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PHN 1
GII THIU TI
I.1. GII THIU
Cng ngh thng tin (CNTT-IT) c mnh danh l ngnh ca mi ngnh. Thc
vy, ta c th thy hu ht mi ngnh, ngh u t nhiu c ng dng CNTT vo cng
vic ca n.
Bo mt thng tin, nn d liu l cng vic cn thit trong ngnh CNTT. T thi xa
xa, con ngi bit gn gi thng tin trong qun s, trong chin tranh nhm lm cho
thng tin c an ton, khng lt vo tay i phng. Mun vy, con ngi c nhiu
cch lm cho thng tin truyn i c gn nh v an ton khi lu thng. thngtin n tay ngi nhn mt cch b mt, gn gng nht th h phi nn thng tin li hay
dng cc k hiu c bit c qui c trc.
Ngy nay vi s tin b ca khoa hc k thut, CNTT c nng ln mt tm cao
mi. Mi ngnh, ngh u phi ng dng CNTT mt cch trit pht trin mt
cch tt nht. Ngay nh c cm t Chnh ph in t m ta thng nghe cho ta thy
tm quan trng ca vic a CNTT vo cuc sng. Khng nm ngoi s pht trin c
tnh qui lut chung ca x hi, CNTT v ang lm ht sc mnh pht trin ngy
mt tt hn, nhanh hn, nh hn, gn hn . CNTT v Vin thng c mi lin h
tng i cht ch vi nhau. Mt trong nhng tiu ch gip ngnh CNTT pht trin l
s dng cng ngh bo mt, nn d liu, thng tin trong lu tr v truyn thng.
Trong k thut truyn s liu, bo mt v nn d liu (ngun tin) truyn i l 2 vn
quan trng, nhiu c s l thuyt v m ha ngun cho ta thy tm quan trng ca
vic m ha v nn d liu. Cc thut ton nn d liu ra i t rt lu nh m nn
Shannon-Fano, Huffman hay Lempel Ziv Welch (LZW) c cho l kinh in ca
cng ngh nn d liu.Trong bi lun vn ny, em s trnh by i nt v cc thut ton nn d liu thng
dng hin nay v so snh tnh hiu qu ca vic nn mt s loi d liu khc nhau gia
2 loi m nn Shannon-Fano v Huffman. Phn m phng tnh hiu qu ca 2 loi m
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nn thng dng ny em s trnh by bng chng trnh c vit trn nn tng ngn
ng lp trnh C v C++.
I.2. MC CH V PHM VI CA TI
n ny tp trung vo cc vn xoay quanh thut ton Shannon-Fano v
Huffman, phn tch cc u, nhc im ca thut ton ny so vi thut ton kia bng
hnh thc m phng t l nn ca n. Cc phn s thc hin trong n ny gm :
1. Tm hiu l thuyt nn d liu, tng hp cc kt qu trn th gii v nn
d liu. Tng quan v cc phng php m ha ngun, nguyn tc lm vic ca cc
phng php, tc v t l nn.
2. Phn loi v ng dng. Nhim v t ra l a ra c ni dung ca cc
thut ton m ha ngun .
3. Ni dung thut ton Shannon-Fano v Huffman . Trong phn ny s i su
phn tch cc u nhc im ca tng thut ton, la chn thut ton Shannon-Fano
v Huffman l cc thut ton nn d liu kinh in so snh tnh hiu qu ca tng
loi vi mt s loi d liu khc nhau.
4. Khi qut v chng trnh, kt qu ca thut ton. Trong phn ny s nu cu
trc ca chng trnh v cc vn khi xy dng chng trnh. Nhim v quan trng
l phi m phng c thut ton, xy dng lu thut ton v coding.
I.3. PHUNG PHP NGHIN CU
- a ra c c s l thuyt
- Tin hnh nghin cu thut ton ca cc loi m nn Shannon-Fano v Huffman.
- a ra nguyn tc m ha v bi ton m ha.
- Chy chng trnh v kim th
- nh gi u im v nhc im
PHN 2
NI DUNG
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CHNG 1
S LC V NN D LIU
II.1.1 TNG QUAN V NN D LIU
Trong khoa hc my tnh v l thuyt thng tin, nn d liu l qu trnh m ha thng
tin dng t bit hn so vi thng tin cha c m ha bng cch dng mt hoc kt hp
ca cc phng php no . Da theo nguyn tc ny giptrnh cc hin tng knh
truyn b qu ti v vic truyn tin tr nn kinh t hn. Nn d liu gip tit kim cc
ti nguyn nh dung lng b nh, bng thng, thi gian. Ngc li, d liu cnn cn phi c gii nn c (thc thi, nghe, xem v.v), qu trnh ny cng i
hi cc ti nguyn nht nh. Mt v d in hnh l vic nn video i c th i hi
phn cng t tin qu trnh gii nn nhanh ta c th xem c. Do vic
thit k mt chng trnh nn d liu ph thuc nhiu yu t nh mc nn, mo
(i vi nn c tn hao), ti nguyn h thng dng thc hin qu trnh nn v gii
nn d liu.
II.1.2 TNG QUAN CC LOI M NN
II.1.2.1. Cc chng trnh nn hot ng nh th no
Nguyn tc ca cc chng trnh nn ni chung ging nhau: Tn dng s lp li
ca d liu, cc chui d liu lp li c thay th bi con tr chung c di b hn.
K thut ny rt c hiu qu i vi d liu dng text, bng tnh, hoc file DBF (nn
trn 70%), v tnh lp li ca d liu loi ny cao: File chng trnh (.EXE hoc .COM)
nn c t hn.
II.1.2.2. Tc v t l nn
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Ngay c khi tt c cc chng trnh nn file u dng chung mt thut ton th hot
ng ca chng cng khc nhau. Mi hng trin khai thut ton mt kiu dung ha
hai vn : thi gian v t l nn. Chng trnh PKZIP thng tri hn cc chng
trnh nn khc v mt tc , v mt t l nn, nhiu khi n cng kh hn. Tnh nnh ca cc chng trnh nn cng l iu cn quan tm. Cc file nn ni chung rt
t khi b hng. Cng cn lu l cc loi file nn khng tng thch vi nhau, tc l
nu gi file nn cho ngi khc th ngi cn phi c chng trnh thch hp mi
gii nn ra c. Tuy nhin gii quyt vn ny, c 3 chng trnh ARC + PLUS,
LHA v PKZIP u cho php to file nn t bung - tc file nn dng chng trnh
thc hin, khi chy s t ng bung ra, trn th trng cng bt u xut hin chng
trnh chuyn i t dng file nn ny sang dng file nn khc, v d chng trnh
D'Compress for Windows chuyn cc file PKZIP, ARC, LHA sang dng ARJ.
Cc chng trnh nn gi khng cao (PKZIP: 47USD, LHA cung cp min ph)
nn c dng kh rng ri. Hn ch hin nay ca chng l giao din ngi dng
khng thun tin, thng phi g lnh vi nhiu tham s du nhc ca DOS thc
hin mt cng vic no . Ci tin theo hng ny ang c thc hin: ARC +
PLUS c giao din kiu menu, PKZIP cng c phn b sung l PKZIP menu.
Nhiu chng trnh qun l file trong DOS v trong Windows bt u dng k
thut nn. Chng trnh Magellan ca hng Lotus dng PKZIP t nm 1990, chng
trnh Xtree Gold a PKZIP vo cng c qun l file nm 1991.
Th mc nn ri sau li phi bung ra dng ca cc chng trnh nn file kh
rm r, chnh bi l do ny m cc chng trnh nn a nh Stacker hoc Super
Store c s dng tng i rng ri. Cc chng trnh nn a cng hot ng trn
nguyn tc ging nh nn file, ch khc l chng t ng nn v bung m ngi dng
khng phi quan tm n. Thi gian v t l nn ca cc chng trnh nn loi ny
khc nhau. bung 3,5 Mb d liu, chng trnh ny ht 12 giy, chng trnh khc
40 giy. T s nn i vi file vn bn cng khc: t 2:1 n 3:1. Tm li khi dng
chng trnh nn a, ngi dng yn tm l dung lng trng ca cng dng nh
tng khong 2 ln.
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Vic bung v nn khi lm vic vi file s lm cng vic chm li i cht. i vi
cc file d liu ln, iu ny th hin kh r. Khi lm vic, cc chng trnh nn a
hot ng dng thng tr, bi th mt mt n chim dng b nh RAM, mt mt c
th gy xung t vi cc chng trnh thng tr khc. Cc chng trnh nn file khic s c ch hng mt vi file, cn chng trnh nn a lm hng c a. Tuy iu
ny rt t khi xy ra nhng n cng lm cho nhiu ngi e ngi khng dm dng.
ci t chng trnh nn a cn phn chia li cng v my tnh cn c khi
ng bng a nn trc khi chng trnh nn hot ng. Nu dng Windows th phn
khng nn cn kh ln (thng thng cn dnh 10 Mb cho vng khng nn, ch nn
vng a cn li).
Mt iu c th lm ngi dng au u l phi quyt nh t l nn l bao nhiu.
Vi t l nn 10:1 chng hn, chng trnh nn s dnh nhiu "con tr" tr n
cc d liu, mi con tr chim 2 byte, khi d xy ra trng hp khng con tr,
chng trnh bo a y m thc ra khng phi nh vy.
Cui cng, vic loi b chng trnh nn a khi ci t cng l mt vn hi
phin toi. Nhiu chng trnh - chng hn Double Density c chc nng loi b. i
vi cc chng trnh khc cn tm cc file n ca chng trnh nn v xa b chng i.
C khi phi format li cng.
