usb soundcard and static issue

Audio and video systems need a reference point for their voltages. Generally referred to as common or ground, although it may not be actually connected with the earth, this reference remains at "zero volts" while other signal voltages "swing" positive (above) and negative (below) it. Physically, the common may be a wire, a trace on a printed-circuit board, a metal chassis, virtually anything that conducts electricity. Ideally it should be a perfect conductor, but in any practical system it is not. As the complexity and size of the system is increased, the imperfect conductivity of the common (ground) conductor inevitably causes problems.

Hum and buzz (50Hz/60Hz and it's harmonics) occur in unbalanced systems when currents flow in the cable shield connections between different pieces of equipment. Hum and buzz can also occur balanced systems even though they are generally much more

The cable shield currents and ground voltage differences are caused by several mechanisms. The second most common source of hum and buzz is the voltage difference between two safety grounds separated by a large distance or the voltage difference between a safety ground and an "Earth" ground (such as a grounded satellite dish or cable TV source). This problem is usually called "ground loop". This is the most common one in severe humming problems.

Hum and buzz can also be magnetically induced or capacitively induced directly into signal cables. Or the noise current can leak from mains input through capacitance between the A.C. power transformer primary and secondary windings which causes that a portion of the A.C. line voltage will ALWAYS be capacitively coupled directly to audio circuit ground. This capacitivly coupled power line signal will usually contain significant harmonics out to 1MHz or more. These signals will cause currents to flow in the cable shields thus adding this noise directly to the audio signal.