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The code which we analyse is an (8,4) parallel concatenated code. We have put into pictures that a turbo decoder is two decoders solving local minima problems in order to solve a global minima problem. We show the suboptimality of turbo decoders by plotting decision boundaries (MAP and turbo) on the plane. A good interleaver may be one where the produced decoding surfaces are as orthogonal to each other as possible. In analog decoders we see clearly that both component decoders act simultaneously.