Abstract: Digital data which is correlatively encoded into discrete plural states and transmitted by modulating a carrier signal whose phase in each baud interval is a function of the data is detected through a tree searching technique which recursively determines the maximum metric for each of the states during each baud interval and generates a data bit stream therefrom. The maximum metric is determined by computing the transition metric for each possible path into a state from a comparison of the baseband signal with a reference signal generated therefrom and adding it to the state metric for the prior state from which the path originated to derive a path metric and then selecting the maximum path metric for each state designated survivor decision metric.