PHAT THANH TRUYEN HINH
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KY THUAT STREAMING
TRONG
MOBILE TV
ThS. Pham Dinh Chung, ThS. Pham Anh Thu
1 . G i a i T H I E U CHUNG
Traydn hinh di dbng (MobUs TV) li ebng nghe ma hoi vi traydn din cic chuong trlrta traydn htah hoac video de eb the thu dugc tren eie thiet bi di dgng nhu dien taoai dl dgng, cic thidt bi hd ttg sd cam tay (PDA), eie thidt bi da phuang tien vb tuydn, eie miy dien thoai cb kha nang thu tta bleu trayen binh di dbng [1].
Vbi Mobile TV ngubi xem cb thd tray nhip mgt dai rbng cie chuong trlrta traydn htah trong khi di ehuyen. Cie chuong trlrta trayen htah cb the duge traydn tal ludng (streamtag) tbi may di dbng de xem b tdc do gidng nhu khi phit; hoac cb the xem cic chuang trinh vbi ttd thbi gian hoae eb the dugc ghi lal toin bb gidng nhu bang cassette video hoac dia DVD. Mobile TV khbng chi eho phbp tmyen din mbt chieu thbng thuang mi cbn eho phep traydn tta bleu traydn hirta tuong tic rtab su dung eie kenh phan hdi cung cap bbi mang te bio.
Tao ludng (Stieamtag) li mbt van de quan trgng trong mobile TV, arta hubng true tiep ddn ehat lugng dieh vu. Bii bio gibi thieu tdng quan vd ky thuit streamtag trong mobile TV bao gdm khii mem vd strsamtag trong mobile TV, qua trirta bit giu vi ma hba nbi dung, bidn ddi file thirta khubn
dang streamtag, Qng dung stream servtag, van de quan ly bang thbng, vi chuong trirta xem vi server streamtag.
2. STREAMING TRONG MOBILE TV Streamtag ngi dung (Am tharta, video) da ttb thirta cbng nghe phd bidn cQng vbi sir phat trien cua tatemet vio rtaimg nam 1990. Mbt phuong phip trayen tai du Ueu video db li ngubi su dung se tai vd eic file am tharta vi video vi bat dau xem chuang trirta khi da tai xong, phuang phip niy eb nhugc diem li do kich thube eua eac file cb the lbn (vi du 20 Mbytes) nen thbi gian chb de tii ve li rat dii.
6 ehd do stteamtag du UBU am thanh vi vidso dugc phan phit true tiep tbi ngubi su dung dl dbng hoae eie thidt bi khie de xem trong khi vin tiep tue tal ve. Vi du, vbi kdt ndi 128 kbps, dQ Ueu video a tdc do 64-100 kbps eb the dugc traydn tal ludng Uen tuc, eho phbp ngubi su dung tray nhap true tidp tbi nbi dung da phuong tien.
Stteamtag thue hien dugc nhb su dung eic bb ma hoi cb tdc do nbn cao, cimg vbi cbng nghe traydn til ludng ngi dung bing each bien ddi khubn dang
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file du Ueu lim giu thirta khubn dang gbi phQ hgp vbi vbi traydn tai qua mang tatemet.
Cb hai phuang phip stteamtag: Du Ueu am thanh, video, eac ttang Web eb thd duge eung cap bing cich su dung giao thQe HTTP. Phuang phip niy duge ggi li tao ludng tisn tidn (progressive stteaming), vi tauc hisn duge khi kerta phan phat eb kha nang phan phat dQ Ueu HTTP b eie tdc do bit yeu eiu. Phuong phip thuc te hon db li phuang phip tao ludng thbi gian thue (real-time stteaming), phuong phap niy yeu eau su dimg eic giao thQc die biet (giao thQe tabi gian thire RTP, cic giao thQc multieasttag...) vi cic server die biet (Apple QmckTlme server, RealTime server, Wtadows Media server).
Stteamtag bao gdm cae bube sau:
- Bat glQ vi ma boa nbi dung;
- Bien ddi file thinh khubn dang stteaming;
- Stteam servtag;
- Trayen tai ludng qua cae mang IP;
- Chay chuong trtah tren media player.
2 . 1 Qua t r i n h b a t g i u ' va m a h o a n o i d u n g
Viec bit giQ nbi dung bao gdm vise ttdp nhan dbng am taarta vi vidso a khubn dang tuong thieh vbi card bit giu video cua may ttnh hoac ssrvsr.
Khubn dang eb the li AVI hoae eic Qng dung chuyen nghiep, dang SDI. Card bit giu gdm eie bb ma hoi phan cQng cb the tidp rtaan nbi dung o khubn dang YC, RGB... Sau khi bit giu, cae file dugc luu giu tten dia vi du a khubn dang .avi trabc khi nbn. Sau khi nbn eie file dugc luu giQ a khubn dang nen phQ hgp nhu .mpg, .mp4,... phu thube vio bb ma hoi dugc sudung.
