Look-ahead Sigma-Delta modulators look forward k samples before deciding to output a “one” or a “zero”. The Viterbi algorithm is then used to search the trellis of the exponential number of possibilities that such a procedure generates. This paper describes alternative tree based algorithms. Tree based algorithms are simpler to implement because they do not require backtracking to determine the correct output value. They can also be made more efficient using “Stack” algorithms. Both the tree algorithm and the more computationally efficient “Stack” algorithms are described. In particular, the effects of different error metrics on the performance of the “Stack” algorithm are described and the average number of moves required per bit discussed. The performance of the “Stack” algorithm is shown to be better than previously thought.
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