Voice Codingl 7 p

8/8/2019 Voice Codingl 7 p

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Speech Coding at GSM


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Speech Coding

A/D SegmentationChannel

coding

Speech

coder

8 KHz

13 bits 160 SamplesOf 13 bits

=2080 bits

(20 msec)

260 bits

20 msec13kbps

456 bitsPer 20ms

22.8 kbps

speech


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RPE-LTP

speech

encoder

160 samples/20 msfrom A/D

(= 2080 bits)

36 LPC bits/20 ms

9 LTP bits/5 ms

47 RPE bits/5 ms

260 bits/20 ms (13Kbps) to

channel encoder

LPC: linear prediction coding filter

LTP: long term prediction filter

RPE: regular pulse excitation signal

Regular pulse excited - long term prediction (RPE-LTP)

speech encoder


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BSSide

8 bit A-Law

to

13 bit Uniform RPE/LTP speech Encoder To Channel Coder 13Kbps

8 K sps

MSSide

LPF A/DRPE/LTP speech Encoder

To Channel Coder 13Kbps

8 K sps,

Sampling Rate - 8Kbps

Encoding - 13 bit Encoding (104 Kbps)

RPE/LTP - Regular Pulse Excitation/Long Term Prediction

RPE/LTP converts the 104 Kbps stream to 13 Kbps

Air Interface

8000 samples/s,13 bits/sample

104 kbps

8000 samples/s,

13 bits/sample

104 kbps


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The 260 bits are divided into three classes:

Class Ia 50 bits - most sensitive to bit errors.

Class Ib 132 bits - moderately sensitive to bit errors. Class II 78 bits - least sensitive to bit errors.

Class Ia bits have a 3 bit cyclic redundancy code added for errordetection = 50+3 bits.

132 class Ib bits with 4 bit tail sequence = 132 + 4 = 136.

Class Ia + class Ib = 53+136=189, input into a 1/2 rate convolutionencoder of constraint length 4. Each input bit is encoded as two outputbits, based on a combination of the previous 4 input bits. The convolutionencoder thus outputs 378 bits, to which are added the 78 remaining classII bits.

Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate

of 22.8 kbps. 456 bits * 50 sps = 22.8 kbps

Hybrid coder block

Class 1A

50

Class 1B

132

Class 2

78


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Hybrid coder block The 260 bits are divided into three classes:

Class Ia 50 bits - most sensitive to bit errors.

Class Ib 132 bits - moderately sensitive to bit errors.

Class II 78 bits - least sensitive to bit errors.

Hybrid coder block

Class 1A

50

Class 1B

132

Class 2

78


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(2,1,5)convolution

coder

260 bits/20 ms

= 13 kb/s

50 class

1a bits

182 class 1b bits

78 class 2 bits

456bits/20ms

= 22.8 kb/s

Class 1a: CRC (3-bit error detection) and convolutional coding (error Correction)

Class 1b: convolutional coding

Class 2: no error protection

*tail bits to periodically reset convolutional coder

Channel encoder

3-bit

CRC

53

bits

Bit

inter-

leaver

378bits

4 tail bits*


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Channel Coder:

The Channel Coder is divided into two blocks:

The Block Coder allows error detection on the Class 1A bits. This is performed

by using a CRC-3 process that generates (50 + 3) bits.

The Convolution Coder allows error detection and correction. It protects the

resulting 53 bits of the Block Coder as well as the Class 1B bits. 378 bits are

generated. The decoding process is performed by the Viterbi decoder

22.8 kbits/s= 456 bits

For each 20 ms block

Channel

Coding

Block codingConvolutional

coding


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Channel Coder

Block coder

Convolutional coder 2

456 bits for 20 ms speech

4 '0' tail bits

378 bits

Class 1A

50

Class 1B

132

Class 2

78

1

1

1A 1B 1B 1A tailCRC

3 66 25 46625

2378Class 1 bits coded

with

convolutional coderClass 2 bits

78


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Air Interface

Di itizi

S rce i

GMSK

M l ti

I terle vi

ipheri

TDMA b rst

F rm tti

el i

Speech13 Kbit/s

22.8Kbit/s

22.8Kbit/s

33.8 Kbit/sAir i terf ce


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Interleaving - Second level

SB#1 block n

SB#2 block n

SB#3 block n

SB#6 block n

SB#4 block n

SB#7 block n

SB#5 block n

SB#8 block n

SB#5 block n - 1

SB#6 block n - 1

SB#7 block n - 1

SB#8 block n - 1

SB#1 block n + 1

SB#2 block n + 1

SB#3 block n + 1

SB#4 block n + 1

Second level interleaving introduces a delay (i.e.40 ms)


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1 2 3 4 5 6 7 8

1 26 8.253 57

20 ms

1 57 3

260 260

456 bit

Speech Coder Speech Coder

Channel Encoding Channel Encoding

Interleaving

NORMALBURST

Out of first 20 ms Out of second 20ms

20 msSpeech

456 bit

Documents

Voice Codingl 7 p