2.2 Bien do'i file t h a n h Idiuon d a n g s t r e a m i n g De cie file dugc trayen tai dbng stteamtag theg
thbi gian thue, cic fUe niy can cb thbng tta didu khien thbi gian dd ssrvsr quan ly tdc do phit. Do db cie flls dugc bien ddi thirta khubn dang stteamtag bing cich them vio thbng tta dieu khien thbi gian vi metadata giup eho viee phan phat cb thQ tu du
Ueu stteamtag ddi vbi cae Qng dung. Vi du QmckTlme su dung ttnh ning Htat Tracks de eung cap thbng tta dieu khien rtaim xac duta thbng tta am tharta vi video da duge stteamtag.
2.3 S t r e a m s e r v i n g
Stteam servtag li mbt Qng dung die biet duge su dung b ehd do cUent server de phit ehudi Uen tuc cic gbi qua mang IP ddn may cUent. Ung dung stteamtag sii dung cac giao thQc ttao ddi file thbi gian thue da phuang tisn duge phit tridn bai IETF.
Cie giao thue niy bao gdm: Giao thQe thbi gian thue RTP, giao thQc dieu khien thbi gian thue RTCP vi giao thue tao ludng thbi gian thue (RTSP).
Qui trtah stteaming bao gdm hai kenh rieng re duge thiet lip cho phien stteaming. Kenh du Ueu cung cap viec trayen tii dQ Ueu im tharta vi video, ttong khi db kenh dieu khien eung cap phan hdi tQ stteamtag cUsnt (vi du nhu media player) tbi server.
Du Ueu am thanh vi video ttong qui trirta stteamtag duge didu khien boi giao thQc RTP, giao thQe niy su dung giao thue UDP vi IP lim cie lbp ben dubi. Do db du UBU dugc phan phit thso chudi datagram mi khbng can xac nhin. Didu nay tao thirta kenh du Ueu ttong qui trlnh streamtag.
CUent eung cap thbng tta nhu sd lugng gbi thu
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Hinh 1 . Ngan xep giao thuc streaming
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Hinh 2 . Stream serving
duge vi ehat lugng kenh qua kbita RTCP. Server dua tren thbng tta thu duge se bidt ttah trang Idi vi tie nghen eua mang vi tdc do mi cUsnt thuc su thu duge eie gbi. Nhb db ssrver eb the thue hien phin phit gbi b tdc do chtah xic. Vi du, dua tren thbng tin phan hdi tu cUent, server eb the chgn mgt trong eae tdc do bit trayen tai dbng kha dung (ching ban 64, 128, 256 kbps) hoae chgn tdc do khung thap ban de dam bao tdc do du Ueu trayen til khbng vugt qui kha ning eua kerta IP. Kenh didu khien su dung giao thue RTSP, giao thQe niy boat dbng tren ndn giao thue TCP vi IP. RTSP do db didu khien viec phin phit nbi dung streamtag tbi cUent qua mang IP. RTSP hd trg eie chue ning dieu khien nhu xem lai (playback), xem (play), tua di (forward), tua lal (rsvsrss) vi dimg tam thbi (pause), kdt hgp vbi media player cua cUent SB cung cap eho ngubi su dung day du chQc ning dieu khien trong qui trlnh xem lai qua tmyen tii dbng streamtag.
2.4 S t r e a m i n g serving va quan ly bang thong
Streamtag ssrvsr vi msdia cUent (Thiet lap kdt ndi ddn server dd trayen tii ludng) boat dgng b ehe do bit tay nhu mb ta tten Hinh 2.
Dbng media dugc gui theo eie khdi du Ueu, cic
khdi niy duge bidn ddi thinh eic gbi RTP. Mdi gbi RTP bao gdm phan tieu dd vi du Ueu. Tieu dd cb phan mb ta dbng trayen tai, dau thai gian, vi sd thQ tu eua gbi dQ Ueu. Thbng tta niy rat can thiet trong viec ghbp lai eie gbi theo thQ tu chirta xic o miy thu dau cudi. Giao thQe RTP duy tri tdc do phan phit du Ueu da duge ed dirta vi dugc thoi thuan qua kdt ndi UDP/IP, trong khi db giao thue RTSP hd trg su tuong tic cUent vbi eic ehQc nang rtau xem vi dimg tam thbi. Khi mbt kdt ndi duge thiet lap, RTSP dugc su dung de tao kdt ndi o tdc do bit cao rtaat mi mang IP hd trg (eb the gdm eic dubng Uen kdt vb tuydn hoac cie mang GSM/GPRS hoae 3G, CDMA). Cac gbi trayen tai (im tharta vi video) duge xu ly Uen tuc khi ChQng tbi media player. CUent can thuc hien luu dsm du Usu vl tdc do bit traydn din thay ddi. Ndu tdc do du Ueu thap hoic ty le ldi eao do didu kien dubng Uen kdt thi cUent se gm tta bleu tbi server yeu cau thue hien chuyen mach sang trayen til ludng thbng minh hoae do tdc do khung hi rbt. Qui trinh niy tao thinh ket ndi don hubng (Unicast). Tuy nhien dang kdt ndi niy khbng ly rabng khi cb mbt sd lugng lbn ngubi su dung mudn tray nhip eimg mbt nbi dung, vl khi db sd ludng trayen tai vi du Ueu duge eung cap ting len nhanh chbng.
