The automated part of a turntable is the arm, the long plastic or metal piece that holds the needle and connects to the player at a rotating joint. On an automatic turntable, pressing the play button will cause a mechanism to move and drop the needle onto the record with no other physical manipulation required from the user. Automatic turntables will also return the arm to it’s home once playing is complete.
Semi-automatic turntables will do most of what’s described above, but require the user to actually move the tonearm back to its support after a side has played.
Ideally, this convenient mechanism leads to a consistent and safe amount of pressure being put on the needle with each play. However, these mechanisms can cause more surface noise, decreasing sound quality and accuracy. When any moving parts are added to a machine, the chances of needing repair increase. Automatic turntable arms are difficult to replace, especially for consumers.