dethitailieuhoctapnlu.files.wordpress.com · Web viewMỗi khung con có 4 bit phụ: Bit giá...
If you can't read please download the document
Transcript of dethitailieuhoctapnlu.files.wordpress.com · Web viewMỗi khung con có 4 bit phụ: Bit giá...
Gio trnh mn hc: K thut Audio- Video s
CHNG I: TNG QUAN V AUDIO- VIDEO S
1.1. TNG QUAN V AUDIO S:1.1.1 Tn hiu AUDIO:
T cui th k 19, m thanh in t c pht trin di dng h thng my in thoi. Sang u th k 20, cc my quay a v my pht thanh m ra nhng ng dng mi cho m thanh in t, n mc gn nh cc gia nh trong th gii vn minh ngy nay u s hu mt vi thit b phc v i sng tinh thn ny.
1. m thanh t nhin:
m thanh l nhng bin i p sut nhanh xy ra trong khng kh do nhiu qu trnh t nhin gy nn. Ting gi thi trn cnh cy, ting sng bin v b, ting chim ht tt c u l m thanh t nhin. Nhiu h thng do con ngi ch to cng to ra nhng bin i p sut tng t, i khi l c ch nh song i khi l khch quan.
Tai con ngi phn ng li vi nhng bin i p sut khng kh phm vi tn s trong khong t 30 Hz n 15.000 Hz sau a n no v chnh l m thanh. ln hay bin dao ng ca nhng bin i p sut ny to nn cm gic v ting n.
2. Ti to m thanh t nhin:
m thanh in t c gi chung l Audio. m c thu t ngun nh mt hoc nhiu micro v nhng tn hiu audio thu c truyn qua h thng cho n khi ti loa pht.
+
=
=
)
,
(
)
,
(
)
,
(
)
,
(
v
u
Q
v
u
Q
v
u
F
v
u
Fq
2
v)/
Q(u,
v)
F(u,
nht
gn
n
nguy
tr
Ga
Hnh 1.1: H thng ti to m thanh in t
Mc ch ti to m thanh in t l ti cc sng m thanh n nhng khong cch xa c v khng gian v thi gian, ngi nghe c th tip nhn nh th h ang nghe m trc tip t ngun. Mt mc ch khc na l chau chut m thanh t nhin, lm cho m thanh in t hay hn m gc, hoc to ra nhng m thanh mi khng c trong t nhin.
1.1.2. Tn hiu AUDIO s:
Van theo truyen thong, cac Audio c so hoa theo 3 cong oan sau:
1. Lay mau:
Qua trnh lay mau thc hien viec nhan tn hieu audio tng t vi chuoi xung co thi gian lap lai vi tan so lay mau. o la qua trnh ieu bien xung (PAM) trong mien thi gian hnh 1.2 va trong mien tan so hnh 1.3.
Hnh 1.2: Qua trnh ieu bien trong mien thi gian
Khi tan so lay mau fs
2fmax th khong co s xuyen nhieu gia hai pho, ch can mot loc (LPF) la tach c tn hieu goc.
- Khi fs < fmax tn hieu qua LPF b xuyen nhieu goi la Aliasing noise khong chap nhan c. Fmax la tan so cao nhat cua tn hieu nguyen thuy, Audio fmax = 20 KHz
chon fs = 44.1 KHz, fs la tan so lay mau.
- Trc khi thc hien ADC, phai gii han bang tan audio en fs, neu khong co the gay ra hien tng chong pho va meo tn hieu khoi phuc.
- Xung lay mau phai co thi gian hep, bang mot chu ky lay mau
s
f
1
.
Hnh 1.3: Qua trnh ieu bien trong mien tan so
Trong thc te bo ADC, gia tr bien o xung cua moi mau c gi en khi nao at mau tiep theo, hnh 7.4, 7.5.
Hien nay thng dung 4 chuan tan so lay mau cho tn hieu Audio:
- 32 KHz (tieu chuan chuyen dung) dung trong truyen dan va phat thanh FM stereo.
- 44.1 KHz (tieu chuan dan dung ) : Dung trong VCR, CD, R_DAT player.
- 48 KHz (tieu chuan phat thanh) : Tieu chuan phat thanh bang tan rong va truyen hnh chat lng cao.
- Lay mau tan so cao (over sampling) 4fs e giam meo lng t
SNR[db] = 6.02n + 1.76 + 10log10d
Vi d : he so lay mau tan so cao (d = 4)
N : So bit / mau lng t.
2. Lng t hoa (Quantizing):
Sau qua trnh lay mau la qua trnh lng t hoa. Cac mc bien o c ri rac tng mc bien o vi gia tr nh phan tng ng. Neu n bit lay mau se co 2n khoang lng t.
s
m
Hnh 1.6: Qua trnh lng t hoa 4 bit
Khoang lng t la khong oi xng vi cac chuyen dch dng va am cua tn hieu audio goc.
So mc lng t cang tang th mo lng t cang tang th meo lng t cang giam.
Goi Q la khoang lng t hoa (
2
2
Q
Q
-
+
) ta co t so tn hieu / nhieu lng t SNR la :
SNR =
5
,
1
2
12
2
2
1
n
n
Q
Q
N
V
=
=
-
Vi tn hieu Sine V =
2
2
1
Q
n
-
[volt rms]
Nhieu lng t N =
12
Q
[Vms]
SNR [db] = 6,02n + 1,76
Vi n la so bit lng t hoa (so bit/mau lng t hoa)
3. Ma hoa (coding):
Moi gia tr lng t hoa nh phan can phai c ma hoa e phu hp vi loai tn hieu lay mau, truyen dan va ghi am. He thong thng dung la PCM (ieu bien ma xung), PWM (ieu che o rong xung), ADM (ieu che delta thch nghi), DPCM (ieu xung ma vi sai), floating point (iem di ong ).
Loai PCM c dung nhieu v n gian, tuy nhien t hieu qua. ac iem cua no la thc hien tuyen tnh cho tac ca cac khoang lng t hoa. Cac mc lng t hoa c gan bang cac t ma theo mot trat t logic. Hnh 1.7 la s o khoi la qua trnh ma hoa va giai ma PCM.
Hnh 1.7 : S o khoi n gian hoa cua he thong PCM
1.1.3. Giao din s AES/EBU:
1. C trc c bn ca giao thc AES/EBU
Tiu chun tn hi video s AES/EBU l mt giao thc cho php cc thit b s c th pht v thu tn hiu audio s.
Khung d liu c to bi hai khung con (khung con A v B). Mi mu d liu c phi hp t mt ngun audio hoc mt knh, d liu ph, d liu m u, cc thng tin v gi tr (V), thng tin thm vo tr gip ngi s dng d liu (U), thng tin v cc tham s h thng (C) v bt chn l (P) pht hin sai s truyn kim tra tin cy ca knh.
Cc khung d liu audio c nhm li thnh cc khi (block) gm 192 khung. Mt tn hiu c c gi i trong dng d liu cho php nhn bit mi block. Khong thi gian tn ti mt khung audio l 20,83 s trong h thng ly mu 48 KHz. Do thi gian tn ti mt khi AES/EBU l: 20,83 s x 192=4000 s.
s
m
D liu m u gm 4 bit, hay cn gi l t ng b, c dng nhn bit s bt u ca mt mu mi v ca mt block mi. Ba t cn li c s dng trong:
- T ng b Z: dy bt ny cho bit s bt u ca khung u tin ca mt khi (block) audio mi. T ny to ra c Z.
- T ng b X: t ny cho bit s bt u ca tt c cc khung con A.
- T ng b Y: t ny cho bit s bt u ca mi khung con B.
Mi khung audio gm 2 khung con, trong mi khung gm 32 bit. Mi mu tn hiu audio, theo l thuyt c th gm 16 n 20 bt hoc ti 24 bt khi 4 bt ph c coi nh cc bt mu tn hiu audio. Bt ph cho bit tnh trng cc dy bit trong knh, n nm trong byte 2, cc bt 0-2.
Mi khung con c 4 bit ph:
Bit gi tr (V): bit ny cho bit cc bit d liu ly mu m thanh trc c ng hay khng.
Bit s dng (U): bit ny cho bit mi thng tin thm vo tr gip ngi s dng d liu.
Bit knh (C): cng ging nh bit s dng, bit knh c mt trong cc khung ph, n c gi ti cc hng i c to bi 28 hng. Ni dung ca hng i rt quan trng trong vic xc nh ni dung ca t d liu audio.
Bit parity (P): bit parity c t ch nh mt parity chn. Bit parity cho php pht hin s l ca li do qu trnh truyn dn. Mt s thit b b qua nhng bit ny hoc thi hnh khng chnh xc trong qu trnh ch nh.
2. Cc c im giao din knh chun AES/EBU:
c im ca nh dng giao din s dng AES/EBU c ch ra trong bng sau:
+ nh dng:truyn dn ni tip ca hai knh s liu c ly mu v m
ho tuyn tnh.
+ Cc thng s pht:tn hiu ra khng cn bng.
Cc b ni: gic cm m thanh RCA.
Bin tn hiu ra: 500mVnh-nh ng vi ti 75( (khng cn bng).
+ Cc thng s thu:Tn hiu vo khng cn bng.
Cc b gic cm m thanh RCA.
Tr khng vo 75(.
nh dng knh ngi s dng ny c dng trong cc thit b CDs v RDATs vi tn hiu vo ra s.
1.2 Tng quan v VIDEO s:
1.2.1. Tn hiu VIDEO:
1.2.2. S ha tn hiu VIDEO:
1. Lay mau tn hieu Video so
Tn hieu vao fv co tan so fb se c bien oi t hnh bao dang song qua cac xung hep vi chu ky lay mau TS. Tan so lay mau la :
S
S
T
f
1
=
Do pho cua tn hieu sau lay mau co the chong len nhau tao thanh nhieu aliasing nen tan so lay mau phai thoa nh ly lay mau, tc la fs
2fb. Vi tan so nay se bao am tai tao lai c tn hieu nguyen thuy khi thc hien qua trnh bien oi so tng t.
Hnh 7.8: Pho cua tn hieu sau khi lay mau
a. Tan so lay mau tn hieu Video tong hp (Composite Video Signal)
oi vi tn hieu video tong hp, tan so lay mau tn hieu video thong thng bang 2, 3 hoac 4 lan tan so song mang mau fsc.
Thong thng th ngi ta chon fs = 3fsc
s
m
NTSC : fs = 10.7 MHz
PAL : fs = 13.3 MHz
The nhng ngi ta thng dung fs = 4fsc
NTSC : fs = 14.32 MHz
PAL : fs = 17.72 MHz
Do tn hieu video so mang ay u nhng khuyet iem cua video tng t nen ngi ta thng s dung phng phap so hoa tn hieu thanh phan.
b. Tan so lay mau tn hieu Video thanh phan (Component Video Signal)
Vi tn hieu video thanh phan, tan so lay mau thng c lay thong qua t le tan so gia tn hieu choi va tn hieu mau. Thong thng co cac t le 4:1:1; 4:2:2; 4:4:4, ta se nghien cu ky phan sau.
2. Lng t hoa bien o tn hieu video (Quantizing).
ay la qua trnh bien oi tn hieu sau lay mau thanh cac khoang ri rac, goi la khoang lng t (Q).
Q = 2n
Vi n la so bit/mau.
Neu n = 8, th se co 28 = 256 khoang lng t.
Co hai cach lay khoang lng t:
- Tuyen tnh: cac khoang lng t cach eu nhau khong phu thuoc tn hieu analog vao.
- Khong tuyen tnh: Cac khoang lng t thay oi theo bien oi bien o cua tn hieu. Cac vung t bien oi th khoang cach lng t tha, cac vung bien oi nhieu th khoang cach lng t ngan.
Qua trnh lng t la lam tron cac nh xung vuong (bien o tn hieu a lay mau so snh vi mot mc lng t gan nhat). Nh vay se co sai so trong qua trnh lng t ha do lam tron. Sai so nay se
Q
2
1
.
Khi lng t hoa tuyen tnh, neu lng t vi so bit/mau thap se xuat hien sai so lng t >
Q
2
1
. Sai so nay se gay ra nhieu lam meo tn hieu va tao thanh ng vien (contouring effects).
Gia tr trung bnh (RMS Root Mean Square) cua moi sai so lng t co gia tr :
12
Q
RMS
=
Gia tr cua tn hieu ra (bo DAC) la:
(2n 1)Q
Ngi ta thng dung ty so tn hieu nh tren nhieu lng t
MRS
Q
S
e lam ch tieu anh gia mot thiet b so hoa.
[
]
dB
n
Q
Q
Q
S
n
RMS
8
.
10
02
.
6
12
2
log
20
10
+
=
=
Vi n = 8 th :
dB
Q
S
RMS
96
.
58
=
Thc te co rat nhieu yeu to anh hng en ty so
RMS
Q
S
nh :
- Viec han che fmax.
- Khoang lng t hoa tn hieu video tch cc
Do o, thc te ty so
RMS
Q
S
c tnh nh sau:
-
-
+
+
=
b
w
q
s
RMS
V
V
V
f
f
n
Q
S
10
max
10
log
20
2
log
10
8
.
10
02
.
6
Trong o :
max
10
2
log
10
f
f
s
la gia tr anh hng cua fmax
-
-
b
w
q
V
V
V
10
log
20
la anh hng cua khoang video tch cc.
N : la so bit/mau
Fs :la tan so lay mau
Fmax : la tan so video cc ai (4.2; 5; 5.5; 6 mhz)
Vq : la ien ap tn hieu toan bo khoang lng t [V]
Vw : la ien ap mc trang [V]
Vb : la ien ap mc xoa
+
=
=
)
,
(
)
,
(
)
,
(
)
,
(
v
u
Q
v
u
Q
v
u
F
v
u
Fq
2
v)/
Q(u,
v)
F(u,
nht
gn
n
nguy
tr
Ga
-
=
-
=
+
+
=
1
0
1
0
2
)
1
2
(
cos
2
)
1
2
(
cos
)
,
(
)
(
)
(
)
(
)
(
2
)
,
(
N
u
N
v
N
v
j
N
u
i
v
u
F
v
C
u
C
v
C
u
C
N
j
i
f
p
p
Vw Vb =
3. Ma hoa (Coding)
ay la bc sap xep cac gia tr so sau khi lng t hoa theo mot khung thuoc cau truc nao o.
