Partial response coding forbids certain sequences of symbols. If M is the number of baseband levels from partial response coding, then the coded signal cannot transverse more than N levels between consecutive symbols, where N is (M 1) / 2. The only way such forbidden sequences can occur is when a symbol has been received in error. The assumption is that errors are caused by white Gaussian noise, and are displaced one level only from the correct level into adjacent levels. This is a valid assumption under normal received S/N ratios of interest. When such a forbidden sequence occurs, it is called a partial response violation, or PRV. The probability of detecting PRV given that a symbol error has occurred in the past D symbols is:
In an ambiguity zone decoder, the eye is divided vertically between the pinpoints (ideal locations of the eye centres) into two types of regions. The symbols sampled in the regions closest to the pinpoints are given ambiguity weight of 0. Other regions, or the ambiguity zones, are further from the tiny pinpoints and given an ambiguity weight of 1. Their relative ambiguity positions above or below pinpoints are also marked for later error correction. Symbols measured in the ambiguity zones are far more likely to be in error than symbols closer to the pinpoints and are considered to be suspect. The decoded symbols and their ambiguity weights and position markers are fed into the error (PRV) detection and correction circuitry.
When a PRV is detected, the decoder will look back D symbols to see if any decision is made in the ambiguity zone, that is with ambiguity weight of 1. A correction is made if and only if both conditions are met. The decoder tracks only one PRV at a time and associates the nearest ambiguity decision to the PRV. The correction is made by pushing the ambiguity decision one level up or down using the position marker.
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