Compression systems that are used to create a fixed rate of output data. This is usually to fit within a given bandwidth such as that available on a video tape recorder or a constant bit rate transmission channel. With video, the amount of useful information contained in the material varies widely both spatially and temporally – with movement. For example, a football match with crowds and grass texture as well as fast panning cameras typically contains far more information than a largely static head-and-shoulders shot of a newsreader. Using constant bit rate means that the video quality may suffer in order to meet a specified maximum bit rate. In the football case, the texture of the grass may go ‘flat’ during a camera pan, and reappear when the camera is still.
As overflowing the available bit rate budget could have disastrous results with bits being lost, the aim is always to use just under the available bit rate. The degree of success in almost filling the available space (not easily done live) is a measure of the quality and efficiency of the compression system.
See also: Variable bit rate