u l cc h sc tn s cao, nng lng thp v c gi tr thp. H s DC v mt vi h stn s thp s hm cha phn ln thng tin c m t trong khi ban u.iu ny c ngha l b lp m c th loi b phn ln h s tn s cao cn

li m khng lm gim ng k cht lng hnh nh ca khi.B lp m chun b cc h s cho cng on ny bng cch qutcho mng li theo ng zig-zag, bt u t h s DC v qua v tr cacc h s ngang dc tng dn. Do vy n to ra c mt chui h s csp xp theo tn s.

* Lng t ho v m ho entropy:Ti y thao tc nn khng gian mi thc s din ra. Da trn mt

h s t l (c th iu chnh bi b m ho), b lng t ho s cn i tt

c cc gi tr h s. Do phn ln h s i ra t DCT u mang nng lngcao nhng gi tr thp nn b lng t ho s lm trn chng thnh 0. Ktqu l mt chui cc gi tr h s c lng t ho bt u bng mt sgi tr cao u chui, theo sau l mt hng di cc h s c lng tho v 0. B lp m entropy c th theo di s lng cc gi tr 0 lin tiptrong mt chui m khng cn m ho chng, nh vy gim bt c khilng d liu trong mi chui.

2.4.4) Cc u im ni bt ca chun nn H.264:

+ u im ca nn khng gian:Chun nn MPEG-4 AVC c hai ci tin mi trong lnh vc nn

khng gian. Trc ht, b lp m ny c th tin hnh nn khng gian ticc macroblock 16x16 im nh thay v cc block 8x8 nh trc y. iuny gip tng cng ng k kh nng nn khng gian i vi cc hnh nhc cha nhiu khong ln cc im nh ging nhau.

Th hai l thao tc nn c tin hnh trong min khng gian trckhi cng on DCT din ra. Chun nn MPEG-4 AVC so snh macroblock

hin thi vi cc macroblock k bn trong cng mt khung, tnh ton chnh lch, v sau s ch gi on chnh lch ti DCT. Hoc l n cth chia nh macroblock 16x16 im nh thnh cc khi 4x4 nh hn v sosnh tng khi ny vi cc khi k bn trong cng mt macroblock. iuny gip ci thin kh nng nn nh chi tit.

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+ u im ca nn thi gian:im ci tin ln nht MPEG-4 AVC l ch m ho gia.

Nhng phng php tin tin ch ny khin cho nn thi gian t nmt cp cao hn nhiu, cng vi cht lng chuyn ng tt hn so vi

cc chun MPEG trc y.

+ Kch c khi: ch gia khi, MPEG-2 ch h tr cc macroblock 16x16 im

nh, khng phn gii m ho chnh xc cc chuyn ng phc tphoc phi tuyn tnh, v d nh phng to thu nh. Ngc li, MPEG-4 AVCli tng cng hiu chnh chuyn ng bng cch cho php b lp m bini kch c thnh phn chi ca mi macroblock. (B lp m s dng thnh

phn chi nh vy l do mt ngi nhy cm vi chuyn ng chi hn

nhiu so vi chuyn ng mu.) Nh c th thy trong Hnh 2, MPEG-4AVC c th chia thnh phn chi ca tng macroblock thnh 4 c: 16x16,16x8, 8x16 hoc 8x8. Khi s dng khi 8x8, n cn c th chia tip 4 khi8x8 ny thnh 4 c na l 8x8, 8x4, 4x8 hoc 4x4.

Hnh 2.9.MPEG-4 AVC c th phn chia thnh phn chi ca tngMacroBlock theo nhiu cch ti u ho vic b chuyn ng.

Vic phn chia cc macroblock cho php b lp m x l c mtvi loi chuyn ng tu theo phc tp ca chuyn ng cng nhngun lc v tc bit. Nhn chung, kch c phn chia ln ph hp vivic x l chuyn ng ti cc khu vc ging nhau trong nh, trong khi kch c phn chia nh li rt c ch khi x l chuyn ng ti cc ch cnhiu chi tit hn. Kt qu l cht lng hnh nh cao hn, t b v khihn.

