kokleunam,
Somewhere I'm sure I saw a description of a device that converted HD SDI to HDMI. Maybe I saw it on this website.
Anyhow, if you have an HDMI signal, you can capture it with an Intensity card from Black Magic Design. The card fits in a PCI Express slot in your computer. Of course, the computer needs to be fast enough to record uncompressed HD in real time. That's a significant requirement. ;-)
Ken Hull