I want to keep two dir/file hierarchies of music in sync.

So that if I add/remove/change files in tree #1, only those changes get updated in tree #2.

It doesn't need to be realtime.

rsync works perfectly for trees 1 & 2 simply containing the same files/formats. E.g., an ogg tree -> ogg tree sync.

In this case, though, tree 2 contains an audio-format conversion of tree 1.

I.e., all the dir/file names & hierarchy are the same, except that every tree 1 .ogg file exists as its converted .mp3 analogue in tree 2.

(How) can rsync be used to keep these 2 different trees in sync, detecting changes in tree 1, then converting-then-rsyncing only those files/folders?

Or, is rsync not the best tool here?