Some background:
I work in a technical field called >
EMC< (ElectroMagnetic Compatibility). This breaks down into two courses of investigation: Emissions, and Immunity.
For emissions testing, we put the test sample in an electrically 'quiet' room, and listen to it with sensitive antennas and analyzing equipment. If it makes more noise than the applicable regulations permit, the product can not be sold in the marketplace.
Immunity testing subjects a product sample to specified levels of interference to see if it will continue to operate as specified by the manufacturer. Again, if it fails, it can't be sold commercially.
These tests weed out the worst offenders, but since a certain amount of noise is allowed, there is always the potential for two pieces of equipment to interfere with each other.
Any time you get a current flowing through a wire, there is a magnetic field produced around that wire. If the amplitude of current changes, the amplitude of the magnetic field will change with it. A changing magnetic field will induce a current in any wire that is in the field. This is how guitar pickups work.
Electric fields are the result of voltages appearing between two points, instead of current flow, but they behave somewhat like magnetic fields, and they can also effect electronic devices.
So, in a computer monitor you have voltages being switched at high frequecy, and current flow being started and stopped at high frequency. Older picture-tube monitors are worse because they use high voltages (reulting in high currents), but even new LCD and plasma displays give off emissions. Both magnetic and electric-field. Same thing with fluorescent lights.
These emissions can get picked up by anything that acts like an antenna. Which means that your pickups do a great job of receiving magnetic pulses, and cables can be very good E-field antennas. And then the amplifiers boost these interference signals along with the music. Articles like that SoundOnSound link give some ideas of what problems are likely to occur and some solutions to try when they do.
Lordathestrings
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