this invention concerns a synchronizing control for an electro - hydraulic seismic vibrator , which control utilizes a magnetic tape cassette reader that puts out a train of pulses of changing time spacing of a preselected pattern . this operates a counter which controls the address of a read - only memory which has a selected number of memory locations , each one holding a corresponding one of a sequence of digitized values forming a single cycle of a sine wave . by continuing to operate the counter , each of the values stored in the memory is sequentially placed in an output circuit to provide a digitized sinusoidal reference sweep signal . the cassette reader signal also drives a second counter through a frequency ratio control of the pulses provided by the cassette . by this means , the second counter can be varied to count at a different rate from the first counter . the second counter also addresses the read - only memory , and the outputted digital signals are converted to an analog signal which is used as the drive sweep for the vibrator . the motion of the vibrator baseplate is detected by an accelerometer , the signal of which is doubly integrated and converted to digital form . one of two identical digital filters filters the reference sweep signal and the other the accelerometer signal . the filter outputs then go to a phase comparison network which controls the frequency ratio control of the second counter . when the reference sweep leads or lags the output signal from the accelerometer , the frequency ratio control advances the second counter at a higher or a lower rate , as necessary , to convert the second digital output of the memory to a correspondingly different phase , in order to correct the phase relation of the vibrator output and the reference sweep .