I am testing a new computer and scanner for my stream but am getting a lot of noise (background hum). I assume its from an impedence mismatch between the scanner and audio card. Is there an off-the-shelf cable to eliminate this noise?
This is a ground loop.
The ground potential of the scanner may be higher or lower than that of the computer and the computer's ground path to earth ground, or ground impedance, may be higher or lower than that of the scanner. The result of a ground loop is audible 60Hz hum.
These unbalanced components share a common ground bond between them via the shields of their interconnect cables, or via the ground wire of their AC power cords.
The ground bond via the AC power cords can be problematic because the cords may be different gauges, have different contact mechanical integrity at the outlet, and have different distances to travel back to earth ground. All these variables can create differences in the ground impedance of each component and opens the door for ground loops and system noise.
Check this web site out...you may find some good suggestions here and you will learn something:http://www.epanorama.net/documents/groundloop/index.html
I usually find that the safest and most convenient approach is to connect the scanner's ground (via the BNC plug or via a screw to chassis ground) to the PC case (usually via a screw on the back of the PC case). Other methods get more complex and sometimes don't work...plus they can pose safety hazards.