You need a 4-in / 4-out usb or firewire audio interface to do that. My suggestion, just buy a Kaoss Pad or some similar external DJ FX unit and you'll be ALOT happier! (no fiddling with a mouse, and the laptop can also get in the way of your mixer...)

Buy something like FS2 and Traktor 3. Does the trick for me.

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if ure using cdjs 200, using traktor seems like a stupid idea. i agree that for using a cpu as a rack fx processor (if thats what i understood) u need a external audiointerface which probaly is more expensive than getting a decent hardware sampler. i suggest also to get an used korg electribe es-1, which has an audio input and its perfect for manipulating incoming audio, or some other external hardware solution. i dont think that is a good use for a cpu.