Tm li, d mt s hn ch, nn d liu l cch thc kinh t nht m rng dung
lng cng. Ngoi ra cn tit kim c kh nhiu thi gian v kinh ph khi nn d
liu trc khi truyn i
T l nn l mt trong cc c trng quan trng nht ca mi phng php nn.
Tuy nhin, v cch nh gi v cc kt qu cng b trong cc ti liu cng cn c
quan tm xem xt. Nhn chung, ngi ta nh ngha t l nn nh sau :
T l nn = 1/ r x %
Vi r l t s nn c nh ngha : r = kch thc d liu gc / kch thc d liu
thu c sau nn. Nh vy hiu sut ca nn l : ( 1 - t l nn) x %
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Trong cc trnh by sau khi ni n kt qu nn, chng ta dng t s nn, th d nh
10 trn 1 c ngha l d liu gc l 10 sau khi nn ch c 1 phn.
Tuy nhin, cng phi thy rng nhng s o ca mt phng php nn ch c gi trvi chnh s nn , v rng hiu qu ca nn cn ph thuc vo kiu d liu nh nn.
nhiu khi t l nn cao cng cha th ni rng phng php l hiu qu hn cc
phng php khc, v cn cc chi ph khc nh thi gian, khng gian v thm ch c
phc tp tnh ton na. Th d nh nn phc v trong truyn d liu : vn t ra l
hiu qu nn c tng hp vi ng truyn khng.
II.1.2.3 Cc loi d tha d liu.
Nh trn ni, nn nhm mc ch gim kch thc d liu bng cch loi b dtha d liu. vic xc nh bn cht cc kiu d tha d liu rt c ch cho vic xy
dng cc phng php nn d liu khc nhau. Ni mt cch khc, cc phng php
nn d liu khc nhau l do s dng cc kiu d tha d liu khc nhau. C 4 kiu d
tha chnh c trnh by cc mc sau y.
II.1.2.3.1. S phn b k t.
Trong mt dy kt, cmt sktctn sut xut hin nhiu hn mt sdykhc. Do vy, ta cthmhodliu mt cch c ng hn. Cc ktctn xut
xut hin cao hn c thay th bi mt tmnhphn vi sbt nh; ngc li cc
dy ctn xut xut hin thp sc mha bi tmcnhiu bt hn. y chnh l
bn cht ca phng php mhoHuffman hay Shannon-Fano.
II.1.2.3.2. Slp li cua cc kt
Trong mt stnh hung nh trong nh, 1 khiu (bt "0" hay bt "1") c l p il p li mt sln. Kthut nn dng trong trng hp ny lthay dy l p bi dy
mi gm 2 thnh phn: s ln l p vkhiu dng m ha. Phng php mho
kiu ny ctn lmha lot di RLC (Run Length Coding).
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II.1.2.3.3. Nhng mu sdung tn sut
Cthcdy khiu no xut hin vi tn sut tng i cao. Do vy, cth
mhobi t bt hn. y lc sca phng php mhokiu tin do Lempel-Ziv a ra vcci tin vo nm 1977, 1978 vdo ctn gi lphng php nn
LZ77, LZ78. Nm 1984, Terry Welch ci tin hiu qu hn v t tn l LZW
(Lempel-Ziv- Welch). Thut ton nn d liu da vo mu s dng tn sut hiu qu
phi k n phng php nn d liu ca Shannon-Fano v Huffman.
II.1.2.3.4. d tha vitri
Do sphthuc ln nhau ca dliu, i khi bit c khiu (gitr) xut hinti mt vtr, ng thi cthon trc sxut hin ca cc gitrcc vtrkhc
nhau mt cch phhp. Chng hn, nh biu din trong mt li hai chiu, mt s
im hng dc trong mt khi dlu li xut hin trong cng vtrcc hng khc
nhau. Do vy, thay vlu trdliu, ta chcn lu trvtrhng vct. Phng php
nn da trn sd tha ny gi lphng php mhodon.
Cch nh gid tha nh trn hon ton mang tnh trc quan nhm biu th
mt ci gxut hin nhiu ln. i vi dliu nh, ngoi c thchung , ncncnhng c thring. Thdnh cng dng khng cn ton bdliu th ca nh
mchcn cc thng tin c trng biu din nh nh bin nh hay vng ng nht. Do
vy, cnhng phng php nn ring cho nh da vo bin i nh hay da vo biu
din nh.
II.2.2 PHN LOI V NG DNG
II.1.3.1 Da vo nguyn l nn:
Theo cch ny ngi ta phn thnh 2 h:
II.1.3.1.1 Cc thut ton nn khng tn hao:
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Trong phng php nn khng tn hao, d liu c nn sau khi gii nn sging
y nh ban u. Trong thng dng nht l thut ton Lempel-Ziv (LZ). DEFLATE,
l mt bin th ca thut ton LZ, c ti u ha nhm tng tc gii nn v t l
nn, b li thut ton ny c tc ca qu trnh nn chm. DEFLATE c dngtrong PKZIP, GZIP, v PNG. LZW (Lemple-Zip-Welch) c dng trong nh dng
file GIF. Hai bin th ca thut ton LZ cng ng ch l thut ton LZX dng trong
nh dng file CAB ca Microsoft (Microsoft cn dng thut ton nn ny trong file
CHM, cc file office 2007) v thut ton LZMA dng trong chng trnh 7-ZIP.
Cc thut ton nn khng tn hao c dng nn cc file nh file thc thi, file
vn bn, word, excel, v.v Cc loi d liu ny khng th sai lch d ch mt bit, nht
l cc file chng trnh.
Cc thut ton nn khng tn hao c bn:
1. Run-length encoding (RLE).
2. Dictionary coders.
3. LZ-77 & LZ-78.
4. LZW.
5. Burrows and Wheeler transform (BWT).
6. Prediction by partial matching (PPM).
7. Context mixing (CM).
8. Entropy encoding.
9. Huffman coding (huffman ng thng dng bc cui cng ca qu trnh nn
file gm nhiu bc).
10.Adaptive Huffman.
11.Arithematic.
12.Shannon-Fano coding.
13.Range coding.
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II.1.3.1.2 Cc thut ton nn tn hao:
Trong cc phng php nn tn hao th d liu c nn khi gii nn ra s khng
ging vi d liu gc, tuy nhin phi m bo d liu sau khi nn vn cn hu ch.i vi hnh nh, m thanh, video, do gii hn ca mt v tai ngi nn mt lng ln
dung lng c th c tit kim bng cch loi b cc phn d tha, trong khi cht
lng hu nh khng thay i.
Trong thc t, cc file hnh nh m thanh hay l video c lu tr trn my tnh
u c nn c tn hao tit kim dung lng v bng thng. i lp vi nn
khng tn hao cc phng php nn c tn hao thng gy gim cht lng rt nhanh
khi thc hin nn v gii nn qui nhiu ln.Cc mu hnh nh m thanh s c chia
thnh cc phn nh v c bin i qua min khc. Cc h s bin i ny s c
lng t ha sau c m ha bng m huffman hoc m ha s hc.
Cc mu hnh nh m thanh trc c s dng d on cc mu tip theo. Sai
s gia d liu d on v d liu thc s c lng t ha ri m ha
u im ca nn tn hao so vi nn khng tn hao l nn tn hao trong nhiu
trng hp cho t l nn cao hn rt nhiu so vi bt c thut ton nn khng tn hao
c bit, trong khi vn m bo c cht lng. Nn tn hao thng c s dng
nn nh, m thanh, video. m thanh c th nn vi t l 10:1 m hu nh khnggim cht lng. Video c th nn vi t l 300:1 vi cht lng gim t.
II.1.3.2 Da vo cch thc thc hin nn
Theo cch ny, ngi ta cng phn thnh hai h:
Phng php khng gian (Spatial Data Compression): cc phng php
thuc hny thc hin nn bng cch tc ng trc tip ln vic ly mu ca nh trongmin khng gian.
Phng php s dng bin i (Transform Coding): Gm cc phng
php tc ng ln sbin i ca nh gc mkhng tc ng trc tip nh htrn.
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CHUNG 2
CC PHNG PHP NN D LIU
II.2.1 PHUNG PHP NN KHNG TN HAO
II.2.1.1. M hnh thng k
II.2.1.1.1 Thut ton Shannon-Fano:
Cc bc thc hin m ho theo thut ton Shanon-Fano:
- Bc 1: Sp xp cc k t theo th t gim dn.
- Bc 2: Tnh xc sut
- Bc 3: quy lm hai phn, mi phn c tng xc sut gn bng
nhau. M ho phn trn bng bit 0 (hoc bit 1), phn di bng bit 1(hoc bit 0).
- Bc 4: V s cy.
- Bc 5: Tnh Entropy, s bits m ho trung bnh v s bit m ho
thng thng.
v V d m t thut ton
Thng k lng tin:
K hiu A B C D E
S ln xut hin 15 7 6 5 6
M ha lng tin:
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K hiu m PiLog2(1/
pi)M Tng bits
A 15 15/39 1.38 0 0 30
B 7 7/39 2.48 0 1 14
C 6 6/39 2.7 1 0 12
E 6 6/39 2.7 1 1 0 18
D 5 5/39 2.96 1 1 1 15
S bits s dng trung bnh: (tng bits/ s ln xut hin).
R = (30+14+12+18+15) / 39 = 2.29 bits
II.2.1.1.2 Thut ton Huffman
Thut ton Huffman c u im l h s nn tng i cao, phng php thc
hin tng i n gin, i hi t b nh, c th xy dng da trn cc mng b hn
64KB. Nhc im ca n l phi cha c bng m vo tp tin nn th pha nhn mic th gii m c do hiu sut nn ch cao khi ta thc hin nn cc tp tin ln.