Cc cuc th nghim ch ra rng vic sp xp hp l cc khungc th tng t l nn thm 15%. MPEG-4 AVC ly phn chi ca nh gc

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v s dng cc macroblock c chia nh ti cc khu vc c nhiu chitit nhm tng cng kh nng hiu chnh chuyn ng.

* chnh xc trong hiu chnh chuyn ng:

Trong a s trng hp, chuyn ng ti ra mi macroblockhay khi thng din ra vi phn gii nh hn mt im nh. Dovy, chun nn MPEG-4 AVC c th m bo chnh xc tronghiu chnh chuyn ng ln ti 1/4 hoc 1/8 im nh, trong khi ccchun MPEG trc y ch dng li mc 1/2 im nh. Kh nngt mc chnh xc 1/8 nh im ca MPEG-4 AVC gip tng hiusut m ho ti tc bit cao v phn gii video cao. Cc thnghim cho thy chnh xc n 1/4 im nh c th lm gim tc bit xung hn 15% so vi chnh xc 1 im nh.

* Chn nhiu hnh tham chiu:Chun nn MPEG-2 ch da trn 2 khung tham chiu d

on cc chuyn ng mang tnh chu k, ging nh trong tr koqun. Tuy nhin, khi camera thay i gc quay hay chuyn quachuyn li gia cc cnh, vic ch s dng 2 khung tham chiukhng cn ph hp d on chnh xc chuyn ng. Tng tnh vy, on trc cc chuyn ng phc tp nh sng bin haymt v n, ta cn phi c nhiu hn 2 khung tham chiu. V th,

chun MPEG-4 AVC cho php c ti 5 khung tham chiu phc vcho vic m ho gia khung. Kt qu l cht lng video tt hn vhiu sut nn cao hn.

* Gii khi tch hp:Video s sau khi nn thng to ra mt hiu ng gi l "kt

khi", c th thy r ti im giao nhau gia cc khi, c bit l khic tc bit thp. Hiu ng ny l do cng on x l s dngnhiu loi chuyn ng v b lng t khc nhau. i vi MPEG-2,

cch duy nht ngn chn hiu ng ny l s dng cc c ch hux l ph hp, tuy nhin cc c ch ny li khng tng thch cvi tt c cc my thu. Chun nn MPEG-4 AVC a vo s dngmt b lc gii khi hot ng hai cp : macroblock 16x16 vkhi 4x4. Vic gii khi thng to ra mt t s tn hiu trn nhiu

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(PSNR) cc im thp hn, tuy nhin nhn mt cch ch quan th nto ra hnh nh cht lng tt hn.

+ u im v lng t ho v bin i:

Chm di ng 8x8 DCT cng vi dung sai ca li lm trn chnh lphn ct li ca cc chun MPEG trc y. MPEG-4 AVC c o hn ch n s dng bin i khng gian nguyn (gn ging nh DCT) i vicc khi 4x4 im nh. Kch c nh gip gim bt hin tng "kt khi",trong khi thng s nguyn tuyt i gip loi b nguy c khng thch nggia b lp m v gii m trong php bin i ngc. Thm vo , dy hs xch lng t ln hn khin cho c ch kim sot tc d liu b lpm hot ng mt cch linh hot hn da trn mt t l phc hp vokhong 12,5% thay cho mt mc tng lng gia khng i.

+ u im i vi m ho entropy:Sau khi tin hnh hiu chnh, bin i v lng t ho chuyn ng,

cc b lp m MPEG trc y s vch ra cc symbol biu din vctchuyn ng v h s lng t ho thnh cc bit thc s. V d nhchun nn MPEG-2 s dng phng php m c chiu di bin thin tnh(VLC) khng th ti u ho trong mi trng video thi gian thc (trong ni dung v cc cnh bin i theo thi gian).

