manchester encoding
(foldoc) | Manchester encoding
A method of transmitting bits which
enables the receiver to easily synchronise with the sender.
A simple way of signalling bits might be to transmit a high
voltage for some period for a 1-bit and a low voltage for a 0
bit:
Bits Sent: 1 1 0 0
Signal: High ___________
Low |___________
Time: -> . . . . .
However, when several identical bits are sent in succession,
this provides no information to the receiver about when each
bit starts and stops.
Manchester encoding splits each bit period into two, and
ensures that there is always a transition between the signal
levels in the middle of each bit. This allows the receiver to
synchronise with the sender.
In normal Manchester encoding, a 1-bit is transmitted with a
high voltage in the first period, and a low voltage in the
second, and vice verse for the 0 bit:
Bits Sent: 1 1 0 0
Signal: High __ __ __ __
Low |__| |_____| |__|
Time: -> . ' . ' . ' . ' .
In Differential Manchester encoding, a 1-bit is indicated by
making the first half of the signal equal to the last half of
the previous bit's signal and a 0-bit is indicated by making
the first half of the signal opposite to the last half of the
previous bit's signal. That is, a zero bit is indicated by a
transition at the beginning of the bit.
Like normal Manchester encoding, there is always a transition
in the middle of the transmission of the bit.
Differential Manchester Encoding
Bits Sent: 1 1 0 0
Signal: High ____ __ __ __
Low |_____| |__| |__|
Time: -> . ' . ' . ' . ' .
With each bit period half as long, twice as much bandwidth
is required when using either of the Manchester encoding
schemes.
(1995-11-23)
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