Nn video theo chun MPEG

A.Nn video theo chun MPEG1) Khi qut v nn MPEG MPEG (Moving Picture Expert Group) l nhm chuyn gia v hnh nh, c thnh lp t thng 2 nm 1988 nhm chun ha cho nn tn hiu m thanh v video. Nn tn hiu video theo chun MPEG l phng php nn nh ng khng nhng lm gim d tha khng gian (nh JPEG) m cn gim d tha thi gian gia cc khung nh, y chnh l khc bit so vi IPEG. MPEG khng n gin l mt cng c nn n l m u im ca nn nh MPEG chnh l ch MPEG c mt tp hp cc cng c m ha chun, chng c th c kt hp vi nhau mt cch linh ng phc v cho mt lot cc ng dng khc.

Nn MPEG l s kt hp hi ha gia bn k thut n gin:

+ Tin x l (Preprocesing): lc ra cc thng tin khng cn thit t tn hiu video v nhng thng tin kh m ha nhng khng quan trng cho cm th ca mt ngi.+ on trc s chuyn ng ca cc frame b m ha (Temporal prediction): da trn nguyn tc l cc nh trong chui video dng nh c lin quan mt thit vi nhau theo thi gian: Mi frame ti mt thi im nht nh s c nhiu kh nng ging cc frame ng ngay pha trc v ngay sau n. Cc b m ha s qut ln lt tng phn nh trong mi frame v ch pht nhng thay i gia cc khi trong frame hin ti v cc khi c b m ha tin on ti pha thu. Nh vy m c th loi b d tha v thi gian.+ B chuyn ng b gii m (motion compensation): ti pha thu c lu tr sn nhng thng tin m khng thay i ca frame ny ti frame khc trong b nh m v chng c dng in thm mt cch u n vo cc v tr trng trong nh c khi phc. y cng chnh l mt trong nhng cng c mnh lm gim d tha v khng gian gia cc bc nh.+ M lng t ha (quatisation coding)2) Cu trc dng bt- Cu trc dng MPEG gm 6 lp:

Lp khi (block) : gm khi 8x8 cc im nh tn hiu chi v tn hiu mu dng cho phng php nn DCT. Lp macroblock: lp t hp cu trc khi l mt nhm cc khi tng ng vi lng thng tin cha ng trong kch thc 16x16 im trn bc nh. Cu trc khi ny cng xc nh c lng thng tin cha trong s thay i ty theo cu trc mu c s dng. Thng tin u tin trong cu trc khi mang dng ca n v cc vector b chuyn ng.

Lp ct lt dng bt (slice): Mng bao gm mt v cu trc khi k nhau. Kch thc ln nht ca mng c th bao gm ton b bc nh v kch thc nh nht ca mng l mt cu trc khi. Thng tin u tin cha ng v tr ca mng trong ton bj nh, v h s cn bng lng t.. Lp nh (picture): cho php b gii m xc nh loi nh c m ha. Thng tin u dng ch th t truyn khung ( b m ha c th sp xp cc nh li theo th t ng) v cc thng tin v ng b, phn gii v phm vi vector chuyn ng. Trong nn MPEG ngi ta s dng 3 loi nh sau: nh I (Intra Pictures): c m ha c lp m khng ccn tham kho cc nh khc. Hiu qu nn tn hiu t c do loi b c d tha v khng gian m khng c yu t thi gian tham gia vo qu trnh. nh I c dng mt cch tun hon to thnh im ta cho dng d liu trong qu trnh gii m.

nh P (Predicted Pictures): c m ho t nh I hoc nh P ngay trc b chuyn ng v chnh n cng c th dng tham kho cho vic tin on nh tip theo. Mi khi nh trong nh P c th c m ha theo kiu tin on hoc m ha mt cch c lp. Do s dng c nn khng gian v thi gian nen hiu qu nn ca nh P cao hn ng k so vi nh I.

