If the level representing the zero logical bit state is lower than the level for the one state, we call this positive logic, and the respective levels are then called low level and high level. Levels change at bit boundaries only if the bit value changes and remain stable for the entire duration of the bit period. The non-return-to-zero (NRZ) format is the prototypical representation of binary data: A logical zero state is transmitted as one signal level, and a logical one state as another level. 1.3.2 Binary Line Codes 1.3.2.1 Non-Return-to-Zero Code Longer runs create stress in many applications, because of either excessive intersymbol interference (ISI) or baseline wander due to local disparity. The run length distribution of a data pattern gives the relative probabilities for runs of identical consecutive bits. Random data is again exactly at the middle of the range: Because the probability that two consecutive bits are identical is 0.5, the transition density is 0.5, too. The transition density ranges from 0.0 to 1.0, where the extremes are marked by static patterns (all-zeros or all-ones) and toggle patterns. Where N T is the number of transitions in the pattern, N One is the number of ones, and N Zero is the number of zeros. The transition density (TD) of a data pattern is defined as the number of transitions in the pattern, divided by the length of the pattern: DC balance is an important property in some applications if it is required to maintain a DC level in the link, then amplifiers and other system components need to be DC coupled, often leading to a more complicated and problematic design. A pattern with a mark density of 0.5 is therefore also called a DC-balanced pattern. It is therefore a direct measure for the DC content of the signal. If we represent a zero bit by 0.0 and a one bit by 1.0, the mark density is equal to the time average over the pattern. If we look only at a subsection of the random data pattern, however, its mark density can be very different. Random data is exactly at the middle of the range: It contains as many one bits as zero bits, and its long-term mark density is therefore 0.5. The mark density ranges from 0.0 to 1.0, where the extremes are marked by all-zeros (N One equals 0) and all-ones data (N Zero equals 0). Where N One is the number of ones in the pattern, and N Zero is the number of zeros. The mark density (MD) of a binary data pattern is defined as the number of one bits in the pattern, divided by the length of the pattern: 1.3.1 Properties of Binary Data 1.3.1.1 Mark Density Coding efficiency determines the required link bandwidth, and the cost of implementation depends on the complexity of the code. Coding can influence the frequency spectrum, the direct current content, and the transition density of the resulting data stream. Numerous coding schemes are available, and which one is best for any given application depends on many factors. Line coding determines how the binary data is represented on the link. In electrical links, that's usually a voltage or current optical systems use the intensity of light and wireless radio links often use the phase and frequency of a signal carrier. When binary data is sent through a link, it is represented by a physical quantity in the transport medium. Learn More Buy 1.3 Line Coding of Digital Signals Digital Communications Test and Measurement: High-Speed Physical Layer Characterization
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