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1
KYÕ THUAÄT ÑIEÀU CHEÁ MAÕ
HOAÙ XUNG
(PCM - PULSE CODE MODULATION)
Giaùo vieân trình baøy: ThS. Hà Duy
Hưng
2
SÔ ÑOÀ KHOÁI NGUYEÂN LYÙ
LowPass
Filter (LPF)
Laáy maãu
(Sampling)
Löôïng töû
(Quantizing)
Maõhoaù
(Coding)
Giaûi maõ
(Decoding)
Giöõmaãu
(Holdcircuit)
LowPass
Filter (LPF)
Khueáchñaïi
(Amplifier)
x (t)
xr (t)
Heäthoáng
truyeàndaãnsoá
ADC
DAC
ADC: Analog-Digital Converter
DAC: Digital-Analog Converter
f = 8KHz
3
NHIEÄM VUÏ CUÛA CAÙC KHOÁI
 Loïc thoâng thaáp LPF (Low Pass
Filter)
− Giôùi haïn phoå taàn tín hieäu tin töùc
 Phoå taàn tín hieäu thoaïi: 300Hz-
3400Hz
 Baêng thoâng cuûa boä loïc: ∆f = 3100Hz
 Phoå taàn cöïc ñaïi cuûa tín hieäu thoaïi:
fmax = 3,4KHz, laøm troøn baèng 4KHz
− Loaïi boû caùc can nhieãu taàn soá cao
4
NHIEÄM VUÏ CUÛA CAÙC KHOÁI
 Laáy maãu (Sampling)
− Laø maïch ñieàu cheá bieân ñoä xung PAM
(Pulse Amplitude Modulation): nhaân tín
hieäu tin töùc x(t) vaø soùng maïng daïng
xung s(t)
− Rôøi raïc hoaù tín hieäu thaønh chuoãi xung
bieân ñoä rôøi raïc Taàn soá laáy maãu fs ≥
2.fmax (ñònh lyù Nyquist)
− Ñoái vôùi tín hieäu thoaïi:
fs = 2.fmax = 2 . 4KHz = 8KHz
5
NHIEÄM VUÏ CUÛA CAÙC KHOÁI
 Löôïng töû hoaù (Quantizing)
− Gaàn ñuùng hoaù caùc xung bieân ñoä PAM
thaønh caùc xung coù giaù trò chuaån (laøm troøn
ñeán möùc luôïng töû gaàn nhaát)
− Laøm ñôn giaûn quaù trình bieán ñoåi tín hieäu
sang daïng soá (xung PAM  chuoãi bit nhò
phaân 0;1 (Bn-1Bn-2…B0))
− Soá möùc löôïng töû: N = 2n
− Trong kyõ thuaät PCM, 1 xung ñöôïc maõ hoaù
thaønh chuoãi nhò phaân 8 bit  coù 28
=256
möùc löôïng töû.
− 128 möùc döông, 128 möùc aâm, moãi phaàn chia thaønh 8
6
NHIEÄM VUÏ CUÛA CAÙC KHOÁI
− Hai loaïi löôïng töû hoaù: löôïng töû hoaù tuyeán tính vaø
löôïng töû hoaù phi tuyeán
− Löôïng töû hoaù tuyeán tính (ñeàu):
+ Bieân ñoä tín hieäu ñöôïc chia thaønh caùc khoaûng
ñeàu nhau
+ Laáy troøn caùc xung laáy maãu ñeán möùc löôïng töû
gần nhaát
 Nhieãu löôïng töû: coâng suaát trung bình: PLT=
∆2
/12
+ PLT chæ phuï thuoäc vaøo möùc löôïng töû maø khoâng
phuï thuoäc vaøo möùc tín hieäu
+ SNR (signal-to-noise ratio) cuûa tín hieäu lôùn seõ
12
max
−
=∆ −
n
PPV
7
NHIEÄM VUÏ CUÛA CAÙC KHOÁI
- Löôïng töû hoaù phi tuyeán (khoâng ñeàu):
+ Khi bieân ñoä tín hieäu caøng lôùn thì böôùc löôïng töû caøng
lôùn
+ Laáy troøn caùc xung laáy maãu ñeán möùc löôïng töû gần
nhaát
+ Neùn - daõn töông töï (analog) vaø neùn - daõn soá (digital)
+ Luaät A (tieâu chuaån chaâu AÂu):
(A=87,6)
+ Luaät µ (tieâu chuaån Baéc Myõ vaø Nhaät):
(µ=255)
( )







≤≤
+
+
≤≤
+
=
1
1
,
ln1
.ln1
1
0,
ln1
.