nh B (Bidirectionally Predictive Pictures) c th s dng cc nh I v P pha tr v pha sau n b chuyn ng v vy cho t l nn cao nht. Mi khi trong nh B c th tin aons theo chiu ngc, xui, c hai hng hoc c m ha c lp. tuy nhin tin aons ngc t mt bc nh pha sau n th b m ha phi p xp li cc bc nh. Do vy s to ra tr do pho sp xp li thng tin, tr ln hay nh ph thuc vo s bc nh B lin tip nhau c truyn. Lp nhm nh (GOP): cc nh I, P, B thng xut hin theo mt th t lp i lp li mt cch tun hon. V vy xut hin nhm nh GOP. Cht lng nh khng ch ph thuc vo t l nn trong tng khun hnh m cn ph thuc vo di ca nhm nh GOP. Chng l n v mang thng tin c lp ca MPEG v bt buc phi bt u bng mt nh I. Tip sau l mt lot cc nh P v B. Cu trc ca mt GOP c m t boi 2 tham s: N(s nh trong GOP) v M(khong cch gia cc nh P)

Lp dy nh (sequence): on video bao gm thng tin u, mt s nhm nh v thng tin kt on.

- Mi lp ny h tr mt chc nng nht nh: mt l chc nng x l n hiu (DCT v b chuyn ng). Hai l chc nng logic (ti ng b, im truy xut ngu nhin).

4:2:2

Khi im nh

8x8 (block) Macroblock

Mng (slice)

Khung (frame)

Nhm nh (Group Of Piture)

Hnh 1: Cu trc dng bt MPEG

Thng tin u on thng tin kt on dng c

s

on video th n-1on video th non video th n+1

Thng tin u

nhm nh

GOP#pGOP#p+1GOP#p+2GOP#p+3GOP#p+4

Thng tin u

khung nh

Khung IKhung BKhung PKhung BKhung I

Thng tin

u mng

MBMBMBMBMBMBMBMBMBMBMBMBMB

Thuc tnh MBVector chuyn ngYYYYCrCb

MB = Macroblock

H s DCTEOB

Khi 8x8 im chi Hnh 2: Kin trc dng d liu MPEGM = Khong cch gia hai im so snh

I B B P B B I

nh chun D on hai chiu

Nhm nh (GOP)

N= KHONG CCH GIA HAI NH I

Hnh 3: Cu trc nh MPEG- Qu trnh to dng bt MPEG l ghp knh: kt hp dng d liu vo, dng d liu ra, iu chnh ng b v qun l b m. C php dng MPEG bao gm: lp dng bit (stream), lp gi (back) v lp gi tin (backet).3) Nguyn l nn MPEG iu khin tc bt

Bng lng tVideo + +

-

+ +

Vc t chuyn ng

iu khin nhm nh (Group of Picture - GOP)Hnh 4: B m ha MPEG tiu biu* C s ca cng ngh nn video MPEG l s kt hp gia nn trong nh (Intra -Frame Compression) v cng ngh nn lin nh ( Inter-Frame Compression). Trong :

- Nn trong nh (Intra -Frame Compression): l loi nn nhm gim bt

thng tin d tha trong min khng gian. Nn trong nh s dng c hai qu

trnh c tn hao v khng c tn hao gim bt d liu trong nh. Qu

trnh ny khng s dng thng tin ca cc nh trc v sau nh ang xt.

- Nn lin nh (Intra -Frame Compression): Trong tn hiu video c cha

thng tin d tha trong min thi gian. Ngha l vi mt chui lin tc cc

nh, lng thng tin cha ng trong mi nh thay i rt t t nh ny sang

nh khc. Tnh ton s dch chuyn v tr ca ni dung nh l mt phn rt

quan trng trong k thut nn lin nh. Trong thut nn MPEG, qu trnh

xc nh Vector chuyn ng c thc hin bng cch chia hnh nh thnh cc Macro-Block, mi Macro-Block c 16 x 16 phn t nh (tng ng

vi 4 Block, mi Block c 8 x 8 phn t nh). xc nh chiu chuyn

ng, ngi ta tm kim v tr ca Macro-Block trong nh tip theo, kt qu

ca s tm kim s cho ta Vector chuyn ng ca Macro-Block * Nguyn l nn MPEG :

Tn hiu u vo c dng 4:2:2 hoc 4:2:0 c nn lin nh nhm to ra nh khc bit u ra b cng. nh khc bit ny sau li c nn trong nh qua cc bc : bin i DCT, lng t ha, m ho. Cui cng c trn cng vi vector chuyn ng a n b khuch i m s thu c nh nn.