4. Tieu chuan Composite so
a. Tieu chuan Digital Composite fs = 4fsc PAL (tieu chuan PAL 4fsc)
STT
Thong so
Gia tr cua PAL 625/50
1
So mau/dong
1135 (tr dong 313 va 625)
2
So mau/dong tch cc
948
3
Tan so laymau fs
Fs = 4fsc = 17.734475 mhz
4
Trang thai pha lay mau
+450;+1350;+2250;+3150
5
Ma hoa
Lng t hoa eu
6
o phan giai lng t hoa
8 hoac 10 bit/mau
7
Toc o d lieu
177.3Mb/s
Bang 7.1: Cac thong so cua tieu chuan PAL 4fsc
- Tai mau fsc (Burst) c thay oi pha lan lt theo hai gia tr +1350 va +225 va no tao ra xung lay mau fs.
- Tai mau PAL co cha mot tan so Offset la 25Hz. Nh vay fsc se la:
Fsc =
(
)
MHz
Hz
f
f
f
n
H
V
H
43361875
.
4
25
75
.
285
2
4
1
2
=
+
=
+
+
, vi n=567
- So chu ky tai mau trong mot anh (frame):
75
.
177344
25
=
SC
f
chu ky/frame
- Tan so lay mau fs co pha la 00 va truc U
Fs = 4fsc = 17.734475 mhz.
+ So mau trong mot dong c tnh gia hai xung ong bo ke nhau:
dong
Mau
Hz
MHz
f
f
H
S
0064
.
1135
15625
734475
.
17
=
=
.
+ So mau tong cong tren mot anh la :
mau
anh
Sodong
Dong
Somau
109379
625
0064
.
1135
=
=
.
Neu chon so mau/dong la 1135 th so mau/anh la 709375, trong o co 948 mau/dong tch cc va 187 mau/khoang xoa dong.
Hnh 7.9: Quan he so mau PAL 4fsc va mot dong tn hieu Analog
Can chu y rang dong th 313 va 625 nam trong khoang xoa manh.
Tren hnh 7.9 cho thay cac mau c tnh t 0
1134, trong o au tien la cac dong tch cc chiem 948 mau t 0
947, ke en la mc ong bo chiem 187 mau t 948
1134.
iem chuan ong bo nam gia mau 958.
Thi gian mot mau la:
ns
s
4
.
56
1134
64
=
m
* Lng t hoa
TT
Thong so PAL composite
o phan giai 8 bit
o phan giai 10 bit
1
Cac mc bao ve tn hieu mau
FF
3FC, 3FD, 4FE, 3FF
2
Mc lng t hoa cao nhat
FE
3FB
3
Mc nh mau
>FE
>3FB
4
Mc trang
D3
34C
5
Mc xoa, mc en
3C
100
6
Mc nh ong bo mau
04
016
7
Mc lng t hoa thap nhat
01
004
8
Cac mc bao ve ong bo
00
000, 001, 002, 003
Bang 7.2: Cac mc chu yeu cua composite soc mau tng t 100/0/100/0 va cac gia tr PAL 4fsc tng ng vi o phan giai 8 bit va 10 bit.
Hnh 7.10: Quan he gia cac mc cua Analog PAL soc mau 100% va PAL 4fsc
Bang 7.2 va hnh 7.10 minh hoa mot so thong so cua tn hieu soc mau composite tng t va cac mc PAL 4fsc tng ng trong trng hp o phan giai 8 bit va 10 bit.
* Cac mc lng t tng ng vi o phan giai 10 bit gom co 2n = 1024 mc t 0
1023 (he DEC) va t 000
3FF (he HEX) vi cac chc nang nh sau :
T 000
003 la mc bao ve, cha thong tin d phong ong bo, khong cha dong so.
T 004
3FB (hay mc 4
1019 DEC)dung bieu dien tnh hieu video so.
Mc 004
(HEXA) hay mc 4 (DEC) la nh xung ong bo cung la mc lng t hoa thap nhat.
Mc 100 hay 256 (DEC) la mc xoa cung la mc en, tng ng 0 mV.
Mc 34C (HEXA) hay 844 (DEC) la mc trang, tng ng 700 mV.
Mc 3FB la mc lng t hoa cao nhat.
Nh vay, co 1016 mc so (004
3FB hay t 4
1019 DEC) dung e bieu dien tn hieu video tong hp.
Cac mc t 3FC
3FF la cac mc bao ve.
Mc nh tn hieu mau vang (Yellow) va xanh cam thach (Cyan) la 933.5mV, nhng khi lay mau tn hieu vang la 886mV nho hn mc lng t cao nhat tng ng 908.3mV, nen ay ton tai mot headroom am tai nh tn hieu so.
ay, vi n = 10 bit/mau, fs = 17.73 MHz, fmax = 5 MHz, Vq = 1.2131 V, Vw Vb = 0.7 V, ta co
dB
Q
S
RMS
711
.
68
=
.
b. Tieu Chuan Digital Composite fs = 4fsc NTSC (tieu chuan NTSC 4fsc)
STT
Thong so
Gia tr NTSC 525/60
1
So mau/dong
910
2
So mau/dong tch cc
768
3
Tan so laymau fs
Fs = 4fsc = 14.32818 MHz
4
Cau truc lay mau
Trc giao
5
Trang thai pha lay mau
+330;+1230;+2130;+3030
6
Ma hoa
Lng t hoa eu
7
o phan giai lng t hoa
8 hoac 10 bit/mau
8
Toc o d lieu
143Mb/s
Bang 7.3: Cac thong so c ban cua NTSC 4fsc
* Tan so lay mau chuan fs = 14.3118 MHz
14.32 MHz, fh = 15734.25 Hz, so mau/dong la :
910
=
H
S
f
f
.
Co 525 38 = 487 dong tch cc, mot dong tch cc cha 768 mau.
* Khoang xoa dong la : 910 768 = 142 mau.
iem chuan ong bo la iem gia sn xung ong bo nam gia mau th 784 va 785.
Khoang video tch cc t 0
767
Tiep theo la khoang xoa t mau 768
909 (142 mau).
Chung c minh hoa tren hnh 7.11.
-
=
-
=
+
+
=
1
0
1
0
2
)
1
2
(
cos
2
)
1
2
(
cos
)
,
(
)
(
)
(
)
(
)
(
2
)
,
(
N
u
N
v
N
v
j
N
u
i
v
u
F
v
C
u
C
v
C
u
C
N
v
u
F
p
p
Hnh 7.11: Khoang xoa dong so NTSC 4fsc
TT
Thong so NTSC composite
o phan giai 8 bit
o phan giai 10 bit
1
Cac mc bao ve tn hieu mau
FF
3FC, 3FD, 4FE, 3FF
2
Mc lng t hoa cao nhat
FE
3FB
3
Mc nh mau
F3
3CC
4
Mc trang
C8
320
5
Mc en
46
11A
6
Mc xoa
3C
0F0
7
Mc nh ong bo mau
04
016
8
Mc lng t hoa thap nhat
01
004
9
Cac mc bao ve ong bo
00
000, 001, 002, 003
Bang 7.4: Cac mc chu yeu cua Composite soc tng t 100/7.5/100/7.5 va cac gia tr NTSC 4fsc tng ng vi o phan giai 8 bit va 10 bit
* Lng t hoa.
Cac mc lng t hoa theo o phan giai 10 bit gom 210 = 1024 mc t 0
1023 t di len co cac mc nh sau :
T 000
003 la khoang d phong e chen cac mc ong bo, khong cha cac dong so.
T 004
3FB (hay mc 1019 DEC) dung cho tn hieu so .
-
=
+
=
1
0
2
)
1
2
(
cos
)
(
)
(
)
(
2
)
(
N
k
N
k
m
k
C
k
X
k
C
N
m
x
p
Hnh 7.12: Quan he gia cac mc cua analog NTSC va
Cac gia tr mau digital 10 bit cac soc mau 100%
T 0 04
010 (4
16 DEC) la mc headroom ong bo.
Headroom nh t 3CC
3FB (927
1019).
T 3FC
3FF la mc bao ve nh.
Mc 3CC la mc tn hieu cao nhat (Yellow va Cyan).
Mc 0F0 la mc xoa .
Mc 11A la mc en.
Ty so tn hieu tren nhieu lng t c tnh :
ay, vi n = 10 bit/mau, fs = 14.33 MHz, fmax = 4.2 MHz, Vq = 1.3042 V, Vw Vb = 0.7143 V, ta co
dB
Q
S
RMS
09
.
68
=
.
* Ket Luan :
oi vi tn hieu video tong hp (NTSC hoac PAL) tan so lay mau cho tn hieu video thng la 2, 3 hoac 4 lan tan so song mang mau (2fsc, 3fsc hoac 4fsc).
Tn hieu video so tong hp con mang ay u nhng khuyet iem cua tn hieu video tng t nhat la hien tng can nhieu choi mau.
Tan so lay mau cang tang, chat lng video cang cao, tuy nhien tan so lay mau ln oi hoi thiet b, ng truyen phai co dai thong rong va cac bo nh co dung lng ln. Chi ph cho toan bo he thong do vay tang len nhieu lan. Tan so lay mau thch hp nam trong khoang t 12 MHz
14 MHz.
So hoa tn hieu video tong hp co toc o bit thap so vi phng phap so hoa tn hieu video thanh phan.
Tn hieu so tong hp boc lo nhieu nhc iem trong qua trnh x ly so, tao ky xao, dng hnh
5. Tieu chuan Component so.
Tieu chuan Component so c dung chau Au va chau My theo chuan Recommendation 601 CCIR. Tieu chuan nay tng thch vi tieu chuan quet 625/50 va 525/60 vi o phan giai 8bit/mau va 10 bit/mau.
a. Ty le lay mau
Cac to chc phat thanh va truyen hnh:
SMPTE (Society of Motion Picture ang Television Engineer) vi chuan ATSC (Advance Television standards Committee).
EBU (European Broadcasting Union) vi chuan DVB (Digital Video Broadcasting) a thong nhat chon chuan CCIR Rec. 601 t nam 1982 tren c s tan so chuan 3.375 MHz.
Ty le lay mau tieu chuan component so la t le ma o tn hieu Luminance (Y) va hai tn hieu mau CB va CR c lay mau cac tan so tng ng vi boi so cua tan so chuan 3.375 MHz. Thong thng co cac ty le lay mau sau :
4:1:1, trong o tn hieu Y co tan so lay mau fsy =
MHz
5
.
13
375
.
3
4
=
, hai tn hieu so mau c lay mau tan so fscb = fscr =
375
.
3
1
= 3.375 MHz.
4:2:2, trong o tn hieu Y co tan so lay mau fsy =
MHz
5
.
13
375
.
3
4
=
, hai tn hieu so mau c lay mau tan so fscb = fscr =
375
.
3
2
= 6.75 MHz.
4:4:4, trong o cac tn hieu Y, CB, CR co tan so la
MHz
5
.
13
375
.
3
4
=
.
b. Cac tn hieu ma hoa, thi gian va tan so lay mau chuan 4:2:2.
Cac tn hieu ma hoa Y, CB, CR c mo ta nh bang sau:
* Cac thong so ma hoa mau 4:2:2 cua tieu chuan 625/50 va 525/60.
Thong so
Chuan 4:2:2 cua 625/50
Chuan 4:2:2 cua 525/60
Cac tn hieu ma hoa sau khi a sa
g
Y = 0.587G + 0.114B + 0.299R
CB = 0.564(B Y)
CR = 0.713(R Y)
So mau/dong
Y : 864
CB : 432
CR : 432
Y : 858
CB : 392
CR : 429
So mau/dong tch cc
Y : 720
CB : 360
CR : 360
Cau truc lay mau
Trc giao
Cac mau CB, CR, lap lai theo dong, manh va anh cung vi cac mau Y le tren dong.
Tan so lay mau
Fsy = 846fh = 13.5 MHz
Fscb =fscr = 432fh = 6.75MHz
Fsy = 858fh = 13.5MHz
Fscb=fscr=429fh=6.75MHz
Loai ma ho
ieu xung ma (PCM), lng t hoa eu.
o phan giai lng t
8 hoac 10 bit/mau cho Y va cac tn hieu so mau
Bang 7.5: Cac thong so ma hoa 4:2:2 cua tieu chuan 625/50 va 525/60
* Thi gian va tan so lay mau 4:2:2.
He 625/50 co thi gian mot dong la 64
s
m
, thi gian mot dong tch cc la 52
s
m
, thi gian xoa dong la 12
s
m
.
He 525/60 co thi gian mot dong la 63.56
s
m
, thi gian mot dong tch cc la 52
s
m
, thi gian xoa dong la 11.56
s
m
.
Fsy = 13.5 cua hai he c tao t bo dao ong PLL-CO (Phase Locked Loop Controlled Oscillator). Vi he 625/50 fsy = 864
15625 Hz = 13.5 MHz. Con he 525/60 fsy =858fh = 858
15734.265 = 13.5 MHz.
Chung c ong bo t tn hieu video in. ac tnh cua mach loc Y bang chang en 5.75 MHz, con CB, CR at en 2.75 MHz.
c. Lng t hoa
* Thanh phan choi Y.
