Polar encoding
Polar Encoding
When the digital encoding is symmetrical--around 0 Volts--it is called a Polar Code. For example, the RS-232D interface uses Polar line encoding. The signal does not return to zero; it is either a +ve voltage or a -ve voltage. Polar line encoding is also called None Return To Zero (NRZ). Polar line encoding is the simplest pattern that eliminates most of the residual DC problem.
There is still a small residual DC problem, but Polar line encoding is a great improvement over Unipolar line encoding. Polar encoding has an added benefit in that it reduces the power required to transmit the signal by one-half.
File:Pds2.jpg RS-232D TXD
Polar and Unipolar line encoding both share the same synchronization problem: if there is a long string of logical 1s or 0s, the receive oscillator may drift and become unsynchronized.