Digidesign Sync I/O is a unit to help you synchronize everything up in your Pro Tools HD system. It has options and connections of all types, including for video, word clock, LTC, AES/EBU, and more. It supports sample rates up to 192 kHz, and will support pretty much any time code format. It is rack mountable and will take up a single space in a traditional rack casing. This isn't a necessary device to run Pro Tools HD with, rather is a tool to help those using other time code devices and formats to have everything in synch. If you need one of these, you will know that you need one, it isn't like it is something that you can add to make your system better, it is mostly a necessity item with all possible accommodations. It is mostly aimed for Pro Tools user in post production and for working with audio and film. Everything about the Sync I/O is very clean cut and since it is a Digidesign product, it will of course be compatible with your HD system. I'm not sure if it would work in tandem with another kind of system, but I would stray away from it unless you do in fact have an HD system. The price of the Digidesign Sync I/O isn't cheap, as you will generally pay a bit more than average for a Digidesign product, but if you need it, you need it. This is just one of those products that I can't necessarily recommend one way or the other. Although it is definitely a high quality product, it isn't like you really have any other choice if you need a synchronizer for your Pro Tools HD system. The Digidesign Sync I/O is a very well built unit that is an ideal addition to your Pro Tools HD system.