John is right, USB will not output live video, must use HDMI or analog. Slight correction, Component video WILL output 1080i if camera supports it, not limited to SD (Composite and S-Video are SD-only).
On a PC desktop, the BlackMagic Intensity Pro will work if you have an open PCI-e slot to put it in. Provides capture from HDMI or analog sources, plus output as well. Records as uncompressed or “Motion-JPEG” .avi – you will want to use the latter for sure, as uncompressed HD files require expensive drive arrays and are unnecessarily large.
Most cameras will send video direct from sensor to HDMI output before it ever gets compressed to camera format, so output is 4:2:2 uncompressed. Then you record to a 4:2:2 format with Intensity and really keep the quality up.
The Matrox MXO2 Mini is another capture device I rely on myself. Costs more, but provides a nice aluminum desktop breakout box, plus if you edit with Premiere it includes several plug-in effects.
EDIT: I’ll second John’s motion to look at Atomos, I use Ninja 2 and it works great to record HDMI source to ProRes 422, ready to edit.
Thanks
Jeff Pulera
Safe Harbor Computers