Look-Ahead Sigma-Delta Modulation

The generic noise-shaping quantizer model from chapter four is modified to support look-ahead, and a hypothetical sigma-delta modulator with look-ahead capability is derived. From here the main principles of look-ahead are explored, including linear modeling of a look-ahead modulator, and an overview of expected benefits and disadvantages of a look-ahead sigma-delta modulator. Although only with an infinite amount of look-ahead the global optimal encoding solution can be found, it is concluded that already with a limited amount of look-ahead an improvement in the signal conversion performance can be obtained. The benefits of look-ahead, primarily an increase of the stability and a reduction of the distortion, are the biggest in the case of a 1-bit converter, since here the quantizer is severely non-linear. Therefore, in the remaining chapters the focus will be primarily on 1-bit converters. The operation of the full look-ahead algorithm, an algorithm that can be found in open literate and that performs an exhaustive search for the best solution within the look-ahead depth, is reviewed. With these insights the feasibility of an analog-to-digital converter with look-ahead is investigated, and it is concluded that the feasibility is low. It is found that in the case of a digital-to-digital converter, in principle, look-ahead can be implemented without major difficulties, but that the traditional full look-ahead approach is not computationally efficient.