MPEG-4 AVC s dng m ho thut ton nh phn theo tnh hungCABAC (Context-Adaptive Binary Arithmetic Coding). Hiu sut m hoca CABAC cao hn hn nh kh nng thch nghi vi cc thay i c thxy ra trong phn b symbol. V d, n c th khai thc s tng quan giacc symbol v t s dng s tng quan bit v thut ton m ho. Cch ny c th gip tit kim thm mt lng bit vo khong hn 5%.

+ Mt v d v u th ca MPEG-4 so vi MPEG-2:Hnh sau l biu so snh cht lng gia MPEG-4 AVC vi

MPEG-2. N so snh hot ng ca cc b lp m ti tn khi m ho mton video 30 khung/s c phn gii CIF ghi hnh mt trn thi u tennis.

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Hnh 2.10. So snh cht lng gia MPEG-4 v MPEG-2.

Biu din mc tit kim tc bit ca MPEG-4 AVC so vi mt s

chun hin c khc.

2.4.5) Kt lun:MPEG-4 AVC nh du mt bc ngot trong lnh vc nn video, p

dng cc k thut tin tin nhm mc ch s dng bng thng hiu qu hn vem li cht lng nh cao hn. Vi cc k thut ny, MPEG-4 AVC c thgim tc bit xung hn 50% so vi chun MPEG-2. Tuy nhin, MPEG-4AVC i hi mt cp phc tp cao hn trong c qu trnh lp m ln giim. Mc d vy, th thch ny hon ton c th chinh phc c nh cnhng tin b mi lin tip trong kh nng x l phn mm cng nh phncng. iu ny c ngha l MPEG-4 AVC l mt ng c vin nng k c khnng thay th MPEG-2 trong nhng nm sp ti.

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IPTV c c hi rt ln pht trin nhanh chng khi m mng bng rng cmt khp mi ni v hin nay c trn 100 triu h gia nh s dng dch v bngrng trn ton cu. Rt nhiu nh cung cp dch v vin thng ln trn th gii ang trinkhai thm d IPTV v xem nh mt c hi mi thu li nhun t th trng hin c ca

h v coi nh mt gii php t bo v trc s ln sn ca cc dch v truyn hnhcp. Ti th trng cung cp dch v Vit Nam, dch v IPTV bt u c thnghim cung cp vi mt s dch v c bn. iu ny xem nh l c hi kinh doanh dchv mi ca cc nh cung cp dch v ti Vit Nam, khi m c s h tng mng bng rng v ang pht trin mnh m cng vi s i hi nhu cu ca khch hng ngy cngcao. Ni dung bi vit s cho chng ta thy r tim nng to ln ca dch v IPTV trongmng bng rng v kh nng ng dng trin khai IPTV trn th trng vin thng Vit

Nam.

3.1. Tnh hnh IPTV trong khu vc:Cui thp k trc, cng s pht trin ca cc dch v truyn hnh v tinh,

s tng trng ca dch v truyn hnh cp s, v c bit l s ra i ca HDTV li du n i vi lnh vc truyn hnh. Tuy nhin, hin nay trn th gii xut hin mt phng thc cung cp dch v mi cn mnh hn vi e da s lmlung lay mi th c. Internet Protocol Television (IPTV) ra i, da trn shu thun ca ngnh vin thng, c bit l mng bng rng, IPTV d dng cungcp nhiu hot ng tng tc hn, to nn s cnh tranh mnh m hn cho ccdoanh nghip kinh doanh dch v truyn hnh. Hng In-Stat, mt hng nghin cu

th trng cng ngh cao c uy tn, gn y d bo rng th trng cc dch vIP video ti khu vc chu Thi Bnh Dng s tng trng ti gn 80% minm t nay n nm 2010 v s to ra mt th trng 4,2 t USD. Hng ny cngd on chu s chim ti mt na trong tng s thu bao TV ca cc cng tyin thoi trn ton th gii vo nm 2009 vi tng s thu bao ti thiu 32 triu.Cc s liu ny cho thy trong nhng nm cn li ca thp k ny, IPTV s trthnh mt dch v c th trng rng ln trn ton cu vi chu tip tc dn utrong vic thu ht khch hng. Cc con s ny cng cho thy y l mt th trngnng ng vi rt nhiu c hi cho cc nh cung cp dch v truyn hnh c m

hnh kinh doanh, hnh thc cung cp dch v v cng ngh hp l.

