Technical Reference

Signal to Noise Ratio

One of the factors determining the clarity or quality of a picture on a monitor is noise. Noise is snowyness or graininess over the entire picture on a monitor. Every time a video signal is processed or transmitted in any way noise is introduced. There are any number of sources of noise, from poor circuit design, to heat, to your cell phone and even the Universe itself. This is one place where literally everything is against you. I am not kidding about the Universe either...whenever you see a tv screen with just snow on the screen, about 1% of that is the Universe's cosmic background radiation.

Great, so noise makes your picture crummy. And noise is literally everywhere. How do you know what sources produce a good picture ? What you want is a source that provides a lot of signal for very little noise. You need to know its signal to noise ratio. Often abbreviated to S/N ratio or even just SNR. A big number is good.

The easiest way to understand signal to noise ratio is a little thought experiment. Set your camera to a specific exposure. Doesn't matter what f-stop or shutter you use, just make sure the camera doesn't change them automatically. Also set your camera to 0dB gain. Now light the camera to 100 IRE. Doesn't matter how much light you use, just make sure the image is all white at 100IRE. Now...put the lens cap on. You are looking at what is supposed to be a black screen. If you have a very good camera it may look black, but it isn't really. Measure it on your waveform monitor ...for a good MiniDV camera you should get about 0.2 IRE.

In this example your signal is 100IRE and your noise is 0.2IRE. Your signal to noise ratio is 100/0.2 = 500. Most often this ratio is described using dB, or decibel, units. Decibels area a logarithmic unit system designed to approximate human sensory response.

To convert our raw SNR number to dB we use this formula:

SNR = 20*(log(signal/noise))

[read as,"The signal to noise ratio equals twenty times the log of signal level divided by noise level."]

For our example this gives 53.979 dB, which is the SNR for a typical "prosumer" DV based recorder.