In receiving quotes from a few avionics shops for a GPS+ADS-B upgrade, I've noticed some shops say I need a separate altitude encoder, specifically the GAE-12 to work with the proposed Garmin GTX345. Other shops say I should get the ACK A.30 encoder. Some shops say I don't need either with the GTX but I can install if I want to.
Can someone shed some light on whether I need a separate encoder or not, and which one is appropriate? (and why?). Assume a GTX345+GTN650 setup.
Thanks!