Cc chun nn MPEG: Cng ngh MPEG l mt chun nn, c s dng rng ri trong cc h thng truyn hnh nh: mt t, cp, v tinh. MPEG l ch vit tt ca Moving Picture Experts Group, l mt hip hi cng nghip chuyn nghin cu v pht trin cc chun nn thch hp cho vic truyn ti video. Nhm ny c to ra bi T Chc Tiu Chun Quc t (ISO) v y Ban K S Quc T (IEC). T khi c thnh lp, MPEG a ra cc chun nn nh: MPEG-1, MPEG-2, MPEG-4(Part 2 v Part 10), MPEG-7, v MPEG-21. Trong cc chun ny, MPEG-2 v MPEG-4 Part 10 c s dng rng ri trong IPTV. Bng tm tt cc nh dng MPEG nh dng MPEG M t nh dng MPEG-1 cpht trin u tin vo nm 1988 v c s dng chnh nn video ti tc bit l 1.5Mbps. Ni dung MPEG-1 c s dng cho dch v nh pht thanh s(DAB). MPEG-1 l nh dng c bn ca chun MP3, c s dng rng ri cho nhc trn Internet. MPEG-2 c xy dng trn kh nng nn mnh m ca chun MPEG-1. MPEG-2 c s dng rng ri trong truyn ti ca truyn hnh qung b cht lng v lu tr ni dung video trn cc a DVD. Mt s cc chun truyn hnh quc t da vo nh dng nn ny. MPEG-4 l nh dng thit k chnh thc ca ISO/IEC vo thng 10/1998 v tr th nh chun quc t vo nm 2000. Chun Part 2 th c chia vo trong mt s thng tin m a ch yu cu ca mt s ng dng video trong in thoi di ng n cc my camera theo di. MPEG-4 Part 10 hay cn gi l H.264/AVC c thit k truyn video qung b v video cht lng DVD ti tc d liu thp nht.

MPEG-1

MPEG-2

MPEG-4 Part 2

MPEG-4 Part 10

Chun nn video.

Tuy c nhng t ph v giao thc truyn nhng vic truyn video trc tuyn s khng th tr thnh hin thc nu khng c nhng tin b vt bc trong k thut m ha, nn v gii nn d liu video. Cc tp tin ni dung video gc hay cha nn thng rt ln, c th lm tc nghn bt k ng truyn no, v vy vic nn nh d liu gim yu cu bng thng c ngha v cng quan trng i vi k thut streaming. C mt s cch gim yu cu bng thng ca dng d liu truyn dn. V d i vi video, ngi ta c th gim tn sut khung hnh (ch dng 15 khung hnh/giy thay cho 30 khung hnh/giy) v phn gii (v d gim phn gii t 640x480 pixels xung cn 320x240 pixels). Tuy nhin, nhng phng php ny lm gim nhiu cht lng hnh nh v vn cha gip gim bng thng truyn dn nh mong mun. V vy ngi ta cn phi s dng cc k thut m ha nn ni dung gc, sau gii m my thu client. Hin c nhiu k thut m ha nn d liu audio v video, nhng tu trung c th phn theo 2 k thut c bn. Mt l tm cch th hin hiu qu thng tin d tha trong ni dung, theo cch ny, khi c ti to li d liu kt qu ging tng bit vi ni dung gc. Tuy nhin, cch thc nn khng tn tht ny ch gip gim s bit khng ng k. a phn k thut m ha v nn video s dng trn mng Internet hin nay s dng k thut c tn tht, tc l ngi ta s lc b bt i mt s thng tin m cc c quan tri gic con ngi (tai v mt) khng nhn bit c. Cc k thut ny cng cho php m ha nn vi nhiu t l bit khc nhau mt cch linh hot, c th cho php truyn ti ty theo bng thng kt ni khc nhau v khi , trn my thu client cng phi c cng c gii m tng ng gip gii m ni dung th mi c th hin th c. y thng l cc cng c gii m c cung cp min ph, khch hng c th ti v t cc my ch ca nh cung cp dch v video online trn mng nh Windows Media (wmv, wma, avi v asf), Realplayer (rm, ram), QuickTime (mov) v MPEG-4 (ISO, DivX)... Bng 1: Mt s chun nn v gii m dng video ph bin trn mng Internet hin nay.

H.264 H.264 gim yu cu bng thng trong khi cht lng nh tng ng MPEG-2 v MPEG-4. Cng ngh ny gip cho tng cng kh nng nn khng gian v nn thi gian, cho hnh nh truyn rt nhanh trn mng LAN, internet. Vi nhng u vit ca mnh, H.264 ang c ng dng rt nhiu vo ngnh an ninh.

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 k ny, 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 nh on chuyn ng, tin x l, x l i ngu v phn b tc bit ty theo tnh 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 gii hn ng dng ca

mnh trong lnh vc truyn truyn hnh t sn xut tin k n hu k v lu tr Video s. Bn cnh , nhu cu nn Video li ang ngy mt 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 cc nh cung cp dch v thng tin c th mang li ngy cng ln, ngoi ra h cn c th cung cp nhiu dch v theo yu cu thng qua h thng cp, v tinh v cc 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 thit k bi nhng nh sn xut khc nhau i vi mi loi ng dng Video; Nh vy 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 chng ng m MPEG-2 b d.

