I have a project coming up in July, and I have been thinking about taping and also trying to cut live on my mac. I have been looking at CamTwist and BoinxTV and they're interesting pieces of software, especially for simple cutting...

My problem is I would need to find an interface, maybe something with 3 coax inputs for SD?

You might want to rethink live cutting on your computer because of the cost alone.
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