Dispersion-Tolerant Optical Transmission Using Duobinary Modulation Description Duobinary data encoding is a form of correlative coding in partial response signaling. The modulator drive signal can be produced by adding one-bit-delayed data to the present data bit to give levels 0, 1, and 2. An identical effect can be achieved by applying a low-pass filter to the ideal binary data signal. The correlated three-level signal can be demodulated into a binary signal again using an optical direct detection receiver. The advantage of this correlative electrical data encoding is that the duobinary modulated optical signal has a narrower bandwidth compared to the binary NRZ modulated signal.
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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. 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.
Synchronization and Zeroes[ edit ] 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. A bipolar encoding is an often good compromise: runs of ones will not cause a lack of transitions.
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: robbed-bit signaling and bit stuffing. T-carrier uses robbed-bit signaling: the least-significant bit of the byte is simply forced to a "1" when necessary. 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. Error detection[ edit ] 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. Other T1 encoding schemes[ edit ] Main article: Modified AMI code 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. Historical Uses[ edit ] 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.