TT
Thong so cua Y component
o phan giai 8 bit
o phan giai 10 bit
1
Cac mc bao ve nh
FF
3FC, 3FD, 3FE, 3FF
2
Mc lng t hoa cao nhat
FE
3FB
3
Mc nh trang
EB
3AC
4
Mc xoa
10
040
5
Gia tr lng t hoa nho nhat
01
004
6
Cac mc bao ve di
00
000, 001, 002, 003
Bang 7.6: Mot so thanh phan Y tn hieu soc mau 100/0/100/0 va cac tn hieu so Y trong he HEX tng ng vi 8 bit va 10 bit
Cac mc lng t theo 10 bit gom 210 = 1024 mc t 0
1023 (he DEC) va t 000
3FF (he HEX) vi chc nang cua cac mc nh sau:
Hnh 7.13: Quan he gia cac mc tn hieu Analog Component Y cua
soc mau 100% va cac gia tr lay mau 10 bit va 8 bit
T 000
003 la 4 mc d phong di.
T 004
3FB la dong tn hieu so.
Mc 004 HEX hay 64 DEC la mc xoa.
Mc 3AC HEX hay 940 DEC la mc nh trang.
Phan xung ong bo cua tn hieu choi khong c lay mau.
Headroom di nho la khong d phong cho phep chnh cac mc analog, t mc 004
040 HEX hay 4
64 DEC.
T 3AC
3FB HEX hay 940
1019 DEC la mc headroom tren.
Mc 3FB HEX hay 1019 DEC la mc lng t cao nhat.
T 3FC
3FF HEX hay 1020
1023 DEC la 4 khoang d phong.
ay, vi n = 10 bit/mau, fs = 13.5MHz, fmax = 5.57 MHz, Vq = 0.8174 V, Vw Vb = 0.7 V, ta co
dB
Q
S
RMS
35
.
70
=
.
Vi n = 8 bit, ta co :
dB
Q
S
RMS
3
.
58
=
* Thanh phan CB va CR.
STT
Thong so CB va CR Component
o phan giai 8 bit
o phan giai 10 bit
1
Cac mc bao ve nh
FF
3FC, 3FD, 3FE, 3FF
2
Mc lng t cao nhat
FE
3FB
3
Mc nh dng
E0
3C0
4
Mc xoa
80
200
5
Mc nh am
10
040
6
Mc lng t thap nhat
01
004
7
Cac mc bao ve di
00
000, 001, 002, 003
Bang 7.7: Cac mc analog component CB va CR cua tn hieu soc mau 100/0/100/0 va cac gia tr cua CB va CR digital ng vi 8 bit va 10 bit
Can chu y rang cac tn hieu CB va CR la lng cc, do o phai dch mc 350mV e a vao bo ADC.
Tng t nh thanh phan choi, cac mc lng t hoa cua CB ch ro hnh 7.14a, cua CR hnh 7.14b.
Hnh 7.14b: Cac mc tn hieu CR
T le:
N = 10 bit/mau
Vq = 0.7792V
MHz
f
B
R
C
SC
75
.
6
=
Vw VB = 0.7V
dB
Q
S
RMS
74
.
70
151
.
1
889
.
0
71
7
.
0
7992
.
0
log
20
5
.
5
75
.
6
log
10
8
.
10
10
02
.
6
=
+
+
=
-
+
+
=
Vi n = 8 bit/mau
dB
dB
Q
S
RMS
7
.
58
698
.
58
d. Cau truc lay mau trc giao
Cac mau c sap xep tren cac dong ke nhau, thanh cac hang thang ng. Cau truc nay co nh theo manh (field) va theo frame.
-
=
+
=
1
0
2
)
1
2
(
cos
)
(
)
(
2
)
(
N
m
N
k
m
m
x
k
C
N
k
X
p
Cac hang thang theo chieu doc goi la cau truc mau trc giao.
Hnh 7.15: Cau truc mau trc giao
CHNG II: X L TRUYN DN AUDIO- VIDEO S
2.1. NN TN HIU AUDIO S
2.1.1. Gii thiu chung
Tn hiu audio s PCM c dng nhiu trong truyn hnh, multimedia v nhiu ng dng khc (cc h thng a ng audio , nh ht ...). Tn hiu m thanh stereo c phn gii 16 bit , ly mu 48 KHz, s cho tc d liu audio l 1,54 Mbit/s.
Audio s c nn c s dng trn CD-ROM, mng, pht thanh,truyn hnh s v tinh DBS...
Cc h thng nn thng tin audio da trn c trng tm sinh l nghe (m hc) v cc gii hn ca tai ngi loi b cc phn d liu khng cn thit (d tha ) trong tn hiu audio.
H thng thnh gic ca con ngi hot ng nh mt b phn tch ph, v phn ph ca m thanh nghe c c lp sau khi qua b lc thng gii, gi l cc di bng chun . Cc di bng chun c rng 100Hz khi tn s di 500 Hz, v khi tn s trn 500 Hz th di bng t l vi tn s. Hin nay ch s dng c to bi 25 di bng con tng ng vi 25 b lc c tai theo quy lut t nhin. Khi tn hiu audio c to ra t cc tn s gn nhau, h thng HAS t hp chng thnh cc nhm tng ng vi cng mt nng lng. Thng th cc tn s cch xa nhau c x l ring r, cc m lng tng ng ca chng c nh gi v xc nh .
nhy ca HAS gim i ti cc tn s thp v cao, th hin vic ng vin mc m lng trung bnh t l nghch vi cc tn s m thanh. C th nhn thy rng, ti cc mc m lng thp, mc thay i nhy ca HAS l rt quan trng v n gim i ti cc mc m lng cao.
2.1.2. K thut nn s liu audio
Cc k thut m ho ngun c dng loi b d tha trong tn hiu audio v cc k thut psychoacoustic- che mt n tm sinh l nghe c s dng dng nhn bit v loi b nhng thnh phn khng thch hp (cc mu m thanh li).
C hai k thut nn ch yu s dng hin nay l:
1.M ho d on trc trong min thi gian:
Phng php ny s dng vic m ho khc nhau i vi cc thnh phn khc nhau ca cc mu lin tip m c th khi phc c .Vic gim tc dng bit s c s dng m ho v truyn dn cc thng tin ca tn hiu audio.
2. M chuyn i trong min tn s:
K thut ny s dng cc khi block ca cc mu audio ra t b PCM u chuyn t min thi gian sang min thi gian sang min tn s nhng di bng khc nhau.
a.Nn khng tn hao.
Nn khng tn hao cho php khi phc li dng bit nhng thng tin nguyn thu sau b gii nn m khng gy ra tn hao. H thng ny loi b d tha thng k, nhng thng tin tn ti trong tn hiu audio c th d bo trc t nhng mu trc . B nn s liu khng tn hao cho cc t s nn thp, t s tt nht t c l 2:1 n ph thuc vo s phc tp ca tn hiu audio ngun.
Nn khng tn hao s dng nhng k thut m d on trc trong min thi gian bao gm:
- Thut ton vi sai: Cc tn hiu m thanh c c tnh l lp i lp li, v vy s xut hin lng d tha s liu ln, ngoi ra cn c nhng d tha nh cc tn hiu m thanh khng lin quan n gic quan con ngi. Nhng thng tin lp i lp li s c loi b trong qu trnh m ho v li a vo ti qu trnh gii m cui cng .K thut DPCM thng c s dng trong qu trnh ny . Cc tn hiu audio u tin c phn tch thnh tp hp cc di bng con bao gm mt s lng cc m thanh ri rc .Sau , DPCM c s dng nhm d bo trc cc tn hiu lp theo mt on chu k. Qu trnh m ho ny to ra s thch ng tr li nng lng tn hiu u vo nhm sa kch c bc lng t ph hp. Bc ny cng c gi l b thch ng DPCM ( ADPCM).
- Cc m entropy tn dng d tha trong cch miu t ca cc h s bng con lng t ho nhm ci thin tnh hiu qu ca qu trnh m ho. Cc h s lng t ny c gi i theo s tng dn ca tn s, kt qu l chng c gi tr ln ti nhng tn s thp v ti nhng tn s cao l mt dy di cc h s nh hoc bng 0. M c di thay i c s dng nh Haffman s to ra bng m ti u thng k cc gi tr min tn s thp v cao.
- Cc thng s qu ti khi d liu : Cc gi tr nh phn t qu trnh bin i ADC, c nhm thnh cc khi d liu trong min thi gian, bng cch ly cc mu k nhau ti u ra ADC, ln trong min tn s, bng cch ly cc h s tn s ti u ra FDCT. Cc gi tr nh phn trong mt khi d liu sau to thang tip, sao cho gi tr va di gi tr ton b thang. H s thang ny l chung cho tt c cc gi tr trong khi. Cho nn, mi gi tr c th biu din bng mt nh tr v bng mt s m(ch th bin rieng ca mu). l mt qu trnh lng t ho khng u ln bc lng t ho c xc nh bng s bit chim trong khi. V tr bit c tnh t m hnh HAS. Gim rng d liu c thc hin bng cch gi cc gi tr m ln 1/khi d liu. Mc d nhiu ph thuc vo ni dung tn hiu , nhng m ho c thc hin tt . K thut che mt n gip gim nhiu audio.
Hnh 4.8 : H thng m ho im qu ti khi d liu Audio.
b. Nn tn hiu c tn hao.
Nn c tn hao c cp n bi s kt hp ca hai hay nhiu hn cc cng ngh x l m n li dng c tnh ca h thng HAS l khng th phn bit c cc thnh phn ph c bin nh gia cc thnh phn ph c bin nh gia cc thnh phn ph c bin ln. Cc phng php gim s liu x l cao c h s nn t 2:1 n 20:1, n ph thuc vo qu trnh nn v gii nn, v vo yu cu cht lng ca audio. Cc h thng nn s liu c tn hao s dng cng ngh m ho tri gic. Nguyn l c bn ca n l loi b nhng thnh phn d tha trong tn hiu audio s bng cch b i nhng tn hiu nm di ng cong ngng m , iu ny gii thch ti sao ngi ta gi cc h thng nn s liu c tn hao l mt cc thnh phn m .
Nn lm mt cc thnh phn m c kt hp t cc k thut nh:
- K thut masking- che i vi cc thnh phn tn hiu trong min thi gian v min tn s. T l signal- mask c s dng xc nh s bit cho qu trnh lng t ho mi bng vi mc ch gim thiu kh nng nghe thy ca m thanh.
- Chn mc tp m lng t cho tng m ca tn hiu m thanh bng cch ch nh s bit va chc chn rng mc nhiu lng t lun lun nm di mc gi tr cn chn. Ti nhng tn s gn vi tn s tn hiu m thanh th t s tn hiu trn tp m SNR c th chp nhn c l t 20 n 30 dB, tng ng vi phn tch t 4 n 5 bit.
- M ho ni: Cng ngh ny khai thc s d tha trong h thng audio a knh, ngi ta thy rng c rt nhiu phn s liu trong cc knh l ging nhau, do ngi ta c th nn s liu bng cch m ho mt phn s liu chung trn mt knh v ch nh cho b gii m lp li tn hiu trn cc knh cn li.
2.1.3. Nn audio theo tiu chun MPEG
2.1. Chun MPEG-1
2
Cc mu audio
S2
S2
S1
S1
F1
1A
F1
1A
PMC@ Chuyn i t min M ho Bit ch nh B m Dng bit
D1
BRIDGE
D1
BRIDGE
D5
DIODE
D5
DIODE
thi gian sang khp - Lng t ho khung
32/44,1/48 KHz min tn s ni - M ho s liu m ho
D2
DIODE
D2
DIODE
D2
DIODE
D2
DIODE
D2
DIODE
D2
DIODE
(a)
m hnh tm S liu ph
sinh l nghe
(b)
Dng bit Khung khng Khi Chuyn i t Cc mu audio
m ho ng gi phc min thi gian PCM @
sang min tn s 32/44,1/48 KHz
S liu ph
Hnh 4.9: S cu trc m ho (a) v gii m (b) MPEG cho tn hiu Audio
Tiu chun MPEG-1 c pht trin ch yu dng cho m ho nh ng v tn hiu audio km theo cho lu tr vi tc 1,5Mbit/s. Chun MPEG-1 da trn nguyn l nn tn hiu audio ca tiu chun MPEG-1. Hnh v m t cu trc c s ca b m ho v gii m MPEG tn hiu audio.
Chun MPEG s dng tn s ly mu ca CD-DA(compact Dise Digital Audio) v DAT (Digital Audio Tape). Bn cnh cc tn s ly mu ny MPEGcn s dng cc tn s: 44,1kHz,48kHz,32kHz ly mu v tt c u s dng 16bits. i vi s liu audio trn a compact th cc knh u c tn s ly mu l 44,1kHz vi 16bit/mu, khi tc s liu audio l 1,4Mbit/s. Bi vy,chng cng cn phi c nn li. Thut ton nn tn hiu MPEG bao gm 3 bc:
- u tin tn hiu audio c chuyn v min tn s v ton b di ph ca n c chia thnh 32 bng con thng qua b lc bng con.
+ Lc bng con: Ph ca tn hiu c chia thnh cc bng con c rng di thng bng nhau . N tng t nh qu trnh phn tch tn s ca HAS, chia ph tn hiu audio thnh cc bng ti hn. rng ca cc bng ti hn c th thay i. Di 500Hz, rng di bng l 100 Hz v n tng ti vi KHz khi tn s trn 10 KHz. Di 500 Hz mt bng con c ti vi bng ti hn. Cc b bng lc bng con c mt phn nh gi ln nhau v thng thng s dng cho cc mu k nhau trong min thi gian. V d : trong mc II chun MPEG, mt khung audio c 1152 mu gc c chia thnh 32 bng con c rng bng nhau ( 750 Hz tn s ly mu 48 KHz ) , mi bng con c 36 mu.Mi tn hiu bng con sau c lng t ho u vi cc bit ch nh c trng nhm bo v di bng con bng t s tch cc masking trn tp m (MNR). T s ny c tnh tch cc khi ng cong cc mc chn trn mc tp m.