Nguyn l:
Nguyn l ca phng php Huffman l m ha cc bytes trong tp d liu
ngun bng bin nh phn. N to m di bin thin l mt tp hp cc bits. y l
phng php nn kiu thng k, nhng k t xut hin nhiu hn s c m ngn hn
(gn ging Shannon-Fano).
Thut ton:
Thut ton nn:
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- Bc 1: Tm hai k t c trng s nh nht ghp li thnh mt, trng
s ca k t mi bng tng trng s ca hai k t em ghp.
- Bc 2: Trong khi s lng k t trong danh sch cn ln hn mt
th thc hin bc mt, nu khng th thc hin bc ba.
- Bc 3: Tch k t cui cng v to cy nh phn vi quy c bn
tri m 0, bn phi m 1.
v Xt v d.
Thng k lng tin:
K hiu A B C D E
S ln xut hin 15 7 6 5 6
M ha lng tin:
K hiu Xc sut M Tng bit
A 15/39 1
13/39 0
0 1 24/39 0
0 11/39
1 15
B 7/39 000 21
C 6/39 001 18
E 6/39 010 18
D 5/39 011 15
- S bit trung bnh: 87/39 =2.23 (
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Thut ton gii nn:
-Bc 1: c ln lt tng bit trong tp tin nn v duyt cy nh phn c
xc nh cho n khi ht mt l. Ly k t l ghi ra tp gii nn.
-Bc 2: Trong khi cha ht tp tin nn th quay li thc hin bc mt, ngc
li th thc hin bc tip theo.
-Bc 3: Khi ht tp tin, kt thc thut ton.
II.2.1.1.3 Thut ton Run-length:
Loi d tha n gin nht trong mt tp tin l cc ng chy di gm cc k tlp li, iu ny thng thy trong cc tp tin ha bitmap, cc vng d liu hng
ca cc tp tin chng trnh, mt s tp tin vn bn...
v V d, xt chui sau:
AAAABBBAABBBBBCCCCCCCCDABCBAAABBBBCCCD
Chui ny c th c m ho mt cch c ng hn bng cch thay th chui k
t lp li bng mt th hin duy nht ca k t lp li cng vi mt bin m s ln kt c lp li. Ta mun ni rng chui ny gm bn ch A theo sau bi ba ch B
ri li theo sau bi hai ch A, ri li theo sau bi nm ch B... Vic nn mt chui theo
phng php ny c gi l m ho di lot. Khi c nhng lot di, vic tit kim
c th l ng k. C nhiu cch thc hin tng ny, tu thuc vo cc c trng
ca ng dng (cc lot chy c khuynh hng tng i di hay khng ? C bao nhiu
bit c dng m ho cc k t ang c m ?).
Nu ta bit rng chui ca chng ta ch cha cc ch ci, th ta c th m ho bin
m mt cch n gin bng cch xen k cc con s vi cc ch ci. V vy chui k t
trn c m ho li nh sau: 4A3BAA5B8CDABCB3A4B3CD
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y "4A" c ngha l "bn ch A"... Ch l khng ng m ho cc lot
chy c di 1 hoc 2 v cn n hai k t m ho.
i vi cc tp tin nh phn mt phin bn c tinh ch ca phng php nyc dng thu c s tit kim ng k. tng y l lu li cc di lot,
tn dng s kin cc lot chy thay i gia 0 v 1 trnh phi lu chnh cc s 0 v
1 . iu ny gi nh rng c mt vi lot chy ngn (Ta tit kim cc bit trn mt
lot chy ch khi di ca ng chy l ln hn s bit cn biu din chnh n
trong dng nh phn), nhng kh c phng php m ho di lot no hot ng
tht tt tr phi hu ht cc lot chy u di.
Vic m ho di lot cn n cc biu din ring bit cho tp tin v cho bn
c m ho ca n, v vy n khng th dng cho mi tp tin, iu ny c th hon
ton bt li, v d, phng php nn tp tin k t c ngh trn s khng dng
c i vi cc chui k t c cha s. Nu nhng k t khc c s dng m ho
cc s m, th n s khng lm vic vi cc chui cha cc k t . Gi s ta phi m
ho bt k k t no t mt bng ch ci c nh bng cch ch dng cc k t t bng
ch ci . minh ho, gi s ta phi m ho bt k mt chui no t mt ch ci ,
ta s gi nh rng ta ch c 26 ch ci trong bng ch ci (v c khong trng) lm
vic.
c th dng vi ch ci biu din cc s v cc k t khc biu din cc phn
t ca chui s c m ho, ta phi chn mt k t c gi l k t "Escape". Mi
mt s xut hin ca k t bo hiu rng hai ch ci tip theo s to thnh mt cp
(s m, k t) vi cc s m c biu din bng cch dng k t th i ca bng ch
ci biu din s i. V vy, chui v d ca chng ta s c biu din nh sau vi Q
c xem l cc k t
Escape"QDABBBAABQHCDABCBAAAQDBCCCDT hp ca k t "Escape", s m v mt k t lp li c gi l mt dy Escape.
Ch rng khng ng m ho cc ng chy c chiu di t hn bn k t, v t
nht l cn n ba k t m ho bt k mt lot chy no.
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Trong trng hp bn thn k t "Escape" xut hin trong dy k t cn m ho ta
s dng mt dy "Escape" vi s m l 0 (k t space) biu din k t "Escape".
Nh vy trong trng hp k t "Escape" xut hin nhiu th c th lm cho tp tin nn
phnh to hn trc.
Cc lot chy di c th c ct ra m ho bng nhiu dy Escape, v d, mt
lot chy gm 51 ch A s c m ho nh QZAQYA bng cch dng trn.
Phng php m ho di lot thng c p dng cho cc tp tin ho
bitmap v thng c cc mng ln cng mu c biu din di dng bitmap
l cc chui bit c ng chy di. Trn thc t, n c dng trong cc tp
tin .PCX, .RLE.
II.2.1.2. M hnh t in
II.2.1.2.1 Thut ton LZ78
Thay v thng bo v tr on vn lp li trong qu kh, m LZ78 nh s tt c
cc on vn sao cho mi on ghi nhn s hiu on vn lp li trong qu kh cng
vi mt k t m n lm cho on khc vi on trong qu kh. Nh vy mi on
mi l mt on k t trong qu kh cng vi mt k t trong qu kh. Chnh v thon mi khc vi on c trong qu kh.
V d: Gi s ta c on vn bn sau: aaabbabaabaaabab
Theo thut ton LZ78 th chng c phn on nh sau:
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Input A Aa b Ba baa baaa bab
on 1 2 3 4 5 6 7
output 0+a 1+a 0+b 3+a 4+a 5+a 4+b
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Nh vy bn nn ca chng ta l: (0,a); (1,a); (0,b); (3,a); (4,a); (5,a); (4,b)
Thut ton nn:
Bc 1: c mt k t -> ch, on c gn bng 1, kt np k t vo t in,
w=ch;
Bc 2: While not eof(f) do
Begin
c tip k t tip theo w:= ww+ch;
If w thuc t in then ww:=w;
Else begin
Code(w,j);
Ghi j v ch vo tp nn.
Thm w vo t in.
End;
End;
Bc 3: Dng chng trnh.
Thut ton gii nn
Bc 1: c thng tin v t in c lu trong tp nn, tl:=false;
Bc 2: while not eof(f) do
Begin
c byte tip theo -> b
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Decode(b,s,t);
If tl=false then w:=w+s
Else w:=ww+s;
TIMCHU(w,t);
If t=false then
Begin
Ghi s ra tp gii nn Thm s vo t in
End
Else Begin
ww:=s;
End; End;
Bc 3: Dng chng trnh.
nh gi: Ni chung thut ton LZ78 l mt thut ton nn vn bn kh tt, c thi
gian chy chng trnh tng i nhanh tuy nhin kh nng tit kim cha c khai
thc.
II.2.1.2.2 Thut ton LZW
Gii thut nn LZW xy dng mt t in lu cc mu c tn sut xut hin cao
trong nh. T in l tp hp nhng cp t vng v ngha ca n. Trong , t vng s
l cc t m c sp xp theo th t nht nh. Ngha l mt chui con trong d liu
nh. T in c xy dng ng thi vi qu trnh c d liu. S c mt ca mt
chui con trong t in khng nh rng chui tng xut hin trong phn d liu
c. Thut ton lin tc tra cu v cp nht t in sau mi ln c mt k t d
liu u vo.
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Do kch thc b nh khng phi v hn v m bo tc tm kim, t in
ch gii hn 4096 phn t dng lu ln nht l 4096 gi tr ca cc t m. Nh
vy di ln nht ca t m l 12 bits (4096 = 212 ). Cu trc t in nh sau.
0 0
255 255
256 256| Clear Code
257 257| End of Information
258 Chui mi
4095 Chui mi
256:M xo CC khc phc tnh
trng mu lp ln hn 4096, nu mu lp
ln hn 4096 th gi CC xy dng tin cho phn tip theo.
Eoi: Bo hiu ht mt phn nn.
- 256 t m u tin theo th t t 0255 cha cc s nguyn t 0255.
y l m ca 256 k t c bn trong bng m ASCII.
- T m th 256 cha mt m c bit l m xo (CC- Clear Code). Mc
ch vic dng m xo nhm khc phc tnh trng s mu lp trong nh ln hn 4096.
Khi mt nh c quan nim l nhiu mnh nh, v t in l mt b t in gm
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nhiu t in con. C ht mt mnh nh ngi ta li gi mt m xo bo hiu kt
thc mnh nh c, bt u mnh nh mi ng thi khi to li t in cho mnh nh
mi. M xo c gi tr l 256.
