6 Line Coding: Unipolar Unipolar Line Coding –uses only one non-zeroand one zero voltage level (e.g.) 0 = zero level, 1 = non-zero level simple to implement, but obsolete due to two main problems: DC component present lack of synchronization for long series of 1-s or 0-s
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7 Line Coding: Polar Polar Line Coding –uses two non-zero voltage levelfor represent. of two data levels - one positive & one negative “DC-problem” alleviated 4 main types of polar coding: NRZ-level NRZ-invert (1) Nonreturn to Zero (NRZ) NRZ-level: signal level represents particular bit, (e.g.) 0 = positive volt. , 1 = negative volt. poor synchronizat. for long series of 1-s & 0-s NRZ-invert: inversion of voltage level = bit 1, no voltage inversion = bit 0 1s in data streams enable synchronization long sequence of 0-s still a problem