Hip hi vin thng quc t (ITU) v t chc tiu chun quc t/ U ban k thut in t quc t (ISO/IEC) l hai t chc pht trin cc tiu chun m ho Video. Theo ITU-T, cc tiu chun m ho Video c coi l cc khuyn ngh gi tt l chun H.26x (H.261, H.262, H.263 v H.264). Vi tiu chun ISO/IEC, chng c gi l MPEG-x (nh MPEG-1, MPEG-2 v MPEG-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 thoi truyn hnh. Mt khc, nhng tiu chun MPEG c thit k hng ti mc tiu lu tr Video chng hn nh trn a quang DVD, qung b Video s trn mng cp, ng truyn s DSL, truyn hnh v tinh hay nhng ng dng 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 part 10, H.26L hoc JVT.

2. Tnh k tha ca chun nn 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 Video trc y. Khng ch th, chun nn H.264 cn k tha phn ln li im ca cc 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 cc hnh nh lin tip bi mt vi m ca nhng Block gc thng qua d on v khng gian, php bin i, qu trnh lng t v m ho Entropy (hay m 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 lin tip. 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 ho trc 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 cc loi Block.

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

3. C ch nn nh ca H.264 (MPEG-4 AVC)

Vi chun nn H264, mi hnh nh c phn chia thnh nhiu Block, mi block tng ng vi mt s lng nht nh cc MacroBlock. V d mt hnh nh c phn gii QCIF (tng ng vi s lng im nh 176x144) s c chia thnh 99 MacroBlock vi kch c 16x16. Mt s phn on cc MacroBlock tng t c s dng cc kch c nh khc. Thnh phn chi ca nh c ly mu tng ng vi phn gii ca nh , trong khi thnh phn mu CR v CB c ly mu vi tn s thp hn theo 2 chiu ngang v dc. Thm vo mi hnh nh c th c phn thnh s nguyn ln cc lt mng (slice), vic ny rt c gi tr cho vic ti ng b trong trng hp li d liu.

Mi hnh nh thu c c xem nh mt nh I. nh I l nh c m ho bi vic p dng trc tip cc php bin i ln cc MacroBlock khc nhau trong nh. Cc nh I c m ho s c kch c ln bi n c xy dng t mt khi lng ln thng tin ca bn thn nh hin ti m khng s dng bt c thng tin no t min thi gian trong qu trnh x l m ho tng hiu qu x l m ho bn trong trong H.264.

3.1. Gim bt d tha

Cng ging nh cc b lp gii m khc, H.264 nn video bng cch gim bt d tha c v khng gian v thi gian trong hnh nh. Nhng d tha v mt thi gian l nhng hnh nh ging nhau lp i lp li t khung (frame) ny sang khung khc, v d nh phn phng nn khng chuyn ng ca mt chng trnh i thoi trn truyn hnh. D tha v khng gian l nhng chi tit ging nhau xut hin trong cng mt khung, v d nh nhiu im nh ging nhau to thnh mt bu tri xanh. Hnh 1 biu din mt cch s lc cc bc m b lp gii m MPEG-4 phi tin hnh nn khng gian v thi gian.

3.2. Chn ch , phn chia v ch ng

B lp gii m bt u bng vic quyt nh loi khung cn nn ti mt thi im nht nh v chn ch m ho ph hp. Ch "trong khi" to ra nh "I", trong khi ch "gia khi" to ra khung "P" hoc "B". Sau , b m ho s chia nh thnh hng trm hng v ct cc im nh ca nh video s cha nn thnh cc khi nh hn, mi khi c cha mt vi hng v ct im nh.

3.3. 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 hin ra 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 ra mt khi "chnh lch" hoc "li". Thao tc ny s gim bt d liu trong mi block mt cch hiu qu do ch phi trnh by chuyn ng ca n m thi. Tip n l cng on bin i csin ri rc (DCT) bt u nn theo 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 thng cng on DCT.

3.4. 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 i my (nu c). Nh vy c ngha l tn s thay i gi tr im nh trong khi ny l rt thp. Nhng khi nh th c gi l khi c tn s khng gian thp. B lp m li dng c im ny bng cch chuyn i cc gi tr im nh ca khi thnh cc thng tin tn s trong cng on bin i csin 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 khi ban u c tn s khng gian thp, DCT s tp hp phn ln nng lng tn s vo gc tn s thp ca mng. Nh vy, nhng h s tn s thp gc s c gi tr cao hn.

