Short term spectral analysis or source-filter modelling gives a parameterised description of the acoustic signal in terms of a sequence of vectors. These parameter vectors change slowly with time corresponding to a slowly moving vocal tract. This paper deals with modelling the time variation by a set of target vectors and interpolation functions that overlap in time; temporal decomposition (Atal, 1983). We present a simple geometric interpretation of the approach and discuss it in relation to hidden Markov modelling.