- T m th 257 cha m kt thc thng tin (EOI End of information).
M ny c gi tr l 257. Nh chng ta bit, mt file nh GIF c th c cha nhiu
nh.Mi mt nh s c m ho ring.Chng trnh gii m s lp li thao tc gii m
tng nh cho n khi gp m kt thc thng tin th dng li.
- Cc t m cn li (t 258 n 4095) cha cc mu thng lp li trong
nh. 512 phn t u tin ca t in biu din bng 9 bit. Cc t m t 512 n 1023
biu din bi 10 bit, t 1024 n 2047 biu din bi 11 bit v t 2048 n 4095 biu
din bi 12 bit.
Nguyn tc hot ng ca n nh sau:
- Mt xu k t l mt tp hp t hai k t tr ln.
- Nh tt c cc xu k t gp v gn cho n mt du hiu (token) ring.
- Nu ln sau gp li xu k t , xu k t s c thay th bng du hiuca n.
- Phn quan trng nht ca phng php nn ny l phi to mt mng rt
ln dng lu gi cc xu k t gp (Mng ny c gi l "T in"). Khi cc
byte d liu cn nn c em n, chng lin c gi li trong mt b m cha
(Accumulator) v em so snh vi cc chui c trong "t in". Nu chui d liu
trong b m cha khng c trong "t in" th n c b sung thm vo "t in" v
ch s ca chui trong "t in" chnh l du hiu ca chui. Nu chui trong b m
cha c trong "t in" th du hiu ca chui c em ra thay cho chui dng
d liu ra.
Qu trnh nn:
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LZW bt u bi 1 t in 256 k t (trong trng hp s dng bng m 8 bits) v
s dng chng nh tp k t chun. Sau mi ln c n c 8 bits (v d 't', 'r', ...) v
m ha thnh con s tng ng vi ch mc ca k t trong t in.
Mi khi LZW i qua 1 chui con mi (gi s "tr") th n thm chui con vo t
in;
Mi khi n i qua 1 chui con m n thy trc , n ch c thm 1 k t mi
na v cng vi chui con bit to ra 1 chui con mi. Ln tip theo LZW bt
gp mt chui con c, n ch c vic s dng s ch mc tng ng trong t in.
Thng th ngi ta s nh sn s lng ln nht cc t trong t in (gi s
4096), v th vic nn LZW khng lm tiu tn ht ton b b nh. V vy m ca cc
chui con trong v d ny l 12 bits (2 ^ 12 = 4096). Cn thit phi lp m di hn s
bits ca mt k t (12 vs 8 bits), do o khi rt nhiu chui con lp li s c thay th
bi mt m duy nht th vic nn c thc hin.
V d: Cc bc m ho chui "ABCBCABCABCD" nh sau:
Cc bc thc hin.
- Bc 1: w = NIL;
- Bc 2: Trong khi c c k t th k trong chui:
- Bc 3: Nu wk tn ti trong t in th w=wk
- Bc 4: Cn khng th thm wk vo trong t in, m ho ng ra cho
w,w=k
- k=k+1
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Count W K wk symbol index output
0 Nil A A
1 A B AB AB 258 65
2 B C BC BC 259 66
3 C B CB CB 260 67
4 B C BC
5 BC A BCA BCA 261 259
6 A B AB
7 AB C ABC ABC 262 258
8 C A CA CA 263 67
9 A B AB
10 AB C ABC
11 ABC D ABCD ABCD 264 262
12 D NIL D 68
Chui ra: 65- 66- 67- 259 -258- 67 (output)
u vo kch thc: 12 x 8 = 96 bits.
u ra kch thc l: 5 x 8 + 3 x 9 = 67 bits
T l nn l: 96 /67 =1.43
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II.2.2 PHUNG PHP NN TN HAO
II.2.2.1 Phng php nn nh MPEG.
MPEG (Moving Picture Expert Group) c ra i vo nm 1988 nhm mc
ch chun ho cho nn tn hiu m thanh v video. MPEG - 1 c th nn tn hiu video
ti 1.5Mbit/s vi cht lng VHS v m thanh lp th (stereo audio) vi tc 192 bit/
s. N c dng lu tr video v m thanh trn CD-ROM.
Vo nhng nm 1990, MPEG-2 ra i nhm p ng cc tiu chun nn
video cho truyn hnh. MPEG-2 c kh nng m ho tn hiu truyn hnh tc 3-
15Mbit/s v truyn hnh nt cao tc ti 15-30Mbit/s. MPEG-2 cho php m
ho tn hiu video vi nhiu mc phn gii khc nhau, chng c kh nng p ng
cho nhiu ng dng khc nhau. Nhiu thut ton tng ng vi nhiu cc ng dng
khc nhau pht trin v c tp hp li thnh mt b tiu chun y ca MPEG.
Vic p dng ton b cc c im ca chun MPEG-2 trong tt c cc b m ho v
gii m l khng cn thit do s phc tp ca thit b cng nh s tn km v di thng
ca ng truyn V vy trong hu ht cc trng hp ta ch s dng mt phn nht
nh trong ton b cc c im ca chun MPEG-2, chng thng c gi l profiles
v levels. Mt profile s xc nh mt thut ton (iu chnh bitstream v phn gii
mu) v mt level s xc nh mt s tiu ch bt buc cho cc tham s ca bc nh (v
d nh kch thc nh v s lng bit).
MPEG-4 tr thnh mt tiu chun cho nn nh k thut truyn hnh s, cc ng
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 truyn hnh cp, Internet video...) vo nm 1999. Ngy nay,
MPEG-4 tr thnh mt tiu chun cng ngh trong qu trnh sn xut, phn phi v
truy cp vo cc h thng video. N gp phn gii quyt vn v dung lng chocc thit b lu tr, gii quyt vn v bng thng ca ng truyn tn hiu video
hoc kt hp c hai vn trn.
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MPEG khng phi l mt cng c nn n l m u im ca nn nh dng
MPEG chnh l ch MPEG c mt tp hp cc cng c m ho chun, chng c th
c kt hp vi nhau mt cch linh ng phc v cho mt lot cc ng dng khc
nhau.
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 frame b m ho (temporal
prediction), b chuyn ng b gii m (motion compensation) v m lng t ho
(quatisation 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
ca mt ngi. K thut on chuyn ng 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 mtthi im nht nh s c nhiu kh nng ging vi cc frame ng ngay pha trc v
ngay pha sau n. Cc b m ho s tin hnh qut ln lt tng phn nh trong mi
frame gi l macro blocks, sau n s pht hin macro block no khng thay i t
frame ny ti frame khc. B m ho s tin on trc s xut hin ca cc macro
blocks khi bit v tr v hng chuyn ng ca n. Do ch nhng s thay i gia
cc khi trong frame hin ti (motion compesated residual) v cc khi c tin on
mi c truyn ti bn pha thu. Pha bn thu tc b gii m lu tr sn nhng
thng tin m khng thay i t frame ny ti frame khc trong b nh m ca n vchng c dng in thm mt cch u n vo cc v tr trng trong nh c
khi phc.
Nh chng ta u bit, nn tn hiu video c thc hin nh vic loi b c s
d tha v khng gian (spatial coding) v thi gian (temporal coding). Trong MPEG,
vic loi b d tha v thi gian (nn lin nh) c thc hin trc ht nh s dng
cc tnh cht ging nhau gia cc nh lin tip (Inter-frame techniques). Chng ta c
th s dng tnh cht ny to ra cc bc nh mi nh vo nhng thng tin t nhngnh gi trc n (predicted). Do vy pha b m ho, ta ch cn gi nhng bc
nh c thay i so vi nhng nh trc, sau ta li dng phng php nn v khng
gian loi b s d tha v khng gian trong chnh bc nh sai khc ny. Nn v
khng gian da trn nguyn tc l pht hin s ging nhau ca cc im nh (pixels)
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ln cn nhau (Intra-frame coding techniques). JPEG ch p dng phng php nn theo
khng gian v n c thit k x l v truyn cc nh tnh. Tuy nhin nn tn hiu
theo phng php ca JPEG cng c th c dng nn cc bc nh mt cch c
lp trong dy tn hiu video. ng dng ny thng c gi l JPEG ng (MotionJPEG). Trong mt chu k gi mt dy cc bc nh theo kiu JPEG ng, nh u tin
c nn nh s loi b d tha v khng gian, sau cc nh tip theo c nn
nh s loi b d tha v thi gian (nn lin nh). Qu trnh c lp i lp li cho
mt dy cc bc nh trong tn hiu video.
Thut ton nn MPEG cng da trn php bin i DCT cho cc khi nh 8x8
picxels tm ra s tha v khng gian mt cch c hiu qu gia cc im nh trong
cng mt bc nh. Tuy nhin, trong trng hp c mi tng quan cht ch gia ccim nh trong cc bc nh k tip nhau tc l trong trng hp hai bc nh lin tip
c ni dung trng nhau, k thut Inter-frame coding techniques s c dng cng
vi vic tin on s d tha v khng gian to thnh k thut tin on b chuyn
ng gia cc bc nh (Motion compesated prediction between frames). Trong nhiu
s nn MPEG, ngi ta thng kt hp c vic tin on b chuyn ng theo
thi gian v php bin i thng tin theo khng gian t hiu qu nn cao (Hybrid
DPCM/DCT coding of video).
Hu ht cc s nn MPEG u dng k thut ly mu b xung
(Subsampling) v lng t ho (Quantization) trc khi m ho. Ly mu b xung
nhm mc ch lm gim kch thc bc nh u vo theo c theo chiu ngang v
chiu dc, nh vy s gim s lng cc im nh trc m ho. Cng nn nh rng
trong mt s trng hp ngi ta cn ly mu b xung theo thi gian lm gim s
lng cc bc nh trong dy nh trc khi m ho. y c xem nh l mt k thut
rt c bn nhm loi b s d tha da vo kh nng lu nh ca mt ngi cm th.
