Based on the author's formalism given in [M. Singh, Optica Acta 31, 1293 (1984), Phys. Lett. A126, 463 (1988) and Phys. Lett. A139, 213 (1989)l we derive here for the first time a unique analytic generating function for the Gaussian signal characterized by a triangular auto-correlation. We utilize this result to show the inadequacy of the approximate formalism due to Bedard, Chang and Mandel [Phys. Rev. 160, 1496 (1967)l to study the behaviour of the time interval distribution of registering a conditional photocount.