Tc bt ca tn hiu video c nn khng c nh, ph thuc vo ni dung nh ang xt (v d mt phn nn t hn hoc nhiu hn), nhng ti u ra b m ho dng bt phi c nh xc nh tc cho dung lng knh truyn. Do ti u ra b m ha phi c b nh m ln. B m ha phi kim tra trng thi y ca b nh m. Khi s liu trong b nh m gn bng dung lng cc i th cc h s bin i DCT ngc c lng t ha t chnh x hn. V vy b nh m cha t s liu th vic lng t ha cc h s s tng ln.4) Nguyn l gii nn MPEG Vo Ra

H s DCT Cc vecto chuyn ngHnh 5: B gii m MPEG tiu biu*Nguyn l gii nn MPEG :

Chui tn hiu vo c gii m Entropy ti VLD, sau tch d liu nh (h s bin i DCT) ra khi cc vector chuyn ng. S liu nh s c gii lng t ho v bin i DCT ngc.

Nu nh l nh loi I bt u mi nhm nh trong chui, u ra s nhn c nh hon chnh bng cch trn ( v nh loi I ch l nn trong nh, khng c b chuyn ng, khng dng d liu ca nh khc). N c lu tr trong b nh nh v c v c dng gii m cc nh tip theo.

Nu nh l nh loi P th cng thc hin gii lng t ha v bin i DCT ngc kt hp vi vic s dng vector chuyn ng v lu vo b nh nh. Trn c s xc nh c d on nh ang xt. Ta nhn c nh ra sau khi cng d on nh (nh d on) v kt qu bin i DCT ngc. nh ny cng c lu vo b nh c th s dng nh l chun khi gii m cc nh tip theo.

B. CC CHUN NN MPEG1) MPEG-1- Tiu chun The Moving Picture Experts Group Phase 1 (MPEG-1) nh dng m thanh, phim nh ca nhm chuyn gia nh ng l mt tiu chun cho nh dng m thanh, phim nh ca t chc ISO/IEC - International Organization for Standardization/International Electrotechnical Commission (T chc tiu chun ha quc t/y ban k thut in quc t) pht trin nhm h tr cc thit b trnh chiu m thanh, phim nh.- Tiu chun MPEG-1 (hay cn gi l ISO/IEC 11172 M ha nh ng v m thanh kt hp cho phng tin lu tr thng tin s 1,5Mbit/s) bao gm 05 phn:- ISO/IEC cng b 03 phn u tin ln u vo thng 08/1993 gm:

+ ISO/IEC-11172-1:H thng (lu tr v ng b m thanh, phim nh v cc d liu khc)

+ ISO/IEC-11172-2: Phim nh (cha ni dung ca phim nh nn)+ ISO/IEC-11172-3:m thanh (cha ni dng ca m thanh nn)- Phn 4 ca MPEG-1 ISO/IEC-11172-4: th kh nng tng thch (kim tra vic trin khai tiu chun) c cng b ln u vo thng 03/1995- Phn 5 ca MPEG-1 ISO/IEC-11172-5M phng phn mm (phn mm mu thc hin vic m ha v gii m nh th no) c cng b ln u vo thng 11/1998.Ni dung cc phn c trnh by tm tt nh sau:+Phn H thngquy nh phng thc s dng, lu tr m thanh, phim nh c m ha v cc d liu khc, ng thi duy tr tnh ng b gia cc ni dung khc nhau.nh dng tp tin MPEG c thit k p ng vic lu tr trn phng tin (on media), truyn ti knh d liu mt cch tin cy;

+Phn Phim nhch ra phng php nn d liu video lm gim tc truyn ti d liu trong mt lung thng tin video truyn ti qua mng;

+Phn m thanhch ra phng php nn d liu m thanh lm gim tc truyn ti d liu m thnh trong mt lung thng tin m thanh truyn ti qua mng;

+Phn Kim th tng thchcung cp hng dn v m hnh tham kho cho vic kim th tnh tng thch ca MPEG-1;

+Phn mm tham chiuch ra phng php m ha, gii m nh th no.

