  We present a thorough analysis of the signal line shapes of Fourier
transform-based noise-immune cavity-enhanced optical frequency comb
spectroscopy (NICE-OFCS). We discuss the signal dependence on the ratio of the
modulation frequency, f${_m}$, to the molecular line width, {\Gamma}. We
compare a full model of the signals and a simplified absorption-like analytical
model that has high accuracy for low f${_m}$/{\Gamma} ratios and is much faster
to compute. We verify the theory experimentally by measuring and fitting
NICE-OFCS spectra of CO${_2}$ at 1575 nm using a system based on an Er:fiber
femtosecond laser and a cavity with a finesse of ~11000.
