Controlling Distortion in a CATV Network 4

While the concept of noise in day to day life is well understood, giving it a specific, technical definition is a difficult task. Noise can probably be defined as random signals that are not part of the intended signal. Let us consider the case of a simple, small cable network transmitting signals on channels E-2 to E-12. Any randomly fluctuating signals on this network that are not part of the specific channel audio and video carriers are noise.
Noise is inevitable and occurs in every system. The key here is to keep noise below a level where it would cause deterioration in the received picture or sound at your most distant subscriber. Noise will occur within each channel. It will also occur across the entire frequency spectrum. The noise occurring across the entire spectrum is referred to as "Broadband Noise".
Figure

Figure 1 shows a Spectrum display of a particular channel of the audio and video carriers as well as the noise within that channel. As you will see from this picture the noise components (spikes) are approximately 50 dB less than the video carrier of this channel. This simply implies that the Carrier to Noise (C/N) ratio for this channel is approximately 50 dB. Noise occurs at every stage of electronic signal processing or amplification. Noise is also added during transmission of a signal even along a passive component such as a cable. This article does not aim to review the physics aspect of noise such as Shot Noise, Thermal Noise, Johnson noise, etc. But will restrict discussions to practical concerns in a Cable TV network. Let us now systematically examine the noise at each stage in a cable network.