Ni dung caPhn Phim nhvPhn m thanhm t cc nh dng c s dng ph bin trong lu tr, trao i, x l m thanh v phim nh (v d:(.mpg), (.mpeg), (.mp1), (.mp2), (.mp3), (.m1v), (.m1a), (.m2a), (.mpa), (.mpv))MPEG-1 phn Phim nh v phn m thanh-MPEG-1 phn Phim nh (ISO/IEC-11172-2) s dng cc phng php nn nh lm gim tc d liu theo yu cu ca mt lung phim nh. Phng php nn nh s gim hoc loi b hon ton thng tin/d liu trong mt di tn s nht nh v cc khu vc hnh nh m mt ngi nhn bit mt cch hn ch. Cc thut ton nn phim nh v cc b m ha/gii m (codecs) u kt hp k thut nn hnh nh theo khng gian v k thut b p chuyn ng theo thi gian. Trong thc t, hu ht cc b m ha/gii m u tin hnh k thut nn m thanh v nn hnh nh song song hay ring bit cho mt tp tin phim nh, tuy nhin, cc dng d liu c kt hp li v gi i nh mt gi d liu. MPEG-1 phn Phim nh h tr phn gii ln n 4095x4095 (12 bt) v tc bt ln n 100 Mbit/s. MPEG-1 phn Phim nh thng s dng phn gii SIF (Source Input Format): 352 x 240, 352 x 288 hoc 320 x 240. phn gii thp kt hp vi mt t l tc bt thp hn 1,5 Mbit/s to ra mt dng d liu bt thp vi thng s hn ch. y l thng s k thut ti thiu m bt k b m ha/gii m no cng x l c, m bo s cn bng gia cht lng phim nh v hiu qu s dng, cho php trin khai trn cc h thng phn cng c chi ph khng cao.

- MPEG-1 phn m thanh(ISO/IEC-11172-3) c chia lm 03 lp: MPEG-1 phn m thanhLp I c t l tc bt t 32 448 kbit/s (32, 64, 96 416, 448 kbit/s), Lp II c t l tc bt t 32-384 kbit/s (32, 48, 56, 64, 80256, 320, 384 kbit/s), Lp III c t l tc bt t 32-320 kbit/s (32, 40, 48, 56, 64, 80224, 256, 320 kbit/s). Vic m ha/gii m s phc tp hn trong lp cao hn v s dng hiu qu t l tc bt thp hn.

- MPEG-1 phn m thanh Lp I c nh dng tp tin l (.mp1) v nh dng tp tin ca Lp II l (.mp2), ch yu c s dng trong pht sng truyn thanh, truyn hnh. MPEG-I phn m thanh Lp III, thng c gi l MP3, c nh dng tp tin l (.mp3), l nh dng m thanh ph bin trong ngnh cng nghip m nhc, gii tr trn Internet do kch thc nh gn v cht lng m thanh ca tp tin .mp3.Cho n nay, cc phn c cp nht nhiu ln. Ln cp nht mi nht cho cc phn ca MPEG-1 l ISO/IEC-11172-1 (1999); ISO/IEC-11172-2 (2006); ISO/IEC-11172-3 (1996), ISO/IEC-11172-4 (2007), ISO/IEC-11172-5 (2007).