+ Bng chuyn i: Thut ton sa i DCT (MDCT) thng c s dng bin i tn hiu audio t min thi gian sang min tn s thnh mt s lng ln cc bng con ( t 256 n 1024). Ging nh lc bng con , trong bng chuyn i cc bng con nm k nhau.
+ Lc bng ghp: y l s kt hp gia cc b lc bng con v cc b lc bng chuyn i. u tin tn hiu c chia thnh 32 bng con bi b lc bng, sau thut ton MDCT c p dng cho 18 mu trong tng bng con, to ra tng cng 576 bng hp. t c phn tch v thi gian l 3,8 ms.
- Vi mi bng con ngi ta xc nh mc bin tn hiu v mc nhiu bng m hnh tm sinh l nghe. y l thnh phn chnh ca b m ho MPEG Audio v chc nng ca n l phn tch tn hiu vo.
- Cui cng mi bng con c lng t ha qua lng t cc thnh phn nghe thy trong mi bng. N i km vi m Hufman m ha cc gi tr ph tn hiu v cho nn s liu tt hn v nh dng s liu .
- Chun MPEG p dng vi audio a ra ba mc nn : Mc I i din cho thut ton nn c bn vi tc bit ln nht 448Kbit/s ,mc II v III l nhng mc m rng ca mc I v tc bit ca chng t c 384Kbit/s v 320Kbit/s.Tuy nhin ,nu chng ta t c t l nn cao th b vo chi ph cng tng cho b gii m v m ha.
Ba mc ring bit trong tiu chun MPEG audio ny ph thuc theo tng ch vi cc ng dng khc nhau:
Mc I: dng trong cc thit b dn dng .
Mc II: dng trong cc thit b chuyn dng v Multimedia.
Mc III: dng trong h thng m ho ting ni 64Kbit/s v thp hn ,dng m ho cht lng cao cho tn hiu m nhc .
Mc I c to t thut ton c bn , trong khi mc II v III l s kt hp gia cc mc I . Tiu chun ny c nh ngha hai m hnh ng dng cho tt c cc lp ca vic nn tn hiu .
2. Chun nn MPEG-2.
Nm 1994 tiu chun th 2 thnh lp , c tn gi MPEG-2(ISO/IEC 13818), l s m rng ca tiu chun MPEG-1 c nh ngha nhm p ng cc nhu cu ca cc ng dng mi nh:
Tiu chun MPEG l a nng , cho php t cht lng cao, tc truyn s liu nhanh v thit b phc tp . Cht lng audio c th thay i trong mt phm vi rng tu thuc vo tc dng bit t thp n cao, tc s liu t 32 n 1066 Kbit/s.Phm vi rng ny c thc hin bi vic chia khung s liu audio MPEG-2 thnh hai phn , mt phn l dnh bit gc thch ng MPEG-1, v phn cn li l dng bit m rng . Vi mc III cho tc dng bit l 64 Kbit/s trn mt knh , c th nn tn hiu c tc l 320 Kbit/s tc l c th m ho c mt tn hiu c rng di thng bng nm knh audio. iu ny ni ln rng c cc thut ton m ho mi c dng pha my pht hnh m khng cn phi thay i pha my thu.
Trong min chun MPEG-2 c thm cc tn s ly mu mi(16; 22,05; 24 KHZ). N cho php truyn bng tn trong khong 7,5 n 11KHz v cho cht lng cao khi tc dng s liu < 64 Kbit/s cho mt knh.
Kh nng ng dng a knh ( Tc bit trong a knh c th m rng t trn 1 Mbit/s n tc cho cht lng cao). Nhng s liu ny c gi trong khong trng dnh cho s liu ph ca cu trc khung s liu audio MPEG-1.
Tiu chun audio MPEG-2 pht trin sau v tng thch vi tiu chun MPEG-1. Nhng b gii m MPEG-1 ch c th gii m c knh tri v phi ca dng s liu audio MPEG-2. Tt c cc lp MPEG-1 v MPEG-2 u ging nhau.
u im ca hai tiu chun MPEG:
Dng bit MPEG-1 c th m rng thnh dng MPEG-2 mt cch d dng.
T dng bit MPEG, mi b gii m MPEG-1 c th tch cc tn hiu Mono hoc cc tn hiu Stereo v cc tn hiu MPEG-2 cn li.
Trong h thng MPEG c th ng thi truyn nhiu thng tin ph , v d thng tin v RDS (Radio Date System= H thng lch ca i), li cc bi ht , thng tin v cc loi chng trnh , thng tin fax hoc modem. Dng s liu ph c th thay i trong qu trnh pht sng . N c th lin kt hai chiu gia pha pht v thu. H thng tng tc cho php thu chng trnh ring m mnh yu cu .
MPEG-1 c dng rng ri vi k thut chuyn dng , v d truyn v phn phi s, Audio, chuyn i-I, Multimedia, dng kinh t s.
Tiu chun MPEG c s dng rng ri trong nhng nm sp ti, cho php s dng pht thanh s trn mt t v qua v tinh DAB ( Digital Audio Broadcasting) v DBV cho truyn hnh v tinh , truyn hnh cp v pht sng trn mt t k thut s.
Header
CRC
Phan bo bit
SCFSI
He so thang o
Mau bang tan con
Heade
R
MC RCR
MC bit phan bo
MC SCFSI
MC SCF
MC
D bao
Mau bang tan con MC
Dong bit s cap (dong bit goc), tng thch vi MPEG 1 (384 Kb/s cho Player II),
Dong bit m rong .
Vi layer III, tai 64 Kb/s tren mot kenh, 5 kenh audio ay u bang tan co the c ma hoa vi toc o 320 Kb/s.
Ma hoa 6 kenh audio, bao gom mot kenh phu nang cao tan so thap, e lam am thanh tron nhieu kenh.
S m rong tren co the thc hien c nh cong them vao moi layer :
2
1
tan so lay mau (16; 22.05; 24 KHz) cho phep truyen bang tan trong khoang 7.5 11 KHz va cho chat lng cao khi toc o dong so lieu < 64 Kb/s cho mot kenh .
Dung lng a kenh ( toc o bit a kenh m rong en 1 Mb/s, cho phep at chat lng cao). Cac d lieu nay c cay vao khong gian d lieu phu cua cau truc MPEG-1 Audio frame.
Tieu chuan MPEG-2 audio tng thch xuong tieu chuan MPEG-1. Tuy nhien bo giai ma MPEG-1 ch co the giai ma cac kenh trai va phai cua dong d lieu MPEG-2 audio. Tai tan so lay mau thap, o phan giai tan so khoang 21 Hz vi tan so lay mau 24 KHz. No cho phep cac bang tan co he so thang o phu hp tot hn vi o rong bang tan ti han va cho chat lng audio tot hn tai cac toc o bit thap, mac du o rong bang tan tn hieu audio c giam toi a la 12 KHz.
Hnh 7.17: Cac chuan nen tn hieu Audio theo tieu chuan MPEG
Cac lp MPEG-2 giong nh MPEG-1 va co ac iem nh sau:
2
a. ac iem layer I
Toc o d lieu : 32
448 Kbps (tong cong).
Tn hieu vao chia thanh cac khung bao gom 348 mau tren mot kenh .
Chu ky khung la 8 ms (12
s
m
83
.
20
32
) cho kenh 48 KHz.
32 bang tan con (Sub band) co kch thc bang nhau, tao ra t cac khoi (block) gom 12 mau (
384
12
32
=
mau).
He so t le 6 bit tren mot bang (dai ong am thanh la 120 dB), he so t le khac nhau cho moi bang.
Phan phoi bit theo phng thc thch ng trc.
Moi mau bang tan con c lng t hoa mot cach chnh xac bang cach tnh toan phn bo cac bit.
Kenh n hoac kep, stereo hoac joint stereo (ma hoa ket hp kenh trai va kenh phai cua tn hieu stereo audio).
Hau het thch hp cho cac ng dung nh ghi am hoac trong studio v kch thc khung (frame) ch la 8 ms.
-
=
+
=
1
0
2
)
1
2
(
cos
)
(
)
(
2
)
(
N
m
N
k
m
m
x
k
C
N
k
X
p
Hnh 2.2: Cau truc Frame cua Player I
b. ac iem cua layer II
Loai nay la ket qua nang cao phng thc hoat ong cua player I, t le nen cao hn. Toc o bit xap x xung quanh 128 Kb/s.
Toc o d lieu 32 384 Kb/s
Tn hieu vao chia thanh cac khung, cha 1152 mau /kenh
32 bang tan con co kch thc bang nhau, tao cac khoi 36 mau (32
36 = 1152 mau).
Chu ky khung la 24 ms cho kenh 48 KHz (
ms
24
83
.
20
3
384
=
m
)
He thong thang o 6 bit/bang (dai ong 120 dB), moi bang khac nhau, cac nhom 12, 24, 36 mau (8, 16 hoac 24 ms) e loai tr meo am thanh.
V tr bit tien thch nghi, khung 24 ms co nh va s dung lng t hoa bit chia nho.
Kenh n hoac kenh kep, stereo hoac joint stereo.
Dung rong rai tieu chuan MPEG trong CD ROM, DVB, DAB, DBS, multimedia
c. ac iem player III
Toc o d lieu 32 384 Kb/s
Tn hieu vao chia thanh cac khung, cha 1152 mau /kenh
Tao khung 24 ms.
32 bang tan con co kch thc bang nhau, c chia tiep thanh 18 bang tan MDCT (tong cong 576 kenh), co kha nang chuyen mach khoi tc thi thanh 192 bang.
Cac he so thang o c dung e cai thien cau truc va mc nhieu lng t.
V tr bit tien thch nghi.
Ma hoa VLC (Huffman) cac gia tr lng t.
Kenh n va kep, stereo hoac joint stereo.
Dung cho toc o bit thap, v du nh ISDN, vien thong, ng truyen ve tinh va am thanh chat lng cao qua internet.
Bang 2.1: Chi tiet ve he thong audio chung.
Toc o lay mau
Toc o lay mau mot na
Am thanh a kenh
32 KHz
44.1 KHz
48 KHz
16 KHz
22.05 KHz
24 KHz
Mono
2 kenh stereo
Joint stereo
a kenh am thanh vong
5. S o khoi c ban nen MPEG
-
=
+
=
1
0
2
)
1
2
(
cos
)
(
)
(
)
(
2
)
(
N
k
N
k
m
k
C
k
X
k
C
N
m
x
p
Hnh 2.3: Cau truc c s cua bo ma hoa MPEG tn hieu Audio
Dong bit audio au vao au tien c chuyen tn hieu t mien thi gian sang min tn s. Tn hieu ngo vao c s ly khi so sanh bi mo hnh tam ly nghe (Psychoacoutic modeler) e ieu khin phan tan bit loi. Mach nay lng t hoa d lieu vi s kiem che gi cho t le S/N audio tren mc nho nhat co the chap nhan c. Cuoi cung, d lieu c ma hoa thanh dong bit cuoi cung.
2.1.5. S o khoi n gian giai nen MPEG
-
=
-
=
+
+
=
1
0
1
0
2
)
1
2
(
cos
2
)
1
2
(
cos
)
,
(
)
(
)
(
)
(
)
(
2
)
,
(
N
u
N
v
N
v
j
N
u
i
v
u
F
v
C
u
C
v
C
u
C
N
v
u
F
p
p
Hnh 2.4: Cau truc n gian cua bo giai ma n gian MPEG tn hieu Audio
au tien dong bit ma hoa c nh dang lai thanh dong bit ma hoa n gian, sau o xay dng lai nhng gia tr bang tan con a c lng t hoa. Cuoi cung chuyen oi nhng gia tr cua phan bang tan con a c lng t hoa thanh dang tn hieu audio mien thi gian.
2.2. NN TN HIU VIDEO S
2.2.1. mc ch nn.
Vi k thut s ho truyn hnh, tc truyn v do vy di thng truyn l qu ln. Do vy i hi rng knh truyn l rt ln. V d : tn hiu video s thnh phn ( s ho 8bit 625 dng ) c tc bit bng 216 Mbit/s. Di ph cn thit truyn tn hiu ny c b rng khng di ( 3/4) x 216 ( 162 MHz.
Nu truyn qua v tinh bng C vi tiu chun 40MHz/ Transponder th phi cn 4 Transponder ghp lin nhau.
Trong studio truyn tn hiu bng cp, Rc ni cht lng cao v vi khong cch ngn, vic nn di tn ch mang tnh kinh t, nn vic truyn tn hiu c th thc hin c m khng cn nn. Song s rt kh khn, thm ch khng th thc hin c vic truyn tn hiu video s qua v tinh vi rng di tn mt knh 27 MHz hoc qua h thng truyn hnh qung b trn mt t vi tiu chun 7 ( 8 MHz cho mt knh truyn hnh tiu chun. Do vy nn video l cng on khng th thiu c khc phc nhng kh khn v yu cu di bng tn cho php.
ii. thc cht ca nn video
Bn cht ca nn video l mt qu trnh trong lng s liu biu din lng thng tin ca mt nh hoc nhiu nh c gim bt bng cch loi b nhng s liu d tha trong tn hiu video. Cc chui nh truyn hnh c nhiu phn t ging nhau. Vy tn hiu truyn hnh c cha nhiu d liu d tha, ta c th b qua m khng lm mt thng tin hnh nh. l qu trnh xo dng xo mnh, vng nh tnh hoc chuyn ng rt chm, vng nh nn ging nhau hoc khc nhau rt t. Thng th chuyn ng trong nh truyn hnh c th d bo, do ch cn truyn cc thng tin bin i. Cc h thng nn s dng c tnh ny ca tn hiu video v cc c trng ca mt ngi ( l km nhy vi sai s trong hnh nh c nhiu chi tit v cc phn t chuyn ng). Qu trnh sau nn l gin nh to li nh gc hoc mt xp x nh gc.