Informa Telecoms Media d bo IPTV s c s dng bi trn 35% cch gia nh s dng dch v truyn hnh s Hng Kng vo nm 2010, con sny s gn tng ng vi s h gia nh dng dch v truyn hnh cp (khong37%). Cng ty ny cng d bo s c n 13% cc h s dng dch v truyn hnh

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s Singapore s nhn tn hiu truyn hnh s thng qua ng dy DSL ca h,iu ny lm cho IPTV tr thnh mt nn tng truy nhp s ph bin hn rt nhiuso vi truyn hnh s mt t (DDT). Informa cng d bo rng DSL s chim ti9,2% cc h gia nh s dng truyn hnh s c, 6,2% New Zealand, 5,8%

i Loan, 5,7% Nht Bn v 4,2% Hn Quc. Truyn hnh cp vn s thngtr n nm 2010, nhng sau IPTV s thc s l i th cnh tranh vi truynhnh s mt t v v tinh i vi ngi xem truyn hnh chu .

S pht trin ca IPTV chc chn s nhanh hn, nhng vi s s ha catruyn hnh cp v v tinh, cc nh cung cp s phi cnh tranh ginh ckhch hng mi. Ty thuc vo th trng c th, cc nh khai thc dch v IPTVs phi b sung vo dch v truyn hnh qung b nhiu knh vi vic m rngcung cp cc dch v nh VoD, Replay-TV (network DVR), In-home DVR, Multi-

room Service, v.v... PCCW Hng Kng, nh cung cp dch v IPTV ln nht thgii vi trn 500.000 thu bao, a HDTV v VoD vo cung cp trn mngDSL ca mnh. SOFTBANK ca Nht Bn cng nhm n xy dng ni dungln n 5.000 gi cho cc phim truyn Nht Bn v Holywood trn dch vDSL/FTTH Video-On-Demand.

3.2. Tnh hnh IPTV ti Vit Nam:Ti Vit Nam, hin c nhiu nh khai thc dch v vin thng ln ang

cnh tranh nhau nhm cung cp cho khch hng cc dch v bng rng vi cht

lng cao v gi r. H cng nhn ra xu hng pht trin ca truyn hnh trctuyn v video theo yu cu, v ang c nhng bc i mnh m. Mt s Websitecung cp th nghim cc chung trnh truyn hnh trc tuyn ca VietNamNet,Cng ty VTC, i truyn hnh thnh ph H Ch Minh ghi nhn s lng truycp rt ln, cho thy sc hp dn ca dch v ny i vi cng chng.

FPT Telecom l doanh nghip vin thng u tin chnh thc khai thc vcung cp dch v IPTV trn h thng mng bng rng ADSL/ADSL2+ t ngy03/03/2006 sau mt nm th nghim v hin ti c 500 khch hng th nghim

u tin. FPT Telecom mua cc thit b nhn sng t v tinh truyn trnmng v cng k kt bn quyn t VTV v HTV pht sng 32 knh truynhnh trn Internet phc v cho cc khch hng ca FPT. Hin FPT ang tmkim cc phng thc hp tc tng t nh vi VTC c thm mt s knh

phim truyn ca i ny. Vi mt thu bao ADSL 2+ ca FPT, khch hng c thxem mt lc 3 knh truyn hnh ng thi. Hin FPT ang c gn 100.000 thu

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Dch v truyn hnh: cc ni dung truyn hnh c qung b theo lchtrnh thi gian c nh nh truyn hnh truyn thng. S la chn cc gi knhtheo yu cu ca khch hng c th bao gm cc knh truyn hnh cng cng(public), cc knh truyn hnh tr tin (pay TV), cc knh truyn hnh c a

thch, cc knh v mua sm, cc knh v thi trang, v.v...

