The authors describe a procedure for the empirical determination of system dynamic characteristics in which the time of measurement may be exchanged for test signal/noise ratio. The technique hinges on the well-known result that, if white noise is applied to a linear system, the cross-correlation of input and output gives the system impulse response.The authors point out that, in practice, it is much more convenient to use a periodic ‘random’ test input rather than a true stochastic input. Furthermore, by employing binary test inputs, the multiplica tion of output and input (in the cross-correlation) becomes simply a switching process. In describing some preliminary experimental work, the authors discuss the problems of mechanization supported by analytical considerations. The errors of such measurements are examined (including the interesting case when the test signal is com parable with system noise), and some illustrative results are presented.