Thng thng, chng ta c th phn bit s thay i v sng ca nh (changes inBrightness) tt hn so vi s thay i v mu (Chromaticity changes). Do trc ht
cc s nn MPEG s tin hnh chia bc nh thnh cc thnh phn Y (Luminance
hay brightness plane) v UV (Chrominance hay color planes) tc l mt thnh phn v
sng v hai thnh phn v mu. Cc tn hiu video thnh phn ny s c ly
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mu (samples) v s ho (digitised) to nn cc im nh ri rc theo t l 4 : 2 : 2
v 4 : 2 : 0.
K thut tin on b chuyn ng c s dng nh l mt trong nhng cng cmnh lm gim s d tha v khng gian gia cc bc nh. Khi nim v b
chuyn ng l da trn s phn on hng chuyn ng ca cc bc nh tc l
cc nh thnh phn trong dy video s c thay th gn ng. K thut tin on b
chuyn ng gia cc bc nh c xem nh l bin php hn ch bt cc thng s
ca chuyn ng bi vic dng cc vector chuyn ng m t s dch chuyn ca
cc im nh. Kt qu tin on tt nht ca mt im nh l da trn s tin on b
chuyn ng t mt bc nh m ho c truyn pha trc ca n. C hai thng
s, sai s chuyn ng (bin ) v cc vectors chuyn ng (hng chuyn ng)u c truyn ti pha bn nhn. Tuy nhin do c mi quan h tng quan cht ch
gia cc im nh v khng gian (trng v khng gian), mt vector chuyn ng c
th c dng cho mt khi cc im nh gm cc pixels ln cn nhau (MPEG -1 v
MPEG -2 dng cc khi 16 x16 pixels).
Trong MPEG-2, c nhiu phng php tin on s chuyn ng. V d mt
khi nh c th c tin on xui t nhng nh c truyn trc n (Forward
Predicted), c th on ngc t nhng nh truyn sau n (Backward Predicted) hoc
theo c hai chiu (Bidirectionally Predicted). Cc phng php dng tin on cc
khi trong cng mt nh cng c th khng ging nhau, chng c th thay i t khi
n sang khi kia. Hn na, hai trng (fields) trong cng mt khi cng c th c
tin on theo hai cch khc nhau dng cc vector c lp nhau hoc chng c th
dng chung mt vector. i vi mi khi nh, b m ho s chn cc phng php
tin on thch hp, c gng m bo cht lng nh tt nht khi c gii m trong
iu kin yu cu kht khe v s bit. Cc thng s lin quan ti chn phng php tin
on cng c truyn ti b gii m cng vi d on sai s nhm khi phc gnchnh xc nh gc.
Trong MPEG, c 3 kiu nh khc nhau c dng m ho cho cc khi nh.
Kiu nh Intra (I-pictures) l nh c m ho mt cch c lp m khng cn tham
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kho ti cc nh khc. Hiu qu nn tn hiu t c do loi b s tha v khng gian
m khng c yu t thi gian tham gia vo qu trnh. I-pictures c dng mt cch
tun hon to thnh cc im ta cho dng d liu trong qu trnh gii m.
nh Predictive (P-pictures) c th s dng cc nh I hoc P ngay st pha
trc n b chuyn ng v chnh n cng c th c dng tham kho cho vic
tin on cc nh khc tip theo. Mi khi nh trong P-picture c th hoc c m
theo kiu tin on (predicted) hoc c m mt cch c lp (intra-coded). Do s
dng c nn theo khng gian v thi gian, hiu qu nn ca P-pictures c tng ln
mt cch ng k so vi I-pictures.
nh Bidirectionally-Predictive pictures hay B- Pictures c th s dng cc nh
I hoc P pha trc hoc pha sau n cho vic b chuyn ng v do vy cho kt qu
nn cao nht. Mi khi trong B-pictures c th c tin on theo chiu ngc, xui,
c hai hng hoc c m mt cch c lp. c th tin on ngc t mt bc
nh pha sau n, b m ho s tin hnh sp xp li cc bc nh t th t xut hin mt
cch t nhin sang mt th t khc ca cc nh trn ng truyn. Do vy t u ra
ca b m ho, B-pictures c truyn sau cc nh dng tham kho pha trc v
pha sau ca n. iu ny s to ra tr do phi sp xp li thng tin, tr ny ln
hay nh l tu thuc vo s cc bc nh B-pictures lin tip nhau c truyn.
Cc nh I, P, B-pictures thng xut hin theo mt th t lp i lp li mt cch
tun hon, do ta c khi nim v nhm cc bc nh GOP (Group of Pictures). Mt
v d ca GOP dng nh t nhin xut hin theo th t nh sau:
B1B2I3B4B5B7B8P9B10B11P12
Th t xut hin ca chng trn ng truyn b thay i do s sp xp li cab m ho nh sau:
I3B1B2P6B4B5P9B7B8P12B10B11
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Cu trc ca mt GOP c th c m t bi hai tham s: N l s cc nh trong
GOP v M l khong cch gia cc nh P-pictures. Nhm GOP ny c miu t nh
N = 12 v M = 3.
S CA B M HO V GII M DNG MPEG-2
Hnh 1. S b m ho v gii m dng MPEG
M ho MPEG-2
Qu trnh m ho cho P pictures v B pictures c gii thch nh sau:
D liu t cc khi nh (macroblocks) 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 khi nh mi c a vo ny vi cc khi nh c a vo trc
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v c lu li nh l cc nh dng tham kho (Reference Picture). Kt qu l b
on chuyn ng s tm ra cc khi nh trong nh tham kho gn ging nht vi khi
nh mi ny. B on chuyn ng sau s tnh ton vector chuyn ng (Motion
Vector), vector ny s c trng cho s dch chuyn theo c hai chiu dc v ngangca khi nh mi cn m ho so vi nh tham kho. Chng ta lu rng vector chuyn
ng c phn gii bng mt na do thc hin qut xen k.
B on chuyn ng cng ng thi gi cc khi nh tham kho ny m chng
thng c gi l cc khi tin on (Predicted macroblock) ti b tr tr vi khi
nh mi cn m ho (thc hin tr tng im nh tng ng tc l Pixel by pixel). Kt
qu l ta s c cc sai s tin on (Error Prediction) hoc tn hiu d, chng s c
trng cho s sai khc gia khi nh cn tin on v khi nh thc t cn m ho.
Tn hiu d hay sai s tin on ny s c bin i DCT, cc h s nhn c
sau bin i DCT s c lng t ho lm gim s lng cc bits cn truyn. Cc
h s ny s c a ti b m ho Huffman, ti y s bits c trng cho cc h s
tip tc c lm gim i mt cch ng k. D liu t u ra ca m ho Huffman
s c kt hp vi vector chuyn ng v cc thng tin khc (thng tin v I, P, B
pictures) gi ti b gii m.
i vi trng hp P-pictures, cc h s DCT cng c a n b gii mni b (nm ngay trong b m ho). Tn hiu d hay sai s tin on c bin i
ngc li dng php bin i IDCT v c cng thm vo nh ng trc to nn
nh tham kho (nh tin on). V d liu nh trong b m ho c gii m lun nh
vo b gii m ni b ngay chnh bn trong b m ho, do ta c th thc hin thay
i th t cc bc nh v dng cc phng php tin on nh trnh by trn.
Gii m MPEG-2
Qu trnh khi phc li nh 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 b bin i ngc IDCT
bin tn hiu t min tn s thnh tn hiu min khng gian. i vi P pictures
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v B pictures, vector chuyn ng s c kt hp vi cc khi tin on (predicted
macroblock) to thnh cc nh tham kho.
II.2.2.2 Phng php nn m thanh
Mc ch:
Biu din chui s ngn gn.
Tc bit thp.
Cht lng cao
ng c:
Gim tc d liu.
Gim chi ph truyn dn (BW).
Gim cc yu cu lu tr.
Cc yu cu:
Cm nhn trong sut.
c lp ngun.
C kh nng a knh.
tr hp l.
v K thut nn MPEG1:
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Hnh 2. Phn lp m ha MPEG 1
Gii thiu
c pht trin trn c s phi hp chun ISO/IEC 11172.
S dng tn s ly mu ca CD-DA, vi fs=32;44.1;48kHz, m ho 16bits/mu
tn hiu.
Tc bt: 32 - 768 kbps/channel.
Cc kiu: Mono, dual-mono, dual-stereo, joint-stereo.
Xc nh cc tham s khc nhau v tc , dng s sau khi nn, s mu trong
header cho mt knh, cu trc thi gian khung, phng php m ho d on v cc
ch lm vic.
c tnh:
Lp I Lp II Lp III
Dng cho thit b dn dng Dng cho thit b chuyn
dng, a mi trng
Dng cho thit b chuyn
dng, a mi trng
Tc dng s liu t 32-
448kbps
Tc dng s liu t 32-
384kbps
Tc dng s liu t 32-
320kbps
384mu/khung/knh 1152mu/khung/knh 1152mu/khung/knh
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MPEG-1
Lp I Lp II Lp III
Mono v Stereo
32, 44.1, 48kHz
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Lc bng con 0
Lc bng con 1
Lc bng con 31
Lc bng con 2
Cc mu
Audio
ng vo
12 mu 12 mu 12
mu12 mu 12 mu 12
mu
12 mu 12 mu 12
mu
12 mu 12 mu 12
mu
Khung
lp I
Khung lp II
v lp III
Hnh 3. Cc mu Audio Khung lp I : 12x32 =384.