X
AA
XA
A
X
A
XA
Y
( )
( )
( )
( )






≤≤−
+
−
≤≤
+
+
=
01,
1ln
.1ln
10,
1ln
.1ln
X
X
X
X
Y
µ
µ
µ
µ
8
NHIEÄM VUÏ CUÛA CAÙC KHOÁI
Ñaëc tính boä neùn vaø
boä daõn
Ñaëc tính boä neùn
Ñaëc tính boä daõn
X
Y
1
1
-1
-1
9
NHIEÄM VUÏ CUÛA CAÙC KHOÁI
- Muoán ñaït SNR=25dB thì:
+ Soá möùc löôïng töû ñeàu: 4096
 moãi töø maõ caàn coù 11 bit (khoâng keå bit
daáu)
+ Soá möùc löôïng töû sau khi neùn: 256
 chæ caàn 8 bit cho moãi töø maõ (keå caû bit
daáu)
 Löôïng töû hoaù phi tuyeán ñöôïc duøng trong caùc thieát bò
gheùp keânh.
10
NHIEÄM VUÏ CUÛA CAÙC KHOÁI
 Maõ hoaù (Coding)
− Chuyeån ñoåi tín hieäu töôïng töï sang tín hieäu soá
− 1 xung ⇒ chuoãi nhò phaân 8 bit B7B6 …B0 coù giaù trò
töông öùng: VPAM = B6.26
+ …+ B0.20
− Trong ñoù:
+ B7: bit daáu, B7=0 tín hieäu aâm, B7=1 tín hieäu
döông
+ B6B5B4: bit xaùc ñònh ñoaïn
+ B3B2B1B0: bit xaùc ñònh möùc trong moãi ñoaïn
Xaùc ñònh chuoãi bit cuûa hai xung PAM coù giaù trò töông
öùng 123 vaø -15
11
NHIEÄM VUÏ CUÛA CAÙC KHOÁI
 Giaûi maõ (Decoding)
− Chuyeån ñoåi tín hieäu soá sang tín hieäu töông töï
− Bieán ñoåi 8 bit noái tieáp  8 bit song song
− Chuoãi nhò phaân 8 bit B7B6 …B0  1 xung PAM coù
giaù trò töông öùng: VPAM = ± VI (B6.26
+ B5.25
+ …+ B0.20
)
vôùi VI = ∆ laø giaù trò moät böôùc löôïng töû
Xaùc ñònh giaù trò cuûa xung PAM cuûa caùc chuoãi bit
sau: 01000101; 10010100
12
NHIEÄM VUÏ CUÛA CAÙC KHOÁI
 Maïch giöõ maãu (Hold circuits)
− Caùc xung PAM vöøa khoâi phuïc ñöôïc giöõ laïi
baèng heä thoáng tuï ñieän coù trong maïch giöõ
maãu
− Noái caùc ñænh bieân ñoä tín hieäu rôøi raïc PAM
thaønh tín hieäu lieân tuïc theo thôøi gian
 Maïch khueách ñaïi (Amplifier)
− Khueách ñaïi coâng suaát
− Heä soá khueách ñaïi ñöôïc ñieàu chænh töï ñoäng
ñeå cho tín hieäu giöõa caùc keânh tín hieäu thu
ñoàng ñeàu nhau
13
BAØI TAÄP
Giaû söû tín hieäu thoaïi coù daïng: v(t) = Cos(4000.π.t) (V)
ñöôïc ñöa vaøo heä thoáng PCM tuyeán tính coù ñaëc tröng:
n = 8 bit; Vp-p = 5V; fs = 8KHz; ñoä roäng xung laáy maãu: τs
= 10 µS
a/. Xaùc ñònh soá möùc löôïng töû, giaù trò möùc löôïng töû?
b/. Veõ tín hieäu thoaïi vaø xung laáy maãu s(t) treân cuøng
heä truïc toaï ñoä?
c/. Veõ xung PAM töông öùng?
d/. Xaùc ñònh töø maõ PCM cho caùc maãu PAM?
e/. Xaùc ñònh sai soá cuûa caùc maãu PAM khi ñöa chuùng
qua maïch giaûi maõ phía ñaàu thu?
