Abstract: A read amplifier generates pulses in response to the transitions of a phase encoded signal and selectively gates the pulses to an output and to a retriggerable one shot multivibrator under the control of inhibit pulses produced by the multivibrator. During a preamble of the data signal in which a known sequence of zeros is produced, a decade counter prevents generation of pulses in response to the polarity return transitions between the zeros and prevents the inhibit pulses from being applied to gate the pulses produced by the data signal until a predetermined number of zero pulses has been counted. During the preamble the inhibit pulses at the output of the multivibrator are averaged and compared with a reference so as to adjust the duty cycle of the multivibrator to produce inhibit pulses which are 70 percent of the duration of the bit intervals of the data signal.