TETRA Voice Coding

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TETRA Voice Coding

• Ranko Pinter
• Simoco Digital Systems

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Agenda

• Why code speech?
• Basic principles of TETRA voice coding
• How was TETRA codec selected?
• Operational performance
• Future enhancements

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Analogue transmission
Continuous variations in electric current
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Digital transmission
De-coder Bit stream to analogue speech
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Types of codec

• Waveform codec
• Transmitted bits represent the speech waveform

• Parametric codec
• Transmitted bits drive a speech synthesiser

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Waveform codecs
Sample
Quantise
Encode
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Parametric codecs

• Parametric model of speech production
• Transmitted bits drive a speech synthesiser

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Speech production - vocal tract
Soft palate
Hard palate
Pharynx
Larynx
Tongue
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Parametric coding - Speech synthesis
Excitation generator
Synthesis filters
Synthetic speech
(Vocal tract)
(Lungs, vocal chords)
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Speech synthesis
Synthetic speech
Pitch prediction filter (Long term)
LPC synthesis filter (Short term)
Perceptual error weighting filter
Excitation generator
LPC - Linear Predictive Coding
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Analysis-by-synthesis predictive coding
Speech input
Perceptual error weighting
Excitation generator
Synthesis filters
Error minimisation
Speech encoder
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TETRA ACELPAlgebraic Code Excited Linear
Predictive
Algebraic codebook
Excitation generator
Adaptive codebook
Pitch prediction filter
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Audio processing (Tx)
Frame stealing
Speech input
Homing function
Speech importance
Speech encoder
Encryption
Channel coding
Digital output
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Audio processing (Rx)
Speech output
Missing frame substitution
Missing frame generation
Homing function
Speech decoder
Comfort Noise
Decryption
Channel decoding
Digital input
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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 _at_ low bit-rate
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Complete Codec - Block Diagram
analogue
TETRA
TETRA
TETRA
TETRA
speech
Voice
Voice
Voice
Voice
Decoder
Decoder
Encoder
Encoder
8 KHz
8 kHz
8 kHz
8 kHz
Importance
4.567 kbps

Sampling
Sampling
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
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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?

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Secret of a stolen Frame
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Codec selection
Speech quality Subjective assessments
Complexity Computational demand of speech and
channel codec
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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
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Codec performance
MOS 4 Excellent quality Imperceptible
impairment MOS 3 Good quality Just
perceptible impairment, but not
annoying
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TETRA Codec performance
Factor Effect on quality Input level
change Insensitive Frame stealing Slight
degradation, not significant Tandeming Bes
t avoided! Background noise at Tx Practical
results impressive Very robust
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Codec performance
Quality (Q) comparison with analogue FM
Quality Q (dB)
TETRA
FM
Audio input level (dB)
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Codec performance
Comparison with analogue FM
Low background noise
TETRA
Quality
Range
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Codec performance
3 4
1 2
Quality
FM
ETSI demo 2 phrases Analogue 2 phrases TETRA
TETRA
Range
1 Male Moderate C/N
2 Female Moderate C/N
3 Male Poor C/N
4 Female Poor C/N
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Future enhancements

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

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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