In telecommunication and data storage, Manchester coding is a line code in which the encoding
of each data bit has at least one transition and occupies the same time. It therefore has
no DC component, and is self-clocking, which means that it may be inductively or capacitively
coupled, and that a clock signal can be recovered from the encoded data. As a result, electrical
connections using a Manchester code are easily galvanically isolated using a network isolator—a
simple one-to-one isolation transformer.
Background The name comes from its development at the
University of Manchester, where the coding was used to store data on the magnetic drum
of the Manchester Mark 1 computer. Manchester coding is widely used; consumer
IR protocols; see also RFID or near field communication). There are more complex codes,
such as 8B/10B encoding, that use less bandwidth to achieve the same data rate but may be less
tolerant of frequency errors and jitter in the transmitter and receiver reference clocks.
Features Manchester code ensures frequent line voltage
transitions, directly proportional to the clock rate; this helps clock recovery.
The DC component of the encoded signal is not dependent on the data and therefore carries
no information, allowing the signal to be conveyed conveniently by media which usually
do not convey a DC component. Description
Extracting the original data from the received encoded bit:
Summary: Each bit is transmitted in a fixed time.
A 0 is expressed by a low-to-high transition, a 1 by high-to-low transition.
The transitions which signify 0 or 1 occur at the midpoint of a period.
Transitions at the start of a period are overhead and don't signify data.
Manchester code always has a transition at the middle of each bit period and may have
a transition at the start of the period also. The direction of the mid-bit transition indicates
the data. Transitions at the period boundaries do not carry information. They exist only
to place the signal in the correct state to allow the mid-bit transition. The existence
of guaranteed transitions allows the signal to be self-clocking, and also allows the receiver
to align correctly; the receiver can identify if it is misaligned by half a bit period,
as there will no longer always be a transition during each bit period. The price of these
benefits is a doubling of the bandwidth requirement compared to simpler NRZ coding schemes.
Manchester encoding as phase-shift keying Manchester encoding is a special case of binary
phase-shift keying, where the data controls the phase of a square wave carrier whose frequency
is the data rate. Such a signal is easy to generate.
Conventions for representation of data
There are two opposing conventions for the representations of data.
The first of these was first published by G. E. Thomas in 1949 and is followed by numerous
authors. It specifies that for a 0 bit the signal levels will be low-high - with a low
level in the first half of the bit period, and a high level in the second half. For a
1 bit the signal levels will be high-low. The second convention is also followed by
numerous authors as well as by IEEE 802.4 and lower speed versions of IEEE 802.3 standards.
It states that a logic 0 is represented by a high-low signal sequence and a logic 1 is
represented by a low-high signal sequence. If a Manchester encoded signal is inverted
in communication, it is transformed from one convention to the other. This ambiguity can
be overcome by using differential Manchester encoding.
See also Coded mark inversion
Differential Manchester encoding Self-clocking signal
Binary offset carrier modulation References
This article incorporates public domain material from the General Services Administration
document "Federal Standard 1037C".