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This paper presents formulations which relate average received signal power, noise density, and bit rate of the to source to the expected probability of bit error in the received information for binary phase modulation systems. The performance of these systems is analyzed for the practical case of demodulation with switching-type phase detectors and the performance is compared with optimal product demodulation. The received signals are assumed corrupted with additive white Gaussian noise. The following modulations are analyzed explicitly: 1) Direct-carrier phase reversal, 2) Squarewave-subcarrier phase reversal, 3) Sinusoidal-subcarrier phase reversal. It is shown that direct-carrier and squarewave-subcarrier phase reversal modulations are optimally demodulated with the switching-type phase detector. The sinuosidal-subcarrier phase reversal modulation, however, is not optimally demodulated by this method. The degradation from optimal product demodulation is slight for modulation indices of 2 radians or less but becomes greater for larger indices.