What is the purpose and advantages of shielded cable?

Answer 1

Shielded cable is commonly used for routing of audio, video, and rf signals. The grounded shield surrounds one or more inner conductors that carry the often low level signals and prevents spurrious external electromagnetic events from inducing an unwanted additional signal into the center conductor(s). Idealy, the signal cables should all be commonly grounded only at one location. Otherwise, a condition known as ground loop can occur which can actually degrade the signal and possibly even damage connected devices. In some cases where even more noise protection is needed, a two wire plus shield method known as balanced wire is used. The signal is applied across the two center conductors and at the recieving end applied to a balanced transformer with its center tap connected to ground and the shield. Therefore, any induced noise will (presumably) be equal in both signal conductors and will cancel itself at the transformer while the out of phase signal passes right through.