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TETRA Voice Coding

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

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  1. TETRA Voice Coding Ranko Pinter Simoco Digital Systems

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

  3. Analogue transmission Continuous variations in electric current

  4. Encoder Analogue speech to bit stream Digital transmission De-coder Bit stream to analogue speech

  5. Types of codec • Waveform codec • Transmitted bits represent the speech waveform • Parametric codec • Transmitted bits drive a speech synthesiser

  6. Reconstruct Decode Waveform codecs Sample Quantise Encode

  7. Parametric codecs Parametric model of speech production Transmitted bits drive a speech synthesiser

  8. Speech production - vocal tract Soft palate Hard palate Pharynx Larynx Tongue

  9. Parametric coding - Speech synthesis Excitation generator Synthesis filters Synthetic speech (Lungs, vocal chords) (Vocal tract)

  10. Speech synthesis Synthetic speech Pitch prediction filter (Long term) LPC synthesis filter (Short term) Perceptual error weighting filter Excitation generator LPC - Linear Predictive Coding

  11. Analysis-by-synthesis predictive coding Speech input Perceptual error weighting Excitation generator Synthesis filters Error minimisation Speech encoder

  12. TETRA ACELPAlgebraic Code Excited Linear Predictive Algebraic codebook Excitation generator Adaptive codebook Pitch prediction filter

  13. Audio processing (Tx) Frame stealing Speech input Homing function Speech importance Speech encoder Encryption Channel coding Digital output

  14. Audio processing (Rx) Speech output Missing frame substitution Missing frame generation Homing function Comfort Noise Speech decoder Decryption Channel decoding Digital input

  15. 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

  16. 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

  17. 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?

  18. 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

  19. Codec selection Speech quality Subjective assessments • Complexity • Computational demand of speech and channel codec

  20. 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

  21. Codec performance MOS 4 Excellent quality : Imperceptible impairment MOS 3 Good quality : Just perceptible impairment, but not annoying

  22. 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

  23. Codec performance Quality (Q) comparison with analogue FM Quality Q (dB) TETRA FM Audio input level (dB)

  24. High background noise FM Codec performance Comparison with analogue FM Low background noise TETRA Quality Range

  25. 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

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

  27. 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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