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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. Encoder

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tetra voice coding

TETRA Voice Coding

Ranko Pinter

Simoco Digital Systems

agenda
Agenda
  • Why code speech?
  • Basic principles of TETRA voice coding
  • How was TETRA codec selected?
  • Operational performance
  • Future enhancements
analogue transmission
Analogue transmission

Continuous variations in electric current

digital transmission

Encoder

Analogue speech to bit stream

Digital transmission

De-coder

Bit stream to analogue speech

types of codec
Types of codec
  • Waveform codec
    • Transmitted bits represent the speech waveform
  • Parametric codec
    • Transmitted bits drive a speech synthesiser
waveform codecs

Reconstruct

Decode

Waveform codecs

Sample

Quantise

Encode

parametric codecs
Parametric codecs

Parametric model of speech production

Transmitted bits drive a speech synthesiser

slide8

Speech production - vocal tract

Soft palate

Hard palate

Pharynx

Larynx

Tongue

parametric coding speech synthesis
Parametric coding - Speech synthesis

Excitation generator

Synthesis filters

Synthetic speech

(Lungs, vocal chords)

(Vocal tract)

slide10

Speech synthesis

Synthetic speech

Pitch prediction filter

(Long term)

LPC synthesis filter

(Short term)

Perceptual error weighting filter

Excitation generator

LPC - Linear Predictive Coding

analysis by synthesis predictive coding
Analysis-by-synthesis predictive coding

Speech input

Perceptual error weighting

Excitation generator

Synthesis filters

Error minimisation

Speech encoder

tetra acelp algebraic code excited linear predictive
TETRA ACELPAlgebraic Code Excited Linear Predictive

Algebraic codebook

Excitation generator

Adaptive codebook

Pitch prediction filter

audio processing tx
Audio processing (Tx)

Frame stealing

Speech input

Homing function

Speech importance

Speech encoder

Encryption

Channel coding

Digital output

audio processing rx
Audio processing (Rx)

Speech output

Missing frame substitution

Missing frame generation

Homing function

Comfort Noise

Speech decoder

Decryption

Channel decoding

Digital input

channel coding
Channel coding

Analogue speech input

Digital output

Channel encoder

Speech encoder

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

slide16

Complete Codec - Block Diagram

analogue

TETRA

TETRA

TETRA

TETRA

speech

Voice

Voice

Voice

Voice

Encoder

Encoder

Decoder

Decoder

8 KHz

8 kHz

8 kHz

8 kHz

Importance

Sampling

Sampling

4.567 kbps

Sampling

4.567 kbps

Factor

16 bits

16 bits

16 bits

TETRA

TETRA

TETRA

TETRA

Channel

Channel

Channel

Channel

Encoding

Encoding

Decoding

Decoding

Bad

FEC+ CRC

FEC+ CRC

Frame

Flag

Three levels of

Hi

Hi

bit stream

TX

TX

RX

RX

Med..

Med..

7.2 kbps

protection FEC

7.2 kbps

No

No

slide17

Usual Questions

  • How to send four speech channels down one digital pipe?
  • How to “steal” 18th time slot to send synchronisation data without loosing speech?
secret of a stolen frame

1 TETRA Multiframe - 1.02 sec

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

1 ch. encoded speech frame

transmitted frame No.

1

3

18

2

15

16

17

18

1

60 msec

1

15

16

17

transmitted slot No.

1

2

3

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

56.67 msec

15

16

17

18

1

2

3

4

1

2

3

4

18th stolen frame

1

1

1

1

1

1

1

2

2

2

2

2

2

2

3

3

3

3

3

3

3

4

4

4

4

4

4

4

1.02 sec

Secret of a “stolen” Frame
codec selection
Codec selection

Speech quality

Subjective assessments

  • Complexity
      • Computational demand of speech and channel codec
codec complexity
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
Codec performance

MOS 4 Excellent quality : Imperceptible impairment

MOS 3 Good quality : Just perceptible impairment, but not annoying

tetra codec performance
TETRA Codec performance

FactorEffect on quality

Input level change Insensitive

Frame stealing Slight degradation, not significant

Tandeming Best avoided!

Background noise at Tx Practical results impressive

Very robust

codec performance1
Codec performance

Quality (Q) comparison with analogue FM

Quality

Q (dB)

TETRA

FM

Audio input level (dB)

codec performance2

High background noise

FM

Codec performance

Comparison with analogue FM

Low background noise

TETRA

Quality

Range

codec performance3
Codec performance

3 & 4

1 & 2

Quality

FM

TETRA

ETSI demo 2 phrases Analogue

2 phrases TETRA

Range

1 Male Moderate C/N

2 Female Moderate C/N

3 Male Poor C/N

4 Female Poor C/N

future enhancements
Future enhancements
  • TETRA provision for 4 codecs
  • Enhanced codec for TETRA telephony
    • AMR (Adaptive Multi-Rate)
  • Provision of a new codec for military
conclusions
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
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