What is dithering?

Short definition

Dithering is a process designed to minimise the unwanted side effects of reducing an image or audio files bit-depth.

Longer definition

Dithering is a process most often associated with reducing the bit depth of audio and image files.

It is often desirable or necessary to reduce bit depth. Here are a few scenarios ...

You have created a 24-bit / 44.1KHz audio mix master of a recording which needs to be converted to 16-bit in order to conform to the red book audio CD standard.
You have taken a high quality photograph with a digital camera and you need to make a smaller file size version to include in a web site.
You are viewing a web site which contains high quality images (thousands of colours) on an old computer which only has an 8-bit (256 colours) graphics card.

When you reduce bit-depth you lose quality. Dithering is a process which attempts to minimise this quality loss.

Audio dithering

Audio dithering is a process whereby low level white noise (random sound) is introduced into the signal to help randomise quantisation errors. The effect of this is to turn the audible effects of quantisation errors from unpleasant distortion into a the more acceptable analogue noise.

Click here to read a fuller explanation of quantisation and audio dithering.

Image / colour dithering

Image / colour dithering is a process whereby a number of pixels of 2 differing colours are arranged in a pattern in order to simulate the effect of a third colour during image optimisation / conversion / "downsampling".

Click here to read a full explanation of the dithering process in image file optimisation.