MPEG-1 l tiu chun nn suy hao cho m thanh, phim nh. MPEG-1 c thit k nn m thanh, phim nh xung 1.5 Mbit/s (t l nn tng ng l 26:1 v 6:1) m khng gim cht lng m thanh, phim nh qu mc.Cc c tnh ca MPEG-1Cu trc dng bt ca MPEG -1 cng tng t nh cu trc dng bt ca MPEG gm:

Sequence (chui nh) : gm nhiu nhm nh GOP, c chc nng l dng bt video.

GOP (Group of Picture ): gm t 1- n nh bt u bng nh I, c chc nng l n v truy xut.

Picture I, P, B: gm nhiu Slice, chc nng l n v m ho c bn.

Slice : gm nhiu cc Macro Block, l n v ti ng b phc hi li.

Macro-Block : gm 16 x 16 pixel, l n v b chuyn ng.

Block : gm 8 x 8 pixel, l n v tnh DCT.

Bng 2: Tham s theo tiu chun MPEG -1: Tham sc im

Tn hiu m ho

Cu trc ly mu

Kch thc nh ti a(im nh x im nh)

Biu din mu

chnh xc ca qu trnh lng t ho v bin i DCT

Phng php lng t ho h s DCT

Cu trc khi trong qu trnh lng t ho thch nghi

chnh xc cc i ca h s DC

Bin i RLC

Bng VLC

H s cn bng cc khi

B chuyn ng

Qut

chnh xc d on chuyn ng

Tc khi nnTn hiu m ho

4:2:0

4095 x 4095

8 bt

9 bt

DPCM tuyn tnh

16 x 16 bt

8 bt

M Huffman

Khng th truyn ti

C th bin i

Trong khung hnh v gia cc khung hnh

Tun t

im nh

Tc khi nn 1.85 Mb/s cho nn tham s

100 Mb/s cho dng y tham s

MPEG-1 khng thc s nh ngha thit k mt b m ha, thay vo chun ny m t c php m ha v b gii m tham kho gi thuyt.

ng dng

MPEG-1 ng dng trong thit k CD chy m thanh, trnh chiu video, truyn hnh cp/v tinh k thut s Ngoi ra, MPEG-1 thng c s dng l tiu chun cho a quang VCD

2) MPEG-2Tiu chunThe Moving Picture Experts Group Phase 2(MPEG-2) nh dng m thanh (audio), phim nh (video) MPEG-2 l mt tiu chun cho truyn hnh k thut s thuc h tiu chun MPEG dng m ha lung d liu m thanh, phim nh.MPEG-2 c cng bthnh tiu chunISO/IEC 13818,gm 11 phn nhbng di y:STTTiu chunNm cng b ln th nhtNm cp nht mi nhtCh

Phn 1ISO/IEC 13818-119962013H thng h tr phng thc m ha kt hp phim nh v m thanh

Phn 2ISO/IEC 13818-219962013Phng php biu din d liu phim nh m ha v quy trnh gii m theo yu cu khi phc nh

Phn 3ISO/IEC 13818-319951998Phng php m ha d liu m thanh

Phn 4ISO/IEC 13818-419982009Th tc kim th tnh tng thch

Phn 5ISO/IECTR1381819972005M phng phn mmngn ng C ca b m ha v gii m

Phn 6ISO/IEC 13818-619981998Cc m rngcho yu cu v iu khin phng tin lu tr s (Digital Storage Media Command and Control - DSM-CC)

Phn 7ISO/IEC 13818-719972006Tiu chun m ha m thanh nng cao

Phn 8

( b thu hi)M rng phim nh 10-bt, ng dng chnh l phim nh trong studio (xng phim)

Phn 9ISO/IEC 13818-919961996M rng giao din thi gian thccho cc b gii m h thng

Phn 10ISO/IEC 13818-1019991999M rng tng thch choDSM-CC

Phn 11ISO/IEC 13818-1120042004Qun l s hu tr tu

Tiu chun MPEG-2 l m rng ca MPEG-1 c s vi mc tiu cung cp nn c cht lng cao cho vic truyn phim nh, c th l to ra hnh nh cht lng cao hn MPEG-1 tc bt (bit rate) cao hn (t 3 - 10 Mbps), tuy nhin ti tc bt thp(