Abstract: A sensor produces a signal indicative of the actual arrival time of each turbine blade at the sensor. A first circuit is responsive to the sensor for producing digital data representing the actual arrival time of each blade at the sensor. A second circuit establishes expected arrival times for each blade at the sensor. A third circuit is responsive to the digital data for generating a plurality of data entries each representing the difference between the actual arrival time and the expected arrival time. A fourth circuit stores the data entries in a first string having a plurality of dummy entries at the beginning thereof and at the end thereof and inserts into the string space saver characters in the event that the data entry for a particular blade is not produced. A fifth circuit establishes a running average value, over a plurality of data entries, for each data entry. The running average value is representative of common mode vibration.