The spectrum of the stationary synchronous interval process in the stochastic point process obtained by superposing p Erlang renewal processes is derived by using relationships based on the Palm-Khinchine formulae and the fundamental identity linking the counting process of a point process to the interval process. The spectra coincide with those of mixed moving average--autoregressive processes. Explicit results are derived for a few simple cases for small p and a computational formula for the more complicated cases. Some general results on the shape of the spectrum of intervals are also given. (Author)