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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Binary coding in which a a zero digit in the input sequence of binary digits is represented by a zero digit in the output sequence, whereas a one digit in the input sequence might cause a change in the output-sequence pulse power level, this change depending on the number of zero digits since the last one digit occurred, b if the number of zero digits since the last one digit in the input sequence is even, no change occurs in the output level, and c if the number of zero digits since the last one digit is odd, a change occurs in the output signal level to the appropriate level at which it was last positioned, positive or negative level.
Note 1: The duobinary coding bandwidth is about half of that required for nonreturn-to-zero NRZ coding. Duobinary coding requires a bandwidth of about 6. Note 2: Duobinary is a ternary signal and thus has a lower signal-to-noise ratio than pure binary. However, this loss is Skip to main content Skip to table of contents.
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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.