14
BAØI TAÄP (2)
Giaû söû tín hieäu thoaïi coù daïng: v(t) = 2.Sin(2000.π.t) (V)
ñöôïc ñöa vaøo heä thoáng PCM tuyeán tính coù ñaëc tröng:
n = 8 bit; Vp-p = 5V; fs = 8KHz; ñoä roäng xung laáy maãu: τs
= 5 µS
a/. Xaùc ñònh soá möùc löôïng töû, giaù trò möùc löôïng töû?
b/. Veõ tín hieäu thoaïi vaø xung laáy maãu s(t) treân cuøng
heä truïc toaï ñoä?
c/. Veõ xung PAM töông öùng?
d/. Xaùc ñònh töø maõ PCM cho caùc maãu PAM?
e/. Xaùc ñònh sai soá cuûa caùc maãu PAM khi ñöa chuùng
qua maïch giaûi maõ phía ñaàu thu?
15
PCM30 vaø PCM24
 PCM30
− Gheùp 30 keânh thoaïi PCM (64Kbit/s)
− Caáu truùc gheùp keânh cô sôû theo tieâu chuaån
chaâu AÂu
− Toác ñoä bit: 2048Kbit/s
− Luaät neùn daõn: A = 87,6/13
 PCM24
− Gheùp 24 keânh thoaïi PCM (64Kbit/s)
− Caáu truùc gheùp keânh cô sôû theo tieâu chuaån
Baéc Myõ vaø Nhaät Baûn
− Toác ñoä bit: 1544Kbit/s
− Luaät neùn daõn: µ=255/15
16
CAÁU TRUÙC KHUNG VAØ ÑA KHUNG
CUÛA PCM30
F0 F1 F8F7F6F5F4F3F2 F9 F15F14F13F12F11F10
0 1 8765432 9 151413121110 16 17 24232221201918 25 313029282726
1 8765432 9 151413121110 16 23222120191817 24 302928272625
TMF = 125µs × 16 = 2ms
Ña khung
16 khung
TF = 125µs
TS
CH
B1 B2 B8B7B6B5B4B3
S0 SAS00
A
0
Khung F0
Si 0 110110
SSi SSSSA1
Caùc khung chaün
Caùc khung leû
MF: Multiframe
F: Frame
TS : Time Slot
CH: Channel
17
CAÁU TRUÙC KHUNG VAØ ÑA KHUNG
CUÛA PCM30
 Moãi ña khung (multiframe) coù 16 khung
(frame) (F0,F1,…,F15) daøi 2ms
 Moãi khung daøi 125µs chia laøm 32 khe thôøi
gian (time slot) (TS0,TS1,…,TS31)
 Moãi khe thôøi gian daøi 3,9µs goàm moät töø
maõ 8bit
 Thôøi gian cuûa moãi bit laø 488ns
 TS1,…,TS15,TS17,…,TS31 cuûa caùc khung
truyeàn 30 keânh thoaïi 1,2,…,30 töông öùng
18
CAÁU TRUÙC KHUNG VAØ ÑA KHUNG
CUÛA PCM30
 Caùc TS0 trong caùc khung chaün goàm:
− Bit Si ñöôïc söû duïng cho quoác teá (neáu khoâng
duøng thì Si=1)
− Baûy bit coøn laïi laø töø maõ ñoàng boä khung
0011011
 Caùc TS0 trong caùc khung leû goàm:
− Bit Si ñöôïc söû duïng cho quoác teá (neáu khoâng
duøng thì Si=1)
− Bit thöù 2 luoân baèng 1 ñeå phaân bieät vôùi töø
maõ ñoàng boä khung
− Bit thöù 3 (A) duøng cho caûnh baùo xa khi maát
ñoàng boä khung (A=1 khi maát ñoàng boä khung)
− 5 bit coøn laïi daønh cho quoác gia
19
CAÁU TRUÙC KHUNG VAØ ÑA KHUNG
CUÛA PCM30
 TS16 cuûa khung F0:
− Bit 1,2,3,4: töø maõ ñoàng boä ña khung 0000
− Bit 6 (A): caûnh baùo xa khi maát ñoàng boä ña khung
(A=1)
− Bit S daønh cho quoác gia (neáu khoâng söû duïng S=1)
 TS16 cuûa khung F1 ñeán F15 duøng ñeå truyeàn
baùo hieäu: abcd abcd
− Moãi keânh thoaïi ñöôïc maõ hoaù baèng 4 bit a,b,c,d
− 4 bit abcd beân traùi: keânh thoaïi 1-15
− 4 bit abcd beân phaûi: keânh thoaïi 16-30
− Neáu khoâng duøng cho baùo hieäu thì b=1, c=0, d=1
 Baùo hieäu keânh keát hôïp CAS

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Pcm slide

  • 1. 1 KYÕ THUAÄT ÑIEÀU CHEÁ MAÕ HOAÙ XUNG (PCM - PULSE CODE MODULATION) Giaùo vieân trình baøy: ThS. Hà Duy Hưng
