The two main variants of biphase encoding techniques are discussed here. Manchester and Differential Manchester encoding scheme are explained with examples. Comparison between several classes of polar encoding techniques are done along with the exposure about the advantages and disadvantages of both schemes.

1. Manchester & Differential
Manchester
Line Coding: Polar Biphase Encoding Techniques
CMS-A-CC-4-8
Lockdown Talk Series: DCNIT-LDTalks-6
Arunabha Saha
Department of Computer Science, Vidyasagar College
University of Calcutta
May 2020

2. Outline
Overview of Biphase encoding techniques
Manchester encoding
Differential Manchester encoding

3. Polar Encoding Variants

4. Biphase Encoding
It solves the synchronization problem
The transition of state in signal happens in the middle of the bit;
does not returns to zero
Two variants:
Manchester Encoding (ME)
Differential Manchester Encoding (DME)

5. Manchester Encoding(ME)(1)
In ME, every bit represented as a transition, low → high or
high → low
Its a self-synchronizing signal with no DC component.
ME is a form of binary phase shift keying(BPSK), where the data
controls the phase of a square wave carrier whose frequency is the
data rate.
ME encodes both the data and the clock in a bit stream.

6. ME(2)
There are two conventions to represent ME
first published by G.E. Thomas(1949)
IEEE 802.3(ethernet) and IEEE 802.4(token bus) standards.

7. ME(3)
Encoding: In ME, XOR logic is used for encoding; XOR the data with the
clock
Conventions:
Each bit has a period .
IEEE 802.3: ’0’ represented by high-to-low and ’1’ represented by
low-to-high( in G.E. Thomas convention, the reverse is true).
Any transition occurs at the mid of the bit-period.
Initial transition of the waveform is just overhead; not represents any
meaningful data.

8. ME(4)
Applications:
ME used is early ethernet physical layer.
Its in use for IR protocols, RFID and NFC systems.
this widely used in LAN such as Ethernet(IEEE 802.3) and token
bus(IEEE 802.4).
Pros:
No DC component
No signal drooping issue
Ease of synchronization
ME is transparent
Cons:
Because of larger number of transitions occupies larger bandwidth.
Lacks error detection capability.

9. Differential Manchester Encoding(DME)(1)
Also known as Biphase Mark Code, Conditioned diphase , etc.
In DME, data and clock are combined to form singel 2-level
synchronizing data stream.
Presence or absence of transitions indicates logical value.
Its not important to know the exact polarity of the signal because
the data encoded as the change of state.
It makes synchronization easier
In DME, the transition in the middle of the bit-period used for
synchronization

10. DME(2)
Transition means "0" and no transition means "1" .

11. DME(3)
DME example:
Each digit in the differential encoded sequence is obtained by comparing
the present input bit with the past encoded bit. A binary 1 is encoded if
the present input bit and past encoded bit are of opposite state. A binary
0 is encoded if the states are the same.
d(n) = m(n) ⊕ d(n − 1)

12. DME(4)
Differential Manchester encoding has the following advantages over some
other line codes: 1
For every bit there is a transition is guaranteed.
In noisy environment, detecting transitions is less erroneous than
comparing agaisnt a threshold.
Unlike ME, here(in DME) presence of transitions is important, not
the polarity.
If the magnitude of high and low signal levels are same but of
different poalrity, it makes the average 0; (theoretically, 0 DC bias).
1 https://en.wikipedia.org/wiki/Differential_Manchester_encoding

13. Thank You
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