In the Spline Modeler you can increase the surface fidelity under geometry>surface fidelity.... Default is 100%; you can increase it quite a bit (I don't know what the upper limit is, but it doesn't balk at 2000%), but I've found that 300 or 400% is usually plenty to hide any un-smooth edges in long curves. Beware, though, because you're asking Carrara to calculate more with higher surface fidelity. - Dex