  • 2. 2 SÔ ÑOÀ KHOÁI NGUYEÂN LYÙ LowPass Filter (LPF) Laáy maãu (Sampling) Löôïng töû (Quantizing) Maõhoaù (Coding) Giaûi maõ (Decoding) Giöõmaãu (Holdcircuit) LowPass Filter (LPF) Khueáchñaïi (Amplifier) x (t) xr (t) Heäthoáng truyeàndaãnsoá ADC DAC ADC: Analog-Digital Converter DAC: Digital-Analog Converter f = 8KHz
  • 3. 3 NHIEÄM VUÏ CUÛA CAÙC KHOÁI  Loïc thoâng thaáp LPF (Low Pass Filter) − Giôùi haïn phoå taàn tín hieäu tin töùc  Phoå taàn tín hieäu thoaïi: 300Hz- 3400Hz  Baêng thoâng cuûa boä loïc: ∆f = 3100Hz  Phoå taàn cöïc ñaïi cuûa tín hieäu thoaïi: fmax = 3,4KHz, laøm troøn baèng 4KHz − Loaïi boû caùc can nhieãu taàn soá cao
  • 4. 4 NHIEÄM VUÏ CUÛA CAÙC KHOÁI  Laáy maãu (Sampling) − Laø maïch ñieàu cheá bieân ñoä xung PAM (Pulse Amplitude Modulation): nhaân tín hieäu tin töùc x(t) vaø soùng maïng daïng xung s(t) − Rôøi raïc hoaù tín hieäu thaønh chuoãi xung bieân ñoä rôøi raïc Taàn soá laáy maãu fs ≥ 2.fmax (ñònh lyù Nyquist) − Ñoái vôùi tín hieäu thoaïi: fs = 2.fmax = 2 . 4KHz = 8KHz
  • 5. 5 NHIEÄM VUÏ CUÛA CAÙC KHOÁI  Löôïng töû hoaù (Quantizing) − Gaàn ñuùng hoaù caùc xung bieân ñoä PAM thaønh caùc xung coù giaù trò chuaån (laøm troøn ñeán möùc luôïng töû gaàn nhaát) − Laøm ñôn giaûn quaù trình bieán ñoåi tín hieäu sang daïng soá (xung PAM  chuoãi bit nhò phaân 0;1 (Bn-1Bn-2…B0)) − Soá möùc löôïng töû: N = 2n − Trong kyõ thuaät PCM, 1 xung ñöôïc maõ hoaù thaønh chuoãi nhò phaân 8 bit  coù 28 =256 möùc löôïng töû. − 128 möùc döông, 128 möùc aâm, moãi phaàn chia thaønh 8
  • 6. 6 NHIEÄM VUÏ CUÛA CAÙC KHOÁI − Hai loaïi löôïng töû hoaù: löôïng töû hoaù tuyeán tính vaø löôïng töû hoaù phi tuyeán − Löôïng töû hoaù tuyeán tính (ñeàu): + Bieân ñoä tín hieäu ñöôïc chia thaønh caùc khoaûng ñeàu nhau + Laáy troøn caùc xung laáy maãu ñeán möùc löôïng töû gần nhaát  Nhieãu löôïng töû: coâng suaát trung bình: PLT= ∆2 /12 + PLT chæ phuï thuoäc vaøo möùc löôïng töû maø khoâng phuï thuoäc vaøo möùc tín hieäu + SNR (signal-to-noise ratio) cuûa tín hieäu lôùn seõ 12 max − =∆ − n PPV
  • 7. 7 NHIEÄM VUÏ CUÛA CAÙC KHOÁI - Löôïng töû hoaù phi tuyeán (khoâng ñeàu): + Khi bieân ñoä tín hieäu caøng lôùn thì böôùc löôïng töû caøng lôùn + Laáy troøn caùc xung laáy maãu ñeán möùc löôïng töû gần nhaát + Neùn - daõn töông töï (analog) vaø neùn - daõn soá (digital) + Luaät A (tieâu chuaån chaâu AÂu): (A=87,6) + Luaät µ (tieâu chuaån Baéc Myõ vaø Nhaät): (µ=255) ( )        ≤≤ + + ≤≤ + = 1 1 , ln1 .ln1 1 0, ln1 . X AA XA A X A XA Y ( ) ( ) ( ) ( )       ≤≤− + − ≤≤ + + = 01, 1ln .1ln 10, 1ln .1ln X X X X Y µ µ µ µ
  • 8. 8 NHIEÄM VUÏ CUÛA CAÙC KHOÁI Ñaëc tính boä neùn vaø boä daõn Ñaëc tính boä neùn Ñaëc tính boä daõn X Y 1 1 -1 -1
  • 9. 9 NHIEÄM VUÏ CUÛA CAÙC KHOÁI - Muoán ñaït SNR=25dB thì: + Soá möùc löôïng töû ñeàu: 4096  moãi töø maõ caàn coù 11 bit (khoâng keå bit daáu) + Soá möùc löôïng töû sau khi neùn: 256  chæ caàn 8 bit cho moãi töø maõ (keå caû bit daáu)  Löôïng töû hoaù phi tuyeán ñöôïc duøng trong caùc thieát bò gheùp keânh.
