N-to-1 elements have been previously mentioned and discussed in both and in . The main noteworthy thing about N-to-1 elements is that each pad is push-based in its own thread, and the N-to-1 element synchronizes those streams by expected-timestamp-based logic. This means it lets all streams wait except for the one that provides the earliest next-expected timestamp. When that stream has passwed one buffer, the next earliest-expected-timestamp is calculated, and we start back where we were, until all streams have reached EOS. There is a helper base class, called GstCollectPads, that will help you to do this. Note, however, that this helper class will only help you with grabbing a buffer from each input and giving you the one with earliest timestamp. If you need anything more difficult, such as "don't-grab-a-new-buffer until a given timestamp" or something like that, you'll need to do this yourself.