running digital sum (code monitoring) (RDS)

In line coding, the signals to be transmitted are adapted to the properties of the transmission media. One aspect of this is freedom from direct current, which ensures that the mean value of the signal level on the transmission line does not change, which could lead to misinterpretations during decoding. DC freedom means that the average signal level shifts only insignificantly during the transmission of digital signals. The Running Digital Sum( RDS), which stands for the running sum of the digital signals, may only shift within a small range.

In line coding, DC shifts are reduced as much as possible by returning the signal level to zero or by biphasic signals. Non Return to Zero Mark( NRZ-M), CMI coding(Coded Mark Inversion) and differential Manchester coding are examples where the average DC voltage value does not change or changes only slightly.