  • 10. 10 NHIEÄM VUÏ CUÛA CAÙC KHOÁI  Maõ hoaù (Coding) − Chuyeån ñoåi tín hieäu töôïng töï sang tín hieäu soá − 1 xung ⇒ chuoãi nhò phaân 8 bit B7B6 …B0 coù giaù trò töông öùng: VPAM = B6.26 + …+ B0.20 − Trong ñoù: + B7: bit daáu, B7=0 tín hieäu aâm, B7=1 tín hieäu döông + B6B5B4: bit xaùc ñònh ñoaïn + B3B2B1B0: bit xaùc ñònh möùc trong moãi ñoaïn Xaùc ñònh chuoãi bit cuûa hai xung PAM coù giaù trò töông öùng 123 vaø -15
  • 11. 11 NHIEÄM VUÏ CUÛA CAÙC KHOÁI  Giaûi maõ (Decoding) − Chuyeån ñoåi tín hieäu soá sang tín hieäu töông töï − Bieán ñoåi 8 bit noái tieáp  8 bit song song − Chuoãi nhò phaân 8 bit B7B6 …B0  1 xung PAM coù giaù trò töông öùng: VPAM = ± VI (B6.26 + B5.25 + …+ B0.20 ) vôùi VI = ∆ laø giaù trò moät böôùc löôïng töû Xaùc ñònh giaù trò cuûa xung PAM cuûa caùc chuoãi bit sau: 01000101; 10010100
  • 12. 12 NHIEÄM VUÏ CUÛA CAÙC KHOÁI  Maïch giöõ maãu (Hold circuits) − Caùc xung PAM vöøa khoâi phuïc ñöôïc giöõ laïi baèng heä thoáng tuï ñieän coù trong maïch giöõ maãu − Noái caùc ñænh bieân ñoä tín hieäu rôøi raïc PAM thaønh tín hieäu lieân tuïc theo thôøi gian  Maïch khueách ñaïi (Amplifier) − Khueách ñaïi coâng suaát − Heä soá khueách ñaïi ñöôïc ñieàu chænh töï ñoäng ñeå cho tín hieäu giöõa caùc keânh tín hieäu thu ñoàng ñeàu nhau
  • 13. 13 BAØI TAÄP Giaû söû tín hieäu thoaïi coù daïng: v(t) = Cos(4000.π.t) (V) ñöôïc ñöa vaøo heä thoáng PCM tuyeán tính coù ñaëc tröng: n = 8 bit; Vp-p = 5V; fs = 8KHz; ñoä roäng xung laáy maãu: τs = 10 µS a/. Xaùc ñònh soá möùc löôïng töû, giaù trò möùc löôïng töû? b/. Veõ tín hieäu thoaïi vaø xung laáy maãu s(t) treân cuøng heä truïc toaï ñoä? c/. Veõ xung PAM töông öùng? d/. Xaùc ñònh töø maõ PCM cho caùc maãu PAM? e/. Xaùc ñònh sai soá cuûa caùc maãu PAM khi ñöa chuùng qua maïch giaûi maõ phía ñaàu thu?
