Many monographs point out that differential encoding and decoding is necessary for effectual information transmission against phase ambiguity while seldom discuss the reason why phase ambiguity will emerge inevitably. Available algorithms are specially designed for certain modulation scheme; these algorithms cannot satisfy the requirement of soft-defined radio, which perhaps demands a uniform algorithm for different modulations. This paper proposes a new opinion on phase ambiguity from the view of probability. This opinion believes that modulating symbol sequence can affect, at optimum sampling epoch, the modulated waveform as oscillating carrier has done, and so the stochastic sequence leads to phase ambiguity. Based on a general signal model, this paper also puts forward a novel universal algorithm, which is suitable for different signals, even some new ones, by configuring several parameters.