1. M hnh nn nh
Tng u tin ca b m ho video, tn hiu video c trnh by di dng hiu qu nn hiu qu nht. im ct yu l phi xc nh ci g c m ho. S biu din c th cha nhiu mu thng tin m t tn hiu hn chnh l bn thn tn hiu, nhng hu ht cc thng tin quan trng ch tp trung trong mt phn nh ca s m t ny. Trong cch biu din hiu qu, ch c phn nh d liu l cn thit truyn cho vic ti to li tn hiu video.
-
=
-
=
+
+
=
1
0
1
0
2
)
1
2
(
cos
2
)
1
2
(
cos
)
,
(
)
(
)
(
)
(
)
(
2
)
,
(
N
u
N
v
N
v
j
N
u
i
v
u
F
v
C
u
C
v
C
u
C
N
j
i
f
p
p
Hnh 2- 1 : M hnh h thng nn video
Hot ng th hai ca b m ho l lng t ho gip ri rc ho thng tin c biu din. truyn tn hiu video qua mt knh s, nhng thng tin biu din c lng t ho thnh mt s hu hn mc. cng on ny xy ra sai s, sai s ny c gi l sai s h thng.
Hot ng th 3 l gn cc t m. Cc t m ny l mt chui cc bit dng biu din cc mc lng t ho. Cc qu trnh s ngc li trong b gii m video.
Mi hot ng c ging loi b phn d tha trong tn hiu video v li dng s gii hn ca h thng nhn ca mt ngi. Nh b i phn d tha, Cc thng tin ging nhau hoc c lin quan n nhau s khng c truyn i. Nhng thng tin b i m khng nh hng n vic nhn cng s khng c truyn i.
2. d tha d liu
Nn s liu l qu trnh gim lng s liu cn thit biu din cng mt lng thng tin cho trc. Cn phi phn bit gia s liu v thng tin. Thc t gia s liu v thng tin khng ng nht vi nhau. S liu( v do l tn hiu) ch l phng tin truyn ti thng tin. Cng mt lng thng tin cho trc c th biu din bng cc lng s liu khc nhau, n bao gm s liu hoc t khng cho thng tin thch hp ln xc nh bit.
d tha s liu l trung tm trong nn nh s. d tha d liu khng phi l mt khi nim tru tng m l mt thc t c th nh lng c bng ton hc.
Nu r1 v r2 l lng s liu trong hai tp hp s liu cng c dng biu din mt lng thng tin cho trc th d tha s liu tng i ca tp hp s liu th nht so vi tp hp s liu th hai c th c nh nghi nh sau :
RD = 1- 1/CN
Trong CN : thng c gi l t s nn, CN = r1/r2.
Trong trng hp r1 = r2 th CN = 1 v RD = 0, c ngha l so vi tp hp s liu th hai th tp hp s liu th nht khng d tha. Khi r2 cache L2 ->RAM) v a cng cn truy cp n cc d liu cha trn n. Khng n thun nh vy CPU c th i hi nhiu hn mt tp tin d liu ti mt thi im, khi s xy ra cc trng hp:
1. a cng ch p ng mt yu cu truy cp d liu trong mt thi im, cc yu cu c p ng tun t.
2. a cng ng thi p ng cc yu cu cung cp d liu theo phng thc ring ca n.
Trc y a s cc a cng u thc hin theo phng thc 1, c ngha l chng ch truy cp tng tp tin cho CPU. Ngy nay cc a cng c tch hp cc b nh m (cache) cng cc cng ngh ring ca chng (TCQ, NCQ) gip ti u cho hnh ng truy cp d liu trn b mt a nn a cng s thc hin theo phng thc th 2 nhm tng tc chung cho ton h thng.
c v ghi d liu trn b mt a
S hot ng ca a cng cn thc hin ng thi hai chuyn ng: Chuyn ng quay ca cc a v chuyn ng ca cc u c.S quay ca cc a t c thc hin nh cc ng c gn cng trc (vi tc rt ln: t 3600 rpm cho n 15.000 rpm) chng thng c quay n nh ti mt tc nht nh theo mi loi a cng.Khi a cng quay u, cn di chuyn u c s di chuyn n cc v tr trn cc b mt ph vt liu t theo phng bn knh ca a. Chuyn ng ny kt hp vi chuyn ng quay ca a c th lm u c/ghi ti bt k v tr no trn b mt a. Ti cc v tr cn c ghi, u c/ghi c cc b cm bin vi in trng c d liu (v tng ng: pht ra mt in trng xoay hng cc ht t khi ghi d liu). D liu c ghi/c ng thi trn mi a. Vic thc hin phn b d liu trn cc a c thc hin nh cc mch iu khin trn bo mch ca a cng.
Cc cng ngh s dng a cngS.M.A.R.T
S.M.A.R.T (Self-Monitoring, Analysis, and Reporting Technology) l cng ngh t ng gim st, chun on v bo co cc h hng c th xut hin ca a cng thng qua BIOS, cc phn mm thng bo cho ngi s dng bit trc s h hng c cc hnh ng chun b i ph (nh sao chp d liu d phng hoc c cc k hoch thay th a cng mi).Trong thi gian gn y S.M.AR.T c coi l mt tiu chun quan trng trong a cng. S.M.A.R.T ch thc s gim st nhng s thay i, nh hng ca phn cng n qu trnh li xy ra ca a cng (m theo hng Seagate th s h hng trong a cng chim ti 60% xut pht t cc vn lin quan n c kh): Chng c th bao gm nhng s h hng theo thi gian ca phn cng: u c/ghi (mt kt ni, khong cch lm vic vi b mt a thay i), ng c (xung cp, r ro), bo mch ca a (h hng linh kin hoc lm vic sai).
S.M.A.R.T khng nn c hiu l t "smart" bi chng khng lm ci thin n tc lm vic v truyn d liu ca a cng. Ngi s dng c th bt (enable) hoc tt (disable) chc nng ny trong BIOS (tuy nhin khng phi BIOS ca hng no cng h tr vic can thip ny).
cng lai
cng lai (hybrid hard disk drive) l cc a cng thng thng c gn thm cc phn b nh flash trn bo mch ca a cng. Cm b nh ny hot ng khc vi c ch lm vic ca b nh m (cache) ca a cng: D liu cha trn chng khng b mt i khi mt in.Trong qu trnh lm vic ca cng lai, vai tr ca phn b nh flash nh sau:
Lu tr trung gian d liu trc khi ghi vo a cng, ch khi my tnh a cc d liu n mt mc nht nh (tu tng loi cng lai) th a cng mi tin hnh ghi d liu vo cc a t, iu ny gip s vn hnh ca a cng ti hiu qu v tit kim in nng hn nh vic khng phi thng xuyn hot ng.
Gip tng tc giao tip vi my tnh: Vic c d liu t b nh flash nhanh hn so vi vic c d liu ti cc a t.
Gip h iu hnh khi ng nhanh hn nh vic lu cc tp tin khi ng ca h thng ln vng b nh flash.
Kt hp vi b nh m ca a cng to thnh mt h thng hot ng hiu qu.
Nhng cng lai c sn xut hin nay thng s dng b nh flash vi dung lng khim tn 256 MB bi chu p lc ca vn gi thnh sn xut. Do s dng dung lng nh nh vy nn cha ci thin nhiu n vic gim thi gian khi ng h iu hnh, dn n nhiu ngi s dng cha cm thy hi lng vi chng. Tuy nhin ngi s dng thng kh nhn ra s hiu qu ca chng khi thc hin cc tc v thng thng hoc vic tit kim nng lng ca chng.
Hin ti (2007) cng lai c gi thnh kh t (khong 300 USD cho dung lng 32 GB) nn chng mi c s dng trong mt s loi my tnh xch tay cao cp. Trong tng lai, cc cng lai c th tch hp n vi GB dung lng b nh flash s khin s so snh gia chng vi cc cng truyn thng s tr ln khc bit hn.
Thng s v c tnhDung lng
Dung lng a cng (Disk capacity) l mt thng s thng c ngi s dng ngh n u tin, l c s cho vic so snh, u t v nng cp. Ngi s dng lun mong mun s hu cc a cng c dung lng ln nht c th theo tm chi ph ca h m c th khng tnh n cc thng s khc.
Dung lng a cng c tnh bng: (s byte/sector) (s sector/track) (s cylinder) (s u c/ghi).
Dung lng ca a cng tnh theo cc n v dung lng c bn thng thng: byte, kB MB, GB, TB.Theo thi quen trong tng thi k m ngi ta c th s dng n v no, trong thi im nm 2007 ngi ngi ta thng s dng GB. Ngy nay dung lng a cng t tm n v TB nn rt c th trong tng lai theo thi quen, ngi ta s tnh theo TB.
a s cc hng sn xut u tnh dung lng theo cch c li (theo cch tnh 1 GB = 1000 MB m thc ra phi l 1 GB = 1024 MB) nn dung lng m h iu hnh (hoc cc phn mm kim tra) nhn ra ca a cng thng thp hn so vi dung lng ghi trn nhn a (v d a cng 40 GB thng ch t khong 37-38 GB).
Tc quay ca a cng
Tc quay ca a cng thng c k hiu bng rpm (vit tt ca t ting Anh: revolutions per minute) s vng quay trong mt pht.Tc quay cng cao th cng lm vic nhanh do chng thc hin c/ghi nhanh hn, thi giam tm kim thp.
Cc tc quay thng dng thng l:
3.600 rpm: Tc ca cc a cng a th h trc.
4.200 rpm: Thng s dng vi cc my tnh xch tay mc gi trung bnh v thp trong thi im 2007.
5.400 rpm: Thng dng vi cc a cng 3,5 sn xut cch y 2-3 nm; vi cc a cng 2,5 cho cc my tnh xch tay hin nay chuyn sang tc 5400 rpm p ng nhu cu c/ghi d liu nhanh hn.
7.200 rpm: Thng dng vi cc a cng sn xut trong thi gian hin ti (2007)
10.000 rpm, 15.000 rpm: Thng s dng cho cc a cng trong cc my tnh c nhn cao cp, my trm v cc my ch c s dng giao tip SCSI
Cc thng s v thi gian trong a cngThi gian tm kim trung bnh
Thi gian tm kim trung bnh (Average Seek Time) l khong thi gian trung bnh (theo mili giy: ms) m u c c th di chuyn t mt cylinder ny n mt cylinder khc ngu nhin ( v tr xa chng). Thi gian tm kim trung bnh c cung cp bi nh sn xut khi h tin hnh hng lot cc vic th vic c/ghi cc v tr khc nhau ri chia cho s ln thc hin c kt qu thng s cui cng. Thng s ny cng thp cng tt.
Thi gian tm kim trung bnh khng kim tra bng cc phn mm bi cc phn mm khng can thip c su n cc hot ng ca a cng.
Thi gian truy cp ngu nhin
Thi gian truy cp ngu nhin (Random Access Time): L khong thi gian trung bnh a cng tm kim mt d liu ngu nhin. Tnh bng mili giy (ms).y l tham s quan trng do chng nh hng n hiu nng lm vic ca h thng, do ngi s dng nn quan tm n chng khi la chn gia cc a cng. Thng s ny cng thp cng tt.
Tham s: Cc a cng sn xut gn y (2007) c thi gian truy cp ngu nhin trong khong: 5 n 15 ms.
Thi gian lm vic tin cy
Thi gian lm vic tin cy MTBF: (Mean Time Between Failures) c tnh theo gi (hay c th hiu mt cch n thun l tui th ca a cng). y l khong thi gian m nh sn xut d tnh a cng hot ng n nh m sau thi gian ny a cng c th s xut hin li (v khng m bo tin cy).
Mt s nh sn xut cng b a cng ca h hot ng vi tc 10.000 rpm vi tham s: MTBF ln ti 1 triu gi, hoc vi a cng hot ng tc 15.000 rpm c gi tr MTBF n 1,4 triu gi th nhng thng s ny ch l kt qu ca cc tnh ton trn l thuyt. Hy hnh dung s nm m n hot ng tin cy (khi chia thng s MTBF cho (24 gi/ngy 365 ngy/nm) s thy rng n c th di hn lch s ca bt k hng sn xut a cng no, do ngi s dng c th khng cn quan tm n thng s ny.
B nh m
B nh m (cache hoc buffer) trong a cng cng ging nh RAM ca my tnh, chng c nhim v lu tm d liu trong qu trnh lm vic ca a cng.
ln ca b nh m c nh hng ng k ti hiu sut hot ng ca a cng bi vic c/ghi khng xy ra tc thi (do ph thuc vo s di chuyn ca u c/ghi, d liu c truyn ti hoc i) s c t tm trong b nh m.n v thng bnh bng kB hoc MB.Trong thi im nm 2007, dung lng b nh m thng l 2 hoc 8 MB cho cc loi a cng dung lng n khong 160 GB, vi cc a cng dng lng ln hn chng thng s dng b nh m n 16 MB hoc cao hn. B nh m cng ln th cng tt, nhng hiu nng chung ca a cng s chng li mt gi tr b nh m nht nh m t b nh m c th tng ln nhng hiu nng khng tng ng k.
H iu hnh cng c th ly mt phn b nh ca h thng (RAM) to ra mt b nh m lu tr d liu c ly t a cng nhm ti u vic x l i vi cc d liu thng xuyn phi truy cp, y ch l mt cch dng ring ca h iu hnh m chng khng nh hng n cch hot ng hoc hiu sut vn c ca mi loi a cng. C rt nhiu phn mm cho php tinh chnh cc thng s ny ca h iu hnh tu thuc vo s d tha RAM trn h thng.
Chun giao tip
Cc chun giao tip ca a cng
Giao tip(vit tt)
Tn ting Anh y
Tc truyn d liu
SCSI
Small ComputerSystem Interface
Nhiu loi, xem thm
Ultra160 SCSI
160 MBps
Ultra320 SCSI
320 MBps
ATA
AdvancedTechnology Attachment
Max = 133 MBps
SATA 150
Serial ATA 150
150 MBps
SATA 300
Serial ATA 300
300 MBps
SATA 600
Serial ATA 600
600 MBps
C nhiu chun giao tip khc nhau gia a cng vi h thng phn cng, s a dng ny mt phn xut pht t yu cu tc c/ghi d liu khc nhau gia cc h thng my tnh, phn cn li cc giao tip nhanh c gi thnh cao hn nhiu so vi cc chun thng dng.
