How do we design systems to avoid noise?

In order to answer this question, we need to know a few basic facts.

Firstly, what do we mean by ground?

Look at Fig 2. This shows the three different sorts of 'ground' in a typical item of audio gear. Two of them, the chassis and earth/safety ground, are used to drain current from the screen of audio connections. The earth ground also acts as described previously as a 'safe' path in case of electrical faults. The chassis and earth ground should be connected together for safety reasons, because if a fault develops that means the chassis or case of an item becomes 'live' i.e. has a high voltage running through it, the current will drain to the safety ground and not through someone who happens to touch the case.

The third ground is the audio signal ground. This acts as a reference of 0V for the audio signals, and is generally seen as a track on the PCB (printed circuit board) in the unit. This ground should not be connected in any way to either the chassis or earth safety ground so that noise currents cannot enter the audio signals. However, in certain situations it is, and this is the main cause of problems.