Chd do trayen din khie li che do multicast, trong db nhieu ngubi su dung cung thu dugc nbi dung, khi
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Hinh 3 . Truyen tai dong va ma hoa video MPEG-4 dupc phan IPp
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Hinh 4. Chuyen mach dong stream xay ra acac khung I.
db du Ueu duge phit multicast. Cic bb dirta tuyen ttong mang thu duge dbng multicast thire hien phit lap du Ueu tbi cac dubng Uen kdt khie ttong mang.
Thay vl hing trim nghta phibn unicast, mdi dubng Uen kdt traydn tal chi mbt dbng nbi dung multicast.
Phuong phip niy eb nhieu uu diem, tuy nhien cac cUent khbng cb eo ehd dieu khien de ybu cau server thay ddi tdc do bit trayen din khi xiy ra ldi hoae tic nghen mang...
Vbi MPEG-4 cb co ehd khic de eung cap tdc do bit cao ban tbi eic cUent khi mang eb bing thbng eao hon. Server traydn tii dbng MPEG-4 se phit dbng eo sb cb do phin dai thap vi mbt sd dbng phii. CUent thu dbng ca sb vi cie dbng phu niy khi bang thbng kha dung lim cho chat lugng hinh inh tdt hon.
2.5 ChUtfng trinh x e m va server s t r e a m i n g
Cb mbt sd loai bb ma hoi vi server stteamtag, trong db mbt sd cbng nghe yeu eiu ban quydn rieng.
Vi du su thiet lap streamtag eua RealNetwork gdm bb ma hoi/giii ma RealVideo vi server stteamtag SureStream. RealVideo dua tren nguyen ly ma hoi MPEG-4, su dung tdc do khung khbng lay miu. Didu niy eho phbp cae tdc do khung yeu cau ddi vbi toe do phan phat duge lira chgn dugc tao ra bing cac vector chuyen dbng vi ngi suy khung. Do db mgt file msdia don gian cb the dugc tao ra cb cae tdc db mi hoi khie rtaau. Khi thuc hien trayen tai ludng, RealNetwork SureStream se thidt lap kdt ndi sau kW thoa thuan vbi khich hing. Tdc do du Ueu thap nhat se dugc trayen tai ludng ttong dieu kien mang
tic nghen rtaat. SureStteam su dung chuyen mach stream dbng de chuyen mach tbi tdc do bit thap hon hoae cao hon phu thube vio dieu kien traydn din vi phan hdi tQ cUent. Vi du, SureStteam cb the chuydn tQ ludng 64 kbps sang ludng 128 kbps hoac nguge lai.
Khubn dang RealMedia su dung giao thuc RTP vi giao thQc ban quyen RDP (giao Uiuc gbi du Ueu thuc) de trayen tai du Ueu. Hg cac giao thue RealMedia trayen tai du Ueu b chd do unicast.
3. KET LUAN
Mobile TV la cbng nghe vb tuydn dugc thidt kd de cb the traydn tal duge tta hieu traydn htah trong mbi trabng vb taydn di dbng eb bang thbng ban chd vi thubng xuyen chiu arta hubng cua fadtag, nhidu vi tap am, ttong khi phai dip Qng duge khi nang hien thi tta hieu tdt tten miy dau cudi cam tay dl dbng cb kich thube man htah nhb, cbng suat pta tisu thu hi ban chd. Streamtag li ky thuat chu ydu dugc su dung dd traydn tai Un bleu Mobile TV tren mang 3G.
Tai lieu t h a m khao
[1] AMITABH KUMAR, iVIobiie TV: DVB-H, Df^B, 3G Systems and Rich IVIedia Applications, Elservier Inc, 2007.
[2] BORKO FURHT, SYED AHSON, Handbool< of IVIobiie Broa±asting: DVB-H, DIVIB, ISDB-T AND iVIEDIAFLO, CRC Press, 2008.
[3] CLINT SMFTH, DANIEL COLLINS, 3G Wireless Networks, McGraw-Hill, 2002.
[4] DANIEL MINOLI, IP Multicast with Applications to IPTV and IVIobiie DVB-H, John Wiley & Sons, 2008.
[5] KAMEL HADDAD, DVB-H in Denmark Technical andEconomic Aspects, Master's Thesis, Technical University of Denmark, 2007.
[6] FRANK HARTUNG, UWE HORN, JORG HUSCHKE, MARKUS KAMPMANN, THORSTEN LOHMAR, and MAGNUS LUNDEVALL,
"Delivery of Broadcast Servtes in 3G Networks ", IEEE Transactions on Broadcasting, Vol. 53, No. 1, March 2007.
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