Trc y, cc chun ATA v SATA th h u tin c s dng ph bin trong my tnh c nhn thng thng trong khi chun SCSI v Fibre Channel c tc cao hn c s ch yu nhiu trong my ch v my trm. Gn y, cc chun SATA th h tip theo vi tc giao tip cao hn ang c s dng rng ri trong cc my tnh c nhn s dng cc th h chipset mi.
Bng di y so snh cc chun ATA thng s dng nhiu vi a cng trong thi gian gn y.
ChunStandard
Pht trin(nm)
Cng b(nm)
Loi b(nm)
PIOModes
DMAModes
UDMAModes
Parallel Speed(MBps)
Serial Speed(MBps)
c tnh
ATA-1
1988
1994
1999
02
0
8,33
H tr ln ti 136.9GB; BIOS issues not addressed
ATA-2
1993
1996
2001
04
02
16,67
Faster PIO modes; CHS/LBA BIOS translation defined up to 8.4GB; PC-Card
ATA-3
1995
1997
2002
04
02
16,67
SMART; improved signal integrity;LBA support mandatory; eliminated single-word DMA modes
ATA-4
1996
1998
04
02
02
33,33
Ultra-DMA modes; ATAPI Packet Interface;BIOS h tr ti 136.9GB
ATA-5
1998
2000
04
02
04
66,67
Faster UDMA modes; 80-pin cable with autodetection
ATA-6
2000
2002
04
02
05
100
100MBps UDMA mode; extended drive and BIOS support up to 144PB
ATA-7
2001
2004
04
02
06
133
150
133MBps UDMA mode; Serial ATA
ATA-8
2004
04
02
06
133
150
Phin bn ph
SMART = Self-Monitoring, Analysis, and Reporting Technology
ATAPI = AT Attachment Packet InterfaceMB = Megabyte; million bytesGB = Gigabyte; billion bytesPB = Petabyte; quadrillion bytesCHS = Cylinder, Head, SectorLBA = Logical block addressPIO = Programmed I/ODMA = direct memory accessUDMA = Ultra DMA
Tc truyn d liu
Tc ca cc chun giao tip khng c ngha l a cng c th p ng ng theo tc ca n, a phn tc truyn d liu trn cc chun giao tip thp hn so vi thit k ca n bi chng gp cc ro cn trong vn cng ngh ch to.
Cc thng s sau nh hng n tc truyn d liu ca a cng:
Tc quay ca a t.
S lng a t trong a cng: bi cng nhiu a t th s lng u c cng ln, kh nng c/ghi ca ng thi ca cc u t ti cc mt a cng nhiu th lng d liu c/ghi cng ln hn.
Cng ngh ch to: Mt st cht ca cc track v cng ngh ghi d liu trn b mt a (phng t song song hoc vung gc vi b mt a): dn n tc c/ghi cao hn.
Dung lng b nh m: nh hng n tc truyn d liu tc thi trong mt thi im.
Bng so snh sau tc gia cc vng cc cng khc nhau di y s gip chng ta nhn ra mt s yu t nh hng n tc truyn d liu ca a cng.
a cng Ultra-ATA/100 Hitachi (IBM) Deskstar 120GXP
Vng
Sectors/Track
Tc quay(vng/pht)
Tc truyn dliu (MB/giy)
Vng ngoi
928
7.200
57,02
Vng trong
448
7.200
27,53
Trung bnh
688
7.200
42,27
a cng Maxtor DiamondMax D540X-4G120J6 120GB ATA
Vng ngoi
896
5.400
41,29
Vng trong
448
5.400
20,64
Trung bnh
672
5.400
Nh vy ta thy rng tc truyn d liu thc s mc trung bnh 42,27 MBps a c giao tip Ultra-ATA/100 (vi tc thit k truyn d liu 100 MBps) ch gn bng 1/2 so vi tc giao tip.
Kch thc
Kch thc ca a cng c chun ho ti mt s kch thc m bo thay th lp rp va vi cc my tnh. Kch thc a cng thng c tnh theo inch (")Kch thc v ngoi cc loi a cng: xem bng.
S s dng in nng
a s cc a cng ca my tnh c nhn s dng hai loi in p ngun: 5 Vdc v 12 Vdc (DC hoc dc: Loi in p mt chiu). Cc a cng cho my tnh xch tay c th s dng ch mt loi in p ngun 5 Vdc. Cc a cng gn trong cc thit b s cm tay khc c th s dng cc ngun c mc in p thp hn vi cng sut thp.
in nng cung cp cho cc a cng phn ln phc v cho ng c quay cc a, phn cn li nh hn cung cp cho bo mch ca a cng. Tu tng loi ng c m chng s dng in p 12V hoc 5 Vdc hn (thng qua nh mc tiu th dng in ca n ti cc mc in p ny). Trn mi a cng u ghi r cc thng s v dng in tiu th ca mi loi in p s dng m bo cho ngi s dng tnh ton cng sut chung.
a cng thng tiu th in nng ln nht ti thi im khi ng ca h thng (hoc thi im a cng bt u hot ng tr li sau khi tm ngh tit kim in nng) bi s khi ng ca ng c ng trc quay cc a t, cng ging nh ng c in thng thng, dng in tiu th nh cc i ca giai on ny c th gp 3 ln cng sut tiu th bnh thng.
cng thng thng ly in trc tip t ngun my tnh, vi cc a cng ngoi c th s dng cc b cung cp in ring km theo hoc chng c th dng ngun in cung cp qua cc cng giao tip USB.
Cc thng s khc
KCH THC V CC LOI CNG
CAO
RNG
DI
TH TCH
Loi 5,25 Dng trong cc my tnh cc th h trc
3,25" (82,6mm)
5,75" (146,0mm)
8" (203,2mm)
149,5 ci (2449,9 cc)
1,63" (41,3mm)
5,75" (146,0mm)
8" (203,2mm)
74,8 ci (1224,9 cc)
Loi 3,5Thng s dng i vi my tnh c nhn, my trm, my ch
1,63" (41,3mm)
4" (101,6mm)
5,75" (146,0mm)
37,4 ci (612,5 cc)
1,00" (25,4mm)
4" (101,6mm)
5,75" (146,0mm)
23,0 ci (376,9 cc)
Loi 2,5Thng s dng i vi my tnh xch tay
19,0mm (0,75")
70,0mm (2,76")
100,0mm (3,94")
133,0 cc (8,1 ci)
17,0mm (0,67")
-nt-
-nt-
119,0 cc (7,3 ci)
12,7mm (0,50")
-nt-
-nt-
88,9 cc (5,4 ci)
12,5mm (0,49")
-nt-
-nt-
87,5 cc (5,3 ci)
9,5mm (0,37")
-nt-
-nt-
66,5 cc (4,1 ci)
8,5mm (0,33")
-nt-
-nt-
59,5 cc (3,6 ci)
Loi 1,8"hoc nh hn dng trong cc thit b k thut s c nhn
9,5mm (0,37")
70,0mm (2,76")
60,0mm (2,36")
39,9 cc (2,4 ci)
7,0mm (0,28")
-nt-
-nt-
29,4 cc (1,8 ci)
Loi 1,8" PC Card
8,0mm (0,31")
54,0mm (2,13")
78,5mm (3,09")
33,9 cc (2,1 ci)
5,0mm (0,20")
-nt-
-nt-
21,2 cc (1,3 ci)
Loi 1,0" Micro Device
5,0mm (0,20")
42,8mm (1,69")
36,4mm (1,43")
7,8 cc (0,5 ci)
Cc thng s di y nhng ngi s dng thng t ch bi chng thng khng nh hng nhiu n hiu sut lm vic ca cng. Cc thng s ny khng nn ly lm ch tiu so snh gia cc a cng trong s la chn trong s s dng thng thng.
n
n ca a cng l thng s c tnh bng dB, chng c o khi a cng ang lm vic bnh thng. a cng vi cc c trng hot ng l cc chuyn ng c kh ca cc a t v cn di chuyn u c, do chng khng trnh khi pht ting n. Do a cng thng c n thp hn nhiu so vi bt k mt qut lm mt h thng no ang lm vic nn ngi s dng c th khng cn quan tm n thng s ny.
Nhng ting lc tc nh pht ra trong qu trnh lm vic ca cng mt cch khng u n c sinh ra bi cn u c/ghi di chuyn v dng t ngt ti cc v tr cn nh v lm vic. m thanh ny c th gip ngi s dng bit c trng thi lm vic ca a cng m khng cn quan st n trng thi HDD.
Chu trnh di chuyn
Chu trnh di chuyn ca cn c/ghi (Load/Unload cycle) c tnh bng s ln chng khi ng t v tr an ton n vng lm vic ca b mt a cng v ngc li. Thng s ny ch mt s hu hn nhng ln di chuyn m c th sau s ln a cng c th gp li hoc h hng.
Sau mi phin lm vic (tt my), cc u t c di chuyn n mt v tr an ton nm ngoi cc a t nhm trnh s va chm c th gy xc b mt lp t tnh, mt s a c thit k cn di chuyn u c t ng di chuyn v v tr an ton sau khi ngng cp in t ngt. Nhiu ngi s dng nng ng c thi quen ngt in trong mt phin lm vic trn nn DOS (bi khng c s tt my chnh thng) ri tho a cng cho cc cng vic khc, qu trnh di chuyn c th gy va chm v lm xut hin cc khi h hng (bad block).
Chu trnh di chuyn l mt thng s ln hn s ln khi ng my tnh (hoc cc thit b s dng a cng) bi trong mt phin lm vic, a cng c th c chuyn sang ch tm ngh (stand by) tit kim in nng nhiu ln.
Chu ng sc
Chu ng sc (Shock - half sine wave): Sc (hnh thc rung ng theo na chu k sng, thng c hiu l vic giao ng t mt v tr cn bng n mt gi tr cc i, sau li tr li v tr ban u) ni n kh nng chu ng sc ca a cng khi lm vic.Vi cc cng cho my tnh xch tay hoc cc thit b k thut s h tr c nhn hay cc a cng ngoi th thng s ny cng cao cng tt, vi cc a cng gn cho my tnh c nhn bn th thng s ny t c coi trng khi so snh la chn gia cc loi cng bi chng c gn c nh nn him khi xy ra sc.
Nhit v s thch nghi
Tt c cc thit b da trn hot ng c kh u c th b thay i thng s nu nhit ca chng tng ln n mt mc gii hn no (s gin n theo nhit lun l mt c tnh ca kim loi), do cng nh nhiu thit b khc, nhit l mt yu t nh hng n qu trnh lm vic ca a cng nht l bn trong n cc chuyn ng c kh cn tuyt i chnh xc.Nhit lm vic ca a cng thng l t 0 cho n 40 C, iu ny thng ph hp vi nhiu mi trng khc nhau, tuy nhin khng ch c vy: m l yu t lin quan v kt hp vi mi trng to thnh mt s ph hoi a cng. cng thng c cc l (cha b lc khng kh) cn bng p sut vi bn ngoi, do nu nh khng kh trong mi trng cha nhiu hi nc, s ngng t hi nc thnh cc git hoc ng bng u bn trong a cng c th lm h hng nu ta hnh dung c tc quay ca n ln th no v khong cch gia u t vi b mt lm vic ca a t nh n u.
Chnh v vy trc khi a mt a cng vo lm vic ln u tin (tho b v nha bc kn n khi sn xut) trong thit b hoc a cng s dng c a n t mt mi trng khc n mt ni lm vic mi (c nhit mi trng cao hn), nn t n vo khoang cha trong mt s thi gian nht nh trc khi kt ni cc dy cp ngun v cp d liu chng lm vic.Thi gian thch nghi ln m bo cho:
1. Cc git nc b bay hi hoc cc cm bng tuyt bin thnh hi nc v cn bng vi mi trng bn ngoi.
2. m bo s ng u v mi trng bn trong v bn ngoi ca a cng, trnh s bin i (do nhit thay i t ngt) vi cc thit b c kh bn trong khi nhit ca a cng tng ln sau mt thi gian hot ng.
Thi gian thch nghi cn thit: xem bng.
Nhit trc khihot ng
Thi giancn thchnghi (gi)
+40F (+4C)
13
+30F (-1C)
15
+20F (-7C)
16
+10F (-12C)
17
0F (-18C)
18
-10F (-23C)
20
-20F (-29C)
22
-30F (-34C)hoc nh hn
27
Tng t vic a mt my tnh xch tay t ngoi tri x lnh vo trong phng lm vic m p cng nn thi gian ch nh vy bi trong my tnh xch tay cng c cc a cng - tr trng hp khi ngoi tri (x lnh) my ang hot ng (m bo n khng b ng bng tuyt bn trong a cng).
Vi nhit theo bng ta c th thy rng kh hu Vit Nam hoc cc nc gn xch o khc c nhit trung bnh cao c l t cn c thi gian thch ng trc khi a a cng vo s dng (tr nhng vng c th c nhit thp v xut hin tuyt nh Sa Pa Vit Nam)
Vi s lm mt a cng, xem thm phn thng tin thm ca bi.
Cc s thng s v sn phm
Phn di y gii thch mt s thng s khc ca cc a cng.
Model: K hiu v kiu sn phm ca a cng, model c th c s dng chung cho mt l sn phm cng loi c cc c tnh v thng s ging nh nhau. Thng thng mi hng c mt cch k hiu ring v thng s model c th gii thch s qua v mt s thng s trn a cng .
