the cable can introduce electrical interference in the signal. The induced alternating current flowing through the resistance of the cable ground conductor will cause a small AC voltage drop across the cable ground. This is added to the signal applied to the input of the next stage. In audio equipment, the 50 or 60 Hz interference may be heard as a hum in the speakers. In a video system it may cause distortion or syncing problems. In computer cables it can cause slowdowns or failures of data transfer. Ground loops can also exist within the internal circuits of electronic equipment, as design flaws. Competing purposes of grounding The causes of ground loops have been thoroughly understood for more than half a century, and yet they are still a very common problem where multiple components are interconnected with cables. The underlying reason for this is an unavoidable conflict between the two different purposes of a grounding system: reducing electronic noise and preventing electric shock. From a noise perspective, it is preferable to have single-point grounding, with the system connected to the building ground wire at only one point. National electrical codes, however, often require all mains-powered components to have third-wire grounds; from a safety standpoint, it is preferable to have each mains component grounded. However, the multiple ground connections cause ground loops when the components are interconnected by signal cables. For this reason, in some large professional installations such as recording studios, it is sometimes the practice to provide two completely separate ground connections to equipment bays. One is the normal safety ground that connects to exposed metalwork, the other is a technical ground for cable screens and the like. Representative circuit The circuit diagram illustrates a simple ground loop. Circuit 1 (left) and circuit 2 (right) share a common path to ground of resistance . Ideally, this ground conductor would have no resistance (), yielding no voltage drop across it (), keeping the connection point between the circuits at a constant ground potential. In that case, the output of circuit 2 is simply . However, if this ground conductor has some resistance (), then it forms a voltage divider with . As a result, if a current () is flowing through from circuit 1, then a voltage drop across of occurs, causing the shared ground connection to no longer be at the actual ground potential. This voltage across the ground