Reduction. Air is compressed by squeezing it into a smaller space. Digital signals are compression by reducing the amount of data space required to hold them. Some digital data compresses readily, due to easily identifiable redundancy within the data. So, for example, a text document typically has a lot of space characters and 'e' characters, so other ways of expressing these can be found. Digital audio and video signal tend not to carry a lot of redundancy, so systems that compress these highly rely on eliminating some of the data and are consequently called lossy compression systems.