Echo Indigo DJ
This is used in conjunction with the Traktor DJ Studio and basically transforms your laptop into a fully functioning DJ workstation.

The basics
Essentially, what this does is pop into your sound card slot and make it possible to monitor a second source of music while the master output plays independently. (ie, you can cue up a records with your headphones.) So, with this tool coupled with the Traktor DJ Studio, you can arrive at a club or party and plug and play. It really is a joy to behold.

From the Echo website
Finally, a portable solution for the digital DJ. Indigo DJ offers two independent stereo 1/8 inch analog outputs; one for setting up your cue mix on a pair of headphones, and the other for the house mix. Each output features 24-bit/96kHz converters for the quality you deserve. Also included is a 6 foot audio cable for RCA or 1/4 inch connections.
 
A unique feature of the Indigo DJ is the use of "virtual" outputs. Indigo DJ appears to software as if it has eight separate outputs, which are digitally mixed down to the physical outputs using Indigo DJ's "console" software and its on-board DSP. This makes Indigo DJ compatible with all popular multi-track software. And, with Echo's "multi-client" drivers, more than one application (such as an editor and a software synth) can be playing back through Indigo DJ at the same time.

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