Frequency domain versus time domain methods in system identification

For some reason I now forget, Keith Glover and I were asked to prepare a discussion paper on frequency and time domain methods in system identi cation by the organizers of the IFAC symposium on System Identi cation in Darmstadt 1979. I visited Keith in Cambridge in April 1979 to prepare our paper. It later evolved into an Automatica arcticle. Since then, there has been a useful development of several of the problems and questions dealt with in that paper. Questions of Maximum Likelihood formulations, asymptotic analysis, choice and design of noise lters and numerical algorithms, for example have been discussed in many references. In this contribution for the Glover Birthday Conference I will focus on three aspects that relate to using frequency domain data for identi cation: The role of periodic input data, the role of the input intersample behavior and packaging a transparent time-frequency domain tool for identi cation.