Dch v truyn hnh theo yu cu: vic pht cc ni dung truyn hnh cla chn bt u khi ngi s dng la chn ni dung . Thng thng, ni dungl cc b phim hay cc phim c ghi li t mt th vin. Dch v ny c thc s dng trong mt thi gian gii hn. Cc chc nng thng ging nh chcnng ca my ghi hnh (VCR) hay u DVD (DVD player): pht hnh (play), dnghnh (pause), tua hnh (fast forward), v.v...

My ghi hnh cc nhn (Personal Video Recorder, PVR): PVR l mt thitb in t dn dng cho php ghi li cc ni dung qung b xem li mt thiim sau .

My ghi hnh c nhn qua mng (Network PVR, NPVR): y l phin bns dng trn mng ca PVR. N c th c xem nh l mt VCR o vi vic lutr v cc chc nng khc cung cp t mng. Ni dung truyn hnh qung b cth c ghi v xem li sau .

Hng dn chng trnh in t (Electronic Program Guide, EPG): mthng dn cung cp cho ngi s dng cc thng tin v cc chng trnh IPTVang v sp pht. C th ni mt EPG l phng thc ngi s dng tm kimcc ni dung ca nh cung cp.

Cc dch v thng tin: cc dch v thng tin c th bao gm tin tc thi s,tin th thao, d bo thi tit, thng tin v cc chuyn bay, cc s kin trong khuvc/a phng, v.v...

Truyn hnh tng tc: knh ph (back-channel) IP khng ch cung cpkh nng ly thng tin m cn cho php tng tc vi cc show truyn hnh hockhi to cc ng dng lin kt n cc chng trnh ang chy. Cc v d inhnh ca truyn hnh tng tc l tham d vo cc tr chi truyn hnh, bnh chnqua truyn hnh, phn hi ca ngi xem truyn hnh, cc chng trnh thngmi, v.v...

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Cc ng dng tng tc: s tng tc khng ch c lin kt n mtchng trnh truyn hnh truyn thng. u gi, mua sm, dch v ngn hng lcc ng dng truyn hnh c s dng rng ri, to ra s hi t ca thit b v s

pht trin cc giao din ngi s dng mi. Truyn hnh khin cho vic s dngcc ng dng tng tc (ging nh vic s dng Internet) tr thnh mt trongnhng thnh phn chim u th ca IPTV/VoD tng lai. y cng l mt yu tkhc bit ch yu nht so vi truyn hnh qung b truyn thng vn khng cmt knh ph no (c chng l mt ng in thoi).

Cc ng dng bng rng: cc ng dng dng cho ngi tiu dng v doanhnghip cng c th c thc hin thng qua h tng IPTV/VoD nh hi nghtruyn hnh, o to t xa, gim st an ninh, v.v...

Pay-per-View (PPV): l hnh thc tr tin xem mt phn chng trnhtruyn hnh, v d: tr tin xem mt s kin th thao hay tr tin nghe mt

bn nhc. H thng cung cp mt knh phim truyn hnh theo hnh thc PPV chocc thu bao.

Tr chi theo yu cu (Games on Demand): dch v ny s cung cp nhiuloi game ty chn n thu bao t mt danh sch c sn. IPTV yu cu game ngin da trn HTML.

m nhc theo yu cu (Muics on Demand): cc thu bao c th xem nhngclip ca nhc theo yu cu, ging nh dch v VoD.

Truyn hnh ca hm trc (TV of Yesterday, TVoY): dch v ny chophp thu bao xem phim truyn hnh c pht nhng ngy trc.

Karaoke theo yu cu (Karaoke on Demand): cc thu bao c th chn vxem cc bi Karaoke qua Set-top Box (ST trn TV. T list cc bi karaoke

c gii thiu, thu bao c th mua mt hoc nhiu bi ht cng lc. Dch v sc trin khai trong tng lai.

