TETRA Voice Coding

Ranko Pinter

Simoco Digital Systems

Agenda

• Why code speech?

• Basic principles of TETRA voice

coding

• How was TETRA codec selected?

• Operational performance

• Future enhancements

Analogue transmission

Continuous variations in electric current

Digital transmission

EncoderAnalogue speech to bit stream

De-coderBit stream to analogue speech

Types of codec

• Waveform codec

•Transmitted bits represent the speech waveform

• Parametric codec•Transmitted bits drive a speech synthesiser

Waveform codecs

Sample Quantise Encode

Reconstruct Decode

Parametric codecs

Parametric model of speech production

Transmitted bits drive a speech synthesiser

Pharynx

TongueLarynx

Soft palate Hard palate

Speech production - vocal tract

Parametric coding - Speech synthesis

Excitation generator

Synthesis filters

Synthetic speech

(Vocal tract)(Lungs, vocal chords)

Excitation generator

Synthetic speech

Pitch prediction filter

(Long term)

LPC synthesis filter

(Short term)

Perceptual error weighting filter

Speech synthesis

LPC - Linear Predictive Coding

Analysis-by-synthesis predictive coding

Speech input

Excitation generator

Synthesis filters

Perceptual error weighting

Error minimisation

Speech encoder

TETRA ACELP

Algebraic Code Excited Linear Predictive

Excitation generator

Pitch prediction filter

Algebraic codebook

Adaptive codebook

Audio processing (Tx)

Speech encoder

Homing function

Frame stealing

Encryption

Speech importance

Channel coding Digital output

Speech input

Audio processing (Rx)

Speech decoder

Homing function

Missing frame generation

Digital input

Speech output

Missing frame substitution

Decryption

Channel decoding

Comfort Noise

Channel coding

Analogue speech input

Speech encoder

Channel encoder

Digital output

274 bits per 60 msec of speech

= 4.567 kbit/s

432 bits per 60 msec of speech

= 7.2 kbit/s

Quality speech @ low bit-rate

Complete Codec - Block Diagram

4.567 kbps

TETRATETRAChannelChannelEncodingEncoding

FEC+ CRCFEC+ CRC

7.2 kbps

ImportanceFactor

Three levels of bit stream

protection FEC

analoguespeech

TXTX

TETRATETRAVoiceVoice

DecoderDecoder

RXRX

TETRATETRAVoiceVoice

EncoderEncoder

TETRATETRAChannelChannelDecodingDecoding

HiHi

Med..Med..

NoNo

4.567 kbps

8 KHz8 kHzSampling

16 bits

8 kHz8 kHzSamplingSampling

16 bits16 bits

7.2 kbps

BadFrameFlag

• How to send four speech channels down one digital pipe?

• How to “steal” 18th time slot to send synchronisation data without loosing speech?

Usual Questions

Secret of a “stolen” Frame

1 ch. encoded speech frame

60 msec

2 31 15 16 17 1

1 TETRA Multiframe - 1.02 sec

transmitted slot No.

transmitted frame No.

15 16 17 181234123412341234 1234

1817

56.67 msec

1234123412341234

18 1 2 3 1615 1

17 x 60 msec segments of encoded speech bits (1.02 sec)

1.02 sec

18th stolen frame

17 x 56.67 msec TDMA frames (0.963 sec) ÷4=241ms/ ch.

Codec selection

Speech quality

Subjective assessments

Complexity

Computational demand of speech and channel codec

Codec complexity

C = MOPS + 0.2 * RAM (kB) + 0.05 * ROM (kB)

Overall encoder complexity 11.9 MOPS

Overall decoder complexity 5.4 MOPS

Very complex Heavy processing demand

Codec performance

0

1

2

3

4

TETRA GSM

Qu

ali

ty (

MO

S)

MOS 4 Excellent quality : Imperceptible impairment

MOS 3 Good quality : Just perceptible impairment, but not

annoying

TETRA Codec performance

Factor Effect on quality

Input level change Insensitive

Frame stealing Slight degradation, not significant

Tandeming Best avoided!

Background noise at Tx Practical results impressiveVery robust

Codec performance

Quality (Q) comparison with analogue FM

0

5

10

15

20

25

30

-32 -22 -12

TETRA

FM

Quality

Q (dB)

Audio input level (dB)

TETRA

FM

Codec performance

Comparison with analogue FM

TETRATETRA

Range

Quality

Low background noise

FM

High background noise

Codec performance

ETSI demo 2 phrases Analogue2 phrases TETRA

1 Male Moderate C/N

2 Female Moderate C/N

3 Male Poor C/N

4 Female Poor C/N

TETRA

FM

Range

Quality

1 & 2 3 & 4

•TETRA provision for 4 codecs

•Enhanced codec for TETRA telephony

• AMR (Adaptive Multi-Rate)

•Provision of a new codec for military

Future enhancements

Conclusions

• Using ACELP technique, TETRA Codec provides a nearly GSM quality at almost half bit rate

• TETRA Codec provides a superior quality to FM and GSM in the high background noise environment

• Current TETRA Standard has a provision for 4 Codecs

• Additional Codec planned for TETRA Release 2 will provide even higher quality for telephony applications