Mt s lng ln cc h s khc cn li trn ma trn u l cc h s c tn s cao, nng lng thp v c gi tr thp. H s DC v mt vi h s tn 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 qut cho mng li theo ng zig-zag, bt u t h s DC v qua v tr ca cc h s ngang dc tng dn. Do vy n to ra c mt chui h s c sp 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 lng cao nhng gi tr thp nn b lng t ho s lm trn chng thnh 0. Kt qu l mt chui cc gi tr h s c lng t ho bt u bng mt s gi tr cao u chui, theo sau l mt hng di cc h s c lng t ho v 0. B lp m entropy c th theo di s lng cc gi tr 0 lin tip trong mt chui m khng cn m ho chng, nh vy gim bt c khi lng d liu trong mi chui. Cc u im ni bt ca chun nn H.264

4.1. 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 ti cc macroblock 16x16 im nh thay v cc block 8x8 nh trc y. iu ny gip tng cng ng k kh nng nn khng gian i vi cc hnh nh c cha nhiu khong ln cc im nh ging nhau.

Th hai l thao tc nn c tin hnh trong min khng gian trc khi 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 c th chia nh macroblock 16x16 im nh thnh cc khi 4x4 nh hn v so snh tng khi ny vi cc khi k bn trong cng mt macroblock. iu ny gip ci thin kh nng nn nh chi tit.

4.2. 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 n mt cp cao hn nhiu, cng vi cht lng chuyn ng tt hn so vi cc chun MPEG trc y.

4.3. 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 tp hoc phi tuyn tnh, v d nh phng to thu nh. Ngc li, MPEG-4 AVC li tng cng hiu chnh chuyn ng bng cch cho php b lp m bin i 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-4 AVC 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 khi 8x8 ny thnh 4 c na l 8x8, 8x4, 4x8 hoc 4x4.

Vic phn chia cc macroblock cho php b lp m x l c mt vi loi chuyn ng tu theo phc tp ca chuyn ng cng nh ngun lc v tc bit. Nhn chung, kch c phn chia ln ph hp vi vic 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 c nhiu chi tit hn. Kt qu l cht lng hnh nh cao hn, t b v khi hn.

Cc cuc th nghim ch ra rng vic sp xp hp l cc khung c th tng t l nn thm 15%. MPEG-4 AVC ly phn chi ca nh gc v s dng cc macroblock c chia nh ti cc khu vc c nhiu chi tit 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 macroblock hay khi thng din ra vi phn gii nh hn mt im nh. Do vy, chun nn MPEG-4 AVC c th m bo chnh xc trong hiu chnh chuyn ng ln ti 1/4 hoc 1/8 im nh, trong khi cc chun MPEG trc y ch dng li mc 1/2 im nh. Kh nng t mc chnh xc 1/8 nh im ca MPEG-4 AVC gip tng hiu sut m ho ti tc bit cao v phn gii video cao. Cc th nghim 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 ko qun. Tuy nhin, khi camera thay i gc quay hay chuyn qua chuyn li gia cc cnh, vic ch s dng 2 khung tham chiu khng cn ph hp d on chnh xc chuyn ng. Tng t nh vy, on trc cc chuyn ng phc tp nh sng bin hay mt 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 v cho vic m ho gia khung. Kt qu l cht lng video tt hn v hiu 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 khi c tc bit thp. Hiu ng ny l do cng on x l s dng nhiu 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 hu x l ph hp, tuy nhin cc c ch ny li khng tng thch c vi tt c cc my thu. Chun nn MPEG-4 AVC a vo s dng mt b lc gii khi hot ng hai cp : macroblock 16x16 v khi 4x4. Vic gii khi thng to ra mt t s tn hiu trn nhiu (PSNR) cc im thp hn, tuy nhin nhn mt cch ch quan th n to ra hnh nh cht lng tt hn.

4.4. u im v lng t ho v bin i.

Chm di ng 8x8 DCT cng vi dung sai ca li lm trn chnh l phn 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 vi cc 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 ng gia b lp m v gii m trong php bin i ngc. Thm vo , dy h s xch lng t ln hn khin cho c ch kim sot tc d liu b lp m hot ng mt cch linh hot hn da trn mt t l phc hp vo khong 12,5% thay cho mt mc tng lng gia khng i.

4.5. 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 vct chuyn ng v h s lng t ho thnh cc bit thc s. V d nh chun 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 hung CABAC (Context-Adaptive Binary Arithmetic Coding). Hiu sut m ho ca CABAC cao hn hn nh kh nng thch nghi vi cc thay i c th xy ra trong phn b symbol. V d, n c th khai thc s tng quan gia cc symbol v t s dng s tng quan bit v thut ton m ho. C ch ny c th gip tit kim thm mt lng bit vo khong hn 5%.

4.6. Mt v d v u th ca MPEG-4 so vi MPEG-2.

Hnh 3 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 mt on video 30 khung/s c phn gii CIF ghi hnh mt trn thi u tennis.

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 v em li cht lng nh cao hn. Vi cc k thut ny, MPEG-4 AVC c th gim tc bit xung hn 50% so vi chun MPEG-2. Tuy nhin, MPEG-4 AVC i hi mt cp phc tp cao hn trong c qu trnh lp m ln gii m. Mc d vy, th thch ny hon ton c th chinh phc c nh c nhng tin b mi lin tip trong kh nng x l phn mm cng nh phn cng. iu ny c ngha l MPEG-4 AVC l mt ng c vin nng k c kh nng thay th MPEG-2 trong nhng nm sp ti.