Khung lp II, III: 12x32x3=1152.
Kin trc:
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Hnh 4. Kin trc 3 lp ca MPEG-1
Thut ton c bn
Tin hnh chia ng vo thnh 32 bng con bi cc bng lc.
Ly 32 mu PCM trong cng mt thi im, kt qu l 32 h s
tn s ng ra. Trong MPEG-1 lp I th tp 32 gi tr PCM c kt hp vo trong khi
gm 12 nhm 32 mu ny.
MPEG-1 lp II v lp III th gm 3 khi 12 nhm ny.
Phn b bit m bo rng mi nhiu lng t nm di cc ngng
che.
Vi mi bng con, xc nh mc bin v mc nhiu bng m hnh
tm sinh l nghe. SMR (signal-mask rate) c s dng xc nh s
bit cho qu trnh lng t ho i vi mi bng con vi mc ch gim
thiu dung lng.
Trang 32
Bng lc
phn tch a
pha 32 knh
Lng t
ho
M ho MU
X
FFT
LI: 512
LII: 1024
Phn tch
tm sinh l
m hc
Phn phi
bit ng
32
D liu
Thng
tin thm
32
s(n)
Bng lc
phn tch a
pha 32 knh
MDCT
FFT
Phn tch tm
sinh l m
hc
SMR
32
s(n)knh
Vng lp ch nh bit
Lng t ho
M ho Huffman
M thng
tin thm
32
MPEG1 lp1,2
MPEG1 lp3
SMR (Signal Mark
Rate): T s tn
hiu/ngng che
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Phn phi bit
L th tc xc nh s bit cho mi bng con.
Da vo thng tin vo t m hnh tm sinh l nghe
V d: Sau khi phn tch, mc ca 16 bng con u l:
Band 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Level (db) 0 8 12 10 6 2 10 60 35 20 15 2 3 5 3 1
Nu mc ca bng con th 8 l 60 th n che 12 dB bng con th 7 v 15 dB bng
con th 9.
Bng con 7 c mc 10dB15dB: gi i.
Ch c cc mc ln hn mc che l c gi i thay v dng 6 bits m
ho, ta ch dng 4 bits.
MPEG-Layer I: B lc DCT 1 khung v tn s bng phng trong mi bng con. M
hnh tm sinh l nghe s dng che tn s.
MPEG-Layer II: C 3 khung trong b lc (trc, hin ti v k), tng l 1125 mu. S
dng vi bits che thi gian.MPEG-Layer III: S dng b lc ti hn p ng tt hn. M hnh tm sinh l nghe
s dng che thi gian, che tn s, tnh ton d tha stereo v m ho Huffman.
Cu trc khung:
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Hnh 5. Cu trc khung MPEG
Header: Gm 12 bits ng b; 20 bis thng tin h thng ch th tc bit
CRC vi a thc sinh x16+x15+x2+1.
Side Info: Gm phn b bit: lp 1 vi 4 bits tuyn tnh cho cc bng con, lp II
4 bits cho cc bng con tn thp, 3 bit tn trung v 2 bits tn cao; h s t l l
6 bits/bng con kt hp vi phn b bits v cc bits m ha cho bng con
xc nh gi tr, lp III m ha m thanh ni. Bit Reservoir: Bit cung cp, cc mu d liu t 1 hoc 2 khung trc.
Samples: 32x12 mu i vi lp I v 32x36 mu i vi lp II v lp III.
Ancillary Data: D liu b sung
v K thut nn MPEG2:
M rng MPEG-1 cho cc ng dng mi.
C kh nng p dng nhiu tc khc nhau, t 32 n 1066kbps. Tn s lymu c th gim 1 na so vi MPEG-1 (16; 22,05; 24kHz).
Kh nng a knh, tc bits m rng c th ln n 1 Mbps cho cc ng dng
tc cao. Cho php nn ng thi nhiu knh.
Cht lng m thanh tu thuc ng dng.
H tr kh nng lng ting, bnh lun nhiu ngn ng trong phn bits m rng
(7 knh).
MPEG-2 s dng m ho cng cao, gim xuyn m, m ho d on linknh v m ho o nh knh trung tm nhn c tc bit kt hp 384
kbps.
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Khung MPEG-2 c chia thnh 2 phn, phn u l MPEG-1stereo, phn m
rng MPEG-2 cha tt c nhng d liu surround khc.
Hnh 6. Cu trc khung MPEG 2
M ha v gii m MPEG2:
Hnh 7. S m ha v gii m MPEG 2
CHNG 3
XY DNG CHNG TRNH NN D LIU
BNG M SHANNON-FANO V HUFFMAN
Trang 35
Mono-stereo
MPEG-1
32;44.1;48kHz
MPEG-2
Layer ILayer II
Layer III
Mono-stereo
MPEG-2
16;22,05;24kHzLayer ILayer II
Layer III
5 channels
MPEG-2
multi channel
32;44.1;48kHz
Layer ILayer II
Layer III
Matrix
MPEG-1
encoder
MPEG-2
Extension
encoder
L
C
R
LS
RS
L0
R0
T3
T4
T5
+
MPEG-1
decoder
MPEG-2
Extension
decoder
L0
R0
T3
T4
T5
Inverse
Matrix
L
C
R
LS
RS
channel
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II.3.1 C IM CA CC THUT TON NN D LIU
Trong gii hn tm hiu nn d liu di dng mt tp tin em nhn thy ccphng php nn c dng ph bin trn c nhng c im ng ch : thut ton
nn di lot (Runlength) khng th p dng cho nhiu loi tp tin c, v d nh
tp tin chng trnh, tp tin c s d liu... v cc lot chy l rt ngn, do nu
p dng thut ton ny khng nhng khng lm b tp tin m cn lm phnh to chng.
Thut ton nn LZW c cc u im l h s nn tng i cao, trong tp tin
nn khng cn phi cha bng m. Nhc im ca thut ton ny l tn nhiu b nh,
kh thc hin da trn cc mng n gin (b hn 64KB).
Cc thut ton khc nh Huffman, Shannon-Fano, LZ77 v LZW u c th
p dng c nn nhiu loi tp tin trn cc my vi tnh.
Thut ton Shannon-Fano c h s nn tng i tt vi cc file dng text
nhng do c tnh ca thut ton v yu cu kh phc tp nn him khi c s dng.
Thut ton Huffman c u im l h s nn tng i cao, phng php thc
hin tng i n gin, i hi t b nh, c th xy dng da trn cc mng b hn
64KB. Thng c s dng trong cc cng on cui cun nn m thanh, hnh nh v
video.
T cc c tnh thut ton ca cc loi m nn trn, em nhn thy rng thut
ton m ha ca Shannon-Fano v Huffman c nt tng ng. Tuy nhin s khc
nhau v thut ton lm cho hiu sut nn (h s entropy) ca Huffman v Shannon-
Fano khc nhau. Do vy, em s tp trung nghin cu, xy dng m phng v nh gi
2 loi m nn ny lm r u v nhc ca 2 loi m nn trn.
II.3.2 PHN TCH THUT TONII.3.2.1 Thut ton Shannon-Fano:
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M Shannon-Fano l mt thut ton m ha dng nn d liu. N da trn bng tn
sut xut hin cc k t cn m ha xy dng mt b m nh phn cho cc k t
sao cho dung lng (s bt) sau khi m ha l nh nht.
Cc tp tin ca my tnh c lu di dng cc k t c chiu di khng i l 8 bits.
Trong nhiu tp tin, xc sut xut hin cc k t ny l nhiu hn cc k t khc, t
ta thy ngay rng nu ch dng mt vi bit biu din cho cc k t c xc sut xut
hin ln v dng nhiu bit hn biu din cho cc k t c xc sut xut hin nh th
c th tit kim c di tp tin mt cch ng k. V d, m ho mt chui nh
sau: "ABRACADABRA"
Nu m ho chui trn trong dng m nh phn 5 bit ta s c dy bit sau:
0000100010100100000100011000010010000001000101001000001
gii m thng ip ny, ch n gin l c ra 5 bits tng thi im v chuyn
i n tng ng vi vic m ho nh phn c nh ngha trn. Trong m chun
ny, ch D xut hin ch mt ln s cn s lng bit ging ch A xut hin nhiu ln.
Ta c th gn cc chui bit ngn nht cho cc k t c dng ph bin nht, gi s ta
gn: A l 0, B l 1, R l 01, C l 10 v D l 11 th chui trn c biu din nh sau:
0 1 01 0 10 0 11 0 1 01 0
V d ny ch dng 15 bits so vi 55 bits nh trn, nhng n khng thc s l mtm v phi l thuc vo khong trng phn cch cc k t. Nu khng c du phn
cch th ta khng th gii m c thng ip ny. Ta cng c th chn cc t m sao
cho thng ip c th c gii m m khng cn du phn cch, v d nh: A l 11,
B l 00, C l 010, D l 10 v R l 011, cc t m ny gi l cc t m c tnh prefix
(Khng c t m no l tin t ca t m khc). Vi cc t m ny ta c th m ho
thng ip trn nh sau : 1100011110101110110001111
Vi chui m ho ny ta hon ton c th gii m c m khng cn du phncch. Nhng bng cch no tm ra bng m mt cch tt nht ?
- Bc u tin trong vic xy dng m Shannon-Fano l m s ln xut hin ca mi
k t trong tp tin s c m ho (trong phn ny ch v d hn ch mt s k t).
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- Bc tip theo l xy dng bng m da vo thut ton Shannon-Fano. Thut ton
ny c thc hin bng cc bc sau:
Bc 1: Sp xp th t cc lp tin tng (hay gim), ngun tin c thng k t tp tincn thc hin m ha.
