Loop sync is Word clock 1x that is circulated to and from all HD interfaces in a Pro Tools|HD System. Loop sync always runs at 44.1 or 48k and all incoming clocks are reclocked by the 192 or 96 I/O interface.Besides the fact that HD systems use 1x wordclock (rather than 256x), there is a new "loop sync" connection. This is a second pair of BNC jacks that pass 44.1/48k sync between the boxes. This replaces the Slave Clock that was used in earlier systems. In configuration practice, the only difference is that you connect the output of the last interface back to the input of the first interface to form a sync "loop". The software "breaks" the loop at the "master" interface. This allows any interface in the system to be declared sync master - not just the top box in the chain as before. This new configuration eliminates sync repatching. The new SYNC peripheral supports this loop sync feature, but the USD (which was constructed with Mix, d24, and Pro Tools III in mind) does not.