Dvc

June 2001 Copyright 2001 Global Wireless Education Consortium DVC ● 1

Digital Voice Coding: Digital Voice Coding:

Vocoding & Vocoding & TechniquesTechniques

(DVC)(DVC)


DVCDVC© Copyright 2001 Global Wireless Education Consortium

All rights reserved. This module, comprising presentation slides with notes, exercises, projects and Instructor Guide, may not be duplicated in any way without the express written permission of the Global Wireless Education Consortium. The information contained herein is for the personal use of the reader and may not be incorporated in any commercial training materials or for-profit education programs, books, databases, or any kind of software without the written permission of the Global Wireless Education Consortium. Making copies of this module, or any portion, for any purpose other than your own, is a violation of United States copyright laws.

Trademarked names appear throughout this module. All trademarked names have been used with the permission of their owners.


DVCDVC

■ Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-9972380 and Advanced Technological Education Program under grant DUE 9950039. ‑

■ GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use.

August 2001 Copyright 2001 Global Wireless Education Consortium DVC ● 4

Table of ContentsTable of ContentsOverview 5Learning Objectives 7Pulse Code Modulation 10Linear Predictive Coders 18

Code Excited Linear Predictive (CELP)

Vector Sum Excited Linear Predictive (VSELP)Algebraic Code Excited Linear Predictive (ACELP)

Summary 32Contributors 34


OverviewOverview

■ In this module, you will learn about Digital Voice Coding (DVC) techniques that make possible more efficient use of the allotted frequency spectrum for wireless telecommunication.


Overview Overview (cont.)(cont.)

■ In this module, you will learn about about Digital Voice Coding (DVC) techniques that makes possible more efficient use of the allotted frequency spectrum for wireless telecommunication.


Learning ObjectivesLearning Objectives

After completing this module and its activities, you will be able to:■ List the process for Analog-to-Digital and Digital-to-Analog

conversion (A/D, D/A).■ Describe the methodology, application, and codebooks associated

with Coded Excited Linear Prediction (CELP).■ Describe the methodology, application, and codebooks associated

with Vectored Sum Excited Linear Prediction (VSELP).■ Describe the methodology, application, and codebooks associated

with Algebraic Coded Excited Linear Prediction (ACELP).


IntroductionIntroduction


IntroductionIntroduction

Mobile SwitchingCenter (MSC)

PSTN

972 555 1212

Cell Site

12

3

4

5

6


Pulse Code Pulse Code Modulation Modulation

(PCM)(PCM)


Analog SignalsAnalog Signals

+V

-V

0

+V

-V

0

Figure 1

Figure 2

Time

Time


DigitizationDigitization

Nyquist’s theorem is basic to the concept of digital sampling.■ We must sample at twice the

highest frequency of the fundamental, or carrier signal

Yeah, Jim, Harry Nyquist here. Yeah, I think I’m onto something. Yeah, well, I think you have to

sample at twice the highest frequency. Yeah, I’m pretty sure

of it. Yeah, I know your way is cheaper, but it’s distorted. Yeah,

distorted. D-I-S-T-O… (sigh)

You won’t eat (pop) lunch in this

town again, Nyquist (crackle)


Sampling Voice SignalsSampling Voice Signals

125 usec.

+V

-V

Sampling Rate = 8,000 sample/sec

0


A-D ConversionA-D Conversion

125 usec.

Sampling gives us PAM

0

Steps in the voice digitization process:■ Sampling

• follows Nyquist theorem■ Quantization

• requires sufficient levels for acceptable noise

■ Companding• improves efficiency for voice

transmission• A-law; µ-law

At this point, depending on the transmission medium, the bitstreams can be time division, or otherwise, multiplexed; fed through a vocoder to further reduce the bandwidth; and combined with error-recovery codes for robustness.

125 usec.

0001

-0010

Quantization gives us PCM

0

-0001

0010


Encoding PAM to Encoding PAM to PCMPCM

125 usec.

Sampling gives us PAM

0

Steps in the encoding PAM to PCM:■ Quantify the amplitude level of

the specific pulse.■ Create an eight bit word that

represents that value.■ Where:

■ 42V = 10101010■ 5V = 10000101■ 36V = 10100100■ -36V = 01000100■ -5V = 00000101■ -42V = 00101010125 usec.

5V

-42V

Quantization gives us PCM

0

-5V

42V36V

-36V


Signal to Noise Signal to Noise Quantizing RatioQuantizing Ratio

++=

max10log2002.676.1A

AnSNQR

Where:n = number of bits in the coded wordA = amplitude of the signal and that point in timeAmax = maximum amplitude of the signal


CompandingCompanding

■ Companding involves compressing larger signals in the coding process so that the quantization interval increases as the signal increases.