Serial number: M s sn phm, mi a cng c mt s hiu ny ring. Thng s ny thng cha ng thng tin c quy c ring ca hng sn xut v thi gian sn xut hoc n thun ch l th t sn phm khi c sn xut.
Firmware revision: Thng s v phin bn firmware ang s dng hin thi ca a cng. Thng s ny c th thay i nu ngi s dng nng cp cc phin bn firmware ca a cng (nhng vic nng cp ny thng rt him khi xy ra).
Mt s hng sn xut phn mm c th s dng cc thng s trn ca a cng nhn dng tnh trng bn quyn ca phn mm trn duy nht mt my tnh, tuy nhin cch ny khng c p dng rng ri do
ng dng
a cng c s dng ch yu trn cc my tnh nh: my tnh c nhn, my tnh xch tay, my ch, my trmVi cc thit b lu tr d liu chuyn dng nh: cc thit b sao lu d liu t ng hoc cc thit b sao lu d liu dng cho vn phng/c nhn bn trn th trng hin nay u s dng cc a cng. Khi a cng c dung lng ngy cng ln, chi ph tnh theo mi GB d liu r i khin chng hon ton c th thay th cc h thng sao lu d liu d phng trc y nh: bng t (m u im ni bt ca chng l chi ph cho mi GB thp).
Ngy nay, mt s hng sn xut a cng c th ch to cc a cng rt nh. Cc a cng nh ny c th c s dng thit b k thut s h tr c nhn, thit b cm tay, in thoi di ng, my nh s, my nghe nhc c nhn, tai nghe khng dy, my quay phim k thut s (thay cho bng t v a quang vi u th v tc ghi v s son tho hiu ng tc thi)...
Nhng thit b gia dng mi xut hin p ng nhu cu ca con ngi cng c s dng cc a cng nh: Thit b ghi li cc chng trnh ti vi cho php ngi s dng khng b st mt knh yu thch no bi chng ghi li mt knh th hai trong khi ngi s dng xem knh th nht, hoc t lch trnh ghi li khi vng nh.
3.1.3 a CD - VCD - DVD
a compact l tm phng trn c ng knh 12 cm c cu to t Poly-Carbonat. Phn tm ca a l l trn c ng knh l 15 mm, phn trong sut bn ngoi c ng knh 26 ( 33 mm gi l vng kp a (Clamping area) c dng gi a c nh trn bn xoay (Turn table) nh vo cn kp a trn my CD. Lp bao ph (bc hi b mt kim loi nhm) c b rng t 46mm ( 117 mm phn chiu tia laser.
a compact disc gm 3 phn: Phn trong, phn ngoi, phn gii hn.
- Phn trong l phn "Read in " (dn nhp). y l ni cha bng ni dung T0C (Table of contents) ca a. Bng ni dung c dng cha cc thng tin bao gm tng s thi gian pht, s cc bn nhc, thi gian dnh cho mi bn nhc....
- Phn ngoi cng ca a c b rng khong 1 mm c gi l "Read out (Dn xut), ni ny dng cha thng tin kt thc ch pht (End of play).
- Phn gii hn gia Read in v 'Read out' l vng chng trnh (programe area) dng cha thng tin, chng hn nh thng tin m nhc v thi gian pht...
- Tn hiu vo c i thnh dng EFM v c ghi trn a theo cc chui vt h (pt) vi cc chiu di khc nhau, c 9 loi vt h khc nhau vi chiu di bin i t 0,87 ( 3,18 (m vi b rng l l 0,5 (m, pit ngn nht c chiu di l 0,87 (m gi l pt 3T v pit di nht 3,18 (m gi l 11T.
Cc h (pit) ny c sp xp mt cch lin tc hnh thnh nn mi track, vi khong cch gia cc track l 1,6 (m.
Dng tia laser c d liu trong cc h ny, tia sng laser c tnh cht l nh sng n sc (Mono Chromaticity) nh hng mnh v l chm tia song song. Cc tia sng phn x ti bin ca h th khng quay tr v theo hng ban u, iu ny gy nn hin tng gim s lng cc tia sng quay ngc theo ng hng. Bng cch o s lng nh sng quay tr v v i chng thnh tn hiu in, ta s c c d liu (data) trn a.
Cu to a compact c minh ho nh hnh 2.1.
Hnh 2.1: Cu trc a CD
Cc tn hiu c ghi trn a di dng cc h ( Pit) c chiu di khc nhau:
+ Pit ngn nht c chiu di 0,87(m ( bng 3T)
+ Pit di nht c chiu di 3,18 (m ( bng 11T)
+ B rng ca mi Pit 0,5 (m
+ Khong cch gia 2 track 1,6 (m
+ Tc quay ca a: T 500 vng/ pht n 200 Vng/ pht khi u c c t trong ra ngoi.
3.2. X l tn hiu lu tr trn a CD VCD DVD:
Tn hiu m thanh tn ti trn a Compact di dng cc bit 0/1 thng qua cc pit v cc plat.
Tn hiu m thanh trc khi ghi ln a l tn hiu thng tin Analog, n phi c thc hin bin i sang tn hiu s Digital trong h thng CD. Tn hiu sau c ng khung v thc hin an cho d liu. Vic an cho d liu c tc dng pht hin v sa li, m sa li c m ha theo thut ton Reed Solomon v c x l theo cch sp xp an xen. Tn hiu c a qua tng bin iu EFM (bin i m 8 bt thnh 14 bit), cng thm tn hiu ng b kiu EFM sau c ghi ln a di cc rnh phn on gi l cc h (pit) d liu.
* S khi vic x l m thanh trc khi ghi ln a:
Hnh 2.5: Qu trnh x l m thanh trc khi ghi ln a
* Chc nng cc khi:
a. Khi bin i A/D:
Bao gm cc cng on sau:
- Ly mu
- Lng t ha
- M ha
* Ly mu:
Ly mu l qu trnh ri rc ho tn hiu Analog theo min thi gian. Tn s ly mu l yu t quan trng trong phng php ly mu, tn s ly mu cng cao th lng mu ly c cng nhiu, dng tn hiu bin i cng ging vi thc t, m thanh ti to cng trung thc. trnh hin tng chng ph v m bo khi to li tn hiu t cc mu ly m khng b mt thng tin th tn s ly mu phi tha mn nh l Shannon: tn s ly mu phi ln hn hoc bng 2 ln tn s cao nht ca tn hiu: fLM > 2fmax
Ph tn hiu m thanh t 20Hz- 20KHz, do trong CD, ngi ta la chn tn s ly mu l 44,1 KHz.
* Lng t ha:
Sau khi thc hin ly mu, ta thu c cc mc tn hiu ri rc theo thi gian, sau cc mc tn hiu ny c gn cc gi tr gin on theo trc tung (chiu biu din bin ). Nh vy, lng t ho l thao tc bin tr s bin tn hiu lin tc m mu ly c thnh dng sng tn hiu khng lin tc m bin bin i c dng bc thang, bng cch dng s tng t in p bin dao ng hu hn khng lin tc theo tng qung thi gian nht nh. Kt qu lng t ho l bin i tn hiu lin tc v tr s bin dao ng thnh tn hiu ri rc v tr s bin dao ng nhng c lin quan n bin dao ng ban u.
Trong qu trnh lng t ho do xy ra qu trnh lm trn nn s gy ra sai s, sai s ny l khng trnh khi v ch c th lm gim bng cch tng s mc lng t ln. Khi m thanh c s ho, sai s ny gi l nhiu lng t ho hay mo lng t ho.
* M ha:
M ho l thao tc bin tr s mu lng t ho thnh dy m. T chc thit k m thanh quc t qui nh cc thit b s s dng m b 2 gm 16 bit (216 = 65.536 mc lng t c th xc nh), tng ng vi di ng l thuyt l D = 201g216 = 92 db.
Qu trnh chuyn i m nh phn tun theo trt t quan trng, bit c ngha ln nht (MSB-Most Significant Bit) ng v tr u tin, bit c ngha nh nht (LSB-Least Significant Bit) ng v tr cui cng. Cc s nh phn c sp xp theo tng t, mi t m bao gm 8 bit.
b. nh dng khung (Frame).
Cc tn hiu knh phi v knh tri bin i lin tc, song song theo thi gian. Mt tn hiu c m ho vi 16 bit c gi l t d liu mu (Sample data word).
T d liu mu ny c chia lm hai thnh phn: thnh phn 8 bit trn v thnh phn 8 bit di; c hai u c gi l "k t biu tng" (Symbol word). Mt "khung" bao gm 6 t d liu mu cho knh tri v 6 t d liu mu cho knh phi. ngha l gm tng cng 24 k t biu tng.
V tn s ly mu h thng CD l 44,1 KHz, nn thi gian cn thit hon thnh mt khung l: (1/44.100) x 6 = 136,05 ((sec).
Vic nh dng khung c minh ho hnh 2.6.
1 Symbol word 8bit
0110 1001 0100 0010
LOA LOB
1/44100
1 Sample Data Work (16 bit)
L0
R0
L1
R1
L2
R2
L3
R3
L4
R4
L5
R5
Hnh 2.6: Vic nh dng khung d liu
c. an cho d liu:
Trong qu trnh ch to a v thit b, kh t c s chnh xc tuyt i nn gy ra li trong qu trnh ghi v c nh hng ti cht lng. V vy phi tm cch khc phc. Hin nay trong h thng CD, ngi ta s dng h thng sa li c tn CIRC (Cross Interleave Reed - Solomon Code).
Li m c th chia lm 2 loi: li n (li ngu nhin) v li kp (li chm). Li n do qu trnh sn xut to ra, v li kp (li xy ra lin tc trn nhiu bit) do a b xc hay bi trong qu trnh s dng to ra. Ngoi ra, nhng bin i in c cng lm mt tnh nht qun trong vic ghi tn hiu s sai khc bit d liu, nhng d liu ny c th lm dng Servo b mt hoc b thiu trong trng hp bng t v trong trng hp tip xc bng - u.
Li ngu nhin c th xut hin trong qu trnh iu bin xung m (PCM Pulse Code Modulation) hoc qu trnh iu ch tn hiu khi nhn mt trng thi nh phn khc l ngn chn, nhng t chn l thnh lp bi 8 bit c thm vo vi 24 k t m thanh. Mt loi bit chn l l bit ph c thm vo vi cc bit thng tin. Trong k thut CD, ngi ta s dng 2 h thng chn l P v Q (P parity & Q parity).
V d s 9 vit di dng nh phn l 1001. i vi h thng chn l - l th 1 bt chn l c thm vo to ra mt s l cc s 1, by gi s 9 c vit li thnh 10011, trong 1001 l thng tin tn hiu cn s 1 cn li l bit chn l. Nu ta k hiu cc ch s nh phn ca 9 l A, B, C, D th trng thi P s l A + B + C + D, nu trong qu trnh ti to, bit A khng nhn ng v n s c bo hiu bng bit chn l v phng trnh A = P - B - C - D. Chn l Q cng l 1 kt qu ca A + B + C + D v chn l P, tuy nhin n xc nhn gi tr ca mnh bng cch nhn tng ch s thnh phn vi mt s khng i bit.
Nhng m sa sai ch hiu qu khi tn hiu mt mt (hoc cng lm l vi bit). Nu sai bit ko di, tc l khi li kp (li chm) xut hin, cn phi c thm mt bin php khc gi l xen k tn hiu. Cn phi t chc sp xp li d liu sao cho li kp tr thnh cc li n v c sa theo qui tc nh trn. S sp xp d liu c gi l an xen d liu, gi tt l CIRC, hay m Reed -Solomon an xen cho. Qu trnh ny c tm tt nh sau:
u tin, trong s 6 t d liu mu ca mi knh, cc t nh s chn c phn b vo ng tr vi thi tr l 2 khung. Sau , 4 k t biu tng cn bng Q ca m Solomon c chn vo chung vi 24 k t biu tng thuc khung mi. Tip theo, ton b 28 k t mi ny li ln lt c lm tr vi chu k 4 khung v thi tr tng dn theo cp s cng, tc l d liu n y c phn tn ri rc 4x27=108 khung. Sau ngi ta thm 4 k t cn bng P ca m Solomon hnh thnh 1 khung mi gm 32 k t. D liu c xo trn ln cui bng cch lm tr xen k, ngha l c cch 1 hng, d liu li c lm tr vi thi tr l 1 khung.
Nh vy d liu sau khi c phn tn s lm cho cc li kp (nu c) tr thnh cc li n By gi c th d dng pht hin v sa cc li ny bng cc m cn bng P v Q.
d. Tn hiu iu khin v hin th: ( C&D Control and Display)
Trong h thng CD, cc rnh ghi tn hiu khng nhn thy bng mt thng c. Do , v tr ca bn nhc, nhp ..., lun lun phi c i din bng cc m. Tn hiu C &D c cng vo l v mc ch ny.
Tn hiu C&D bao gm 8 bit, t tn knh cho cc bit ny ln lt l P, Q, R, S T, U, V, W. Hin nay ch c knh P v Q c s dng, vic nghin cu cc bc nh tnh m chim ch cc knh cn li (R, S, T, ...) v dng cc knh ny cho cc lnh vc khc ang c tin hnh.
Nh vy 1 khung gm 32 k t, k c m sa li v cng thm c k t iu khin v hin th l 33 k t biu tng (trong 24 t biu tng, 8 biu tng sa sai v 1 biu tng cho iu khin v hin th. Mi t biu tng gm 8 bit).
e. Bin i EFM ( Eight to Fourteen Modulation)
Gi s sau khi m ho, tn hiu s c ghi ln a v tr thnh cc chui bit 0 v 1. Khi c li s xy ra thnh phn 1 chiu v thi gian c cc chui bit ln (mt tn hiu). i ph vi vn ny, ngi ta thc hin bin iu EFM.
