Bipolar encoding

[1] The use of a bipolar code prevents a significant build-up of DC, as the positive and negative pulses average to zero volts.

Little or no DC-component is considered an advantage because the cable may then be used for longer distances and to carry power for intermediate equipment such as line repeaters.

Bipolar encoding is preferable to non-return-to-zero whenever signal transitions are required to maintain synchronization between the transmitter and receiver.

These alternative approaches require either an additional transmission medium for the clock signal or a loss of performance due to overhead, respectively.

Where frequent transitions are a requirement, a self-clocking encoding such as return-to-zero or some other more complicated line code may be more appropriate, though they introduce significant overhead.

The coding was used extensively in first-generation PCM networks, and is still commonly seen on older multiplexing equipment today, but successful transmission relies on no long runs of zeroes being present.

[4] If the characteristics of the input data do not follow the pattern that every eighth bit is '1', the coder using alternate mark inversion adds a '1' after seven consecutive zeros to maintain synchronisation.

Because of the way Duobinary was coupled to the NICAM like digital audio subsystems for the MAC family, up to 50% of data reduction was possible in both Stereo and Mono transmission modes.