Figured out a new trick this weekend. I have a Focusrite Saffire audio interface connected to my computer. (Seems to have been discontinued :-( When I got it I learned that all that 4in/10out stuff around audio interface marketing is a bit misleading. As far as I was concerned I only had 2 ins. I can use XLR or 1/4", I have a line/inst switch, a gain knob and some level LEDs for each of these ins. The third and fourth inputs are SPDIF which is a single RCA style connector and thus completely useless.
Or so I thought.
Later on, I got a Boss GT-10 for mo' better guitar wankery. It has a lot of stuff on it including its own USB audio interface. I tried this out and quickly determined that to my untrained ears it sucked. I didn't spend a lot of time thinking about why, I just plugged the analog outs to my Saffire and moved on. Of course I have guesses. One guess is that the GT-10's USB interface presents 16bit/44kHz audio whereas the Saffire is capable of delivering 24bit/96kHz... Another guess is that I just like something that the Saffire preamps are doing. Another guess is that I didn't spend enough time figureing out how to setup the USB stuff to sound good. The final nail in the coffin comes from the way Logic will only deal with a single CoreAudio entity. What that means is if you have an input only device, like a GT-10 and you want outbound sound to go to a Saffire, you need to create an aggregate device (which is a totally cool feature but not without its little problems).
I lived happily in this way for a while until somebody asked me if the GT-10 had a digital out. Dunno, but I looked and surprise it had one... Quick check of the manual, I learn that digital out is actually SPDIF. Hmmm. I wonder if I can free up my analog inputs (so I could hook up a mic or something) by using this SPDIF thing?
Long story short, it works. And it sounds great.
Some minor details did need to be worked out. The Saffire comes with a program called Saffire Control and I needed to enable the SPDIF input. Additionally, the incoming level was a little low. Turns out the GT-10's digital out level is controlled by a configuration in one of the System menus. The main output level knob on the unit only controls the analog output level. This is actually kind of cool since you could, in theory, record the digital signal in addition to mic'ing an amp.
Interesting hobby we have. Lots of shiny toys but like Lego it's up to us to figure out what to do with them.