■ A section of the µ-law compression curve illustrates this process at left. The compressed signal has increasingly greater quantization intervals as the input signal gets stronger.

A/D D/A

Compression Linear PCM

encoder

Linear PCM

decoder

Expansion

Compressed digital code

words

Slope = 1/8

Slope = 1/4

Slope = 1/2

Slope = 1

Linear signal

Com

pres

sed

sig

nal


Linear Predictive CodersLinear Predictive Coders(CELP)(CELP)

(VSELP)(VSELP)(ACELP)(ACELP)


Codebooks and Codebooks and VocodersVocoders 1. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

2. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx3. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx4. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx5. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx6. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx7. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx8. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx9. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx10. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx11. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx12. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx13. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx14. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx15. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx16. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx17. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx18. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx19. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx20. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx21. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx22. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx23. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx24. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx25. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx26. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx27. xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

1. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx2. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx3. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx4. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx5. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx6. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx7. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx8. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx9. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx10. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx11. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx12. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx13. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx14. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx15. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx16. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx17. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx18. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx19. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx20. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx21. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx22. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx23. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx24. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx25. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx26. Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx27. xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx





Codebook

2 X

H

L

X

+

b (n)L

Codebook 1 X

ex(n)

A(z)

synthesis filter

spectral postfilter

output speech

I α i

βq

γ2q

γ1q

Long Term Filter State


VocodersVocoders

■ The basic goal of a vocoder is to encode only perceptually important aspects of speech with fewer bits than PCM.

■ Types of vocoders include:❑ Channel vocoders❑ Formant vocoders❑ Linear predictive coders

In wireless communications, we are primarily interested in the last class.


VSELP VocoderVSELP Vocoder

Codebook

2 X

H

L

X

+

b (n)L

Codebook 1 X

ex(n)

A(z)

synthesis filter

spectral postfilter

output speech

I α i

βq

γ2 q

γ1q

Long Term Filter State

IS-136 systems use a type of code-excited, linear predictive vocoder called a VSELP vocoder.


VSELP VocoderVSELP VocoderShort-term filter coefficients, αi 38 bits/frame

Frame energy, R(0) 5 bits/frame

Lag, L 7 bits/subframe 28 bits/frame

Codewords, I, H 7+7 bits/subframe 56 bits/frame

Gains, β, γ1, γ2 8 bits/subframe 32 bits/frame

The basic data rate of the speech codec is 7950 bps. There are 159 bits per speech frame (20 msec) for the speech codec. These 159 bits are allocated as shown above.


ACELP VocoderACELP Vocoder

-

•

• •

Digital Output

LPC anal., quantiz.

and interp.

Perceptual weighting

Open Loop Pitch

analysis

Adaptive Codebook

Algebraic Codebook

Gain VQ

MSE Search

Synthesis filter

Perceptual weighting

MTPX

Input speechUnquantized LPC info

To

Past excitation

T

k

gp

gc

LPC info

Gains (7+7)

Pitch lag (8+5+1)

Index (16+16)

LPC info (19+1)


IS-136 Channel CodingIS-136 Channel Coding

In IS-136, class 1 bits are convolutionally coded and interleaved with class 2 bits and transmitted over two time slots.

12 most perceptually significant bits

77

7Coded Class-1 bits

178

82

260 260

Spe

ech

Cod

er7-bit CRC Computation

Rate 1/2 Convolutional Coding

Voi

ce C

iphe

r

2-S

lot

Inte

rleav

er

Speech frames x and y

Speech frames y and z

Class-1 bits

Class-2 bits


VSELP Bit Class VSELP Bit Class AssignmentAssignment

ParameterTotal

Codeword BitsClass 1

BitsClass 2

BitsMost Perceptually

Significant Bits

R0 5 4 1 3

LPC1 6 4 2 3

LPC2 5 3 2 2

LPC3 5 3 2 2

LPC4 4 2 2 1

LPC5 4 1 3 1

LPC6 3 0 3 0

LPC7 3 0 3 0

LPC8 3 0 3 0

LPC9 3 0 3 0

LPC10 2 0 2 0

LAG_1 7 7 0 0

CODE1_1 7 0 7 0

CODE2_1 7 0 7 0

GSP0_1 8 8 0 0

LAG_2 7 7 0 0

CODE1_2 7 0 7 0

CODE2_2 7 0 7 0

GSP0_2 8 8 0 0

LAG_3 7 7 0 0

CODE1_3 7 0 7 0

CODE2_3 7 0 7 0

GSP0_3 8 8 0 0

LAG_4 7 7 0 0

CODE1_4 7 0 7 0

CODE2_4 7 0 7 0

GSP0_4 8 8 0 0

■ For the various parameters produced by the VSELP vocoder, bits are classified according to the table at left.❑ Class 1 and Class 2 bits

are designated.❑ Class 1 bits will have

convolutional coding applied.

