The signal-to-noise ratio (s/n ratio) is the ratio of the video signal to the
amount of noise present in the image. Noise in a video image is seen as
snow or graininess, resulting in a poorly defined image on the monitor or video
recording. The unit for expressing s/n ratio is decibels (dB), but it
can also be expressed as a ratio.
A signal-to-noise ratio of 40dB is equivalent to a ratio of 100:1, which means
that the signal is 100 times the noise level. Conversely, the noise is
one hundredth of the signal. At a signal-to-noise ratio of 20dB, the noise
is 10% of the signal and would produce an unacceptable picture. The following
table provides a guide as to the quality you can expect at various signal-to-noise
ratio levels.
There are many sources of noise, including poor circuit design, heat, over-amplification,
external influences, and automatic gain control, as well as transmission systems
such as microwave and infrared. The signal-to-noise ratio is an important
measure of video quality: the higher the camera's signal-to-noise ratio,
the better the image quality delivered by the camera.