  • 14. 14 BAØI TAÄP (2) Giaû söû tín hieäu thoaïi coù daïng: v(t) = 2.Sin(2000.π.t) (V) ñöôïc ñöa vaøo heä thoáng PCM tuyeán tính coù ñaëc tröng: n = 8 bit; Vp-p = 5V; fs = 8KHz; ñoä roäng xung laáy maãu: τs = 5 µS a/. Xaùc ñònh soá möùc löôïng töû, giaù trò möùc löôïng töû? b/. Veõ tín hieäu thoaïi vaø xung laáy maãu s(t) treân cuøng heä truïc toaï ñoä? c/. Veõ xung PAM töông öùng? d/. Xaùc ñònh töø maõ PCM cho caùc maãu PAM? e/. Xaùc ñònh sai soá cuûa caùc maãu PAM khi ñöa chuùng qua maïch giaûi maõ phía ñaàu thu?
  • 15. 15 PCM30 vaø PCM24  PCM30 − Gheùp 30 keânh thoaïi PCM (64Kbit/s) − Caáu truùc gheùp keânh cô sôû theo tieâu chuaån chaâu AÂu − Toác ñoä bit: 2048Kbit/s − Luaät neùn daõn: A = 87,6/13  PCM24 − Gheùp 24 keânh thoaïi PCM (64Kbit/s) − Caáu truùc gheùp keânh cô sôû theo tieâu chuaån Baéc Myõ vaø Nhaät Baûn − Toác ñoä bit: 1544Kbit/s − Luaät neùn daõn: µ=255/15
  • 16. 16 CAÁU TRUÙC KHUNG VAØ ÑA KHUNG CUÛA PCM30 F0 F1 F8F7F6F5F4F3F2 F9 F15F14F13F12F11F10 0 1 8765432 9 151413121110 16 17 24232221201918 25 313029282726 1 8765432 9 151413121110 16 23222120191817 24 302928272625 TMF = 125µs × 16 = 2ms Ña khung 16 khung TF = 125µs TS CH B1 B2 B8B7B6B5B4B3 S0 SAS00 A 0 Khung F0 Si 0 110110 SSi SSSSA1 Caùc khung chaün Caùc khung leû MF: Multiframe F: Frame TS : Time Slot CH: Channel
  • 17. 17 CAÁU TRUÙC KHUNG VAØ ÑA KHUNG CUÛA PCM30  Moãi ña khung (multiframe) coù 16 khung (frame) (F0,F1,…,F15) daøi 2ms  Moãi khung daøi 125µs chia laøm 32 khe thôøi gian (time slot) (TS0,TS1,…,TS31)  Moãi khe thôøi gian daøi 3,9µs goàm moät töø maõ 8bit  Thôøi gian cuûa moãi bit laø 488ns  TS1,…,TS15,TS17,…,TS31 cuûa caùc khung truyeàn 30 keânh thoaïi 1,2,…,30 töông öùng
  • 18. 18 CAÁU TRUÙC KHUNG VAØ ÑA KHUNG CUÛA PCM30  Caùc TS0 trong caùc khung chaün goàm: − Bit Si ñöôïc söû duïng cho quoác teá (neáu khoâng duøng thì Si=1) − Baûy bit coøn laïi laø töø maõ ñoàng boä khung 0011011  Caùc TS0 trong caùc khung leû goàm: − Bit Si ñöôïc söû duïng cho quoác teá (neáu khoâng duøng thì Si=1) − Bit thöù 2 luoân baèng 1 ñeå phaân bieät vôùi töø maõ ñoàng boä khung − Bit thöù 3 (A) duøng cho caûnh baùo xa khi maát ñoàng boä khung (A=1 khi maát ñoàng boä khung) − 5 bit coøn laïi daønh cho quoác gia
  • 19. 19 CAÁU TRUÙC KHUNG VAØ ÑA KHUNG CUÛA PCM30  TS16 cuûa khung F0: − Bit 1,2,3,4: töø maõ ñoàng boä ña khung 0000 − Bit 6 (A): caûnh baùo xa khi maát ñoàng boä ña khung (A=1) − Bit S daønh cho quoác gia (neáu khoâng söû duïng S=1)  TS16 cuûa khung F1 ñeán F15 duøng ñeå truyeàn baùo hieäu: abcd abcd − Moãi keânh thoaïi ñöôïc maõ hoaù baèng 4 bit a,b,c,d − 4 bit abcd beân traùi: keânh thoaïi 1-15 − 4 bit abcd beân phaûi: keânh thoaïi 16-30 − Neáu khoâng duøng cho baùo hieäu thì b=1, c=0, d=1  Baùo hieäu keânh keát hôïp CAS