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Such a signal is called a duobinary signal. Thus, the line always returns to the "zero" level to denote optionally a separation of bits or to denote idleness of the line. One kind of bipolar encoding is a paired disparity code , of which the simplest example is alternate mark inversion.
In this code, a binary 0 is encoded as zero volts, as in unipolar encoding , whereas a binary 1 is encoded alternately as a positive voltage or a negative voltage. The name arose because, in the context of a T-carrier , a binary '1' is referred to as a "mark", while a binary '0' is called a "space".
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.
Other systems must synchronize using some form of out-of-band communication, or add frame synchronization sequences that don't carry data to the signal. These alternative approaches require either an additional transmission medium for the clock signal or a loss of performance due to overhead, respectively. A bipolar encoding is an often good compromise: However, long sequences of zeroes remain an issue. Long sequences of zero bits result in no transitions and a loss of synchronization.
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. There are two popular ways to ensure that no more than 15 consecutive zeros are ever sent: T-carrier uses robbed-bit signaling: The modification of bit 7 causes a change to voice that is undetectable by the human ear, but it is an unacceptable corruption of a data stream.
Data channels are required to use some other form of pulse-stuffing,  such as always setting bit 8 to '1', in order to maintain a sufficient density of ones. 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.
On the decoder side, this extra '1' added by the coder is removed, recreating the correct data. Another benefit of bipolar encoding compared to unipolar is error detection. In the T-carrier example, the bipolar signals are regenerated at regular intervals so that signals diminished by distance are not just amplified, but detected and recreated anew.
Weakened signals corrupted by noise could cause errors, a mark interpreted as zero, or zero as positive or negative mark. Every single-bit error results in a violation of the bipolar rule. Each such bipolar violation BPV is an indication of a transmission error. The location of BPV is not necessarily the location of the original error. For data channels, in order to avoid the need of always setting bit 8 to 1, as described above, other T1 encoding schemes Modified AMI codes ensure regular transitions regardless of the data being carried.
A very similar encoding scheme, with the logical positions reversed, is also used and is often referred to as pseudoternary encoding. This encoding is otherwise identical.
B-MAC , and essentially all family members of the Multiplexed Analogue Components Television Transmission family used Duobinary to encode the digital audio, teletext, closed captioning and selective access for distribution. At least with some data transmission systems, duobinary can perform lossless data reduction though this has seldom been utilized in practice.
From Wikipedia, the free encyclopedia. Line coding digital baseband transmission. Unipolar encoding Bipolar encoding On-off keying. Carrier-suppressed return-to-zero Alternate-phase return-to-zero.