Everyone is well aware of the fact, that noise in an amplifier or .system, serves to degrade the information of the desired signal. In a cable system it sets the limit on the minimum signal that can be transmitted along the cable and still provide a good quality picture. In order to describe the quality of the picture in a more concise manner, we establish a relationship between the signal and the noise, namely the signal to noise ratio. Since noise fluctuates randomly over a period of time, it would be meaningless to try to relate an instantaneous signal voltage to instantaneous noise voltage. Instead, we deal with averages over a long period of time and use the mean squared signal voltage and mean squared noise voltage. The ratio of these two voltages is then referred to, as the signal to noise ratio. Since the signal and the noise are measured at the same point in a system and therefore appear across the same load, the ratio of mean squared voltages is also the ratio of powers and thus represents the signal to noise power ratio. Using this relation the quality of a T.V. picture can now be described with a number and the performance of a system with respect to noise, can be rated by the signal to noise ratio.