Dr. Dóra Maros
Institute of Telecommunication
Óbuda University
Telecommunications Techniques II.
Source Coding (speech)
Error detection and Error correction
Transmission from source to
destination
Source
(analog)
-Microphone
-Camera
-Pressure,
temperature etc
sensors
Source
coding
Source
decoding
Channel
coding
Channel
decoding
Channel
(digital)
Destination
(analog/digital)
-Speaker (ears)
-Displays (eyes)
-Alarms (see and
hear!)
Bandwidth allocation
Source
information
Filter
Source information
A/D
Filter
Compression
A/D
Channel coding
Compression
Modulation
Channel
coding
Channel
How we „generate” voice?
Glottis
Vocal cords
Vocal tract
(resonator
filter system)
Excitation
Characterization
of Human Speech
• Frequency: 300-3400 Hz
• The charcaterization of a 20 ms duration sample does not
change (excitation, filter system)
• We can find some correlations between two consecutive
20 ms samples
• Using a long time analysis we can predict the next pattern
(differencial coding, eg. DPCM)
•Our brain can correct a lot of „errors”!
Classification of
Speech Encoders
Waveform encoder: describes the analog
signals in the time and frequency domain
Vocoders: describes the rules of phonation
(how we generate the voice)
Hybrid encoders: combines good properties
of two coders (low rate, good quality)
Subjective MOS
MOS: Mean Opinion Score
Theory of Waveform Encoders
Sampling
(sampling
frequency)
Quantization
(number of levels)
Coding
(length of
codeword)
Benefits: good voice quality
Disadvantage: big rate (PCM: 64 kbit/s)
Theory of Vocoders
A, E, O, I etc.
vowels
Excitation
B, F, P, M etc.
consonants
Benefit: low rate (pár kb/s)
Disadvantage: bad quality
Voiced - Vowels
Typical amplitude/time
signal
Pitch frequency: 50-500 Hz
Typical
Power Spectrum Density/
Frequency signal
Voiceless - Consonants
Typical amplitude/time
signal (white noise)
Typical
Power Spectrum Density/
Frequency signal
Sub-band Coding
•Baseband is divided into sub-bands regarding PSD
•Each sub-bands are encoded separately
•Benefits: low noise sensitivity (different sampling
frequencies and codeword length are used)
•Bitrate: 16-32 kbit/s
•Receiver: sub-bands are superimposed (added)
Speech Codecs MOS
AMR: Adaptive
Multirate Codec
(3G és 4G mobile)
G723: VoIP (mobile)
G728: LD-CELP, LowDelay code excited
linear prediction
G729: CS-ACELP,
Coding of Speech,
Adaptive Code
GSM FR and EFR: Full rate, Enhanced Full Rate Excited Linear
Prediction (VoIP)
Internet Low Bitrate Codec (iLBC)
Theory of Hybrid Encoder/Decoder
Analysis by Synthesis (AbS)
Excitation: MPE and RPE
RPE
MPE: Multiple Pulse
Excitation
MPE
RPE: Regular Pulse
Excitation, GSM13kbit/s
MPE: aplitudes and phases are encoded
RPE: only amplitudes is encoded
Excitation: CELP (Code Excited
Linear Prediction)
Excitation
Table
Index
Excitation
Table
•Code table: 1024 excitation samples
•Only the index of table is sent!
•Bitrate: 4,8-16 kbit/s
Codec Rate
tin
tout
Encoder
Decoder
Codec rate:
tin-tout
Typical: 50-100 ms
Fast: G728 CELP (DoCELP) codec, 2-5 ms
GSM Speech Codec
Voice
activity
detector
Waveform
encoder
Bitrate: 260 bit/20ms, 13kbit/s
Voice Activity Detector (VAD)
Discontinous Transmission (DTX)
and SID Frame
SID: Silence Indicator
Fletcher-Munson Curves
(sensitivity of ears)
Equal-loudness
contours for
the human ear
Phon: Unit of
relative
loudness level
for pure tones
Frequency
Color Sensitivity of Eyes
Luminosity sensitivity is 3 x more faster then
color sensitivity
Compression of Source Information
Lossless compression: small part of information is
lost
Lossy compression: huge part of information is lost
Compression Rate:
original information/compressed information
image:
lossless : .raw
lossy:
.jpeg (eg. 25/1)
Error Correction (general)
Noisy channel
Demodulator
Received bit
sequence (BER)
Error detection
procedures
Automatic Repeat
Request (ARQ)
Forward Error Coding
(FEC)
Error detection
Error correction
Error Detection I. (repetition)
• We use small blocks
• Each block is sent repeatedly
• Efficient, when we have only one bit error
Sender
Receiver
1011 1011 1011
1010 1011 1011
1011 1011 1011
1010 1011 1010
Error Detection II. (parity)
• Even or odd parity
• Even: we add „1” parity if the number of „1”s is
odd, otherwise parity is „0”.
