Uni-polar and Polar Signals
. Generally digital signal is visualized as
a train of discrete-amplitude rectangular pulses. In
fact, this is often the way it comes from the data source.The
below illustration shows the binary message 10110100
as it might appear at the output of a digital
computer. This waveform, a simple on-off sequence, is
said to be uni-polar, because it has only one polarity.
It is also synchronous because all pulses have equal
duration and there is no separation between them.
The disadvantages of uni-polar signals are:
They contain a nonzero dc component that is difficult
to transmit, carries no information, and is a waste
of power.
Synchronous signals require timing co-ordination
at transmitter and receiver (like synchronous demodulation),
which means design complications.
The polar (two-polarity) return-to-zero
signal of below illustration overcomes both of these
problems. However, the "spaces" making the signal self-clocking
are a waste of transmission time. If efficiency is a
dominant consideration, the polar synchronous signal
of below illustration would be preferable. As in case
of analog systems, there is a trade-off between efficient
transmission and simpler design.
Binary messages can also be represented as shown in
below illustration. The decode wave consists of polar
pulses denoting only the symbol transitions. The quaternary
(four-level) signal is derived by grouping the binary
digits in blocks of two.
