For example, in a 10 seconds fade in. With PWM the servo travels from one point to the other in a constant movement. What happens if I use digital?

That's a good point, Mytx, but

for the full DC voltage approach. This type of slow fade feature comes out more reliable and more flexible if implemented in software.

You would have a software routine that takes as arguments the index of a stored setting, and a number of seconds with which to fade, and possibly a time increment, probably defaulted to say 100 ms. The routine would then go in a loop which computes a new temporary set of values, each n 100ths of the way there, where n is the loop index and repeatedly sets all the sliders to those intermediate values.

That way the overall fade takes exactly 10 seconds in 100 imperceptible steps and you don't have to worry that the fader motors will get stuck if given only 25% PWM. Or that it is hard to calculate the PWM that corresponds to 10 seconds.

Another feature (for once you have this linear fade to Settings #3 in n secs command).

You can do a non-linear or exponential fade. In the above example of a loop 100x, instead of using the loop index "n" for the number of 100ths of the way there, use "en", where en = (int)(0.5 + pow (n/100.0, 1/2.0) * 100).

This zooms faster at first and slows down towards the end. (example: at index n=25, which is 1/4 of the way to 100, en is the square root of 1/4, or 1/2 of the way there, or index 50. You can even change the exponent by increasing the "2.0" for more effect or bringing it down to 1.0 for none or regular linear fade.