Bc 2: Chia tng ngun tin ra lm 2 nhm sao cho tng ca mi nhm xp x bng
nhau (hiu ca 2 nhm l nh nht).
Bc 3: Gn cho mi nhm k hiu l 0 hay 1.
Bc 4: Lp li bc 2 cho n khi ch cn li 1 nhm tin.
- Sau khi c bng m Shannon-Fano, ta c th m ha cc gi tin. u im ca
phng php m ho Shannon-Fano l t c h s nn tng i cao (H s nn
tu thuc vo cu trc ca cc tp tin). Nhc im ca phng php ny l bn
nhn mun gii m c thng ip th phi c mt bng m ging nh bng m
bn gi, do khi nn cc tp tin b h s nn khng c cao.
- gii m gi tin, nh ni trn ta phi c bng m. Ln lt ta so snh tng
nhm tin, nu trng vi bng m th ta c c gi tin nh ban u.Chng trnh m ha ngun dng m Shannon-Fano l chng trnh dng thut
ton Shannon-Fano (c trong l thuyt truyn tin hay k thut truyn s liu) gii
quyt bi ton m ha theo xc sut xut hin ca cc k t c trong bng m ASCII.
Sau , da vo bng m ny ta c th m ha cc chui k t hay dng vn bn ra
thnh m my tnh gm cc k t 0 v 1. Cc k t c m ha s c di khc
nhau, tuy nhin xt v tng th th di ca chui hay ca vn bn c m ha s
ngn hn khi ta cha m ha. Nh vy, khi lu tr s t tn b nh hn cng nh khi
truyn tin s chim t bng thng hn.
Cc chc nng ca hthng/chng trnh :
Tng th cu trc ca chng trnh ln lt s thc hin nh sau :
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1. Khai bo cc hng, bin ton cc, cc cu trc s thc hin trong sut chng
trnh.
typedef struct Node
{
char kytu;
float xacsuat;
char code[32];
};
Node a[256]; // bng m ASCII gm 256 k t, bin a cha mng cc k t trong
bng m, l bin ton cc.
2. Khai bo cc chng trnh con hay cu trc ca cc chng trnh con :
a) Hm read: c file v thng k xc sut hin ca n : Ta tin hnh c cc
k t t file ri tng s ln xut hin ln, lu vo trong mng a. Mng a l
ni lu tr s ln xut hin ca k t c trong file, v tr trong mng a l
m ASCII ca k t .
b) Hm sapxeptang : sp xp theo s ln xut hin ca k t trong mng a
theo hng tng dn.c) Hm mahoa : Hm ny s m ha ngun dng m Shannon-Fano cc k
t c s ln xut hin ln hn 0 trong file. Mi k t u c 1 m bng nh
phn ring.
d) Hm giaima: Gii m file m ha theo bng m.
e) Hm insapxep: In cc k t sau khi sp xp theo xc sut xut hin ra
mn hnh tin theo di.
f) Hm thuchien: y l hm lm cng vic chnh ca chng trnh. Hm ny
s thc hin cng vic m ha file ri lu vo file khc m ca chng trnhnn.
g) Hm main: cha cc thng tin cn thit ca ti v thc hin gi cc
chng trnh con.
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V d : cho ngun tin gm 10 lp tin nh sau :
X A B C H N U G O M I
Px 0.20 0.11 0.08 0.10 0.07 0.09 0.13 0.06 0.12 0.04
a) Hm sp xp : Hm ny s tin hnh sp xp li cc k t theo xc sut xut
hin c trn theo th t gim dn nhm phc cho qu trnh m ha cc
k t ny theo thut ton Shannon-Fano.
X A G M B H U C N O I
Px 0.20 0.13 0.12 0.11 0.10 0.09 0.08 0.07 0.06 0.04
b) Hm m ha : Hm ny c nhim v ti quan trng i vi bi ton ny.
Hm s m ha cc k t nhp hay cho trc sao cho cc m ny khng
c trng nhau. Cc bc thc hin nh sau :
Bc 1 : Da vo th t sp xp trn chia mng a ra lm 2 phn. iu
kin l hiu ca tng cc xc sut c trong tng phn s l nh nht.
Bc 2 : Gn cho mi k t ca nhm u gi tr 0 vo trong phn code
ca n (k t) v nhm th 2 gi tr 1.
Bc 3 : Lm li bc 1 vi nhm th nht sau l vi nhm th 2.
iu kin dng l khng th chia nh c na (mng ch cn 1 phn
t).
Nh vy, sau khi hon tt cc bc trn, ta c c 1 bng m vi cc ch
ci c gn vi mt m tng ng nht nh, cc m ny s khng trng
nhau.V d : Bng m sau khi c m ha :
X A G M B H U C N O I
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Px 0.20 0.13 0.12 0.11 0.10 0.09 0.08 0.07 0.06 0.04
Code 00 010 011 100 1010 1011 1100 1101 1110 1111
Trnh t cch phn chia nh sau :
X Px Code
A 0.20 0 0
G 0.13 0 1 0
M 0.12 0 1 1
B 0.11 1 0 0
H 0.10 1 0 1 0
U 0.09 1 0 1 1
C 0.08 1 1 0 0
N 0.07 1 1 0 1
O 0.06 1 1 1 0
I 0.04 1 1 1 1
c) Hm gii m : Hm ny s da vo bng m trn gii m mt chui
gm cc k t 0 v 1. Hm c kim tra xem c phi d liu nhp vo c
ng nh yu cu khng v cc k t phi c trong bng m thng k
trn. Trnh t cc bc nh sau :
Bc 1 : Kim tra xem k t u tin c trong bng m hay khng. Nu
c th lu vo trong mng d v tip tc lm bc 1 nhng v tr u tin l v
tr k tip (cha kim tra) , nu khng c th tip tc bc 2. Bc 2 : Kim tra cp k t u tin v k t th 2 xem c trong bng
m hay khng. Nu c th lu vo trong mng d, v quay li bc 1 vi k
t u tin l k t k tip, nu khng c th lm li bc 2 vi 3, 4,,n (s
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phn t ca chui nhp vo) v tr k tip.
Bc 3 : Kim tra xem nu kim tra n phn t cui cng m khng
thy trng vi phn t no trong bng m th pht thng bo D liu nhp
vo b li. Lm n khi khng cn phn t no trong mng th dng. Bc 4 : in cc k t va c gii m hoc thng bo li ln mn hnh.
d) Hm main : Hm ny hin th thng tin ngi thc hin ti. Cc la
chn m ha file hay gii m. Hin ln mn hnh thng bo ngi s
dng bit thc hin m ha hay gii m xong cha, file m ha hay gii
m c lu u.
II.3.2.2 Thut ton Huffman:
Thnh lp cy nh phn t tp hp cc k hiu trong thng bo, mi k hiu
l mt nt l ca cy. Cch thnh lp cy nh sau :
Chn hai nt a,b c xc sut nh nht trong tp hp cc nt, gi s xc sut nt a nh
hn hoc bng xc sut nt b. Thnh lp cy nh phn c nt gc x, con tri l a, con
phi l b. Nt x c xc sut bng tng xc sut ca a v b.
Tp hp cc nt by gi l cc nt cn li ( loi b a, v nt x. Lp li mt
cch qui qu trnh trn tp hp ang xt cho n khi tp ny ch cn li mt nt.
M ca a, b s tm c bng cch ly m ca x ni thm 0 cho a v 1 cho b.
M ca nt gc l rng.
Nh vy thc cht qu trnh trn l ta xy dng mt cy nh phn t tp hp
cc k t mun m ho, cui cng ta c mt cy nh phn c l l cc k t . M
ca mt k t l mt ng i trn cy t gc n l cha k t, vi 0 i sang tri cn
1 i sang phi. tng ca gii thut m ho cng ht sc n gin, ta tm b m cho
cc k t sao cho cc k t c tn sut xut hin cao (xc sut xut hin l ln) s cm ngn (gn vi gc) di trung bnh m ho mt k t l nh nht.
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V d:
Cho bng tn sut ca 5 ch ci A,B,C,D,E nh sau tng ng l 0.10; 0.15; 0.30;
0.16; 0.29.
A B C D E
0.10 0.15 0.30 0.160.2
9
Qu trnh xy dng cy Huffman din ra nh sau:
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Hnh 8. Bc 1 v Bc 2
Hnh 9. Bc 3 v Bc 4
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Hnh 10. Bc 5
Nh vy b m ti u tng ng l:
A B C D E
010 011 11 00 10
- c file vn bn, lu vo mng a.
- Xy dng cy Huffman gii m dng vn bn
- Hin th cy Huffman v bng m Huffman ra mn hnh
- Thc hin m ha dng vn bn v di m
- M rng m ha v gii m mt file vn bn. Kt qu gii m v m ha c ghi
vo 2 file vn bn khc.
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Cu trc chng trnh:
- File huffman.cpp dng ci t cy huffman. N bao gm cc hm:
- mahoa_chuoi(): M ha chui.
- giaima_chuoi(): Gii m chui.
- general(): Ci t cy Huffman, c s dng trong cc hm trn.
- make_table(): To bng m cho cc k t, c s dng trong cc hm m ha.
- quit(): Cc thao tc cui cng trc khi return.