3.4. Tng lai IPTV ti VN:Xt trn kha cnh cng ngh, xu hng cng ngh hin nay l s hi t

ca nhiu cng ngh a ra nhng loi hnh dch v tng hp (nh kt hp cc

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dch v thoi, s liu v bng rng) cho khch hng, ng thi tn dng cnhng c s h tng sn c gim thiu chi ph u t nng cp. Dch v IPTVchnh l mt sn phm ca s hi t khi m ch vi mt thit b u cui khchhng c th s dng khong 6-7 loi hnh dch v con (truyn hnh qung b,

truyn hnh theo yu cu, in thoi thng thng, in thoi IP, in thoi truynhnh, truy cp Internet, v.v...). Hn na vic p dng cng ngh trin khainhng dch v vi cc chi ph nh, ti u ho h tng vin thng sn c s tngsc cnh tranh khi m Vit Nam gia nhp WTO.

ng thi c th khng nh vi h tng mng truy nhp hu tuyn v vtuyn bng rng trn c s mng NGN hin i m cc nh khai thc cung cpdch v ca Vit Nam v ang hng ti xy dng th vic trin khai dch vIPTV l hp l v kh nng bo m p ng yu cu trin khai dch v ny l

hon ton kh thi.

Cc dch v IPTV ngy cng tr nn l dch v ph bin c cc cng tyvin thng cung cp. Trong khi , h thng khng dy WiMAX c kh nng htr bng thng rng v tr thp rt ph hp cho vic phn phi cc dch vmultimedia. Hn na, h thng ny cng cung cp vng ph sng rng, h tr thu

pht di ng, v khng yu cu tm nhn thng (line-of-sight). Do , WiMAX lgii php ha hn phn phi cc dch v IPTV vi bt c khi no v bt k niu. Vi cc u im ca h thng WiMAX th vic phn phi cc dch v IPTV

s ha hn mang n chi ph phn phi thp nhng h cht lng dch vaudio/video cao v c bit l h tr kh nng di ng.

Vi s ph bin ca truyn thng khng dy hin nay, vic ng dng IPTVtrn mng khng dy ha hn s gip cho cc dch v IPTV hin nay c nhiuthun tin, linh hot, chi ph thp nh cc u th v k thut ca mng din rngkhng dy WMAN (wireless metropolitan area network) nh IEEE 802.16d/e (lmt phin bn ca chun WiMAX). K thut WiMAX da trn chun 802.16-2004 v 802.16e-2005 h tr tc d liu ln n 70Mbps trn khong cch

30Km v h tr kh nng di ng. Hin nay, WiMAX l h thng khng dy duynht c kh nng h tr cht lng QoS tc d liu cao trn nn tng IP. LpMAC (Medium Access Control) ca WiMAX h tr rtPS (time real PollingServices) bo m yu cu v bng thng v tr ti thiu cho cc dch v videoyu cu cht lng QoS. K thut ny dng iu ch OFDM (OrthogonalFrequency Division Multiplex) v lp vt l OFDMA (Orthogonal Frequency

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Division Multiple Access). N cng s dng cc c ch iu ch, m ha v sali FEC (Forward Error Correction) gia tng cht lng dch v. Hn na,WiMAX l mt mng truy cp IP v h tr chuyn gi d liu da trn mng li.Do , WiMAX c xem l h thng ph hp nht m rng cc dch v IPTV

theo hng khng dy v di ng.

Hin nay, nhiu nh cung cp vin thng trin khai cc h thng IPTVnhng khng phn phi dch v trn WiMAX m phn phi trn cp quang kotrc tip n nh thu bao. Trong tng lai, WiMAX s l hng pht trin cquan tm pht trin IPTV nhm p ng nhu cu ca cc thit b di ng, vtrong nhng vng kh trin khai c s h tng mng.

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TI LIU THAM KHO

1. http://www.tvover.net2. http://vi.wikipedia.org3. http://xahoithongtin.com.vn4. http://pctoday.vn
http://www.tvover.net/http://vi.wikipedia.org/http://xahoithongtin.com.vn/http://pctoday.vn/http://www.tvover.net/http://vi.wikipedia.org/http://xahoithongtin.com.vn/http://pctoday.vn/

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