My introduction to AP was GFC 700 (with a G1000 set up), as i understand , its one of the best in GA , now before we get into garmin vs other other auto pilots.. thats not the intent of this post (though i am fairly certain it wont matter). anyway, after i got the century 2000 AP and flew around witt it, my first impression was it was broke. why? while flying in NAV mode it would do S turns trying to capture the magenta line or the green line. later on it dawned upon me that thats pretty much how the older AP works that do not have GPSS. now, in GFC 700 when i used ALT hold, it actually did ALT hold, all the time. in century, it does hold but every now and then it raises/lower the nose for now apparent reason trying to go to a diff altitude, i have never actually waited to see where it settles. as i understand, this behavior happens when the pressure changes, most of the times i usually get a new altimeter from center around the same time. what i want to understand is the technology behind the ALT hold and why a newer AP compensates for the pressure change. is there some kind of altitude encoder connected to the newer digital APs?