People hum when they don't know the words. Audio circuits hum when they
don't know what silence should sound like. The sensitive circuits that
boost your camera's audio before it's recorded, or shuttle sound around
your editing suite, need a reference they can be sure is zero volts.
They compare the input signal to this reference, amplify or proces the
difference, and generate an output voltage that's also compared to the
reference. Designers designate one point within a piece of equipment
(often connected to the chassis or grounding pin of the power plug) and
call it "ground": all voltages inside the equipment are measured with
respect to it.
That's fine for a single piece of equipment, but when you hook two devices together, both have to agree on the reference. Since the cable shield has to be grounded at least at one end, the usual scheme is to use it to connect the two devices' reference points together. It works in very simple systems.
But remember, that shield is picking up hum from the building wiring. And if the shield is carrying current -- something unavoidable if it's part of the audio path -- it has a slight voltage drop. Both these factors mean that the two devices are going to have slightly different references, and the difference is constantly varying. The input circuit can't tell that this variation isn't part of the signal, so it amplifies it. Again, in a simple NLE with short wires this interference may be tolerable. But in a complex room or studio shoot, it becomes hum and noise.
Complex setups have other problems as well. If there are multiple ground paths, they combine to make a very efficient loop antenna for the 60 Hz noise. These "ground loops" are almost impossible to predict, since you don't know the internal details of your equipment, and can crop up even in very simple setups... particularly if both pieces of audio equipment also share a ground connection through their power plugs' grounding pins. In a practical video studio, the situation is apt to be far worse: the non-audio cables -- RS-232 and RS-422 control, video wires, and even cable TV -- all have their own grounds.
- (Source : http://www.dplay.com)
That's fine for a single piece of equipment, but when you hook two devices together, both have to agree on the reference. Since the cable shield has to be grounded at least at one end, the usual scheme is to use it to connect the two devices' reference points together. It works in very simple systems.
But remember, that shield is picking up hum from the building wiring. And if the shield is carrying current -- something unavoidable if it's part of the audio path -- it has a slight voltage drop. Both these factors mean that the two devices are going to have slightly different references, and the difference is constantly varying. The input circuit can't tell that this variation isn't part of the signal, so it amplifies it. Again, in a simple NLE with short wires this interference may be tolerable. But in a complex room or studio shoot, it becomes hum and noise.
It's called "60 Hz hum", but it's not just 60 Hz
When power-line frequencies leak into an audio circuit, they generate harmonics. The 60 Hz base signal also hums at 120 Hz, 240 Hz, and up the band. That's why filters don't do a good job removing hum... you have to fix it at the sourceComplex setups have other problems as well. If there are multiple ground paths, they combine to make a very efficient loop antenna for the 60 Hz noise. These "ground loops" are almost impossible to predict, since you don't know the internal details of your equipment, and can crop up even in very simple setups... particularly if both pieces of audio equipment also share a ground connection through their power plugs' grounding pins. In a practical video studio, the situation is apt to be far worse: the non-audio cables -- RS-232 and RS-422 control, video wires, and even cable TV -- all have their own grounds.
- (Source : http://www.dplay.com)