❑ Twelve of the Class 1 bits are designated the most perceptually significant and will have a 7-bit CRC computed for them.

■ Class 2 bits are transmitted without any error protection.


Convolutional EncodingConvolutional Encoding■ The convolutional coding

process takes the Class 1 bits and processes them in order to protect them from error.❑ There are 32 states in the

convolutional code, each with a defined output for a given input.

❑ For each input Class 1 bit, two output bits are produced.

❑ The bits are ordered according to information in the spec.

■ The CRC is also included in this calculation.

77 Class 1 bits plus CRC and tail bits in (89 total)

Ordering of bits

Application of generator polynomials

178 convolutionally coded bits out


InterleavingInterleaving

0x 26x 52x 78x 104x 130x 156x 182x 208x 234x

1y 27y 53y 79y 105y 131y 157y 183y 209y 235y

2x 28x 54x 80x 106x 132x 158x 184x 210x 236x

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

12x 38x 64x 90x 116x 142x 168x 194x 220x 246x

13y 39y 65y 91y 117y 143y 169y 195y 221y 247y

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . .

24x 50x 76x 102x 128x 154x 180x 206x 232x 258x

25y 51y 77y 103y 129y 155y 181y 207y 233y 259y

■ In the interleaving process, the bits are spread across two speech frames to guard against instances of Rayleigh fading and interference.❑ Class 2 bits are

mixed with coded Class 1 bits.

■ In the interleaving array at left, X is the previous speech frame and y is the current speech frame.


PitchFilter

FormantFilter

DSP QCELP VOCODER

Coded Result

20ms Sample

Feedback Loop

Codebook

Other Vocoder TypesOther Vocoder Types■ Various vocoder types:

❑ IS-136 specifies use of two vocoders and has “hooks” for incorporation of others:

■ VSELP: a “codebook” vocoder operating at 8 kb/s

■ ACELP: an improvement on VSELP, designed to perform better in the presence of impairments

❑ IS-95 uses QCELP - a “variable rate” codebook vocoder

■ ranges from 13kb/s to 1kb/s (average 4kb/s)

■ takes advantage of natural pauses in speech

■ Research continues into specification of “half rate” vocoders.


Codebook

Codebook DevelopmentCodebook Development

■ A Book of “Voices”:❑ Speech patterns were

sampled:■ English: more than 60% ■ Spanish: approximately 38%■ Other languages

❑ These patterns were cataloged ■ 2 Volumes■ 156 bit line items


Codebook ContentCodebook Content

Volume I

Volume 2







■ Each codebook is made up of two volumes

■ Each volume has multiple pages

■ Each page contains multiple line items

■ Each line item is a bit pattern based on random samples of speech patterns.


Digital Format ElementsDigital Format Elements

■ Each VSELP or ACELP encoded signal consists of two basic elements:❑ The conversation data itself (7.4Kb/s)❑ The overhead information needed to process the

signal (8.8Kb/s)


SummarySummary


SummarySummary

Mobile SwitchingCenter (MSC)

PSTN

972 555 1212

Cell Site

12

3

4

5

6


Industry ContributorsIndustry Contributors

■ AT&T Wireless (http://www.attwireless.com)■ Ericsson (http://www.ericsson.com)■ Nortel Networks (http://www.nortelnetworks.com)■ Telcordia Technologies, Inc. (http://www.telcordia.com)

The following organizations provided materials and resource support for this module:

http://www.attwireless.com/

http://www.ericsson.com/

http://www.nortelnetworks.com/

http://www.telcordia.com/


Individual ContributorsIndividual ContributorsThe following individuals and their organization or institution provided materials, resources, and development input for this module:■ Mr. Jeff Cobb

❑ Verizon Wireless❑ http://www.verizonwireless.com

■ Dr. Philip DiPiazza ❑ Florida Institute of Technology ❑ http://www.fit.edu/

■ Dr. Kaveh Heidary❑ Alabama A&M University❑ http://aamu.edu

■ Mr. John Wakeman❑ Award Solutions, Inc.❑ http://www.awardsolutions.com

http://www.verizonwireless.com/

http://www.fit.edu/

http://www.fit.edu/

http://www.fit.edu/

http://aamu.edu/

http://aamu.edu/

http://www.awardsolutions.com/

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