Cc vn chung trong xy dng chng trnh:
Cc cu trc d liu s dng trong chng trnh:
Code:
typedef struct {
char kytu;
int tansuat;
char code[MAXBITS];}hlist;
- Kiu d liu ca mng a[] cha tp k t (a[i].kytu), tn s tng ng
a[i].tansuat) v chui m ha k t (a[i].code)
Code:
typedef struct{
char kytu;
int contrai,conphai,nutcha;int tansuat;
int isleft;
}hnode;
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- Kiu d liu ca mng node[] dng ci t cy Huffman. Cc node tng ng vi
k t (node[i].kytu nu c). Node.contrai, node.conphai, node.nutcha tng ng l ch
s ca nt con tri, con phi, v nt cha (Nu khng c th c gi tr l -1).Node.tansuat cha tng tn s cc nt l thuc nhnh ca n. Node.isleft bng 1 nu
nt l con tri ca cha n, bng 0 nu l con phi, bng -1 nu l nt gc (goc).
Ci t cy Huffman t tp k t c t file:
- Th tc general()
- Sp xp li cc a[i]
- Khi to cc node[i] (i=0 n n-1), node[i].kytu v node[i].tansuat tng ng vi
a[i].kytu v a[i].tansuat sau khi sp xp. Cc thnh phn cn li c gi tr l 1(cha xc nh).
- To cy Huffman bng cch chn thm nt mi ng thi sp xp li theo th t tn
s tng dn.
Cc vn khc cn gii quyt:
1. Lp b k t (a[i].kytu) v tn s tng ng (a[i].tansuat) t mt xu k t (s):
- c k t u tin ca xu cho vo a[0].kytu tng ng l a[0].tansuat bng 1.
- Duyt tng k t cn li ca xu, nu gp k t no c trong mng a[i].kytu thtng a[i].tansuat ln 1, nu cha c k t th thm phn t mi vo mng v cho tn
s tng ng bng 1.
2. Lp tp k t trong file text v tn s tng ng: (Tng t vi xu k t)
3. Xy dng bng m Huffman (lu trong a[i].code): Th tc make_table()
- Duyt tt c cc node[i] (i t 0 ti 2*n-2)
- Nu gp node no c cha k t (node[i].kytu khc NULL) th duyt ln ti root
c xu m ha (a[i].code) vit theo chiu ngc li.
- o ngc a[i].code bng hm strrev(), ta c xu m ha ca k t tng ng.
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4. M ha:
- c tng k t ca chui (hoc file), gp phn t no th hin th xu m ha tng
ng hoc ghi thm xu m ha tng ng ca k t vo file m ha (fileout).
5. Gii m:
- Goc tr vo node gc (goc=2*n-2)
- Duyt cy Huffman t trn xung, gp 0 th nhy xung con tri, gp 1 th nhy
xung con phi, cho ti khi gp node c thnh phn kytu khc -1.
- Nu gp node c thnh phn kytu khc -1 th hin th k t ca node v nhy v
gc (goc=2*n-2).
II.3.2.3 Lu thut ton :
II.3.2.3.1 Lu chng trnh m phng thut ton Shannon-Fano:
-
Begin
Lu thut ton hm main:
Gii thiu ti
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File tt
End
T
- Lu thut ton m ha Shannon-Fano.
Trang 49
Gi hm read c d liu
trong file v thng k ngun tin
Nhp tn file cn m
ha
Gi hm sapxeptang sp xp
ngun tin theo tn sut xut hin.
Gi hm insapxep m ha
ngun tin v in bng m ra mn
hnh
Gi hm thuchien thc hin
m ha ngun tin, a ra t l nn
F
T
Begin
Mng a (ngun tin thng k)
Phn mng a ra thnh 2 phn (phn u t
0n i, phn sau t i+1n n-1 )vi tng
xc sut ca mi phn c hiu l nh nht
Mng ch c 1
pt
Gn gi tr 0 vo phn code
cho phn t
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- Lu thut ton m ha Huffman :
Trang 51
T
Begi
n
Nhp chui
ly t file
m s ln xut hin mi k
t trong chui
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CHNG 4
KT QU TH NGHIM V NHN XT
II.4.1 KT QU M PHNG
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Lu vo 1 file cha d liu l k t
v tn sut xut hin tng ng
Sp xp cc k t theo th t tn
sut tng dn (hm general)
To bng m cc k t dng cy
Huffman (hm make_table)
Xut
chui
c
m haEnd
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Hnh11. mn hnh m phng m Shannon-Fano vi v d 1
Hnh 12. Thng k tn sut xut hin v bng m ca Shannon-Fano trong v d 1
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Hnh 13. Hin th t l nn ca m Shannon-Fano trong v d 1
Hnh 14. Gii thiu ti dng m Huffman
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Hnh 15. Menu iu khin chng trnh nn m Huffman
Hnh 16. Hin th trc quan thng tin nn ca m Huffman trong v d 1
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Hnh 17. Mn hnh gii thiu nn file vidu7.txt vi m Shannon-Fano
Hnh 18. Thng k d liu trong v d 7
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Hnh 19. Kt qu nn Shannon-Fano vi v d 7
Hnh 20. Giao din chng trnh nn m Huffman vi v d 7
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Hnh 21. Thng k d liu trong v d 7 ca m Huffman
Hnh 22.Kt qu chng trnh nn m Huffman trong v d 7
II.4.2 CC KT QU
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Hnh 23. Thng k t l nn Shannon-Fano v Huffman vi 10 v d khc nhauII.4.3 NHN XT V NH GI
Series1 : M Shannon-Fano (ct mu nht)
Series2 : M Huffman (Ct mu m)Hnh 24. Biu so snh t l nn ca Shannon-Fano v Huffman
u im, nhc im ca phng php m ho Shannon-Fano v Huffman
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u im:
- Thut ton nn Shannon-Fano v Huffman l thut ton nn khng tn
hao. Do vy, d liu sau khi gii nn l nguyn vn.
- Hai thut ton nn trn u s dng phng php nn d liu bng cch
m ha cc k t sao cho s ln xut hin ca k t t l nghch vi s bits c m
ha, lm cho d liu c lu tr hay truyn i c kch thc nh hn rt nhiu so vi
ban u.
- Tc nn nhanh do vic xy dng bng m khng phi mt nhiu thigian.
- S dng hai thut ton nn trn khng lm tn b nh nhiu, vic lu tr
d dng.
- Nhn vo bng thng k v biu , ta c th thy c t l nn file text
ca 2 m nn ny l rt tt, file c kch thc cng ln th t l nn cng cao (nht l
m nn Huffman).
Nhc im:
- Vic nn d liu l tng i tt i vi cc loi file vn bn, tuy nhin n
khng hiu qu i vi cc loi file khc.
- Bng m lun phi i km vi d liu nn phi mt mt khong vng nh
cho n.
- Lun lun phi duyt thng tin n 2 ln. Mt ln c d liu to bng
m v mt ln m ha.
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- Vic m ha file c kch thc nh thng khng cho t l nn tt so vi
file c kch thc ln hn.
nh gi tnh hiu qu ca phng php Shannon-Fano v Huffman :
Vi file text c dung lng nh v va th t l nn chnh lch ca 2 m ny
l khng cao, thng th m Huffman c t l nn cao hn Shannon-Fano nhng
khng ng k.
Vi nhng file text c nhiu k t ging nhau th t l nn l tng ng nhau.
Vi nhng file text c dung lng tng i ln tr ln th chng ta c th
nhn thy rng m nn Huffman vt tri v kh nng nn d liu do thut tonnn ca Huffman gim c lng tin tt hn. V vy, m ha file text c dung
lng ln th nn chn m nn Huffman cho t l nn tt hn rt nhiu so vi m
Shannon-Fano.
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PHN 3
KT LUN V HNG PHT TRIN
Vi phn m phng thut ton nn d liu vi m Shannon-Fano v Huffman,
em phn no c kt c u v nhc im ca 2 thut ton ny qua vic so snh
thut ton v t l nn ca n. Em cng tm hiu, thng k c mt s thut ton
nn khc vi cc d liu khc nhau nh file vn bn, file audio v video...Sau y l
nhng mt lm c v nhng mt cn hn ch ca ti ny:
Nhng mt lm c :
-M phng thnh cng 2 loi m nn Shannon-Fano v Huffman bng
chng trnh C.
- a ra c t l nn ca tng loi, so snh 2 m nn ny v a ra ckt lun.
- Tm hiu, nghin cu cc thut ton nn khc.
- Nn thnh cng vi nhng file text c kch thc ln.
Nhng mt cn hn ch :
- Cha thc hin c nn d liu ln hn 8 bits nh m ting Vit (c du)
hoc cc k t c bit.
- Cha thc hin nn c vi cc file d liu khc ngoi file text.
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- Cha thc hin c vic so snh mt cch tng qut v trc quan ca tt
c cc loi m nn hin nay, dn n vic cha ua ra c kt lun loi m
nn no l ti u, m nn no l thng dng nht hin nay.
- Cha thc hin c vic nn cc loi d liu khc nh nn folder
Hng pht trin ca ti:
Trong thi gian thc hin n theo quy nh, em thc hin c cc cng
vic nh nu cc phn trn. Mt s ni dung trong phn hn ch em s tip tc
hon thnh trong thi gian ti nh nn d liu vi nhiu dng file khc, nn d liu vi
folder v nghin cu, ci tin thut ton nn nhanh hn, c giao din p mt hn v
d s dng vo thc t hn.
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TI LIU THAM KHO
[1]. Thut ton trong tin hc V c Thi NXB KHKT
[2] Gio trnh l thuyt m Nguyn L Anh, Nguyn Vn Xut, Phm Th Long
Trng HDL ng 1997
[3] The Data Compression Book 2nd edition - Mark Nelson and Jean-loup Gailly
[4]. Ngn ng lp trnh C - Quch Tun Ngc.
[5]. Ngn ng lp trnh C++ - Nhm Ngc Anh Th Press
[6]. Gio trnh Multimedia GV Nguyn Duy Nht Vin (Lu hnh ni b)
[7]. Cc ngun t Internet.
