Undesired electrical, molecular, radio frequency or other energy that interferes with a signal as it moves from one place to another. Like cacophony is to the human ear — some flavors of acid rock, fingernails down a blackboard, each instrument in an orchestra playing a different song at a different tempo at the same time — noise is taxing to signal passage.
The IEEE defines noise as “unwanted disturbances superposed upon a useful signal that tend to obscure its information content.” Cable engineers use a more strict definition, classifying noise as “thermal,” meaning it is both unpredictable, and doesnt repeat.
Thermal noise is a function of HFC transmission characteristics and bandwidth-per-channel, and is usually expressed in decibels relative to microvolts (mV), or dBmV. When designing cable systems, engineers calculate the minimum thermal noise power figure, expressed as “the noise floor.” The noise floor in cable systems is generally recognized to exist at (-) 59 dBmV.
In general, when the noise floor rises, signals degrade or slow down (in the case of some digital transmission systems.) This is no different than driving on a heavily pocked and pitted road. It makes you slow down. The potholes, in this case, are the noise. On a smooth road — no potholes, no noise — you can go faster. Same idea.
Noise is also an element in two of the most commonly used design parameters in cable systems: The carrier-to-noise ratio (also known as “C/N” and “CNR”) and the signal-to-noise ratio (also known as “S/N” and “SNR.”) Carrier-to-noise ratio quantifies noise figures of individual amplifiers, input levels to each amplifiers, and the number of amplifiers in cascade, to come up with an end-of-line “budget” of acceptable CNR. SNR measures noises as a function of signal level, and is also used to calculate an end-of-line “budget” of acceptability.
Noise is particularly prevalent in the 5-40 MHz spectral area used by cable operators to send signals from homes to the headend. Here, two other types of noise enter the discussion: Impulse noise, generated by random bursts of unwanted electricity, and ingress, which happens when signals leak into a section of coaxial cable.