Qu trnh bin i EFM l thc hin chuyn i 8 bit d liu thnh 14 bit d liu tho mn lut 2 n l0 bng cch chn thm 3 bit ghp. Qu trnh c thc hin v c ci sn my tnh. Lut 2 n 10 qui nh: "Gia 2 bit 1 lin tip nhau bao gi cng phi c t 2 n 10 bit 0"
V d: 110000101 khng tho mn lut "2 n l0"
01000000000001001 khng tho mn lut 2 n 10
100100001001 tho mn lut "2 n l0"
Trong h thng CD s dng dng xung ghi NRZI (Non Retum Zero Invened). NRZI c ngha l dng xung s b o mc ti thi im d liu EFM l 1. Do tn hiu EFM c hnh thnh theo lut t 2 n 10 bit 0 c kp gia hai bit 1 nn ta d dng thy dng xung s o mc vi sn ln hoc sn xung tng ng vi t bin 0 n 1 hoc 1 n 0 ca d liu EFM.
V d: ng vi d liu EFM (a ) ta nhn c dng xung (b).
0 1 000 1 0000 1 00000 1 00 . . . (a)
(b)
Sau bin i 8 bit sang 14 bit, s phi hp cc bit gia cc mng d liu k nhau c th khng tho mn lut "2 n l0". Do , trong EFM ngi ta cng thm 3 bit ghp (Merging bit) i ph vi vn ny.
V d, khi bit cui cng ca mng d liu ng trc v bit u tin ca mng d liu tip theo u l "l", th l s phi hp ca cc bit khng cn tho iu kin na. Lc ny h thng gim st pht hin, v 3 bit ghp c gi tr "000" s c in vo p ng ng theo yu cu.
f. Tn hiu ng b.
Sau khi hon tt bin iu EFM ngi ta cng thm tn hiu ng b khung 24 bt cng vi 3 bit ghp vo u mi khung. Tn hiu ng b ny c to theo mt mu m khng th ln vi bt k mt tn hiu no khc. N dng nhn dng im bt u ca mt khung khi my ang ch pht (Play back), ng thi tn hiu ng b cng cn c nhim v kim sot vn tc quay ca a.
g. Dng tn hiu ghi.
Tn hiu Analog u vo qua tin trnh x l nh cc phn trn cui cng thnh tn hiu Digital, bao gm mt chui cc khung, mi khung gm 588 bit h thng, v c ghi ln a di hnh thc cc h (pit) d liu:
di ca pit d liu thay i trong phm vi t 0,9 - 3,3 (m trong 9 bc. Phng thc ny c p dng do yu cu ca bin i EFM. Ngoi ra, phc v cho EFM, k thut NRZI cng c s dng, theo "l" tng ng vi trng hp o mc. Nh vy s ni tip ca hai bit "0" hnh thnh pit ngn nht (shostest pit). v mt chui lin tc 10 bit "0" hnh thnh pit di nht (longest pit). Nu gi T l di 1 bit th di 1 pit trong CD tng ng s l 3T - 1lT.
1FRAME: 588 CHANNEL BIT
M
SYNC
C&D
DATA-1
CIRC-Q
DATA-2
CIRC-P
SYNC
Hnh 2.7:Ni dung ca tn hiu ghi
SYNC (Synchronous):
Tn hiu ng b.
M (Merging bit):
Bit ghp
C&D (Control & Display):
Tn hiu iu khin v hin th.
DATA - 1 , DATA -2 :
Tn hiu d liu .
CIRC- Q, CIRCP- P:
M cn bng P, cn bng Q.
Cn c vo s trnh by trn hnh 2.7, s bit trong 1 khung s l:
- Sync. Word
= 24 + 3 (bit ghp)
= 27 bit
- Control Word
= 14 + 3
= 17 bit
- Audio Symbols (L)= (14 + 3). 12
= 204 bit
- Audio Symbols (R)= (14 + 3). 12
= 204 bit
- P Parity
= (14 + 3). 4
= 68 bit
- Q Parity
= (14 + 3). 4
= 68 bit
588 bit
Thi gian x l 1 khung: 136,05 (s ng vi vn tc di khng i 1,25 m/s.
- Tn s xung nhp khung: FFCK = 1/136,05 MHZ = 7,35 KHz
Thi gian c 1 bit: 136,05/588 = 0,231 (s
- Tn s xung nhp m bit: FBCK = 588/136,05 = 4,3218 MHZ
phn Servo, i vi Spindle servo, ngi ta s dng tn hiu FCK cung cp cho tng chnh th v BCK cung cp cho tng chnh tinh.
Nh vy thi gian c mt khung l 136,5 (sec nn di ca Pit cho mt bit s c tnh ton khong chng 0,3 (m. Do , pit ngn nht (Shortest pit) cho 3 bits s vo khong 0,9 (m v pit di nht (Longest pit) cho 11 bits s c di xp x 3,3 (m.
3.3 Qu trnh ghi CD VCD DVD
Tia Laser c iu khin theo cng sng bi b bin iu quang v n lp ph cm quang (Photo resist coating), vi cng sng ph thuc vo mc tn hiu. Sau vic ph cm quang thc hin sao cho cc phn li v phn lm c lu li trn cc track tn hiu ca a gc. Cc thu knh phi lun c iu chnh hi t tia Laser ln lp ph cm quang tng ng vi chuyn ng ln xung ca a gc.
Vic ct CD yu cu c chnh xc cao hn rt nhiu so vi a thng. Thm vo , qu trnh to a compact sau khi a gc c ph lp cm quang, n c to khun, a ch (master disc) c to ra, ri a m c to ra t a ch... Trong qu trnh ch to ny c s khc bit gia cc a Digital v Analog.
Hnh 2.3: S khi khi ghi tn hiu ln a Compact
Cui cng, a compact c to ra t a con bng cch p cht do cc pit c b rng l 0,4(m v chiu di ln nht l 3,3(m, kh c th sao chp chng bng cch p, nn. Thm vo , sau khi nn p, ngi ta bao ph mn phn x v mn bo v. Trn mn phn x, ngi ta ph ln lp nhn theo phng php bc hi chn khng.
3.4 Qu trnh c CD VCD DVD
Hnh 2.4: S khi my ht a Compact khi pht.
Chc nng ca cc khi:
Khi RF:
Nhim v ca khi RF l bin i tn hiu quang thnh tn hiu in v khuch i tn hiu ny cp cho khi iu chnh v khi x l tn hiu m thanh.
Khi tch d liu (Data Strobe):
Khi ny c nhim v nhn tn hiu RF-Amp tch cc bit clock gii iu ch EFM tr li m nh phn 8 bit ca tn hiu nguyn thy. Ngoi ra khi tch d liu cn c nhim v tch tn hiu ng b c ci sn trong qu trnh ghi m tn hiu ln a Compact.
Khi x l tn hiu s (DSP: Digital Sinal Processing).
Khi ny c nhim v nhn tn hiu t khi tch d liu cp cho mch gii an xen, mch sa sai v mch tch m ph.
Khi x l tn hiu m thanh (Audio):
Khi ny c nhim v nhn cc m m thanh t khi DSP cp cho mch chuyn i D/A (Digital/Analog). Tn hiu knh tri (L) v knh phi (R) ng ra c ly ra nh mch LPF cp cho ng ra Lv R hoc khuch i Headphone.
Khi iu chnh Servo:
iu chnh vn tc a quay (Spindle Servo): Khi ny nhn tn hiu phn hi t mch x l tn hiu s DSP cung cp in p iu khin vn tc quay ca motor lm quay a. Khi ny phi m bo rng vn tc quay ca a c bin thin t khong 500 vng/pht khi cm quang hc trong cng v 200 vng/pht khi cm quang hc ngoi cng.
iu chnh hi t (Focus Servo): Khi ny nhn tn hiu t RF-Amp iu chnh cun dy hi t (Focus coil) lm dch chuyn cm quang hc theo phng ng.
iu chnh vt qut (Tracking Servo): Mch ny nhn tn hiu t khi RF-Amp cp in p thay i cho cun tracking (tracking coil) lm dch chuyn cm quang hc theo chiu ngang m bo tia Laser ri vo ng track m n ang chy.
iu chnh v tr cm quang hc theo phng ngang (Mch Sled Servo): Mch Sled Servo nhn tn hiu iu khin t khi iu chnh vt qut a ra in p iu chnh Sled motor to tc ng dch chuyn cm quang hc theo tng bc t trong ra ngoi. Ngoi ra trn my CD cn c trang b cc h thng np a hoc a a ra ngoi. Ton b qu trnh vn hnh ca my c iu khin bi khi vi x l.
Khi vi x l (System Control):
C nhim v nhn cc tn hiu t h thng phm n, t cc kha in bo tnh trng h c... ra lnh iu khin thch hp. Ngoi ra khi vi x l cn c nhim v to ra cc tn hiu data, clock, giao tip vi cc mch vi x l tn hiu s, mch iu chnh.
3.5 Khi LASER PICKUP
3.5.1 Laser bn dn:
1. Tia Laser (Laser bn dn).
Tn gi Laser l do thut ng ting Anh Light Amplification of Stimulated Emission of Radiation: Khuch i nh sng bng pht x kch thch. Nh vy Laser l s pht i nhng bc x in t c kch thch, nhng bc x ny trong vt liu ca dng c do s pht x cc bc x cng bc.
Thc t ta thng gp 4 loi laser
+ Laser trng thi rn.
+ Laser lng.
+ Laser kh.
+ Laser bn dn.
nh sng phn x
2. Laser bn dn.
Khi ta phn cc thun mi ni P-N ca mt cht bn dn, cc in t v l trng c khuch tn qua min tip gip gia 2 cht bn dn, lc cc ht mang in thiu s c kt hp vi cc ht mang in a s. Do pht sinh ra mt loi nh sng tng ng vi chnh lch v nng lng. nh sng laser c to ra theo phng thc ny gi l laser bn dn.
Laser bn dn hot ng theo nguyn l pht x kch thch. Cu to c bn trong Laser c hai mt phn x hai u lp tch cc to nn mt hc cng hng quang. Phn nh sng pht ra theo chiu dc ca hc cng hng s b phn x qua li gia hai mt phn x. Trong qu trnh di chuyn theo chiu dc ca hc nh sng kch thch cc in t kt hp vi cc l trng phng ra cc photon mi. Phn nh sng thot ra theo cc phng khc b tht thot dn. Nh vy ch c phn nh sng pht ra theo chiu dc mi c khuch i.
Mt sau ca Laser c ph mt lp phn x cn mt trc c ct nhn mt phn nh sng phn x cn mt phn chiu ra ngoi.
Hnh 2.9: Nguyn l cu to Laser bn dn
nh sng phn x di ra bn trong l trng c cung cp bi cc mt knh song song v tr thnh nh sng ng, pht x nh sng laser sng dng.
3.5.2 Cm quang hc(Laser pickup hay u c):
1. Cm quang hc loi 3 tia:
* Laser diode (LD)
B phn ny dng to nh sng laser, bc sng ca nh sng laser bng 780nm.
Hnh dng ca diode laser: Diode laser c hnh dng 3 chn, trong gm mt chn chung, mt chn dnh cho diode LD, mt chn dnh cho diode MD.
- LD (Laser Diode): Dng pht tia laser cp cho cm quang hc v diode MD.
- MD (Monitor Diode - Diode gim st): Nhn nh sng t Diode laser ti, cp cho mch APC (Automatic Power Control - T ng iu chnh cng sut tia sng).
* Li nhiu x (Diffraction Grating):
Khi chm tia laser c xuyn qua li nhiu x, mt tia chnh v hai tia ph hnh thnh bng cch tn dng hin tng nhiu x ca tia laser.
* Bn lng knh v lng knh phn tia.
Bn lng knh (half prism) c s dng cho phn cc thng. Lng knh phn tia (Beam Splitter) c s dng cho phn cc vng.
Bn lng knh cho php truyn nh sng theo t l 50% theo hng truyn i v 50% theo hng vung gc. Thu knh phn tia c nhim v truyn ton b 100% nh sng ph thuc vo gc phn cc ca nh sng. Khi s dng nh sng phn cc thng, gi s rng s lng nh sng t thi im m n xuyn qua li l 100%, n b gim i 50% do i qua bn lng knh, 25% khi n Photor Detector theo bn lng knh mt ln na. Mt khc trong vic s dng phn cc vng, khi gc phn cc thay i do s tn x i ca a, s lng ng sng i ti Photor Detector b gim.
* Thu knh chun trc (Collimator lens):
nh sng i qua bn lng knh hoc b tch tia c sa dng thnh mt chm tia song song bi thu knh chun trc.
* Phin i hng ((/4 Wave lens plate):
Phin i hng (/4 c ch to bi tinh th c tnh d hng, chit sut ca chng thay i theo hng nh sng. nh sng i qua phin ny s b lch pha 900 so vi thnh phn nguyn thu. Do , nh sng phn cc thng s i thnh phn cc vng v ngc li nh sng phn cc vng s i thnh nh sng phn cc thng.
* Vt knh (Objective Lens):
Thu knh ny c dng hi t tia laser trn b mt a, thu knh c iu khin bi hai cun dy: cun Focus v cun Tracking, khong cch gia thu knh v b mt a c iu chnh bi cun hi t. Cun dy hot ng sao cho thu knh dch chuyn theo tn hiu b mt a.
* Thu knh lm (Concave Lens):
Thu knh c dng gim i nh hng ca s bin i theo chiu di ca ng dn nh sng trn photodeteror (b tch quang) do s thay i khong cch gia vt knh & b mt a.
* Cc thu knh hnh tr (Cylinder Lens):
Thu knh ny c s dng trong khi nhn din Focus. Tia sng xuyn qua thu knh ny ban u bin dng thnh hnh elip theo chiu dc, sau bin dng thnh vng trn v cui cng bin thnh elip theo chiu ngang.
Khi khong cch gia vt knh v b mt a thay i, v tr ny s thay i.
* B tch quang (Photo Detector) hay Ma trn diode nhn.
i vi b tch quang hot ng theo phng thc 3 tia, ngi ta s dng 6 cm
