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The author computes the error probability for conventional and for symmetric differential phase shift keying (CDPSK and SDPSK) with differential detection in systems with binary, ternary and quaternary symbols, and with two kinds of filters. In the first system, the system response is a Butterworth filter with two, three and four poles, which causes both intersymbol interference (ISI) and noise correlation which depend on the normalised 3 dB filter bandwidth B. In the second system the system response is a Nyquist filter with excess bandwidth beta . In this system the author considers both ISI caused by errors in sampling time and adjacent channel interference (ACI) caused by a signal in a neighbouring channel with frequency separation fA.