Quote Originally Posted by Jay Pace View Post
To record a mix your computer has to digitise an incoming analog signal, which requires an AD converter and processor speed enough to covert the signal into the required format (16/44100)

If you are recording a mix you are sending four signals out of your computer through a soundcard (2 stereo signals) which are then being mixed through an analog mixer, the output of which is essentially a new audio signal. To record a mix you have to route the output of your mixer through an AD converter and tell your computer to encode it and make a digital file from the incoming signal.
Ah, of course... I'm forgetting that you don't mix in the box with * Scratch.

If you were mixing with Ableton, for example, you could just load up SoundForge and hit record as you play.