Tetra voice coding
This presentation is the property of its rightful owner.
Sponsored Links
1 / 27

TETRA Voice Coding PowerPoint PPT Presentation


  • 128 Views
  • Uploaded on
  • Presentation posted in: General

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

Download Presentation

TETRA Voice Coding

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


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


The tetra mou association

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)


The tetra mou association

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


The tetra mou association

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


The tetra mou association

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 changeInsensitive

Frame stealingSlight degradation, not significant

TandemingBest avoided!

Background noise at TxPractical 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

1MaleModerate C/N

2FemaleModerate C/N

3MalePoor C/N

4FemalePoor 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


  • Login