Greg,
and think that the AC that goes to the engine is what is being referred to as using DC
He would be correct! 
As I stated in my initial post, "Use DC voltage for track power as opposed to AC voltage".
The issue is not how the TIU is powered, rather, how the PS3 engine is powered. What is being discussed is using DC voltage as input to the TIU channel (must be a fixed channel) rather than AC voltage.
As an FYI, one of my close friends who is also a DCS beta tester and MTH-trained tech called me one evening to discuss problems he was having transferring a PS3 sound file while using DC voltage.
The engine kept coming up in conventional mode as soon as track power was applied and the DCS Loader Program couldn't find it.
As it turned out, the problem was that the TIU itself was being powered by AC power through its Aux. Power port. When that power source was removed or also changed to DC, the transfer worked without a hitch.
Unfortunately, we never did any further research into the issue. However, as I recall, he was using an older TIU model rather than a Rev. L.