• Odd: we add „1” parity, if the number of „1”s is
even
• Applications: mikrocontollers, data bus, data
storage
Sender
Receiver
Even parity
10110010
10110110
Odd parity
00110010
00110110
Error Detection III. (polarity change)
• We send the original and negated bit sequency
• If the bit polarity is the same at the receiver error
• Disadvantege: big redundancy
• Application: multicarrier systems
Sender
Receiver
Receiver
10110010
10110010
10100010
01001101
01011101
01011101
Error Detection IV.
(Cyclic Redundancy Check -CRC)
• A polimon is generated from bit sequence (data)
• Generator polinom is the same (known) at sender and
receiver
• CRC= data polinom/generator polinom
• Application: mobile, RFID, Bluetooth, Ethetnet..etc
MSB
Sender
LSB
10110010101001 CRC
polinom
division
generator polinom
Receiver
10100010101001 CRC
?
polinom
division
CRC
generator polinom
Error Detection V.
(CheckSum)
•
•
•
•
•
The message is segmented into parts
We binary add the parts (CS)
The message and CS are sent
We binary add the parts again at receiver side
If two CSs are not equal -error
Sender
Error Detection VI.
(Hamming coding)
Receiver
Code i
Code i
Hamming
distance
calculator
The Hamming distance of the code
words is greater or equal d+1
Eg: 4/7 Hamming code …….4 data bits – 3 parity bits
d=2 error can be detected, but only one is corrected
Number of parity bits = largest exponent (23)
Hamming coding (even parity)
DATA
PARITY
7653
0001
HD:4
0010
HD:3
0011
HD:3
0100
p1 p2 p3
110
101
011
011
p1: (3,5,7) – 20=1
P2: (3,6,7) – 21=2
P3: (5,6,7) – 22=4
7654321
Error Correction
(ARQ:Automatic Repeat Request)
Four types:
•Stop and Wait
•Go-Back-N
•Selective
•Hybrid
Stop and Wait ARQ
Data block
t1
Sender
Receiver
Data block
Sender
t1
Receiver
Data block
t2
After time out the sender repeats the transmission
Problem: receiver does not know whether it received the repeated
or next data block
Solution: 0 and 1 flag bits in consecutive data blocks
Go-Back-N ARQ
1. We define a sliding
time window
2. The packets are sent
one by one (without
ACKs) in this window
3. If the first ACK is
detected window slides.
4. If not, all packets are
sent again
Selective ARQ
Message
Long message – one ACK
Advantage: if no errors – fast transmission
Disadvantage: If errors – the message is sent again –
slow transmission
Small block – more ACKs
Advantage: we repeate only those blocks where errors are
detected
Disadvantage: slow ( ACK)
Hibrid ARQ (Chase combining)
NACK is sent back, receiver does not drop faulty packets
Using an errors correction procedure we can repair the
data from faulty packets
Hibrid ARQ (Incremental redundancy)
A FEC (Forward Error Coding) is used at sender side
The redundancy bits are sent after NACK
Forward Error Coding: FEC
Two main types:
•Convolutional coding
•Block coding
We add redundancy bits which help correct
data at the receiver
Convolutional Coding (theory example)
(3,1,3) coder
(n,k,m) coder
D laches and modulo2 adders
• n: number of outputs
• k: number of inputs
• m: number of D laches
• number of input state : (m-1)2
Generator polinoms
G1(1,1,1)
G2 (0,1,1)
G3(1,0,1)
Types of Convolutional Coders
(Mother coder and „punctured” coder)
Mother coder:1/n
„Punctured” coder: k/n
eg: 2/3 coder
Systematic coder
•We send original data (V3,V2,V1 and
redundancy (v4)
Non systematic coder
•We do not send original data
•We send only convolutional coded info
We code „1”
(find the error!)
Output:
11111011
1011 vs. 1 1 1 1 0 1 1 1 0 1 0 1 1 1 ?
But we use convolution !
Input bits
Respones (coded data)
1
11111011
0
00000000
(two bits shift)
1
11111011
1
11111011
----------------------------------------------------------------------------Kódolt info
11110111010111
We use modulo2 adder for outputs!
Shift: L-1 (L: number of inner D laches)
Operation table
State graph
Info: 1011
Coded info:
11110111010111
Tree structure
Info: 1011
Coded info:
11110111010111
Trellis Graph
Coding with Trellis Graph
Info: 1011
Coded info:
11110111010111
Error Correction – Fano algorythm I.
Info: 1011
Coded info: 11 11 01 11 01 01 11 - Detected info: 01 11 01 11 01 01 11
Error Correction – Fano algorythm II.
Error Correction – Fano algorythm III.
Error Correction – Viterbi algorythm I.
We use Hamming metric
Hamming metric
Error Correction – Viterbi algorythm II.
Error Correction – Viterbi algorythm III.
Error Correction – Viterbi algorythm IV.
Error Correction – Viterbi algorythm V.
Error Correction – Viterbi algorythm VI.
Error Correction – Block Coding (general)
G= 1+ X11+X13+X14+X16
XOR Gates
LFSR: Linear Feedback Shift Register
Error Correction – Block Coding
(GSM Speech)
XOR gate
Data
50 bits
Block
code bits
SW-ON – coding (50 clocks)
SW-OFF – Reading Data from D laches