hdb3 code

EEE358S Communications Engineering508 April 7, 2023

EEE358SFundamentals of Communications Engineering

Pulse Code Modulation

Emmanuel O [email protected]

http://www.uct.ac.za/depts/staff/rebejide/Department of Electrical Engineering

University of Cape Town


Analogue to Digital

After sampling, the analogue amplitude value of each sampled (PAM) signal is quantized into one of a number of L discrete levels. The result is a quantized PAM signal.

A codeword can then be used to designate each level at each sample time. This procedure is referred to as “Pulse Code Modulation”.

Low-pass Filter

Encoder; Pulse

modulateSampler Quantizer

Continuous-time message signal

PCM wave

Quantized PAM signal

Updated 9/2004


Encoding

After quantization, a digit is assigned to each of the quantized signal levels in such a way that each level has a one-to-one correspondence with the set of real integers. This is called digitization of the waveform.

Each integer is then expressed as an x-bit binary number, called codeword, or PCM word.

The number of codewords, L , is related to x by: 2x = L

Updated 9/2004

Quantized PAM signal

A real integer

PCM codeword (bit stream)

digitization To binary


Codeword

Quantization followed by digitization maps input amplitudes into PCM words.

A cell is the set of input amplitudes mapped to a codeword.

There are L integers, PCM words, or codewords to correspond to the L allowed output amplitudes of the quantizer.

Codebook is the set of all these L codewords.

Updated 9/2004


Analogue to Digital

time

Analogue waveform

voltage

00000011010101111001101111011111

0010010001101000101011001110

01

1110 1000 0100 0010 0000 0011 0101 1011

Bit stream

Sign bit

1 1 1 0 1 0 0 0 0 1 0 0 0 0 101 0 0 0 0 0 0 0 1 1 0 1 0 1 1 1

PCM Codeword

3. Digitize into real integers

1. Sample analogue waveform at discrete times

2. Q

uant

ize

into

dis

cret

e le

vels

Updated 9/2004

01234567

-1-2-3-4-5-6-7

-7 -4 -2 -1 0 1 2 5

4. To binary


Types of Encoding(to PCM Waveforms)

Unipolar

RZ

NRZ

Bipolar AMI

Biphase Polar

M

L

S

Updated 9/2004

Polar

Bipolar

Unipolar


Encoder Attributes

RZ (return to zero), NRZ (non-return to zero) Unipolar, Polar, Bipolar L (level), M (mark), S (space) Biphase AMI (alternate mark inversion)

Updated 9/2004


NRZ – Unipolar/Polar – L/M/S Coding

Unipolar

RZ

NRZ

Bipolar AMI

Biphase Polar

M

L

S

Updated 9/2004

Polar

Bipolar

Uniolar


NRZ coding may be unipolar or polar

1 0 1 1 0 11 0 01

UnipolarNRZ-L

PolarNRZ-L


NRZ-L Coding

NRZ-L (level):1 higher level; 0 lower level

Used in SONET XOR bit sequence, and in early magnetic tape recording

Long sequence of same bit causes difficulty in clock recovery; also in detecting the average DC level


(Polar) NRZ Coding

1 0 1 1 0 11 0 01

NRZ-L

NRZ-M

NRZ-S


NRZ – Unipolar/Polar – L/M/S Coding

NRZ-L (level):1 higher level; 0 lower level

NRZ-M (mark):1 (mark): change level0 (space): no change in level (clock recovery problem with

successive 0’s)used primarily in magnetic tape recording used in Synchronous data link control (SDLC)

NRZ-S (space):0 (space): change in level 1 (mark): no change level (clock recovery problem with

successive 1’s)

Updated 9/2004


RZ – Bipolar/Unipolar Coding

Unipolar

RZ

NRZ

Bipolar AMI

Biphase Polar

M

L

S

Updated 9/2004

Polar

Bipolar

Unipolar


RZ Coding may be unipolar or bipolar

1 0 1 1 0 11 0 01

UnipolarRZ

BipolarRZ

RZ-AMI

Include RZ-AMI here for completeness, defer discussion

Updated 9/2004


RZ Coding may be unipolar or bipolar

Unipolar-RZ:1 is represented by positive for the first half of T and zero for

the second half.0 is represented by 0

Bipolar-RZ:1 is represented by positive for the first half of T and zero for

the second half.0 is represented by negative for the first half of T and zero for

the second half.

Used in baseband data transmission, magnetic recording. The transitions at T/2 may be used for synchronization.


Biphase Coding

Unipolar

RZ

NRZ

Bipolar AMI

Biphase Polar

M

L

S

Updated 9/2004

Polar

Bipolar

Unipolar


Biphase Coding

1 0 1 1 0 11 0 01

biphase-L

biphase-M

biphase-S


Biphase Coding may be L, M, or S

Biphase-L (level) / Manchester:0 is represented by +ve for first half and –ve for 2nd half1 is represented by -ve for first half and +ve for 2nd half

used in digital logic circuits including IEEE 802.4 standard, Ethernet.

An alternate scheme adopted by some authors has the above coding for 0’s and 1’s in reversed manner.


Biphase Coding may be L, M, or S

biphase-M (mark) / Differential Manchester:Always transition at beginning of bit1 (mark) is represented by second transition at T/20 (space) is represented by no second transition at T/2

Alternately: always transition at center1 is represented by additional transition at start of T.

biphase-S (space):Always transition at beginning of bit0 (space) is represented by second transition at T/21 (mark) is represented by no second transition at T/2


AMI Coding

Unipolar

RZ

NRZ

Bipolar AMI

Biphase Polar

M

L

S

Updated 9/2004

Polar

Bipolar

Unipolar


(Bipolar) AMI Coding may be NRZ or RZ

1 0 1 1 0 11 0 01

NRZ-AMI

RZ-AMI


AMI Coding may be NRZ or RZ

AMI (Alternate Mark Inversion) coding: 0 is represented by 0

NRZ-AMI: The 1’s are alternately positive and negative. RZ-AMI: The 1’s are represented by equal amplitude

opposite polarity RZ pulses

Used in the signaling scheme in telephone systems.


B8ZS coding

B8ZS (bipolar with eight-zero substitution) coding: With AMI coding, user data containing too many

successive 0’s are difficult to find bit boundaries. Bipolar violations are deliberately inserted if user data

contains a string of 8 or more consecutive zeros. A violation bit has the same polarity as that for the

previous non-zero bit. Is used in the T1 rate.


B8ZS coding versus bipolar NRZ-AMI

0 0 0 0 0 10 0 01

NRZ-AMI

B8ZSV V


HDB3 coding

HDB3 (High density bipolar order 3) code modifies AMI to prevent long runs of 0’s by introducing violation (V) and balance (B) bits. It is used in E1.

4 0’s are replaced by 000V if the number of 1’s from the previous V is odd.Receiver turns back to 0 all V preceded by 000

4 0’s are replaced by B00V if the number of 1’s from the previous V is even.B is such at V is of opposite polarity to the previous V. Its

purpose is to prevent DC introduced by the V’sReceiver turns back to 0 all V preceded by 00, together the bit

(B) before the 00.


HDB3 coding versus bipolar NRZ-AMI

0 0 1 1 01 0 0 00

NRZ-AMI

HDB3VB

0

V

0

V

hdb3 code

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