The ratio of the wanted signal (image, sound…) information to noise, usually expressed in dB. Noise can be high frequency, making pictures look grainy or adding a hiss to sound. Digitally generated images or sounds are theoretically capable of being pure; noise-free, having an infinite signal to noise ratio. But for pictures, their purity may cause contouring artifacts if they are processed without enough care; a reason for Dynamic Rounding.
A rule of thumb to express the realistic signal-to-noise capability of a digital system is given by the expression:
S/N (dB) = 6N + 6
where N is the number of bits. Hence an 8-bit system has 54 dB S/N and a 10-bit system 66 dB. This would be the noise level of continuous LSB dither and would only be produced over the whole picture by digitizing a flat field (i.e. the same gray over the whole picture) set at a level to lie midway between two LSBs. If it were set exactly on a digital level, there would be no noise. Other test methods give a variety of results, mostly producing higher S/N figures.