Chủ đề: VoIP_Part_II_2: Các kiểu mã hoá analog và digital

2.Analog to digital voice encoding
cac cach so hoa giong noi; cac ki thuat nen cho phep chuyen giong noi voi di xa trong dieu kien bang thong hep. Cac ki thuat do chat luong giong noi de co the chon duoc thuat toan nen tot nhat

Muc tieu:
• Cac buoc chuyen doi tu analog sang so
• Cac buoc chuyen doi tu so sang analog
• Muc dich cua li thuyet nyquist
• Luong tu hoa
• Hai ki thuat nen
• Mo ta chuan G.729 va G729
• Ba chuan nen kem theo bang thong
• Do chat luong voice

2.1.Convert analog to digital
so hoa giong noi duoc thuc hien boi bell system vao nam 1950. Muc dich chinh la trien khai nhieu mach voice voi chi mot so it day. Phat trien phuong thuc van chuyen E1 va T1

cac buoc thuc hien:
• Lay mau tin hieu analog: toc do lay mau can phai gap hai lan tan so cao nhat cua tin hieu
• Luong tu hoa tin hieu: chia ra cac muc; sau do xep cac mau vao cac muc
• So hoa: ma hoa cac gia tri da luong tu hoa thanh dang 8-bit
• Nen mau nham giam bang thong

2.2.Chuyen doi tu so sang analog
Sau khi nhan duoc tin hieu so PCM; thiet bi can phai chuyen tu dang so sang tin hieu tuong tu; thong qua hai buoc:
• Decoding: giai ma tin hieu co duoc bang cach dinh ra bien do cua mau
• Filtering: cho tin hieu PAM di qua cac bo loc duoc thiet ke thich hop nham kien tao lai tin hieu analog cu

2.3.Cac thuat toan nen va chuan codec
Hai ki thuat nen voice:

Waveform alg (coders): co cac chuc nang:
• Tan so lay mau: 8000 lan /s
• Su dung phuong thuc du doan vi sai de giam bang thong
• Anh huong lon den chac luong voice vi giam bang thong
• Khong mang theo uu diem cua dac tinh giong noi

Source alg (coders)
• Con duoc goi la voice coder: day la mot thiet bi chuyen doi giong noi analog thanh so su dung cac thuat toan dac biet duoc toi uu hoa cho ma hoa giong nguoi
• Co cac dac tinh quan trong cua giong noi
• Giam bang thong chi xay ra khi gui cai dat dang loc tuyen tinh
• Codebook luu tru cac dang song du doan duoc cua giong noi. Chung co the duoc dung de match giong noi; ma hoa cac phase; giai ma dang song tai phia nhan bang cach tim cac phase da duoc ma hoa.
2.3.1.Waveform compression
co hai dang:
• PCM (pulse code modulation): chuan ITU G.711
• ADPCM( adaptive differential PCM): cung giong ma hoa waveform nhung cho phep du doan thich nghi bang cach nhin cac ket qua trung gian da thuc hien. Cho phep giam so bit cho moi s ma PCM da dung

Cac chuan ITU cho nen dang song bao gom:
• G.711 rate: 64 kbps = (2 * 4 kHz) * 8 bits/sample
• G.726 rate: 32 kbps = (2 * 4 kHz) * 4 bits/sample
• G.726 rate: 24 kbps = (2 * 4 kHz) * 3 bits/sample
• G.726 rate: 16 kbps = (2 * 4 kHz) * 2 bits/sample

2.3.2.Source compression
ba dang ma hoa nguon:
• CELP
• G.728: LDCELP-16 K
• G.729: CS-ACELP: 8 K

CELP (code excited linear prediction): ma hoa theo cac buoc:
• Dau vao cua coder duoc chuyen doi tu mot mau 8-bit thanh mot mau 16-bit tuyen tinh PCM
• Mot codebook duoc su dung de hoc va du doan dang song cua voice
• Coder duoc kich hoat bang bo phat nhieu trang
• Cac cong thuc toan hoc duoc gui toi bo giai ma dau xa de tong hop va phat ra dang song

2.3.3.Yeu cau bang thong
Ðính Kèm 201
ba ki thuat nen chinh duoc chuan hoa boi ITU:
• PCM: bien do cua tin hieu thoai se duoc lay mau va luong tu hoa voi tan suat 8000 lan /s. Moi mau duoc bieu dien boi 8 bit. Voi qua trinh lay mau: su dung a-law hoac muy-law de giam SNR
• ADPCM: ma hoa: tinh toan su sai khac giua mau hien thoi va gia tri du doan cua no (dua vao cac mau truoc): duoc bieu dien boi 2; 3; 4; 5 bit. Phuong thuc nay lam giam yeu cau bang thong mac du chat luong tin hieu van tot
• Excitation value va dat cac bo loc tuyen tinh

So sanh giua cac chuan va bang thong yeu cau:

Ðính Kèm 202
2.4.Do chat luong thoai
hai phuong thuc do chat luong thoai tren duong telephone line. Do cac phuong thuc nen khac nhau lai cho cac ket qua do chat luong khac nhau nen can co mot phuong thuc de so sanh chat luong giua cac phuong thuc ma hoa
Ðính Kèm 203
tren day la MOS (mean opinion score) la he thong danh gia chat luong cuoc goi; he thong nay bao gom cac do do thong ke bat nguon tu su danh gia cua nhieu thue bao.

Ngay nay su dung bo PSQM (perceptual speech quality measurement):
Ðính Kèm 204