Transcript 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.

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
Encoder
Analogue
speech to
bit stream
De-coder
Bit 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
Reconstruct
Decode
Encode
Parametric codecs
Parametric model of speech production
Transmitted bits drive a speech synthesiser
Speech production - vocal tract
Soft palate
Hard palate
Pharynx
Larynx
Tongue
Parametric coding - Speech synthesis
Excitation
generator
(Lungs, vocal chords)
Synthesis
filters
(Vocal tract)
Synthetic
speech
Speech synthesis
Synthetic
speech
Excitation
generator
Pitch
prediction
filter
LPC
synthesis
filter
(Long term)
(Short term)
LPC - Linear Predictive Coding
Perceptual
error
weighting
filter
Analysis-by-synthesis predictive coding
Speech input
Excitation
generator
Perceptual
error
weighting
Synthesis
filters
Error
minimisation
Speech encoder
TETRA ACELP
Algebraic Code Excited Linear Predictive
Algebraic
codebook
Excitation
generator
Adaptive
codebook
Pitch prediction
filter
Audio processing (Tx)
Frame
stealing
Speech
input
Homing
function
Speech
importance
Speech
encoder
Encryption
Channel coding
Digital output
Audio processing (Rx)
Speech
output
Missing frame
substitution
Comfort Noise
Missing frame
generation
Homing
function
Speech
decoder
Decryption
Channel decoding
Digital input
Channel coding
Analogue
speech
input
Digital
output
Channel
encoder
Speech
encoder
274 bits per 60
msec of speech
432 bits per 60 msec
of speech
= 4.567 kbit/s
= 7.2 kbit/s
Quality speech @ low bit-rate
Complete Codec - Block Diagram
analogue
speech
TETRA
Voice
Encoder
TETRA
Voice
Decoder
88 KHz
kHz
Sampling
16 bits
Importance
Factor
4.567 kbps
4.567 kbps
TETRA
Channel
Encoding
FEC+ CRC
Three levels of
bit stream
protection FEC
Hi
Med..
No
TETRA
Channel
Decoding
TX
7.2 kbps
RX
7.2 kbps
Bad
Frame
Flag
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
60 msec
1 ch. encoded
speech frame
transmitted
frame No.
transmitted
slot No.
1
2
18
15
3
1
2
3
1234 1234 12341234
16
17
1
15
16
17 18
1
1
1
1
1
1234 1234 1234 12341234
5
6
7
8
56.67 msec
17 x 60 msec segments of encoded speech bits (1.02 sec)
18th stolen frame
17 x 56.67 msec TDMA frames (0.963 sec) ÷4=241ms/ ch.
1.02 sec
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
Quality (MOS)
Codec performance
4
3
2
1
0
TETRA
GSM
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 impressive
Very robust
Codec performance
Quality (Q) comparison with analogue FM
30
Quality
25
Q (dB) 20
TETRA
TETRA
FM
15
FM
10
5
0
-32
-22
-12
Audio input level (dB)
Codec performance
Comparison with analogue FM
Low background noise
TETRA
Quality
High background noise
FM
Range
Codec performance
1&2
Quality
ETSI demo
2 phrases Analogue
2 phrases TETRA
3&4
FM
TETRA
Range
1
Male
Moderate C/N
2
Female
Moderate C/N
3
Male
Poor C/N
4
Female
Poor C/N
Future enhancements
•TETRA provision for 4 codecs
•Enhanced codec for TETRA telephony
•
AMR (Adaptive Multi-Rate)
•Provision of a new codec for military
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